Abstracts No.21: 9th AGC Achievements in Australian Geoscience, 1988, Brisbane by GSAustralia

Australia 1788-1988

NINTH AUSTRALIAN GEOLOGICAL CONVENTION GEOLOGICAL SOCIETY OF AUSTRALIA

ABSTRACTS NUMBER 21

FEBRUARY 1-5,1988 UNIVERSITY OF QUEENSLAND, BRISBANE

Geological Society of Australia

ABSTRACTS NUMBER 21

NINTH AUSTRALIAN GEOLOGICAL CONVENTION

FEBRUARY 1-5, 1988 UNIVERSITY OF QUEENSLAND, BRISBANE

Published by the Geological Society of Australia Incorporated Chains House, 10 Martin Place, Sydney

ISBN 0 9 0 9 8 6 9 56 1

1988

ISSN O y S S - O U X

Geological Society of Australia Incorporated

O f f i c e B e a r e r s 1986-1987

P r o f . D . M . Boyd P r o f . J.B. Waterhouse M r I . R . Johnson Dr A.P. Belperio Mr J.W. Hunt P r o f . J. R o b e r t s Mr D.H. Probert

President Vice Presidents Hon. Secretary Hon. Treasurer Hon. Editor Hon. A d m i n i s t r a t i v e O f f i c e r

Ninth Australian Geological Convention, 1988

Organising C o m m i t t e e

CONVENER:

Mr Rod Allen

SECRETARY:

Dr G e r h a r d H o f m a n n

M r Bob H a l l Dr Neil Williams

Scientific Program:

Dr C l i f f M a l l e t t

Excursions:

Dr L l o y d H a m i l t o n Dr E r i c H e i d e c k e r

Trade Exhibition:

M r A n d r e w Simpson

Promotions:

Mr Bill Whitaker Mr Neil Krosch

Accommodation and T r a v e l :

Mr Brian Coxhead Mr Graham Boyd

Social Program:

Mr Lance Grimstone

GSA E x e c u t i v e :

P r o f . B r u c e Waterhouse

Finance:

Geological Society of Australia Abstract Series Number 21

CONTENTS LIST OF P A P E R TITLES Keynote Papers Scientific Papers Scientific Posters

PAGE 4 4 21

A B S T R A C T S (alphabetically by author) Keynote Papers Scientific Papers Scientific Posters

23 32 438

AUTHOR INDEX

452

K E Y N O T E PAPERS Professor Kenneth A . Eriksson, V i r g i n i a Polytechnic I n s t i t u t e and State U n i v e r s i t y , Blacksburg, USA: 'Precambrian basin f o r m a t i o n and evolution comparison w i t h possible Phanerozoic counterparts', Dr Dennis V . Kent, L a m o n t - D o h e r t y Geological Observatory and D e p a r t m e n t of Geological Sciences of Columbia U n i v e r s i t y , Palisades, USA: 'Pangea, Gondv^ana and paleomagnetic problems in Paleozoic f o l d belts'. Professor Bruce E. Hobbs, Monash U n i v e r s i t y , C l a y t o n : in the minerals e x p l o r a t i o n industry'.

' C o m p u t e r modelling

Professor Tom G . Vallance, The U n i v e r s i t y of Sydney: Finding the way f r o m s c r a t c h - o p p o r t u n i t y and r e a l i z a t i o n in A u s t r a l i a n geology over t w o centuries'.

SCIENTIFIC PAPERS SESSION 1: H I S T O R I C A L PERSPECTIVES 1.1

DAVIES, H . L . Geologists in Papua New Guinea - M a i t l a n d , Stanley, Wade and others.

1.2

ROBERTSON, A . D . Copper mining, b r i c k m a k i n g and furnace building at K a r i b o e , Dawes Range, c e n t r a l Queensland.

1.3

ROBERSTON, A . D . H i s t o r y of sapphire mining in c e n t r a l Queensland.

1.4

J O Y C E , E.B. ^ . . , Looking a f t e r s c i e n t i f i c sites in A u s t r a l i a - a decade of work by the Geological Society of A u s t r a l i a .

SESSION 2: D Y N A M I C S O F BASIN F O R M A T I O N 2.1

DAVIES, G.F. M a n t l e superswells - a d i s t i n c t r i f t i n g mechanism?

2.2

LISTER, G.S., E T H E R I D G E , M . A . and SYMONDS, P . A . Modelling the e f f e c t of detachment f a u l t i n g on the uplift-subsidence histories of passive c o n t i n e n t a l margins.

2.3

ETHERIDGE, M.A. Ramp basins - a new class of sedimentary basin in extensional settings.

2.4

W I L L I A M S O N , P.E., SWIFT, M.G. and O'BRIEN, G.W. Two-stage L o w e r Cretaceous r i f t i n g in the O t w a y Basin.

2.5

H E G A R T Y , K.A. and H O U S E M A N , G.A. A model for the Tertiary evolution of the lithospheric stress field of the Gippsland Basin.

2.6

H A M M O N D , R.L. and M A L L E T T , C.W. The Bowen Basin - an upper crustal extension model for its formation.

2.7

B O N E , Y. and R U S S E L L , N. Palaeotemperatures in basin analysis - direct reflectivity with fluid inclusion microthermometry.

correlation

vitrinite

2.8

M I D D L E T O N , M.F. Subsidence trends in Western Australian sedimentary basins - implications for formation mechanisms.

2.9

H A R R I N G T O N , H.J. Oroclinal basins.

2.10

P I G R A M , C.J. and S Y M O N D S , P.A. Development of the Moresby Trough region - passive margin to foreland basin.

2.11

L A M B E C K , K. The Perth Basin - a possible framework for its formation and evolution.

2.12

L A M B E C K , K. Seismic travel time anomalies in central Australia and implications for deep crustal structure.

2.13

G O L E B Y , B.R., WRIGHT, C., C O L L I N S , C.D. and K E N N E T T , B.L. Central Australian crustal structure - results from seismic profiling and associated geophysical studies.

2.14

K O R S C H , R.J. and L I N D S A Y , J.F. Origin and evolution of the Amadeus Basin, central Australia.

2.15

SHAW, R.D. E T H E R I D G E , M.A., Z E I T L E R , P., L A M B E C K , K. B L A C K , L.P. Basement uplift as a constraint on basin formation in central Australia.

2.16

L I N D S A Y , J.F. and K O R S C H , R.J. Timing the breakup of a Proterozoic Australian intracratonic basins.

supercontinent

- evidence

and

from

2.17

D R U M M O N D , B.J., W Y B O R N , L.A.I., W Y B O R N , D. and T A R N E Y , J.F. Temporal changes in continental growth patterns revealed by siesmic crustal structure and granite geochemistry.

2.18

M c N A U G H T O N , N.J., P A R T I N G T O N , G.A., SEET, L.H. and K E P E R T , D.A. Craton margin tectonics and the origin of Sn-Ta pegmatites in the southwest Yilgarn Block - the Pb isotope approach.

2.19

M c N A U G H T O N , N.J., G R E E N , M.D., C O M P S T O N , W. and W I L L I A M S , I.S. Are anorthositic rocks basement to the Pilbara Craton?

2.20

B O U R K E , D.J., M c C O N A C H I E , B.A., S E N A P A T I , N. and S L A D E , J.C. A tectonic reconstruction for the basement rocks beneath the Carpentaria Basin.

2.21

GRIMSTONE, L. The Thomson Terrane r e v i s i t e d .

2.22

W I L S O N , C . J . and W A T C H O R N , R . B . F o l d i n g , t h r u s t i n g and s t r i k e slip f a u l t i n g in the southwest p o r t i o n of Lachlan Fold Belt.

2.23

STUART-SMITH, P.G. E x t e n s i o n a l t e c t o n i c s and development Wales.

of the T u m u t Trough, N e w

the

South

2.24

L E V E N , J., S T U A R T - S M I T H , P . G . , R I C K A R D , M . and C R O O K , K . A deep seismic survey across the T u m u t Trough, N e w South Wales.

2.25

W I T H N A L L , I.W. and L A N G , S . C . T e c t o n i c history of the P a l a e o z o i c B r o k e n R i v e r P r o v i n c e , north Queensland.

2.26

SCOTT, M. S t r a t i g r a p h y of the C l a r k e R i v e r Basin, north Queensland.

2.27

W A K E - D Y S T E R , K . D . , S E X T O N , M . J . , J O H N S T O N E , D.W. and F I N L A Y S O N , D.M. S e i s m i c f e a t u r e s of the Thomson F o l d B e l t under the w e s t e r n Surat Basin.

2.28

B R O W N L O W , J.W. T h e r m a l systems and t h e r m a l provinces in the L a t e C a r b o n i f e r o u s to M i d d l e T r i a s s i c development of eastern A u s t r a l i a .

2.29

H A R R I N G T O N , H.J. R i b b o n t e c t o n i c s and low dip s t r i k e slip f a u l t s .

2.30

W E L L M A N , P . and K O R S C H , R . J . O r o c l i n a l bending of the N e w England O r o g e n - a r e f i n e m e n t using m a g n e t i c and g r a v i t y anomalies.

2.31

B R O W N L O W , J.W. T e c t o n o t h e r m a l r e g i m e s and the m i d - C a r b o n i f e r o u s e v o l u t i o n of northeastern N e w South Wales.

2.32

L E N N O X , P . G . , R O B E R T S , J . and J E F F R E Y , S. The enigma of the Hastings B l o c k , N e w England O r o g e n .

2.33

F I N L A Y S O N , D.M., W A K E - D Y S T E R , K.D., J O H N S T O N E , D.M. S E X T O N , M.J. and M U R R A Y , C . G . C r u s t a l s t r u c t u r e s across the N e w England F o l d B e l t under the eastern Surat and C l a r e n c e - M o r e t o n Basins.

2.34

S E X T O N , M . J . , W A K E - D Y S T E R , K . D . and J O H N S T O N E , D.W. S e i s m i c r e f l e c t i o n p r o f i l i n g on the B e e n l e i g h B l o c k , Queensland.

Middle

SESSION 3: PALAEOMAGNETISM AND TECTONICS IN AUSTRALASIA 3.1

M c E L H I N N Y , M.W. A r e v i e w of the status of A u s t r a l i a n p a l a e o m a g n e t i s m .

. . Triassic

3.2

M c F A D D E N , P.L. The origin of the geomagnetic field and its palaeomagnetic consequences.

3.3

BARTON,.C.E. Magnetostratigraphy on a timescale geomagnetic excursions and reversals.

of 0-10^ years - secular

3.4

C L A R K , D.A. Magnetic fabric - principles and applications to tectonics.

3.5

E T H E R I D G E , M . A . and W Y B O R N , L.A.I. Tectonic constraints on palaeomagnetic Proterozoic.

variation,

models for the Early to

Middle

3.6

G I D D I N G S , J.W. and I D N U R M , M. A review of the Precambrian apparent polar wander path for Australia.

3.7

I D N U R M , M. Accumulated apparent polar wander - a new phenomenon in palaeomagnetism.

3.8

E M B L E T O N , B.J. Low palaeolatitudes Australia.

for

late

Precambrian

periglacial

deposits

South

3.9

T A N A K A , H. and I D N U R M , M. Paleomagnetism of Precambrian mafic dykes and intrusives around Mount Isa.

3.10

POWELL, C.McA. Palaeozoic tectonics of eastern and central Australia - implications palaeomagnetic studies.

3.11

S C H M I D T , P.W. Current palaeomagnetic constraints on Tasman terrane analysis.

3.12

L A C K I E , M.A. Magnetic fabric and palaeomagnetism of the Dundee comagmatic granites of the Moonbi Suite, New England.

Rhyodacite

for

and

3.13

T H R U P P , G. Preliminary paleomagnetic results from mid-Paleozoic rocks of the Tasman Orogen, Victoria.

3.14

L I , Z.X., S C H M I D T , P.W., P O W E L L , C . M c A . and E M B L E T O N , B.J. P r e - and syn-deformational remanent magnetizations from mid-Palaeozoic sedimentary rocks of Australia, and their tectonic implications.

3.15

K L O O T W I J K , C . and G I D D I N G S , J. A n alternative A P W P for the middle to late Palaeozoic of Australia implications for terrane movements in the Tasman Fold Belt.

3.16

P I G R A M , C.J. Review of terrane tectonics in New Guinea and eastern Indonesia, in relation to palaeomagnetic problems.

3.17

D A V I E S , H.L. The Finisterre collision.

3.18

H I L L , K.C. and H E G A R T Y , K . A . T e c t o n i c i n t e r a c t i o n s b e t w e e n New i m p l i c a t i o n s f o r b a c k a r c spreading.

Guinea

and

the

Caroline

Plate

3.19

MUSGRAVE, R.J. P a l a e o m a g n e t i s m and t e c t o n i c h i s t o r y of the Solomon Islands a r c .

3.20

T H R U P P , G . A . , S L I T E R , W.V., S I L V E R , E . A . , P I G R A M , C . J . , P R A S E T Y G , H . and C O E , R.S. P a l a e o m a g n e t i s m of t h e L a t e C r e t a c e o u s c a l c a r e o u s s e d i m e n t , f r o m t h e M i s o o l A r c h i p e l a g o , I r i a n Jaya.

3.21

G I D D I N G S , J.W., S U N A T A , W. and P I G R A M , C . J . P a l a e o m a g n e t i s m of the Bird's Head, I r i a n J a y a .

3.22

K L O O T W I J K , C . , G I D D I N G S , J., P R I G R A M , C . J . , L U X T O N , C . , D A V I E S , H., R O G E R S O N , R . and F A L V E Y , D. P a l a e o m a g n e t i c c o n s t r a i n t s on the t e c t o n i c e v o l u t i o n of New Guinea - r e s u l t s f r o m the Highlands and N o r t h Sepik Regions of Papua N e w Guinea.

SESSION 4 :

METALLOGENY AND TECTONIC EVOLUTION OF THE EARLY TO MIDDLE PROTEROZOIC

4.1

W Y B O R N , L . A . , W A R R E N , R . G . , P A G E , R . W . and E T H E R I D G E , M . A . M e t a l l o g e n i c o v e r v i e w of the P r o t e r o z o i c of A u s t r a l i a w i t h special r e f e r e n c e t o g o l d , p l a t i n u m and r a r e e a r t h s .

4.2

PLUMB, K.A. M e t a l l o g e n y and t e c t o n i c s of the m i d - P r o t e r o z o i c of n o r t h e r n A u s t r a l i a , w i t h p a r t i c u l a r r e f e r e n c e t o the M c A r t h u r Basin.

4.3

P A G E , R . W . and H A N C O C K , S.L. G e o c h r o n o l o g y of a r a p i d 1.85-1.86 orogen, n o r t h e r n A u s t r a l i a .

4.4

4.5

Ga t e c t o n i c

transition -

Halls

Creek

B E A R D S M O R E , T . J . , N E W B E R Y , S.P. and L A I N G , W.P. The M a r o n a n Supergroup - oldest of t h r e e s i m i l a r v o l c a n o s e d i m e n t a r y sequences in the middle P r o t e r o z o i c of n o r t h w e s t Queensland. H A N N A N , K . W . , H E R B E R T , H . K . , G O L D I N G , S.D. and K R O U S E , H . R . A n update on M o u n t Isa - g e o c h e m i c a l evidence f o r e p i g e n e t i c mineralisation.

rift

copper

4.6

LOOSVELD, R.J.H. N u m e r i c a l s i m u l a t i o n of h i g h - T m e t a m o r p h i s m and c o e v a l c r u s t a l t h i c k e n i n g , w i t h r e f e r e n c e t o the M o u n t Isa I n l i e r .

4.7

S T E W A R T , A . J . and W I L L I A M S , P . R . E a r l y e x t e n s i o n in the M o u n t Isa I n l i e r , and a s o l u t i o n to the p r o b l e m of the Deighton Quartzite outliers.

4.8

H A R R I S , C . W . , GIBSON, R . G . , E R I K S S O N , K . A . and SIMPSON, C . B a s e m e n t - c o v e r relatioQships in southwest C o l o r a d o - i m p l i c a t i o n s f o r E a r l y t o M i d d l e P r o t e r o z o i c c r u s t a l e v o l u t i o n of the southwest U S A .

4.9

WILDE, A.R. The o r i g i n of m i d - P r o t e r o z o i c u n c o n f o r m i t y - r e l a t e d mineralisation, Alligator Rivers, Northern Territory.

SESSION 5:

uranium-gold-P.G.E.

GEOLOGICAL SUBDIVISION IN TIME AND SPACE PRINCIPLES AND PROBLEMS

5.1

G A T E H O U S E , C . G . and C O O P E R , B . J . E v o l v i n g a Quide to A u s t r a l i a n stratigraphy*

5.2

M U R R A Y - W A L L A C E , C . V . and K I M B E R , R . W . Q u a t e r n a r y marine a m i n o s t r a t i g r a p h y - P e r t h Basin, Western A u s t r a l i a .

5.3

M U R R A Y - W A L L A C E , C . V . and B E L P E R I O , A . P . R a d i o c a r b o n and amino a c i d r a c m i s a t i o n t i m e f r a m e w o r k s for l a t e P l e i s t o c e n e and H o l o c e n e s e d i m e n t a t i o n , G u l f St V i n c e n t , South A u s t r a l i a .

5.4

C O O P E R , B.J. C h r o n o s t r a t i g r a p h i c units and their a p p l i c a t i o n in A u s t r a l i a n geology.

5.5

PLUMB, K.A. Subdivision of the P r e c a m b r i a n Precambrian Stratigraphy and chronostratigraphy.

proposals by the Subcommission on comparison with Phanerozoic

5.6

H I L L , R.I. C h r o n o s t r a t i g r a p h y of greenstone sequences, Y i l g a r n B l o c k .

5.7

P R E I S S , W.V. P r i n c i p l e s and problems of l i t h o s t r a t i g r a p h i c c o r r e l a t i o n and synthesis in a f o l d e d P r o t e r o z o i c basin - a case history of the A d e l a i d e G e o s y n c l i n e .

5.8

C A R T E R , R.M. A p p l i c a b i l i t y of the V a i l et a l . " G l o b a l " S e a - l e v e l C u r v e to a southern ocean margin - G r e a t South Basin, southeast N e w Z e a l a n d .

SESSION 6: ECONOMIC GEOLOGY 6.1

A N D R E W , A.S., H E I N R I C H , C . A . , W I L K I N S , R . W . and P A T T E R S O N , D . J . Sulphur sources for copper ores at Mount Isa, A u s t r a l i a .

6.2

B E A R D S M O R E , T.J. The S e l w y n - s t y l e C u - A u deposits in the Mount Isa B l o c k - a late orogenic, granite and d e f o r m a t i o n - r e l a t e d s t y l e .

6.3

M Y E R S , I.A., G O L D I N G , S.D. and T A Y L O R , R . G . O x y g e n and hydrogen isotope studies of t i n deposits t o u r m a l i n i f e r o u s granite at C o o k t o w n , north Queensland.

6.4

associated

with

L A I N G , W.P., R U B E N A C H , M . J . and S W I T Z E R , C . K . The Starra gold-copper deposit syndeformational metamorphic m i n e r a l i s a t i o n l o c a l i s e d i n a folded early r e g i o n a l zone of d e c o l l e m e n t .

6.5

O D L I N G , N.W. C - O - S - H fluids - redox reactions as a mineralization mechanism in layered mafic intrusions.

6.6

H E I N R I C H , C.A., A N D R E W , A.S., W I L K I N S , R.W. and P A T T E R S O N , D.J. A fluid inclusion and stable isotope study of synmetamorphic copper ore formation at Mount Isa.

6.7

H U T T O N , L.J. Tectonic evolution of the basal Drummond Basin sequence and its relationship to gold mineralisation.

6.8

A R N E , D.C., G R E E N , P.P., D U D D Y , I.R., G L E A D O W , A.J., L Q V E R I N G , J.P. and L A M B E R T , I.B. A n apatite fission track study of Zn-Pb mineralisation on the Lennard Shelf, Western Australia.

6.9

M A R S H A L L , B. The interpretation piercement veins.

6.10

L A I N G , W.P. A system of structural analysis in drillcore, and how to use it to find ore deposits.

6.11

SCOTT, K.M. , ^ ^^ ^ Mineralogical mapping of alteration outcrop at Mount Leyshon, northeast Queensland, using X - r a y diffractrometry.

6.12

R O C K , N.M.S. Which rocks really contain diamonds?

6.13

P R E M O L I , C. Primary chromite deposits of Nev/ Caledonia.

6.14

L L O R C A , S. ^ . , Supergene cobalt mineralization. New Caledonia - geology and mineralogy.

6.15

O S T W A L D , J. Recent mineralogical investigations on terrestrial and marine oxides of the Australian region.

6.16

R E C O V E R , S.R. Zeolite deposits in New South Wales - commercial prospectivity.

6.17

C O L L I N S , L.B. and H A M I L T O N , N.T.M. Development of heavy mineral placers in Holocene barrier systems, southwest Australia.

Durchbewegung

structure,

piercement

cusps

and

manganese

SESSION 7: SEDIMENTOLOGY 7.1

A H M A D , R . and H O S T E T L E R , P.B. Recent advances in the^study of Holocene dolomitic carbonate sedimentation in the Coorong area of South Australia.

7.2

G R E E N , P.M., D O M A G A L A , J. and B U L T I T U D E , R.J. Chillagoe Formation - a record of a collapsed Silurian-Devonian carbonate shelf.

7.3

A R A K E L , A.V. and J A C O B S O N , G. Sediment-water interaction in the central Australian groundwater discharge zone - Lake Amadeus, Northern Territory.

7.4

A R A K E L , A.V., J A C O B S O N , G. and C H E N , Y . Palaeohydrologic and environmental significance inland drainage basins of the Australian arid zone.

7.5

D E D E C K K E R , P. and L A S T , W.M. Modern non-marine dolomite in evaporitic Australia.

of silicified calcrete

playas

western

Victoria,

7.6

S T E P H E N S O N , A.E. and B R O W N , C . M . The palaeogeography of the Murray Basin, southeastern Australia.

7.7

L A N G , S.C., J E L L , J.S., T A L E N T , J., M A W S O N , R . and W I T H N A L L , I.W. Early to Middle Devonian carbonate-terrigenous sedimentation in the Broken River Group, Graveyard Creek Subprovince, north Queensland.

7.8

G O R T E R , J.D. Late Cambrian shoaling cycles in the Amadeus Basin.

7.9

J O H N S T O N , D. A simple thick Australia.

shoaling-upward

cycle

in the

northern

Georgina

Basin,

7.10

L A N G , S.C. Late Devonian-Early Carboniferous sedimentation in the Bundock Creek Group of the Broken River Province, north Queensland.

7.11

G A G A N , M.K., J O H N S O N , D.P. and C H I V A S , A . R . The Cyclone Winifred storm bed, central Great Barrier Reef shelf, Australia.

7.12

D Y E , J.E. The Late Pleistocene/Holpcene palaeogeography and stratigraphy of the Maori Reef area, central Great Barrier Reef province, Australia.

7.13

J O N E S , B.G., N A N S O N , G.C., Y O U N G , R.W., S E N A P A T I , N. and B O U R K E , D.J. Channel avulsion and sand geometry in the Gilbert River fandelta, northern Queensland.

7.14

HILL, C.M. Catchment sedimentation as an active geomorphic process.

7.15

HILL, C.M. Application of sedimentological studies - a case study.

7.16

techniques

to catchment

H A Y W I C K , D.W. Pliocene cyclothemic inner shelf sedimentation assemblages, eastern North Island, Nev/ Zealand.

patterns

hydrogeological

and

lithofacies

7.17

7.18

B E L P E R I O , A . P . and G O S T I N , V . A . T h e s e d i m e n t a r y f r a m e w o r k and n a t u r e of L a t e sediments, G u l f St V i n c e n t , South A u s t r a l i a .

Pleistocene

and

Holocene

SIMPSON, C . S u b a q u e o u s and s u b a e r i a l v o l c a n i c f a c i e s i n t h e E a r l y D e v o n i a n F o r m a t i o n , southeastern N e w South Wales.

Tangerang

7.19

F I E L D I N G , C . R . and L A N G , S . C . A f a c i e s a n a l y s i s of the S t a i r c a s e S a n d s t o n e M e m b e r ( E a r l y P e r m i a n ) in t h e Springsure area, southwestern Bowen Basin.

7.20

S E N A P A T I , N . and B O U R K E , D . J . E n v i r o n m e n t s of d e p o s i t i o n in the n o r t h e a s t e r n C a r p e n t a r i a B a s i n .

7.21

L E A C H , J.H. S e d i m e n t s o u r c e f o r the M a r t i a n n o r t h p o l a r d u n e f i e l d .

7.22

F R A K E S , L . A . and F R A N C I S , J . E . Australian Cretaceous ice.

7.23

J O N E S , B . G . , C A R R , P . F . and M I D D L E T O N , R . G . G l e n d o n i t e s - o c c u r r e n c e and s i g n i f i c a n c e .

SESSION 8: METAMORPHISM AND MAGMATISM IN CRUSTAL EVOLUTION 8.1

SIVELL, W.

. .

8.2

W A R R E N , R . G . , W Y B O R N , L . A . and M c C U L L O C H , M . T . G e o c h e m i c a l s t u d i e s i n the A r u n t a B l o c k , c e n t r a l A u s t r a l i a .

8.3

M c N A U G H T O N , N . J . and R O S M A N , K . J . T i n i s o t o p e f r a c t i o n a t i o n in c a s s i t e r i t e as a p e t r o g e n e t i c i n d i c a t o r ?

8.4

WYBORN, L.A., WYBORN, D., CHAPPELL, B.W., T A R N E Y , J . F . , C O L L I N S , W . J . and D R U M M O N D , B . J . G e o l o g i c a l e v o l u t i o n of g r a n i t e c o m p o s i t i o n s w i t h t i m e c o n t i n e n t - i m p l i c a t i o n s f o r t e c t o n i c and m a n t l e p r o c e s s e s .

Eastern A r u n t a orthogneiss suites - implications for a repeated transition f r o m r i f t to s u b d u c t i o n - r e l a t e d m a g m a t i s m d u r i n g s u c c e s s i v e P r o t e r o z o i c e n s i a l i c orogenies.

SHERATON, in t h e

J.,

Australian

8.5

C O L L I N S , W . J . , F L O O D , R . H . and V E R N O N , R . H . G r a n i t e g e n e r a t i o n in the S t r a n g w a y s M e t a m o r p h i c C o m p l e x , A r u n t a B l o c k , c e n t r a l A u s t r a l i a - i m p l i c a t i o n s f o r the o r i g i n of h i g h t e m p e r a t u r e , l o w pressure granulite facies terranes.

8.6

F O D E N , J . , B U I C K , L S . and M O R T I M E R , G . E . G r a n i t i c gneisses f r o m the E n t i a D o m e , eastern A r u n t a B l o c k - i m p l i c a t i o n s f o r m a g m a t i s m in an i n t r a c r u s t a l m o b i l e z o n e .

8.7

F E R M I O , S . J . and F O D E N , J . P e t r o g e n e s i s of the D e l a j n a r i a n R e e d y C r e e k G r a n o d i o r i t e , S o u t h A u s t r a l i a .

8.8

S T E P H E N S O N , P . J . and C H A P P E L L , B . W . The g e o l o g y and p e t r o l o g y of the P a l m Islands, n o r t h Q u e e n s l a n d .

8.9

STEPHENSON, P . J . , C H A P P E L L , B.W. and F R O S T , M.T. Aspects of the geology and petrology of arfvedsonite granites, Hinchinbrook Island, north Queensland.

8.10

WILLIAMS, I.S., C H E N , Y . D . , C H A P P E L L , B.W. and COMPSTON, W. Dating the sources of Bega Batholith granites by ion microprobe.

8.11

WORMALD, R . J . The peralkaline granites near Temora, southern petrological, geochemical and tectonic implications.

8.12

McLENNAN, T . P . T . The boundary between the Broken River Province and the LolworthRavenswood block, north Queensland - a major crustal suture, the Clarke River Fault Zone.

8.13

LAW, S.R. The Bulgonunna Volcanics - a Carboniferous ignimbritic cauldron complex in central Queensland.

8.14

MACKENZIE, D.E. Petrological and structural evolution of the Per mo-Carboniferous Featherbed Volcanics, northeastern Queensland, and their relationship to mineralisation.

8.15

McPHIE, J . Proximal and distal facies associations of subaerial silicic caldera volcanoes.

8.16

O V E R S B Y , B. Late Palaeozoic magmatism in northeastern Queensland - incipient rifting at a passive margin of Gondwanaland?

8.17

McLENNAN, T . P . T . Igneous intrusions and their relationship to the regional structural fabric of the central northern Bowen Basin.

8.18

PATTISON, C . I . , HAMILTON, L . H . , HAMMOND, R . L . and M A L L E T T , C.W. The intrusive history of the northern Bowen Basin - tectonic implications.

8.19

Y O U N G , D.N. Petrology of intrusive charnockites, Mawson Coast, Antarctica.

8.20

C R O O K , K . A . W . , T A Y L O R , B., E X O N , N.F. and JOHNSON, R.W. Structure and tectonics of the Woodlark triple junction - arc volcanoes on an incoming oceanic plate.

8.21

F O D E N , J . , V A R N E , R . , S T O L Z , A . J . and J E N N E R , G . A . Sangeang Api - alkali arc magmatism ultramafic and mafic xenoliths, arc growth by underplating.

8.22

RUXTON, B.P. Magma evolution on the eastern Managalase Plateau, Papua New Guinea.

8.23

W A R R E N , R . G . and DAVIES, H . L . Eclogite-bearing, domec^, layered metamorphic complexes ("core complexes") in the D'Entrecasteaux Islands, Papua New Guinea.

New

South

Wales

8.24

T A Y L O R , W . R . , S T O L Z , A . J . and A D A M , J . D . M i n e r a l o g y a n d g e o c h e m i s t r y of m e l i l i t e n e p h e l i n i t e and i j o l i t e f r o m S h a n n o n Tier, central Tasmania.

8.25

T A Y L O R , W.R. Melting behaviour

u of p e r i d o t i t e in the p r e s e n c e of r e d u c e d C - O - H f l u i d s

i m p l i c a t i o n s f o r r e d o x m e l t i n g of t h e m a n t l e . 8.26

O D L I N G , N.W. S y n t h e t i c high t e m p e r a t u r e , high pressure f l u i d inclusions - a new technique.

8.27

M c N A U G H T O N , N . J . , F R O S T , K . M . and G R O V E S , D.I. G r o u n d m e l t i n g by k o m a t i i t e s a t K a m b a l d a , W e s t e r n A u s t r a l i a - e v i d e n c e f r o m U-Th-Pb systematics.

8.28

A D A M , J.D. . . D r y , H o O - s a t u r a t e d and C 0 2 - b e a r i n g l i q u i d u s phase r e l a t i o n s h i p s i n the C M A b s y s t e m a t 28 k b , a n d t h e i r b e a r i n g on t h e o r i g i n of a l k a l i b a s a l t s .

8.29

C H E N , Y . D . and P R I C E , R . C . E q u i l i b r a t i o n b e t w e e n m a f i c inclusions and host granites.

8.30

W A R R E N , R . G . and H E N S E N , B . J . D i s t r i b u t i o n of m e t a m o r p h i c f a c i e s i n the A r u n t a B l o c k , c e n t r a l A u s t r a l i a .

8.31

O L I V E R , R . L . , P U R V I S , A . J . and T A Y L O R , M . J . E x t r a h i g h g r a d e ( ? ) m e t a m o r p h i s m in t h e n o r t h w e s t Australia.

Gawler

Craton,

8.32

K A T Z , M.B. W i l l y a m a shear f a u l t mobile b e l t . Broken H i l l , N e w South Wales.

8.33

GIBSON, G.M. M e t a m o r p h i s m in the w e s t e r n p r o v i n c e o f N e w Z e a l a n d - a r e v i e w .

8.34

C H E N H A L L , B . E . , C A R R , P . F . and J O N E S , B . G . C o n t a c t m e t a m o r p h i s m a d j a c e n t to the e a s t e r n m a r g i n o f the Tonalite, B r a y t o n d i s t r i c t . N e w South Wales.

South

Arthursleigh

8.35

R U B E N A C H , M . J . and M O R R I S O N , G . W . P e t r o g e n e s i s of s k a r n s f r o m the C h i l l a g o e a r e a of n o r t h e a s t Q u e e n s l a n d .

8.36

O F F L E R , R . , H A N D , M . and B A L E , R . b a n d i l l i t e c r y s t a l l i n i t y s t u d i e s of K - w h i t e m i c a s in r o c k s f r o m f o r e a r c basins and a c c r e t i o n a r y c o m p l e x sequences, southern N e w England F o l d B e l t , N e w South Wales.

SESSION 9: GEOLOGICAL HAZARDS AND E A R T H Q U A K E RISK 9.1

D E N H A M , D. S e i s m i c i t y of t h e A u s t r a l i a n P l a t e and i t s P a c i f i c P l a t e m a r g i n .

9.2

G A U L L , B . A . , A D A M S O y , D . A . and P I C K A R D , J . C a l v i n g i c e b e r g s and s e i s m i c a c t i v i t y f r o m an o u t l e t g l a c i e r , M a w s o n S t a t i o n , Antarctica.

9.3

M I C H A E L - L E I B A , M.O. Macroseismic effects, locations and magnitudes of some early earthquakes.

Tasmanian

9.4

JONES, L.E., M I C H A E L - L E I B A , M.O. and KINGSTON, D. The Bream Creek earthquake swarms of 1986-1987.

9.5

C H O P R A , P. and D R U M M O N D , B.J. A laboratory study of compressional and shear wave velocities in a suite of rocks from southwest Western Australia.

9.6

G R E E N H A L G H , S.A., D E N H A M , D. and R Y N N , J.M. Magnitude-intensity relations for Australian earthquakes.

9.7

R Y N N , J.M. Studies of earthquake risk - a multidisciplinary approach.

9.8

WILLIAMS, D.J. Earthquake induced liquefaction potential in Queensland.

9.9

B O Y C E , W.H. Earthquake codes.

9.10

B I C K F O R D , J.L.C. The role of the State Emergency Service in the community.

9.11

F E A T H E R S T O N E , C. Earthquakes and insurance.

9.12

JUST, G.D. Damage risk from blasting vibrations.

9.13

L Y N A M , C., R Y N N , J.M. and B A R L O W , B. Tsunamis along the Australian coastline.

9.14

MORRIS, P.H. Application of statistics earthquake data.

in geotechnical engineering - an example

from

SESSION 10: ENGINEERING GEOLOGY AND HYDROGEOLOGY 10.1

B U R T O N , N. and LAWSON, T. Engineering geology of the Burdekin Falls Dam, north Queensland.

10.2

H E R C Z E G , A.L., TORGERSEN, T., H A B E R M E H L , M.A. and CHIVAS, A.R. Geochemical evolution of groundwaters from the Great Artesian Basin, Australia.

10.3

JONES, R.M. Urban geology of the Balmain Peninsula and recent geotechnical problems.

10.4

R U X T O N , B.P. Colluvium and slope staljility in Hong Kong.

SESSION 11: G E O L O G I C A L S O U R C E D A T A H A N D L I N G 11.1

GERDES, L . and S M I T H , K . Towards a standard f o r geoscience r e f e r e n c e databases.

11.2

S H E L L E Y , E.P. I n t e g r a t i o n of geoscience databases in the Bureau of M i n e r a l Resources.

11.3

L A N G , S.C., W I T H N A L L , L W . , GRIMES, K . G . and M U R P H Y , P . R . R E G M A P - a r e g i o n a l mapping f i e l d data management s y s t e m .

11.4

A G G S T I N I , A . and G I L L I G A N , L . B . M R L I S - a g e o - r e l a t i o n a l database used by the New South Wales D e p a r t m e n t of M i n e r a l Resources.

11.5

L U C E Y , C.J. Queensland Land I n f o r m a t i o n System.

11.6

E L L I O T T , B.G. Databases and database d e v e l o p m e n t Bureau of M i n e r a l Resources.

in the M i n e r a l C o m m o d i t i e s

Branch,

SESSION 12: P L A N E T A R Y G E O L O G Y A N D G L O B A L EVENTS 12.1

L E A C H , J.H. T e c t o n i c r a m i f i c a t i o n s of a s t e r o i d i m p a c t s .

12.2

HEIDECKER, E.J. M i n e r a l landscaping towards 'blooming' events - Devonian e x t i n c t i o n r e c o r d w i t h modern management i m p l i c a t i o n s .

12.3

YOUNG, G.C. Frasnian-Famennian vertebrates.

extinction

event

the

evidence

from

Devonian

12.4

N I C O L L , R.S. and P L A Y F O R D , P.E. Upper Devonian i r i d i u m anomalies and the F r a s n i a n - F a m e n n i a n boundary in the Canning Basin, Western A u s t r a l i a .

12.5

DICKINS, J.M. A u s t r a l i a n P e r m i a n c o r r e l a t i o n and sea l e v e l changes.

12.6

JAMES, P.M. On polar m o b i l i t y and e x t i n c t i o n s .

12.7

S H A F I K , S. C r e t a c e o u s sea l e v e l m o v e m e n t s .

12.8

W A T E R H O U S E , J.B. and F L O O D , P. The P e r m i a n - T r i a s s i c c o n t a c t in the H i m a l a y a s of Nepal, and its relevance to episodic l i f e c a t a s t r o p h e .

SESSION 13: C O A L G E O L O G Y 13.1

B A M B E R R Y , W.J., JONES, B.G. and H U T T O N , A . C . I n t e r d i s t r i b u t a r y bay deposits in the I l l a w a r r a Coal Measures, southern Sydney Basin.

13.2

M A L L E T T , C.W. and G O D F R E Y , N . H . H . The depositional s e t t i n g of the German Creek and Moranbah Coal Measures, Bowen Basin.

13.3

SAPPAL, K.K. Maceral and mineral m a t t e r associations of C o l l i e coal. C o l l i e Basin, Western Australia.

13.4

FALKNER, A. Sedimentology of the Brassall Subgroup - a comparison of sandstone mudstone-dominated coal measure sequences.

and

B O G A C Z , W. and K I V I O R , I. Specific t e c t o n i c c h a r a c t e r of the T e l f o r d Basin and i m p l i c a t i o n s f o r planning, Leigh Creek C o a l f i e l d , South A u s t r a l i a .

mine

13.5

13.6

FIELDING, C.R. Sedimentary environments of the Middle Jurassic Walloon Coal Measures in the Rosewood-Walloon C o a l f i e l d , southeast Queensland.

13.7

S T R U C K M E Y E R , H. and F E L T O N , E . A . Organic and sedimentary facies of the Early Cretaceous O t w a y Group, Otway Basin, southern A u s t r a l i a .

13.8

F R A N K E L , E. Eastern A u s t r a l i a n f r e s h w a t e r coastal swamps.

13.9

HUTTON, A.C. Aspects of the geology of Queensland T e r t i a r y o i l shales.

13.10

H A M M O N D , R . and M A L L E T T , C. Observations on the behaviour of coal measures during d e f o r m a t i o n .

SESSION 14: P A L A E O N T O L O G Y 14.1

14.2

A H M A D , R. Rare e a r t h elements in biogenic a p a t i t e - r e c o r d of secular variations b o t t o m seawater redox during the Phanerozoic.

CHIVAS, A . R . , DE D E C K K E R , P. and S H E L L E Y , J . M . G . Trace elements and stable isotopes of ostracod shells palaeosalinity, p a l a e o t e m p e r a t u r e and p a l a e o p r o d u c t i v i t y .

for

tools

14.3

A H M A D , R., D A L E , L.S. and F A R D Y , J.J. C e r i u m anomaly in siliceous microfossils - r e c o r d of modern and ancient b o t t o m seawater redox.

14.4

A R C H B O L D , N.W. Permian brachiopod faunas of the Perth Basin, Western A u s t r a l i a - a study of progressive isolation.

14.5

RIGBY, J.F. The Permian flora of the Nychum Volcanics, north Queensland.

14.6

C A N N , J.H. Quaternary foraminifera and late Pleistocene and Holocene sea levels, Gulf St Vincent, South Australia.

14.7

HENDERSON, R.A. Albian-Cenomanian biogeography of Australasian ammonites Cretaceous first fleet and its palaeoceanographic interpretation.

14.8

mid-

T U R N E R , S. Palaeozoic fish help to date Australian rocks.

SESSION 15: EVOLUTION OF THE TASMAN SEA 15.1

L I S T E R , G.S., E T H E R I D G E , M . A . and S Y M O N D S , P . A . Extensional history of the margins of the Tasman Sea.

15.2

W I L L C O X , J.B. and S Y M O N D S , P . A . Structural development of the western Lord Howe Rise.

15.3

S Y M O N D S , P . A . , W I L L C O X , J.B. and K U D R A S S , H . R . Dampier Ridge in the Tasman Sea - a continental fragment.

15.4

R O O T S , W.D. The marine evidence for subduction of Tasman sea floor beneath Australia and a pre-Tasman continental reconstruction which includes the lost area.

15.5

C O L W E L L , J.B., C O F F I N , M.F., P R I T C H A R D , T. and S P E N C E R , R . Structure of southern New South Wales and northeast Gippsland Basin margins - results of R i g Seismic research cruise 13.

15.6

JENKINS, C.J. Regional sediment facies and the history of bottom-water circulations in the Tasman Sea.

15.7

M c D O U G A L L , L and D U N C A N , R . A . Age progressive volcanism in the Tasmantid Seamount chain, a hotspot trace.

15.8

Q U I L T Y , P.G. Biostratigraphy of seamounts in the Tasman Sea.

15.9

EGGINS, S.M. Petrology and geochemistry of Tasmantid Seamounts.

15.10

MIDDLEMOST, E.A. Petrological features of lavas from the Derwent-Hunter Tablemount.

15.11

M c C U L L O C H , M.T. Nd-Sr isotope geochemistry hotspot trace.

15.12

the Tasmantid Seamounts - evolution of a

JENKINS, C.J. SeaBeam and reflection-seismic imaging of the flanks and volcanic aprons of Tasmantid Seamounts.

15.13

C O L E M A N , R., JENKINS, C.J., P A C K H A M , G.H. and L A M B E C K , K . Lithosphere loading and seamount subsidence history - s a t e l l i t e and seismic r e f l e c t i o n data.

SESSION 16: G E O L O G I C A L R E M O T E SENSING 16.1

H A R R I S , P.T. Tidal sedimentary processes in Torres S t r a i t - a p p l i c a t i o n of L A N D S A T , air photograph and sidescan sonar images as c o m p l e m e n t a r y r e m o t e sensing techniques.

16.2

ERASER, S.J., H U N T I N G T O N , J.F. and GREEN, A . A . Iron oxides - their s u r f i c i a l d i s t r i b u t i o n and r e m o t e sensing.

16.3

L E A C H , J.H. S a t e l l i t e observations of the major s t r u c t u r a l elements of the Bowen Basin.

16.4

H E G A R T Y , R . A . , L A I N G , W.P. and C A T T , P. D i g i t a l i n t e g r a t i o n of geology w i t h geophysical and satellite-sensed data sets in the A t h e r t o n 1:250 000 Sheet area.

SESSION 17: EAST A U S T R A L I A N C A I N O Z O I C V O L C A N I S M 17.1

JOHNSON, R.W. and T A Y L O R , S.R. The I n t e r n a t i o n a l Lithosphere P r o j e c t .

17.2

D U N C A N , R . A . and M c D O U G A L L , L Time-space relationships f o r Cenozoic Australia.

intraplate

volcanism

eastern

17.3

W E L L M A N , P. Intrusions beneath large i n t r a p l a t e volcanoes.

17.4

O ' R E I L L Y , S.Y. and G R I F F I N , W.L. Highlights of a mantle perspective on Phanerozoic eastern A u s t r a l i a .

17.5

E W A R T , A., C H A P P E L L , B.W. and MENZIES, M. Petrogenesis of the eastern A u s t r a l i a n Cainozoic volcanic provinces.

17.6

SHEN-SU, S. C h e m i c a l and isotope systematics of i n t r a p l a t e basalts - i m p l i c a t i o n s magma genesis of Cenozoic basalts in eastern A u s t r a l i a and New Zealand.

for

17.7

SMITH, I.E., WEAVER, S.D. and G A M B L E , J . A . I n t r a p l a t e basaltic volcanism in New Zealand.

17.8

PRICE, R.C., G R A Y , C . M . and F R E Y , F . A . Geochemistry of plains basalts of the Western D i s t r i c t s volcanics province of Victoria.

17.9

STEPHENSON, P.J. The eastern A u s t r a l i a n Qainozoic volcanic zone, northern Queensland.

17.10

D U G G A N , M.B., K N U T S O N , J. and C H A P P E L L , B.W. Petrology of the Warrumbungle Volcano, New South Wales.

17.11

S U T H E R L A N D , F . L . , ROBERTSON, A . D . and MOLLIS, J . D . The R o c k h a m p t o n Province - a Cretaceous c e n t r a l volcano m i g r a t i o n ?

17.12

ROBERTSON, A . D . . Cainozoic volcanism along the n o r t h w e s t e r n margin of the Maryborough basin.

17.13

S U T H E R L A N D , F . L . , HOLLIS, J . D . and ROBERTSON, A . D . The Boyne Basalts, southern Queensland - t h e i r relevance to the hthospheric state.

17.14

BROWN, M . C . , C L A R K E , L , M c Q U E E N , K . G . and T A Y L O R , G. Early T e r t i a r y volcanism in the southern Monaro region. New South Wales.

17.15

RECOVER, S.R. Cainozoic maar volcanism and the o r i g i n of sapphire and possibly diamond in eastern A u s t r a l i a .

SESSION 18: G E O S C E N C E E D U C A T I O N - CONSENSUS OR C O N F L I C T 18.1

ROSE, J . M . The Australasian I n s t i t u t e of Mining and M e t a l l u r g y a c c r e d i t a t i o n proposals.

18.2

EVANS, P.R. The A u s t r a l i a n I n s t i t u t e of Geoscience perspective.

18.3

T A Y L O R , R.G. Training a geologist.

18.4

SVENSON, D. A recipe for graduating an economic geologist.

18.5

R O C K , N.M.S. Geological numeracy and c o m p u t e r a c y in A u s t r a l i a - where do we go t r o m here?

18.6

CARTER, R.M. The r e s t r u c t u r i n g of geoscience in A u s t r a l i a n t e r t i a r y i n s t i t u t i o n s .

SESSION 19: TOWARDS AN A U S T R A L I A N B I O S T R A T I G R A P H I C T I M E SCALE 19.1

J E N K I N S , R . J . , HAINES, P.W. and GOSTIN, V . A . The Ediacaran r e v i s i t e d .

19.2

SHERGOLD, J.H. A u s t r a l i a n C a m b r i a n biochronology.

19.3

WEBBY, B.D., COOPER, R . A . , V A N D E N B E R G , A . H . , N I C O L L , R.S., STEWART, I., SHERGOLD, J.H., BURRETT, C.F., ST A I T , B. and C O O P E R , B.J. Towards an Ordovician b i o s t r a t i g r a p h y of A u s t r a l i a - a progress r e p o r t .

19.4

STRUSZ, D . L . and Y O U N G , G.C. T i m e scales for the Silurian and Devonian in A u s t r a l i a .

19.5

J E L L , J.S. D e v o n i a n series and stage boundaries in A u s t r a l i a .

19.6

JONES, P.J. C a r b o n i f e r o u s b i o s t r a t i g r a p h i c c h a r t f o r A u s t r a l i a - i n t e r i m notes.

19.7

D I C K I N S , J . M . and A R C H B O L D , N.W. A standard g e o l o g i c a l scale for the P e r m i a n s y s t e m in A u s t r a l i a .

19.8

A R C H B O L D , N.W. The P e r m i a n of A u s t r a l i a - the 1988 position.

19.9

D A Y , R.W. The C r e t a c e o u s S y s t e m in A u s t r a l i a .

19.10

TRUSWELL, E.M. A C a i n o z o i c t i m e s c a l e for A u s t r a l i a .

SESSION 20J OCEAN DRILLING PROGRAM PREVIEW 20.1

C O L W E L L , J.B., C O F F I N , M . F . , D A V I E S , H . L . and S T A G G , H . M . F i r s t ever d r i l l i n g on the K e r g u e l e n P l a t e a u and in P r y d z B a y .

20.2

E X O N , N . F . , V O N R A D , U., W I L L I A M S O N , P . E . and B O Y D , R . O c e a n d r i l l i n g on the E x m o u t h P l a t e a u and A r g o A b y s s a l P l a i n in mid-1988.

20.3

D A V I E S , P . J . , S Y M O N D S , P . A . , F E A R Y , D . A . and P I G R A M , C . J . The e v o l u t i o n of the c a r b o n a t e p l a t f o r m s in northeast A u s t r a l i a - the goal of the O c e a n D r i l l i n g P r o g r a m .

SCIENTIFIC POSTERS B A R T H , W.H. Compositional mineralisation.

variation

zoned

tourmalines

associated

v^ith

Sn-W

KEENE,

J.B.,

BARTON, C.E. M a g n e t o s t r a t i g r a p h y of sediments f r o m the L o r d Hov»/e R i s e . C A R T Y , S.J., C O L E M A N , R . , H U B B L E , T . C . , J E N K I N S , C . J . , P R I T C H A R D , T . R . and S C H N E I D E R , P . M . G e o p h y s i c a l , S e a B e a m and s e a f l o o r c a m e r a i n v e s t i g a t i o n s Seamounts.

Tasmantid

Devonian

G A U L L , B.A. Studies of ground motion a t t e n u a t i o n in Western A u s t r a l i a . G A U L L , B . A . , M I C H A E L - L E I B A , M . O . and R Y N N , J . M . P r o b a b i l i s t i c earthquake risk maps of A u s t r a l i a . H E I D E C K E R , E.J. C h l o r o f l u o r o c a r b o n generation - g e o l o g i c a l e n v i r o n m e n t s during p l a n e t a r y change.

models

based

M c C U E , K., B A R L O W , B.C., D E N H A M , D., M I C H A E L - L E I B A , M.O. Marryat Creek earthquake, 30 March 1986.

JONES,

T.,

GIBSON,

and

M I C H A E L - L E I B A , M.O. and G A U L L , B.A. Probabilistic earthquake risk maps of Tasmania. M I D D L E T O N , R.G., C A R R , P.F. and J O N E S , B.G. Geological exploration by J.D. Dana and the Reverend W.B. Clarke in the Illawarra, 1840. O F F L E R , R., G A M B L E , J. and F A R D Y , J. Palaeozoic forearc volcanism in the southern New England Fold Belt, New South Wales. S I V E L L , W. and W A T E R H O U S E , J.B. The Patuki ophiolite - implications of sequential pre to magmatic episodes within a fast-slipping fracture zone.

post-spreading

ABSTRACTS OF KEYNOTE PAPERS

PRECAMBRIAN BASIN FORMATION AND EVOLUTION - COMPARISON WITH POSSIBLE PHANEROZOIC COUNTERPARTS Kenneth A. Eriksson Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia USA

Precambrian sedimentary basins commonly display a similar stratigraphic evolution to those of Phanerozoic age. For some of these Precambrian basins, Phanerozoic examples serve as good analogues for understanding basin formation and subsidence, whereas for others, such comparisons are more tenuous. Sediment compositions together with stratigraphic and structural evidence are compatible with 3.3-3.2 Ga-old Fig Tree and Moodies Groups in the Barberton Mountain Land having accumulated in a foredeep or foreland basin. The 5 km-thick stratigraphic succession records progressive infilling of the trough under deep-water followed by shallow-water conditions. Influx of siliciclastic detritus was related to uplift of a gneiss terrain to the south; massive uplift at ca. 3.3 Ga age resulted from simple shear upwards and to the north. Basin deepening is attributed to crustal loading related to this thrusting. Stacked, upward-shoaling sequences averaging 1 km thick reflect repeated drowning of the depositional interface by reactivation of thrust faults, followed by basin infill. The ca. 3.0 Ga-old and 3 km-thick upper Gorge Creek Group in the Pilbara Block developed in strike-slip basins comparable to those in the California Borderlands. Strike-slip faulting took place during the late compressional history of the Pilbara Block. Stratigraphic and sedimentological evidence supporting the contention that basin formation and subsidence were controlled by marginal strike-slip faulting include: (1) the presence of intraformational, low-angle unconformities; (2) the great stratigraphic thickness relative to the small basin area; (3) the elongate shape of the basin parallel to a northwestern boundary fault; (4) the presence of numerous alluvial fans within the basin implying that boundary faults were active during sedimentation; (5) the asymmetry of basin fill and f a d e s distribution; (6) the vertical stacking and limited lateral migration of depositional environments; and (7) the dominance of longitudinal infilling. Early Proterozoic (2.2-1.8 Ga) sequences bordering the Superior Province in North America display a similar stratigraphic evolution to that in Phanerozoic orogens such as the Appalachians. Examples include the Marquette Range Supergroup (Lake Superior Region), Kaniapiskau Supergroup (Labrador Trough), and Belcher Group (eastern Hudson Bay), and their correlatives. Each sequence contains an autochthon and fold-and-thrust belt within which a three-fold stratigraphic subdivision is recognizable. Basal mafic volcanics and immature siliciclastics represent synrift deposits. An overlying m i x e d carbonate-siliciclastic interval is interpreted as a passive-margin prism related to thermotectonic subsidence. Unconformities, which may be defined by karst breccias, cap the passive-margin deposits and developed in response to migration of the peripheral bulge at the onset of compression. Subsequent basin foundering resulted from thrust loading. Basal t r a n s g r e s s i v e quartz arenites, iron formations, black shales,

turbidites^ volcanics in the foredeep.

and fluvial conglomerates

and sandstones

accumulated

The 2.8 to 2.2 Ga-old Ventersdorp-Transvaal (ca. 16 km) and Mount Bruce (ca. 9 km) Supergroups also contain a three-fold stratigraphic subdivision. Early rift-related bimodal volcanics and immature siliciclastic and subordinate carbonate sediments are overlain by carbonate-iron formation intervals. The latter display an asymmetric distribution of facies reflecting a platform to platform-edge to basinal transition. Platform and platform-edge facies predominate in the Transvaal Supergroup, whereas basinal facies are prevalent in the Mount Bruce Supergroup. An areally extensive karst breccia caps the platform carbonates in the Transvaal Supergroup. The uppermost stratigraphic subdivis ion in both supergroups consists of siliciclastic sediments and subordinate volcanics; sediments are of shallow-water origin in the Transvaal Supergroup and deep-water origin in the Mount Bruce Supergroup. The Mount Bruce Supergroup on the southern margin of the Pilbara Block consists of an autochthon and fold-and-thrust(?) belt comparable to the North American Proterozoic sequences. By analogy with Phanerozoic orogenic belts, the three stratigraphic intervals in the Mount Bruce Supergroup can be related to rift, thermotectonic and thrustloading stages of subsidence. Despite a similar stratigraphic evolution, subsidence mechanisms are equivocal for the ca. 6 km-thick upper subdivision in the Transvaal Supergroup in particular. This is because the supergroup is entirely autochthonous and occupies an area of ca. 500, 000 km^ on the Kaapvaal Craton. The ca. 1.0 Ga-old and 5 km-thick Bangemall Basin displays a similar stratigraphic evolution to aulacogens such as the Oklahoma Aulacogen. Basal, coarse-grained siliciclastic sediments accumulated in grabens. An overlying, predominantly carbonate interval reflects gradual onlap with drowning of basement topography; basin enlargement is attributed to thermotectonic subsidence. Widespread drowning of the carbonate shelf was followed by two cycles of sedimentation involving resedimentation of shelf and slope carbonates into base-of-slope environments, followed by radial influx of siliciclastic turbidites. Increased rates of subsidence were related to thrust loading in the east. Sedimentation in the basin was terminated by longitudinal influx of siliciclastic sediments from the fold-andthrust belt to the east and their deposition in alluvial and shallow-marine environments.

PANGEA, GONDWANA AND PALEOMAGNETIC PROBLEMS IN PALEOZOIC FOLD BELTS Dennis V.

Kent

L a m o n t - D o h e r t y G e o l o g i c a l O b s e r v a t o r y and D e p a r t m e n t G e o l o g i c a l S c i e n c e s of Columbia U n i v e r s i t y , P a l i s a d e s , New Y o r k USA

The P a l e o z o i c f o l d b e l t s of the c i r c u m - A t l a n t i c region can be a t t r i b u t e d t o the i n t e r a c t i o n s o f t h e m a j o r l a n d m a s s e s o f Gondwana. B a l t i c a and Laurentia. plus a v a r i e t y of intervening d i s p l a c e d t e r r a n e s , whose movement h i s t o r y c u l m i n a t e d in the f o r m a t i o n of Pangea. New and r e v i s e d p a l e o m a g n e t i c data have become a v a i l a b l e which a l l o w improved p a l e o g e o g r a p h i c r e c o n s t r u c t i o n s but at t h e same t i m e , e m p h a s i z e t h e importance of remagnetizations in the rockmagnetic record. In the Appalachians of North America, redbed u n i t s have been an important s o u r c e o f p a l e o m a g n e t i c data f o r t h e a p p a r e n t p o l a r wander path o f L a u r e n t i a . Renewed i n v e s t i g a t i o n s o f the Upper O r d o v i c i a n J u n i a t a Fm., Upper S i l u r i a n Bloomsburg Fm., Upper Devonian C a t s k i l l Fm., and the Lower Carboniferous Mauch Chunk Fm., combined with new work on the Lower Devonian Andreas Fm., show t h a t the d i r e c t i o n s p r e v i o u s l y r e p o r t e d f r o m many o f these u n i t s are contaminated by Late P a l e o z o i c o v e r p r i n t s . I t has now been p o s s i b l e t o i s o l a t e high unblocking temperature components that pass f o l d t e s t s in these r o c k s , and these p r e - f o l d i n g magnetizations i n d i c a t e that L a u r e n t i a was g e n e r a l l y f a r t h e r s o u t h than p r e v i o u s l y d e t e r m i n e d : about 350s in the Late Ordovician and Early Devonian, 20°S by the Late Devonian, IS^'S i n t h e E a r l y C a r b o n i f e r o u s , and a p p r o a c h i n g near e q u a t o r i a l p a l e o l a t i t u d e s only by the Late Carboniferous. These r e v i s e d p a l e o l a t i t u d e estimates do not support a r e l a t i v e l a t i t u d i n a l displacement of the Avalon terrane s i n c e the Devonian. On the other hand, the magnetizations suggest approximately 25® of o r o c l i n a l bending of the Pennsylvania s a l i e n t . A s c e n a r i o f o r P a l e o z o i c m o t i o n s b a s e d on t h e s e and o t h e r p a l e o m a g n e t i c d a t a i n c l u d e s t h e e x i s t e n c e o f a w i d e ocean in C a m b r i a n - O r d o v i c i a n t i m e between Laurentia and B a l t i c a ( s i t u a t e d in more e q u a t o r i a l l a t i t u d e s ) and the f a c i n g margin of Gondwana, along with the peri-Gondwanide terranes such as Armorica, Avalon, the Appalachian Piedmont, and F l o r i d a ( s i t u a t e d in h i g h s o u t h e r n l a t i t u d e s ) . The s u b s e q u e n t s o u t h e r l y m o t i o n o f L a u r e n t i a culminated in i t s c o l l i s i o n with the northwest margin of South America by t h e E a r l y D e v o n i a n , g i v i n g r i s e t o the C a l e d o n i a n - A c a d i a n o r o g e n and t h e a m a l g a m a t i o n o f t h e p e r i - G o n d w a n i d e t e r r a n e s . Gondwana s e p a r a t e d f r o m L a u r e n t i a , B a l t i c a and t h e i r a c c r e t e d t e r r a n e s . t o f o r m a L a t e Devonian ocean t h a t w i d e n e d t o the e a s t ; s u b s e q u e n t c l o s u r e o f t h i s ocean i n t h e Carboniferous r e s u l t e d in the Alleghanian/Hercynian orogenic b e l t and the formation of Pangea with northwest A f r i c a against Laurentia. Evidence of remagnetization has become more widely recognized although the o r i g i n of s t a b l e secondary components i s not always c l e a r . Most P a l e o z o i c c a r b o n a t e s in the A p p a l a c h i a n s had been c o m p l e t e l y r e m a g n e t i z e d but secondary components in the redbeds are t y p i c a l l y s y n f o l d i n g , that i s , are

best grouped after only partial tectonic tilt correction. The directions of these magnetizations indicate acquisition over a ca. 50 My interval of apparent polar wander in the Late Carboniferous and Perimian (Kiaman) time; an association with the Alleghanian orogeny is further suggested by a spatially-dependent pattern of timing of the remagnetizations. The high unblocking temperatures of the secondary magnetizations in the hematitebearing redbeds argue for a chemical origin but blocking temperaturerelaxation time relationships do not exclude a thermoviscous origin for remagnetization in the magnetite-bearing limestones. These partial to complete remagnetizations nay be due to thermochemical effects associated with flow of hot fluids and this magnetic signature could be used to develop an improved understanding of the p l u m b i n g of an orogen. Remagnetization must also have occurred during earlier phases of orogenic activity and should be looked for.

FINDING THE WAY FROM SCRATCH ~ OPPORTUNITY AND REALIZATION IN AUSTRALIAN GEOLOGY OVER TWO CENTURIES

T.G. Vallance The University of Sydney

'No prejudices or scholastic disputations have retarded our progress, for those who have aided in the work were disciples in the modern school.' That was Ralph Tate, as president of AAAS in 1893, explaining what he saw as the proud success of the first hundred years of geological effort in this country. Another century on, with so much more detail on record, and before a geological convention (itself a remarkable sign) dedicated to 'Achievements in Australian Geoscience' it may seem churlish not to continue Tate's triumphalism. Yet I desist. Tate, for whose work I have great respect, sets me thinking along different lines. First, that 'modern school'. Where was it in 1788 when European settlement of Australia began? James Mutton's paper on the Theory of the Earth may have been printed that year but most of his contemporaries remained content to leave earth history constrained by Scripture. Those then who bothered about strata simply expected the rocks found in Europe to occur in the same arrangement all over the world. Lithology was the key and Europe the sufficient model. Button's theory, assailed as irreligious, in fact languished until Charles Lyell in the 1830s began to gain for it lasting recognition. It was Lyellian geologists who were Tate's scientific progenitors. Anglophone Australia, however, even a sort of geological interest in it, began before Lyell was born. Our country and our science acquired their respective present characters over much the same period. Neither arose without trauma and their conjunction in a land remote from Europe and unknown to all but its aboriginal inhabitants raised particular problems. In January 1788 two groups of Europeans reached Botany Bay. The British stayed to colonize. The French went on with their exploration, leaving buried on shore one of their two staff appointed to study rocks and minerals. Joseph Banks, the main adviser of the British venture, had seen no need to provide governor Phillip with anyone to examine his unknown territory. For people like Banks, himself a mine-owner, minerals were for private venturers, not a government establishment. The French might see a challenge in mineral nature; he did not. With no encouragement from London, discoveries relevant to geology in the colony came by accident. Coal, for instance, was soon found along the coast near Sydney. The occurrences led George Bass in 1797 to suggest a basin lay under the settlement. Some rather futile attempts at boring followed and by 1801, in response to prompts by a British government anxious to economize on the colony, coal was being mined for export, by convicts at the Hunter River. Australia's first mining venture began without much geological interest. Men like the surgeon Bass were few. Convict miners matched what they saw in Australia with particular seams known at home. Others who remarked on our coal likewise equated it with the British Coal Measures. Even the advent of palaeontological stratigraphy seemed to yield no surprises, William Buckland of Oxford in 1821 applied the then new method to fossils from Australia, concluding that shells from Tasmania matched those of the English Mountain Limestone whereas coal plants from the Hunter River resembled those of his local Coal Measures. In the terminology then emerging both would be called Carboniferous.

Curious travellers in Australia might gather fossils but the specimens had to be sent to Europe for study. It seemed the discovery of stratigraphical order in Australia was not to be done situ but where such order was first recognized. Decisions and revisions affecting Australian geology continued to be made elsewhere. Thus among Buckland's fossil plants Adolphe Brongniart of Paris in 1828 recognized what he termed 'Glossopteris browniana', a plant present also in Indian coal but not in Europe. This Glossopteris had characters 'younger' than those of the European Carboniferous flora. Indeed, Brongniart had first used the name for a Jurassic fossil and later added another species from coal in Yorkshire that had been shown by field study to be Jurassic. There Brongniart left the matter, content to have Glossopteris Carboniferous in Australia and India but Jurassic in Europe. It was to prove a troublesome legacy. By the 1830s Australia was losing its convict character. Settlement by free and freed had already spread far and explorers, some claiming awareness of Lyell, were pressing further inland. The surveyor Thomas Mitchell even produced a geological map of the Wellington Valley, N.S.W., at this time. Earlier, in 1825, the merchant Alexander Berry had described in outline the Sydney Basin and reported its unconformable base in the south. These purposeful activities, however, passed without much notice in Europe. The little interest most European geologists had in Australia centred on oddities, like the living plants and animals that seemed to have relatives in the European fossil record. Disciples of Cuvier's catastrophism found interest in bone caves like those at Wellington and other opponents of Hutton and Lyell seized on the Burning Mountain at Wingen, N.S.W., as proof that volcanoes were due to combustion. Scholastic disputation involving Australia was wider than Tate supposed. By the 1830s also there were other colonies besides New South Wales, in situations where rocks different from those of the coal basin existed. As a rule old lithological methods had to suffice for correlation even after some, like the slates of South Australia, began to support metalliferous mining. Nevertheless, coal in New South Wales retained a peculiar fascination. As the explorer Ludwig Leichhardt claimed a few years later, the geology of Australia began at Newcastle. C o a l was a datum in Europe. It seemed likewise here but was it Carboniferous, or Jurassic, or what? Three men destined to address that question came separately to Sydney in 1839: W . B . Clarke, J . D . Dana and P . E . Strzelecki. They brought geological experience beyond any hitherto available in Australia and although only Clarke settled each left his mark. Clarke the clergyman could mix duty with an enthusiasm for field geology acquired at Cambridge from Adam Sedgwick. We remember the other two, the American Dana and the Pole Strzelecki, chiefly for monographs published respectively in 1849 and 1845. A l l could agree with Berry about the unconformity below the southern Sydney Basin but Strzelecki, who worked apart and kept a pre-Lyellian faith in catastrophes, claimed a break also existed within the basin, under the coal in the Illawarra district. Clarke and Dana had found no such break and only became aware of Strzelecki's claim after it was printed in London. Clarke meanwhile had been collecting fossils which he sent to his revered professor in Cambridge for expert study, a task Sedgwick handed to his young assistant Frederick McCoy. Wirthout warning Clarke, McCoy in 1847 launched into print: marine fossils from below the coal were Carboniferous (Mountain Limestone) but the coal plants were Jurassic. Brongniart's legacy had settled on McCoy who pointed to Strzelecki for congenial support. The

geologist J. B. Jukes had been over the disputed ground and defended Clarke in Cambridge but McCoy was adamant and Sedgwick vacillated. In the ensuing fuss and because gf its limited edition, Dana's report drew little notice. He agreed with Clarke on field relations and introduced the thought, from a study of fish remains, that some at least of the coal might be Permian. Clarke let that one pass; he remembered the English Permian had no coal and England for him was the source of authority, even unpalatable palaeontological authority. To his lasting credit, however, Clarke kept going, compiling field data on the coal-bearing succession until gold discoveries in the eastern colonies from 1851 created a temporary diversion. Gold, of course, changed direction for more than Clarke. It transformed Australian society. Golden riches promoted universities in Sydney and Melbourne, the establishment of geological surveys and the growth of scientific societies. Systematic mapping of sheet areas by the Victorian Geological Survey under A. R. C. Selwyn introduced to Australia methods practised in England and Wales. And McCoy came to Melbourne, as professor, with equally striking result. Gold also changed European geological patronage for Australia, creating new dependences, and divisions. Roderick Murchison, for instance, director of the geological survey in London, emerged as a claimant to credit as golddiscoverer. Hitherto Sedgwick, through Clarke and McCoy in their opposite ways, had been a dominating influence. Now, Murchison's men were running surveys. Shades of the Cambrian/Silurian controversy between the 'schools' of Sedgwick and Murchison appeared in Australia, with survey men going Silurian and McCoy at least Cambrian along English party lines. Some old relations suffered; Clarke began to cultivate Murchison. With Clarke and McCoy now practically neighbours it was inevitable coal problems would re-emerge. Clarke sniped when McCoy claimed, with what has proved convincing reason, that certain coal in Victoria was Jurassic. McCoy answered Clarke's report of beds containing Glossopteris lying under marine strata with 'Carboniferous' fossils near Maitland by insisting Clarke had failed to notice the fault which must be there. It was not found either there or in Queensland when similar associations in that colony received geological inspection. McCoy remained unmoved by mere field evidence. I have trailed this coal business not just to question Tate's rosy notion of untroubled progress but to emphasize what seems a real weakness in our early geological style. Neither Clarke nor McCoy, our two dominating early figures, managed to grasp that Europe was no necessary model for geology, or so it seems to me. I am ready, however, to admit that Clarke by his willingness to look in the field came nearer to independence than McCoy. The field indeed held clues that could have averted so much wasted effort in our first century, and those clues had been seen in Australia. Yet the credit for perceiving their significance, the Late Palaeozoic glaciation which Europe escaped, belongs to field men of the Geological Survey of India, men chosen for their critical intelligence and ready confidence to judge for themselves. Indian geology then was distinctly better served than ours. Their conjunction in 1886 when R. D. Oldham from India and T. W. Edgeworth David of the N.S-W. Geological Survey together recognized glacial remains in the northern Sydney Basin marked for most Australian geologists a breaking of ties too long considered both precious ajid necessary. It was a tardy liberation. The century since has seen our geological knowledge expand under more independent auspices, and certainly more convenient ones. A profession counted in tens a hundred years ago now runs to thousands, most of them

trained in Australia and many with well-deserved reputations. I excuse myself from naming names; each of us has his or her particular scientific heroes and heroines. As Australian geologists we have ample reason for pride but little, I think, for complacency. Some things continue to nag. The old conceptual dependence on Europe has gone but are we in our science as self-governing as our once-colonial nation is supposed to be? Have we yet lost the inclination to follow, to set our work, after the manner of Procrustes, into models and perspectives generated elsewhere? As our seirvant technologies increasingly help to spawn data it may be tempting to resort to established prescriptions but innovation and originality are not likely to lie that way. When I ask myself how many truly creative new insights, how many new lines of research, have Australian geologists over two centuries contributed to earth science I face no risk of losing count in the tally. It seems to me we have become pretty expert at filling in detail. Surely it is time we set our sights higher.

A B S T R A C T S OF S C I E N T I F I C P A P E R S

8.28 DRY, H2O-SATURATED AND CO2-BEARING LIQUIDUS PHASE RELATIONSHIPS IN THE CMAS SYSTEM AT 28 kb, AND THEIR BEARING ON THE ORIGIN OF ALKALI BASALTS J. Adam Geology Department, University of Tasmania

Dry, H20-saturated, and CO^-bearing liquidus phase relationships were studied within the CMAS system fCa0-Mg0-Al203-Si02) at 2 8 kb. In the dry system garnet, diopside, enstatite and rorsterite, were found to co-exist with a liquid of the approximate composition 50% SiO^, 16% Al.O., 2 5% MgO, 9% CaO. Under H^O-saturated conditions this liquid is relatively more CaAl20^-rich, and MgO-poor, than it is under dry conditions. Addition of CO^ causes a large expansion of the garnet and enstatite liquidus fields. This causes C02-rich liquids in equilibrium with garnet Iherzolite to be SiO^poor and nave high Ca0/Al202. ^ The compositional trace of CMAS liquids in equilibrium with plagioclase, spinel and garnet-lherzolite, between latm and 40kb, is shown in Fig. 1. Also plotted sre the 28kb HpO~Sciturated and CO«~bearing (lOWt.%) liquid compositions. The simple system liquid trends can be compared with the compositional trends of primitive alkali basalts from southeastern Australia, central Europe, Hawaii and the Canary Islands (Fig.l) . With decreasing Si02 alkali-basalts maintain relatively constant CaAl,0,/Mg,Si0., but increase in CaO/Al O^. This later feature is not particulaflj apparent from Fig.l and is better shown in a plot of CaO/Al-0- vs. SiO,,Fig.2. Within the CMAS system the alkali basalt trends most closely match those that would be produced by variable C02-contents in liquids equilibrated with garnet Iherzolite. In natural systems the presence of alkalis and volatile components may make the pyroxene-olivine cotectic more sensitive to pressure in the range 2 8kb-40kb, than it is in the CMAS system. This could enable the alkali basalt trends to be explained as largely the result of garnetIherzolite melting over a range of pressures. REFERENCES Clague,D.A. and Frey,F.A. (1982) J. Petrol. 23, 447-504. Davis,B.T.C. and Schairer,J.F. (1965) Carnegie Inst. Wash.Ybk 64, 123-126. Irving,T.N. and Green,D.H. (1976) J. Geol Soc. Aust. 23, 45-66. Presnall,D.C.,Dixon,J.R.,0'Donnell,T.H. and Dixon.S.A. (1978) J. Petrol. 20, 3-35. von Rad,U.,Hinz,K.,Sarnthein,M., and Seibold,E. (Eds.) (1982) Geology of the northwest African continental margin.. Springer Verlag. Wedepol,K.H. (1983) Geol. Jaharb. Hessen 111, 261-302.

Fiq 1 A l k a l i b a s a l t s from Honolulu (Clague & Frey,1982), north Hessian Depressian (Wedepol, 1983), south-eastern Australia (Irving & Green, 197 6) , Tnd the Canary I s l a n d s (Schminke in von Rad et a l . , 1982) Original analyses,all with MgO/(MgO+FeO) = 69.0-71.0, have been recalculated a c c o r d ing to the following scheme: SiO„=(Si0,-2Na,0-2K,0)60.09 A1,6,= (Al^O.+F^O.+Ra 0+K 0) 101.98 MgO="tMgO+FeO+Mn6) 40.32 CaO=(Ca0+Na20+K20-5/3P205)56.08 All oxides in molecular proportions. Phase relations at <28kb from Presnall et al.(1978).

SiO •

HjO-SATURATED

28kb

PROJCCTtON FROM CaSiOj 10% C 0 2

28kb

PRIMITIVE ALKALI - BASALTS

Fig. 2

MggSiO^

CaAl204

1.5

CaO AI2O3 1.0

PROJECTION r n o M $ / o .

0.5

CaO

35.0

40.0

45.0

Si02 Wt.%

50.0

11.4 MRLIS - A GEO-RELATIONAL DATABASE USED BY THE NEW SOUTH WALES DEPARTMENT OF MINERAL RESOURCES A. Agostini

and

New South Wales Department

L.B.

Gilligan

of Mineral

Resources

The New South Wales Department of Mineral Resources has installed computer hardware and software to establish MRLIS (Mineral Resources Land Information System). The Department contracted Computer Power to supply Synercom*s Informap II, a geo-relational database system. The system comprises a VAX 8200, supporting IBM PCAT graphics workstations, Tektronix colour graphic terminals, and DEC VT220 alphanumberic terminals on a local area network. The geo-relational nature of the database means that geographic data held in the system can be viewed in the electronic equivalent of transparent map overlays, with the type and scale of the overlays being selected by the user. Initial applications which are being developed for MRLIS are: Index and Mines Administrative Mapping: digitizing and production of maps showing regions and administrative area specific to the Department; Titles Administration: titles;

incorporating information on both coal and mineral

Borehole Inventory: comprising the location of and information on boreholes drilled in the State (initial intake being for coal boreholes); Mineral Deposits Inventory: an extension of the existing hard copy database of the Department's metallogenic mapping programme, and comprising location of and information on metalliferous, industrial mineral, and construction material deposits; Geological Mapping: initially equivalent to the current production of standard series geological maps, the application within MRLIS will make use of the truly scaleless and continuous map within the digital database. Subsidence Engineering: entailing the collation of subsidence grid survey data and its related gaining layout and local geological information, and will permit plotting of scaled overlys of subsidence with respect to topography, mining layouts, etc. Software is currently being written to enable the continued development of this application. For the other five applications, a pilot area of the State was selected so that data relevant to the applications could be input, displayed, and enquired on. The pilot area is a defined window with corners at 150''E 30'45'S, 150'E 31"S, 151''E 31'S, 151'E 30'45'S, i.e. the southern 1:50,000 sheet areas of the Boggabri and Manilla 1:100,000 standard sheets, in the New England region of N.S.W. Within this window, surface geological data, and data relating to 100 mineral deposits, 530 coal boreholes, 35 coal titles and 22 mineral titles have been input. Use of this concept has allowed (a) familiarisation'' with the system for a group of users, (b) tailoring of the system for these applications, and (c) a test of the system's versatility and potential with respect to the applications.

Development to date has addressed input and enquiry of the data, using Informap functionalities. Input is achieved by digitizing point and line data (e.g. geological boundaries, trace of faults, and title boundaries), and by using macros (programs using Informap commands) for input of attribute data (such as mineral deposit name, title number, information relating to the mineral deposit or the title). In the Mineral Deposits application enquiry macros have been developed to allow selective display of mineral deposits based on user specified criteria, e.g. commodity/mineral; deposit name, type, or size; ore genesis; host rock lithology or age. The user could also use a combined choice question such as "deposits occuring as gold veins" (i.e. two separate attributes of a deposit: (i) mineral = gold (ii) form of deposit = vein). In the Geological Mapping application, drawings are produced in the first instance as "Working Storage" drawings (by digitizing boundaries, placing symbols on the drawing and annotating them). The drawing is then placed in "Permanent Storage", where it becomes part of a continuous map, which can be constantly and readily updated. Development in the near future should proceed in two strands: firstly to investigate ways to utilize the system's versatility more fully, and secondly to render the use of the five applications more user-friendly. Across the State take-on of live data for all six applications is currently planned to commence in early 1988.

14.1

RARE EARTH ELEMENTS IN BIOGENIC APATITE - RECORD OF SECULAR VARIATIONS IN BOTTOM SEAWATER REDOX DURING THE PHANEROZOIC R. Ahmad School of Earth Sciences, Macquarie University, North Ryde

A large number of samples comprising of biogenic apatite of marine, living fish, fossil fish-debris and conodonts collected from different localities around the world were analyzed by Instrumental Neutron Activation Analysis (INAA) for rare earth elements. Average shalenormalized patterns of the rare earths in these samples resemble to that of modern seawater. The living fish apatite contains very low concentrations of the rare earths that they aquire from the seawater in which they live. However, the apatite of fossil fish-debris and conodonts are enriched in rare earths by orders of magnitude relative to the living fishes. The enrichment of these elements in the fish-debris and conodonts took place at or near the sediment-water interface after they died and settled down on the sea-floor. Among the rare earths, the concentration of cerium varies significantly in the apatite of Recent fish-debris depending on the redox state of bottom seawater at the sites of their deposition. Previous workers have shown that the fractionation of cerium in marine environments is redox controlled. Under oxidizing conditions cerium is fractionated by coprecipitation with metallic oxides, especially of iron, producing a negative cerium-anomaly (Ce-anom) numerically defined by Elderfield and Greaves (1982), whereas no such fractionation occurs in anoxic conditions and produce a normal or a positive Ce-anom. The apatite of living fishes show a large negative Ce-anom closely resembling that of the modern oxiding seawater in which they live. Recent fish-debris deposited in the Pacific Ocean with oxidizing bottom water show negative Ce-anom. On the other hand. Recent fish-debris deposited in the bottom of Perru continental shelf/slope. Gulf of California etc. with anoxic bottom-water show normal or a positive Ce-anoms. Based on these observations, it is interpreted that the Ce-anom in the apatite of Recent fish-debris record a signature of the redox state of the bottom seawater, and this signature remains preserved in them during their subsequent burial into the marine sediments with geological time. The same phenomenon is believed to be true for conodonts because both conodonts and the fish-debris are consist of carbonate apatite or francolite, and also show similar Ce-anoms if these are sampled from the same formation and locality. A plot of the Ce-anoms in the apatite of fish-debris and conodonts from the DSDP-Cores from the Pacific and Atlantic oceans, Australia, North America, Europe, and the Russian platform etc. vs. geological time shows secular variations. This plot indicates that largely anoxic conditions

prevailed in the bottom-water column of the Cambrian - Silurian, Lower Triassic and Cretaceous oceans, whereas Devonian - Permian, Jurassic and the Cenozoic oceans remained generally oxic. It also shows that the North Pacific bottom-water remained highly oxic during Eocene Oligocene, oxic during Miocene, suboxic to anoxic during Pliocene Pleistocene, oxic to highly oxic during Holocene, whereas the North Atlantic bottom water remained anoxic during Eocene - early Miocene. Variations in bottom seawater redox were possibly related to the changing patterns of oceanic circulation caused by tectonism, change in global climate, large-scale stratification of oceans and/or other unknown phenomena.

Reference Elderfield, H. and Greaves, M. J., 1982. The rare earth elements in seawater. Nature, vol. 296, p. 214 - 219.

14.3

CERIUM A N O M A L Y IN SILICEOUS MICROFOSSILS - RECORD OF MODERN A N D A N C I E N T BOTTOM SEAWATER REDOX Rl Ahmad^, L.S. Dale^ and J.J.

Fardy^

^School of Earth Sciences, Macquarie University, North Ryde ^Lucas Heights Research Laboratories, CSIRO Division of Energy Chemistry

Samples of siliceous microfossils obtained from shallow cores taken from the Guaymas Basin, Gulf of California (Lat. 27^55.2' N, Long. 111^36.6' W) with anoxic bottom seawater, and from DSDP cores obtained from sites 159, 40, and 51 with oxic bottom waters were analyzed by Instrumental Neutron Activation Analysis (INAA) for rare earth and other trace elements. These microfossils include mainly diatoms and siliceous sponge spicules of ages ranging from Recent to Eocene. The rare earth elements in these siliceous microfossils as well as those analyzed by other previous workers show variation in concentrations among the samples from different localities. Average shale normalized rare earth patterns of the siliceous microfossils resemble that of modern seawater suggesting that the rare earths of the microfossils were derived from the sea water. The values of the cerium-anomaly (Ce-anom) as defined by Elderfield and Greaves (1982) vary among samples. The samples from the Gulf of California with anoxic bottom water show slightly negative to positive Ce-anoms, whereas the samples from the Pacific and Indian oceans with oxic bottom waters show varying degrees of negative Ce-anoms suggesting that the Ce-anom in the siliceous microfossils records a signature of the bottom seawater redox in which these were deposited. This observation agrees with the independently varified Ce-anoms in biogenic apatite from the same/similar depositional environments by us as well as by other workers. A plot of the Ce-anoms in the siliceous microfossils obtained from the cores of Pacific Ocean vs. geologic time shows that the Pacific Ocean bottom water remained highly oxic during Eocene - Oligocene, oxic during Pliocene - Pleistocene, and moderate to less oxic during the Holocene. This observation agrees well with our findings based on Ce-anoms in biogenic apatite. Reference Elderfield, H. and Greaves, M. J., 1982, The rare earth elements in seawater. Nature, vol. 296, p. 214 - 219.

7.1

RECENT A D V A N C E S IN THE STUDY OF HOLOCENE DOLOMITIC CARBONATE SEDIMENTATION IN THE COORONG AREA OF SOUTH AUSTRALIA R. Ahmad and P.B,

Hostetler

School of Earth Sciences, Macquarie University, North Ryde The ephemeral Coorong Lagoon and adjacent chains of ephemeral lakes make up an area of 5 X 100 kilometers wherln a variety of carbonate minerals have been forming throughout the Holocene. These minerals include dolomite, magnesite, hydromagnesite, low- and high-magnesian calcite and aragonite. The dolomite varies from disordered to partially ordered; none is fully ordered. Since the discovery of dolomite formation in lakes of the area by Mawson (1929), about fifteen papers have been published by various workers dealing with formation aspects of these carbonate sediments. These papers have generally concentrated on providing explanations for the origin of dolomite, based on limited sedimentological and geochemical investigations. Actualistic, physico-chemical modeling must, of course, consider dolomite formation within a framework of genesis and diagenesis for all of the carbonate minerals. In the past several years, a large number of sediment- cores were obtained from the lakes and the lagoon. Piezometers were set in many locations in and around the lakes and the lagoon, and waters from different depth-levels in the subsurface as well as the surface were collected on seasonal basis. Sedimentological investigations were carried out on a large number of cores, and selected ones were analyzed for major and trace elements. Water samples were analyzed for pH, PCO2, major anions and cations and isotopic ratios of hydrogen and oxygen. Surface films of carbonate minerals were collected from various lakes and lagoon locations and investigated for carbonate mineralogy.The flux of elements through rainwater and aerosol was monitored at different locations in the area. Surface facies distributions of the sediments were mapped in the ephemeral Coorong and in some lakes. Based on a study of the sedimentology and preliminary geochemistry of the carbonate sediments and associated waters of the lagoon and some lakes, together with hydrologic data of other previous workers. Von der Borch et al. (1975) present a model for the formation of dolomite in the area. According to their model, the bulk of dolomite in the Coorong Area is formed through evaporation of seaward-flowing groundwaters that emerge seasonally in the lagoon and lakes from unconfined carbonate aquifers beneath them. Possible sources of soluble Mg include Mg depletion of high Mg-calcite allochems in the carbonate aquifers and leaching of ash deposits around Quaternary volcanic centers near ground water source areas. However, our interpretation of the chemical data obtained fron surface and subsurface waters in and around the lakes and lagoon is that the emerged ground water is derived from rainfall in

local catchment areas. These rain waters are rich in dissolved solids contributed by marine salt spray and aerosols. Entering the Holocene carbonate sequence in the lakes, these waters from semi-confined aquifers owing Jto underlying seals made up of lithified Pleistocene calcrete overlain by impermeable organic-rich sapropel at the base of the Holocene section. Waters of the ephemeral Coorong are more complex, derived from a mixture of seawater, Murray River water, local catchment water and, possibly, continental groundwater that emerges in the deeper part of the Coorong. Based on our investigations of the sedimentology, major and trace element geochemistry of the sediments, hydrogeochemistry, and aerosol, we present here a major modification of the Von der Borch et al. (1975) ground water model for the formation of dolomite in the Coorong Area. Accordingly, the dolomite is forming by direct precipitaion from surface waters of the lakes and the restricted, hypersaline, ephemeral Coorong as a result of degassing, photosynthesis and eventually evaporation in a closed basin situation. The Mg/Ca ratio and their absolute concentrations in the lake and lagoon waters are probably the contolling factors for the formation of dolomite. Upon burial, dolomite grain-size increases via diagenetic processes in the subsurface with waters having relatively high Mg/Ca ratios. The major source of Mg is salt apray and aerosol brought in from the southwest by wind and rain; some contributions are also made by dissolution of Holocene allochems in the subsurface or wind-blown from the modern barrier dunes of the Younghusband Peninsula.

6.1

SULFUR SOURCES FOR COPPER ORES AT MOUNT ISA, AUSTRALIA

A.S. Andrew^, C.A. Heinrichl>2^

R.W.T. Wilkins^, D.J.

Patterson^

ICSIRO Division of Mineral Physics and Mineralogy, North Ryde 2Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Mount Isa Mines Pty Ltd, Mount Isa

The Pb-Zn-Ag and Cu ores at Mount Isa are hosted by the Urquhart Shale, a metamorphosed Proterozoic sequence of dolomitic siltstones and shales. The Cu ores at Mount Isa differ from other well studied shale-hosted Cu deposits, in that Cu mineralization was temporally associated with the later of several phases of regional deformation and greenschist fades metamorphism. Previous structural studies have shown that metasomatic replacement of dolomitic-pyritic Urquhart Shale was separated by 1-2 phases of deformation from earlier, probably syn-sedimentary to diagenetic formation of the stratiform Pb-Zn-Ag orebodies (Perkins, 1984, Swager, 1985). Recent fluid inclusion and stable C-O-H isotope data (Heinrich et al., 1988) indicate that the metasomatic evolution of the siliceousdolomitic breccia (locally termed "silica dolomite") which hosts the Cu ores occurred at temperatures consistent with the lower-greenschist regional metamorphic assemblages in mafic and metacarbonate rocks. A major question of direct consequence for exploration for similar ores concerns the sources of Cu and S. Was the S transported with Cu in the same ore fluid, or did chaicopyrite form by reaction of a Cu-rich but Sdeficient fluid with a local source of S? An obvious possibility for the latter is the abundant layered pyrite, which preexisted in the Urquhart Shale (Robertson, 1982). The values of sulfides from the Mount Isa mine area show a wide variation, which is systematically related to rock type and mineral texture. Variations in S^^S values can be explained by varying proportions and degrees of homogenization of two major ^^s contributions, a) a relatively depleted ^^S in sediments predating "silica dolomite" alteration and Cu mineralization, and b) a component of enriched ^^S introduced into the sediments, probably together with Cu by the NaCl-rich ore-forming brine. The lowest S isotope values (1 to 5 per mil) occur in individual pyrite grains (Eldridge, pers. comm., 1986) and bulk samples from sedimentary layers rich in fine cubes of pyrite. This texture is interpreted to be the least altered, and the depleted S^^S values are thought to approach most closely the composition of the primary pyritic sediments. This pattern is widespread as shown by SHRIMP ion microprobe results (Eldridge et al., 1985) of depleted S^^S values (below 10 per mil) for earliest pyrites within dolomitized samples from the No. 8 Pb-Zn-Ag orebody. Some of the most enriched 634s values (20 to 30 per mil) have been measured in texturally different parts of the same samples that also contain sulfides on the extreme light end of the S^'^S spectrum, for example, in minor pyrite-chalcopyrite veins cross-cutting pyritic sediments near the fringe of the "silica dolomite" and in small pyritic overgrowths in layered pyrite-rich sediments (Eldridge et al., 1985). Similarly high values occur in the greenstones beneath the "silica dolomite" (with the highest S values in the most highly altered quartz-chlorite-sericite rocks), and in some of the samples from th^fe smaller Cu orebodies (e.g. 650). Other Cu ores, including most samples from the large orebodies (e.g. 1100), occupy a much narrower, intermediate range of 9 to 13 per mil.

All plausible sources of S in the mineralizing fluid involve direct or indirect participation of oxidized S (SO^^") which was then reduced prior its introduction into the Urquhart shale. Possible sources of sulfate include leaching of S of any origin under oxidized conditions, or early interaction of' the brines with sulfate-bearing evaporites. Sedimentological evidence suggests that evaporitic conditions were widespread in the Mount Isa region, and early interaction of the proto orebrines with evaporites would explain their enriched ^^S signature as well as their high salinity. These sulfate-bearing brines may have been reduced by interaction with the Eastern Creek metabasalts, from which they may have also leached Cu. The sediments that provided the ultimate source of the fluids, their salts, and their enriched 34s have not been identified. If the overgrowth in "coalescing" textures of layered pyrite occurred at the same time as the (? diagenetic) Pb-Zn-Ag mineralization (Neudert, 1984), then the bulk S isotope composition of the pyritic Urquhart Shale, immediately prior to "silica dolomite" formation, was probably somewhere between the depleted values of the early sedimentary pyrite and the variable intermediate composition of the Pb-Zn-Ag orebodies. The bulk isotopic composition of the sulfides encountered by the Cu mineralizing fluids (Heinrich et al., 1988) is thus poorly constrained, but was probably depleted in ^^S compared with the bulk of the present Cu ores. The proportions of sedimentary/diagenetic S to introduced S are also poorly constrained, but was probably much greater than one. The isotopic evidence for large-scale incorporation of sedimentary sulfide S into the epigenetic Cu ores is consistent with the suggestion that the sulfides in the replaced metasediments were an important ingredient in the "chemical trap" that contributed to the localization of the Cu ores at Mount Isa. References Eldridge, C.S., Compston, W., Williams, I.S., Patterson, D.J., Williams, N., Both, R.A., and Walshe, J.L., 1985, Ion microprobe analyses of sulphur and lead isotopic compositions of sulphides from the Pb-Zn ore bodies at Mt. Isa, Queensland, Australian In Herbert, H.K. (Ed.) Conference on stable isotopes and fluid processes in mineralization. Univ. of Queensland, p. 22-23. Heinrich, C.A., Andrew, A.S., Wilkins, R.W.T., and Patterson, D.J., 1988, A fluid inclusions and stable isotope study of synmetamorphic copper ore formation at Mount Isa: Geol. Soc. Aust. Abstr., v.13. Neudert, M.K., 1984, Are the Mount Isa lead-zinc ores really syngenetic?: Geol. Soc. Aust. Abstr., v. 12, p.402-404. Perkins, W.G., 1984, Mount Isa silica dolomite and copper orebodies: the result of a syntectonic hydrothermal alteration system: Econ. Geol., v. 79, p. 601-637. Robertson, C.W., 1982, The role of preexisting sulfides in copper-ore formation at Mount Isa, Queensland: BMR J. Austral. Geol. Geophys., v. 7, p. 119-124. Swager, C.P., 1985, Syndeformational carbonate-replacement model for the copper mineralization at Mount Isa, Northwest Queensland: A microstructural study: Econ. Geol., v. 80, p. 107-125.

7.3

SEDIMENT-WATER INTERACTION IN THE CENTRAL A U S T R A L I A N GROUNDWATER DISCHARGE ZONE - LAKE A M A D E U S , NORTHERN TERRITORY A.V. Arakel^ and G.

Jacobson^

^Department of Applied Geology, Queensland Institute of Technology ^Bureau of Mineral Resources, Geology and Geophysics, Canberra The regional groundwater discharge zone of Central Australia is an elongated valley, 500 km long, extending from Lake Hopkins in Western Australia, through Lakes Neale and Amadeus, to the Finke River in the Northern Territory. This great valley incorporates numerous playas, which act as drainage sumps for discharge from large regional flow systems. In Lake Amadeus, the playa? contain a variety of neogenic silicate, carbonate and evaporite minerals, which are characteristically zonated, reflecting shallow transmission and progressive evaporative concentration of groundwater solutions down the local hydrologic gradients. Documentation of sedimentary features and geochemical signatures of the playa sediments indicate diagenetic modifications due to variation in water levels, chemistry and probably climatic pulses. The playa-marginal areas contain extensive groundwater calcrete deposits. The most advanced stage of diagenesis corresponds with the zone of active groundwater circulation in the calcrete deposits. Here, partial-to-total replacement of the host calcrete by a variety of silica species attests to complex migration _ history of the silica in the palaeodrainage channels and fluctuations in calcite-silica saturation of the pore solutions. Basinward, diagenesis is mot pronounced in a narrow transitional zone developed around the playas, where the shallow groundwater approaches the surface. Textural and fabric features of gypsum assemblages recorded from this zone indicate sedimentation due to a combination of evaporative concentration, groundwater-level fluctuations and selective lon-exchange reactions. Gypcrete crusts are also developed in the playa margins and islands as a result of induration of bordering gypsum dunes. In playa-central parts, the hyper-saline waters of variable ionic concentrations, confined to clay, carbonate and evaporite layers of heterogenic mineral compositions, also testify for complicated interplay of the sediments and pore waters. Overall, the available evidences indicate a geochemical evolutionary trend dictated by a historical sequence of increasing aridity, geomorphic evolution and adjustments in the groundwater discharge pattern in subrecent times. Field observations suggest that with a long-term reduction in groundwater head, the playa sediments progressively become degraded by illuviation, pedogenic processes and eolian sand drift. The term •playa capture', an analogy to river capture, is proposed as a process by which the playas may migrate through time.

7.4

PALAEOHYDROLOGIC A N D ENVIRONMENTAL SIGNIFICANCE OF SILICIFIED CALCRETE IN INLAND DRAINAGE BASINS OF THE A U S T R A L I A N A R I D ZONE A.y. Arakel^, G. Jacobson^ and Y. Chen^ ^Department of Applied Geology, Queensland Institute of Technology ^Bureau of Mineral Resources, 2 Geology and Geophysics, Canberra Institute of Geology, Beijing, China

Quaternary calcretes are common in channel fill deposits of major palaeodrainage systems in the arid zone of Western and Central Australia, Silicification of calcrete has been recorded from both the vadose and phreatic hydrologic zones of shallow groundwater aquifer, and represents a late stage diagenetic event. Opaline and quartzose silica occur as void filling or as replacement of calcite groundmass. Diagenetic changes in the textures of silicified calcretes commonly correspond with silica-phase transformation and reflect replacement of carbonate through dissolution and precipitation. Petrographic evidence also suggest that silica precipitation proceeds concurrently with dissolution of host detrital material in the aquifer. In the large arid drainage basins, the shallow groundwater regime controls chemical sedimentation, and is an important landscape forming agent. Low hydraulic gradient (owing to limited groundwater discharge) is conducive to slow groundwater velocities, thus allowing time for sediment-water interactions to proceed. The C-14 and ESR dates determined for calcrete drillcores from selected internal drainage basin (23-55,000 years B.P.) indicate a remarkable short span of time required for calcrete/silcrete cogenesis. Similar silicified calcretes have also been recorded from the same drainage basins, as raised ridges standing several meters above the present groundwater level. They are considered to represent older analogues, developed from considerably elevated groundwater systems. The occurrence of silicified calcrete horizons in ancient terrestrial (lacustrine and groundwater) carbonate deposits may be related to the migration history of the silica in palaeohydrologic systems. The presence of silica as thin grain coatings, veins, and pore and cavity linings is a manifestation of the intricate balance between the solution of the host calcrete and precipitation of silica under arid/semi-arid hydrologic conditions. Such cogenesis, although spatially highly variable, has an important bearing on preservation and redistribution of economic mineral ores, and on the occurrence of fresh groundwater aquifers in the internal arid drainage basins of Australia.

14.4 PERMIAN BRACHIOPOD FAUNAS OF THE PERTH BASIN, WESTERN AUSTRALIA - A STUDY OF PROGRESSIVE ISOLATION

N.W. Archbold Department of Geology, University of Melbourne

The Perth Basin is the southermost basin of Western Australia with a significant marine Permian sequence. It is separated from the Carnarvon Basin to the north by structural features (the Hardabut Fault and Ajana Ridge) which are interpreted as having controlled differential subsidence on either side. Less than 300 kilometres separates the marine Permian sequences of the two basins but the duration of marine Permian influence was considerably shorter in the Perth Basin (Asselian? to Middle Artinskian) than in the Carnarvon Basin (Asselian? to Late Kungurian) . Rich benthonic sequences of brachiopod assemblages range from Late Sakmarian to Middle Artinskian age in the Perth Basin. Elements of this faunal succession were described in 1907 by Robert Etheridge Junior but few other brachiopods were described and figured until P.J. Coleman's study of productids K.S.W. Campbell's study of terebratulids and recent studies by the author. Three diverse faunas are recognised. They are those of the Sterlitamakian (Late Sakmarian) Fossil Cliff member of the Holmwood Shale; the Aktastinian (Early Artinskian) High Cliff Sandstone and the Early Baigendzhinian (Middle Artinskian) Mingenew Formation. These faunas demonstrate progressively a marked increase in local endemism when compared with the correlative faunas of the Carnarvon Basin to the north. The limestone, siltstones, sandy siltstones and shales of the Fossil Cliff Member contain the following diverse brachiopod assemblage indicative of an open marine environment: Neochon^tes pratti. St,rophalosia i rwinensis. Hetf^ralosia rqommerif=>i 1 Pl-.h^ridafî , Au.l oRtâes ha r a nooden s i s , AulOStegeS .^plnOSUSf Taeniothaerus irwinensis. Callytharrella caHytharr^ngis, Cancrinella irwinensis > ?Stenoscisma sp. nov, glQbi^ll^ fgordir Neospirifer hardmani . Triaonotreta n^Q^y?tra1is, Elivin^ hn.qkinaaf^ - Punrt or-yrt^ 11 a sp ., relytha f r d r iCKS 1. Phricodothyris o c r i d e n t a l i s , T o m i o p s i s WO O d w a I'd i r r Ifîothyridina b;^ r;::^ rood^ns is ajld F1, f^tcherithyr IS—sp . cf hardmani. Although the assemblage is not as diverse as that of the Callytharra Formation (Carnarvon Basin) virtually all species are shared with that formation. The fine to course sandstones of the basal High Cliff Sandstone contain a less diverse assemblage, possibly indicative of a regressive sequence prior to deposition of the Irwin River Coal Measures. Brachiopods present include: Neochonetes (ggmm^yi^lj^) sp. nov., Aulosteaes cf A. inaens. Taeniothaerus sp. nov., r;^nrrinella sp. nov., Neofipirlfer sp. nov., neospirif erid gen. et sp. nov. (allied to / Punrrtocyrtella sp. nov..

Tomiopsi.^ rarusr Cleiothyridinc^ sp., Giiifidia wQoiaafin.qi .g and Hoskingla sp. nov. Over half the species are endemic to the Perth Basin as is the new neospiriferid genus. The siltstones ahd fine to coarse sandstones of the Mingenew Formation, separated from other Permian units by fault relationships, contain a diverse assemblage including the following: StreptorhynchuR sp. , Tornquistia sp. , Neochonf^t^R (Sommerie^l^) robystVS/ Strpph^Xosi^ sp., Echinalosia sp. nov., Ming^n^wi^ ^ncpm^l^, Taeniothaeru.q sp. nov., Cancrinella sp. nov., permosyrinxinid gen. nov. A (a highly transverse genus), permosyrinxinid gen. nov. B (a small compact form), Fusisoirifer byroensis f Neospir i f f^r sp., neospiriferid gen. et sp. nov. (allied to CraS£?if?pirifer) . Cleiothyr idina sp., Hoskinai a n o b i l i s . G i l l e d i a sp . cf home va l en s i and Gilled i a wQol^gensj-S. Sufficient species indicate correlation with the Madeline - Coyrie faunas of the Carnarvon Basin while other species and up to four genera are endemic to the Perth Basin. The progressive increase in endemdsm shown by the Perth Basin faunas from late Sakmarian to mid-Artinskian times is of note considering the relatively short distance from the correlative faunas of the Carnarvon Basin. Open marine conditions with free migration of species is indicated for the Sterlitamakian. Regressive sequences dominate the Aktastinian of both basins and presumably differential movements on the Hardabut Fault and Ajana Ridge partially isolated the faunas of the two basins at this time. Partial isolation continued into the Early Baigendzhinian after which marine sedimentation ceased in the Perth Basin. Nevertheless this partial isolation did not prevent diverse brachiopod assemblages evolving within the basin and some migration of species between the two basins. Salinity was apparently that of normal marine waters in view of the diverse brachiopod assemblages present. There is no indication of impoverished or dwarf faunas being present as would be expected if salinity was abnormally high or low. Differences in facies or palaeotemperatures cannot be readily proposed because the facies of correlative stratigraphical units in the two basins are comparable and the distance between the faunas is rather short. A futher question of interest concerns the relationship of the Perth Basin faunas to those that may have occurred to the west. Two broad models exist for Permian reconstructions of the western margin of Australia, specifically the Peninsula IndiaWestern Australia model and the Greater India (Tibet) Western Australia model. Tibetan and Himalayan Permian brachiopod faunas are becoming well known and no direct relationships with Perth Basin faunas are indicated. Instead, more subtropical elements are present in the range of Tibetan faunas suggesting palaeogeographical positions more related to northwest Australia and Timor. Peninsular India appears to lack correlative faunas with the Perth Basin Sterlitamakian to Artinskian succession although an exception appears to be the faunas from the undescribed succession at the north west Indian locality at Badhaura. The discovery of analogous faunas to those of the Perth Basin within India-Tibet would provide critical constraints on the Permian palaeogeography of India and Australia.

19.8

THE PERMIAN OF AUSTRALIA - THE 1988 POSITION N.W. Archbold

Department of Geology, University of Melbourne

Perhaps the longest studied of any system in Australia, The Permian System yielded Australia's first described fossil (the brachiopod, Triannot-rel-a stokesi) and Australia's first described fossil plant nssopi-p-ri s hrnwnii). both being described in the 1820's. However, despite this long period of study, investigations have been hampered by a number of inherant difficulties. The nature of the type Permian sequences in the U.S.S.R. (including tropical marine, saliferous, marginal marine and continental deposits) has continued to cause debate as to the most appropriate International Stages within the System and the fixing of the CarboniferousPermian and Permian-Triassic boundaries. The highly provincial nature of Permian shallow marine faunas and terrestrial floras has substantially contributed to problems of global correlation within the framework of an evolving international time scale. Despite difficulties such as those above, substantial efforts have achieved considerable results in unravelling the Australian Permian. This work is continuing and involves detailed investigations into stratigraphical sequences and the documentation of preserved marine micro and macro faunas and continental mega plant and palynological floras, the necessary first steps for detailed local biostratigraphy. Following these studies meaningful international correlations can be attempted. This review briefly examines some of the current work and advances in the knowledge of the Australian Permian time-scale. The Eastern Australian successions are revealing critical information for detailed biostratigraphies. The Tasmanian succession, through the work of M.J. Clarke and others, is a series of tightly defined lithostratigraphical and biostratigraphical sequences as outlined in the mid - 1970's. A local stage scheme of three major formal associations (consisting of 10 faunizones) documents the three major episodes of sedimentation, transgressions and regressions. A refined summary is in press for the Tasmanian geology Bicentennial Volume, the brachiopod assemblage of Faunizone 10 will be published by the time of this conference and the brachiopods assemblages of Faunizones 1 to 3 are in press. The lower part of the Tasmanian succession is providing critical data on the correlation of the marine faunas with the palynostratigraphical subdivisions of eastern Australian continental sequences. Marine faunas of the Sydney Basin have been analysed by D.J.C. Briggs (1987) and a refined zonation based on brachiopods has been outlined. As with the Tasmanian sequence, the lower part of the Sydney Basin sequence is providing data for defining the Carboniferous - Permian boundary in Eastern Australia. Both the continental and marine sequences of Queensland are the subjects of detailed investigations. Advances in the palynostratigraphical scheme of the Southern Galilee Basin through the investigations of

M.J. Jones and E.M. Truswell have implications not only for local Carbo-Permian biostratigraphy but also for correlations with the Western Australian palynostratigraphical units and defining the Carboniferous-Permian boundary in Australia. Other palynostratigraphical work (eg McMinn, 1987; Price, 1985) continues the refinement of such schemes. The Marine faunas and stratigraphy of Queensland are the subject of much attention (e.g. Waterhouse and Briggs, 1986; Murray and Waterhouse, 1977 and Parfrey, 1986) and other investigators (e.g. J.M. Dickins and V. Palmieri) also have studies in preparation. Debates on details of correlation indicate the need for continued work. The Western Australian Permian sequences occupy a pivotal position between the eastern Australian provincial marine faunas and the faunas of the Tethyan seas. A summary of work on Western Australian faunas and floras has been prepared by Skwarko (in press) and includes information published in 1987 such as the study of Carnarvon Basin crinoids (Webster, 1987). Nevertheless work continues in a variety of disciplines in order to accrue further data for an appreciation of these critical sequences. This all too brief and inadequate review has indicated three points to its author. 1. Diverse and considerable work is adding substantially to an Australian Permian time-scale. 2. Further resolution of an Australian Permian time-scale can only come from accruing a wider data base of palynostratigraphical zonations, mega-plant associations, marine faunal zonations and lithostratigraphical investigations and 3. That further c o m m u n i c a t i o n b e t w e e n workers may be enhanced by an annual Australian Permian Review Newsletter. I express my thanks to the many Permian workers who responded to my search for information and my apologies to those "left out". Again, increased communication is indicated. Briggs, D.J.C., 1987. Permian productid zones of New South Wales. Advances in the studv of the Svdnev Basin, Svmposium 21. Abstract.^. 135-142. McMinn, A., 1987, Palynostratigraphy of the Stroud-Gloucester Trough, N.S.W. Alcheringa 11: 151-164. Murray, C.G. and Waterhouse, J.B., 1987. 1987 Field Conference, Gympie District. Geological Society of Australia Queensland Division, Brisbane. 133pp. Parfrey, S.M., 1986. Early Permian invertebrates from the Camboon Andesite near Biloela, Southeastern Bowen Basin. Publication. Geological Survev of Oufi^n.^i ^nri, 387: 57-67. Price P.L., 1985. An historical review of the Bowen Basin Stratigraphy. Supplement to Geological Society of Australia Abstracts 17,.8pp. Skwarko, S.K., in press. Palaeontology of the Permian of Western Australia. Bulletin of the Geological Survey of Western Aust.ra 1 i . Webster, G.D., 1987. Permian crinoids from the type-section of the Callytharra Formation, Callytharra Springs, Western Australia. Alcherinaa 11: 95-135.

6 8

AN APATITE FISSION TRACK STUDY OF Zn-Pb MINERALISATION ON THE LENNARD SHELF, WESTERN AUSTRALIA D.C. Arnel, P.F. Green^, I.R. Duddyl, A.J.W. Gleadowl J.F. Lovering2 ^nd I.B. Lambert-^

^Department of Geology, University of Melbourne 2Department of Geology, Flinders University, Adelaide ^Baas Becking Geobiological Laboratory, Canberra

Epigenetic, carbonate-hosted Pb-Zn sulphide minerahsation of the Mississippi Valley-type (MW) is considered to precipitate from warm basinal brines in the temperature r ^ g e 50-250 C. A variety of genetic fluid flow models have been proposed to explain individual MVT ore districts but all suffer from a lack of constraint concerning the timmg of ore formation. In addition, assumptions concerning the maximum temperatures attained by host rocks on a regional scale are often unsupported. Fission tracks in apatite result from the spontaneous decay of trace amounts of 238U and are thermally unstable over the range of temperatures proposed for MVT ore formation. At temperatures above approximately 70°C oyer geologic time penods, the rate of shortening for fission tracks in apatite increases and results in a reduction of Ae appênt apatite age. If peak temperatures attained during the thermal event ^ e high enough (le: >110 C), apparent apatite fission track ages wUl reflect the time of cooling frona these temperatures Aus constraining the time of heating. Provided sufficient time is allowed, fission tracks m apatite will record thermal events related to MVT ore formation. Zinc mineralisation in Devonian carbonates of the Lennard Shelf, north Canning Basin is similar in many respects to that of the Mississippi Valley-type, includmg estimated minimum tLperYmres of sulphide precipitation between 70 and 110°C An apatite fission track smdy was midertaken in tiie êas of known mineral occurrences in order to place within a regional thermal context, and to perhaps constrain the timing of its fomation M e ^ apatite fission track ages for Precambrian granitic basement and for êmtal apatites in Devom^^ carbonates near Zn-Pb mineralisation generally range between 260 and 340Ma ™ Precambrian samples tending to have slightly older apatites than the Devonian c^bonates These mean ages are younger than the soratigraphic age of the matenal analyzed, indicating that a p p S l e a L a l i n I of fission tracks in apatite has occurred in PO^t-^fvom^ t i m e ^ ^ horizontal confined trick lengths range between 12 and 13um and reflect slow coolmg from peak temperatures attained during annealing. Studies of well sequences (Grevillea#l and S e d i a # l ) indicate a period of rapid uplift in the area during the Late Tnassicârly tossic Assuming a constant geothermal gradient of 30°C/km approximately 1-Sk® of uîft is estimated Immediate thermal effects related to Miocene lamproite intrusion into Precambnan basement appear to be restricted to within 200 meters of the contact zone. For Devonian carbonate outcrop samples, a thermal history is proposed involving burial in the Late Paleozoic/Eariy Mesozoic, followed by uplift and cooling from peak temperatures around 70°C in mid-Mesozoic times. The possibiUty of a phase of even higher temperatiês dunng the Late Devonian/Early Carboniferous cannot be excluded solely on the basis of fission track data, but may be doubtful given conodont colouration results. The absence of anomalous annealing X c t s near Zn-Pb occurrences, coupled with other evidence, suggests that the mineralizing episode was of short duration and precludes any conclusions concerning the timmg ot ore formation.

13.1 I N T E R D I S T R I B U T A R Y B A Y D E P O S I T S IN T H E ILLAWARRA COAL MEASURES, SOUTHERN SYDNEY BASIN W.J.

Bamberry,

Department

B.C.

Geology,

Jones

and A . C .

University

Button Wollongong

In the lower reaches of river-dominated deltas, distributary channels form the framework controlling the distribution of sediment on the delta-plain. Interdistributary bays form laterally extensive areas between the major distributaries and represent sites for significant delta growth. These areas receive sediment by a variety of flood-generated processes, including crevasse splays and flocculation of suspended particulate matter. The resultant bay-fill sequences provide a platform for plant colonization and represent an important coal-forming environment. Descriptions of present-day interdistributary bay environments and their deposits are well documented, particularly for the Mississippi Delta. There are, however, few detailed accounts of ancient examples. Within the progradational succession of lower to upper delta-plain deposits forming the Late Permian Illawarra Coal Measures in the southern Sydney Basin, several bay-fill and distributary channel sequences have been recognised. The upper part of the Wilton Formation and overlying Tongarra Coal consist of a lower delta-plain succession in the middle of the Illawarra Coal Measures. This succession is particularly well exposed in brickpits and coastal headlands between Woonona and Wombarra (Fig. 1), north of Wollongong. It is 12 m to lA m thick and has been informally subdivided into four units reflecting different stages in the development of interdistributary bay sequences. The lowermost unit (Unit I) is about 2 m thick and coarsens upwards. It grades from laminated carbonaceous lagoonal shale of the middle Wilton Formation, through alternating beds of flat-bedded and ripple-bedded sandstone with thin siltstone laminae, to thicker beds of predominantly flat-bedded sandstone. Driftwood, distorted bedding and water escape structures are common features of this unit. Unit I is interpreted as crevasse splay deposits which prograded into a shallow, muddy interdistributary bay. Locally, sharp-based beds of planar cross-stratified sandstone occur near the top of the unit and represent the deposits of minor crevasse channels. Rootlets in the upper part of Unit I reflect partial emergence of the splay surface and the completion of a thin interdistributary bay-fill sequence. Subsequent abandonment of this delta lobe is indicated by a gradual deepening of the depositional environment. During the initial stages of this deepening event the upper part of the splay deposit was reworked by wind-generated currents to produce extensive exposures of symmetrical ripples in sandstone beds separated by thin silty laminae. Unit I is overlain by consisting of about 2 m of dark grey, welllaminated siltstone and mudstone (Unit II) with a thin (5 cm) coal seam composed of allochthonous plant matter. This unit contains abundant organic matter including many Glossopteris leaves. Growth faults and scattered dropstones are also present. These features, together with the absence of rootlets, indicate a placid period of lagoonal or lacustrine sedimentation. Unit II represents a protected interdistributary bay

succession

deposited

during

the

progressive

isostatic

subsidence

the

abandoned delta lobe. Unit III is typical of a coarsening-upward interdistributary bay-fill deposit. It consists of broadly lenticular beds of interlaminated siltstone and very fine-grained sandstone. Smaller coarsening-upward packages occur within the unit , and broad shallow erosional scours are filled with sandstone. The sequence is characterised by the common occurrence of bioturbation, small-scale soft sediment deformation structures, scattered dropstones and asymmetrical ripple marks. At this stage of deposition, the interdistributary bay was open to circulatory currents and may have been either brackish water or marine. The coarsening-upward sequence filling the bay represents the transition from reworked distal splay sands to splay deposits more proximal to an adjacent distributary channel. The major 100 Unit m ciay .siit-.sgndi % sand incursion of crevasse splay deposits into 0the bay environment coincides with a reduction in salinity and the common development of sideritic concretions. The proportion of siltstone decreases upwards through the lower two thirds of IV the bay-fill sequence but then increases towards the top of sequence. The interlaminated siltstone and mudstone in the upper part of the sequence represents overbank flood-plain deposits capped by a soil containing abundant rootlets.

') s s s s

Ht. ir ^

iQj

The Tongarra Coal (Unit IV) represents autochthonous marsh and swamp deposits formed as a blanket peat on the delta platform following progradation of the interdistributary bay-fill. The coal seam consists of alternating dull and minor bright coal plies reflecting periodic fluctuations of the water level in this environment. Light-coloured bands of tuffaceous claystone in the coal seam are up to 41 cm thick and, in places, infill swales at the top of the coal. These tuffaceous beds result from episodic deposition of ash-fall material onto the lower delta-plain.

The recognition of interdistributary bay-fill sequences in the upper Wilton Formation has implications regarding the lateral continuity and thickness of the Tongarra Coal. On a regional scale the coal should thicken towards the distributary channels but should be s siderife Q dropstones locally interrupted by levees and JL rootlets d i s f o r t e d bedding burrows ripple-bedding channel-fill sequences and split by Q plant ripple marks crevasse splay deposits. The coal should fragments s flQt-bedding become thinner, dirtier but brighter • 1 common S rare (less oxidized) towards the distal Fig. 1. Stratigraphic section of margins of the interdistributary bay-fill sequence where the greater degree of the upper Wilton Formation and compaction of muddy bay-fill deposits Tongarra Coal at Austinmer. produces conditions less conducive for thick peat accumulation.

3.3 MAGNETOSTRATIGRAPHY ON A TIMESCALE OF 0-10^ YEARS SECULAR VARIATION, GEOMAGNETIC EXCURSIONS AND REVERSALS

C.E. Barton Bureau of Mineral Resources, Geology and Geophysics, Canberra

Three properties of the geomagnetic field can be exploited for magnetostratigraphic correlation and dating on timescales up to millions of years: long-period secular variation in direction and intensity of the field, geomagnetic excursions, and magnetic polarity reversals. A review of these is given and some related studies in Australia are outlined. Palaeomagnetic analyses of rapidly deposited sediments have established that the geomagnetic secular variation has time constants varying up to thousands of years, and possibly longer. A characteristic pattern of secular variation can often be recognised over regions of up to one or two thousand kilometer in size. When combined with adequate chronological control this provides a master curve for dating other sequences provided they preserve a record of fluctuations of the geomagnetic field at, or shortly after, the time of deposition. Southeastern Australia is one of the few areas of the world where a dated secular variation master curve exists for the Holocene. The record was obtained from radiocarbon-dated cores from Lakes Keilambete, Gnotuk and Bullenmerri (KGB) in SW Victoria, and includes archaeomagnetic data, historical and present-day observations. New palaeomagnetic results from two long cores from Lake Terang, SW Victoria, indicate that it may be possible to extend the secular variation master curve back to about 50 Ka. If so we will have one of the longest continuous secular variation records ever obtained, and the basis for a valuable dating tool. Geomagnetic excursions are short lived (hundreds to thousands of years in duration) perturbations of the geomagnetic field. On a global scale they probably result from aborted reversals and can be expected to have the same probability of occurrence as actual reversals (presently about one in 250 000 yr) . On a regional scale they may result from abnormally high amplitude secular variation. The value of such excursions as stratigraphic and chronological markers is considerable. However, although there have been numerous claims for excursions during the Quaternary, the palaeomagnetic evidence for nearly all of them is weak. The Lake Mungo excursion at about 30 Ka (based on data from aboriginal fireplaces on the shores of Lake Mungo in western NSW) is the only excursion that has been detected in the Australian region. It is considered by many to be one of the best documented excursions, and may correlate with the nort-hern hemisphere Laschamp event.

Several sedimentary sequences in the region have been examined for further palaeomagnetic evidence of the Lake Mungo excursion - at Lake Mungo itself and adjacent lakes, at Lake Terang in southwestern Victoria, at Darwin Crater in western Tasmania, and in DSDP cores from the Lord Howe Rise and the Chatham Rise. None of these supports the hypothesis of a geomagnetic excursion at about 30 Ka. This new evidence is not sufficient to disprove the Lake Mungo excursion, but does cast doubt on its validity. The record of geomagnetic polarity reversals during the last few million years based on data from marine sediments and K/Ar-dated lava flows is now well established. Even so there may still be very short polarity intervals that have not been identified. This is illustrated by the "Cobb M o u n t a i n Event" at 1.1 Ma which has only just been recognised. In Australia considerable use has been made of polarityreversal stratigraphy for dating sequences from dry and ephemeral lakes and other Quaternary deposits. Despite the severe problems that arise from post-depositional chemical alteration it is nearly always possible to locate the Brunhes-Matuyama boundary (0.73 Ma). Reversals become increasingly difficult to identify in older sediments and are seldom recognizable prior to the Gauss chron (3.4 Ma). Clay lithologies are noticeably more efficient as recorders of the geomagnetic field than sandy facies. The value of magnetostratigraphy in dating marine sediments is illustrated by results from cores of pellagic carbonates from the Lord Howe Rise, collected during Leg 90 of the Deep Sea Drilling Project (see poster in technical session 15) . There is considerable scope for expanding the use of magnetostratigraphy in marine environments and in using rapidly deposited marine sediments for studying long-period secular variation and geomagnetic excursions. In addition to the above "geomagnetic" dating methods the natural variations in m i n e r a l m a g n e t i c p r o p e r t i e s of sediments may be used for stratigraphic correlation. Such "rock-magnetic" methods can be simple and rapid to apply, but are generally restricted to stratigraphic correlation within particular basins and cannot be used for dating.

6.2

THE SELWYN-STYLE CU-AU DEPOSITS IN THE MOUNT ISA BLOCK - A LATE OROGENIC, GRANITE AND DEFORMATION-RELATED STYLE T.J. Beardsmore Geology Department, James Cook University of North Queensland

Selwyn-style breccia-hosted copper-gold deposits form a 70kilometre long, north-trending lineament south of Cloncurry in the Mount Isa Block. All show broadly similar geological settings. The Mount Dore copper deposit belongs to this group, and lies about 130 kilometres south of Cloncurry. It is the best known deposit of its type, with a large data base accumulated during mineral exploration. Detailed examination was expected to produce a regional genetic model. Regional mapping has placed the Mount Dore deposit into its geological context. Host litholgies are steeply-east-dipping guartz+mica schists and carbonaceous slates of the uppermost Maronan Supergroup (Beardsmore et al., in press.), structurally overlying the younger meta-calcarenites, marbles and meta-basalts of the Staveley Formation because of early, subhorizontal (D^) deformation and later (D2) upright, northtrending, tight to isoclinal folding. Peak metamorphic grade, in the lower to middle amphibolite facies (Jaques et al., 1982), occurred syn-D2. Discontinuous, northwest-trending "corridors" of D3 deformation, each generally less than one kilometre wide, occur sporadically across the Selwyn region. These have produced local shallowing of the steeply-dipping bedding and other foliations. Where these "corridors" intersect the newly-recognized Mount Dore Fault, areas of extensive brecciation and fault-block "shuffling" have occurred, and mineralization may be present. The Mount Dore deposit is an example of this. The Mount Dore Fault is a north-trending structure marking the boundary between the Maronan Supergroup and the Staveley Formation in the Selwyn area. It dips moderately to steeply east, and movement along it is interpreted to be east-overwest. In the vicinity of Mount Dore, the hanging wall comprises the Mount Dore Granite, indicating faulting may be syn- to post-granite intrusion, which has been dated at about 1510 Ma (Nisbet at al., 1983). The fault is actually a zone averaging 250 metres wide. Preliminary drill-core logging and petrologic examination of selected samples from the mineralized zone reveal a complex history of brecciaton and alteration. Early breccias have angular, often tabular clasts, many of which are strongly foliated and crenulated. The crenulation, identified with S3, shows no consistent orientation between clasts, suggesting disruption of host rocks post-dates D3.

All brecciated lithologies show extensive replacement and infill within and around the orebody. Early, angular breccias show variable degrees of K-fe1dspathization , tourmalinization, sericitization and silicification. The last of these is the most pervasive. Later alteration is associated with hydrothermal milling of the early breccias, and includes dolomite, apatite, biotite and chlorite. Mostof the mineralization is associated with the first of these phases, and sulphide assemblages vary with depth in the deposit. Shallow mineralization consists of chalcocite, which replaces earlier pyrite. Deeper-level mineralization comprises pyrite and chalcopyrite, with minor sphalerite and galena. Both assemblages are associated with the hydrothermal milling of early breccias. A preliminary model is proposed to explain mineralization at Mount Dore. Movement along the Mount Dore Fault was accompanied by gaping and hydraulic brecciation where the fault intersects D3 "corridors" of shallowly-dipping bedding and S2 foliation. The nature of alteration suggests involvement of late-magmatic fluids. These may have emanated from a late phase of the Mount Dore Granite, moving into the highly permeable breccia bodies. The early fluid contained K"*", B and SiOj. Fluid-rock interaction during hydraulic brecciation depleted it in these elements, and enriched it in Ca2+, Fe2+, Mg2+, and ore metals. These elements later contributed to alteration and mineralization during hydrothermal milling of the early breccias. Boiling of the fluid may have occurred at this time. Interaction with downward-percolating meteoric water is not discounted. Initial pressure and temperature conditions of the fluid are currently unknown, but the extensive brecciation argues strongly for a shallow, relatively cool hydrothermal system. REFERENCES Beardsmore, T.J., Newbery, S.P. and Laing, W.P., in press. The Maronan Supergroup - an early volcanosedimentary rift sequence in the Middle Proterozoic of northwest Queensland. Submitted to Precambrian Research. Jaques, A.L., Blake, D.H. and Donchak, D.J.T., 1982. Regional metamorphism in the Selwyn Range area, northwest Queensland. B.M.R. J. Aust. Geol. Geophys., 7, 181-196. Nisbet, B.W., Devlin, S.P. and Joyce, P.J., 1983. Geology and suggested genesis of cobalt-tungsten mineralization at Mount Cobalt, northwestern Queensland. Proc. Aus. I.M.M., 287, 9-17.

4,4 THE MARONAN SUPERGROUP - OLDEST OF THREE SIMILAR VOLCANOSEDIMENTARY RIFT SEQUENCES IN THE MIDDLE PROTEROZOIC OF NORTHWEST QUEENSLAND T.J. Beardsmore, S.P. Newbery

and W.P. Laing

Geplogy Department, James Cook University of North Queensland

Mapping in the Cloncurry-Selwyn region of the Proterozoic Mount Isa Inlier has rationalised the stratigraphy there, and revealed an early volcanosedimentary basinal cycle, which probably predates others in the Inlier. The Soldiers Cap Group (sensu stricto) has been expanded to incorporate most of the Kuridala Formation. Quartzofeldspathic rocks in the latter, and in undifferentiated Soldiers Cap Group, are now interpreted to conformably underlie the Soldiers Cap Group, in a 5 kilometre thick sequence termed the Fullarton River Group. The Soldiers Cap and Fullarton River Groups collectively comprise the Maronan Supergroup, an apparently conformable sequence about 10 kilometres thick, consisting of terrigenous and volcaniclastic sediments and bimodal volcanics, which mature texturally and chemically up-sequence, and which were probably deposited in a marine environment. Grainsize indicators suggest an easterly sediment source, but some quartzofeldspathic material may have been derived from Barramundi Igneous Suite rocks to the west. We interpret the Maronan Supergroup as a complete cycle of basin sedimentation, probably related to ensialic rifting. The Supergroup crops out over an area of about 15000 km^. It is unconformably overlain in the east and south by Mesozoic sediments, and bounded in the west by the Mount Dore Fault Zone, a major east-dipping structure separating it from the Mary Kathleen Group along the entire length of the contact. The Gin Creek Block is a quartzofeldspathic inlier tectonically emplaced in the Mary Kathleen Group, and may be stratigraphically equivalent to the Fullarton River Group. The stratigraphic relationship of the Maronan Supergroup to the two cycles of rift sedimentation already recognised in the Mount Isa Inlier needs geochronological justification, but circumstantial evidence suggests it is older than either of these. It is certainly distinct from the other sequences in having, for example, an overall higher metamorphic grade, different geochemical and geophysical signatures for volcanic and volcaniclastic units, and a range of lithologies unique to the Mount Isa Inlier, such as a thick turbidite package in its upper central part, plagioclase-rich sediments, and true banded iron formations (containing Pb+Zn±Cu mineralization). Assuming the Maronan Supergroup is older than other cover units in the Mount Isa Inlier, we interpret the history of sedimentation in the I^plier to have involved three cycles of ensialic rifting, each showing a temporal trend of evolving volcanicity and increasing sediment maturity. Lithologies show a natural three-fold subdivision into distinct packages.

based on the dominant lithologies. These are characterised, from oldest to youngest, by (1) felsic volcanics and associated volcaniclastics; (2) mafic volcanic and maturing coarse- to fine-grained clastic sediments; and (3) finegrained, clastic and carbonate-bearing sediments. A complete rift cycle displays all three rift packages, which are interpreted to relate to early rift, maturing rift, and sag phases of the cycle. This subdivision relates specifically to the Mount Isa Rift Orogen, but may be applicable elsewhere, as the patterns of evolution of volcanosedimentation in other eastern Australian Proterozoic basinal sequences show broadly similar characteristics. From existing geochronological data and, where necessary, inferred ages of sedimentation, the three rift cycles have ages of 1880-1810 Ma, 1810-1750 Ma and 1680-1660 Ma. The Barramundi Igneous Suite may be broadly synchronous with initiation of rifting in the Mount Isa Inlier.

7.17 THE SEDIMENTARY FRAMEWORK AND NATURE OF LATE PLEISTOCENE AND HOLOCENE SEDIMENTS, GULF ST VINCENT, SOUTH AUSTRALIA A.P. Belperio^ and V.A. Gostin^ ^Geological Survey of South Australia ^Geology Department, University of Adelaide The intracratonic St Vincent Basin is an area of Cainozoic sedimentation associated with the separation of Australia fran Antarctica. It contains up to 600m of interdigitating continental clastics and biogenic carbonate sediments that provide an excellent record of Tertiary and Quaternary sea level fluctuations and sedimentation of cool-ternperate biogenic carbonates. Present-day Gulf St Vincent resulted from the latest (Holocene) transgression and covers an area of some 7000 km . There is little present-day input of terrigenous sediment from fluvial systons and sediment accumulation within the Gulf is largely controlled by local biogenic production with limited redistribution. The entrance to the gulf (30-70m) is scoured by strong tidal flows, exposing cemented Pleistocene carbonates. Rocky surfaces are colonised by sponges, algae, ascidians, bryozoans, crinoids and gastropods. Calcareous debris is swept into the Gulf where it contributes to sandy, biogenic sea-floor sediments. In the central part of the gulf (20-40m depth), a wide depression of muddier sediment supports a filter feeding benthos of sponges, bryozoa, bivalves, brachiopods, crinoids, ascidians and sea pens. Skeletal detritus fran these, and from benthic foraminifera and predatory molluscs and echinoderms, make up much of the sediment. Boring by sponges, algae and gastropods, and bioturbation by a varied infauna including prawns, crabs, polychaetes and holothurians, produce a well mixed, poorly sorted, imiddy fine sand sediment. The gently sloping gulf margins (0-15m) are geologically and biologically iirportant areas of extensive growth of seagrass meadows. Seagrasses provide shelter and a subnstrate for a diverse biota of red algae, bryozoa, foraminifera, diatcsns, sponges, molluscs, hydroids, echinoids and crustraceans. The meadows thus act as a carbonate factory and trap and bind the poorly sorted skeletal sand resulting in accumulation of organically bound biogenic carbonate banks. Seagrass bank accumulation results in a shallowing of the nearshore profile, aggradation to low tide level and concomittant intertidal and supratidal progradation. Zonation of plants, animals and sediments is well d e v e l o p across intertidal areas and is largely related to degree of tidal inundation. At about spring low tide level Posidonia seagrasses are replaced by Zostera and Heterozosterar species that are able to tolerate periods of emergence. Mangrove woodlands are developed from mean seal level to just below spring high tide level. Their pieumatoj^ores further facilitate sediment trapping and aggradation. Saline marshes containing halophytic vegetation such as Halosarcia and Sarcocornia are formed behind the mangroves and merge landwards with increasingly dessicated, bare supratidal pans. Cyanobacterial mats colonise the floor of the m^grove woodland, the saline marshes and sane higher intertidal and supratidal flats. During the last glacial maximum, the gulf was an extensive low gradient plain, with a large lake occupying the central depression. This area was

also inundated by interstadial marine transgressions that reached levels of -22 to -28ra relative to present between 45 000 and 30 000 years B.P. These sediments consist of unlithified calcitic mud with variable proportions of shallow marine bivalves, gastropods and foraminifera. During the last interglacial (ca. 125 000 yr B.P.)r sea level was about the same as the present in a slightly enlarged gulf. Similar facies and distribution of facies to that of the Holocene are recorded, though the deposits are variably cemented and surficially calcreted. Accurate identification of the Pleistocene shoreline has enabled differential subsidence rates to be calculated for the Melaide region (.04ranper year) relative to the margins of the basin. The style of cool-tarperate carbonate sedimentation documented for the Holocene has dominated the St Vincent Basin for the past 40 Million years since the Late Eocene.

9.10

THE ROLE OF THE STATE EMERGENCY SERVICE IN THE COMMUNITY J.L.C. Bickford Queensland State Emergency Service

Implications for counter-disaster planners. As a lead into this short paper, a brief sumnary of counter-disaster (CD) arrangements is necessary to create an awareness of the function of the CD conbat agencies in the State. Further details may be obtained by reading the Act and the Handbook for the State Emergency Service and State Counter-Disaster Organization. Background The (^jeensland ' Government has established two major disaster combatting agencies under legislation entitled the State Counter-Disaster Organization Act 1975/78. These are: The State Counter-Disaster Organization (SCDO); and The State Elnergency Service (SES) . In essence, the SCDO performs the top and middle managonent role in the coordination of resources to ensure that all steps are taken to plan for and counter the effects of disaster in the State. These layers of managenent reside at the State and Disaster District levels. The Local Authority counter-disaster officials perform the line management role. The trained workforce to augment the conventional Statutory Authorities^ is made up from volunteer State D:nergency Service units. The objective of all three levels of disaster response is to minimize the impact of a disaster on the comiunity and the alleviation of hardship and distress. The permanent staff of the SES is involved with both organizations on a day to day basis. They have a responsibility to provide advice to planning authorities, delivery of training to response organizations, manning of the State Bnergency Operations Centre, public education, and such other duties as may be required. These functions are controlled by the Director, SES, who is also Executive Officer of the SCDO. Planning and Preparation Counter-disaster (CD) plans provide the framework for CD operations, and form the basis of training for response organizations. Plans are drawn up at three levels. They are: Local Authority; Disaster District; and State. Ihe Local Authority plan is the basis for all other plans, and is therefore the key for CD preparation and response. Local Authority plans are prepared in accordance with guidelines set out in the Handbook for the State Einergency Service and the State Counter-Disaster Organization. Assessing the threat (hazard analysis) is an essential element planning process. Threats are assessed on the following basis: The mTUEE (flood, cyclone, fire, earthquake, etc); The DEGREE (severe, moderate, slight); and The PATTERN (return period).

the

These factors are considered in terns of the populations effected, ge^raphy of the area and resources available to combat the various threats. Information on which the threat assessnent is made is derived from official statistics and local experience. Because of the relatively low incidence of seismic events, earthquake does not figure proninently in many Local Authority counter-disaster plans. Recent works and studies under the auspices of the Department of Geology and Mineralogy of the University of Queensland are providirig useful information for disaster planners in assessing seismic risk potential for their area. The publication entiled Seismological Studies at the University of Queensland 1935-1985 compiled by Dr John M.W. Rynn, and Seismic Risk in Queensland by the same author are two such useful references. As well as being vitally concerned with the preparation, response and recovery stages of disaster, CD authorities also have a vested interest in the prevention phase. This is an area where Government authorities can influence and minimize disaster potential, by zoning development and evolving suitable building codes and engineering standards for construction work. Ei^ineers require definitive information relevant to the Australian scene in order to adequately address this problem. It is consider^ that research being carried out in the area of seismic risk is invaluable to design and construction engineers. The Risk Potential A major earthquake in a significant centre of population has ^ e potential to produce mass casualties involving substantial rescue and medical effort, as well as creating a large number of homeless and distressed persons. A severe tropical cyclone has similar potential for destruction, aggravated by heavy rain and floodir^. The lack of warning of the earthquake event is a unique characteristic which effects the activation of respons^ the basic skills and techniques used to co-ordinate cyclone and other major operations, are equally appropriate. The chief among these functions are: Multi-Service Co-ordination; Mass Rescue; Mass Casualty Handling; and Evacuation and Welfare.

These functions are currently addressed as topics of workshops, saninars, skills training and exercises as part of normal CD preparation. Conclusion The work being carried out in seismic risk and engineering practice is extranely valuable to Counter-Disaster Authorities in Queensland, and should continue. Efforts should be made to ensure that a continuing n o w of adequate information on seismic risk, where it exists, is available to building and construction authorities and CD planners. counter-disaster measures designed to cope with large scale disaster events should be adequate for a major seismic incident, provided.a flexible approach is maintained by those responsible for the coordination of

response.

13.5 SPECIFIC TECTONIC CHARACTER OF THE TELFORD BASIN A N D IMPLICATIONS FOR MINE PLANNING, LEIGH CREEK COALFIELD, SOUTH A U S T R A L I A W. Bogacz^ and I. Kivior^ « ^Geo-Mining Consultants, Adelaide ^The Electricity Trust of South Australia

The Leigh Creek sub-bituminous coal deposit is situated about 500 km north of Adelaide, South Australia. It consists of four rather small isolated subbasins developed upon pre-Cambrian rocks and filled with Upper Triassic Lower Jurassic coal bearing sediments. Current mining takes place within the largest sub-basin called the Telford Basin. This Basin forms an elliptical depression in which beds are inclined towards the centre at 10^ to 55^. The tectonic evolution of the Telford Basin is coded in rock defects and has been recorded based on genetic, statistical and geometrical analysis of these defects. Particularly important for this study were structures in mesoscale. These exhibit both shear and extensional mechanisms of origin with many transitional types of failure. At least three genetic types of faults contribute to the complex structure of the coal bearing formation in the Telford Basin: steeply dipping faults (75^ 90°) which parallel the basin's elliptical shape. These display a dip-slip type of displacement, being especially frequent in marginal parts of the basin forming a system of "peripheral faults". complementary conjugated faults of normal-slip type (dipping 45° 75°). These represent longitudinal and near longitudinal tectonic structures of both homothetic and antithetic sets conjugated at about 60 . During folding these faults have been rotated up to 15° towards the middle of the basin. normal-slip faults (50° - 70°) of tensile character. These display NE-SW trends and are limited to the north-western and northern part of the basin. Characteristic tectonic structures are observed along bedding planes. These are bedding defects indicating a movement (or tendency towards movement) of the rock mass inwards both as synsedimentary and ^igenetic processes (Figure 1). Indications of such movement correspond to shear processes contributing to the origin of the following structures: "clay seams" which are thin (up to a few centimetre) layers in the bedding plane with related derived deformation. "shear zones" along drastic lithology changes, particularly on contact between clay, claystone or mudstone with frequently occurring very hard layers (locally called "hard bars") originating from sideritization of mudstone. Indications al so exist of extensional opening of the bedding anisotropy surfaces which are considered^as weak horizons as well. The geometric and genetic character of the faults and fractures indicate that the oval shape and structure of the Basin sequences has resulted from:

basement instability, particularly reactivation diapirism which is widely known in the region,

of faults and possibly

gravitational and gravitational-extensional stresses. Deposition patterns of the coal-bearing sequences of the Leigh Creek Coalfield correspond also to the above geological factors. Based on analysis and recording of the character and evolution of rock defects the structural model and implications for mine planning will be discussed. An understanding of the specific and complex structure of the coal bearing sequences in the Telford Basin is a necessary condition to control geotechnical rock evaluation, low and high wall stability, mining methods applied and resources estimation.

FIGURE 1: TECTONIC INTERPRETATION OF BEDDING FAULT AND RELATED STRESS FIELDS, M13 MINING AREA. A - Zone of gravitational sliding along bedding plane and related stress field. B - Zone of space compensation by thrust-type shearing and related stress field,

2.7 PALAEOTEMPERATURES IN BASIN ANALYSIS - DIRECT CORRELATION OF VITRINITE REFLECTIVITY WITH FLUID INCLUSION MICROTHERMOMETRY

Y. Bonel and N. Russell^ ^Department of Geology and Geophysics, University of Adelaide ^CSIRO Division of Mineral Physics and Mineralogy, North Ryde

Palaeotemperatures are one of the most important parameters to be determined when petroleum exploration is the underlying reason for basin analysis. Vitrinite reflectance analysis is the most commonly used, rapid, method of assessing the thermal maturity of organic matter with respect to hydrocarbon generation. Currently, palaeotemperatures are estimated by comparing theoretical vitrinite reflectance values, calculated from subsidence rate and geothermal gradient modelling of a petroleum well, with the observed vitrinite reflectance values for the well. This paper reports the direct correlation of vitrinite reflectivity, determined by microscope photometry, and fluid inclusion homogenisation temperatures, determined by microthermometry, for samples from a comprehensive suite of petroleum exploration wells in the Cooper Basin, South Australia. In this study the microthermometry was confined to very small (1-5 um in length) fluid inclusions contained within silica overgrowths developed on detrital quartz grains present in siliciclastic sedimentary rocks. The temperatures measured are in the range associated with hydrocarbon generation, expulsion and migration. Many of these inclusions contain fluids that are autofluroescent under ultravoilet, or blue light, excitation, suggesting the presence of hydrocarbons. The homogenisation temperature values were plotted against vitrinite reflectance values, obtained, wherever possible, from organic matter within the same, or an adjacent core sample. The resultant plot permits a real temperature value to be assigned to a given vitrinite reflectance value, i.e. vitrinite reflectivity can be regarded as a true palaeothermometer. Conversely, in the absence of indigenous vitrinite, the homogenisation temperature of fluid inclusions within diagenetic overgrowths, or cements, can be used as a minimum basin temperature indicator at the particular site and as a guide to the thermal maturation of nearby organic matter. For comparative purposes microthermometric determinations were carried out on primary fluid inclusions within the detrital quartz grains. These inclusions usually gave homogenisation temperature in excess of 300 C, indicating a probable igneous or metamorphic provenance. It is planned to extend this study to other basins, in particular to basins that also contain carbonate rocks, e.g. Canning Basin. It is also planned to expand the fluid inclusion microthermometry facet of the study to include detailed analyses of the contained fluids, and, by the use of cathodoluminescence, to assign the individual fluid inclusions to a sequential fluid event, i.e. cement stratigraphy, thus leading to the construction of a comprehensive history of the basin.

2 20

A TECTONIC RECONSTRUCTION FOR THE BASEMENT ROCKS BENEATH THE CARPENTARIA BASIN

D.J. Bourke, B.A. McConachie, N. Senapati and J.C. Slade Comalco Aluminium Ltd, Brisbane

A new tectonic model for the basement rocks beneath the Carpentaria Basin has been constructed using regional magnetic, gravity and Landsat data. From the model it is possible to predict the nature and extent of basement rocks and thereby select target areas for mineral and petroleum exploration. The Carpentaria Basin is a very large epicratonic down warp located in the north-eastern quarter of the Australian continental plate. It contains up to 1200m of Jurassic-Cretaceous clastic sediment. Because of the rarity of deep drillholes, very little is known about the true geology of the basin or its underlying basement. Much of the exposed Carpentaria Basin is covered by Tertiary sediments of the Karumba Basin. The gravity coverage of the onshore portion of the basin was recorded by the B.M.R. on an 11 kilometre grid. This information has been reprocessed and filtered to produce new Bouguer Anomaly and Residual Gravity maps. The magnetic data came from three sources; the BMR's regional magnetic surveys, the Wenlock River Airborne Survey and the Weipa Aeromagnetic Survey. A composite model of all the total magnetic intensity maps was made for the purposes of correlating the regional magnetic units. In addition, qualitative interpretations were made on the depth to magnetic basement using a Werner deconvolution technique. A comparative study of the gravity and magnetic data over the exposed crystalline rocks, which flank the eastern edge of the basin, demonstrated that consistent and predictable responses occur over the various basement lithologies. These responses can be reliably extrapolated under the sediments of the Carpentaria Basin, thereby making it possible to confidently predict the nature and the extent of subsurface basement rocks. Using Landsat imagery, it is possible to recognise major faults which separate basement blocks and deform the Carpentaria Basin sediments. It is postulated that the Carpentaria Basin sits astride an Early Proterozoic rift complex, which can be traced along the west coast of Cape York Peninsula and south into the Eromanga Basin as a linear zone of dense, highly magnetically active rocks. The crystalline rocks of the Mt Isa Inlier represent the western flank of the rift sequence. The eastern edge of the complex is covered by the sediments of the Carpentaria Basin. Here the basement rocks form a zone of northerly trending magnetic and gravity anomalies which separate the rocks of the Mt Isa Inlier from those of the Georgetown Inlier. The extent of rifting was probably small, not exceeding that seen in the modern rift systems of East Africa. The innumerable northerly trending, mafic dykes which intrude the Mt Isa Block are believed to represent the surface expression of the rifting period. After rifting and between 1670 and 1450 Ma, rocks of the Mt Isa area were regionally deformed and metamorphosed. 66

A period of wrench faulting, dominated by a series of north - south trending faults, further broke up the basement rocks. Extensional basins, such as the Georgina and Hodgkinson Basins formed adjacent to the faults and filled with carbonate, evaporite and clastic sequences. From the age of these basins, it appears that the focus of fault activity may have migrated from west to east and covered a period from Late Adelaidean to Late Devonian. At least two periods of Palaeozoic granitic intrusions are associated with the basement rocks. The first probably occurred in the Early Palaeozoic and gave rise to large, linear massifs which are characterised by both low magnetic and gravity responses. The second period probably occurred in Late Palaeozoic. These igneous bodies are smaller and are typically circular in shape. They produce residual gravity lows but are highly magnetically active; the magnetic activity resulting from the associate volcanic rocks. This second suite of igneous rocks appears to be closely related to the Permo-Carboniferous granitoids and volcanics, which can be traced north-west from the Townsville area, through the Georgetown Inlier and below the basin sediments to the edge of the Gulf of Carpentaria. Several Late Palaeozoic infrabasins, can be inferred from the potential field data. They are postulated in areas where large regional gravity lows are associated with very quiet magnetic activity. The infrabasins probably have a tectonic style which is similar to that of the Galilee or Bowen Basins. At the close of the Palaeozoic, the Carpentaria Basin basement comprised a complex of crystalline and sedimentary rocks which ranged in age from Archean to Permo-Triassic. The palaeotopography was subdued with fluvial sedimentation occurring in valleys along the eastern and southern edge of the basin.

9.9

EARTHQUAKE CODES W.H. Boyce

Cameron McNamara Pty Ltd, Brisbane

The code covering the design of Australian structures for earthquake effects is A S 2121 'The Design of Earthquake Resistant Buildings', known as the 'SAA Earthquake Code' and published by the Standards Association of Australia. The code, drawing on seismological information available to 1976 and seismic risk studies published by McEwin, Everingham and Denham (1976), was published in 1979 after a gestation period of about 8 years. It was based on a document produced by the Structural Engineers Association of California ( S E A O C ) - known as the "blue book" in engineering circles. At the time the Earthquake Code was produced, the Queensland data was not available for processing by the Bureau of Mineral Resources. Token recognition of the seismic activity in the Wide Bay-Burnett region was given by classifying it as Zone A . Recent studies at the University of Queensland (Rynn, 1986) have confirmed that this is not an adequate zoning for Queensland. Indeed, the earthquake activity in the State was underestimated. Building Design Considerations The paramount consideration in the design of buildings is life safety in the event of a severe earthquake. With this in mind the earthquake provisions should enable most buildings to: 1. resist minor earthquakes without damage 2. resist moderate earthquakes without significant structural damage, but with some non-structural damage 3. resist major or severe earthquakes framework and maintain life safety.

without

major

failure of

the structural

It is recognised, however, that because of the random and unpredictable nature of earthquake motions and the uncertainties concerning the strength and response of buildings, the seismic provisions cannot fully ensure that there will be no injury or loss of life. Ultimately the acceptable seismic risk for incorporation in the code is a matter of public policy. Towards A Revised Earthquake Code Recent seismological studies using earthquake data to 1984 have produced revised probabilistic earthquake risk maps for Australia (Gaull, Michael-Leiba and Rynn, 1987). In addition, detailed studies in Queensland have commenced to determine the various limitations and uncertainties relevant to the complete understanding of seismic risk estimates (Rynn and Bpyce, 1987).

In many quarters, it is considered that sufficient information is available to permit a revision of the current Earthquake Code. At the recent Institution of Engineers, Australia, seminar on Earthquake Risk in Sydney, 2-3 December 1986, it was evident that such a revision .would prevail in terms of Limit State Code wherein each of the conditions that a structure must resist during its lifetime are defined. (Bubb, 1986). References Bubb, C.T.J., 1986: Towards a Limit State Code for Earthquake Resistant Design. The Institute of Engineers Australia. Gaull, B . A . , Michael-Leiba, M . O . and Rynn, J . M . W . , 1987: New Probabilistic Earthquake Risk Maps of Australia. Bureau of Mineral Resources Journal of Geology and Geophysics (in press). McEwin, A . , Underwood, R. and Denham D., 1976: Earthquake risk in Australia. Bureau of Mineral Resources Journal of Geology and Geophysics (in press). Rynn, J . M . W . , 1986: Queensland Seismic Risk Study. Final Report to the State Government of Queensland. University of Queensland, Department of Geology and Mineralogy and State Government of Queensland Publication, 191 pp. Rynn, J . M . W . and Boyce, W . H . , 1987: Earthquake risk assessment and engineering design - A vital consideration for the Pacific Rim. Proceedings of the Pacific Rim Congress 87, 26-29 August 1987, Gold Coast. The Australasian Institute of Mining and Metallurgy Publication, 729-735.

17.14

EARLY TERTIARY VOLCANISM IN THE SOUTHERN MONARO REGION, NEW SOUTH WALES

M.C. Brown^, I. Clarke^, M.K. McQueen^ and G. Taylor^ ^School of Applied Science, Canberra College of Advanced Education ^Geological Survey of New South Wales

Mafic volcanic rocks of Palaeocene to Eocene age cover a large part of the Monaro district of south eastern N.S.W.. Recent detailed studies in the southern part of this volcanic province have revealed that the volcanics were erupted over a land surface with relief of up to at least 500 m and a drainage network well adjusted to the structural grain of the Palaeozoic bedrock. Lavas buried considerable thicknesses of Early Tertiary lacustrine and fluvial sediments and formed unusual hyaloclastite deposits where they entered some of the lakes. Similar sediments are interbedded with the flows. Lake sediments at the base of the volcanic sequence contain Late Palaeocene pollen. Lava flows of alkali basalt and basanite dominate the volcanic pile. They include even-grained and porphyritic varieties with phenocrysts (5-15 vol.%) of olivine, clinopyroxene and in some cases plagioclase. Some thick flows (and possible sills) are doleritic and contain concentrations of cumulate, pyroxene and olivine. Minor pyroclastic deposits occur with the basalts and include poorly sorted, coarse-grained tuffs and wellbedded basaltic ash deposits with bedrock and fragments and plant debris. Some laterally-extensive bauxite horizons may represent weathered distal pyroclastics. A number of dykes and volcanic pipes have been recognised and these probably represent feeders to the basalt flows. Many of the pipes contain abundant mantle and crustal xenoliths and xenocrysts. Many are nephelenitic in composition and some contain abundant kaersutite and biotite megacrysts. These eruptive centers are relatively small and widely scattered, with a concentration near the present Great Divide. Their distribution does not support the suggestion of Oilier and Taylor (1987) that the Monaro Volcanic Province is a large shield volcano with its center near Brown Mountain. Kesson (1972) was able to show that the basalts of the Monaro region have a relatively restricted compositional range and could be divided chemically into alkali basalts, basanites and nephelinites. Major and trace element analyses of 29 samples from this study confirm these findings and indicate a range in Si02 content of 41-49%. Magmatic differentiation of the rocks is best indicated by the lOOxMg/(Mg+Fe*) ratio, which varies from 55 to 75 (Fe203/Fe0 set at 0.25). Each of the alkali basalt, basanite and nephelinite series extends for the full range of ratio values. Or and Ni contents decrease with increasing differentiation in all the rock series. Incompatible element contents are usually highest in the nephelinites, lowest in the alkali basalts and intermediate in the basanites, although considerable overlap occurs. Nephelenites generally display the narrowest compositional trends and show enrichment in incompatible "elements with differentiation. The alkali basalts show parallel but broader compositional trends for K^O, Rb, Sr and Ba, but the Zr, P2O5 and LREE contents are remarkably invariant.

To date we have' not been able to find any systematic chemical variation throughout most of the volcanic stratigraphy. However, it does appear that the nephelinites represent the latest stage of volcanic activity in the area as these occur in minor flows and exposed volcanic plugs near the top of the sequence. References Kesson, S.E., 1S72: Basic alkaline rocks. Ph.D. Thesis, Aust. Nat. Univ., Canberra. Oilier, C.D., and Taylor, D., 1987: Geomorphology of the Cooma-Bega region. In Galloway, R.W. (ed.). The Age of Landforms in Eastern Australia. Tech. Memorandum 87/2, CSIRO Div. of Water and Land Resources, Canberra.

2.28

THERMAL SYSTEMS AND THERMAL PROVINCES IN THE LATE CARBONIFEROUS TO MIDDLE TRIASSIC DEVELOPMENT OF EASTERN AUSTRALIA J.W. Brownlow New South Wales Geological Survey

Rapid, regional-scale heating of the lower crust is a func3ainental and powerful geological process. Its recognition and systematic analysis is critical in understanding the evolution of many geological provinces, particularly where granitoid plutonism is involved (e.g. New England Brownlow 1982). The direct effects of high heat flow are episodes of coeval plutonic, volcanic, metamorpAiic and hydrothential activity. Regional uplift and possibly rifting accompany heating. Subsidence occurs during cooling, and sustained subsidence may produce sedimentary basins. Thermal systems are prdDably caused by large scale intrusion of mafic mantle diapirs into of the lower crust (Brownlow 1982). This large scale mass transfer is considered both necessary and sufficient condition to explain the thermal and geografMc features described above. Thermal systems are episodic. Activity coimonly lasts 10 to 40 my, and cooling lasts 50-100 my. These events typically follow major episodes of regional deformation - the canbined proceeds ccxnprising a "tectonothermal regime" which may be the regional scale manifestation of subduction (see Brownlow, this synposium). Ttiermal activity is confined to discrete thermal provinces. These provinces may be oblique or parallel to the structiaral grain of the country rock. Individual thermal provinces may be segmented longitudinally, with different segments being parallel and offset, or may oblique and contiguous. Different segments may exhibit different thermal and geomorphic histories, and different petrological and metallogenic signatiares. Lateral zoning of segments, svdh as paired plutcsiic and volcanic belts, is ccmmon. Plutonic belts are coimonly sites of contenporary uplift, which may be preserved as basanent ridges (e.g. western margin of Sydney Basin). Volcanic belts are coimonly sites of contenporary of subsequent basin formation (e.g. Sydney Basin). Five thermal provinces can be recognized in the Late Carboniferous to Middle Triassic development of eastern Australia. 1. A major Permo-Triassic thermal system developed along the b o u n d ^ between the Lachlan and New England provinces, following midCarboniferous deformaticn. Paired plutonic and volcanic belts are recognized in NSW and possibly north to Townsville. Ihe plutonic belt formed as an elevated region and comprises Late Carboniferous Bathursttype granitoids which intrude the eastern Lachlan Fold Belt and possibly the eastern Drummond Basin. The volcanic belt comprises Late Carboniferous felsic volcanics and Early Permian mafic volcanics v^ch form the basement of Sydney-Bowen Basin. Glaciation accompanied Late Carboniferous uplift, shedding several kilometres of fluvioglacial sediment and felsic volcaniclastics eastward into western New England.

The volcanic belt may have been a rift during the Late Carboniferous, and a marine volcanic basin during the Early Permian volcanism, subsiding and filling to form the Sydney-Bowen Basin. The segment north of Townsville developed partly on the Precambrian shield, as a volcano-plutonic province, apparently without a succeeding sedimentary basin. 2. A second' Permo-Carboniferous system developed in New England following latest Carboniferous deformation. Paired igneous belts are inferred. S~type Hillgrove plutons developed in an area of uplifted basement. Metamorphic complexes developed early in this interval, overlapping with the preceeding defonration. Felsic and later mafic volcanism accompanied subsidence to form the marine Nambucca Basin. This thermal province extends into southern Queensland, where further Hillgrove plutons occur. The northern extension of the associated volcanics in not established. 3. An Early Permian syston developed in western New England, possibly following megafolding in northern New England. S~type Bundarra plutons developed in an area of inferred uplift in western New England. Contanporary volcanics are not recognized, but might be inferred from the composition of volcanogenic sediments in the upper Nambucca Basin sequence. The separate identity of this systen is not conclusively established, and it could be related to the previous syston. 4. Extensive Middle Permian activity occurred along the eastern margin of Australia, following Middle Permian defomnation. The thermal province is a series of NE-trending segments, each offset to the NW relative to its southern neighbour. Latite volcanism occurred along the south coast of New South Wales. Massive granitoids of Nundle, Uralla and Moonbi Plutonic Suites and various leucogranitoids, and their and oogenetic felsic and intermediate volcanics developed mainly in a NE trending belt in central New England. Low—K granitoids (Nundle Plutonic Suite) developed at Barrington Tops, along an offset. Belts of Sn, Mo, Au and base metals developed parallel to the NE trend in central New England. Segments in Queensland corprises granitoids of the Yarraman Block, Rawbelle Batholith, and probably the Mirium Vale Granodiorite. Uplift sourced the Singleton Super Group and Blackwater Coal Measures in the Sydney-Bowen Basin. 5. NNW trending Middle Triassic activity followed Permo Triassic deformation. Mineralized I-type and leucogranitoids developed in the Gundle Belt in eastern New England. The Esk Rift developed further east, and its southern extension includes the volcanic basement of the Clarence-Moreton Basin, the Coramba-Orara Gold Field and a belt of small mineralized granitoids along the mid-north coast of New South Wales. A further northerly trending belt of granitoids, and probably the Gympie Gold Field, developed between Brisbane and Rockhamptai. Subsidence in the volcanic belt produced the Clarence-Moreton Basin. Uplift in the granite belts probably sourced the Wiannamatta Group-Napperby bedsMoolayonftoer Formation in the Sydney-Bcwen Basin. Brownlow, J.W., 1982, Diapirism and the development of four thermal provinces in northern New South Wales, in NEW ENGLAND GEOLOGY, Flood, P.G. and Bruce Runnegar, (eds.), p 229-237 Department of Geology, University of New England and AHV Club, Armidale, N.S.W. Published with permission "of the Secretary, NSW Department Resources.

of Mineral

2 31 TECTONOTHERMAL REGIMES AND THE MID-CARBONIFEROUS TO MIDDLE TRIASSIC EVOLUTION OF NORTHEASTERN NEW SOUTH WALES J.W. Brownlow New South Wales Geological Survey-

There is a regular relationship between tectonism and thermal activity in regional-scale geological processes: an episode of major deformaticn is typically followed by an episode of major deformation is typically followed by an episode of mjor thermal activity. Each has its own characteristic geological and geomorphic expression. Partial overlap in time of the deformational and succeeding thermal episode, and substantial overlap of the affected regions, suggests a close genetic relationship and a cotmon underlying cause. The overall regional-scale process producing deformation and thermal activity is called a "tectonothermal regime". These regimes may be the regional-scale manifestation of subduction. Ttectonothermal regimes represent a longer term ordering of geological processes than do separate tectonic or thermal processes (cf. Brownlow 1977) Recognizing a thermal episode, invites the search for an accon^ying tectonic episode, and vice versa. This can be a valu^le aid in the systematic analysis of "orogenic belts", especially complex, and poorly dated provinces such as the New England, Yarrol and Gympie provinces. Tectonic processes are caused by episodically-active external stress fields (either ccmpressive or wrench), possibly aided by high pore water pressure, which reduces the effective strength of rocks. These processes cause regional deformaticn, crustal thickening and uplift. Sedimentation coTTOonly ocCTors in adjacent basins, or newly formed faialt-angle depressions. The deformed regions are commonly elongate belts, that are segmented longitudinally. Different segments m y deform differently, depending on variations in the stress field and the pre-existing crust. Thermal processes probably result from mantle diapirism, and cause an even more diverse range of effects (see Brownlow, this Symposium). The mid Carboniferous to Middle Triassic evolution of eastern Australia is envisaged as the product of five tectonothermal regimes. These caused not only regional deformation, and thermal activity, but also produced several major sedimentary basins (e.g. Sydney-Bowen Basin, ClarenceMoretcxi Basin). 1. Western Permo-Carboniferous tectonothermal regime A Deformation occurred in the eastern Lachlan and Thomson Fold Belts and the Drummond Basin during the mid Carboniferous. North-south trending structures resulted fran east-west conpression. Tectonic transport was generally west over east. This event may have juxtaposed the New England and Lachlan provinces. Deformation in New England may have included movement on the Peel Fault System. New South Wales may have been pushed westward relative to Queensland, due to right-lateral offset along the Darling River and associated Lineaments. B. Permo-Carboniferous thermal activity occurred along the New England-

Lachlan join, prcxSucing paired igneous belts, and later, the Sydney-Bowen Basin (see Brownlow, this symposium). 2. Eastern Permo-Carboniferous tectonothermal regime A. A NS oriented; left-lateral shear system developed along the eastern margin of Australia during the latest Carboniferous. It caused asymmetric megafolds east of the Peel Fault System and in developing metamorphic complexes in central New England. Serpentinite was prob^ly QTiplaced along the Peel Fault Systen. Megakinks in the eastern Lachlan Fold Belt, may have developed at tliis time. B. Permo-Carboniferous thermal activity followed in central New England, and led to the development of the subsiding Nambucca Basin (see Brownlow, this symposium). 3. Early Permian tectonothermal regime A. Early Permian megafolding may have occurred in northern New England, rotating both metasediments and Hillgrove granitoids from a northerly trend to a south-easterly trend. Conpression fron the northeast or right lateral movenent along major northerly trending faults has been advocated. B. Subsequent thermal activity produced the Bundarra Plutonic Suite granitoids in western New England (see Brownlow, this symposium). 4. Middle Permian tectonothermal regime A. SE-NW shortening produced intense deformatiai, extensive block movenent and crustal thickening in New England during the early Middle Permian. Left lateral drag was imposed on the southern Tamworth Belt, and northern Sydney Basin. The Yarrol Province was also deformed. The southern Sydney Basin only experienced minor deformatiai, possibly being butressed by the crust of the underlying Lachlan Fold Belt. B. A Middle Permian thermal province developed along the east coast as a series of offset NE-trending segments (see Brownlow, this symposium). 5. Permo-Triassic tectonothermal regime A. SW ccarpression in eastern New England during the Permo-Triassic caused thrust or oblique movement on the Hunter-Mooki and related fault systems, and probably left lateral movanent on the Donon Fault. Extensive over thrusting occurred on the northeastern margin of the Gunnedah Basin. Deformation may also have occurred in the Bowen Basin. Slightly younger deformation (Early Triassic) of the Kin Kin beds suggests south to north younging of deformation. B. A NNW-trending thermal province developed along the east coast during the Middle Triassic, producing the Clarence-Moreton Basin during cooling (see Brownlow, this symposium). Brownlow, J.W., 1977, Structural and tectonic framework of the N.S.W. portion of the Sydney-Bov^n Basin. Abstracts and programme for the 11th Newcastle Symposium on "Advices in the Study Basin" p. 12. Published with permission of the Secretary/ NSW Department of Mineral Resources.

10.1

ENGINEERING GEOLOGY OF THE BURDEKIN FALLS DAM, NORTH QUEENSLAND N. Burton and T. Lawson Queensland Water Resources Commission

The recently constructed Burdekin Falls Dam is located on the Burdekin River at AMTD 159.3 km, approximately 150 km south of Townsville. Two stages of construction have been allowed for. The recently completed first stage consists of a mass concrete gravity dam (Main Dam) and three zoned earthfill saddle dams and has a storage capacity of 1.86 x 10^ ML. The dam will be the major water supply for the Burdekin Irrigation Area which is currently under construction. An additional raising of these dams and construction of a fourth saddle dam will give a storage capacity of approximately 8 x 1 0 ML after stage II, which will be utilized for a 500 MW hydroelectric power station. Engineering geological investigation of the site dates back to the early forties but began for the present project in 1979 and was completed in 1983. gWRC Geological personnel were present during the construction p^ase from 1984 to 1987. The Main Dam is a mass concrete gravity structure, 876 m long and 60 m wide. A central spillway section 504 m in length is 37 m above the river bed. The spillway can accommodate a height of water 15.1 m above it to give a maximum it the largest capacity spillway of any discharge of 70 000 m^s^"* , making dam in Australia. The Main Dam is founded on an unweathered, thick, uniform and gently dipping sequence of moderately welded, trachyandesitic ashflow lapilli tuff belonging to the Upper Carboniferous Bulgonunna Volcanics. Uniaxial compressive strength of the fresh trachyandesitic tuff typically ranges from 250 to 450 MPa. All foundations of the Main Dam were geologically mapped at a scale of 1:200 with 1:100 scale mapping used on the abutments. Rock mass defects were mapped and information was recorded on their shape, spacing, orientation and infilling. Approximately 6000 defect orientations were measured and have been analysed using equal area stereographic projections. The presence of conspicuous, persistent subhorizontal joints and the occurrence of rock spalling during periods of high temperature indicates that there is a significant residual stress field. The CSIRO and the James Cook University were employed as consultants to measure the residual stress field using borehole slotting and hydrofracturing methods ( B o c k e t a l , 1986). Values of a^ of 30 to 35 MPa were measured in the river bed near the left abutment, immediately downstream of the dam. Rock bolting of the rock mass in the river bed immediately downstream of the spillway is currently underway, to reduce potential scouring of rock blocks along subhorizontal stress-relief joints.

The residual shear strength of subhorizontal stress-relief joints was investigated during construction using the method of Barton and Choubey (1977). Values of i = and ^ = 27® were obtained giving a residual friction angle of 35® at 1.2 MPa normal load which compared favourably with the design parameter (Russo et al^ 1985). Chemical weathering along closely spaced stress-relief joints appears to be responsible for the presence of kaolinite-infilled weathered zones in the abutments with values of = 11® and c' = 20 kPa. Stability calculations showed that the plane-failure along a weathered zone in the left abutment above the storage may be possible under saturated conditions. Remedial action was taken by pattern drilling of percussion holes to ensure adeguate slope drainage. On the right abutment stability calculations show that the abutment is stable using worst-case piezometric surface models, and piezometers have been installed to monitor the groundwater conditions. Main Dam grouting was carried out in two stages. Initial consolidation grouting after foundation stripping infilled open horizontal defects. Final grouting to a depth below the foundation of 30 metres was used to construct a continuous grout curtain below the structure. The Left Bank Saddle Dam is 2 kilometres north of the Main Dam and consists of a zoned earthfill embankment approximately 1 kilometre long and up to 29 metres high. A headrace channel for a future hydroelectric power station, 600 metres long and up to 100 metres wide and 20 metres deep has also been excavated in the ponded area. The^Left Bank Saddle Dam foundations and Headrace Channel are excavated into a complex sequence of andesitic and dacitic volcaniclastic and pyroclastic rocks belonging to the Lower Carboniferous Star of Hope Formation. The Left Bank Saddle Dam area was mapped at a scale of 1:1000 and 1:500 and cleaned off strips in the core trench, downstream and upstrecun filter zone areas were mapped at a scale of 1:100, to provide information on frequency and orientation of rock mass defects. Extensive thin-section petrography of the rock types was used to delineate lithological units. Consolidation grouting of the Left Bank Saddle Dam was carried out in one stage and areas of high grout takes have been related to shear zones along microgranite dykes. Two additional saddle dams are located near Mt. Graham, some 8 kilometres northwest of the Main Dam, and are only inundated during flood periods. The foundation areas were mapped at a scale of 1:1000 following foundation stripping. Some 626 000 m^ of concrete were used in the Main Dam using rock aggregate up to 150 mm in size. A quarry was developed in part of a PermoCarboniferous microgranite intrusion some 3 kilometres northwest of the Main Dam. Rock from stripping of trachyandesitic tuff in the river-bed was not utilized because of potential problems of alkali-silica reactivity.

REFERENCES Barton N. & Choubey V. 1977. The shear strength of rock joints in theory and practice. Rock Mech., v. 10, pp. 1-54. Bock H., Otto B. , Enever J.R. s Crawford G. 1986. Bwndekin Falls Dam: Report on Rock Strees Meaeuremente, ISBS-lSdS. James Cook University, Dept. Civil Systems Townsville, 70 pp. Russo R., Richardson J.K. & Allen P.H. 1985. Design of the Burdekin Falls Dam. Trans. Inst. Eng., v.21, pp. 362-370.

14,6 QUATERNARY FORAMINIFERA AND LATE PLEISTOCENE AND H O L O C E N E S E A L E V E L S , G U L F ST V I N C E N T , S O U T H A U S T R A L I A J.H.

Cann

South Australian College (Salisbury

of A d v a n c e d Campus)

Education

Within Gulf St Vincent, South Australia, benthic foraminifera are abundant in all surficial sediments. The distribution of many species is closely related to water depth. For example, Dlscorbls dlmidiatas is characteristic of shallow, subtidal sea grass iPosldonia australis) meadows while Massilina millettl favours deeper waters c. AO m (Fig. 1). m.ANOIiaULmA MCmTCRffAl

mscorais

OM«OUTU«

f>CMMn.lS l»UANATUS mtOCULINA

TRMONUCA

mHjOCULmA TKICAMWATft

Percentage distribution Fig. species of benthic of in surficial foraminifera (size fraction sediment along a transect 1.00-0.50 mm) to Edithburgh, St Kilda Gulf St Vincent, South Australia.

riiH.ocui.iiu rrwuTor TWtOCUUtU AFflMH MASSILIMA MltLCTTI

Data is based on analysis of grab samples, from which foraminifera were concentrated by dense liquid flotation.

Nubecularia luclfuga, not shown in Fig. 1, constitutes up to 70% of species at depths of 2-3 m, but is rare to absent in deeper Gulf waters. Two species of Elpbidium, F. crlspum and F. macelliforme provide a useful numerical ratio. The former favours shallow subtidal environments while the latter is a deeper water species. Their logarithic relative abundance, in the sediment size fraction 0.50 -0.25 mm, correlates strongly with water depth (Fig. 2).

Y 5 9 - 4 3 LoqX

Fig;. 2. Log~linear plot of water depth versus ratio of numbers of individuals of F . macelliforme to numbers of individuals of E. crispum for surficial samples from Gulf St Vincent. Also shown is the line of best least squares fit, the calculated regression equation and Pearson's correlation coefficient for the data set.

+ 26 17

R = 0-95

001

100

ELPHIDIUM RATIO (x)

Two vibrocores each recovered c. 4 m of Holocene and Late Pleistocene sediments from the deepest part of Gulf St Vincent, c. 40 m . ^^C and AAR age determinations show that, together, the cores record Late Pleistocene sedimentation for the time interval 45,000 to 30,000 yr B.P. Down core analyses of foraminifera from these vibrocores (Fig. 3) has been compared with present day relationships of species abundance and water depth. In this way it has been possible to infer changes in water depth for the duration of deposition of the recovered sediments. In particular, the Elphidium ratio, when plotted against sample depth in core, provides a de facto sea level curve (Fig. 4). SIZE

FRACTION

0 5 0 - 0 25 M M

200 ^ 200

Fig. 3. Down core percentage distributions of species of foraminifera in vibrocore SV 5 for the size fraction 0.50-0.25 mm. On lithological evidence, the Holocene/ Pleistocene boundary occurs at 53 cm.

S u m of Individuots E

GULF VINCENT SV5

Mocelliforme

S u m of tndividuols E . C r i s p u m

FiR. 4. Log-linear plot of the Elphidium ratio versus sample depth in core. Trends to larger numbers imply rises in sea level and vice versa.

Sea level maxima, so inferred, can be correlated with those determined from study of Huon Peninsula coral reef terraces. Initial estimates of tectonically corrected sea lervels for transgressions in Gulf St Vincent at 31,000 and 40,000 yr B.P. are -22 m and -22.5 m , respectively. The intervening regression lowered sea level to -28 m .

5.8

APPLICABILITY OF THE VAIL et al. 'GLOBAL' SEA-LEVEL CURVE TO A SOUTHERN OCEAN MARGIN (GREAT SOUTH BASIN, SOUTHEAST NEW ZEALAND) R.M. Carter

Geology Department, James Cook University of North Queensland

The Kaikoura Sequence (Cretaceous-Cainozoic) stratigraphy of the southeastern margin of South Island comprises four seismic sequences, encompassing eight major sedimentary Phases. The seismic sequences correspond to litho-stratigraphic Groups already established in eastern South Island, namely Matakea Group (Cretaceous), Onekakara Group (late Cretaceous to early Oligocene), Kekenodon Group (Oligocene) and Otakou Group (early Miocene to Recent). These sequences achieve maximum thickness within three sedimentary basins underlying the continental shelf (Canterbury and Foveaux Basins) and slope (Great South Basin). MEGASEQUENCE/PHASE DOMINANT FACIES

AGE RANGE

SYN-OROGENIC 8c 8b 8a

Offshore pelagic drape Shelf clinoforms Piedmont gravel

Early Miocene to Recent

PARACONFORMITY 7

Platform greensand, carbonate

Middle to Late Oliogocene

POST-RIFT

6 5 4 3

Marls/chalks Shelf clastic wedges Paralic clastic wedges Quartzose coal measures

Paleocene-Olig. Cretaceous to Eocene Cret. to Eocene

SYN-RIFT

2 1

Immature coal measures Fanglomerates

mid-Late Cret. mid-Cretaceous

The Great South Basin is demarcated from the Canterbury Basin across the Waipounamou Fault system, a northeasterly orientated zone of faults which controlled rifting and Matakea Group sedimentation along the western edge of a Cretaceous aulacogen now manifest as the Bountry Trough. The Canterbury and Foveaux Basins contain transgressive sequences (Onekakara Group) that were deposited in response to post-rifting thermal subsidence of the margin. By the Oliogocene, land areas were greatly reduced and at peak transgression there was the development of the widespread Marshall Paraconformity at the junction of the Onekakara and Kekenodon Groups. Subsidence continued until the Miocene, though punctuated by phases of mild faulting and volcanism in the Paleocene and late Eocene-Oligocene. Regression commenced in the Miocene (Otakou Group), consequent upon uplift of the Southern Alps in the west. Crustal unrest reached a climax in east Otago in the late Middle Miocene, with substantial volcanism and block faulting.

The eastern South Island margin sequence forms and appropriate southern hemisphere test of the global nature of the sea level curves of Vail et aL (1977) and Haq et aL (1986). Although eustatic sea-level cycles may have played some part in the differentiation of sedimentary Phases 3-7 (Onekakara and Kekenodon Groups), the overall stratigraphy is adequately interpreted in terms of intrinsic sedimentary evolution within a local tectonic context.

18.6

THE RESTRUCTURING OF GEOSCIENCE IN AUSTRALIAN TERTIARY INSTITUTIONS R.M. Carter

Department of Geology, James Cook University of North Queensland "... the Government believes that important changes are required in our higher education system to improve its response to a range of priority national objectives. The achievement of these changes will require a close examination of existing barriers and the identification of new policy responses to overcome them". John Dawkins, Minister for Education, Training & Employment, 22nd September, 1987. The mining industry is destined to remain a fundamentally important contributor to Australia's overseas earning capacity, and is therefore influential with government. For at least the last 10 years, that industry has also been seeking to achieve a radical restructuring of geology teaching and research at tertiary level. The combination of industry and government pressure (see quotation above) will inevitably result in major changes in the organisation of Australian tertiary geoscience in the near future. Similar pressures for tertiary restructuring have been apparent in the United Kingdom for the last ten years. Coincidentally, Geology was the first science to be restructured, according to recommendations contained in the Oxburgh report (May, 1987). In the restructuring, the 33 United Kingdom departments are to be reorganised such that only 10-12 major geoscience centres remain, each with about 30 staff and a full range of research instrumentation. There will also be a second level category, comprising departments with c.l5 staff that will specialize in honours teaching together with some research. Third level geoscience units may form part of related departments, and will generally only teach general courses at first and second year level. It is inevitable that the Oxburgh plan will be applied in some form in Australia. With a few honourable exceptions, mining companies active in Australia have shown little interest in the long-term health of basic geoscience research, or of the general (as opposed to vocational) educational importance of geoscience; and the truth is that commercial intrusions into academic science can damage science as much as academic excursions into industry can damage industry. It is therefore particularly important that the academic geoscience community take the leadership in national plans for restructuring geoscience teaching and research. It is a sobering thought that those whose overriding concern is vocational efficiency will view the most obvious (and politically practical) restructuring as comprising one geoscience department per state. On the contrary, introductory geoscience should be taught in all secondary schools and in every tertiary institution, implying an expansion from present levels. This will only be achieved if academic geoscientists produce a rational plan for restructuring, based on academic/educational excellence and relevance, economic importance, and efficient use of resources. Perhaps the four most important problems requiring resolution are: *

Increasing the teaching of geoscience in secondary schools

Encouraging most CAE geoscience departments to specialize in the production of adequately trained school teachers

•

Producing a plan for greater collaboration, and equipment rationalisation, between geoscience departments located in the same city

•

Agreeing on criteria for the designation of a number of centres of concentration of the various geoscience subdisciplines.

"... the old assumption that excellence and nothing but excellence should rule has lost its persuasive force ... The themes of selectivity and concentration that have been central to recent CTEC and ASTEC reports, combined with the scarcity of public money, are pushing us towards asking questions about the kinds of excellence we can afford ... The Australian Research Council ... will have to make some decisions about which areas of research need special support, and which do not ... It will be up to research groups and disciplines to argue for the importance and potential of their own work: ultimately it will be the researchers themselves who determine national research priorities". Don Aitken, Chairman ARCS, 1987.

8.29

EQUILIBRATION BETWEEN MAFIC AND HOST GRANITES

INCLUSIONS

Y.D. Chen and R.C. Price Department of Geology, LaTrobe University,

Bundoora

Simple petrographic and geochemical studies of inclusions in granites have provided important information upon which many models for granite genesis and evolution are based. Such studies are in many cases regarded as more relevant than investigations based on sophisticated instrumental methods such as stable isotope studies (Didier, 1982). Hornblende-bearing mafic inclusions in l-type granites have been interpreted as representing magma blobs surviving magma-mixing or magma-mingling (Vernon, 1984; Didier, 1982), or as restite inherited from partially melted igneous source for the host l-type granites (e.g., Chappell, 1978). Mafic inclusions in two granites in central Victoria have been studied in detail to provide data about the field occurrence, petrography, mineralogy, and major and trace element chemistry of inclusions in l-type granites. The two granites are the Mt. Saw Baw Granodiorite and the Lysterfield Granodiorite. Both are l-type in terms of the Chappell and White (1974) nomenclature but both are llmenite-series granites according to Ishihara (1977). Ilmenite is the only Fe-Ti oxide accessory mineral in the two granodiorites. Mafic inclusions in the two granites are less than 50 cm with the majority being less than 10 cm in the maximum exposed dimension. Their shapes are variable but show a general trend towards ellipsoidal. They are mostly rounded or subrounded, and normally show a sharp boundary with the host granites. Mafic inclusions in the Lysterfield Granodiorite are more abundant (0.95 - 2.06%) and larger in size than those in the Baw Baw Granodiorite (0.8% in abundance). Mafic inclusions and the host granites contain exactly the same mineral species but in different proportions and grain sizes. In the Baw Baw Granodiorite minerals common to both host granodiorite and inclusions are plagioclase, quartz, K-feldspar, biotite, amphibole and accessory pyroxene, zircon, apatite, sphene, ilmenite, chalcopyrite and pyrite; whereas in the Lysterfield Granodiorite the common minerals are plagioclase, quartz, K-feldspar, biotite, hornblende and accessory zircon, apatite, sphene and ilmenite. The most significant mineralogical aspects are: (1). Ilmenite is the only Fe-Ti oxide phase in both inclusions and host rocks of the two granites. (2). Pyroxene is absent from inclusions and host rocks of the Lysterfield Granodiorite. Most mafic and intermediate magmas crystallize magnetite or magnetite + ilmenite. Situations where only ilmenite crystallizes are rare. The presence of ilmenite without magnetite in granites implies very low oxygen fugacity conditions ( Ishihara, 1977). The llmenite-series I types require a mixed source with both igneous and sedimentary components. If the restite hypothesis is valid mafic inclusions should be representative of original igneous material in the source and the absence of magnetite within them implies that they have re-equilibrated under very low oxygen fugacity conditions imposed by the sedimentary component. If the magma-mixing or magma-mingling hypothesis is valid then the inclusions would

represent blobs of a relatively mafic magma presumbably generated under oxygen fugacity conditions where magnetite would crystallize. The host granite would represent a felsic melt derived from partial melting of sedimentary material with an inherently low oxygen fugacity prevailing during melting. Mixing or mingling of the magmas must have been accompanied by re-equilibration of the inclusions to oxygen fugacity conditions largely controlled by the host granite melt. Equilibration of mafic inclusions with their host magmas could be a general feature of most mafic inclusion-bearing granites regardless of the models by which they are considered to have been generated. Consequently the earlv history of inclusions may be obscured and may not be resolvable. References: Chappell, B. W., 1978: Granitoids from the Moonbi District, New England Batholith, eastern Australia. Journal of the Geological Society of Australia, v. 25, Pt 5, p. 267 -283. Chappell, B. W. and White, A. J. R., 1974, Two contrasting granite types. Pacific Geology, v.8, p. 173-174. Didier, J., 1982, The problem of enclaves in granitic rocks, a review of recent ideas on their origin. In: Xu, K. and Tu, G. (editors). Geology of granites and their metallogenetic relations, Beijing, Science Press, 954p, p. 137-144. Ishihara, S., 1977, The magnetite-series and ilmenite-series granitic rocks. Mining Geology, v.27, p.293-305. Vernon, R. H., 1984: Microgranitoid enclaves in granites - globules of hybrid magma quenched in a plutonic environment. Nature, v.309, p. 438 -439.

8 34 CONTACT M E T A M O R P H I S M A D J A C E N T TO THE EASTERN M A R G I N OF THE ARTHURSLEIGH TONALITE, BRAYTON DISTRICT, NEW SOUTH W A L E S B.E. Chenhall, P-F. Carr and E.G. Department

Jones

of Geology, University of Wollongong

The Early Devonian Marulan Batholith of the eastern Lachlan Fold Belt crops out over an area of more than 140 square kilometres and comprises at least 16 plutons which extend in a meridional belt from Windellama to Bullio. Northeast of Brayton, the Arthursleigh Tonalite, a member of this plutonic association, intrudes volcanic and sedimentary rocks ranging in age from Late Ordovician to Early Devonian. The I-type Arthursleigh mass exhibits an unusual zonation from a relatively mafic core to a more felsic rim and it has an asymmetric contact aureole. The western portion of the aureole is narrow (400 m wide) and is developed in units of the Bindook Volcanic Complex. In contrast, an extensive (> 4 km wide) zone of thermal metamorphism, consistent with a very shallow easterly dipping contact between the pluton and country rocks, is developed in the eastern part of the aureole. Three zones of progressive contact metamorphism can be defined within Late Ordovician psammitic to pelitic flysch in the eastern part of the aureole (Fig. 1). The outer zone is characterised by the development of the assemblage muscovite + biotite + chlorite in both rock types. Pelites in the intermediate zone contain muscovite + biotite + pseudomorphs after cordierite. The innermost zone, representing the maximum thermal grade within the aureole, is denoted by the development of either andalusite or coexisting microperthite and cordierite in pelites. Psammites in both the intermediate and inner zones contain microperthite. The three zones cannot be mapped with confidence throughout the entire aureole due to paucity of outcrop and intense, pervasive retrogression. quartz piagioctase muscovite chlorite biotite

a jt C Q.

microperthite

+ Wollondilly

River 444-

4+ Tonalite

(D CO Q)

'Sericite! chlorite

Fig. 1.

Metamorphic zones along the Wollondilly River east of Arthursleigh. 86

Metasedimentary enclaves within the Arthursleigh Tonalite are characterised by assemblages containing either cordierite + microperthite, or sillimanite + corundum + microperthite. Metamorphosed derivatives of ? Silurian calcareous sediments are confined to the innermost portion of the aureole where they crop out as discontinuous pods and roof pendants along the eastern portion of the pluton. Calcsilicate hornfelses are represented by the assemblages quartz + calcic plagioclase + ferrosalite + sphene and by wollastonite + diopside + calcite. Some wollastonite-bearing assemblages contain minor vesuvianite or grossularite • Biotites developed within the pelites show a systematic decrease in Mg/(Mg + Fe) and a systematic increase in Ti with increasing grade of metamorphism. Decrease in Mg/(Mg + Fe) is attributed to the decomposition of chlorite and ilmenite and the consequent formation of cordierite. Increase in Ti with increasing metamorphic grade is attributed initially to the decomposition of rutile and then to the breakdown of ilmenite coexisting with biotite. The pyroxene from assemblages containing calcic plagioclase 4- quartz + sphene is ferrosalite whereas the pyroxene in wollastonite-bearing assemblages is diopside. Chemical variation within both pyroxenes is very limited. Mineral assemblages within the eastern portion of the aureole attest to progressive contact metamorphism to upper hornblende hornfels facies grade in a low pressure regime. Field, compositional and mineralogical data place constraints on the pressure, temperature and nature of the fluid phase attending contact metamorphism. In the Arthursleigh area the maximum thickness of the Bindook Volcanic Complex, which has a close temporal and spatial relationship to the Marulan Batholith, indicates a maximum confining pressure of approximately 1 kb. Estimates of metamorphic temperature deduced from two-feldspar thermometry yield a maximum temperature of 600®C in the inner part of the aureole and 700°C in the xenoliths. These estimates are compatible with experimentally determined equilibria for the stability of phases observed in the aureole. Based on the abundance of hydrous metamorphic phases in the aureole, pelitic and psammitic rocks probably evolved under conditions where PcoNFINING = ^FLUID = ^WATERFluid composition is an important factor in determining the evolution of paragenetic sequences in metamorphosed calcareous sediments. Based on the realistic assumption that the metamorphic fluid composition approximates binary carbon dioxide-water mixtures, experimental data indicate that grossularite-bearing assemblages within the aureole have formed under conditions where the mole fraction of carbon dioxide is less than 0.2. Vesuvianite development is also favoured by a low mole fraction of carbon dioxide. The hornfelses within the aureole thus appear to have evolved under metamorphic conditions dominated by a water-rich fluid. There are several potential sources for this fluid, including water from the igneous body and water released by dehydration reactions accompanying metamorphism. The extensive development of hydrous phases throughout the eastern part of the aureole suggests continued infiltration of a water-rich fluid into the hornfelses during the waning stages of thermal metamorphism.

14.2 TRACE ELEMENTS AND STABLE ISOTOPES OF OSTRACOD SHELLS - TOOLS FOR PALAEOSALINITY, PALAEOTEMPERATURE AND PALAEOPRODUCTIVITY A.R. Chivas^, P. De Deckker^ and J.M.G. Shelley^ ^Research School of Earth Sciences, Australian National University, Canberra ^Cenozoic Research Unit and Department of Geography, Monash University, Clayton

Ostracods are bivalved crustraceans which secrete low - Mg calcite shells. The uptake of trace elements (Mg and Sr) and stable isotopes in these ostracod shells has been monitored in in vitro experiments and also using field collections. Partition coefficients (for Mg and Sr) and fractionation (for stable isotopes) have now been established for numerous ostracod species. Overall, it is now established that the uptake of Sr by ostracod shells is only related to the Sr/Ca of the host water whereas the Mg uptake is affected by both water temperature and the Mg/Ca of the water. The temperature dependance of Mg uptake for some ostracod species has already been established through a series of experiments, and this permitted to calculate the temperature range for the water in which some fossil ostracods lived. Knowing also that b^^O increases w ^ h salinity but decreases with water temperature, and that the b'^C in the ostracod shell increases if organic productivity in the waterbody also increases, it becomes possible to reconstruct changes in palaeo-temperature, -salinity and -productivity in lakes. The above concepts will be documented using examples from Australia, China and Africa, for a variety of lacustrine and estuarine environ-

ments.

9.5 A LABORATORY STUDY OF COMPRESSIONAL AND SHEAR WAVE VELOCITIES IN A SUITE OF ROCKS FROM SOUTHWEST WESTERN AUSTRALIA P. Chopra and B. Drummond Bureau of Mineral Resources, Geology and Geophysics, Canberra

This talk presents the results of a series of 300 laboratory experiments on rocks from the S W Seismic Zone in W . A , The compressional- and shear-wave velocities of 15 rock samples were measured at confining pressures up to 1 G P a (10 Kbar) at room temperature. V and V have been measured at ultrasonic frequencies for samples of orthogneiss, qufirtzo-feldspathic gneiss, amphibolite, charnockite, mafic granulite, meta-granite and granite under dry conditions. Additional measurements have been made on some of the samples to investigate the influence of fluid saturation of cracks on the velocities. When used in conjunction with seismic models of the region, the measurements constrain the lithologies likely in the lower crust. The V ^ and Vg measurements allow estimates to be made of the bulk and shear moduli, w h i c h ^ r e important when assessing earthquake risk in the region.

3.4

MAGNETIC FABRIC - PRINCIPLES AND APPLICATIONS TO TECTONICS D.A. Clark CSIRO Division of Mineral Physics and Mineralogy, North Ryde

The magnetisation, J, induced in a rock by the application of a weak magnetic field, F, is reversible and its vector components are linearly dependent on the components of the applied field, i.e. J^ = (i,j = 1,2,3). The nine quantities k^^ represent the elements of a second order tensor known as the susceptibility tensor. It can be shown that reversibility of the magnetisation implies that the susceptibility tensor is symmetric (k-• = k--), so that there are only six independent components. Symmetric second order tensors can be characterised by a representation quadric. For the usual case, where the susceptibility of the rock is positive, the representation quadric is an ellipsoid. The properties of the representation ellipsoid are summarised in Fig. 1. The major (kj^) susceptibility axis corr^ponds to the direction along which the susceptibility magnitude, k = J J IfU is a maximum. The minor (k3) susceptibility axis is perpendicular to the major axis and corresponds to the direction along which the susceptibility is a minimum. The intermediate (k^) axis is orthogonal to the other two principal axes. With respect to the principal axes eqn (1) becomes simply J^ = k^-J^ (i = 1,2,3). For an isotropic rock k^ = k2 = k3^= k and the orientation of the principal axes is arbitrary. In this case J = kF, the magnetisation is parallel to the field and the susceptibility k is a scalar quantity. Most rocks are somewhat anisotropic, however, due to preferred orientation of the constituent mineral grains. Then the induced magnetisation is parallel to the applied field only along the principal axes. When the applied field is oblique to the principal axes the magnetisation is deflected away from the minor axis. The magnetic fabric ellipsoid has semiaxes k^, k2, k3. The magnetic foliation plane contains the k^ and ^^ ^^ ^ ^^^^^ ^^ relatively high susceptibility. The k3 axis corresponds to the magnetic foliation pole. The magnetic lineation coincides with the k^ axis. The magnitude of the susceptibility for an arbitrary applied field direction is equal to the radius of the magnetic fabric ellipsoid along the corresponding magnetisation direction. The symmetry elements of the magnetic susceptibility must include the symmetry elements of the sample. Provided the sample is homogeneous and the rock fabric can be considered penetrative at the scale of sampling, it follows that the symmetry elements of the susceptibility include the symmetry elements of the petrofabric. Because the magnetic fabric ellipsoid has orthorhombic or higher, symmetry, rock samples with petrofabric of lower-than-orthorhombic symmetry exhibit a magnetic fabric of higher symmetry than the petrofabric. For example, intersecting foliations, corresponding to monoclinic symmetry, produce a magnetic foliation, lying between the petrofabric foliations, which contains a magnetic lineation parallel to the intersection of the foliations. There is no fixed relationship between petrofabric elements with triclinic symmetry (e.g. two obliquely intersecting foliations, each containing a lineation ) and magnetic fabric axes. This aliasing of fabric symmetry

must be borne in mind when interpreting magnetic fabric data. However the magnetic fabric method can still be applied to analysis of petrofabrics of low symmetry, either by measuring a number of samples with differing development of various fabric elements, or by resolving the total susceptibility into contributions from paramagnetic and ferromagnetic minerals and analysing the paramagnetic and ferromagnetic subfabrics separately. The most important ferromagnetic minerals in rocks are magnetite, haematite and monoclinic pyrrhotite. In most magnetite-bearing rocks the susceptibility anisotropy reflects preferred orientation of inequidimensional magnetite grains (i.e. it is a dimensional fabric), which are more easily magnetised along the long axis then parallel to a short axis. The susceptibility of haematite and pyrrhotite crystals is much higher within the basal plane then along the c-axis. Thus the magnetic fabric of haematite- and pyrrhotite-bearing rocks arises from preferred crystallographic orientation of these minerals (which is usually correlated with dimensional orientation due to preponderance of a platy crystal habit). Instruments for measuring susceptibility anisotropy are very sensitive. The main advantages of the magnetic fabric studies are the rapidity and sensitivity of the method. Large volumes of data can be gathered quickly and very weak fabrics can be measured, often enabling identification of fabric elements which are not evident in outcrop or thin section. Applications of magnetic fabric include: (i) determination of bedding planes and palaeocurrent directions in weakly deformed sediments, (ii) studies of progressive deformation and formation of cleavage in tectonites, (iii) determination of fold axis plunges and axial planes in folded rocks, (iv) determination of the mode of emplacement and flow directions in igneous rocks, (v) correction of palaeomagnetic data for the effects of deformation, allowing extension of palaeomagnetic studies of tectonic movements to more highly deformed rocks (magnetotectonics). Magnetic fabric data from the Broken Hill Northern Leases, the Cobar area, the Tennant Creek area and the Mt Lyell area will be presented and discussed.

SUSCEPTIBILITY ANISOTROPY REPRESENTATION ELLIPSOID k.x^^,

ANISOTROPY ELLIPSOIDS - 1

r^ -1

DEFINITION

magnetic k ellipsoid shown

k3 ( m i n i m u m susceptibility)

RESPONSE » TENSOR x S T I M U L U S l/Vkj

Jm.g-k„F

Jcd - f y t j E E L E C T R I C A L C O N D U C T I V I T Y (a) E L L I P S O I D substitute:

KiX

(maximum susceptibility)

•^^CJy - r n j i

for k „ . E for F. Jg,, for J ^ , , (intermediate susceptibility)

F magnetic field, E e l e a r i c field, J n m magnetisation, J^j,

Bulk susceptibility k « ( k , + k 2 + k j ) / 3

current density.

DEFLECTION OF MAGNETIZATION

SUSCEPTIBILITY ELLIPSOID : MAGNETIC FABRIC (not to be confused w i t h representation ellipsoid) Magnetic foliation pole

F Applied field along r

along k j

Magnetization J deflected towards k j axis

Magnetic llneation

owing to anisotropy

along k , Magnetic foliation plane containing k , and k j Anisotropy Magnitude

kj/kj

Lineation Magnitude

k.A:

Foliation Magnitude

k,/k3

Ellipsoid Prolateness

L/F

uniaxial oblate

k(r)

m U k j n ' « 1/r^

ellipsoid (disc) l,m,n » direction cosines of r

uniaxial prolate ellipsoid (needle)

15.13

LITHOSPHERE LOADING AND SEAMOUNT SUBSIDENCE HISTORY SATELLITE AND SEISMIC REFLECTION DATA R. Cplemanl,2^

C.J. Jenkins^ G.H. Packham^ and K. Lambeck^

^School of Civil and Mining Engineering, University of Sydney ^Ocean Sciences Institute, University of Sydney ^Research School of Earth Sciences, Australian National University, Canberra

Much insight has been gained in the last decade on the rheoiogy of the ocean iithosphere from flexure studies of seamounts. The most widely used model of flexural response assumes that the Iithosphere behaves as a thin elastic plate overlying a fluid substratum although viscoelastic and elastic-viscoelastic models have also been considered. In recent years, increasing use has been made of satellite altimeter data for studying the thermal and mechanical properties of the ocean Iithosphere. To a first approximation, the altimeter data may be considered as estimates of the shape of the geoid. The geoid data, unlike gravity data, give a smoother representation over very local topographic features, such as seamounts (see Figure 1). Thus, a clearer picture of the underlying structure is possible but satisfactory estimates of key rheological parameters must be supported by other geophysical and geological constraints. Using cruise data (collected during 1986/87 and on past ELTANIN cruises) and all available altimeter data, the loading and subsidence along the Tasmantid seamount chain is described. The available cruise data consisted of rock and sediment samples, bathymetric data including SEABEAM swaths, seafloor photography and single channel seismic lines. Corrections to the observed guyot depths were made for the effects of sediment loading, thermal contraction of the ocean Iithosphere and guyot capping subsidence. Both elastic and viscoelastic models were used to compute theoretical estimates of the flexural rigidity of the Iithosphere. Matching the altimeter information with the model results allows estimates of the subsidence and flexure of the seamount chain to be made. Preliminary results show high effective flexural rigidities (of order 10^3 Nm) compared to other seamount chains of comparable load magnitudes such as the Society Island and Cook-Austral chains. The character of the geoid signal over the seamounts changes with progression from south to north - i.e., as the age of the seamount increases.

0 Hz <

Hoe Z.

a UJ <

Ui o — CC X OQUI Ui z X UJ

—

z 0 Q IAJ

fli z

a a: 0 u. u.

0 a z

UJ z

z ID X 1

z Ui > (X UJ a

i Zrr^

i 20 m T

C'S) Lonare)

12-5 148-1

1^7 Ia-8

lfe'9 l&5'5

I3Z

2/-5 lt2-7

23-8 Itl.Z

25-9 159-8

281 158-3

30.4. IS6-7

32.6 (S5(

Figure 1. Geoid height anomalies f r o m satellite altimeter observations f o r one satellite pass over the Tasman Sea. The lower curve represents the observed heights (shape of "geoid") and the upper curve represents the observed less a regional trend.

6.17 DEVELOPMENT OF HEAVY MINERAL PLACERS IN HOLOCENE BARRIER SYSTEMS, SOUTHWEST AUSTRALIA L,B. Collins and N.T.M. Hamilton Department of Geology and Geophysics, Curtin University of Technology, Perth

Heavy mineral deposits along the cxDast of Geographe Bay in the Minninup shoreline (Fig. 1) are localised deposits within an arcuate^ Holocene barrier system of over 50 km in length. Mines have operated at Koombana Bay (near Bunbiiry) and Wonnerup (near Busselton); in 1986 mining commenced at Minninup. The study area lies within the Bunbury Trough in the southern Perth Basin, where a d e ^ graben contains 10,000 m of Late Jurassic to Quaternary sediments. The graben is bounded by the Dunsborough Fault and the Leeuwin Block to the west, and the Darling Fault and Yilgam Block to the east. The Minnint^) shoreline lies at the seaward border of the Swan Coastal Plain, which is veneered by up to 75 m of Quaternary sediments. Minninup shoreline sediments are within the CXiindalup Dune System and the Safety Bay Sand. Hiey contain significant concentrations of heavy mineral at Wonnerup, Peppermint (Srove, Forrest Beach and Minninup (Fig. 1). The coastal barrier system at Minninup can be divided into seven geomorphic \inits, on the basis of physiography, sediment type, and biological communities. From west to east these are : i) nearshore zone; ii) beach and beach ridge; iii) vegetated linear dunes; iv) vegetated parabolic dunes; v) mobile parabolic dunes; vi) estuarine flats, channels and lagoons; and vii) wetlands. The climate is Mediterranean, with hot dry summers and mild wet winters. Winter storms (average 4 per year) with mean wind speeds of up to 20 m/sec and summer sea breezes (up to 15 m/sec) are important components of the wind spectrum. Tropical cyclones are capable of producing extreme winds and waves, and storm surges. The coast is microtidal and swell wave-dominated. The west to southwest swells are refracted through 45^ before reaching the coast. Northerly littoral currents are generated for much of the year. Bie Minninup barrier system consists of seven Holocene units overlying estuarine sands and clays (Fig. 2). Unit 1 is a diachronous package of dune sands which transgressed over back-barrier sediments following the post-glacial marine transgression. Units 2, 3 and 4 form a shoaling sequence of nearshore, beach and dune sands deposited between 6000 and 3000 years B.P., at a sea level 3m above AHD. Units 5, 6 and 7 are contemporary sediments from nearshore, beach and foredune environments. All the Holocene barrier sediments are fine to medium-grained quartzcarbonate-heavy mineral sands. Ancient and contemporary upper shoreface units contain an average of 10% heavy minerals; beach sediments have up to 95% (average 40%) and dune sands approximately 5% heavy minerals. The concentration of heavy minerals (ilmenite, leucoxene, zircon, garnet, monazite, xenotime) within the sequence b^an about 8000 years B.P., and has persisted throughout barrier evolution. Heavy minerals in the Minninup shoreline are derived from several source terrains. Concentration Jias resulted from a combination of wave refraction patterns in an arcuate embayment and differing hydraulic behaviour of light and heavy minerals. Selective removal of light minerals by wave refraction and littoral transport has resulted in high

shoref ace concentrations of heavy minerals at Minninup throughout the Late Holocene. Shoreward transport and swash-backwash separation have produced heavy mineral-rich beaches, and dunes derived from beach sediment have thin deflation lags composed of heavy minerals within quartz-carbonate sand. The deposit consists of high grade, low tonnage beach, and low grade, high tonnage dune sediments. 115-SO' BUNBURY/

115'

33-M

334 ' S'

It. ^^'Ullilllltt^. Which® I. I\

)

LOCATION MAP

Fig. 1. Minninup shoreline showing heavy minerals ocxioentiatixxis. M = Minninup; P = Peppermint Grove; F = Forrest Beach; H = Wcnnerup.

'J ;

.; ,. ^ ii S • 2 o ?' P " o«TJ ^ c' T j ° j ° u J ^

. o"

flr

! | ! j t | ^ \ ! | ?| | | ! • !

^ «

• • o-••

• • .." J. - -

•• • •

6« H.M4>>|8af*ty Bay Sand mmIi i unit 2 »0% Ijij unit 3 e% fn unit 4 Fig. 2.

•o'El INlI • unit 7 ^ • • m UnoaaforMlty

|T^L»«tf«rvltU

fm.

Stratigraphic cross-section, Mijiini?) shoreline.

8.5 G R A N I T E G E N E R A T I O N IN T H E S T R A N G W A Y S M E T A M O R P H I C COMPLEX, ARUNTA BLOCK, CENTRAL AUSTRALIA - IMPLICATIONS FOR THE ORIGIN OF HIGH TEMPERATURE, LOW PRESSURE GRANULITE FACIES TERRANES W . J . Collins^, R.H. Flood^ and R.H.

Vernon^

^ D e p a r t m e n t of G e o l o g y , T h e U n i v e r s i t y of N e w c a s t l e ^ S c h o o l of E a r t h S c i e n c e s , M a c q u a r i e U n i v e r s i t y , N o r t h R y d e

Rocks of the Strangways Metamorphic Complex in the Arunta Block, central Australia, constitute a supracrustal assemblage of bimodal volcanics, immature terrigenous sediments and less abundant calc-silicates and quartzites that underwent granulite facies metamorphism at 1800 Ma (e.g. Iyer et al., 1976). Peak metamorphism occurred at -850-900^C and 8+1 Kbar (Warren, 1983), associated with at least three phases of folding that ultimately produced upright, regional Type 1 interference structures. Isobaric cooling ensued. At Wuluma Hills, migmatitic quartzofeldspathic gneisses are intimately associated with a small, variably deformed pluton, called the Wuluma Granite. Contacts between the granite and gneisses are invariably gradational. At the pluton margin, banded gneisses grade along strike into granite containing abundant biotite schlieren that parallel regional structures. Granite and pegmatite dykes cut these rocks. Inwards from the contact, the granite is more homogeneous, containing diffuse parallel schlieren and small aligned feldspar crystals, indicating flow of magma. Rafts of unmelted granulite form ghost stratigraphy; they mimic macroscopic folds and show little, if any, retrogressive metamorphic effects. At the pluton core, the granite is typically homogeneous and structurally isotopic, containing some subrounded granulite xenoliths, very diffuse schlieren and disaggregated pegmatite dykes. The variation of granite phases from pluton margin to core also reflects a temporal arrangement, whereby quartzofeldspathic gneisses are progressively modified from stromatic and nebulitic migmatites through schlieren-rich granites, to foliated, K-feldspar crystal-rich granite. Ultimately an isotropic homogeneous granite is produced, containing relics of earlier granite phases and remnants of pegmatitic and granitic dykes. Thus, it appears that an isoclinally folded, vertical body of quartzofeldspathic gneiss was melted "in situ" during granulite facies metamorphism, a more mobile core rising from slightly deeper levels. However, the core did not break away from the source to form intrusive contacts. Geochemical data also suggest the granite formed by anatexis of the quartzofeldspathic gneisses. The chemical similarity of both rock types implies derivation of the granite by either partial melting and retention of residual material in the magma (restite model of White & Chappell, 1977) or "eutectic" melting of the granites, followed by solidification virtually "in situ". Those gneisses with appropriate composition melted instantaneously as water was released during biotite breakdown. Crystallisation of the granite resulted in a composition similar to that of the precursor, but with a coarser granitic texture where flow aligned crystals are concordant with host gneisses. Field evidence supports this interpretation, because "in situ" melting of slightly different gneisses to produce different granite compositions can be demonstrated, even on outcrop scale.

Melting of quartzofeldspathic rocks to produce granites has important implications for understanding some granulite facies terranes. If the process is general, it provides an explanation for the chemically "undepleted" nature of many granulite terranes, because no other rocks were involved in magma genesis. Therefore, these "undepleted" terranes may be the roots of granite batholiths. Also, removal of quartzofeldspathic material by granite generation and ascent would facilitate isobaric cooling of granulite facies terranes, because most of the buoyant, felsic material is removed, leaving denser, more refractory rocks in the source region. References Iyer, S.S., Woodford, P.J., & Wilson, A.F.; Lithos 9, 211-224 (1976) Warren, R.G.; Nature 305, 300-303 (1983) White, A.J.R. & Chappell, B.W.; Tectonophys. 43, 7-22 (1977)

20.1

FIRST EVER DRILLING ON THE KERGUELEN PLATEAU AND IN PRYDZ BAY J.B. Colwell, M.F. Coffin, H.L. Davies and H.M.J. Stagg

Bureau of Mineral Resources, Geology and Geophysics,

Canberra

Legs 119 and 120 of the Ocean Drilling Program, scheduled for December 1987-April 1988, will be drilled on the Kerguelen Plateau, in the southern Indian Ocean, and on the Antarctic continental margin in Prydz Bay (Figure). The objectives of the legs will be * Investigate Neogene palaeoceanography by drilling along latitudinal and depth transects * Determine the origin and Mesozoic-Palaeogene evolution of the Kerguelen Plateau * Determine the evolution of Antarctic climate before and during development of the present continental ice sheet. Sites on the Kerguelen Plateau were selected from those proposed jointly by Australian and French scientists, and are based on multichannel seismic reflection data collected in 1985 and 1986 by both nations. Predictions of stratigraphic succession and nature of basement are based on results of French dredging and coring, to which Australian scientists contributed, and on earlier coring by U.S. scientists. Sites in Prydz Bay were selected from those proposed by American and Australian scientists, and are based on seismic data collected by the Bureau of Mineral Resources in 1982. The Kerguelen Plateau is 2100 km long, NW-SE, and about 500 km across, and stands 2-4 km above adjacent sea floor; water depths are generally less than 1000 m in the north, and 1000-3000 m in the south. The plateau takes its name from the volcanic Kerguelen Islands (French). Australia claims as 'legal continental shelf all of the plateau south of a median line between the Kerguelen Islands and Heard and the McDonald Islands (Australian). Debate continues as to the nature of the basement of the plateau. Early Cretaceous basaltic volcanics were dredged recently from basement scarps, but it remains possible that these represent a volcanic veneer on a basement of (say) thinned continental crust. Certainly the thick sediment sequence on part of the southern plateau (3-4 km of sediment) is more characteristic of continent-margi|^ than of purely oceanic environment. Dipping seismic reflectors within basement may represent gently-folded pre-rift continental sediments, or rift-stage lavas and volcanogenic sediments, comparable with those encountered on the Rockall and Voring Plateaus in the North Atlantic. On the southern plateau, six seismic sequences, A (top) to F (bottom), probably represent Neogene-Quaternary ooze and volcanic and glacial detritus (sequences A,B), over Eocene and Paleocene pelagic sediment (C,D) which, in turn, rests on a sequence which includes Upper Cretaceous chert and mounds of biohermal or volcanic character (E). The nature of the lowest sequence, F, is not known, but it almost certainly includes detritus eroded from the Lower Cretaceous (?) basement. The basement volcanics were erupted at about 115 Ma in a partly-subaerial environment, probably during the initial rifting of Australia from Antarctica. The Plateau apparently subsided through much of the Cretaceous, but was arched and faulted, and possibly partly emergent, at about the end of the Cretaceous (top of sequence E). Paleocene subsidence 99

was followed by partial emergence in late Middle Eocene (end of Eocene in the south), approximately coincident with the onset of rapid spreading at the Southeast Indian Ridge (40-43 Ma) and separation of Broken Ridge from Kerguelen Plateau. Volcanism began on the northern Kerguelen Plateau at this time and continues to the present; it is thought to be hot-spot related. Parts of the Plateau probably remained emergent through the Oligocene. Subsidence through the Neogene-Quaternary is marked by an incomplete sediment record, especially on the southern part of the plateau, due to strong bottom currents. The Prydz Bay sites are at the northern end of the 700 km long Lambert Graben. The planned four holes will step through a series of prograding seismic reflectors, and should yield a stratigraphic record ranging from Permian, Jurassic or Early Cretaceous to Holocene. REFERENCES Leclaire et al., 1987, Geo-Marine Letters 7(4) (in press) Colwell et al., 1988, Marine and Petroleum Geology (in press)

jl^ y \

)

(Base map after Johnson and others. 1976)

Figure shows proposed drill sites on Kerguelen Plateau (KHP, SKP) and in Prydz Bay (PB); previous DSDP drill dites are shown without prefix. Heavy line is 1985 Rig Seismic track.

100

15.5

STRUCTURE OF SOUTHERN NEW SOUTH WALES AND NORTHEAST GIPPSLAND BASIN MARGINS - RESULTS OF RIG SEISMIC RESEARCH CRUISE 13 J.B. Colwell^, H.F. Coffin^, T. Pritchard^ and R. Spencer^

^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Ocean Sciences Institute, University of Sydney ^Geological Survey of New South Wales In early 1987, BMR collected 3300 km of high-quality multichannel seismic reflection (MCS) data along the southern New South Wales and northeast Gippsland Basin margins. These data comprise the first such data collected along the southern New South Wales margin and add substantially to the data available for the deep-water, northeast Gippsland Basin. Magnetic, gravity, seismic refraction, bathymetric, and geological data were also acquired. The southern New South Wales MCS data show a uniformly narrow continental shelf, a steep continental slope usually bereft of sediment except that ponded and/or preserved in apparent half-grabens behind a basement ridge occurring at mid-slope levels, and a poorly-developed continental rise at the foot of the slope. The mid-slope basement ridge is similar in style and location to that observed farther to the south along the Gippsland Basin's eastern margin. The Tasman Basin shows the thickest accumulations of sediment, commonly in excess of 2 s (two-way travel time). Basement of the Tasman Basin lies at deepest levels along the base of the continental slope, and rises gradually away from the Australian margin. Preliminary analyses of the northeast Gippsland Basin MCS data confirm that the eastern margin of the basin and the interpreted continent-ocean boundary are marked by a large structural high which trends approximately NNE at the base of the continental slope. They also indicate that the Gippsland Basin sequences are downfaulted and/or downflexed into the deep-water areas of the Bass Canyon, and that relatively thin sequences overlie probable Palaeozoic basement along the northernmost margin of the basin. The origin of the basement high is uncertain at this stage. However it appears to be fault-bounded in part and commonly flanked by a graben on its western side suggesting formation during north-south rifting. It is cut by the Bass Canyon. Thirteen sonobuoy experiments show numerous wide-angle reflection events emanating from the sediment column as well as reflected and refracted arrivals from the interface between sediments and assumed oceanic crust. Furthermore, several of the experiments recorded refracted energy from the crust-mantle boundary. In general the most prominent gravity (^50 mGal) and magnetic (<.500 nT) anomalies are associated with the basement ridge. High amplitude, short wavelength magnetic anomalies are common on the continental shelf, and are presumably related to shallow-lying, highly-magnetised igneous rocks. Magnetic anomalies which correlate with previously identified Late Cretaceous seafloor spreading anomalies are observed in places in the Tasman Basin.

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5.4

CHRONOSTRATIGRAPHIC UNITS AND THEIR APPLICATION IN AUSTRALIAN GEOLOGY B.J. Cooper

South Australian Department of Mines and Energy Chronostratigraphic Units (ie Eonothems, Erathems, Systems, Series, Stages, and Chronozones) define strata according to their age relations, and the International Stratigraphic Guide is the accepted guide to their use in Australia. In general, chronostratigraphy has heen less considered than lithostratigraphy by Australian geologists, a fact resulting from the pioneering nature of Australian field geology and the lack of well documented fossil faunas. This paper suggests that Australian geologists could fruitfully give greater consideration to chronostratigraphy through 1.

increased use of regional chronostratigraphic units to complement use of the Global Chronostratigraphic Scale. 2. establishment of local reference standards for the Global Chronostratigraphic Scale. Following introduction of a national Stratigraphic Code in 1950, regional chronostratigraphic stages for the Ordovician of Victoria and the Cainozoic of SE Australia evolved from earlier undifferentiated rock, fossil and time terminologies. The regional Cainozoic, Stages have subsequently been revised and two additional stages have been suggested. Since 1950, regional chronostratigraphic scales have also been proposed for the Late Precambrian (Adelaidean) of South Australia, Cambro-Ordovician of Western Queensland and Permo-Carboniferous of Tasmania. Opinion differs as to the value of these regional scales. Regrettably most Australian geologists do not use them because they are seen as a temporary expedient pending adoption of the global scale. Australian geological successions are also seen to be lacking a satisfactory data base to support local chronostratigraphic units. Such attitudes are contrary to international practice, and the International Guide (p.8l) declared that "It is better to refer strata with accuracy to local or regional (chronostratigraphic) units rather than to strain beyond the current limits of time-correlation in assigning these strata to units of a global scale". Regional chronostratigraphic Units are common in Europe, North America, China, Japan and New Zealand where they complement correlation with the evolving international scale. New regional stages have also been proposed recently for such widely different situations as the Lower Ordovician marine succession in North America and the Tertiary terrestial deposits of western Europe. It is concluded in general that regional chronostratigraphic units are applied where correlation within a limited geographic area is useful and achievable, and means of international correlation are absent, poorly available, little needed or inferior to the regional scale. Such circumstances certainly prevail in Australian geological successions. From specific cases it may be recognised that regional stages exist where:- There are no biostratigraphically useful fossils but correlation can be reliably achieved by some physical means. - Provincialism of the marine biota inhibits international correlation. - Non marine faunas, floras or environments can only be equated with diagnostic marine biotas with difficulty. - Information and/or chronological resolution is available that is not incorporated in the international standard.

102

The important factor in introducing new and revised regional chronostratigraphic units in Australia must be practicality. Pertinent questions which might assist any such decision are: Will regional stages facilitate better understanding of regional stratigraphy and palaeoenvironments? How accurate is correlation at present with the global scale? Many stratigraphic problems within Australia today demand detailed studies within a single sedimentary basin or between adjacent basins. Correlations with the global standard, in many cases, is also incredibly indirect or based on isolated occurrences in the biota or stratigraphic sections. In these circumstances, regional stages may provide a more useable chronological framework. The Cambrian, Permian and Tertiary marine successions probably provide the most appropriate parts of the Australian stratigraphic col\imn where regional stages could be revised and expanded. Given the substantial Late Palaeozoic, Mesozoic and Cainozoic terrestial component of the Australian stratigraphic record, nonmarine stages in these parts of the column may also be valuable. At the higher ranks of chronostratigraphic classification (viz. Systems and above) and in some cases at the Series and Stage levels, the value of only applying international terminology in Australia cannot be denied. However, such acceptance should not entail any abrogation of responsibility to deal with chronostratigraphic problems. A practical step in encouraging appreciation of international chronostratigraphic units in Australia would be to search out, document and list those sections in which international boundaries can be located. "Silver spikes" could thus be recognised to support the "golden spikes" that various international sub-commissions are currently establishing. Local reference sections for internationally determined boundaries would be valuable because:- Long geographic distances and vastly different geological settings separate almost all Australian sections from international stratotypes. - Direct correlation of Australian sections with international stratotypes has become the exclusive domain of specialists, while use of international names remain common currency amongst earth scientists. - Some Australian geological sections have international value or potential for providing refined international chronostratigraphical subdivision, and the international geological community should be more aware of this. As a consequence of the two proposals made herein, the Stratigraphic Nomenclature Committee has agreed to compile a set of definitions of currently-used Australian-based chronostratigraphic units. The Committee has also been asked to consider compiling a list of reference sections of precisely known internationally recognised boundaries in Australia.

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8 20

STRUCTURE AND TECTONICS OF THE WOODLARK TRIPLE JUNCTION ARC VOLCANOES ON AN INCOMING OCEANIC PLATE K.A.W. Crookl, B. Taylor2 N.F. Exon^ and R.W. Johnson-^ loepartment of Geology, Australian National University, Canberra 2Hawaii Institute of Geophysics, Honolulu, USA ^Bureau of Mineral Resources, Geology and Geophysics, Canberra

Sea MARC II imagery and bathymetry enable precise delineation of geomorphic, volcanic, structural and tectonic and features in the triple-junction area. The principal geomorphic features in the region are: (1) The Solomons Platform which lies northeast of the triple-junction region, to which the frontal scarp of the platform is the NE boundary; (2) The nokh-trending Simbo Ridge, up to 4.5km high, which emerges at Simbo island, and is attached at its north end to the Solomons Platform; (3) The Ghizo Ridge, ESE-trending and dextrally-segmented, attached to the central part of the Simbo Ridge at its western end but not attached to the Solomons Platform at its eastern end; (4) A closed bathymetric depression flanked by the Solomons Platform, Simbo and Ghizo Ridges, with a sediment drape on the northeast flank of the Ghizo Ridge and a sediment pond at the base of the depression; (5) A double line of closed bathymetric depressions adjacent to the foot of the southern flank of the Ghizo Ridge; (6) Regular rid^e-and-valley topography to the west of the Simbo Ridge, trending 087 ; j (7) A flat-floored sediment-filled depression between the Simbo Ridge and the area of ridge-and-valley topography to the west. Nine volcanic edifices are present in the triple-junction region, all being associated with the Simbo or Ghizo Ridges. The edifice that forms the island of Simbo may be composite. Limited bathymetric data from an area south of Simbo not covered by SeaMARC II bathymetry indicate a second edifice may be present, coalescent at depth with that to the north. The Simbo composite edifice rises 2.3km above the general crest level of the Simbo Ridge, being 13 5km diameter. Two other large edifices, Kana Keoki and Coleman submarine volcanoes, occur at the eastern end of the Ghizo Ridge. Both are 22 km in diameter and rise 2.8km above the surrounding seafloor, the Kana Keoki summit being above -700m and the Coleman summit above -1000m. Five smaller edifices are present on the western Ghizo Ridge and adjacent parts of the Simbo Ridge. These are <lkm high, with basal diameters of 2.5-9.5km. Petrologically, Simbo consists of andesitic to dacitic cumulo-domes and Kana Keoki volcano consists of dacite and rhyolite (Johnson et , 1987). Hyaloclastite of high-silica andesite composition, which may have been derived from Coleman volcano, was sampled at RD40 (Crook ^ al. , 1987). These are all island arc-type volcanics. The composition of the volcanics produced by the other vents is unknown. Many faults are present on the Simbo and Ghizo Ridges, scarps being evident on the SeaMARC imagery and on single-channel seismic reflection profiles which show that the ridge-and-valley topography west of the Simbo Ridge is formed by fault-bounded horsts and grabens. Bench-like features on the frontal scarp of the Solomons Platform may be due to landslip-like collapse on the scarp face. Some of the salients at the foot of this scarp may consist of debris accumulated from such mass-wasting processes.

104

The geomorphic, volcanic and structural features are expressions of the tectonics of the triple-junction region. The foot of the scarp flanking the Solomons Platform is a subduction front, marking the junction between the over-riding Pacific plate and the incoming plates that make up the floor of the Woodlark Basiji. The absence of a trench is due to the youthfulness and small flexure of the incoming Woodlark Basin oceanic crust (Taylor, 1987). The horst and graben structure west of the Simbo Ridge is a spreading fabric formed during the growth of the oceanic crust at the Woodlark spreading center to the south, and carried northwards by seafloor spreading. The Simbo Ridge and flanking trough evidently marks either a fracture zone or an active transform fault. There are no features indicative of contemporary seafloor spreading on the eastern side of the Simbo Ridge; thus the ridge is the site of a transform. A trench-trench-fault (TTF) triple-junction is located at the intersection between the subduction front and the Simbo transform, northwest of Simbo. The great height of the Simbo Ridge, and the presence of volcanic centers along it, suggest that the associated transform fault may have become leaky, contributing to the building of the ridge. The pattern of faults associated with the Ghizo Ridge and the adjacent area to the south is suggestive of disrupted, rotated and partly reactivated spreading fabric. This, and the location of several volcanic edifices along the crest of the Ghizo Ridge, suggest that the ridge developed from a former spreading center segment that was postulated in this position by Taylor (1987) on the basis of seafloor magnetic anomalies. If this is so, the former spreading center has been rotated, uplifted, segmented and extended along its length; and spreading-center basaltic volcanism on an axial volcano has given way to construction of volcanic cones, the largest of which have extruded island arc andesite, dacite and rhyolite. A fracture zone in the Woodlark Basin oceanic crust on the Australian plate impinges on the subduction front opposite the eastern end of the Ghizo Ridge (Taylor, 1987), i.e., at the same point at which the now-extinct spreading center meets the subduction front. Magnetic anomalies indicate that the spreading center was originally offset 11km northwards by the transform fault that generated this fracture zone. Consequently there is no trace of the former spreading center e^st of the fracture zone. The distribution of volcanic centers in the triple-junction area is notable in that the largest edifices, definitely known to be of island arc type, lie closest to the subduction front towards the ends of major ridges. A copious supply of magma is required to build edifices of such size. The volcanic center on Simbo is extinct. Kana Keoki volcano appears to be in the early stages of dissection. In contrast, the cone of Coleman volcano appears pristine, with a total absence of gullying. Crook, K. A. W. and Taylor, G. R. (1987) Sedimentary petrology and mineral chemistry, Woodlark Basin-Solomon Islands offshore region. Johnson, R. W. ^ al. (1987) Ridge subduction and forearc volcanism: petrology and geochemistry of rocks dredges from the Western Solomon arc and Woodlark Basin. Taylor, B. (1987) A geophysical survey of the Woodlark-Solomons region. All the above in: Circtim-Pacific Council for Energy and Mineral Resources, Earth Science Series, 7, in press

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2.1

MANTLE SUPERSWELLS - A DISTINCT RIFTING MECHANISM? G.F. Davies -Research School of Earth Sciences, Australian National University, Canberra

Observations and theory suggest that there are very large-scale (5000 km) swells of up to about 1 km amplitude deriving from the lower mantle. Such superswells might be responsible for the largest-scale lithospheric stretching and rifting events, including continental breakup. Mantle convection necessarily involves lateral variations in temperature, and these include very large scale variations of tens of degrees in temperature which are distinct from the small-scale variations in boundary layers. Numerical models show that these variations can make a significant (though usually not dominant) contribution to surface topography. Furthermore, if the viscosity of the mantle increases moderately with depth (by 2-3 orders of magnitude), these variations are magnified, and can yield topography of about a kilometer amplitude. The relatively rapid evolution of plate boundaries can then effectively decouple this topography from instantaneous plate configurations. Two regions of the earth seem to stand anomalously high: Africa and the western Pacific. Africa is the highest of the continents (about MOO m), the site of many hotspots and the site of the main current example of continental rifting. The western Pacific is about 1 km high relative to simple models of cooling and subsiding lithosphere and is also the site of many hotspots and hotspot tracks. Residual hotspot swells explain some of the excess topography, but there seems to be a broader regional residual as well. (Conventional theories explaining the so-called "flattening" of ocean bathymetry in terms of small-scale convection have recently been shown to be inadequate.) These two regions are also the sites of the major long-wavelength geoid highs, a strong indication that the effects arise from the lower mantle. Seismic tomography models have yielded low seismic velocities in the lower mantle which account for most of these geoid highs if they are assumed to be of thermal origin. There is thus strong observational evidence for large regions of warm mantle under Africa and the western Pacific. Such broad uplifts can be expe#.€ci to cause rifting on equally large scales. A problem with the postulated association of continental breakup with hotspots has been the relatively small scale of the latter. Convective shear stresses on the base of the lithosphere have also been envisaged as causing continental breakup, but it is not clear that such stresses can reach the required magnitude. However, Houseman and England have recently shown that uplift and gravitational sliding is a very effective rifting mechanism. Thus it is plausible to suggest that the proximate cause of major continental breakups is mantle superswells. Hotspot swells are probably associated with superswells, and may control the detailed path and timing of rifting. Mobile lithospheric plates are the dominant tectonic influence earth^s surface, and hotspots are a clear secondary influence. superswells may be a third, distinct influence.

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on the Mantle

1.1

GEOLOGISTS IN PAPUA NEW GUINEA - MAITLAND, STANLEY, WADE AND OTHERS H.L. Davies Bureau of Mineral Resources, Geology and Geophysics, Canberra

Written geological records for Papua New Guinea begin with accounts of volcanic eruptions by 16th and 17th century European navigators (e.g., see [1]. The first geological observations were by 19th century naturalists, including J. MacGillivray aboard HMS Rattlesnake in 1849, and M. Mikluho-Macleay on the north coast in the 1870s. In 1885, an expedition from the Geographical Society of Australasia explored the Strickland River and recovered fossils which demonstrated the presence of Jurassic and Cretaceous strata in the southern highlands (Etheridge in [2]); and, in 1887, the discovery of gold triggered the first of a series of gold rushes. The first review of the geology of Papua [3,4] arose from an invitation extended by Administrator and explorer, Sir William McGregor, to A. Gibb Maitland, then of the Queensland Department of Mines, to spend the months May-October, 1891, in the Territory. Contemporary geological investigations in the German New Guinea territory are not well documented in the English-language literature, but include the geological mapping of New Ireland by Karl Sapper in 1908 [cited in 5]; studies of foraminifera by Schubert [cited in 5]; the exploration of the Ramu River and reported discovery of gold and platinum by Ernst Tappenbeck [6]; and the exploration of the Sepik region by Leonard Schultze in 1910 and Walter Behrmann in 1912-13 [cited in 7]. Evan R. Stanley commenced duty as Government Geologist for Papua (formerly British New Guinea) in January 1911. He was a long time associate of Sir Douglas Mawson, as student and assistant, at the University of Adelaide. In the 14 years until his untimely death in December 1924, Stanley did much to establish the foundations of Papua New Guinea geology, as witness his accounts of the geology of both Territories, published in 1923 and 1924 [8,9]; the paper on the geology of Papua superceded the pithy review published by David and others in 1910 [10]. No Government Geologist for Papua was appointed to succeed Stanley. The discovery of indications of oil in the Gulf Province in 1911 caused great excitement in Australia. The region was immediately dubbed 'The Oil Fields*, and was compared with the oil fields of Burma. Stanley and NSW Assistant State Geologist, Joseph Carne, made initial investigations, which were followed by the appointment of an expert consultant, Arthur Wade, in October 1913. Wade continued in this role until the 1930s. Active exploration, at Government direction, became the prerogative of the Anglo-Persian Oil Company after 1919. N.H. Fisher was appointed first Government Geologist for the Territory of New Guinea in September 1934; he was recruited from Mount Isa Mines Ltd. A second geologist, L.C. Noakes, was appointed in 1935, and, a volcanologist, C.L. Knight, in 1940, following on the eruption of Rabaul Volcano in 1937. After the outbreak of hostilities, in 1942, all three were retained in the Australian Government Service, and went on to become foundation members of the national geological organisation, the Bureau of Mineral Resources, Geology and Geophysics (BMR), when it was established in 1946; Fisher was appointed Chief Geologist. 107

The Australian Army published a geological map of Papua New Guinea in 1945, and accompanying Explanatory Notes were released as a restricted document in December 1944; authors of the map and explanatory notes were J.N. Montgomery, N. Osborne, a n d M . F . Glaessner of the Australasian Petroleum Company, Melbourne. The Explanatory Notes were subsequently reproduced in [11], with permission but without author credits. After the Second World War, government geological operations in Papua New Guinea were undertaken by BMR staff based in Port Moresby - the Resident Geological Group - and in Canberra (Cabinet decision of December 1946). A.K.M. Edwards was appointed first Resident Geologist for the two Territories in 1949, J.E. Thompson as geologist in Wau in 1950, and G.A.M. Taylor as volcanologist in Rabaul, also in 1950. The Group functioned as a subdivision within the PNG Department of Lands, Surveys and Mines, and was funded by the PNG Administration. The first Canberra-based field operations were conducted in 1948 [12]; larger-scale regional geological and geophysical, regional investigations began in 1955 and continued until 1973. A Geophysical Observatory was established in Port Moresby in 1957 as an arm of the BMR, but with support staff provided by the PNG Administration. Private sector geological activity accelerated in the 1960s, following the recognition of the Panguna copper-gold ore body. The Geological and Volcanological Branch, as it was then known, became the Geological Survey Division of the PNG Administration on 22 October 1970, and the nexus with the BMR was severed on 1 September 1972, as part of the necessary preparations for Self Government in 1973. The Geophysical Observatory was absorbed into the Geological Survey in 1976. After Edwards, the successive Senior Resident or Chief Government Geologists were: Thompson 1951-61, Taylor 1961-63, J.G. Best and P.W. Pritchard in the interval 1963-65, A. Renwick 1965-73, H.L. Davies 1973-77, A.C.M. McKinlay 1977-81, and K.W. Doble 1981-86. REFERENCES 1.

Palfreyman WD, and Cooke RSJ, 1976, In Johnson RW (ed.), Volcanism in Australasia. Elsevier, Amsterdam, pp 117-132 2. Jack RL, and Etheridge R (jun), 1892, The Geology and Palaeontology of Queensland and New Guinea. Government Printer, Brisbane 3. Maitland AG, 1892, Geological observations in British New Guinea. Queensland Geological Survey Publication 85 4. Maitland AG, 1905, The salient geological features of British New Guinea. Western Australian Natural History Society Journal, 11, 32-56 5. Hohnen PD, 1978, Bureau of Mineral Resources, Australia, Report 194 6. Gash N, and Whittaker J, 1975, A pictorial history of New Guinea. Jacaranda, Milton Qld 7. Dow DB, and others, 1972, Bureau of Mineral Resources, Australia, Bulletin 133 8. Stanley ER, 1923, Report on the salient geological features and natural resources of the New Guinea Territory, Government Printer, Melbourne 9. Stanley, E.R., 1924 - The Geology of Papua. Government Printer, Melbourne 10. David TWE, Skeats EW, Hall TS, Dun WS, and Chapman F, 1914, Geology of the Commonwealth. In Knibbs, G.H., ed., Federal Handbook. Government Printer, Melbourne, pp 241-325 11. David TWE, ed. Browne WR, 1950, The Geology of the Commonwealth of Australia. Arnold, London 12. Ward HJ, 1949, Bureau of^Mineral Resources, Australia, Record 1949/79

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3.17

THE FINISTERRE COLLISION H.L. Davies

Bureau of Mineral Resources, Geology and Geophysics, Canberra

The Huon Peninsula and adjacent Finisterre Range are formed of an anticlinal nappe bounded by several N-dipping thrusts. The frontal thrust, the Ramu-Markham Fault, can be traced offshore to the ESE as a deformation front along the line of the Markham submarine canyon. The S-dipping Trobriand subduction system and the N-dipping New Britain Trench system converge and appear to join about 100 km offshore. From where they join, the anticlinally-folded Solomon Sea Plate plunges westward beneath the PNG mainland. The folded plate is seen in seismic reflection profiles and earthquake epicentre plots; doubling of crustal thickness probably is the cause of the extreme negative gravity anomaly in this region. Because of contrasts in geology N and S of the Ramu-Markham Fault, the Finisterre-Huon massif has long been recognized as an exotic terrane, accreted to the PNG mainland by arc-continent collision. However, opinions differ as to the timing of the collision. One school, mostly geological, sees it as an Early Miocene event, after which the S-dipping Trobriand and co-linear Wewak trenches developed by arc reversal. The subsequent collision of the Trobriand and New Britain subduction systems ('Schouten Islands collision*) started in the Pliocene, and migrated rapidly to the ESE. During the latter collision, the Finisterre-Huon massif was part of the Indo-Australian Plate, which eventually rode northwards over the trace of the colliding trenches. The model requires a separate evolution of the Finisterre and New Britain Palaeogene volcanic arcs; current convergence along the line of the Ramu-Markham Fault and Markham Canyon are explained as reactivation of the Early Miocene suture. The other school, mostly geophysical, sees the Finisterre collision as a (Late Miocene-)Pliocene-Quaternary event, part of the Trobriand-New Britain trench-trench collision. In this model, the Finisterre-Huon massif developed as a lateral continuation of the New Britain Palaeogene volcanic arc, and, with New Britain, was always on the South Bismarck Plate. This model is at odds with onshore geological data which show that the north coast ranges, including the western part of the Finisterre-Huon massif, were in place prior to the development of the North Sepik sedimentary basins in the Early Miocene, and the Ramu sedimentary basin in the early Middle Miocene. REFERENCE Davies and others, 1987, Convergent tectonics in the Huon Peninsula region, Papua New Guinea. Geo-Marine Letters 7(3) (in press)

109

20.3

T H E E V O L U T I O N O F T H E C A R B O N A T E P L A T F O R M S IN N O R T H E A S T AUSTRALIA - THE GOAL OF THE OCEAN DRILLING PROGRAM P.J. Davies, P.A. Symonds, D.A. Feary and C . J . Pigram B u r e a u of M i n e r a l R e s o u r c e s , Geology and Geophysics, Canberra

The Great B a r r i e r R e e f , a n d the M a r i o n a n d Q u e e n s l a n d P l a t a e u x r e p r e s e n t a n a t u r a l g e o l o g i c a l l a b o r a t o r y , the like of w h i c h occurs no w h e r e else on earth t o d a y , a n d one w h i c h r e p r e s e n t s a recurrent t h e m e in p l a t f o r m e v o l u t i o n t h r o u g h o u t g e o l o g i c a l t i m e . T h e y c o l l e c t i v e l y define a m o d e l of p a s s i v e m a r g i n e v o l u t i o n from b i r t h , t h r o u g h m a t u r i t y , senility a n d d e a t h a n d e n c a p s u l a t e a r e c o r d of t h e t e c t o n i c , e u s t a t i c , p a l a e o c l i m a t i c a n d p a l a e o - o c e a n o g r a p h i c d y n a m i s m w h i c h has c r i t i c a l l y d r i v e n t h e i r formation. The s t r u c t u r a l a n d s e d i m e n t o l o g i c a l features of the c a r b o n a t e p l a t f o r m s of n o r t h e a s t A u s t r a l i a a p p e a r to have b e e n p r i m a r i l y c o n t r o l l e d b y four i n t e r - r e l a t e d forcing f u n c t i o n s , h o r i z o n t a l tectonics(plate motion), climate/oceanography/sea level,vertical sag t e c t o n i c s ( s u b s i d e n c e ) a n d c o l l i s i o n . T h e s e factors have i n t e r a c t e d t o give rise to: 1 . Late C r e t a c e o u s /Palaeocene rifting a n d f r a g m e n t a t i o n of a c o n t i n e n t a l m a r g i n effecting the formation of d e e p i s o l a t e d rifts a n d m a r g i n a l p l a t e a u immediately offshore from a m a r g i n ,the b o u n d a r y faults of w h i c h o c c u r b e n e a t h the p r e s e n t m i d to o u t e r s h e l f . W i t h respect to p r e s e n t latitude this o c c u r r e d w h e n n o r t h e a s t A u s t r a l i a was in latitudes 28^S to 4 3 ^ S . 2 .Temperate clastic fluvio/deltaic a n d c a r b o n a t e s e d i m e n t a t i o n a l o n g the c o n t i n e n t a l m a r g i n w i t h carbonates e s p e c i a l l y a l o n g the n o r t h e r n m a r g i n , a n d temperate (?) carbonate p r o g r a d a t i o n a l o n g the m a r g i n of the Q u e e n s l a n d a n d M a r i o n P l a t e a u x in the E o c e n e a n d O l i g o c e n e u n d e r conditions of g e n e r a l l y h i g h s e a l e v e l a n d sea surface t e m p e r a t u r e s w h i c h decrease m a r k e d l y t h r o u g h o u t the period . 3 . Late O l i g o c e n e to early M i o c e n e subsidence p r o b a b l y i n d u c e d b y c o l l i s i o n in N e w Guinea a n d coincident w i t h a rise in s e a l e v e l led to w i d e s p r e a d t r a n s g r e s s s i o n o v e r parts of a l l t h r e e p l a t f o r m s . A n incipient f o r e l a n d b a s i n d e v e l o p e d in the n o r t h . Reef i n i t i a t i o n d i d not o c c u r immediately due to the inhibiting e f f e c t s of t e m p e r a t u r e a n d ocean c h e m i s t r y . A t e m p e r a t e climate a n d h i g h n u t r i e n t s e f f e c t e d r e d a n d green a l g a l b i o h e r m s f o r m a t i o n p a r t i c u l a r l y on the o u t e r s h e l f . E a r l i e s t f o r m a t i o n of reefs o c c u r r e d on fault blocks w h i c h d e v e l o p e d into a b a r r i e r in the G u l f of P a p u a ; t h e first reefs p r o b a b l y f o r m e d u p o n e a r l i e r r e d a l g a l b i o h e r m s but o n l y a f t e r n o r t h w a r d drift h a d e n a b l e d the region to e n t e r the t r o p i c s . 4 . I n i t i a t i o n of reefs on the Q u e e n s l a n d a n d M a r i o n P l a t e a u x in the e a r l y M i d d l e M i o c e n e as a consequence of p l a t e m o t i o n . E x p a n s i o n of the shelf carbonate p l a t f o r m in the n o r t h e r n Great

110

B a r r i e r Reef a n d throughout m u c h of the M a r i o n a n d Q u e e n s l a n d p l a t e a u x due to the c o n t i n u e d drift into the t r o p i c s a n d the marked, increase in surface w a t e r t e m p e r a t u r e s . C o n t r a c t i o n of the e a r l i e r b a r r i e r reef in the Gulf of Papua into a series of i s o l a t e d p i n n a c l e s as a consequence of subsidence e f f e c t e d b y the s o u t h e r n m i g r a t i o n of the f o r e l a n d b a s i n . 5 Late M i o c e n e subsidence more than c o u n t e r a c t e d the late M i o c e n e s e a l e v e l f a l l a n d r e s u l t e d in stepback of the reefs on the Q u e e n s l a n d a n d M a r i o n P l a t e a u a n d r e - e s t a b l i s h m e n t at a h i g h e r l e v e l . At the same t i m e , contraction of reef g r o w t h a n d b u r i a l o c c u r e d in the n o r t h e r n r e g i o n . M u c h of the c e n t r a l Great B a r r i e r R e e f , a l t h o u g h in the t r o p i c s , was t e r r i g e n o u s d o m i n a t e d a n d largely e x p o s e d . 6 . In the extreme n o r t h , p r o g r e s s i v e forearc subsidence a n d m i g r a t i o n to the south led to a s p h y x i a t i o n of reefs b y f l u v i o c l a s t i c s as a consequence of m o u n t a i n b u i l d i n g in N e w G u i n e a . P l i o c e n e subsidence led to s u b s t a n t i a l stepback on the Q u e e n s l a n d P l a t e a u , drowning of the M a r i o n P l a t e a u a n d stepback of the M i o c e n e B a r r i e r a n d P l a t f o r m reefs to t h e i r p r e s e n t p o s i t i o n on the Great B a r r i e r R e e f . Post P l i o c e n e e v o l u t i o n of the Great B a r r i e r Reef is r e l a t e d to sea l e v e l c o n t r o l on fluvio d e l t a i c d e p o s i t i o n a n d reef g r o w t h . The c o n s e q u e n c e s of the above h y p o t h e s i s are that the Great B a r r i e r Reef t r o p i c a l shelf carbonate p l a t f o r m thins to the south a n d overlies a t e m p e r a t e facies; that reefs grew first in the n o r t h , p r o b a b l y a l o n g the west to east d e v e l o p i n g forearc b a s i n a n d a l o n g its e a s t e r n m a r g i n , a n d that the p l a t f o r m s of the Q u e e n s l a n d a n d M a r i o n P l a t e a u x are the carbonate p l a t f o r m p r e c u r s o r s to m u c h of the c e n t r a l a n d southern Great B a r r i e r reef. The O c e a n D r i l l i n g P r o g r a m in northeast A u s t r a l i a w i l l test this h y p o t h e s i s b y drilling a series of holes a l o n g t w o t r a n s e c t s (Fig 1 ) . The p r i n c i p a l objectives are t w o f o l d : To define a n d compare the effects of s e a l e v e l v a r i a t i o n in the same t i m e span along a m i x e d c a r b o n a t e / c l a s t i c p r o v i n c e and a p u r e c a r b o n a t e p r o v i n c e . The objective demands that the n u m b e r of o p e r a t i n g v a r i a b l e s be r e d u c e d to a m i n i m u m a n d this w i l l be a c h i e v e d t h r o u g h sampling along a l a t i t u d i n a l t r a n s e c t a n d b y sampling at the areas of negligable shelf subsidence where the s e a l e v e l signature w i l l be s t r o n g . Coring a n d d r i l l i n g at sites 1,2,3,4 5,6+8 w i l l help achieve this a i m . Sites 1-5 w i l l define the r e l a t i o n s h i p of seismic s t r a t i g r a p h y , s e a l e v e l v a r i a t i o n a n d the change from slope to b a s i n along a m i x e d c a r b o n a t e / t e r r i g e n o u s p l a t f o r m e d g e . Sites 6+8 w i l l do the same along a p u r e carbonate p l a t f o r m m a r g i n . 2 . To define the effects of p l a t e m o t i o n , a n d t h e r e f o r e p a l a e o c l i m a t e a n d p a l a e o - o c e a n o g r a p h y , on carbonate p l a t f o r m e v o l u t i o n a n d in partictilar the factors e f f e c t i n g the turn on and t u r n off of reef p l a t f o r m s . This objective is b e s t s t u d i e d in a pure c a r b o n a t e system b e c a u s e it is the c a r b o n a t e signature w h i c h w i l l v a r y most as a consequence of strong l a t i t u d i n a l

111

movement. A longitudinal transect would therefore best enable the objective to be achieved with at least one site as far south as possible to increase the climatic contrast. Coring and drilling at sites 8,9A,lOA,11,13 and 14 will achieve this aim. Sites 8,9A and lOA will,in particular, define the palaeo-oceanographic signal in the Neogene: site 11 will define both the relations of platform and basin development and the palaeoclimatic and oceanographic signal in the late Palaeogene and Neogene, and sites lOA, 13 and 14 will define the history of reef growth in the Marion and Queensland Plateaux. During the Cenozoic Australia has traced a unique structural and climatological course , the history of which is encapsulated in the carbonate platforms which have formed along her northern margin. The Ocean Drilling Program will help to unravel questions of fundamental global interest and magnitude, not the least of which is the Origin of the Great Barrier Reef.

20°

E3/0Q/III

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7.5

MODERN NON-MARINE DOLOMITE IN EVAPORITIC PLAYAS OF WESTERN VICTORIA, AUSTRALIA P. De Deckkerl and W.M. Last^

^Cenozpic Research Unit and Department of Geography, Monash University, Clayton ^Department of Geological Sciences, University of Manitoba, Winnipeg, Canada

Microcrystalline dolomite occurs in the surface sediment of several groundwater-fed, shallow playa lakes in the Western Volcanic Plains District, West of Melbourne. There is evidence that this carbonate, which is nearly stoichiometric in composition, but with poorly ordered peaks, directly precipitates from the lake waters. In most lakes, dolomite is associated with magnesite. Three main types of dolomite-precipitating lakes are recognized in the area. A list of their characteristics which appear to be of significance for the formation of dolomite is presented below: (1)

TYPE A has a salinity > A5%o, water Mg/Ca^'ZS and which fluctuates substantially, p H < 9 , water most often undersaturated with respect to calcite and aragonite, dolomite and magnesite in surficial sediment, water turbid and white (= indicating precipitation and resuspension of carbonates in the water column), and the lake dries up occasionally.

(2)

TYPE B has a salinity > 130%o, Mg/Ca;>70 with few fluctuations, p H > 9 , water nearly always just saturated with respect to calcite and most often undersaturated in aragonite, dolomite only in surficial sediment, and water remaining permanent.

(3)

TYPE C has a salinity <15Zo, M g / C a < 1 0 , pH > 9, water saturated with respect to both calcite and aragonite, dolomite and magnesite in the surficial sediment with dolomite in much larger proportions than magnesite and milky water. [However, we possess little long-term information on the water chemistry for this type of lake].

Calcium values of the waters for all 3 lake types are extremely low: ^ 1% equiv.Ca of all cations except on one occasion (1.1%). Alkalinity values are high, sulfate levels are moderate and do not seem to be inhibiting dolomite precipitation. A Holocene sequence recovered over 125 cm of sediments for one of the lakes (Beeac) also consists of dolomite and magnesite, thus suggesting that the process of dolomite precipitation in the area is a continuing phenomenon. We believe that the Victorian localities where dolomite forms at present provide new insights into the formation of dolomite which is still the subject of numerous controversies.

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9.1

SEISMICITY OF THE AUSTRALIAN PLATE AND ITS PACIFIC PLATE MARGIN D, Denham

Bureau of Minetal Resources, Geology and Geophysics,

Canberra

The patterns revealed in plots of earthquake hypocenters are crucial for interpreting tectonic processes within the earth. At active plate boundaries, where a satisfactory model has been developed, hypocentral locations provide important information on tectonic details. Even in complicated areas, studies of the spatial distribution of earthquakes can provide considerable information on the geometries of the plate margins and the subduction zones. For example in the New Guinea region, where the Australian and Pacific plates are interacting strongly, there is not one simple plate boundary but a cluster of small plates, with each boundary accommodating a component of the total interaction process. Furthermore, there is not one single subduction zone but several. However, for the Australian continent, which is an intra-plate environment, there is no comparable tectonic model which can be used successfully to interpret the seismicity patterns. W e know very little about the causes of this seismicity, except that the earthquakes are all shallow and caused by compressive forces, and that large earthquakes can and do occur in the Australian region. The task of developing a successful model to describe intra-plate seismicity patterns is one of the most important seismological problems to be tackled in the next decade.

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12.5

AUSTRALIAN PERMIAN CORRELATION AND SEA LEVEL CHANGES J.M. Dickins

Bureau of Mineral Resources, Geology and Geophysics, Canberra

Three major regressive-transgressive phases are recognizable in the Permian of Australia. These correspond to major world-wide regressive -trangressive changes (Dickins, ms.; Dickins and others, ms.)* The first of these is associated with the Carboniferous-Permian boundary. Here this boundary is taken to coincide with the base of the Asselian of the standard European Soviet sequence (Archbold, 1982). In western Australia, the youngest Carboniferous marine sediments seem to be of Visean-Namurian boundary age {Bonaparte Gulf Basin) and in eastern Australia marine deposits with an authenticated age, may not be younger than early Namurian (northern New South Wales and Queensland) - P.J. Jones (pers. comm.). The beginning of the Permian is marked by the marine transgression at the base of the Lyons Group and equivalents in western Australia and the Lochinvar Formation in the Sydney Basin, eastern Australia. The mid-Permian regression is well marked by hiatus in the Perth Basin, and by development of terrestrial deposits at the end of the Lower Permian in the Carnarvon, Canning and Bonaparte Basins. In the New England area it is represented by a hiatus (Runnegar, 1970) and in the Bowen by regressive coal measures - the Collinsville Coal Measures and equivalents. The transgression is particularly strongly marked in eastern Australia where it is associated with the beginning of the Hunter-Bowen (Indosinian) ''Orogeny** (Dickins, 1985a; 1 9 8 7 ) . The third major regression is well-known and is associated with the end of the Permian. It is described in some detail by Dickins (I983). This regression is world-wide and is one of the most wide-spread lowerings of sea-level in the Phanerozoic, corresponding to a major change in biota. The published correlation chart for the Permian System of Australia (Dickins, 1976) together with a modified correlation chart (Dickins and Archbold, in prep.) indicates further important sea level changes, considered here from oldest to youngest. The transgression of the Carrandibby Formation of the Carnarvon Basin and the Allandale Formation of the Sydney Basin apparently represents a eustatic rise at the end of the main Carboniferous- Permian glaciation (Dickins, 1985b). If the Nura Nura Member is not equivalent to the Callytharra-Fossil Cliff but to the High Cliff and the base of the Wooramel Group, then an Australian wide emergence followed by submergence corresponds to that of the SakmarianArtinskian elsewhere in the world as this appears equivalent to the basal transgression of the Tiverton Subgroup of the Bowen Basin. Similarly if the Sirius Formation of the south-west Bowen Basin is equivalent to the base of the" Gebbie Subgroup of the north-eastern part of the Bowen Basin, the base of the Branxton Formation of the Sydney Basin and

115

the Byro Group of the Carnarvon Basin another important transgression coincides with that of the beginning of the Baigendzhinian in other parts of the world. Recognition of other possible Australia-wide sea levels is possible but is dependent on the refinement and imroved reliability of palaeontological information as well as improved understanding of the stratigraphy. The three major regressive- transgressive changes are so wide-spread and strong that they almost certainly represent major tectonic changes reflecting fundamental changes in the deeper parts of the earth. Because the main glaciation ends at the end of the Asselian when the Carrandibby and Callytharra Formations register a post-glacial rise in sea level, the other sea level rises probably reflect tectonic activity. REFERENCES ARCHBOLD, N.W., 1982. Correlation of the Early Permian faunas of Gondwana: implications for the Gondwanan Carboniferous-Permian boundary. Journal of the Geological Society of Australia, 29 267-276. DICKINS, J.M., 1976. Correlation chart for the Permian System of Australia. Commonwealth of Australia, Bureau of Mineral Resources, Bulletin, I56B. DICKINS, J.M., 1985a. Late Palaeozoic and Early Mesozoic "Orogeny" in eastern Australia.Advances in study of Sydney Basin, Proceedings of the 19th Symposium, Department of Geology,University of Newcastle, 1985, 8-9. DICKINS, J.M., 1985b., Late Palaeozoic glaciation. B m Journal of Australian Geology & Geophysics, 9, I63-I69. DICKINS, J.M., 1987. Tethys - a geosyncline formed on continental crust ? J^ K.G.McKenzie (ed.). Shallow Tethys 2, Rotterdam, Boston , A.A. Balkema, 149158. DICKINS, J.M., ms. Major sea level changes, tectonism and extinctions. Prepared for 11th International Carboniferous Congress, 1987• DICKINS, J.M., and ARCHBOLD, N.W. in prep. A standard geological the Permian System in Australia.

scale for

DICKINS, J.M., N.W.ARCHBOLD, G.A.THOMAS and H.J.CAMPBELL, ms. Mid-Permian correlation. Prepared for 11th International Carboniferous Congress, I987. RUNNEGAR, B.N., 1970. The Permian faunas of northern New South Wales and the connection between the Sydney and Bowen Basins. Journal of t ^ Geological Society of Australia, I6 (2), 696-7IO.

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19.7

A STANDARD GEOLOGICAL SCALE FOR THE PERMIAN SYSTEM IN AUSTRALIA J.M. Dickins^ and N.W. Archbold^

^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Department of Geology, University of Melbourne Following the recent discussions of the Subcommission on Permian Stratigraphy at the Carboniferous Congress, Beijing 198?, a two-fold subdivision of the System into Lower and Upper is favoured for the World Standard Stratigraphic Scale. Similarly a major fourfold subdivision is recognized for the Lower Permian with Asselian, Sakmarian, Artinskian and Kungurian Stages. The Sakmarian is divided into two substages - the Tastubian and Sterlitamakian and the Artinskian into Aktastinian and Baigendzhinian. A five-fold division is used for the Upper Permian with Ufimian (or Kubergandian), Kazanian (or Murgabian), Midian, Dzhulfian and Changhsingian. Taking into account recent work indicating the complexity of the Upper Permian, an age of 270 ma (approx.) for the Lower - Upper Permian boundary seems realistic on the basis that the Lower Permian boundary is 295 (approx.) and the Upper Permian 250 (approx,). An integrated correlation chart for Australia has been prepared using the schemes of Clarke & Banks (1975)• Dickins (1963, 1984). Kemp et al. (1977) and Runnegar and McClung (1975) based on invertebrate faunas and microfloras as well as additional new information (inter alia: Archbold, Dickins and Thomas, in press; Dickins, in press and Waterhouse, I983). REFERENCES ARCHBOLD, N.W., DICKINS, J.M., THOMAS, G.A. In press. Correlation and age of the Western Australian marine faunas. In S.K.Skwarko (ed.), Permian Palaeontology of Western Australia. Perth, Geological Survey of Western Australia. CLARKE, M.J., and BANKS,M.R. 1975- The stratigraphy of the lower (PermoCarboniferous) parts of the Parmeener Super-Group, Tasmania. In K.S.W. Campbell (ed.),Gondwana Geology. Canberra, A.N.U. Press, ^53-467. DICKINS, J.M. 1963. Permian pelecypods and gastropods from Western Australia. Bureau of Mineral Resources, Geology and Geophysics, Bulletin, 63. DICKINS, J.M. 1984. Evolution and climate in the Upper Palaeozoic. In P. Brenchley (ed.). Fossils and Climate. New York, Wiley, 317-327. DICKINS, J.M. In press. Youngest Permian marine macrofossil fauna from the Bowen and Sydney Basins, eastern Australia. BMR ^ i ^ n a l of Geology & Geophysics. KEMP, E.M., BALME, B.E., HELBY, R.J., KYLE, R.A., PLAYFORD, G., and PRICE, P.L. 1977- Carboniferous and Permian Palynostratigraphy in Australia and Antarctica: a review. BMR Journal of Australian Geology & Geophysics, 2 (3), 177-208. RUNNEGAR. B., and McCLUNG, G. 1975- A Permian time scale for Gondwanaland. In K.S.W. Campbell (ed.), Gondwana Geology. Canberra, A.N.U. Press, 425-441. WATERHOUSE, J.B. I983. Systematic description of Permian Brachiopods, Bivalves and Gastropods below Wall Sandstone Member, northern Bowen Basin. Papers Department of Geology, University of Queensland, 10 (3), I55-I79. 117

2.17 TEMPORAL CHANGES IN CONTINENTAL GROWTH PATTERNS REVEALED BY SEISMIC CRUSTAL STRUCTURE AND GRANITE GEOCHEMISTRY B.J. Drummondl, L.A.I. Wyborn^, D. Wyborn^ and J.F. Tarney^ ^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Department of Geology, University of Leicester, UK

I-type granites are a second stage melt of magmas from the mantle emplaced at the base of the crust. They can be grouped into those for which the source was removed from the mantle only a short time before the formation of the granite magma, and those for which the source was removed a long time before the granite magma formed. Archaean, late Palaeozoic, Mesozoic, and Cenozoic granites comprise the first group, while Proterozoic and early Palaeozoic granites make up the second (Wyborn et al., I988). Seismic data can be used to probe the lower crustal source regions of granites and indicate a systematic change in the nature of the lower crust with increasing province age, and this in turn indicates temporal changes in the growth patterns of continental crust. Lateral accretion at volcanic arcs is thought to dominate the present generation of new continental crust. At volcanic arcs, oceanic crust is subducted below continents, continental fragments (eg. the western coast of southern America, Japan, New Zealand, Java, Alaska) or island arcs (eg. New Britain, Tonga, Kermadec) . Igneous rocks within volcanic arcs are predominantly andesitic in composition, reflecting derivation from a source in the lower crust or upper mantle that has itself been derived from partial melting of the upper mantle, with a contribution from material stripped from the downgoing slab. Granites in continental volcanic arcs are commonly heavy rare earth depleted, indicating the presence of garnet in their source. However, seismic models of the lower crust underneath continental volcanic arcs indicate a mafic (basaltic) underplate rich in feldspars and quartz but poor in garnet (Gill, I98I). The source region of granites within volcanic arcs may therefore lie in garnet-bearing basaltic components within the mantle wedge. The small crustal residence time of granites formed from the underplate is consistent with the dynamic character of volcanic arcs, where the high temperatures associated with raised geothermal gradients quickly remelt the underplate to form granites. Proterozoic and Lower Palaeozoic granitic magmas were generated from partial melting of rocks emplaced into the lower crust, in some cases at least 1000 Ma earlier. The residue left behind after the granite magmas were removed can be seen today as thick crustal roots with seismic velocities near jA-^.S km/s. Drummond & Collins (I986) canvassed several models by which the crustal roots may have formed. Lateral accretion could not be excluded, but large areas of northern Australia have the high velocity lower crust. The region contains provinces that appear to have been ancient cratons as well as areas classified as mobile belts, and a subduction-related process is difficult to apply to all crust in such a large and tectonically varied area. Drummond & Collins (1986) favoured vertical accretion (underplating) as the dominant mechanism for emplacing the high velocity rocks into the lower crust. Thus even if the protocrust of Proterozoic and lower Palaeozoic crust formed by lateral accretion in a plate tectonic environment, vertical accretion by mafic underplating is

118

required by both the granite genesis and the seismic models of the lower crust. The generation of Archaean magmas is difficult to fit into either a lateral or vertical accretion model for the generation of continental crust. Archaean granites had short crustal prehistories, but unlike Proterozoic, Lower Palaeozoic and Cenozoic regions, the crust in Archaean cratons today contains no seismic signature of a laterally or vertically underplated mafic source for the granites. The low present-day geothermal gradients of Archaean shields may have resulted in the conversion to eclogite of the residue left after granite magma generation. As eclogite, it would be seismically indistinguishable from upper mantle rocks (cf. Griffin & O^Reilly , I986). The geochemistry of Archaean granites indicates derivation from a garnet-rich source and is therefore consistent with such a model. Thus the underplated source of the granites would have to have been converted to eclogite in the Archaean, prior to the formation of the granites. However, this is inconsistent with the high geothermal gradients common throughout the Archaean and exemplified by the areally and temporally extensive basaltic magmatism and granitic plutonism. An alternative model suggests that Archaean granites were derived by a small degree of partial melting of the basalts in greenstone belts. However, greenstone belts form only a small proportion of surface outcrop and on the basis of seismic and electrical sounding and gravity modelling are usually no more than 7-IO km thick. Small degrees of partial melting of greenstone belts would not have produced the volume of granites observed in the crust. Another model for the generation of Archaean granites that involves some form of derivat.i^ by partial melting of a protocrust which is predominantly felsic may n e S to be constructed . References: Drummond, B.J. & Collins, C.D.N., I986. Seismic evidence for underplating of the lower continental crust of Australia. Earth and Planetary Science Letters, 79, 361-372. Gill, J, 1981. • Orogenic andesites and plate tectonics. Berlin, 390p. Griffin, W.L. and O'Reilly, S.Y., I986. mantle boundary? Geology, 15, 241-244.

Springer-Verlag,

Is the continental Moho the crust-

Wyborn, L.A.I., Wyborn, D., Chappell, B.W., Sheraton, J.W., Tarney, J.F., Collins, W.R., and Drummond, B.J., this volume. Geological evolution of granite compositions with time in the Australian continent: implications for tectonic and mantle processes.

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17.10

PETROLOGY OF THE WARRUMBUNGLE VOLCANO, NEW SOUTH WALES M.B. Duggan^, J. Knutson^ and B.W. Chappell^ ^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Department of Geology, Australian National University, Canberra

The Warrumbungle Mountains comprise the eroded remnants of a Miocene shield volcano with an estimated original diameter of 50 kilometres, a height of about 1000 metres and volume of about 400-500 cubic kilometres. The volcano originally formed a broad lava shield locally modified by some exogenous lava domes and intruded by lava plugs, endogenous domes and dykes. Pyroclastic rocks are abundant in the central portion of the shield. The volcanic rocks form a mildly alkaline series from hawaiite and mugearite to peralkaline trachyte and minor alkali rhyolite with a distinct hiatus in the benmoreite range. Thin flows of hawaiite and mugearite form an important component of the shield-building sequence. Most contain phenocrystic plagioclase, usually accompanied by minor olivine and augite*. The lava shield sequence is also characterised by thick flows of relatively mafic trachyte carrying Fe-rich olivine and ferroaugite. More mafic rocks with Mg-numbers approaching those expected of unmodified mantle melts are extremely rare or absent from the main body of the volcano but do form isolated occurrences outside the flanks of the shield. Some of these contain pyroxenite inclusions and megacrysts of pyroxene and spinel. Peralkaline trachytes, occurring as domes, plugs and dykes, include both nepheline- and quartz-bearing types, each with characteristic mafic mineral assemblages. In quartz-bearing trachytes and alkali rhyolites, arfvedsonite is dominant with minor aenigmatite and aegirine-hedenbergite. Where aegirine-hedenbergite is a minor phase, it is sometimes highly enriched in Zr (up to 14 wt % ZrO^). Nepheline-bearing trachytes are dominated by aegirine-hedenbergite and Ti-poor "ferri-aenigmatite" which in exceptional circumstances becomes Ti-free. Ti depletion in aenigmatite results from a combination of low Ti, low silica activity and low fO^ in the host trachyte. A range of silica saturation levels is evident throughout the range hawaiite to trachyte. The series is similar in gross geochemical characteristics to other eastern Australian central-type volcanoes although the extensive development of silica-undersaturated felsic rocks is apparently unique. There is no geochemical evidence to support the existence of separate petrogenetic lineages in the volcano. Instead, a continuous spectrum of mafic compositions straddles the low pressure thermal divide separating silica-undersaturated and oversaturated mafic magmas. There is strong geochemical coherence between most incompatible elements throughout the series. Some peralkaline trachytes and rhyolites show geochemical features characteristic of extended fractionation including extreme depletion in Mg, Ca, Sr, Ba, P and Ti and enrichment in Rb, Pb, Th, Zr, Nb and LREE. However, Sr and Nd isotopic data for some of the felsic rocks suggest that a significant crustal component may have been involved in their formation. The geochemical features of the series are broadly consistent with a model of fractional crystallization, perhaps modified by assimilation, but there are problems in detail. Thesd include variations in isotopic data, extreme enrichment in some incompatible elements (and the consequent extent of fractionation and volume of complimentary cumulates required) and inconsistencies revealed least squares mixing and fractionation modelling.

120

17.2

TIME-SPACE RELATIONSHIPS FOR CENOZOIC VOLCANISM IN EASTERN AUSTRALIA

INTRAPLATE

R.A. Duncan^ and I. McDougall^ ^College of Oceanography, Oregon State University, Corvallis, USA ^Research School of Earth Sciences, Australian National University, Canberra

Cenozoic volcanism in eastern Australia is confined to a zone averaging about 400 km wide more or less parallel to the coast, within and adjacent to the highlands. Volcanism clearly is in an intraplate setting, having erupted through continental crust involved in the Tasman Orogenic ^ n e . Isotopic ages, mainly K-Ar dates on whole rock samples, are available for most of the volcanic provinces. Estimates of volumes indicate that the rate of eruption has been approximately constant throughout the Cenozoic. Two main types of volcanic province, recognized by Wellman and McDougall (1974), and termed centrd volcano and lava field, remain a convenient division with genetic significance. The central volcano provinces comprise relatively large volumes of basaltic lava erupted from a vent area to produce a substantial volcano in most cases. An essential distinguishing characteristic of this type of province is the presence of some felsic flows or intrusives. The central volcanoes are distributed over a relatively broad zone, extending south from - - i r S in central Queensland toward Victoria. Alignment of groups of volcanic centres in narrower longitudinal zones is evident. Many of the provinces are well dated; typically, activity has occurred over <5 Ma at an individual centre. As documented by Wellman and McDougall (1974), this volcanism exhibits a systematic younging southward, with the rate of migration of volcanism averaging 65 ± 3 nWyear. The observed pattem continues to be interpreted in terms of a hotspot or plume trace, albeit much broader than normal, perhaps reflecting the effect of thick continental lithosphere on the surface expression of volcanism over a hotspot. Together with the much better defined hotspot tracks comprising the Tasmantid and Lord Howe Seamounts, these essentially parallel volcanic chains record passage of the Indo-Australian plate across their respective magma sources situated within the sublithospheric mantle. The age-progressive central volcano provinces have dominated volcanic activity in eastern Austrdia over most of the last 35 Ma. The observed pattem correlates well with onset of rapid spreading between Australia and Antarctica. The rate and direction of Indo-Australian plate motion derived from modelling based upon the assumption that hotspots throughout the world are fixed in the mantle relative to one another, yields a predicted hotspot track for eastern Australia which matches quite well the observed migration path of the central volcano provinces in space and time. Lava field provinces exhibit a diversity of form and size and are widely distributed geographically in eastern Australia. Volcanism is basaltic, dominantly of the alkali basalt association. Within individual provinces, volcanism generally is restricted to an interval of several million years, but in some provinces two or more periods of activity are recognized. No obvious pattem of distribution of lava field provinces with age or geographic position is evident, except that most volcanism occurred prior to the central volcano activity at the same latitude. Modelling of plate motions in the hotspot reference frame indicates that subsequent to initiation of breakup of Gondwanaland in the Mesozoic, the Indo-Australian plate moved only slowly until rapid northward separation from Antarctica began about 40 Ma ago. Much of the lava field volcanism took place in the early Cenozoic in eastern Australia during the latter stages of the opening of the large marginal basin underlying the Tasman Sea, and the opening of the Coral Sea Basin. Uplift of the highlands of eastern Australia also probably was genetically related to the rifting and opening of the Tasman and Coral Sea basins. Although we lack a detailed understanding of the processes that have caused lava field volcanism, it is apparent that regio^nal tensional stresses in the highland region during and subsequent to rifting and uplift facilitated extensive basaltic volcanism in eastern Australia during the Cenozoic. Wellman, P. & McDougall, I. (1974). Cainozoic igneous activity in eastern Australia. Tectonophysics, 23, 49-65. 121

7.12 THE LATE PLEISTOCENE/HOLOCENE PALAEOGEOGRAPHY AND STRATIGRAPHY OF THE MAORI REEF AREA, CENTRAL GREAT BARRIER REEF PROVINCE, AUSTRALIA J.E. Dye Department of Geology, James Cook University of North Queensland

Maori Reef is a modem NE/SW elongate platform reef situated adjacent to Flora Passage on the Central Great Barrier Reef outer shelf (17 06.8'S; 146 20.5'E). Hie post-glacial stratigraphy of the area around Maori Reef has been defined using 3.5 KHz shallow seismic profiles and vibracores. Isopach maps show up to 16m of post-glacial sediment overlie a regional, erosional unconformity (seismic reflector A) formed during the most recent (ca. 18 kybp) glacial sea level low. Maximum sediment accumulations occur ad:)acent to reef platforms and in fluvial palaeo-channels. Post-glacial stratigraphy present in vibracores can be divided into three units: 1) a basal, pretransgressive facies (below seismic reflector A) conposed of dark brown, orqanic-rich, silty clays (kaolinite and illite); unconformably overlam by, 2) a transgressive shallow marine facies composed of dark grey, bioclastic and quartzose terrigenous sandy muds (kaolinite, illite and smectite); grades upward to, 3) a post-transgressive reef-derived carbonate sand and mud facies. Palaeogeography of the outer shelf is dominated by two major feati^es: 1) sub-reef platforms forming topographic highs; and, 2) fluvial chamels which incise the palaeo-coastal plain. The sub-reef platforms have a st^per face on one side and a more gentle, comraonly terraced, slope on the o ^ ^ . Narrow, fluvial palaeo-channels meander across the featureless mid-shelf, coalescing onto the outer shelf and diverging around reef platforms (Figure 1) Detailed mapping of palaeo-channel systems show that Noggin Passage, a modem cross-shelf passage, is located along the course of a palaeo^annel system, and could have been maintained by fluvial erosion during the ^ level low. However, Flora Passage, located adjacent to Maori Reef, contains no palaeo-channels, and therefore cortprises part of the regional mter-reet drowned terrestrial plain.

Figure 1: Location map of fluvial palaeo-channels in the Maori Reef area. 122

15.9

PETROLOGY AND GEOCHEMISTRY OF TASMANTID SEAMOUNTS S.M. Eggins Department of Geology, University of Tasmania

Basaltic samples dredged from 4 Tasmantid Seamounts range in composition from olivine tholeiite and transitional, to alkali basalt/hawaiite. They are geochemically and mineralogically similar to other ocean island basalts, and share petrological features with contemporaneous east Australian volcanism. Not all variation in major and trace element characteristics within, and between, individual volcanoes can be attributed to fractionation processes, as illustrated by incompatable trace element ratios dominated by partial melting(e.g. Zr/Nb) and source abundances(e.g. La/Nb;see Fig.l). These features may be resolved by variable degrees of partial melting of a heterogeneous sub-Tasman Sea mantie. Additional complexity is evident in the form of moderately aluminous pyroxene xenocrysts, indicating moderate pressure evolution of some transition^ and hawaiitic compositions. Furthermore, variable Mg# of orthopyroxene xenocrysts(65-85) in individual samples suggests complex origins involving variously fractionated magma batches. Primitive, glassy, ol+chrome spinel+-plag phyric basalts(Mg#>66, Ni>250ppm) from Gascoyne, Derwent Hunter and Britannia seamounts are primary magma candidates. Their major and trace element abundances closely match those of Hawaiian shield-building tholeiites, in particular REE profiles characterised by moderate LREE enrichment (La/SmcN 1.45-2.37) and fractionated middle to HREE(Gd/YbCN L92-4.06:see Fig.2). As in the Hawaiian case, modelling of partial melting systematics using REE data indicate melt extraction leaving garnet and clinopyroxene in the residuum. However, experimental studies of pertinent ol-tholeiite compositions place melting at 8-20kbar, leaving spinel harzburgite and Iherzolite residues. Equilibrium with a garnet-bearing assemblage can only be reconciled if these erupted basalts have fractionated from highly magnesian picrites (Mg#>78), with liquidus olivine >Fo92. Little direct evidence exists for such liquids, either as whole rock conipositions or as magnesian olivine phenocrysts. Maximum Fo content in Tasmantid olivines is Fo 85.5, while Hawaiian olivines rarely exceed Fo 88. The necessity for such highly magnesian parental compositions may be circumvented if the sub-Tasman Sea mantle was enriched in middle REE relative to HREE prior to melt generation. Migration of a small melt fraction from deeper lying garnet Iherzolite would provide a suitable mechanism for this enrichment. This process must be both pervasive and conmion to all sub-oceanic mantle.

7.5

ZR/NB

12.5

Fig. 1. La/Nb versus Zr/Nb systematics of Tasmantid basalts: o - Gascoyne, + - Derwent-Hunter, O - Taupo, * - Britannia. 123

La C« Pr Nd

Sm tu Gd Tb Dy Ho Cr Tm Yb Lu

Fig.2. REE profiles of selected primitive Tasmantid basalts: o - Gascoyne, + - Gascoyne, * - Derwent-Hunter, O - Britannia.

3.8

LOW PALAEOLATITUDES FOR LATE PRECAMBRIAN PERIGLACIAL DEPOSITS IN SOUTH AUSTRALIA B.J.J. Embleton CSIRO Division of Mineral Physics and Mineralogy, North Ryde

Periglaciai clastic varvites of the late Precambrian Elatina formation yield a magnetic signature, acquired approximately at the time of deposition, inconsistent with classical models of circum-polar glaciations. The contradiction of very cold climate near sea level in low palaeolatitudes during the late Precambrian brings into question basic tenets in palaeomagnetism and celestial mechanics• The inferred palaeolatitude of the sampling locality is 5°. Critical to the interpretation of the magnetisation contained in the laminated facies is the fact that it is wholly consistent with results from other" rock units of similar age from different geological provinces in Australia displayed in Figure !•

Figure 1. Palaeomagnetic pole positions (large dots), group averages (small dots and circles of confidence) and the apparent polar wander path (arrows) for the late Precambrian through Cambrian with respect to Australia. 124

KEY: WV, Wooltana Volcanics; NI, Lower Arumbera Sandstone; MI, Upper Arimbera Sandstone; RI, Todd River Dolomite; MTl, Merinjina Tillite; AFl, Angepina Formation; NDD, Northampton Dykes; EF, Elatina Formation (this stidy); BRK, BRM, Brachina Formation; TF, Tempe Formation; IS, Illara Saidstone; DF, Deception Formation; KI, Kangaroo Island; LFG, Lake Frome Group; DCF, Billy Creek Formation; HQ, Hawker Group; GCL, GCU, Giles Creek Dolomite; SF, Shannon Formation; HRS, Hugh ' River Shale; HF Hudson Formation; The occurrence of ice sheets and a periglacial climate near sea-level in low palaeolatitudes is a major enigma. The most obvious explanation of low latitude glaciation in late Precambrian time is that the Earth was then glaciated from pole to pole during a very severe ice-age. This model of global glaciation, however, is inconsistent with numerous observations. Two other possible explanations include (i) the axial geocentric dipole model of the Earth's magnetic field is invalid for that time interval, and (ii) the obliquity of the ecliptic was considerably increased (> 54®). The high obliquity model appears favoured.

125

2.3

RAMP BASINS - A NEW CLASS OF SEDIMENTARY BASIN IN EXTENSIONAL SETTINGS M.A. Etheridge Bureau of Mineral Resources, Geology and Geophysics, Canberra

The concept of a ramp syncline was introduced by Gibbs [1984], who pointed out that where a low-angle normal fault has a ramp-flat geometry similar to that developed in thrust terranes, a syncline will develop above the ramp as the hanging wall of the fault is pulled off the upper flat. Ramp synclines are therefore corollaries of ramp anticlines in thrust terranes. Since they are primarily upper crustal features, they may have a surface expression, giving rise to potentially important depositional traps called ramp basins. Gibbs [1984] and Coward [1986] briefly alluded to ramp basins, but both considered that they would be relatively minor features related to small scale ramps and flats on high level detachment faults. However, recent application of the detachment model of continental extension to passive continental margins by Lister et al.[1986; in press] concluded that crustal scale detachments may have ramps that extend for tens of km or more down dip with heights of more than ten km. If so, ramp basins of similar scale might be expected, and they could be major depositional features within extensional terranes, especially on passive continental margins. Gibbs [1987] referred to these basins as 'hanging wall basins', and described a possible example (Beryl Embayment) from the Viking Graben of the North Sea. The main features of a ramp basin and its fill are:1) The maximum depth of the basin is related to the height of the ramp, and ramp basins achieve maximum depth after the displacement on the detachment is equal to the width of the ramp. Consequently, ramp basins above major crustal detachment faults that ramp from the mid-crust to the Moho [Lister et al., in press] may be 10 km or more deep, depending on the flexural strngth of the thinned lithosphere and the availability of sediment fill. 2) The final width of a ramp basin is approximately the same as the total amount of displacement on the controlling fault, although it is limited by the horizontal extent of the upper flat. The width of the depocentre at any instant is equal to the horizontal length of the ramp, and the rate of subsidence is controlled by the slope of the ramp. 3) The basin fill is distinctly asymmetric and progrades from the 'downslope' side of the ramp towards the head of the detachment fault. However, at any instant, the basin depocentre (as distinct from the earlier fill) is approximately symmetrical and directly above the ramp. 4) Unlike the half graben that are characteristic of extensional basin formation, there need be no major faults within the ramp basin fill. Hpwever, the ability of the upper plate basement to the ramp basin to remain coherent as it pulled down the ramp depends on its shear strength and the shear resistance on the detachment.

126

The influence of ramp/flat geometry and the amount of fault displacement on the final shape of the basin and its fill will also be discussed. The combination of a shorty shallowly dipping ramp and large fault displacement leads to a broad, thin 'crustal downwarp'. At' the other end of the spectrum is the deep, narrow trough that overlies a long, steep ramp on which the displacement has been relatively small. The shape of a ramp basin in the third dimension depends on the along-strike geometry of the master detachment fault. Variations along the strike of normal faults are commonly accomplished on steeply dipping transfer faults [Gibbs, 1984; Etheridge et al., 1985], although more shallowly dipping lateral ramps analogous to those in thrust terranes may also be present. Where transfer faulting is dominant, sudden variations in the position and/or shape of the ramp basin may result, and the transfer fault buondaries may form scarps which shed coarse sediment into the ends of the basin. References Coward, M.J. 1986. Heterogeneous stretching, simple shear and basin development. Earth and Planetary Science Letters, 80, 325-336. Etheridge, M.A., Branson, J.C. and Stuart-Smith, P.G. 1985. Extensional basin-forming structures in Bass Strait and their importance for hydrocarbon exploration. Australian Petroleum Exploration Association Journal, 25, 344-361. Gibbs, A.D. 1984. Structural evolution of extensional basin margins. Journal of the Geological Society of London, 141, 609-620. Gibbs, A.D. 1987. Development of extension and mixed-mode sedimaentary basins. In: Continental extension tectonics. Coward, M.P., Dewey, J.F. & Hancock, P.L. (eds), Geological Society Special Publication, 28, 19-33. Lister, G.S., Etheridge, M.A. and Symonds, P.A. 1986. Application of the detachment fault model to the formation of passive continental margins. Geology, 14, 246-250. Lister, G.S., Etheridge, M.A. and Symonds, P.A. In press. Detachment model for the formation of passive continental margins. Tectonics.

127

3.5

T E C T O N I C C O N S T R A I N T S O N P A L A E O M A G N E T I C M O D E L S FOR THE EARLY TO MIDDLE PROTEROZOIC M . A . Etheridge and L.A.I. Wyborn B u r e a u of M i n e r a l R e s o u r c e s , Geology and Geophysics, Canberra

T h e k e y q u e s t i o n c o n c e r n i n g t h e E a r l y to M i d d l e P r o t e r o z o i c t e c t o n i c e v o l u t i o n of A u s t r a l i a , e s p e c i a l l y from the v i e w p o i n t of a s s e s s i n g p a l a e o m a g n e t i c d a t a , is w h e t h e r ; - t h e v a r i o u s p r o v i n c e s d e v e l o p e d l a r g e l y on A r c h a e a n s i a l i c b a s e m e n t in t h e i r p r e s e n t r e l a t i v e l o c a t i o n s , without significant continental fragmentation by sea-floor spreading (e.g., R u t l a n d , 1973, 1976; E t h e r i d g e et a l . , 1 9 8 7 ) , o r - t h e c o n t i n e n t e v o l v e d b y l a t e r a l a c c r e t i o n o f t e r r a n e s to A r c h a e a n n u c l e i , w h i c h f r a g m e n t e d in one o r m o r e p e r i o d s of plate m o t i o n similar to modern tectonic settings (e.g., H o f f m a n , 1980; W i n d l e y , 1 9 8 1 ) . The p r e s e n t g e o l o g i c a n d p a l a e o m a g n e t i c d a t a b a s e s b o t h support the former view, although neither dataset is conclusive (Etheridge et a l . , 1987; I d n u r m a n d G i d d i n g s , in p r e s s ) . This p a p e r o u t l i n e s t h e e v i d e n c e f o r t h e t e c t o n i c s e t t i n g o f the A u s t r a l i a n E a r l y to M i d d l e P r o t e r o z o i c p r o v i n c e s . T h e r e is a h i g h degree of c o n s i s t e n c y in t h e style a n d t i m i n g of t e c t o n i s m a c r o s s t h e A u s t r a l i a n c o n t i n e n t , e s p e c i a l l y in t h e Early Proterozoic. The t e c t o n i c e v o l u t i o n is c h a r a c t e r i s e d b y d i s c r e t e e v e n t s , w h i c h are r e c o g n i s e d in s e v e r a l p r o v i n c e s . 1) F r o m t h e e n d of the A r c h a e a n (2500Ma) t o about 2 0 0 0 M a , there was a p e r i o d of t e c t o n i c i n a c t i v i t y . 2) A m a j o r p e r i o d of b a s i n f o r m a t i o n , a p p a r e n t l y t r i g g e r e d b y w i d e s p r e a d e x t e n s i o n of the p r e - e x i s t i n g A r c h a e a n c r a t o n ( s ) , is r e c o g n i s e d a c r o s s m u c h of t h e c o n t i n e n t b e t w e e n a b o u t 2000Ma a n d 1 8 7 0 M a . S i m i l a r t e c t o n o s t r a t i g r a p h i c sequences of t h e s a m e a g e h a v e b e e n r e c o g n i s e d in t h e G a s c o y n e , K i n g Leopold, Halls Creek, Pine Creek, Granites/Tanami, Arunta, T e n n a n t C r e e k , M o u n t Isa a n d G a w l e r p r o v i n c e s . Basin formation is i n f e r r e d t o h a v e b e e n p r e c e d e d and/or accompanied by widespread crustal underplating by enriched m a f i c m a g m a s . This m a f i c u n d e r p l a t e p r o v i d e d the source for m u c h of the subsequent felsic m a g m a t i s m , a n d is r e f l e c t e d in the w i d e s p r e a d S m / N d signature of about 2000Ma t h r o u g h o u t the E a r l y to M i d d l e P r o t e r o z o i c (McCulloch, 1 9 8 7 ) . 3) T h i s p e r i o d of b a s i n f o r m a t i o n was t e r m i n a t e d b y a b r o a d l y s y n c h r o n o u s o r o g e n i c e v e n t t h a t is w e l l d a t e d in m o s t p r o v i n c e s at b e t w e n 1880 a n d 1850Ma ( B a r r a m u n d i O r o g e n y of E t h e r i d g e et a l . , 1 9 8 7 ) . The o r o g e n y was a c c o m p a n i e d b y i n t r u s i o n a n d e x t r u s i o n of a v o l u m i n o u s felsic igneous suite o f r e m a r k a b l y u n i f o r m c h a r a c t e r from p r o v i n c e to p r o v i n c e (and i n d e e d g l o b a l l y ) . A l t h o u g h of b r o a d l y c a l c a l k a l i n e c h a r a c t e r , t h i s (Barramundi) suite is q u i t e d i s t i n c t f r o m modern arc-derived suites ( W y b o r n , in p r e s s ) , a n d is i n t e r p r e t e d to have b e e n d e r i v e d by p a r t i a l m e l t i n g of the 2000Ma u n d e r p l a t e . 4) T h e r e w a s s o m e w h a t less c o n s i s t e n c y o f t e c t o n i c e v o l u t i o n b e t w e e n the v a r i o u s p r o v i n c e s a f t e r the B a r r a m u n d i o r o g e n y . H o w e v e r , v o l c a n o - s e d i m e n t a r y s e q u e n c e s of b e t w e e n 1860 a n d

128

1750Ma are present in most provinces, and there is again a strong petrogenetic similarity between igneous rocks of apparent continental rift affinity and of the same age in widely separated provinces (Wyborn et al., in press). The time of termination of orogenic activity varied widely, ranging from about 1840Ma in some of the northern and westernmost provinces to about 1450Ma in Mount Isa. The final phases of tectonism were commonly accompanied by strike slip faulting, which was rare in earlier events. Low pressure metamorphic facies were developed throughout. 5) The final part of the Early to Middle Proterozoic was marked, like the beginning, by relative inactivity, especially between about 1400 and 1200Ma. This inactivity preceded another major period of basin formation, the Adelaidean. Despite the widespread occurrence of extensional basins similar to those on modern passive continental margins, and of thrust style tectonics, there is very little evidence for large relative horizontal motions between Australian Early to Middle Proterozoic provinces. We suggest that the Australian Archaean craton remained virtually intact, although subject to local stretching and shortening, throughout the intense period of tectonic activity between about 2000 and 1400 Ma. Apart from the consistency of style and timing of tectonism between provinces, the 'fixist' view is supported by (a) absence of ophiolitic sequences, (b) presence of Archaean ensialic basement in some provinces, (c) paucity of igneous suites of modern arc affinity, (d) predominance of broad, shallow sedimentary basins, and (e) almost ubiquitous low pressure prograde metamorphism. References Etheridge, M.A., Rutland, R.W.R. and Wyborn, L.A.I. 1987. Orogenesis and tectonic process in the Early to Middle Proterozoic of Northern Australia. In: Proterozoic lithospheric evolution. Kroner, A. (Ed). 131-147. Hoffman, P.F. 1980. Wopmay Orogen: A Wilson cycle of early Proterozoic age in the northwest of the Canadian shield. In: The continental crust and its mineral deposits, Strangway, D.W. (Ed), Geological Association of Canada Special Paper, 20, 523-549. Idnurm, M. and Giddings, J.W. In press. Australian Precambrian polar wander: A review. Precambrian Research. McCulloch, M.T. 1987. Sm-Nd constraints on the evolution of Precambrian crust in the Australian continent. In: Proterozoic lithospheric evolution, Kroner, A. (Ed) American Geophysical Union Geodynamics Series 17, 115-130. Rutland, R.W.R. 1976. Orogenic evolution of Australia. Earth Science Reviews, 12, 161-196. Windley, B.F. 1981. Precambrian rocks in the light of the plate tectonic concept. In: Precambrian plate tectonics. Kroner, A. (Ed), Elsevier, Amsterdam, 1-20. Wyborn, L.A.I. In press. Petrology, geochemistry and origin of a major Australian 1840-1880Ma felsic volcano-plutonoic suite. Precambrian Research. Wyborn, L.A.I., Page, R.W. and Parker, A.J. In press. Geochemical and geochronological sigr^atures in Australian Proterozoic igneous rocks. In: Geochemistry and mineralization of Proterozoic Volcanic suites, Pharaoh, T.C., Beckinsale, R.D.and Rickard, D.T. (Eds). Geological Society Special Publication, 377-394.

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17.5

PETROGENESIS OF THE EASTERN AUSTRALIAN CAINOZOIC VOLCANIC PROVINCES A. Ewart^,

B.W.

Chappell^

and M .

Menzies^

^'Department of Geology and Mineralogy, University of Queensland ^Department of Geology, Australian National University, Canberra ^University of London, UK A broad zone of intra-plate anorogenic volcanicity has occurred for some 3000 km along E. Australia. The provinces are dominated by mafic lavas which have D.I. Between 25-45 and mg ratios 30-72. Regional differences in overall magma compositions are evident, but are dominated by hawaiites (337o of all analysed lavas). Compositions include leucitites, melilitites, nephelinites, analcimites, basanites, hawaiites, alkali and transitional basalts, and various tholeiites. Differences of chemistry exist between the central-type volcano provinces, and the lava-field provinces; the former contain the majority of the intermediate to silicic evolved magma compositions, but their mafic lavas are also generally more evolved than equivalent mafic lavas of the lava fields. Evolved lavas include mugearites, benmoreites, peralkaline and non-peralkaline trachytes, phonolites, icelandites, comendites, low-silica and high silica-potassic rhyolites. Intermediate to silicic lavas exhibit progressively Fe-enriched Fe-Mg silicate assemblages. Trace element geochemistry is very variable, ranging from relatively incompatible element depleted Q-tholeiites to the highly enriched leucitites of central N.S.W. Gross heterogeneities must exist in the E. Australian lithosphere, but it is apparent that the majority of mafic lavas have been modified by fractional crystallisation processes, involving olivine + augite + plagioclase + Fe-Ti oxides. The critical role of fractional crystallisation is even more apparent in the trace element geochemistry of the evolved lavas. Isotopically, the lavas are again variable; for the mafic lavas, 206 Pb/204 Pb varies from 17.48 19.66; + 7.2 to -2.7; 0.7026 - 0.7055. MORB-like compositions occur in Tasmania and isolated nephelinites elsewhere. Data for the central volcano provinces tend to exhibit more radiogenic The data also exhibit broad Sr, lower S ^ ,, and less radiogenic Pb. correlations between, for example, Ba/Th and Ba/Nd ratios, and Sr, Nd, and Pb isotopic compositions. AFC processes can explain such correlations, but involvement of Palaeozoic arc-type lithosphere is also indicated. The various trachytic and rhyolitic lavas exhibit -1 to -4.8, with rhyolites especially having relatively radiogenic Sr.

130

20.2

OCEAN DRILLING ON THE EXMOUTH PLATEAU AND ARGO ABYSSAL PLAIN IN MID-1988

N.F. Exon^, U. von Rad^, P.E. Williamson^ and R. Boyd^ ^Bureau of Mineral Resources, . Geology and Geophysics, Canberra ^Bundesanstalt fur Geowissenschaften und Rohstoffe, Hanover, West Germany Soalhousie University, Halifax, Canada

BMR and company seismic data, and company drilling on and near the Exmouth Plateau, have .supported very strong proposals for two six-week Ocean Drilling Program legs which will take place in 1988. The holes will be around 1000 m deep, continuously cored and wireline logged. It is probable that five holes will be drilled on the Exmouth Plateau and one on the Argo Abyssal Plain (Figures 1 & 2). The drilling program will provide detailed information on the geological history of an area of considerable resource potential, and some Australians will participate. The central Exmouth Plateau sites (EP6 and 7) are designed to (1) refine the global sea level curve, and (2) study the palaeooceanography and sedimentation history of the Late Jurassic to Cainozoic sequences on this starved continental margin. The western Exmouth Plateau site (EP2), in water 4000 m deep, is near the ocean-continent boundary, and will test subsidence and stretching models for rift and subsequent continental margin evolution, by drilling basement, Mesozoic and Cainozoic sequences. EP12, an alternative site to EP2 in shallower water, is designed to elucidate the geological history of the western margin above a rotated fault block. The northern Exmouth Plateau sites (EP9, EPIO) are designed to examine (1) Early Mesozoic pre- and syn-rift sedimentation, facies, palaeobathymetry and subsidence history, (2) Late Jurassic to Cainozoic breakup and post breakup development on a stretched passive margin, (3) correlation of unconformity-bound sequences with sedimentation, tectonics, and global sea level fluctuations. The objectives of the Argo Abyssal Plain drilling (AAPl) are (1) to study Late Jurassic to Early Cretaceous eastern Tethys palaeo-circulation and palaeo-ecology, (2) to date abyssal plain magnetic anomalies, (3) to study the subsidence history.

EXMOUTH

PLATEAU EAST

ARCH

SOITTH

DFFLOWAGE ' Oceanic crust ctoi Mature O c e a n

Mid C r e t a c e o u s m a r i n e detrital sediments

B r e a k u p m south

Figure 1: Schematic cross-sections showing proposed ODP drilling sites.

E a r l y C r e t a c e o u s d e l t a i c sediments

B r e a k u p m north Rilling

131

A g e a n d lithology

SB B L a t e C r e t a c e o u s a n d Cainozoic m a r i n e c a r b o n a t e s

Jurassic c o a l m e a s u r e s a n d c a r b o n a t e s

Tnassic sandstones a n d shales

!Z1 0Q* 5cft to ft r X tJ o o* >-l p p o" 3 o c/> 3 O * "O ST 3 — •o P o CA s ft a a. *-i ft m ft X o 3 o o" c 3 Er l/l 2. 2 U5* p* 3 ft a' Pc a p O p a

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ST o

C/i

3 n ft P* o' 3 C/5 ET •a' r-t o ar p 3* 3m •-i x T3 ft »-i o

Eendracht I

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^.Scarborough 1

0 Investigator 1 Vinck 1

ABYSSAL

PLAIN

q Angel 1

/RankifTT # North Rankin 1 / / / 0 \^"M"Goodwyn 1 Saturn 1 )//\Malus 1 * 0 Dampier 1 0 North Tryal Rocks 1 /<//vSultan 1 O V / M West TrYal Rocks 1 (§ 0 Tryal Rocks 1 ^ I r, /? JI /f JX -i y a Barrow Island Y

Zeewult 1

CUVIER

" 0 Depuch 1 Picard 1 / / 0 Brigadier^ ^ p Ronsard 1 Gandara \0 ^ ^ ^Sable 1 J J

0 Mercury 1 TJupiter 1

EP2Bi

rv C 3

ABYSSAL

ARGO

Port Hedland

100 km

Edge of detailed company seismic grid Hilda 1A

Multichannel R.V. Rig Seismic seismic profiles

test Muiron 1

Detailed ODP site surveys by R.V. Rig Seismic and R.V. Sonne

Exmouth

2 3/0 W A/71

13.4 SEDIMENTOLOGY OF THE BRASSALL SUBGROUP - A COMPARISON OF SANDSTONE AND MUDSTONE-DOMINATED COAL MEASURE SEQUENCES A. Falkner Department of Geology and Mineralogy, University of Queensland

The Brassall Subgroup is the upper unit of the late Triassic Ipswich Coal Measures, a 1200m thick sequence that accumulated in alluvial plain environments within the intermontane Ipswich Basin, southeast Queensland. The West Ipswich fault, a major structural feature that trends north-northwest marks the western edge of the Ipswich Basin and the Palaeozoic D'Aguilar and Beenleigh Blocks bound it to the north and southeast respectively. The extent of the basin is uncertain with possible equivalents known from the subsurface to the south in New South Wales and to the east offshore from Moreton Island. The Ipswich Coal Measures have an abundant pteridosperm dominated macroflora and a microflora that indicates a Carnian age. The Kholo Subgroup, the lower unit of the Ipswich Coal Measures is about 350m thick and consists predominantly of conglomerates with minor sandstones, mudstones and volcanics that accumulated in alluvial plain environments. The Brassall Subgroup overlies the Kholo Subgroup conformably and comprises the Tivoli and overlying Blackstone Formations. Both the Tivoli and Blackstone Formations have been worked in opencut and underground mines and have been penetrated by numerous Geological Survey of Queensland boreholes. The coal seams of the Brassall Subgroup are high volatile bituminous, with a high ash ( 1 0 - 3 0 7 o ) , low sulphur (O.37o) contents, that commonly split, coalesce and contain numerous intraseam clastic bands. Coals of the two formations are similar petrographically and are used largely for power generation. The sedimentology of the two units show a marked contrast: the Tivoli Formation is dominated by coarse grained pebbly sandstones which comprise over half the compacted volume of the unit while sandstones make up only one third of the Blackstone Formation. This difference of sedimentary style has a profound effect on the thickness, areal extent, continuity and quality and of associated coal seams. The Tivoli Formation is approximately 480m thick in the North Ipswich area where it is poorly exposed and no longer mined. It is penetrated by about 120 boreholes and can be divided into two parts: a lower section 160m thick that consists predominantly of mudstones and contains a number of poor quality coal seams. The upper section is 320m thick and contains sandstones, mudstones and seven coal seams that vary from 1 to 9m and average 4m in thickness. The Blackstone Format ion is about 300m thick and is currently worked in the Bundamba area in numerous opencut mines which provide excellent exposure. The unit is penetrated by over 350 boreholes and contains seven coal seams that average 6m in thickness and vary from 3 to 10m. A facies model that integrates data from interseam, organic seam and clastic interseam sediments suggests that the Blackstone Formation accumulated in floodplains, laterally migrating high sinuosity river channels and floodplain-hosted peat-mires (Falkner, 1986). This model is supported and amplified by facies maps for 13 intervals within the unit and by cross-sections that elucidate the depositional and subsidence history. Vertically stacked channel sequences and the preferential

133

occurrence of channel sandstones above thick coal seams suggests sediment accumulation was influenced by both structurally and compaction controlled subsidence, respectively. The Tivoli Formation in comparison appears to have accumulated in an alluvial plain setting similar to the Blackstone Formation. However, river channel sandstones are more abundant in the Tivoli Formation and floodplain and peat-mire sediments are relatively poorly represented in comparison. Although channel sandstones in the Tivoli Formation are laterally extensive they do not show evidence of lateral accretion. The coal seams of the Tivoli Formation are thinner, less laterally extensive and more commonly contain washouts and intraseam clastic bands than those of the overlying unit. The Brassall Subgroup accumulated in an alluvial plain setting within river channel, floodplain and floodplain-hosted peat-mire environments. The contrast of depositional style between the Tivoli and Blackstone Formations has had a profound effect on the nature of the coal seams: the Tivoli Formation is dominated by channel sandstones and as a result the coal seams are generally less economically viable. The implications for coal exploration and mine planning are that alluvial plain sequences dominated by overbank sediments constitute more prospective targets than those where channel sandstones predominate. Reference FALKNER, A.J. 1986. Sedimentology of the Blackstone Formation, Ipswich Coal Measures, southeast Queensland. Ph.D. Thesis, University of Queensland (unpubl.).

134

13.6 SEDIMENTARY ENVIRONMENTS O F THE M I D D L E JURASSIC W A L L O O N COAL MEASURES IN THE ROSEWOOD-WALLOON COALFIELD, SOUTHEAST QUEENSLAND C.R.

Fielding

De^partment of Geology and Mineralogy, University of Queensland The Middle Jurassic age Walloon Coal Measures and equivalents form a part of the scdhnontary fill of the Mesozoic Great Artesian Basin in eastern Australia. The type area for the unit is the Rose wood-Walloon coalfield, some 80 km west of Brisbane, which lies in the eastern part of the Clarence-Moreton Basin. In this area some 200 m of Walloon Coal Measures are preserved, truncated by erosion and overlain in places by Tertiary sedimentary rocks and basaltic volcanics. The Walloon Coal Measures are composed of interbedded sandstones, siltstones, claystones and coals, with occasional beds of bentonite (altered volcanic ash). Eighteen persistent coal seams have been recognised within the sequence, after the work of Cameron (1970). These coals were formerly the focus of a thriving mining industry in the area. Present mining activity, however, is restricted to one underground (Oakleigh No. 3) and three opencut mines (Oakleigh, Jeebropilly, Ebenezer). The Coal Measures outcrop poorly, as a consequence of their labile mineralogy and intense, recent, sub-tropical weathering. Natural exposures are therefore rare and of poor quality. The present study has concentrated on opencut mine exposures and borehole cores. Petrographic examination of fresh, unweathered specimens reveals a dominantly volcaniclastic mineralogy throughout the formation. Most sandstones may be classified as volcanic litharenites (Folk, 1968) or lithic arenites (Dott, 1964). This contrasts starkly with the dominantly quartzose/feldspathic character of the underlying Marburg Sandstone Formation, and offers a more geologically meaningful and practically recognisable boundary criterion than the current boundary definition ("the top of the uppermost bed of coarse pebbly sandstone or pebble conglomerate" - Cameron, 1970). A detailed f a d e s analysis has been undertaken to ascertain the depositional environments of the Walloons, in the type area. Palaeontological data (specifically an abundance of plant macrofossils and several occurrences of dinosaur footprints), the absence of marine fossils and the apparent intracontinental nature of the Great Artesian Basin, strongly suggest a continental setting. The following f a d e s have been recognised:Assodation

Interpretation

Fades

Major alluvial channel A1

- sharply-based fine-grained cross-bedded sandstone units 20 m thick with shoestring geometry. Associated levee deposits.

Floodbasin

- medium grey, laminated claystones - oolitic ironstone (rare) - sharply-based, fine-grained sandstone, beds 1 m thick, confined to channels lO's m wide, becoming sheet-likfi downpalaleocurrent. - Crudely coarsening upward, interbedded siltstone and very fine/fine-grained sandstone (several m thick)

B1 - Quiet-water lake B2 - Lake B3 - Crevasse channel/ splay

B4 - Minor delta

135

B5 - Major channel levee

Swamp C I - Swamp peat

- poorly laminated siltstones, becoming interlaminated with very fine/fine-grained sandstone laterally - Bituminous coal, carbonaceous shale, dark grey claystones. - Montmorillonite-rich claystone/siltstone beds^O.7 m, sheet like

C2 - Ash fall

Detailed analysis of organic sediments (coals, carbonaceous shales) has revealed a number of unusual characteristics. The coals are thinly banded on a 5-50 cm scale, with considerable interlayering of coal plies and fine-grained clastic sediments. Microscopic interlayering is also prominent. The coal itself is highly variable in character both vertically and laterally, though individual plies are traceable for W s to lOO^s m laterally. Interbedded within some seams are bentonitic claystone/siltstone beds up to 70 cm thick which are interpreted as diagenetically altered volcanic ash falls. Coal plies enclosing these beds commonly contain ovoid pellets of kaolinite and smectite/illite-smectite up to 3 cm in diameter, suspended within intermediate lustre coal. The facies assemblage is interpreted as the deposits of an extensive alluvial basin, possibly draining inwardly towards the central Australian depocentre of the Great Artesian Basin. Alluvial channels of as yet uncertain sinuosity drained the Basin, and were separated by substantial flood basin and swamp tracts. Vertical sequence analysis of core data and invoking Walther^s Law suggests that at any time peat-forming environments accounted for some 5-10% of the basinal area. Volcanic ash periodic^ly fell across the area, and was preserved in swamp environments, protected from dilution by clastic detritus. The interval is unusual in two respects - 1. in the common development of minor mouth bar-style lacustrine delta deposits, which are unusual in alluvial coal basins, though prominent in delta plain systems (cf. Fielding, 1984), and 2. in their enormous lateral extent, orders of magnitude larger than other described (intracontinental) alluvial coal basins. Seams of coal are individually up to about 2 m in thickness, though locally two seams may coalesce to form more substantial deposits. Coals show moderate ash contents (18-25% inherent) low sulphur (0.5%) and very weakly coking character. They are classed as high volatile bituminous, per-hydrous, canneloid, low rank coals, suitable for steam-raising, gas generation and coal-to-oil conversion (Queensland Coal Board, 1986). On a regional basis the Walloon coals show a prominent tendency to split into two component seams and elsewhere rejoin with the original or another seam. This splitting character is suggestive of a differential compaction control on peat swamp evolution, and ultimately on coal geometry (Fielding, 1987). References CAMERON, J.B. 1970: The Rosewood-Walloon Coalfield. Geological Survey of Queensland, Publication 344, 34 pp. DOTT, R.H. 1964: Wacke, greywacke and matrix - what approach to immature sandstone classification? J. Sedim P e t r o l . , ^ , 625-632. FIELDING, C.R. 1984: Upper delta, plain. Lacustrine and fluviolacustrine facies from the Westphalian of the Durham coalfield, NE England. Sedimentology, 31, 547-567. FIELDING,

C.R.

1987:

Coal depositional

models for deltaic

and alluvial plain

sequences. Geology, J^, 661-664. FOLK, R.L. 1968:

Petrology of Sedimentary Rocks.

Austin, Texas, Hamphiirs Book

Store, 170 pp. QUEENSLAND COAL BOARD, 1%6: Queensland Coals - typical physical ^n^ ^^erm^^^^ properties and classification. Brisbane, Qld, Queensland Coal Board (7 th Edition) 55pp.

136

7.19 A FACIES A N A L Y S I S OF THE STAIRCASE SANDSTONE MEMBER ( E A R L Y P E R M I A N ) I N T H E S P R I N G S U R E A R E A , S O U T H W E S T E R N BOWEN B A S I N

C.R. Fielding^ and

B.C.

Lang^

^Department of G e o l o g y and M i n e r a l o g y , U n i v e r s i t y of Q u e e n s l a n d ^Queensland Department of Mines

The Staircase Sandstone Member (SSM) of the Cattle Creek Formation forms a part of the sedimentary fill of the Denison Trough, an elongate, fault-controlled sub-basin of the Permian/Triassic Bowen basin. The SSM ranges up to 250 m in thickness and is a relatively coarse-grained clastic sedimentary unit enclosed by thick claystone/siltstone intervals. The latter contain marine microfossils, and parts of the SSM contain brachiopods and bivalves suggesting a marine origin. On the other hand, preservation of rootlets in situ at several levels within the SSM indicates a periodically emergent setting, of perhaps coastal aspect. The present study involves detailed examination of road cut exposures on the Dawson Highway east of Springsure, and of continuous core from G S Q Stratigraphic Boreholes drilled in the same area. Palaeocurrent data from outcrop suggest bipolar sediment transport directions, oriented east-west, i.e. perpendicular to the nearby western margin of the Denison Trough. Wave-generated structures are similarly oriented. These facts, together with the interbedded lithological character of the SSM, suggest that it formed as a tidally-influenced coastal complex. Detailed facies analysis of road cuts and cores has allowed identification of two major facies associations, as follows: ASSOCIATION A - INTERTIDAL FACIES A1 (Tidal Channel Fill)

Erosively based, fine to coarse-grained sandstone with small pebble basal lags, unimodal to bimodal - bipolar trough and planar cross-bedding, occasional large-scale convolute bedding (water escape) structures. Mud drapes, rare wave ripples, scattered bioturbation.

FACIES A2 (Tidal Lagoon/Flat)

Fine-grained sandstone, and interlaminated siltstone and fine-grained sandstone, current ripple cross-lamination and flat lamination, in situ rootlets, colour mottling, moderate to intense bioturbation.

F A C I E S A3 (Wave/Tide-Influenced Foreshore-Shoreface)

Amalgamated units of mostly medium-grained, well-sorted sandstones showing flat lamination, some trough cross-bedding and hummocky crossstratification (HCS), and wave ripples. Modest bioturbation. Small pebble lags.

FACIES A4 (Transgressive Foreshore-Shoreface)

Similar to above, but intensely bioturbated, particularly around bed boundaries.

ASSOCIATION B - SUBTIDAL F A C I E S HI (High energy Inner Marine Shelf)

firosively-based pebble conglomerate; massive, normally graded or planar cross-bedded, units up to 20 cm thick as basal or top-surface lags.

137

FACIES B2 (High energy Inner Marine Shelf)

Sharply-based, mostly medium-grained sandstones showing abundant amalgamated HCS & flat/low angle lamination, some trough cross-bedding, wave ripples, small pebble lags and isolated pebbles, and occasional escape burrows.

FACIES B3 (Low Energy Marine Shelf)

Siltstones with fine and very fine-grained sandstone, laminae intensely bioturbated.

The identification of sedimentary structures associated with storms and waves in addition to tides suggests that the SSM was deposited under the influence of all these processes in addition to fluvial input. Significantly, coarsest-grained detritus (large pebble, small cobble) is confined to the shelf association, where perhaps only tides and storm currents operating in sympathy were capable of entraining large particles. The Staircase Sandstone Member in the Springsure area is interpreted as the deposits of a tide/wave-dominated deltaic complex which prograded eastward into the northern Denison Trough. The unit displays abundant evidence of the importance of both tides and storm waves/currents in dispersing sediment within the marine shelf environment of the SW Bo wen Basin.

138

2.33 C R U S T A L S T R U C T U R E S A C R O S S T H E NEW E N G L A N D UNDER THE EASTERN SURAT AND CLARENCE-MORETON

FOLD BELT BASINS

D.M. Finlayson^, K.D. Wake-Dyster^, D.W. Johnstone^, M.J, Sexton^ and C.G. M u r r a y ^ ^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Queensland Department of Mines

In 1984-86 the Bureau of Mineral Resources, Geology and Geophysics ( B M R ) recorded an east-west deep seismic reflection traverse from Meandarra to Beenleigh in southeastern Queensland across the N e w England Fold Belt. The traverse crossed the cover rocks of the eastern Surat and Clarence-More ton Basins. Some of the deep seismic events under B M R Line 17 are masked by the near-surface Tertiary volcanics to the west of Toowoomba. Associated with this traverse was a north-south seismic refraction traverse across uplifted and exposed areas of the N e w England Fold Belt from Warwick to Woolbrook, principally across the Late Permian-Triassic granites and volcanics of the Woolomin-Texas Block. Offset refraction shots were also fired at Oakey (near Toowoomba) and in the Hunter Valley (coal mining shots). During the late Palaeozoic, the N e w England Fold Belt was an active continental margin above a west dipping subduction zone. From west to east, parallel belts can be identified which represent continental margin volcanic arc, forearc basin, and accretionary wedge assemblages. The deep seismic reflection line crosses these three belts and also cuts several major tectonic zones of the fold belt:1.

The Goondiwindi-Moonie-Burunga-Leichhardt Thrust, which forms the western boundary of the fold belt with the Bowen Basin. This has been interpreted as a Late Carboniferous transform fault with about 500 km of dextral movement (Harrington & Korsch, 1985).

The northern extension of the Peel Fault, which forms the boundary between forearc basin and accretionary wedge sequences in N e w England.

The Gogango-Baryulgil Fault Zone, a postulated Late Carboniferous-Early Permian transform fault along which at least 500 km of dextral movement was associated with oroclinal bending and repetition of accretionary wedge and forearc basin assemblages (Murray & others, 1987).

The southern extension of the Great Moreton Fault, which may correspond to a shallow east dipping structure on the seismic profile. This structure truncates a west dipping deep layered sequence, possibly the accretionary wedge (Korsch &: others, 1986), and may form the base of the tectonically emplaced Beenleigh and D'Aguilar blocks (Murray &. Lohe, in press). These non-reflecting strata may represent displaced fragments of the accretionary wedge with steep internal dips.

The major features of the seismic data are as follows:1.

There is a clearly defined band of energy at 10.5-12.5 s two-way time ( T W T ) along the traverse which is interpreted as the crust/mantle boundary.

These Moho reflections cross "all the major tectonic boundaries in the region, indicating that they were probably reset since the major tectonic events of the late Palaeozoic.

139

The clear Moho reflecting band with a time thickness of about 1 s T W T (about 3 km) contrasts with the more diffuse reflections at Moho depth under the Devonian basins in the central Eromanga Basin region.

The T W T of 10.5-12.5 s from the reflection profiling is similar to the 11.0-11.2 s T W T determined from seismic refraction shooting across the central WoolominTexas Block where the Moho is modelled at 34 km depth.

The crustal velocity in the central Woolomin-Texas Block is in the range 6.03-6.15 km/s to depths of about 22 km and only slowly increases to 6.45 km/s near the crust/mantle boundary. The upper crustal velocities are 0.1-0.2 km/s lower than those determined for the Lachlan Fold Belt at similar depths. The lower crustal velocities under New England are much less than the velocities at similar depths under the Lachlan Fold Belt.

The crust/mantle transition as determined from both refraction and reflection seismic data is much sharper under New England than under the Lachlan Fold Belt.

The upper mantle velocity of 7.7 km/s under the New England Fold Belt is the lowest yet reliably recorded on continental Australia. There are very few reflections that could undeniably be interpreted as originating from below the crust/mantle boundary.

Reflection events extend throughout the crust, in marked contrast to the pattern under the central Ero manga Basin where an upper crustal "transparent" zone contrasts with strong reflection events in the lower crust.

The character of reflection events under the Taroom Trough and Undulla Embayment contrasts with that to the east. Deep events west of the Moonie F a ^ t are mostly subhorizontal and generally decrease in amplitude and continuity with depth until the crust/mantle reflective zone is reached. Under the New England Fold Belt events tend to be dipping or have an anticlinal form. Some of the latter come close to the surface under the Kumbarilla Ridge (about 0.5 s T W T below the basement horizon).

10.

Under the Burunga and Moonie Faults there are "transparent" zones, perhaps indicators of extreme dislocation of crustal rocks at depth causing defocussing of seismic wavefronts.

This seismic research contributes to the Eromanga-Brisbane Geoscience Transect, an Australian component of the Global Geoscience Transect ( G G T ) Project of the International Commission on the Lithosphere (ICL). References H A R R I N G T O N , H.J., & K O R S C H , R.J., 1985: Tectonic model for the Devonian to middle Permian of the New England Orogen. Australian Journal of Earth Sciences, 32, 163-179. K O R " ^ H , R.J., L I N D S A Y , J.F., O^BRIEN, P.E., S E X T O N , M.J., <5c W A K E - D Y S T E R , K . D . , 1986: Deep crustal seismic reflection profiling, New England orogen, eastern Australia: Telescoping of the crust and a hidden, deep, layered sedimentary sequence. Geology, 14, 982-985. MURRAY, C.G., FERGUSSON, C.L., FLOOD, P.G., WHITAKER, W.G., & K O R S C H , R.J., 1987: Plate tectonic model for the Carboniferous evolution of the New England Fold Belt. Australian Journal of Earth Sciences. 34, 213-236. M U R R A Y , C . G . , <5c L O H E , E.M., in press: Comment. Deep crustal seismic reflection profiling. New England orogen, eastern Australia: Telescoping of the crust and a hidden, deep, layered sedimentary sequence. Geology.

140

8.6

GRANITIC GNEISSES FROM THE ENTIA DOME, EASTERN ARUNTA BLOCK - IMPLICATIONS FOR MAGMATISM IN AN INTRACRUSTAL MOBILE ZONE J. Foden^, I.S. Buick^ and G.E. Mortimer^

^Department of Geology and Geophysics, University of Adelaide ^Department of Earth Sciences, Cambridge University, UK

The Entia Gneiss Complex is an Early to Middle Proterozoic inlier composed of upper amphibolite f a d e s ortho- and para-gneisses in the eastern end of the Arunta Block in central Australia. The Entia Gneiss Complex is separated from overlying, structurally distinct, compositionally different, gneisses of the Harts Range Complex, by a sub-horizontal detachment zone. This zone is partly occupied by the gneissic and partly mylonitised Bruna granite gneiss. The Entia Gneiss complex is characterised by a distinctive, sub-horizontal, recumbently folded schistosity. Orthogneisses ranging from gabbro to true gmite f o m a large volume of the Entia Gneiss Complex, the remainder of which is composed of metamorphosed supracrustal sedimentary and volcanic rocks. Migmatite and pegmatite are also abundant in the gneiss complex. The intrusive orthogneisses are S3m-tectonic and sheet-like and are also involved in the recumbent fold events that deform the early fabric. Some of the most leucocratic granite sheets may be formed by segregation of minimxim melts formed locally and some hornblende- leucogneisses in the gneiss complex have trondjhemitic compositions consistent with an origin as complimentary residues to these melts. In general however, many of the orthogneiss units are tonalitic to granodioritic in composition and are unlikely to have been generated locally. These are I-type, broadly calc-alkaline granitoids with compositions like those in many m o d e m cordilleran Belts. They show some similarities to I-type granite suites from the Lachlan Fold belt in eastern Australia and are quite unlike the wisespread potassium-rich, alkalic granite suites that occur in many of the northern Australian middle Proterozoic inliers. The Bruna granitic gneiss is younger than the granitic gneisses within the Entia Dome, is partly post- or late tectonic and shows geochemical characteristics that are like those of the K^O-rich granites common elsewhere in the northern Australian Proterozoic. The granitic gneisses in the Entia Dome differ from some possible Phanerozoic analogues in that the more felsic end members are more enriched in K^O, Rb and Ba and more depleted in REE, Y, Nb and TiO^. This is explaxned as a result of the sheet-like form of these plutons with their high surface area to volume ratios. This results in more efficient exchange between the granitic magmas and their host gneisses. Marginally compatible elements are extracted and incompatible elements, particularly those concentrated in the migmatitic minimum melts, are enriched. The Entia granitic gneisses imply that this area of the east Arunta Block was the site of a Mid Proterozoic suture. The study also highlights the strong influence tectonic setting has on the geometry of granitic intrusion and in turn the influence that has on its geochemical evolution. It indicates that intra-continental crustal detachment zones may have important localising influence on magmatism with subesequent implications including for instance, the distribution of heat producing elements in the crust.

141

8.21

SANGEANG API - ALKALI ARC M A G M A T I S M U L T R A M A F I C A N D M A F I C XENOLITHS, ARC GROWTH BY UNDERPLATING

J. Foden^,

R. Varne^, A,J. Stolz^ and G.A.

Jenner^

^Department of Geology and Geophysics, University of Adelaide ^Department of Geology, University of Tasmania ^Department of Earth Sciences, Memorial University of Newfoundland, St Johns, Canada

Sangeang Api is an active volcano in the eastern Sunda Arc, Indonesia. It lies between the islands of Flores and Sumbawa and erupted throughout most of 1985 and 1986. This volcano is an unusual island arc type in having erupted alkaline, undersaturated (5 - 15 % normative Ne) lavas. These carry a wide range of mafic and ultramafic xenoliths. Mineral assemblages in the xenoliths include; olivine-clinopyroxene, clinopyroxene-magnetite, clinopyroxenehornblende-magnetite, clinopyroxene-plagioclase-hornblende-magnetite and clinopyroxene- phlogopite-hornblende. The. xenoliths are often complex with inclusion- in- inclusion relationships. Most are "cumulate rocks", some of which have been modified by reaction with cooler melts to produce hornblende and/or phlogopite in a mechanism described by Foden (1986) as wall-rock reaction. Other modifications include cooling towards the local geotherm and cataclastic textures due to shearing in faults. These xenoliths represent disrupted sub-crustal underplate. The Sangeang Api lavas are potassic, hydrous and clinopyroxene-rich. Phenocrysts include olivine, clinopyroxene, hornblende, plagioclase and magnetite. Xenocrysts are common due to disaggregation of the xenoliths. Sangeang Api is about 190 km above the north-dipping Benioff Zone and adjacent active volcanoes include Tambora, about 100 km to the west, and Ebulobo, Inerie and Ija on the island of Flores to the east. Sangeang Api occupies a tectonically important site with very significant geological distinctions between situations to the east and the west. On Flores, the western volcanoes all occur on the southern coast of the island about 130 km above the Benioff Zone. The lavas from these volcanoes are less alkaline than most of those from Lombok and Sumbawa to the west. On Flores there is southwards progression of the locus of active volcanism; some chains of cones young to the south and most of the active crater systems breach and grow to the south . By contrast, on Sumbawa, to the west of Sangeang Api, Tambora, is on the north coast and there is northwards progression of the locus of volcanism of about 30 km in the past 1 m.y. Sangeang Api is probably situated on a sinistral, cross-arc wrench fault. Such a structure has been proposed in the past (e.g. Audley-Charles, 1975) and may result from the collision of the Australian continental plate with the eastern end of the Sunda Arc from the south. There is also, evidence that in addition to segmentation of the arc due to sinistral wrenching, there is uplift of western ends of segments and relative depression of eastern ends. Western Flores shows strong geomorphological evidence of uplift, with exposure of Miocene basement rocks, the deep incision of river vallies and the development of gorges in plains. By contrast the eastern end of the island ''is dominated by very young volcanic products, with the base elevations of these volcanoes at sea level.

142

The apparent differences between the east and western end of Flores, are also reflected on Sumbawa, Lombok and Bali. Each of these islands also have uplifted, and fringing coral reefs on their western coasts, but not on the east. These features suggest that the island chain of the east Sunda Arc is a series of imbricated segments, each showing west-up and east-down rotation. This probably reflects some component of east-west compression due to the collision of the south-west Pacific plate with the east Banda Arc. Wheller et al (1987) have demonstrated that there is east-west K2O variation within each segment of the arc. The model presented here suggests that this reflects westwards lateral migration of magmas welling up against the easterly dipping base of the crust. The path length of this migration can be as much the length of a segment, which in the case of the Flores one, is over 100 km. This would allow complex melt-wall rock reactions to procede, including thermal erosion of the hanging wall, precipitation of hornblende and phlogopite and zone refinement. With time a gradual east to west gradient of increasing incompatible elements and volatiles in this underplate occurs. This is reflected in Sangeang Api, a potassic, undersaturated volcano at the western edge of such a segment. The entrainment of the actual underplate material is due to its fault disruption and to the rapid ascent of the volatile—rich magma. It becomes obvious that the magnitude of the effects of this process on the chemistry of erupted magmas is very large. It clearly indicates the difficulty in assessing the role of other components, such as the subducting slab, which are involved in the primary generation of the melts at depth. Audley-Charles, M.G. (1975). The Sumba Fracture: a major discontinuity between eastern and western Indonesia. Tectonophysics, 26, 109-133. Foden, J.D. (1986). Role of amphibole in evolution of Experimental and natural evidence. International Volcanological N.Z., Sympsium 3, abs vol. pl56.

andesite: Congress,

Wheller, G.E., Varne, R., Foden, J.D. and Abbott, M.J. (1987). Geochemistry of Quaternary volcanism in the Sunda-Banda arc, Indonesia, and three -component genesis of island -arc basaltic magmas. J. Vole. Geotherm. Res., 32, 137-160.

143

7.22

AUSTRALIAN CRETACEOUS

ICE

L.A. Frakes and J,E. Francis Department of Geology and Geophysics, University of Adelaide

During the early Cretaceous central Australia was occupied by an extensive intracratonic basin, the Eromanga Basin. In South Australia, shallow marine accumulations of early Neocomian to Albian age consist of marginal fluvial-lacustrine and paralic sandstones (the Algebuckina and Cadna-owie formations), and a black, organic-rich marine mudstone, the Bulldog Shale, deposited below wave base. In the south-western part of the basin the Bulldog Shale is commonly bioturbated and the trace-fossils Chondrites, Planolites and Zoophycos are very common. Occasionally conditions within the basin became anoxic, indicated by the absence of this fauna and the presence of fine laminations. Benthic foraminifera show low diversity, small size and collapsed thin tests, suggesting they lived in stressed environments. Large exotic blocks occur within the Bulldog Shale and related units in this area. The blocks consist of quartzite, Gawler Range Volcanics and other rock types mostly traceable to Proterozoic/Paleozoic basement of the basin margins. They range in size from small pebbles to large boulders over 3 metres in diameter, and in roundness from well rounded to (rarely) angular. The clasts occur as isolated lonestones, or stringers, or in small pockets. Quartz sand grains also form an important component of Bulldog Shale, either admixed with mudstone or as discontinuous laminae. Storm deposits exhibiting hummocky cross-bedding occur near basin margins and in the form of intermittent lag gravels. The hydrodynamic paradox presented by the occurrence of such outsized boulders in mudrocks can only be resolved if the clasts were carried in laterally by swift currents or mass movement processes, or were dropped at the deposition site by falling from some sort of raft. Evidence of mass-movement processes is lacking in the Bulldog Shale. The anomalous size of the boulders compared to matrix grain size, the occurrence of penetration structures in laminae below the clasts and the presence of quartz sand grains with surface textures characteristic of glacial regimes suggest that the blocks were emplaced by dropping from a raft and that the raft was ice. This new investigation supports David's (1926) proposal that boulders in the Bulldog Shale were emplaced by ice-rafting. We propose that ice formed during cold winters in rivers or on the shores of the basin and rafted boulders out into the basin, later to be dropped into the muds. The south western area of the Eromanga Basin was situated in high latitudes of approximately 65°-75°S during the early Cretaceous. Boulder shales interpreted as ice-rafted deposits have also been recorded from other Cretaceous high-latitude sites. These include Siberia, Spitsbergen, Canadian Arctic and possibly Alaska and New Zealand. Ice-rafting thus occurred between palaeolatitudes of 65 -78 . This implies

144

that the Cretaceous climate was at least seasonally cold at high latitudes, allowing ice to form. The existence of semi-permanent glaciers is problematic due to the apparent lack of early Cretaceous tillites. This geologic evidence agrees with results of computer modelling of the Cretaceous climate, which consistently produce freezing winter conditions in continental interiors at high latitudes. However, due to the extreme continentality effect stammers were hot (an annual range of -18 to +27^C was computed for Australia in one model) which would have allowed the growth of temperate floras. This evidence for at least episodic cold winters challenges the widely-held view that the Cretaceous climate was globally warm and ice-free.

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16.2

IRON OXIDES - THEIR SURFICIAL AND REMOTE SENSING

DISTRIBUTION

S.J. Fraser, J.F. Huntington and A.A. Green CSIRO Division of Mineral Physics and Mineralogy, North Ryde

As a result of our analyses of high-quality remotely sensed data, we have formulated a simplified remote sensing model for the surficial distribution of hematite and goethite in arid, weathered terrains. This model although empirical, is in accordance with trends and observations made by soil scientists. Iron oxides have characteristic spectral responses in the visible-to-near-infrared (VNIR:0.3-1.1 ym) spectral region (Hunt and Ashley, 1979) and, because of the technological advances in detector systems, subtle reflectance differences, such as those distinguishing goethite from hematite, can be mapped from aircraft (Fraser et al., 1985; Fraser et al., 1986). In youthful terrains, such as the western USA, the mere presence of iron oxides can have mineral exploration significance. Because iron oxides are so widely distributed in the typically weathered Australian terrain, there is a need to determine the actual iron-oxide species present, and interpret their distribution in terms of a model. Anomalous areas, perhaps related to mineralization, can then be distinguished from normal background occurrences. Ferrous iron (Fe^"*") readily oxidizes to ferric iron (Fe^"^) under surficial conditions. Although hematite is thermodynamically more stable, goethite is by far the most common species in soils, where the critical factor appears to be water activity. Elevated temperatures, dry conditions and a neutral pH environment have been found by soil scientists to favour hematite. Lower temperature, wet conditions, low pH and the presence of chloride, ferrous sulphate or organic compounds, favour goethite. Our remote sensing studies in arid terrains have shown that hematitic responses dominate the landscape, especially the older paleo-surfaces where chemical rather than physical weathering predominates. Areas of active erosion and deposition, along with transported sands/gravels in river beds tend to be goethitic. On imagery of the Hammersley Basin, near Paraburdoo, we have observed, an apparent ageing of alluvial scree fans from younger goethitic to older hematitic. (A similar ageing from goethitic to hematitic has been observed with dune sands - Folk, 1976). We must stress that remote sensing techniques are affected by the outer few surficial microns of the rock and soil. Because this is a particularly harsh environment in terms of temperature and aridity, hematite rather than goethite is the more stable species. The Model Hot, arid, deeply weathered and tectonically stable terrains develop a veneer of hematitic material, because time, elevated temperatures and lack of water (thermodynamically) will favour hematite. With increasing activity of water either with depth towards the water table or along joints, the amount of goethite will increase. Goethite exposed by active erosion will exist metastably, but gradually transform to hematite; this is a kinetically slow process which rain and high humidity could temporarily reverse.

146

There are several economically significant factors that potentially could upset the model; these include hydrothermal bleaching (lack of total iron), and the presence of weathering sulphides, which lower the pH (i.e. gossans, pyritic lithologies, hydrocarbon leakage) and thus promote goethite rather than hematite. Prior to the availability of specialized remote sensing data such as we have used, geologists have not been able readily to discriminate and map synoptically the distribution of goethite and hematite over large areas. Our analyses to date have shown that spatially coherent mineralogic patterns are produced which can then be related to either geologic or geomorphic controls. Obviously further research is needed to extend our deeply-weathered, arid-terrain model into other environments. Our experience has shown us that it is essential to understand the geomorphic processes operating in an area, before attempting to distinguish iron-oxide anomalies that may be economically significant. Alternatively, the iron-oxide distributions may help to determine the geomorphic processes affecting an area. References Eraser, S.J., Huntington, J.F., Green, A.A., Stacey, M.R. and Roberts, G.P. 1985. Discrimination of iron oxides and vegetation anomalies with the MEIS narrow band imaging system. Proceedings of the 4th Thematic Conference. "Remote Sensing for Exploration Geology" San Francisco, California, April 1-4, 1985. Vol. 1, p.233-253. Fraser, S.J., Gabell, A.R., Green, A.A. and Huntington, J.F. 1986. Targeting epithermal alteration and gossans in weathered and vegetated terrains using aircraft scanners: successful Australian case histories. Proceedings of the 5th thematic Conference "Remote Sensing for Exploration Geology" Reno, Nevada September 29 - October 2, 1986. Folk, R.L. 1976. Reddening of desert sands: Simpson Desert, N.T., Australia. Jour, of Sed. Petrol. 3, p.604-615. Hunt, G.R. and R.P. Ashley. 1979. Spectra of Altered Rocks in the Visible and Near Infrared. Econ. Geology 74, 1979 pp 1613-1629.

147

7,11

THE CYCLONE WINIFRED STORM BED, CENTRAL GREAT BARRIER REEF SHELF, AUSTRALIA

M.K. Gagan^, D.P. Johnson^ and A.R.

Chivas^

^Department of Geology, James Cook University of North Queensland ^Research School of Earth Sciences, Australian National University, Canberra Shelf sediments were collected iinmediately before (8-20 January 1986), after (9-10 February 1986) and one year after (27 February 1987) Cyclone Winifred crossed the central Great Barrier Reef shelf on 1 February 1986 (Fig. 1). The storm produced a normally graded, mixed terrigenous-carbonate storm bed more than lion thick, covering an area at least 1200km2, and extending more than 30km offshore to water depths greater than 40m (Fig. 2). In contrast to storm sedimentation models ^ c h predict thinning and fining offshore, Winifred created a layer that beccsmes thicker and coarser grained in deeper water in response to cross-shelf substrate changes. The storm layer enconpasses two lithologically distinct, shelf-parallel facies belts. The nearshore (0-25m depth) storm bed is 3-llcm thick, thins offshore, and grades upward from medium sand to silty clay. Offshore (25-43m depth) the storm bed thickens to more than 11cm and consists of normally graded, well sorted, relict quartz and skeletal gravelly sands capped by a thin mud veneer.

FIGURE 1. Location of the pre-, post-, and one year after Winifred saitpling sites. The area of the shelf directly affected by the eye of Cyclone Winifred is stippled.

148

KEGCJRE 2. Distribution of storm bed thickness. Sites marked with square symbols represent sairples not penetrating the base of the storm bed and, therefore, show only a minimum thickness.

FIGCJRE 3. Stom bed preservation one year after Cyclone Winifred. Excellent = original depositional textures clearly visible; Fair = remnants of storm bed visible; Poor .= storm bed cortpletely destroyed.

The cross-shelf distribution of sediment grain-size and mineralogy were very similar before and after Winifred suggesting the storm bed did not result from offshore-directed storm currents but, rather, in situ resuspension and settling of the shelf sand. In contrast, suspended sediment transport was extensive. Subtle changes in the the cross-shelf distribution of carbonate and carbon isotope ratios in the mud-fraction suggest that: (1) floodderived Johnstone River sediment did not extend more than 15km from shore, (2) resuspended reef mud was transported up to 1.5km shoreward to the midshelf, and (3) shoreward transport of resuspended mid-shelf mud to the inner shelf may have been extensive. Most storm bed preservation models in the literature imply that the most likely place for a storm bed to be preserved is in deeper water below fairweather wave base. Resaitpling of the layer one year after Winifred showed the exact opposite. The Winifred storm layer was well preserved near^hore (<20m water depth), but cocrpletely bioturbated offshore (>30m water depth) below fairweather wave base (Fig. 3). Preservation may not be a matter of insulating the layer from physical reworking below fairweather wave base, but of burying it vAiere rates of sediment accumulation outpace bioturbation. The effects of storms similar in size to Winifred may be well concealed in ancient shelf sequences subject to rapid biological re-mixing of storm layers.

149

5.1

EVOLVING A GUIDE TO AUSTRALIAN STRATIGRAPHY C.G. Gatehouse and B.J. Cooper

South Australian Department of Mines and Energy

Australia has been a pioneer in the field of stratigraphic classification. Its first stratigraphic code (1950) is predated only "by those of the U.S.A. and Canada, while it was the first country to adopt the International Stratigraphic Guide (1978). Australia has also long maintained arguably the most effective system of providing unambiguous lithostratigraphic nomenclature. Australian interest in stratigraphic classification dates from the formation of the ANZAAS Research Committee on Stratigraphic Nomenclature in 19h6. Like today, that Committee had as its objective "to encourage the orderly use of names and definitions for stratigraphic units". After World War 2, a need to regulate stratigraphic classification was recognised as rock, fossil, and time terms were often confused, and duplication of and irregular existing names were common. Geological investigations across the continent were then expanding rapidly. The 1950s and 1960s were a particularly fruitful period. Four editions of the Stratigraphic Code were published: 1950, 1956, 1959 and I96I1 (reprinted with minor alterations 1973), while the Code was transformed from a set of definitions and recommendations into a tried and proven guide which demanded adherance by geologists. Emphasis was placed on lithostratigraphic nomenclature and tems, such as Group and Formation,-became regular Australian terminology. During the 1960s and early 19TOs, the Stratigraphic Nomenclature Committee, by now under the auspices of the Geological Society of Australia, turned its attention to basin nomenclature and soil nomenclature. In 1972, a major report was published on the latter subject. In 1 9 6 7 , the Stratigraphic Nomenclature Committee, became the Australian organisational member of the International Subcommission on Stratigraphic Classification, as the Committee, since its inception, had always been sensitive to international developments and has continued to contribute to these. In the late 1960s, consideration of chronostratigraphic and biostratigraphic units in the Australian Code was thus deferred pending consensus on the international debate covering these topics. The almost unanimous acceptance and subsequent publication of the International Stratigraphic Guide in 1976 led to its adoption by Australian geologists in 1978. Detailed recommendations on the use of chronostratigraphic and biostratigraphic nomenclature were thus provided. This decision also led on to the compilation of a concise Australian guide to lithostratigraphic nomenclatiire (1985) incorporating relevant parts of the International Guide. Currently the Stratigraphic Nomenclature Committee continues its involvement in regulating and encouraging the correct use of lithostratigraphic names. Procedural revisions are being planned for the classification of igneous and metamorphic rocks. The Committee is also compiling the first listing of Australian-based chronostratigraphic units and is involved with international efforts to revise the biostratigraphic chapter of the International Guide. In addition a proposed new Chapter in the International Guide dealing with Unconformity-Bounded-Units, is being critically examined.

150

In the future 5 the Conmiittee seems likely to become involved in several new areas of stratigraphic classification, eg. Magnetostratigraphic and Pedostratigraphic. Units, which are currently being considered in international forums. Tectonostratigraphic Units and Morphotectonic Regimes represent other new terminologies which may warrant attention. In the realm of chronostratigraphy it has been suggested that the Committee consider Australian-based supplementary reference section of the international standard. In conjunction with the Australasian Association of Palaeontologists, the Committee might also examine the application of fossil datum planes and the increasing use of formal alpha-numerical biozonations which are implicitly rejected -by the International Guide. REFERENCES Australian Code of Stratigraphic Nomenclature 1st Edition, 1950. Aust. J. Sci. 12, 170-173. 2nd Edition, 1956. Aust, J. Sci. l8, 117-121. 3rd Edition, 1959. Geol. Soc. Aust. J. 6, 63-70. hth Edition, 1961+. Geol. Soc. Aust. J. 11, 165-171. kth Edition, Revised with additional notes, 1973. Geol. Soc. Aust. J. 20, 105-112. Brewer, R. (Convenor), Crook, K.A.W. and Speight, J.G. (Sub-Committee for Soil-Stratigraphic Nomenclature), 1970. Proposal for Soil-Stratigraphic Units in the Australian Stratigraphic Code. Geol. Soc. Aust. J. 17, 101-111. Glaessner, M.F., Raggatt, H.G., Teichert, C. and Thomas, D.E., I9U8. Stratigraphic Nomenclature in Australia. Aust. J. Sci. 11, 7-9. Hedberg, H.D. (Editor for International Subcommission on Stratigraphic Classification), 1976. International Stratigraphic Guide. John Wiley, New York, 200 pp. North American Committion on Stratigraphic Nomenclature, I983. North American Stratigraphic Code. Amer. Assoc. Petrol. Geol. Bull. 67(5), 8IH-875. (provides a guide to future trends in stratigraphic classification) Staines, H.R.E. (as Convenor, Stratigraphic Nomenclature Committee), I985. Field Geologist's Guide to Lithostratigraphic Nomenclature in Australia. Aust. J. Earth Sci. 32(2), 83-106.

151

9.2

CALVING ICEBERGS AND SEISMIC ACTIVITY FROM AN OUTLET GLACIER, MAWSON STATION, ANTARCTICA B.A. Gaull^, D.A. A d a m s o n ^ and J. Pickard^

^Bureau of Mineral Resources, Geology and Geophysics, Mundaring Geophysical Observatory ^School of Biological Sciences and Quaternary Research Unit, Macquarie University, North Ryde ^Western Lands Commission, Sydney

Large icefalls occurred from the terminus of a small outlet glacier on the seaward margin of the Antarctic ice sheet near Mawson Station (67®36»S, 62®53'E) in June and November 1979. Debris from both falls was trapped by sea ice, photographed and measured. The seismic station at Mawson recorded both falls. The seismic records are interpreted to show catastrophic iceberg calving from the ice cliffs as the ice sheet separated from the plateau and slowly descended into the submarine glacial valley. Icequakes are thus not always caused by simple cracking in ice sheets, ice shelves or glaciers. Parameters of foreshocks which occurred prior to the major icefall in November could have been used to predict the time and place of the fall. These parameters included the rate of change of event frequency, direction of first motions on the Mawson seismographs, as well as the S-P times. The frequency and amplitude distributions of the foreshocks can be represented as n(t) = and N(A) = where n(t) is the number of foreshocks per day, t days after the foreshocks began and N(A) is the number of events whose amplitudes were in the range A to (A + 2)mm on the Mawson seismograph. These data generally compare favourably with those from other icequake and earthquake studies.

152

11.1

TOWARDS A STANDARD FOR GEOSCIENCE REFERENCE

DATABASES

L. Gerdes^ and K. Smith^ ^South Australian Department of Mines and Energy ^Cultus Resources NL, Perth

Although it is recognised that computers can handle a wide variation in technology through conversion programs, the lack of standards in geoscience databases is not satisfactory to the end user, particularly if the user has access to geoscience databases from several organisations. Standards are also invaluable as a guide to those who are about to develop a new database. The need for standard approaches to manual and particularly computerised database entry has often been discussed amongst the Australian geoscience information community. The Australian Geoscience Information Association (AGIA) took the initiative in early 1987 to establish a Standards Subcommittee whose membership was to be flexible and dependent upon the expertise required. Guidelines for bibliographic (or reference) databases were tackled first. Guidelines have been developed for the style, following data items: map sheet numbering geoscience indexing terms (subject company names stratigraphic names geographic names well names tenement nomenclature authors serial titles collation chemical data

format and/or usage of the

descriptors)

To assist in the application of some of these guidelines, the subcommittee has recommended that relevant authority files be made readily available, preferably by mounting them on the CLIRS network. Three bibliographic databases - MINFINDER, AESIS and SAMREF - are now available on the CLIRS Information Services network. The subcommittee has strongly recommended that these databases should conform to the guidelines as far as practicable so as to avoid confusion to the user and make them easier to use.

153

8.33

M E T A M O R P H I S M IN THE W E S T E R N PROVINCE OF NEW ZEALAND - A REVIEW G.M.

Gibson

School of Applied Science, Darling Downs of Advanced Education

Institute

New Zealand has traditionally been subdivided into two provinces separated by the Median Tectonic Line (Landis & Coombs, 1967). The eastern province comprises several late Palaeozoic - Mesozoic magmatic arc assemblages and accretionary complexes whereas the western province, encompassing the regions of Fiordland, Westland and west Nelson, contains a plutonic-metasedimentary sequence (Tuhua Sequence) which has been affected by at least two major tectonothermal events: the first in midPalaeozoic times about 350-380Ma ago and the second during the late Mesozoic with a strong pulse about 100 - 120Ma. These have been identified as the Tuhua and Rangitata orogenies respectively and are based primarily on radiometric ages obtained from plutonic rocks associated with each event (Aronson, 1965; 1968; Adams & Nathan, 1978; Mattinson £t al., 1986). Only a few of the metamorphic host rocks have been dated and most of these have been given Cretaceous mineral ages (Aronson, 1968). As a result, regional metamorphism in the western province (Tasman Metamorphic Belt) has largely been attributed to the Rangitata Orogeny. Metamorphism during this event was not only considered sufficiently intense to have almost completely erased the earlier Tuhuan metamorphic imprint but it was also inferred to be predominantly of the high T-low P type (andalusitesillimanite facies series) and thus in sharp contrast with the relatively low T-high P type (glaucophane-jadeite facies series) of the eastern province (Wakatipu Metamorphic Belt). This led to the concept of Mesozoic paired metamorphicbelts in New Zealand (Miyashiro, 1961; Landis & Coombs, 1967). Metamorphism in the western province has now been firmly established as polyphase. It not only encompasses a high temperature Cretaceous event but a substantial component dating from the Tuhua Orogeny or earlier. Thus VJestland slates have given K-Ar whole rock ages of 395-A38Ma whilst much of the low grade metamorphism elsewhere in the region is inferred to have accompanied emplacement of the (dominantly) mid-Palaeozoic Karamea Batholith. Mid-Palaeozoic ages have also been obtained from central Fiordland (Oliver, 1980; Gibson & McDougall, unpubl. data) where metamorphism of the Tuhua Sequence took place under low pressure (3-4Kb) and produced sillimanite-K feldspar ± cordierite assemblages without garnet in pelitic lithologies. Progressively younger ages are encountered towards the west where deeper crustal levels, and thus high pressure (5-9Kb) assemblages, are preserved. Rocks from these levels yield mineral ages in the range 130-80Ma and include metapelites in which the assemblage kyanite-garnet ± K feldspar is commonly developed. However, because rocks from these different crustal levels share the same deformational history and contain the same deformational fabrics, it follows that the westward trend towards progressively younger ages must be a secondary effect. The decrease in age with increasing structural level more likely reflects varying degrees of isotopic resetting following a later thermal disturbance. Thus not only does the Tuhua Sequence in Fiordland exhibit elements of both low and high P metmorphi^m, but most if not all of this metamorphism stems from the Tuhua, rather than the Rangitata, Orogeny. The Tuhua Sequence in Fiordland is deformed into p. broad, dome-like structure or "anticlinorium, the core of which is occupied by high pressure 154

Cranulite facies orthogneisses dated at 116Ma (Mattinson £t , 1986). These orthogneisses constitute an entirely separate structural entity and are separated-'from the overlying Tuhua Sequence by a zone of strongly lineated, early Cretaceous mylonites. This zone formed immediately prior to opening of the Tasman Sea and is interpreted here as a ductile shear zone separating upper and lov/er crustal plates generated during continental extension. Thus in several important respects Fiordland has much in common with the metamorphic core complexes of North America and probably developed in much the same way. It was the development of this core complex rather than the Rangitata orogeny that governed the late Mesozoic tectonothermal history of the western province. Following the initiation of transcurrent faulting in New Zealand during mid-Tertiary times, this core complex was fragmented and its various elements dispersed to their present positions on either side of the Alpine Fault. Continental extension led to increased heat flow and thus was probably ultimately responsible for a great deal of the early Cretaceous acid magmatism and reset mineral ages observed throughout the western province. Much of the thermal activity attributed to the Rangitata Orogeny may therefore owe more to the tectonothermal processes that accompanied continental rifting and the splitting of New Zealand away from Australia. This raises questions about the very existence of the Rangitata Orogeny in rocks west of the Median Tectonic Line. Early Cretaceous terrestial deposits developed in fault-bounded sedimentary basins throughout the West Coast Region, and sometimes previously interpreted as the erosional products of a rising Rangitata Orogen, are no more likely to be a manifestation of the Rangitata Orogeny than a response to a rapidly extending continental rift (Nathan £t , 1985). Thus the late Mesozoic thermal history of the Western province appears to have little in common with the accretionary and/or collision events that constitute the Rangitata Orogeny in eastern New Zealand (eg. Howell, 1980). Either the Rangitata Orogeny did not affect this region or else its thermal effects overlap so closely in time with those of the core complex as to be indistinguishable from them. In either case, current ideas about the origin and nature of metamorphism in the western province may need substantial revision. Adams, C.J.D. & Nathan, S (1978): N.Z. Jl. Geol. Geophys. 21, 455-62. Aronson, J.L. (1965): N.Z. Jl. Geol. Geophys. 8, A01-23. Aronson, J.L. (1968): Geochim. Cosmochim. Acta 32, 669-97. Howell, D.G. (1980): Geology. 8, A87-491. Landis, C.A. & Coombs, D.S. (1967): Tectonophys. A, 501-18. Mattinson, J.M., Kimbrough, D.L. & Bradshaw, J.Y. (1986): Contrib. Mineral. Petrol. 92, 383-392. Miyashiro, A. (1961): Jl. Petrol. 2, 277-311. Nathan, S. et ad (1986): N.Z. Geological Survey Basin Studies 1, .90pp. Oliver, G.J.O. (1980): N.Z. Jl. Geol. Geophys. 23, 27-41.

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3.6

A REVIEW OF THE PRECAMBRIAN APPARENT POLAR WANDER PATH FOR AUSTRALIA J.W. Giddings and M. Idnurm Bureau of Mineral Resources, Geology and Geophysics, Canberra

Palaeomagnetic studies on Precambrian rocks have an important role to play in multidisciplinary research programs aimed at elucidating the nature of Precambrian geodynamics and the more general problem of evolution of the Earth's crust. Pertinent questions that may be addressed include: whether the characteristic patchworks of Archaean and Proterozoic cratons separated by younger mobile zones that are often found in the ancient parts of continents, have remained structurally intact through the Precambrian; and what was the nature of relative motion between these various assemblages. Palaeomagnetic findings often conflict with analyses based on geological data so there is a need to update constantly the different datasets in the search for agreement. Palaeomagnetic input to the debate is based on the delineation and analysis of the Precambrian apparent polar wander paths (apwps) for the various continents. From the palaeomagnetic viewpoint therefore, the exercise reduces to continuing attempts to refine the apwps for each continent. For Australia, six years have elapsed since publication of the last Precambrian apwp syntheses. Much new palaeomagnetic data, both published and of preliminary nature, have become available in that time; they derive from many of the structural elements that constitute Precambrian Australia. New isotopic age determinations have also been made which update ages previously assigned to some of the poles. It is appropriate therefore to review the new data in the context of the existing apwp interpretations. A significant upgrade of the path is required, and a refined apwp is presented.

156

The new data allow extension of the path back to 3.45 Ga from the previous oldest datum of 2.5 Ga and provide detail for those periods for which the old path was deficient. The proposed apwp is fairly simple in shape and comprises a number of loops. Certain segments of the path remain uncertain due to lack of data: the tracks from about 0.75 Ga - 0.90 Ga and 1.9 Ga - 2.3 Ga, and the apparently stationary pole periods at 1.7 Ga - 1.9 Ga and 2.5 Ga - 2.7 Ga which may conceal more complex path behaviour. Unfortunately, the quality of the majority of Precambrian results in the Australian pole data bank remains low: surprisingly, only 4 of the 61 poles may be regarded as Key poles when judged against criteria which may be regarded as lenient (ages of poles known to within 0.06 Ga and positions, at the 95% confidence level, to within 2 0 degrees). As a consequence, we must still regard the proposed apwp as a first-order definition. Considering the general standard of the data and the uncertainties inherent in the method, palaeomagnetic evidence still favours the 'single-continent' model for the continental evolution of Australia, in which the structural elements have retained their current or similar spatial relationship since at least 1.6 Ga. Absence of data from some of the elements precludes extension of this conclusion to older periods. The path indicates that Australia has moved through three cyles of low-high-low palaeolatitude displacement. In those cases where palaeoclimatic indicators are available for comparison with this regime, the palaeolatitudes do not generally support the palaeoclimates expected from uniformitarianism.

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3.21

PALAEOMAGNETISM OF THE BIRD'S HEAD, IRIAN JAYA J.W. Giddings^, W. Sunata^

and C.J. Pigram^

^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Geological Research and Development Centre, Bandung, Indonesia

The Bird's Head is a block of Australian continental crust lying close to the junction of three major plates: the Eurasian, Indian-Australian, and Pacific. Interaction of these plates since the Mesozoic has spawned a complex array of island arcs, microcontinents, and zones of spreading, subduction and faulting to the north, west and east. Not surprisingly, in such a tectonically active zone, palaeomagnetic evidence is now accumulating which indeed demonstrates that many of the islands in the region have undergone large local rotations. To date, hypotheses on the tectonic history and origin of the Bird's Head have been based on syntheses of geological and regional tectonic data only, with little quantitative evidence to discriminate them. Proposals include: rotation with respect to Australia in the Neogene, in either a clockwise or counter-clockwise sense; fixed position since either the Mesozoic or Palaeozoic; and a separate drift history as a composite microcontinent. In an effort to inject the controversy with some quantitative measure of motion, a palaeomagnetic study was undertaken on nine formations from the Bird's Head. The results of the study are presented. The rocks form part of a sequence of clastics and carbonates that drape over, and crop out, along the southwestern margin of the Kemum block. The formations range in age from Late Carboniferous to Middle Miocene and respectively are: the three formations of the Aifam Group, Tipuma Formation, Jass Formation, Faumai Limestone, Sirga Formation, Kais Limestone, and Klasafet Formation. Remanence of the Sirga Formation and Kais Limestone is very scattered

158

and yields no u s e f u l palaeomagnetic information. F o r the remaining units h o w e v e r , generally two components of magnetization are p r e s e n t . One component is s i m i l a r in direction for a l l and is interpreted as overprint remanence acquired during the late T e r t i a r y ; the o t h e r component is generally different for each and at this stage, apart the lower part of the J a s s F o r m a t i o n , is regarded as p r i m a r y . T h e Bird's Head poles are in gross agreement w i t h the apparent p o l a r w a n d e r path for eastern G o n d w a n a l a n d . In d e t a i l , the Late Cretaceous and Tertiary poles are discrepant. They point to s m a l l clockwise and counter-clockwise rotations with respect to A u s t r a l i a in the periods Late Cretaceous to Middle-Late Eocene and post-Early-Middle Miocene r e s p e c t i v e l y , and some north-northeastward directed motion in the p e r i o d b e t w e e n the two rotations. The data do at least rule out large clockwise rotation in the N e o g e n e . A t this stage there is no strong palaeomagnetic evidence f o r o r against a derivation of the Bird's H e a d from the eastern side of A u s t r a l i a .

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7.8

LATE CAMBRIAN SHOALING CYCLES IN THE AMADEUS BASIN J.D. Gorter John D. Gorter Pty Limited,

Sydney

Petroleum exploration wells in the eastern Aitiadeus Basin have indicated three discrete shallowing cycles in the latest Cambrian sequence. The lower two cycles are correlated with the upper Goyder Formation, and the upper cycle with the lower part of the Pacoota Sandstone. Application of depositional sequence analysis to the sediments has refined the depositional history of the northeastern Amadeus Basin during the Late Cambrian and Early Ordovician: After period of weathering following the withdrawal of the sea in Mindyallan time, marine depositional conditions returned to the Amadeus Basin in the Payntonian and transgressed the deeply weathered surface. The basal transgressive sediments of the first cycle are represented in the northeastern area by a thin upward coarsening sandstone sequence. Deepening of the sea is indicated by the deposition of fine grained clastics, hummocky crossstratified sandstones (HCS) and glauconitic sandstones. Shoaling resulted in the deposition of interbedded sandstones and an upward increase in carbonate. In the eastern area, oolitic carbonate deposition predominated in the upper part of the cycle. Further westward, sedimentation was dominantly clastic, and lensoidal channel sandstones with brecciated infill occur within the upper part of the sequence, approximately at the level of the oolitic carbonates. After a brief hiatus of uncertain duration, transgression was renewed. There was no basal sandstone laid down in the initial stages of this second cycle. The depositional sequence followed that of the earlier cycle in the northeast culminating in ooid grain shoals. Minor evaporitic conditions prevailed in the central area. In the west, sedimentation was again dominated by clastics. Channel horizons were developed towards the top of the cycle in the western part of the Basin. Trace and body fossils, including trilobites, are sometimes abundant in this sequence. The second cycle overstepped the earlier one onto the Mid Basin Platform, and is represented by shallow water clastics in the Mereenie Oil Field and to the southwest at Tempe Vale No. 1, where a weathered, unconformable contact is noted in core with carbonates of the Mindyallan Jay Creek Limestone.

160

The top of the second cycle is marked by a highly radioactive interval reflecting a concentrated lag of glauconitic material at an unconformity surface. In outcrop, beds below this surface are usually friable and contain few trace fossils, and may be capped by a thin limonitic band. Skolithos pipe rock, large U-shaped burrows and sedimentary structures indicative of tidally influenced conditions are present in the overlying cycle. Unlike the earlier two cycles, the upper sequence is clastic throughout the area studied. The unit has a characteristic gamma ray pattern and can be confidently correlated across the Basin from Tempe Vale No. 1 in the west to Wallaby No. 1 in the east. The sandstone occurs in the Mereenie Oil Field (P4 unit), where it forms a generally poor reservoir for hydrocarbons. However, the basal coarsening upward sandstone in the lower cycle is consistently porous in all wells in which it was encountered and may prove to be a reservoir for hydrocarbons in the northeastern Amadeus Basin. The ooid grainstones at the top of the lower cycle may also form a potential reservoir rock and are overlain by sealing shales of the following cycle. There appears to be no similar seal to the ooid grainstones of the middle cycle.

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7.2 CHILLAGOE FORMATION - A RECORD OF A COLLAPSED SILURIAN-DEVONIAN CARBONATE SHELF P.M. Green, J. Domagala and R.J. Bultitude Queensland Department of Mines

The Chillagoe Formation crops out along the western edge of the Hodgkinson Province. The formation consists of interbedded limestone, arenite, shale, conglomerate, chert and basalt. Most previous workers have interpreted this sequence as representing in place shelf sedimentation. However, the presence of turbidites and chert in close association with postulated shallow-water carbonates appears to conflict with the interpretation of the Chillagoe Formation as a stable shelf sequence. A recent detailed study' of the Chillagoe Fromation in the Mungana and Bellevue areas indicates that much of the limestone in the unit was derived from the episodic collapse of an adjacent carbonate shelf. The study was able to identify limestone facies associated with the original carbonate shelf, collapse of the shelf and slope-basin sedimentation. The original shelf consisted of four distinct facies: a. bivalve mudstone facies; b. clast-peloidal grainstone facies; c. stromatoporoid mudstone facies and d. fossiliferous marly facies. The bivalve mudstone facies is characterised by a diverse fauna of thick shelled bivalves, corals, brachiopods and stromatoporoids - generally wackestone, locally packstone. A distinctive coral-stromatoporoid sub-facies is present at the top of the facies. This sub-facies is characterised by tabulate corals, solitary rugose corals (Tryplasma sp.) with stromatoporoid envelopes and hemispherical stromatoporoids. Amphipora sp. is commonly present between the coral and stromatoporoid colonies. The clast-peloidal grainstone facies is characterised by the dominance of sand size grains and grainstone textures, scarcity of fauna and lack of sedimentary structures. This facies occurs between the bivalve mudstone and stromatoporoid mudstone facies. The stromatoporoid mudstone facies contains a relatively restricted but abundant fauna dominated by stromatoporoids and minor corals. Amphipora sp. are common at the base of this facies. The fossiliferous marly facies consists of a succession of marly beds separated by mudstone. These marl beds may be dominated by one of either braciopods, corals or stromatoporoids. This facies appears to record an input of argillaceous material onto the original shelf but its relationship to the other facies is not known. Reconstruction of the original shelf is difficult mainly because of the problem of interpretating the significance of the different facies. The bivalve mudstone facies is considered to be the most seaward facies of the original shelf. Grain size, presence of grainstone textures and uniformity of the rock types in clast-peloidal grainstone facies are probably indicative of reworking by wave or current activity. This facies may represent a sand shoal on the shelf. The stromatoporoid mudstone facies was probably deposited in a relatively restricted environment, possibly in a shelf lagoon that formed behind the sand shoal. Presence of the clast-peloidal facies well up on the shelf suggests that the original depositional setting may have been a type of carbonate ramp. Collapse of the carbonate shelf was probably triggered by rifting related to basaltic volcanism and the development of the Hodgkinson Basin. This activity produced lines of weakness represented by internal breccias and fractures within the shelf. Large blocks of limestone broke away from the shelf along these lines of weakness and slowly moved down into deeper water. Reorientation of some of these blocks occurred during transportation resulting in the internal bedding within the block becoming discordant with the regional bedding. InfUling of the internal breccias by either carbonate or argillaceous dominant matrix is relatively common. Variations in composition may be a reflection of where the infilling occurred with the carbonate-dominant infills related to 162

the shelf break-up and the argillaceous infills forming further away from the shelf. Deformation of the matrix in these internal breccias is indicative of continued relative motion between clasts probably during the emplacement of the block. The blocks were transported into deep water where they formed three main associations: a. limestone-basalt; b. limestone-siliciclastic, and c. limestonelimestone. The limestone basalt association is present mainly in the Palmerville area and appears to be related to the extrusion of the basalts during the collapse of the shelf. Details of this association have yet to be worked out. The limestone-siliciclastic association is well developed in the Bellevue area. Here four distinct limestone belts are interbedded with siliciclastic turbidites. These belts consist of carbonate allochthonous blocks and debris flows of shelf material. Associated with some of these ridges is a mixed carbonate-siliciclastic sequence. This sequence indicates mixing between two source areas which may reflect reworking of some of the basin sediments or the tapping and transport of siliciclastic detritus from the shelf. The third assoication, limestone-limestone is well developed in the Mungana area. This association consists of allochthonous blocks of shelf limestone in contact with a thin bedded mudstone facies. This facies consists of thin-bedded mudstone deposited from suspension and shallow water material transported downslope by mass flow processes, mainly turbidites to form fossiliferous wackestones and packstones. Probable in place carbonate reef mounds are also present. Slumping is relatively common. This facies is interpreted to represent carbonate slope deposition. Much of the Chillagoe Formation therefore records the final resting place for large allochthonous limestone blocks after their transportation into relatively deep-water.

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9.6

MAGNITUDE-INTENSITY RELATIONS AUSTRALIAN EARTHQUAKES

S.A. Greenhalghl,

FOR

Denham2 and J.M. Rynn^

^School of Earth Sciences, Flinders University of South Australia ^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Department of Geology and Mineralogy, University of Queensland

Intensity data can be used to derive basic earthquake parameters, such as hypocenters and magnitudes, particularly when instrumental observations are not available. We used the intensity information from the 133 earthquakes contained in the Isoseismal Atlases of Australian Earthquakes to derive relations which enable earthquake magnitudes to be determined. The relations are as follows: M l = 0.33 ( ±

O.U){logRpf + 0.74 ( ± OAO)ilogRp) + 1.63 ± (0.34)

M l = 0.31 ( ± OAO)(logRi^ f + 0.65 ( ± M / = 1.24 ( ± 0.33) + 0.6 ( ± 0.06)/^

) + 2.25 ± (0.23) and

M l = 0.026 ( ± 0.013)/s:2 - 0.059 ( ± 0 m 5 ) K + 2.38 ( ± 1.26)

Where is the maximum observed Modified Mercalli Intensity, K the Sponheuer earthquake parameter, Rp the equivalent radius of the MMjij isoseismal, and M^ the Richter magnitude. We used earthquakes ranging in size from M^ 1.3 to M^ 7.2 which were felt over equivalent areas ranging from 8 kn? to 2.5M krr?-. Some of the maps are shown on this display.

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2.21

THE THOMSON TERRANE REVISITED L.R. Grimstone

Lance Grimstone & Associates Pty Ltd, Brisbane

Reconstructions of the SW Pacific over the last of Earth History are used to demonstrate the composite, and generally o b l i q u e , interaction of divergent, convergent, and transform plate boundaries in the development of fold belts, terrane accretion and cratonic margins. Resultant tectonic regimes are therefore commonly unique to a particular region of the crust. It is postulated that such v/as the case in the region of the Tasmanides in the 15$ of E a r t h history since the Late Proterozoic. A consequence of this line of enquiry is a growing appreciation of the role played by Precambrian rocks in the evolution of the Tasmanides, and the relationship of the terranes within it. Of particular benefit is a new perception of the concealed Thomson Terrane. Features parallel to the Diamantina Lineament are thought to represent fundamental crustal fractures which were initiated by Late Proterozoic detachment of the cratonic borderland along a passive continental margin. Because this NE structural trend has not been destroyed by subsequent compressive events, it still forms the most visible component of the structural fabric of the subsurface Thomson Terrane. The Palaeozoic history of the Tasmanides is dominated by easterly compression, which maintained a vector of tension along these NE structures and periods of strike slip movement were common. Transfer fault trends associated with the Late Proterozoic extension were gradually deformed (in a dextral sense) from their original NVJ allignment. Structural intersections between these two sets are believed to provide the structural plumbing at depth for the derivation of mantle material and associated volcanics during these periods of transtension. Although published geological mapping of the region is more than a quarter of a century old, it can be enhanced with compilations of company mapping to demonstrate that magmatic activity has repeatedly exploited terrane boundaries, particularly at dislocations by major NE faults. Preservation of the HE set has provided pathways for transport of magma and hydrothermal fluids to sites of deposition within upper crustal levels. Both gold and coal are preferentially hosted by late Palaeozoic grabens on the leading edge of the Thomson Terrane. Research indicates that the gold is not alluvial but epithermal. The pressure drop afforded by the porosity contrast

165

across the basal unconf orniities caused adiabatic expansion and precipitation from upwelling solutions. The abundance of carbon may also have played a critical role as a fixer. Moreover, the gold is belie^ved to have been remobilized from mineralisation within the Proterozoic rocks upon which the Thomson Terrane is founded. Therefore, a clearer understanding of the plate tectonic controls upon recent thermal episodes beneath the Thomson Terrane will be important in guiding exploration strategy for gold, base metals, uranium, platinoids, rare earths, petroleum, groundwater and coal. The timing of regional features or events such as -the fixing of coal rank in the Bowen Basin, -the maturation of Cooper Basin oils, -the heating of groundwaters of the Great Artesian Basin, -or perhaps the north to south transition of weathering profiles across the continent, may be related to the passage of the Australian Plate over a thermal anomaly. Reconstructions of the SW Pacific put forward by Grimstone et. al. at PACRIM 8? could suggest the mechanism. The mineral potential of the Thomson Terrane is not obscured by tropical vegetation, weather, logistics, hazards, disease, or politics. Any obscurity that does exist can simply be removed by a teamwork approach to geological interpretation. By comparison with the money, effort and teamwork which we have poured into the SW Pacific Rim, we have a virtual geological frontier in our own backyard - a highly prospective backyard at that. At the regional level, improvements must come in our ability to identify the original tectonic components of the old composite plate margin represented by the Thomson Terrane. This covers a broad spectrum of geological effort, traditional and technological, academic and commercial, government and industry.

166

CRATOH

ANIDES

BALCOOMA METAVOLCANICS PADDYS CREEK FORMATION & JUDEA BEDS

MOUNT WINDSOR VOLCANiCS FORK LAGOON BEDS

GRANITES MET AMORPHICS •

ACID VOLCANICS

•

BASIC VOLCANICS

SEDIMENTARY ROCKS & LOW GRADE METAMORPHICS NOT DETERMINED

3 6 1 RADIOMETRIC DATE ON PLUTONIC & MET AMORPHIC EVENTS S-D FOSSIL DATE AFTER HENDERSEN 1980 & MURRAY & KIRKEGAARD 1978

PRECAMBRIAN

ACCRETED TERRANES OF PALAEOZOIC EASTERN AUSTRALIA

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2.6

THE BOWEN BASIN - AN UPPER CRUSTAL EXTENSION MODEL FOR ITS FORMATION R.L. Hammond and C.W. Mallett

CSIRO Division of Geomechanics, Indooroopilly

The Bowen Basin, in central Queensland, contains a locally thick, Permian to mid-Triassic succession of fluvio-deltaic and shallow marine sedimentary rocks and coal measures. In the late Permian the basin comprised upper plate half graben and sag phase fill in an extended terrane. The Bowen Basin is the northern part of the Bowen-Gunnedah-Sydney Basin (Murray 1985) which is developed in the hinterland of, and partly overlies the New England Orogen. The conventional interpretation of the Bowen Basin is that it represents a retro-arc basin associated with the resumption of westward subduction in the Early Permian (Murray 1985; Murray et al. 1987). Uplifted basement terranes along the basin's eastern margin (Connors & Auburn Arches) are mantled by arc-type volcanics equivalent in age to the oldest sediments present in the basin. Morpho-tectonic features of the basin are: i) Springsure & Collinsville Shelves on the western flank, 11) Denison Trough, a marginal sub-basin to the west, ill) Comet Ridge in the south, iv) Taroom Trough (= Mimosa Syncline), v) Dawson Tectonic Zone and vi) Nebo Synclinorium. The Connors and Auburn Arches are separated by the Gogango Over folded Zone (GOZ - includes Grantleigh Trough = basal Bowen basin sequence). The basin's structure is superficially dominated by thrusting (Hobbs 1985). Basement terranes along the basin's eastern margin have been uplifted on buried ramps and have over-ridden the basin succession. The GOZ can be interpreted as shallower level thrust imbrication of basement and rocks in part equivalent to the Bowen Basin sequence. Smaller scale structures are consistent with this assessment, and in particular include spectacular thrust structures exposed in some open cut coal mines. The morphology of the basin changes from north to south in a step-like manner across numerous "corridors" resembling buried tear faults and lateral ramps. Some of these mark significant regional changes in the basin's character. Ziolkowski & Taylor (1985) illustrated the early development of deep half grabens synchronously with the accumulation of thick sequences of volcanics along the present eastern margin of the basin. This unequivocal evidence for an early extension phase has prompted an extension model {e.g. Lister et al. 1986 ) involving belts of half graben development. The model explains transverse corridors as major transfer faults (Gibbs 1984), though their present character can be attributed to subsequent utilisation as lateral ramps and tear faults. The localisation of intrusive activity on some corridors is consistent with the deep seated nature of first order transfer faults. Major, out of place reverse faults are interpreted as half graben bounding faults reactivated during mid-Triassic thrusting. The model also explains the rapid development of oceanic sedimentation east of the present basin as an area of more extreme extension, and the apparently restricted extent pf early Permian volcanics in the north as a consequence of their confinement to half grabens. The Comet Ridge can be interpreted as a marginal piateau-like area where half-grabens did not

168

develop, and which was potentially enhanced by an inversion episode (Ziolkowski & Taylor 1985). Similar arguments can be applied to basement arches on the basineastern margin, though they were subsequently thrust over the basin .on reactivated extensional faults. An early Permian extensional episode has not yet been recognised or considered in tectonic reconstructions of the New England Orogen. The Bowen basin can be subdivided into domains that contain internally consistent basinal structure, but which are commonly markedly different from adjacent domains. This allows more accurate prediction of the type and style of structure likely to be encountered in coal mining operations. The regionally significant transfer faults are deep seated structures providing an ideal "plumbing" system for the rise and emplacement of magma, for fluid circulation, and hence for mineralisation over prolonged periods. Numerous Cretaceous intrusives, some large, mark the trace of postulated transfer faults. Gibbs, A. D. , 1984, Structural evolution of extensional basin margins. J. Geoi. Soc. Lond., 141, 609-620. Hobbs, B. E. , 1985 , Interpretation and analysis of structure in the Bowen basin, Geol. Soc. Aust., Abs., 17, 151. Lister, G. S., Etheridge, M. A. and Symonds, P. A., 1986 , Detachment faulting and the evolution of passive continental margins. Geology, 14, 246-250. Murray, C. G., 1985, Tectonic setting of the Bowen Basin. Geol. Soc. Aust., Abs., 17, 5-16. Murray, C. G. , Fergusson, C. L. , Flood, P. G. , Whitaker, W. G. and Korsch, R. J., 1987, Plate tectonic model for the Carboniferous evolution of the New England Fold Belt. Aust. J. Earth Sci., 34, 213-236. Ziolkowski, V. and Taylor, R., 1985, Regional structure of the north Denison Trough. Geol. Soc. Aust., Abs., 17, 129-137.

COMORS ARCH

GOONYELLA CROSS SECTION R E I N T E R P R E T E D T O INCLUDE P O S T U L A T E D EARLY E X T E N S I O N A L F A U L T S

Section across the northern Bowen Basin illustrating postulated half grabens and fill of early Permian volcanics (stipled areas). This interpretation accounts for the abrupt faulted western limit of volcanics in the far north of the basin and the presence of 'out-of-place' high angle reverse faults.

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13.10

OBSERVATIONS ON THE BEHAVIOUR OF COAL MEASURES DURING DEFORMATION R. Hammond and C. Mallett

CSIRO Division of Geomechanics,

Indooroopilly

Deposition of lacustrine and fluvio-deltaic sediments in the Bowen Basin of northeastern Australia was arrested in the mid- to late Triassic by a major deformation event which involved over-thrusting from the east and the development of thrust and fold belt deformation styles in the sedimentary rocks of the basin. The intensity of deformation generally increases eastwards across the basin but there is considerable lateral variation. Numerous open pit coal mines have been established in the basin, some of which have exposed excellent examples of the effects of deformation on coal bearing rocks. In particular, this presentation discusses structures observed at Curragh, South Blackwater and Moura mines, all in the Rangal Coal Measures. Similar structures are also known from Collinsville. The character of deformation observed indicates that the rheology of coal is markedly different than that of the enveloping sedimentary rocks. In particular two significant points about the behaviour of coal bearing rocks at geological strain-rates and shallow crustal levels can be made: 1.

A coal layer appears to be much stiffer than enveloping rocks (thus influencing their behaviour).

Carbonaceous and coalified plant material within sedimentary rocks in contact with seams appears to weaken the rock thus localising much of the deformation into the coal-rock interface.

Nonetheless, in intensely deformed rocks coal has deformed in a "pseudoductile" manner such that the coal mass accomodated shape change by developing sets of discrete striated slip planes. This is essentially the same deformation process observable in strongly sheared, very low grade sedimentary rocks (Cf. Bosworth 1984a; 1984b). These characteristics have had a spectacular influence on the nature of the thrust horizons passing through coal measures. The presence of coal localises thrust flats and its stiffness induces the development of duplexes. Two types of duplex are observable at Curragh, which can be regarded as end members of a range in styles. They are: 1.

"In-seam" duplexes where slices of the coal seam are imbricated without the involvement of the enveloping rock (recognised at Curragh and South Blackwater)

conventional duplexes which involve a substantial volume of the immediate rock envelope (see Figure, duplexes are very common in Curragh mine).

The latter have been observed to extend for more than a kilometer. In-seam duplexes may reach 500 m in across strike dimensions.

170

In contrast, coal measure rocks at Moura are notable for the absence of duplexes despite favourable structural environs {i.e. very low angle thrusts). Massive, competent sandstone units in the coal measures at Moura were apparently too stiff and thrust displacements too small for duplexing to develop. However, drag fold formation around minor thrusts was facilitated by significant movement localised in the coal-rock contacts. Black coal commonly exhibits closely spaced (1 - b mm) joint-like structures that in many examples form regular orthogonal patterns. Two types of these structures, referred to as cleats, can be recognised. Face cleats are usually more widely spaced and contain a fine layer of clay material or rarely carbonate, butt cleats are more closely spaced, tightly joined (i.e. no gape) and resemble a cleavage. In the Bowen Basin a geometric relationship between cleats, joints and cleavage, and systematic swing in joint trends around faults implies a genetic relationship between higher rank coalification processes, palaeostress orientations and deformation of the coal measures.

Bosworth, W. , 1984a, Foreland deformation in the Appalachian Plateau, Central New York: the role of small-scale detachment structures in regional overthrusting. J. Struct. Geol., 6, 73-81. Bosworth, W., 1984b, The relative roles of boudinage and ^structural slicing' in the disruption of layered rock sequences. J. Geol., 92, 447456. Butler, R. W. H. , 1982, The terminolgy of structures in thrust belts. J. Struct. Geol., 4, 239-245.

CET04

CET03

Interpretation based on highwalll maps prepared by J. Woods (Curragh) showing thrust duplexes involving coal and sedimentary rock (upper), and a composite duplex involving locally duplicated seams (in-seam duplex) as well as rock from the seam floor (lower).

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4.5

AN UPDATE ON MOUNT ISA - GEOCHEMICAL EVIDENCE FOR EPIGENETIC COPPER MINERALISATION K.W. Hannan^, H.K. Herbert^, S.D. Golding^ and H.R. Krouse^

^Department of Geology and Mineralogy, University of Queensland ^Department of Physics, University of Calgary,

Canada

The pendulum of debate on the formation of Mount Isa's Pb-Zn and Cu orebodies has swung erratically during the last 50 years from Grondijs and Schouten's (1937) interpretation of complete epigenesis, through a spectrum of syngenetic Pb-Zn and epigenetic Cu models, to arguments supporting total syngenesis. Although the Pb-Zn ore side of the debate has settled somewhat during the last 30 years (syn/diagenesis), opinion on the timing of Cu mineralisation remains sharply divided. For example, the weight of interpreted structural evidence falls in favour of a syn-tectonic Cu mineralisation event which post-dated at least 2 regional phases of deformation (Perkins, 1984; Swager, 1985). In contrast, geochemical evidence recently collected from the Pb-Zn and Cu orebodies has been interpreted in terms of early, co-genetic, Pb-Zn-Cu mineralisation followed much later by syntectonic hydrothermal alteration, recrystallisation and remobilisation of the (primary) Cu orebodies (P.J. McGoldrick and R.R. Keays, in prep.). New geochemical evidence is now briefly outlined which is interpreted to support the syn-tectonic model of Cu ore genesis. The evidence is provided by metabasalts of the Eastern Creek Volcanics (ECV) that crop out extensively in the Mount Isa area and which also occur as an in-faulted block, termed the Greenstone Basement, adjacent to mineralised metasediments of the younger Mount Isa Group. The replacement of regional-style metamorphic sphene and Ti-magnetite by rutile clusters in greenstones of the Greenstone Basement, and an accompanying loss of a regionally strong, trace element inter-correlation, indicates that these metabasalts had interacted with a relatively CO^-rich fluid, after regional metamorphism of the ECV. 13 18 Carbonate 6 C and 6 0 values from the Greenstone Basement define an almost one-to-one positive correlation which extends from -10 to -3"/ 00 and 7.5 to 12.5^/oo respectively. This trend is co-linear with a small number of published dolomite analyses (Smith et al. , 1978) from the 200 and 1900 Cu orebodies. One possible exploitation for the jtrend is the mixing of a deeply sourced fluid, with a 6 C of -5 to -7 /oo at about 300^C, with fluid dominated by carbon derived from carbonates of the Mount Isa Group. 18 Regional calcites, in contrast, define a tight cluster with a mean 6 0 of 10.1 + 1.3^/oo and mean 6 C of -3.2 + 0.8^/oo. These values are consistent with a derivation of carbon from disseminated marine carbonates and oxygen by the dehydration of a volcano-sedimentary pile at moderate metamorphic grades (Taylor, 1979; Golding et al., 1987). 34 Sulphides from the Greenstone Basement have uniformly high 6 S values with a consistent mean of 2Q /oo. These compositions overlap completely with sulphide from the Cu orebodies and indicate a significant contribution of sulphur from ocean sulphate (Smith et al. , 1978) to the greenstone-copper ore system. Regional ECV sulphide-sulphur

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compositions, on the other hand, cluster aroung 3°/oo, a value of primary mafic igneous rocks.

typical

These observations, by themselves, do not rule out the possibility that greenstone development occurred during a post-copper mineralisation event which resulted in local remobilisation of abundant Mount Isa Group S, C and 0 into the ECV. However, the occurrence of an identical greenstone assemblage in a similar structural setting, 12 km south, at Native Bee, where the overlying Native Bee Siltstone is only very weakly sulphide bearing, makes this proposition less sustainable. To conclude, the systematic relationship between the isotopic compositions of carbonates in the Greenstone Basement and dolomite from the Cu orebody sequence, the overlap in sulphur isotopic compositions of sulphide in both units, and the timing constraints imposed by mineralogic and trace element patterns in metabasalts of the ECV, provide more evidence of a post-regional metamorphic hydrothermal event associated with copper mineralisation at Mount Isa. Golding, S.D., Clark, M.E., Keele, R.A., Wilson, A.F. and Keays, R.Ro , 1987, Geochemistry of Archaean epigenetic gold deposits in the Eastern Goldfields Province, Western Australia: Spec. Pubis, geol. Soc. Aust., V. 13, in press. Grondijs, M.F. and Schouten, C., 1937, A study of the Mount Isa ores: Econ. Geol., v. 32, p. 407-450. Perkins, W.G., 1984, Mount Isa 'silica-dolomite' and copper ore-bodies: The resutl of a syntectonic hydrothermal alteration system: Econ. Geol. , V. 79,' p. 601-637. Smith, J.W., Burns, M.S. and Croxford, N.J.W., 1978, Stable isotope studies of the origins of mineralisation at Mount Isa: Mineralium Deposita, v. 13, p. 369-381. Swager, C.P., 1985, Syndeformational carbonate-replacement model for the copper mineralisation at Mount Isa, N.W. Queensland: a microstructural study: Econ Geol., v. 80, p. 107-125. Taylor, H.P., Jr., 1979, Oxygen and hydrogen isotope relationships, ^ Barnes, H.L. ed., Geochemistry of hydrothermal ore deposits: New York, Wiley-Interscience, p. 236-277.

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4.8 BASEMENT-COVER RELATIONSHIPS IN SOUTHWEST COLORADO ' IMPLICATIONS FOR EARLY TO MIDDLE PROTEROZOIC CRUSTAL EVOLUTION OF THE SOUTHWEST USA C.W. Harris!, R.G. Gibson^, K.A. Eriksson^ and C. Simpson^ ^Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, USA ^Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, USA A juvenile Early to Middle Proterozoic terrane in the southwest U.S.A. adjoins the Archean Wyoming Province to the north. Basement rocks in this terrane consist of 1800 to 1730 Ma-old volcano-plutonic suites intruded by ca. 1710 to 1670 Ma-old granitoids. Cover rocks are represented by 1700 to 1650? Ma-old siliciclastic sediments that locally overhe felsic ashflow sequences. In the Needle Mountains in southwest Colorado, basement rocks include ca. 1750 Ma-old amphibolite grade gneisses derived from mafic and felsic volcanic, volcaniclastic and plutonic protoliths; the gneisses are intruded by ca. 1690 Ma-old granitoids. Sy^mernatic metamorphism, polyphase folding, and foliation development (Z)^ deformation) affected the gneiss complex prior to intrusion of the ca. 1690 Ma-old granitoids. Basement rocks are overlain by parautocthonous conglomerates, quartzites and pehtes of the Uncoinpahgre Group. The 3 km-thick Uncompahgre Group occupies an E - W trending synclinorium bounded by steeply dipping tectonic contacts with basement that are interpreted as remnants of a deformed unconformity. The Uncompahgre Group consists of a basal retrogradational, alluvial to outer shelf sequence followed by three progradational sequences. The progradational sequences exhibit two phases of sedimentation. The first phase was exemplifiied by shoaling outer- to inner-shelf to shoreface and alluvial sedimentation associated with an accelerated rise in relative sea level. Phase 2 sedimentation followed inundation of the delta plain and was characterized by tidal shelf sedimentation. Sediment compositions and the geometiy and thickness of the depositional sequences militate against a passive margin setting. Rather, basin formation in an extensional or transtensional regime is recorded in the basal sequence whereas the progradational sequences reflect deposition possibly m a foreland or transpressional setting. Post-Uncompahgre Group, D^c deformation involved northwarddirected, thin-skinned thrusting in cover rocks followed by the formation of upright, Etrending folds in both the basement and cover and the development of a subv^tical, E-striking foliation in the ca. 1690 Ma-old granitoids. Infolding of the Uncompahgre Group into basement generated a cuspate synclinorium. To relieve space problems within the cusp, deformation zones along basement-cover contacts accommodated upward movement of the Uncompahgre Group. Coincident with cusp tightening ESE-striking dextral and NE-striking sinistral strike-slip shear zones developed in basement and cover and were associated with local cross-folding. D^c structures are consistent with N N W - S S E shortening prior to ca. 1430 Ma-old anorogenic plutonism. The lithologic suites and polyphase deformation history of the Needle Mountains are comparable to those found elsewhere in the southwest U.S.A. The volcano-plutonic gneisses and Uncompahgre Group, respectively, resemble the Moppin Series and Ortega Group of northern New Mexico, and the Yavapai Series and Mazatzal Group of central Arizona. N- to NW-directed tectonic transport in these areas is kinematically and temporally consistent with Z)j,<-.The areally extensive, post-1690 Ma-old deformational episode is interpreted to be the product of regional crustal shortening.

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16.1 TIDAL SEDIMENTARY PROCESSES IN TORRES STRAIT APPLICATION OF LANDSAT, AIR PHOTOGRAPH AND SIDESCAN SONAR IMAGES AS COMPLEMENTARY REMOTE SENSING TECHNIQUES P.T. Harris Ocean Sciences Institute, University of Sydney

Torres Strait is at once one of Australia's most important seaways and one of the least well studied or understood in scientific terms. The Strait contains one of Australia's most important prawn fisheries. It is of strategic import ance for defence. Politically, it forms an international boundary and it is a vital shipping route, linking Australia's eastern seaboard with the northwestern parts of the country. Also, the Strait is fed by rivers found in increasingly mined catchments. Remotely sensed data have greatly increased our knowledge of sedimentary processes in Torres Strait. LANDSAT, vertical aerial photography and sidescan sonar remotely sensed images provide an insight into net bedload transport pathways, determined on the bases of bedform distributions and the morphology of individual bedforms. Also, sediment transported in suspension is indicated by high/low turbidity distributions apparent in LANDSAT images and in aerial photographs. Sand banks aligned sub-parallel to flow and sandwaves alingned subnormal to flow are composite bedforms which, by virtue of their asymmetry and interrelationships, indicate directions of net bedload transport. These bedforms are patchily distributed in predictable locations with respect to scoured tidal channels in the sequence: (1) scoured bedrock channels between coral reefs; (2) flow parallel sand ribbons; (3) sandwaves with headland associated sand banks; and (4) fine-grained sediment depocentres. Transport pathways diverge from the scoured channels (see Figure). Such a sequence has been described by Belderson and Stride (1966; Mar. Geol., 4:237-257) in relation to the decreasing strength of tidal currents down drift from ''bedload parting zones" in European tidal seas. Suspended sediment "fronts", or waters of contrasting turbidity, are evident in LANSAT images and aerial photographs. A region of increased turbidity is located directly south of the Papuan coast and to the west of the Warrior Reef complex. Air photos show mixing of the waters located to the east and west of the Warrior Reefs by means of tidally-induced eddies established adjacent to Reef passages. Other eddies form by the "island wake effect" described by Wolanski Imberger and Heron (1984; J. Geophys. Res., 89(C6):10,553-10569). Low turbidity water intrudes southward from the Gulf of Papua into the Great North East Channel. The remotely sensed data can be compared with sediment distributions determined by limited seabed sampling to generate a facies model for sedimentation. Surficial sediments in Torres Strait are not laterally well mixed but reflect a local dominance in supply. Thus, some areas of sediment high in quartz content are juxtopposed to areas of sediment high in carbonate content. Tidal currents maintain these deposits and transport sand-sized sediment over a restricted portion

175

of T o r r e s S t r a i t , where t i d a l c u r r e n t s are enhanced through constricted passes. This is represented by a Low mud (bedform) facies and ''sand b a n k " d e p o c e n t r e s . Away from the p a s s e s , a H i g h mud (no bedform) facies is developed in relation to "lagoon" depocentres among patch c o r a l r e e f s . The H i g h mud - High c a r b o n a t e d e p o s i t s of the Great North East Channel show evidence of terrigenous input (carbonate d i l u t i o n ) towards the Papuan coast and probably r e f l e c t s the influence of the Papuan rivers.

i A U S T R A L 1 A\

Distribution of bedforms in Torres Strait mapped by combined remote sensing techniques. Pathways of net beload transport represent bedload parting zones and convergences. Postulated depocentres (D) are indicated.

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7.16 PLIOCENE CYCLOTHEMIC INNER SHELF SEDIMENTATION PATTERNS AND LITHOFACIES ASSEMBLAGES, EASTERN NORTH ISLAND, NEW ZEALAND D.W. Haywick Department of Geology, James Cook University of North Queensland

Pliocene sedimentary rocks of eastern North Island New Zealand are primarily products of inner shelf sedimentation and were deposited in an active fore-arc basin during multiple eustatic sea level fluctuations. The shallowly dipping strata are cyclothemic and five regionally correlative siltstone ( 2 - 8 5 metres) - sandstone/limestone (20 - 80 metres) couplets recur within a 450 square km, tectonically undeformed, uplifted block. Sediment input to the basin was from three sources: 1) reworking of pre-Tertiary greywacke (dominant), 2) in situ skeletal carbonate production and 3) contemporary volcanism (minor). Cyclothems may contain up to five distinct lithofacies assemblages: 1) Mid-shelf; thoroughly bioturbated siltstone containing diverse and diagnostic micro- and macrofossil assemblages. 2) Tidally dominated shallow shelf; very well sorted fine sandstone and coquina limestone, bi-directionally planar to dune cross-bedded with a very diverse and diagnostic macrofossil assemblage. 3) Intertidal-beach; very well sorted, planar laminated to low angle cross-bedded fine sandstone and greywacke conglomerate. 4) Estuarine; poorly sorted, sandy, laminated to structureless siltstone, commonly containing conglomeratic lenses, rootlets and a restricted, but diagnostic macrofossil assemblage. 5) Non-marine; poorly sorted greywacke conglomerate with lenses of ripple cross-bedded medium to coarse sand and laminated siltstone, and regionally correlative pumice beds. Rootlets and minor coal seams are common within siltstones. Lithofacies assemblages are characterised by unique, but overlapping, sediment compositions. Vertical lithofacies successions in cyclothems reflect sea level fluctuations and suggest rapid transgressions followed gradual regressions. Lateral interdigitation of lithofacies is rare. Event stratigraphy was the dominant regional control on lithofacies distribution resulting in vertically stacked, laterally continuous rock units of "layer cake" disposition.

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2.5

A MODEL FOR THE TERTIARY EVOLUTION OF THE LITHOSPHERIC STRESS FIELD OF THE GIPPSLAND BASIN K.A. Hegarty^ and G.A. Houseman^

lOeotrack International Pty Ltd, University of Melbourne ^Research School of Earth Sciences, Australian National University, Canberra

There are two major sets of structures present in the Gippsland Basin (Threlfall et al, 1976): i) the basin-forming normal faults which were active principally from the Early Cretaceous to the Early Eocence, and ii) en echelon anticlines and associated shear faults commencing in the Late Eocene. By assuming that each set of structures is generally diagnostic of the local lithospheric stress field at the time of their formation, we infer the major features of the stress field and its evolution in the region of the Gippsland Basin. The first phase of deformation (Phase 1) was related to basin-forming processes resulting in NE-SW extension and normal faulting. The orientation of preserved Cretaceous normal faults both on- and off-shore in the Gippsland Basin imply that the principal extensional stress was oriented to the northeast, the principal compressional stress was vertical and the intermediate stress axis was northwest (see figure below). The main features that formed at this time include the north and south bounding faults (Rosedale and Foster Fault systems) of the central Gippsland graben in which the Strzelecki and LaTrobe units rapidly accumulated.

PHASE 1

PHASE 2

Cretaceous-Eocene

Post- Late Eocene

Schematic representation of the change in stress regime from Phase 1 to Phase 2 m the Gippsland Basin during the Eocene. Length of arrows is proportional to the magnitude of principal deviatoric stress component. Absolute magnitude and sign of intermediate axes are undetermined. (Outward-directed arrows = extension; inward-directed arrows = compression)

The second phase of deformation is manifest as an en-echelon series of anticlines and monoclines throughout the basin and reflect a major realignment of the lithospheric stress field beginning in the Eocene. These structures include the Yallourn and Morwell Monoclines, the Baragwanath Anticline and the Rosedale Monocline/Fault. These features are oriented NE-SW and broadly indicate northwest compression.

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There is usually little significant faulting associated with these features and vertical movements are normally accomodated by monoclinal warping. Amplitude of the displacement can be tens of meters. The inferred stress regime for Phase-2 tectonism (shown above right) is not consistent with the stress field of Phase 1. The general location of the younger compressional features may be largely controlled by the position of earlier Cretaceous failure, but the sense of movement requires a different stress system. The principal extensional axis is now vertical, the principal compressional axis is oriented northwest and the northeast axis is the intermediate stress axis. Earthquake studies (Denham et al, 1981) show that the Gippsland region is largely in compression with the principal stress axis horizontal and oriented NNW Presently forming structures (e.g. Snake Ridge) oriented parallel to the Eocene deformational features are consistent with this stress configuration and suggest that the present stress regime has prevailed from at least the Late Eocene to the present. To explain the transition from extensional to compressional tectonics, we consider the forces that act on the lithosphere to produce the non-lithostatic stress field. The horizontal components of stress are determined by the forces applied to distant plate boundaries. The vertical stress at any point in the lithosphere is proportional to the mass of the overlying load. The evolution of the stress state is controlled by the changing relative contribution of the horizontal and vertical stress components. There are two plausible physical mechanisms that might explain the change in these components defining Phases 1 and 2. The first mechanism is plate-wide and is accomplished by changing the horizontal stresses on the plate boundaries (e.g. Cloetingh & Wortel, 1985), thus altering the balance of horizontal and vertical stresses everywhere in the plate. The second possible mechanism is local and is accomplished by changing the vertical stress in an area largely restricted to the Gippsland Basin. If the extensional phase of basin formation during the Cretaceous was caused by a hot mantle plume pushing the lithosphere upward, then the gradual decay of the plume would significantly change the vertical stress component. The amplitude of the variation in vertical stress may be great enough to produce uplift or depression of the surface by as much as 1 km (Houseman & Hegarty, 1987). A decrease of sufficient magnitude of the vertical stress on the base of the lithosphere is all that is required to transform the stress state from extensional to compressional. Strike-slip or wrench tectonics can be introduced by modifying the relative contribution of the stress components. Previous models of Gippsland's Tertiary compression usually invoke a late overprinting tectonic event apparently unrelated thermo-mechanically to the earlier Mesozoic extension. Our model implies that today's compressional regime is directly related to the evolution of a stress system which began as Cretaceous extension when the lithosphere was uplifted, and presently describes net compression as a result of lithospheric subsidence. Cloetingh, S. and R. Wortel, Regional Stress Field of the Indian Plate, Geophys. Res. Lett., 12(2), 77-80,1985. Denham, D., J.Weekes and C. Krayshek, Earthquake Evidence for Compressive Stress in the Southeast Australian Crust, J. Geol. Soc. Australia, 28,323332 1981 Houseman! G.A. and K.A. Hegarty, Did Rifting on Australia's Southern Margin Result from Tectonic Uplift?, Tectonics, 6(4), 1987. Threlfall, W.F., B.R. Brown and B.TI. Griffith, in Economic geology of Australia and Papua New Guinea, 3 - Petroleum, 41 -66,1976. 179

16.4

DIGITAL INTEGRATION OF GEOLOGY WITH GEOPHYSICAL AND AND SATELLITE-SENSED DATA SETS IN THE ATHERTON 1:250 000 SHEET AREA R.A. Hegartyl, W.P. Laing^ and P. Catt^ ^Queensland Department of Mines 2james Cook University of North Queensland

Numerical integration of regional data has been largely confined to geophysical (generally airborne) and satellite sensed data. This study sets out to integrate these datasets with geology, by digitising existing geological maps to produce an additional dataset with lithology as a variable. Each dataset is sampled/resampled on a 200m spaced grid over the study area to form a database of coregistered images (or "geographic information system") which allows digital integration of the sets. The variables in use are lithology, mineral occurrences, total magnetic intensity, radiometric values (4 channels), LANDSAT MSS (4 channels), LANDSAT TM (7 channels), and NOAA AVHRR (5 channels). The study has four broad aims: (a)

to draw qualitative interpretations from analysis of enhanced images of individual datasets and hence define characteristic responses for the various lithologic units;

(b)

to analyse and integrate the datasets quantitatively using statistical software and supervised classification;

(c)

to document the methodology of these procedures;

(d)

to assess the potential of these methods and datasets regional geological mapping program.

for assisting a

A senario we are particularly exploring is the routine production by Government surveys of computer integrated "total data compilation sheets", for every 1:100000 and 1:250000 sheet area. These sheets would be used: (a)

as maps of a "new style of geological unit", which is not purely lithological but "ultra-lithological";

(b)

by comparison with the existing geological sheet, to outline areas of erroneous or incomplete geology which would be targeted first in a remapping program;

(c)

in regions of thin but total overburden, as "best guess" mapping sheets, to stand in the survey map in inventory with an appropriate reliability rating.

The Chillagoe and Mungana 1:100000 Sheet areas were selected for the study (50km X 100km area) as there are recent datasets of all types, including recently completed mapping by the Geological Survey of Queensland. The area is particularly suited to digital integration methods as it contains a wide variety of rocktypes which have sharply defined boundaries; these include Precambrian metamorphics, Siluro-Devonian sedimentary rocks, Permian to Triassic volcanics and granitoids, and Cretaceous and Quaternary sediment cover. Any internal variations recognised within some of the major units, using the datasets, would be of much importantance in areas of poor outcrop (such as the Precambrian metamorphics) or poor access (such as the Featherbed Volcanics). The study area has been, and still is, the scene of intensive mineral exploration, and mineral occurrences form an additional dataset for integration.

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The first stage of the project (completed) was to assemble the datasets as coregistered images on the operating system COMTAL/SLIP at the Department of Mapping and Surveying, Brisbane. Considerable time and effort was required to trace, videodigitise, tidy, paint, and patch the lithologic maps for the two 1:100000 Sheet areas, and to rectify (warp to AMG) and window the exact study area in the satellite datasets. The lithologic map physical and satellite images to aid qualitative interpretations) and a painted image with a "value" allocated for each lithologic unit to allow numeric integration with the other datasets. The second stage of the project (still underway) is the qualitative and quantitative interpretation of the data. Standard image enhancement techniques provided on the operating system are used, but the methods of intergrating the datasets have required rethinking on the basis of other workers' results and advice. Classification techniques widely used in geographical applications of remote sensing have rarely been successful for geological mapping, particularly with unrelated datasets such as magnetic values and LANDSAT bands, but a supervised classification using representative training areas and well correlated variables will be tested for the study area to produce "predictive" geological maps based on the datasets. The project methodology is necessarily very flexible to allow testing of many techniques. The final stage of the project will involve ground truthing of geological and geophysical interpretations, IRIS spectral radiometer measurements, and presentation of visual, numeric, and interpretive results. Recommendations may be made on the practicability of the computer integrated compilation sheet approach to regional mapping.

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12.2

MINERAL LANDSCAPING TOWARDS 'BLOOMING' EVENTS DEVONIAN EXTINCTION RECORD WITH MODERN MANAGEMENT IMPLICATIONS E.J. Heidecker Department of Geology & Mineralogy, University of Queensland

Proliferation of algae, even to the point of cataclysmic 'blooming', is a developing problem in seas, lakes, and waterways. The geological record of 'blooms' and associated mineral landscaping is likely to assist identification of key controls. 'Blooming' events associated with Devonian mass extinctions have been studied in Germany and Western Australia. Mineral landscapes along the shores of 'blooming' seas can be reconstructed from the Upper Devonian record of the Burdekin Basin, northeastern Queensland, and are characterised by: 1.

Mineralised mud accumulation in littoral structural traps These traps were sited to collect connate fluids collected by growth structures from thick underlying limestone piles disturbed by volcanism. Solutions leached from uplands subjected to oxidation and acid volcanic fallout appear to have gravitated to these traps.

Absence of ecosystems which might have dissipated or accumulation of mineralised mud and microbial products.

restrained

Rippled laminated sediments are superficially marked by trace fossils with limited release of mineral matter. Protective forests are not indicated by the fossil record. These littoral accumulations were perched for cataclysmic release of nutrient and toxic minerals favoring microbial blooms. Release might have been by tsunamis, seiching, storm surges, meteoric impact, and sea level changes associated with geological or planetary disturbances. Accumulations of mineralised industrial and urban effluent along shores pose similar threats if restraining mangroves are destroyed and if reduced numbers of shell-fish and crustaceans are unable to dissipate waster mineral matter.

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6.6 A FLUID INCLUSION AND STABLE ISOTOPE STUDY OF SYNMETAMORPHIC COPPER ORE FORMATION AT MOUNT ISA C.A. Heinrichl'2, A.S. Andrew^, R.W.T. Wilkins^ and D.J. Patterson3 ^CSIRO Division of Mineral Physics and Mineralogy, North Ryde ^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Mount Isa Mines Pty Ltd, Mount Isa

Previous structural studies have shown that the dolomitic and siliceous breccias and contained copper mineralization at Mount Isa formed during regional deformation and greenschist facies metamorphism of their host Proterozoic metasediments. The breccia body (called "silica dolomite") is zoned, with an outer halo of sparry dolomitic alteration replacing finely laminated dolomitic - pyritic metasediments. The core of the dolomitic breccia next to the underlying altered greenstones is in turn overprinted by siliceous breccia containing the bulk of the chalcopyrite ore. Fluid inclusions in quartz and dolomites were studied by microthermometry, Raman microspectrometry, and semiquantitative electron microprobe analysis of inclusion salts. Two types of aqueous inclusions are restricted to dolomitic breccia: (1)

A CaCl2-rich brine with a cation ratio Na:Ca:K:Mg 30:10:3:1 and salinity (25 wt percent NaCl equivalent) similar to recent Salton Sea geothermal brines; (Group 1 inclusions; homogenization temperature Tj^ = 190-290°C); and

(2)

A low-salinity fluid with 10-20 mol percent CO2 (Group 2 inclusions; Tu = 270-290®C). These two aqueous inclusion types are intimately associated in the same crystals and have probably both been simultaneously trapped during dolomitic alteration.

Siliceous alteration which has overprinted dolomitic breccia contains NaCl-rich fluid inclusions (Group 3: Na:Ca:K:Mg 300:10:30:1, CH^ probably exceeding CO2; Tu = 140-180®C). Textures suggest that a higher-salinity (10-20 wt percent NaCl equivalent) and a lower-salinity (4-9 wt percent NaCl equivalent) variants of this NaCl-rich fluid bracket the main stage of chalcopyrite introduction. There are no low density vapour inclusions indicative of fluid boiling at any stage, but rare high-density CH4 (-C02)-rich carbonic inclusions occur in a few samples in association with NaCl-rich aqueous inclusions with slightly higher than average homogenization temperature. Secondary inclusions of a highly calcic low-temperature brine (Group 4 inclusions) occur on late cracks and postdate all major stages of mineral precipitation. The 0, H and C isotope compositions of dolomites, quartz, and hydrous silicates and of fluid inclusion extracts from samples of the siliceous ore stage were dominated by an externally derived, weakly CH4+CO2 bearing fluid at high fluid/rock ratios and slightly variable temperatures. This NaCl-rich fluid may have originated as a highly evolved basin brine or an evaporite-derived metamorphic fluid. The early CaCl2-rich fluid may have had a similar origin to the NaCl-rich ore fluid. However, differences in the D/H ratio, and mass balance constraints placed by the cation ratios, suggest that the two brines were not derived from a single incoming fluid by progressive rock-interaction

183

within the zoned alteration system. Similarly, an independent source is likely for the C02-rich fluid. The absolute pressure - temperature ranges for the two alteration processes seem to be slightly different, but it is uncertain whether the quartz - chalcopyrite mineralization occured at slightly lower temperature or at slightly higher pressure compared with the dolomitic alteration. The latter is 270-350®C and 700-1500 bar by isochore and mineral stability data. The following working hypothesis for the syntectonic and synmetamorphic fluid/rock interaction and copper ore formation at Mount Isa is proposed. In an initial stage, dolomitic alteration occurred by chemical interaction between the Urquhart Shale and moderate amounts of two fluids of different chemical and hydrological origin, the CaClo-rich and the C02-rich fluids. Fluid mixing may have been aided by an anisotropic (regional-tectonic) stress distribution which temporarily lowered fluid pressures, thus leading to hydraulic brecciation and favouring communication between two fluid regimes. It is not yet clear which of the two fluids represents the local "metamorphic" fluid of the Urquhart Shale, and which was introduced from the underlying altered greenstones. The initial stage of brecciation and dolomitic alteration was followed by the main stage of silicification, copper introduction and by the much more copious NaCl-rich fluid. In the main 1100 orebody, this may have occurred at near-lithostatic fluid pressure. Quartz and chalcopyrite precipitation probably occurred by a combination of slight cooling, and a pH and/or sulfur activity change, as the initially reduced, acid and probably sulfur-deficient brine reacted with the previously dolomitized pyritic dolimitic Urquhart Shale.

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14.7 ALBIAN-CENOMANIAN BIOGEOGRAPHY OF AUSTRALASIAN AMMONITES - A MID-CRETACEOUS FIRST FLEET AND ITS PALAEOCEANOGRAPHIC INTERPRETATION R.A. Henderson Department of Geology, James Cook University of North Queensland

Ammonite faunas from continental margin locations provide new insights into the biogeography of Australasian marine invertebrates. Traditional views have been biased by the low-diversity, endemic aspect of marine faunas which developed in the epicontinental sea of the Great Artesian Basin. An Austral faunal province has been widely accepted for Australasia in mid-Cretaceous time, characterized by such elements as Labeceras, Myloceras and Dimitobelus It has generally been interpreted as representing a cold-water fauna consistent with the high latitudinal position of Australasia in mid-Cretaceous time and enhanced by a circum-polar southern hemisphere oceanic circulation cell. Mid-Cretaceous ammonite faunas from shelf seas of continental margin location are well represented in the Beagle Gulf area of northern Australia and Marlborough, New Zealand. They contrast strikingly with those known from the Great Artesian Basin in that they are dominated by cosmopolitan elements well known from western Europe and elsewhere. The Great Artesian Basin assemblages are best interpreted as the product of the restricted marine conditions of an epeiric sea rather than as resulting from low ambient water temperatures. The cosmopolitan flavour of shelf sea assemblages from the continental margin of Australasia is an expression of pandemic midCretaceous oceanic circulation. They suggest warm-water conditions which have been independently confirmed by palaeotemperature estimates based on oxygen isotope data.

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10.2

GEOCHEMICAL EVOLUTION OF GROUNDWATERS FROM THE GREAT ARTESIAN BASIN, AUSTRALIA

A.L. Herczeg^,^, T. Torgersen^, M.A. Habermehl^ and A.R. Chivas^ ^Research School of Earth Sciences, Australian National University, Canberra ^CSIRO Division of Water Resources Research, Glen Osmond ^Department of Marine Sciences, University of Connecticut Avery Point, Groton, CT, USA ^Bureau of Mineral Resources, Geology and Geophysics, Canberra

The Great Artesian Basin covers about 20% of the Australian continent and includes two major aquifer sequences in the Jurassic and Cretaceous respectively. The Jurassic (or J-) aquifer is the most important source of water in terms of quality and abundance for town, pastoral and mining supplies. Recharge occurs primarily in the eastern and southeastern margins of the basin with discharge through springs in the interior of Australia near the salts lakes (Eyre, Frome). The groundwaters in the basin proper are relatively fresh (up to 50 meq TDS) and are of the Na-HC03-Cl type in the central part of the basin while the south- westem portion waters are characterized by Na-Cl-S04. This study focuses on maior (^Na, K, Mg, Ca, CI, HCO3, SO4, SiOj) and minor (Sr, Br, F) element and isotopic O^^Cr^C) mass- balances throughout the basin along with an equilibrium chemical model to elucidate the important mineral-water reactions and other processes that control the chemical composition of the waters. The results indicate that with progressive distance along inferred flow lines, Na and K are removed from the waters. (relative to CI - see Fig. 1) by reconstitution reactions involving kaolinite, a Na-smectite (Fig. 2) and illite. Ca and Mg are removed rapidly near the recharge areas and behave essentially conservatively throughout the interior of the basin. Silica concentrations are a factor of 4 or more undersaturated with respect to amorphous silica thereby supporting the suggested clay mineral control of at least Na and K. Furthermore, XRD studies of aquifer solid material show that quartz, kaolinite, smectite, illite and a carbonate to be the dominant minerals within the J-aquifer matrix. Na/Ci v*s log aCI

Seawater

log aCI

Fig. 1 The ratio of aNa/aCl vs log aCl for groundwaters within the J-aquifer of the GAB. Na is removed from solution relative to chloride with increasing concentration of chloride.

186

log aNa+ZaH-h v*s log aSi02

-2.5

-5.0 log aSi02

Fig. 2 Log activity diagram for the Na-system show that the J- aquifer waters plot within the stability fileds of kaolinite and Na-beidellite. Na concentration is therefore probably controlled by reactions involving these minerals with the dissolved species. of total dissolved inorganic carbon tends to become more positive along the inferred flow paths (Fig. 3). Mass balance calculations indicates this trend could be produced by open-system addition of CO2 and dissolution of CaC03 and /or production of CO2 via methanogenesis.

g Q-

-2

-4

-6

-8

I 5

- 1 0

- 1 2

- 1 4

T A l k (meq/kg)

Fig. 3. of dissolved inorganic carbon as a function of total alkalinity for J-aquifer waters.

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7.14

CATCHMENT SEDIMENTATION AS AN ACTIVE GEOMORPHIC C.M.

PROCESS

Hill

Department of Applied Geology, , Queensland Institute of Technology

Erosion rates in the South Johnstone River Drainage Basin near Innisfail in North Queensland are some of the highest recorded in Australia. Major sediment problems exist in the area due to a high annual rainfall and the presence of erodible soils in the catchment areas. In addition, land uses not best suited to the steep land types exist over much of the catchment. Hydrological modelling of the lower catchment has indicated substantial suspended sediment loads associated with high run off. Most hydrological investigations and subsequent models rely on very limited and sometimes no data related to suspended or bed load sediments. The integration of easily obtainable basic sediment data greatly increases the accuracy of model predictions of sediment loads and thus the ensuing social and/or economic effects of sediment aggradation or erosion. This paper therefor discusses basic and cheaply available methods of collecting channel bed, core and suspended sediment samples. These methods are currently being employed in the study of the South Johnstone River drainage basin. The data will ultimately provide the basis for the calibration and testing of a mathematical hydrosedimentological model to simulate the sediment budget of this and similar drainage basins. The application of standard sampling methods for both suspended and channel bed sediments has been modified by restricted funds available for a significant sampling program. Use has been made of existing resources, local involvement and the utilization and development of cheaper sampling equipment. Despite these problems the sampling program has been more than adequate to date and, given favourable weather conditions during the 1987/88 wet season, sufficient data will be obtained to complete the requirements of the project. The bulk of sediment sampling throughout the drainage basin was accomplished using the standard technique of grab sampling. In excess of 200 sediment samples have been taken to date with sample coverage extending through -out the 530km' catchment but with the greatest sample density occuring within the lower drainage basin and caneland creeks. Sample weights of 1kg were returned to the laboratory where they were oven dried prior to sieving and microscopic examination. Standard grain size statistics were Computed and details from the microscopic examination were entered directly onto a computer data base. All sediment samples are catalogued on a main data base which lists location, number, drainage line, cross references and details of other analyses carried out on the sample. Other analyses include. X-ray Diffraction for mineral indentification, microscopic analysis, elemental analysis of clays using a scanning electron miscroscope, the method of sieving, (wet, dry or disaggregation) and other relevant details. Coring was attempted at selected sites, especially on large, exposed, midchannel bars in the lower reaches of the river. PVC pipe with an internal • diameter of 50mm and a wall thickness of 3mm was cut into 2m lengths and forced, manually, into the^ sediment. Depth penetration was limited by cobble and/or clay layers. Recovery was excellent and the cores were subsequently split in the laboratory using a band saw.

188

A 100mm diameter manual soil auger with a maximum depth penetration of 5m was used at selected sites to establish cross-sectional profiles adjacent to the river. This provided acceptable profiles of soils and overbank flood deposits at locations, identified from air photography, where there had been an obvious migration of the main channel. Data from these profiles provides a history of sediment change and alterations in the flow regime of the river. Water sampling to provide data on suspended sediment loads was untertaken using a gulp/depth integrated water sampler. However, unfavourable weather conditions during all field visits to date meant that suitable suspended sediment samples were unobtainable as there was no significant above average flow recorded in the river or creek system during these times. The problem of sampling during flood events therefore posed a significant problem. The need for some form of permanent, automatic sampling device was evident. Review of literature and discussions with other workers indicated that a rising-stage water sampler would be the most effective instrument. Commercially available products were cost prohibitive due to the limited budget therefore the author designed and built a sampler. Six prototype samplers were constructed using readily available materials. Each sampler is 2m in height with five water intake nozzles 40mm apart on one side of a length of 100mm x 60mm hardwood which provides the main support. The nozzles, formed of 8mm diameter copper pipe, feed through the support into a 100mm gooseneck which empties into a 1000ml plastic bottle. An air exhaust from the bottle culminates in an airlock 70mm above the intake. Under zero flow conditions a 1000ml sample is collected in 11 seconds, this time decreases as flow increases. This design has proved to be relatively easy to maintain once positioned in the field. Unfortunately the reliability of the samplers has yet to be tested due to a lack of significant flood events during the 1986/87 wet season. One small event in a caneland creek filled the lower bottles of one sampler but the suspended sediment load during this event was not significantly above background values. The samplers are backed up by Foxborough stream stage recorders serviced by the Queensland Water Resources Commission. The application of standard sedimentological techniques to catchment studies enables the sedimentologist or engineer to quantify predicted sediment loads and sediment budgets derived from normal hydrological modelling. A simple, yet comprehensive, channel sediment and suspended sediment monitoring program can be carried out relatively inexpensively. The extra cost involved in the sedimentological study enables more accurate hydrosedimentological modelling and thus decreases the possibility of errors in the prediction of sediment aggradation/erosion in catchment studies.

189

7.15 APPLICATION OF SEDIMENTOLOGICAL TECHNIQUES TO CATCHMENT HYDROLOGICAL STUDIES - A CASE STUDY C,M.

Hill

Department of Applied Queensland Institute of

Geology, Technology

Some of the highest rates of soil erosion recorded in Australia have occured in canelands of the South Johnstone River Catchment in North Queensland. Significant sediment aggradation has been noted in both drainage basin creeks and the river itself within the past decade. Inappropriate land use in the lower catchment and a highly weathered granitic upper-catchment are the major contributors of sediment in the drainage basin. This paper combines historical, sedimentalogical and hydrological data as well as multi-dimensional numerical modelling in an effort to document and quantify the sediment budget. This work is part of an ongoing multidisciplinary study aimed at delineating the sources and extent of sediment input and the rate and extent of soil and sediment redistribution within the South Johnstone River Drainage basin. Detailed grab sampling of channel sediments augmented by both coreing and augering indicates a considerable difference in the deposited sediment found in the river and caneland creeks. Channel sediments within the South Johnstone River range from medium gravel to coarse sand and are pale yellow to dull yellow-orange in colour. The dominant river sediments are quartzose sands derived from the upper catchment Tully Granite complex. The minor presence of ferruginous nodules in sediments of the middle and lower catchments indicates input of basalt derived material. However, significant percentages of silt/clay sized material from eroded krasnozem caneland soils are not evident in the river sediments. Large quartzose sand bars have been observed along the entire length of the South Johnstone River both from aerial photography and in the field. These migratory bars pulse vast quantities of sand down the river channel in response to high flow and flood events. The practically unlimited supply of freshly eroded granitic material available within the upper catchment ensures a continuous and consistant supply of quartzose sand for this ongoing morphological process. The result of this process is the formation of large sand islands and extensive cross-channel bars in the low gradient tidally affected reaches of the river. The composition of channel sediments of the lower catchment caneland creeks reflects a basaltic source with minor input of metamorphic material and reworking of alluvial deposits. Sediment size ranges are significantly controlled by the grain size of caneland soil material which provides in excess of 90% of the channel sediment. Preliminary modelling indicates that in excess of 80% of eroded sand/gravel sized material from the canelands is deposited within the caneland drainage system. South Bamboo Creek has been used as a representative caneland creek system within the South Johnstone Drainage basin. Extensive sampling throughout this sub-catchment shows that the sand/gravel size fraction comprises <30% of the total volume of derived sediment. Thus the fate of only 25% of the total volume of eroded caneland material can be adequately accounted for. Grain size distributions ofv caneland material were affected by the presence of ferruginous nodules with sizes in the sand to gravel range. Initial sizing involved the disaggregation of these nodules into their composite

190

clay/silt fraction. Sediment and hydrological modelling" suggested that all size fractions less than fine sand should be removed from the caneland drainage lines during moderate storm events. Field observations showed that in practice this was not occuring and that there were substantial deposits of ferruginous nodule material along caneland drainage lines. Selected grab samples were then sieved using both the wet and dry methods but without disaggregation of the sample. The results of this treatment showed that when treated as composite particles the nodules returned a clay fraction of <5% as against a clay fraction of >90% when disaggregated. Under both wet and dry sieving conditions there was no noticable deterioration of the nodules. The conclusions drawn from this exercise were that the nodula material should be treated as composite particles and further modelling using this data has provided more realistic estimates of sediment load. Preliminary modelling suggests that >70% of the eroded caneland material reaches the river. However, except at the confluence of caneland creeks with South Johnstone River, where there is an increase in nodula material, there is no evidence of the expected quantities of caneland material (800000 tonnes/annum) within the river channel sediments. The significance of these observations is that sedimentation in the South Johnstone River is controlled and dominated by the availability of plutonic quartzose sediment from the granitic upper catchment. Sedimentation within the caneland creeks appears to have no significant effect on sedimentation patterns within the river channel and silt/clay sized material is readily flushed from the river under average conditions. Unfavourable weather patterns during the past eighteen months have not allowed sufficient suspended load sampling throughout the river and upper catchment channels to allow for the final calibration of the computer model and subsequent quantification of the sediment budget. Monitoring will be continued through the 1987/88 wet season (November to March) in an effort to define the relationship between natural erosion and river morphology and the effects of man made erosion in this drainage basin.

191

3.18 TECTONIC INTERACTIONS BETWEEN NEW GUINEA AND THE CAROLINE PLATE - IMPLICTIONS FOR BACKARC SPREADING K.C. Hill^ and K.A. Hegarty^ ^Department of Geology, University of Melbourne ^Geotrack International Pty Ltd, University of Melbourne

At the northern margin of New Guinea there is a fundamental problem related to the relative timing of arc-continent collision(s) represented in the Papuan Fold-Belt (PFB) and backarc spreading in the Caroline basins (Figure 1). New fission track dates from basement and sediments within major anticlines of the PFB indicate that uplift occurred at 4.0 ± 0.5 Ma. The timing is consistent with the fold and thrust shortening (more than 100 km) of the PFB which occurred in the Late Miocene/Pliocene as evidenced by the deformed Miocene carbonates and Pliocene molasse deposits. The Middle and Late Miocene volcanics and intrusives of the New Guinea Mobile Belt (Figure 1) were intensely folded and thrusted during syn to post Late Miocene compression. The Oligocene island arc deposits of the Northern Volcanic Province (Figure 1) were also thrust in the Late Miocene and Pliocene. However, earlier work has suggested that ophiolite emplacement and associated metamorphism in the New Guinea Mobile Belt occurred in the Late Oligocene/Early Miocene. These two compressive deformations were separated by a period of tectonic quiescence in the Middle Miocene. The Late Miocene/Pliocene deformation was pervasive through all three structural provinces, involving basement in each. In contrast, the Late Oligocene/Early Miocene deformation is restricted to metamorphism and ophiolite emplacement in the New Guinea Mobile Belt, with negligible deformation of the continental crust. Thus the evidence suggests that the Late Miocene/Early Pliocene deformation was of considerably greater magnitude than the earlier obduction event. The Caroline Plate lying north of New Guinea has been interpreted as a backarc basin which commenced spreading in the Eocene and ceased spreading in the Late Oligocene (about 28 Ma). A remnant arc presentiy docked to New Guinea, the Northern Volcanic Province, was active in the Eocene and Oligocene and is believed to be the arc related to the formation of the Caroline Plate. The cessation of spreading of the Caroline Plate has previously been interpreted to be due to arc continent collision during the Late Oligocene/Early Miocene, but such a collision is here thought not to have occurred In the Cretaceous and Palaeocene subduction was to the south beneath New Guinea, but "flipped" in the Eocene to give a north-dipping subduction zone far to the north of New Guinea, driving Caroline Plate backarc extension (Figure 2). This backarc basin reached a limiting size at about 28 Ma resulting in the end of subduction. With the continued northward movement of Australia, Mesozoic oceanic crust was obducted southward onto the New Guinea margin in the Late Oligocene/Early Miocene. In the Miocene south-dipping subduction beneath New Guinea recommenced until the now extinct Caroline Plate arc collided with New Guinea in the Late Miocene/Pliocene, causing extensive regional compression and probably sinistral strike-slip faulting. This new tectonic framework suggests that backarc basins reach a limiting ridge-trench distance of 1000-1500 km, beyond which "subduction-pull" cannot drive further spreading so subduction and spreading are terminated.

192

MOBILE STABLE BELT P L A T F O R im| I

N G M B - N E W GUINEA 1 1 0 MOBILE B E L T PFB-PAPUAN FOLD-BELT P N G - P A P U A NEW GUINEA ^ ^ • - O P H I O L I T E / U L T R A B ASICS

I NORTHERN VOLCANIC PROVINCE

Cretaceous and Miocene v o l c a n i c s

NE EARLY

PLIOCENE

Eocene and Ollgocene volcanics

Island arc collision

TRUK

PHILIPPINE PLATE ,

MIDDLE

Shallow water limestone

MIOCENE Reefal limestone

M U M

LATE OLIGOCENE/EARLY

CAROLINE PLATE

MIOCENE Remnant Eocene/Oltgocene arc CAROLINE PLATE

Ophiollte obductlon

C e a s e s u b d u c t l o n , volcanlsm and b a c k a r c spreading

Metan.orphism

Initiate b a c k a r c s p r e a d i n g

BISMARCK SEA

to c r e a t e C a r o l i n e Plate

SSTASLE LATFORM CORAL SEA

Late Cretaceous and/or Palaeocene C l a s t l c s from o c e a n i c crust A u s t . eraton

CRETACEOUS/PALAEOCENE Mesozolc Oceanic c r u s t

1 0 0 0 km

AUSTRALI Figure 1:- T h e structural provinces of New Guinea (shaded) and the Caroline Plate.

F i g u r e 2z-

A p l a t e t e c t o n i c m o d e l for t h e e v o l u t i o n of N e w G u i n e a a n d t h e C a r o l i n e P l a t e .

13.9

ASPECTS OF THE GEOLOGY OF QUEENSLAND TERTIARY OIL SHALES A.C. Button

Department of Geology, University of Wollongong

In 1968, the granting of a permit to evaluate the economic grades of vanadium in the Toolebuc Formation at Julia Creek heralded the beginning of the modern phase of interest in Australia's oil shales. In the decade from 1968 to 1987, many previously-known deposits were re-evaluated and a number of new deposits were discovered. Initially, activity centred on exploration and the geology of the deposits whereas in the latter half of the decade, activity was directed towards retort technology and related fields. Now in 1988, the first year of the second decade of the modern oil shale industry, it is fitting to summarize what is known of the geology of Australia's oil shale deposits. However, to cover all aspects of all deposits evaluated over the past ten years would be a lengthy presentation. Consequently only the Tertiary deposits of Queensland will be considered and for these deposits, only four aspects of the oil shales will be considered - organic petrography, mineralogy, elemental composition of the oil shales and chemistry of the oils. Organic Petrography. Organic petrographic studies of the Tertiary oil shales of Queensland is of historical importance because it established, beyond doubt, that most of the oil-producing organic matter in the majority of the oil shales was derived from algae. In addition, organic petrography was important for two other reasons. i. The recognition of organic matter using fluorescence mode microscopy was quickly used to distinguish the Tertiary oil shales from other types of oil shales such as torbanite from Alpha and marinite from Julia Creek. Consequently, a useful classification of all oil shales was erected and adopted. ii. Organic petrographic techniques established that the Tertiary oil shale successions are composed of several lithotypes and that vertical variations in the type and abundance of organic matter, and therefore variations in the type and abundance of lithotypes, are more pronounced than lateral variations. These data are useful in understanding the genesis of the deposits, for mine planning and retorting behaviour. The composition of the five main lithotypes found in Queensland Tertiary oil shales can be summarized as follows: Cannel Coal - generally a dark greyish-brown to black rock composed of abundant vitrinite with liptinite (such as sporinite, cutinite, resinite and suberinite) derived from terrestrial plants; the organic matter constitutes greater than 50 volume percent (vol%) of the bulk rock; cannel coal is formed in a peat swamp. Carbonaceous Shale - generally a dark greyish-brown to black rock composed of less than 50 vol% organic matter (vitrinite + liptinite); carbonaceous shales contain the same organic assemblage as cannel coal and were probably formed when the peat swamped vwas flooded and the input of clastic detritus restricted the growth of terrestrial plants; Lamosite - a brown or olive green to greyish-green lacustrine rock composed of abundant liptinite derived from algae (alginite) with minor vitrinite; in some samples, vitrinite may be co-dominant or even dominant over the alginite (for such samples the term carbonaceous lamosite is used); 194

Claystone - a grey, white or green lacustrine rock containing less than 5 vol% alginite; in many intervals claystones grade into lamosites; Carbonate - a white to greenish-grey rock composed predominantly of one or more carbonates; in alginite-dominated intervals limestone, dolomite and sideritic layers occur but in vitrinite-dominated intervals sideritic layers are the dominant carbonate; organic matter is a minor component of the carbonate with the type of organic matter related to the associated lithotypes in which the carbonate is found (for example alginite in limestone or dolomite and vitrinite-terrestrial liptinite or alginite in sideritic layers). In several deposits, the succession contains one or more cannel coaldominated sequences and one or more lamosite-dominated sequences. The former is commonly termed carbonaceous oil shales and the latter, brown oil shales. Using lithotypes, petrographic properties and mineralogy, it is possible to divide the oil shale into four types: Rundle Type - dominantly lamosite, containing orange to yellowish-orange fluorescing alginite, with interbeds of cannel coal and carbonaceous shale; Condor Type - lamosite, containing orange fluorescing alginite and in part sideritic, without interbeds of cannel coal; however cannel coal may occur in units above or below the oil shale sequence; Duaringa Type - lamosite, with yellow to green fluorescing alginite, without cannel coal interbeds or coal in units above or below the oil shale» Mt Coolon Type - dominated by cannel coal. Mineralogy. In most Queensland Tertiary oil shales, quartz and clay minerals are the dominant species with locally abundant carbonate (calcite, dolomite or siderite). The clay minerals are predominantly montmorillonite, illite, mixed layer clays and kaolinite. Other minerals recorded include apatite, pyrite and feldspars (all deposits), analcime (RundleStuart) the ammonmium feldspar buddingtonite (Condor, Byfield and RundleStuart), cristobalite, gypsum, geothite, hematite and opaline quartz (various deposits), vivianite (Duaringa), alunite and anatase (Lowmead). Elemental Composition of Oil Shale. The elemental composition of the oil shale reflects the type and abundance of contained macerals. Most Rundle, Condor and Duaringa type oil shales have relatively high H and relatively low 0 and, where the vitrinite content is less than 5 vol%, the H/C ratios are generally greater than 1.2. However, where the vitrinite content increases, 0 increases and H decreases correspondingly. The Mt Coolon type oil shales have relatively low H and relatively high 0 as would be expected where the organic matter is vitrinite-rich, or is dominated by vitrinite, as these oil shales are. The H/C ratio is generally less than or equal to 1. For most oil shales sulphur, nitrogen and O/C ratios do not appear to be dependent on the maceral content as much as the H content and H/C ratio. Chemistry of the Oils. As with the elemental composition of the oil shales, the chemistry of the oils is related to the type and abundance of macerals. Oils from the Rundle, Condor and Duaringa type oil shales are dominated by alkene/alkane doublets with flash pyrolysate oils having a greater proportion of alkenes and a much higher proportion of higher carbon number (>C][7) homolog^ies than Fischer assay oils. 1-pristene and 2-pristene are generally more abundant in the flash pyrolysate. In oils from Mt Coolon type oil shales, the flash pyrolysate oil is characterised by alkene/alkane homologues which are most abundant at higher carbon numbers. 1-pristene, phenols and other aromatics are also abundant. The Fischer assay oil is rich in low carbon number alkene/alkane homologues. 195

6.7 TECTONIC EVOLUTION OF THE BASAL DRUMMOND BASIN SEQUENCE AND ITS RELATIONSHIP .TO GOLD MINERALISATION L.J. Button -

Queensland Department of Mines

The Drummond Basin is a large intracratonic basin early carboniferous age which crops out over 25 000 km^ in Central Queensland. It overlies cratonised lower Palaeozoic basement.

The basin formed as a foreland basin in response to a west dipping subduction zone which lay to the east during the late Devonian. It is postulated that the Drummond Basin developed in stages with the major stages of development co-inciding closely with the "Cycles" of Olgers (1972).

During "Cycle I" time (late Devonian), the basin is characterised by the development of a series of north-south trending horsts and grabens, particularly in the northeastern part of the basin. Acid, intermediate and mildly peralkaline pyroclastics and minor lavas make up the bulk of the fill in these grabens. In the northeastern corner of the basin, the volcanics (Bimurra Volcanics, Silver Hills Volcanics and unnamed volcanics) are interbedded with sediments and epiclastics (Mount Wyatt Formation). The sediments are derived from the volcanics with little or no sediment sourced from outside the basin. Elsewhere in the basin, the Silver Hills Volcanics make up the bulk of this cycle with little or no component of sediment. Large thickness variations, which occur between adjacent blocks, is interpreted as being due to graben formation.

Late in Cycle I time (early Carboniferous) the tensional phase of basin development ceased resulting in the cesation of deepening of the grabens. The major phase of basinal subsidence then began with sediment being derived from outside the basin with little or no volcanic component.

Gold mineralisation occurs in sediments and volcanics of the northeastern Drummond Basin, particularly at Bimurra and Wirralie. This mineralisation appears to be an epithermal type. The age of the mineralisation is not known. However it is postulated to have formed during a late Carboniferous to early Permian intrusive phase. The gold may have been introduced during this intrusive phase or may have been leached from the late Devonian volcanic pile by circulating connate waters driven by the heat of the intrusion and deposited in pre-existing fractures (margin faults to the grabens?). At Mount Coolon, gold occurs in a fracture zone in the late Carboniferous Bulgonunna Volcanics, possibly related to a similar epithermal system to those which operated at Bimurra and Wirralie. The Bulgonunna Volcanics at Mount Coolon are intruded by tonalite of possibly similar affinities to the intrusives at Bimurra and it is proposed that the mineralisation is related to the same event even though it occurs in rocks of a different age.

OLGERS,F., 1972: Geology of the Drummond Basin. Resources, Australia, Bulletin 132.

196

Bureau, of Mineral

3.7

ACCUMULATED APPARENT POLAR WANDER - A NEW PHENOMENON IN PALAEOMAGNETISM M. Idnurm Bureau of Mineral Resources, Geology and Geophysics, Canberra

The apparent polar wander paths for different continents contain many bends and loops, giving the impression that throughout geologic time the continents moved under a complex, even chaotic, system of forces. However, a new type of analysis of the polar wander data now suggests that this impression is not correct; that there exists a certain order among those forces. In this analysis the accumulated angle of apparent polar wander (AAPW) is plotted against time, and it is found that for the last 2Ga the plots c o m p r i s e a small number of segments which are linear and w h i c h join at abrupt angles. Such segments indicate that the polar wander proceeded regularly, at a constant rate over long periods, giving way eventually to another different, but still constant, rate. The linearity in the AAPW plots may have several important implications for Earth sciences. First, it promises to give new insights on mantle dynamics. A simple interpretation of the phenomenon can be found is in terms of mantle convection models in which the convective cells have bilateral rolls on two mutually orthogonal axes, the latter lying respectively in latitudinal and meridional planes. Also, the abrupt changes in the rates of polar wander seem especially significant from the viewpoint of mantle evolution. Second, the phenomenon of linearity suggests that the geocentric axial dipole model is a reasonably good approximation for geomagnetic fields extending at least as far back in time as the late Archaean - important evidence for the validity of the palaeomagnetic method in w h i c h such a model is a fundamental hypothesis, and for the behaviour of the geomagnetic field in general. Third, any significant departures from linearity may signal special problems such as local tectonic rotations of the sampling region or unresolved components in the remanence data. And fourth, the linearity suggests a refined technique for dating and correlation of rock units which could prove valuable especially for Precambrian studies.

197

12.6

ON POLAR MOBILITY P.M.

Consulting

AND

EXTINCTIONS

James

Engineering

Geologist,

Brisbane

The geological sequence is studded with evidence of climatic change occurring at rates several orders of magnitude faster than available under a plate tectonics framework; some of these are accompanied by dramatic, nontectonic, sea level fluctuations, Fairbridge (79). A model of polar mobility is proposed herein, aiming to link climatic change with large (transient) redistribution of the oceans. THE MODEL The theoretical height of a column of water at any point on a spinning planet can be simplistically obtained by equating potential and kinetic energy. For a spherical earth, this approach yields almost 12 km depth of water at the equator and zero at the poles. Such an oceanic distribution does not correspond with reality since the earth itself distorts hydrostatical l y , with a similar equatorial bulge. If the geographic poles were to move, the pattern of centripetal forces would alter, finding an immediate response in the oceans. That is, the equatorial water bulge would migrate immediately to a new position, causing sea levels to rise in locations moving from a higher to a lower latitude, and vice versa. If the earth were to hold the new polar orientation for any length of t i m e , it could be argued that the earth body itself would have to adjust, e.g. by creep. In such a case, the status quo between shapes would be restored and the oceans would thus take up a distribution similar to their original one. Leaving aside the theoretical objections to geoidal migrations, there is quite a body of geological evidence which can be called on in support of a model of (transient) oceanic redistribution. EXAMPLES Two examples will be offered :i ) Submarine Valleys. Submarine valleys cut through hard rock to several kilometres depth; abyssal fan sediments at gradients of less than a metre per kilometre: th^se features can not satisfactorily be explained by any submarine process, Shepard & Dill (55). A mechanism which allows large and transient changes in sea level - without regional tectonism - would be vaulable since almost 90% of submarine valleys have a close connection with terrestrial drainage systems. ii) Ice Age. The generally accepted 100 m sea level drop during the last Ice Age leaves a number of anomalies on a global scale. A relationship between ice cap centres and pole positions produces some surprising results in terms of relative sea levels, using the above model. The model of a migrating polar cap ( through polar mobility) also avoids the problem of massive waxing and waning of the ice sheets in what have been relatively short periods.

198

A MECHANISM FOR EXTINCTIONS? Changes in climate alone - even rapid ones - need not lead to massive or global extinction patterns. However, if the climatic changes are the result of polar mobility and are therefore accompanied by large, transient, sea level changes^ then a mechanism for extinction becomes feasible. This has the advantage of avoiding reliance on chance astronomical events. REFERENCES FAIRBRIDGE R., (1979). Glacial traces from the desert: Winters of the Wovld, Ed. B. John. Jacaranda Q.

Ordovician.

The

SHEPARD F.P. & DILL R.F., (1965). Submarine Canyons and Other Sea Valleys^ Rand McNally Geology Series, Chicago. VITA FINZI C., (1976).

Recent Earth History,McMillan.

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15.6

REGIONAL SEDIMENT FACIES AND THE HISTORY OF BOTTOMWATER CIRCULATIONS IN THE TASMAN SEA C. Jenkins Ocean Sciences Institute, University of Sydney

Late Cenozoic sediment accumulations on the floor of the Tasman basin at depths of 3000m and more are organized into facies by patterns of supply and bottom-current erosion/transport. Current-moulded features include deep-sea channels, sediment drifts in Stillwater zones, wide terrains of eroded seafloor and scour moats alongside obstructions like seamounts. Pre-Oligocene deposits show little evidence of any bottom-water circulation. Seismic coverage of the Tasman basin is relatively dense and this offers a comprehensive view of the deep-water palaeoceanographic development, especially when combined with information on the present-day circulations from physical ocesinography and seafloor photographs. The analysis illustrates the complexity which can attend facies and unconformities even in a small semi-closed and deep ocean basin in times of polar-driven circulation. Other applications are in the understanding of seabed erosion for local submarine cable and ocean waste-dumping activities. In the South Tasman Sea, bottom-water effects are first clearly marked in Eocene-Oligocene sediments, their introduction coinciding with the SW Pacific regional deep-sea unconformity. As oceanographic climates changed through the later Cenozoic, strongly erosive and carbonate-corrosive thermohaline flows of Antarctic Bottom Water appear to have switched between three different paths which are marked by deep-sea channels. Eastwards forcing of bottom-waters by the Circumpolar Current at times of global climatic cooling is the proposed mechanism. The accompanying figure (next page) shows erosional and depositional effects of these flows where they turn NNW from near the Macquarie Ridge into the central Tasman Sea. At depths >4000m along the east Australian margin, northwards bottomwater flow is predicted under the dynamics of Coriolis westwardsintensification. Between the levels of Tasmania and Smokey Cape (NSW) large migrating surface warm-core eddies shed from the East Australian Current have a dynamic effect to the abyssal seafloor and disrupt this steady flow. This is observed in recent deep current-meter records and seafloor photography programs east of NSW. A 'smeared' facies distribution results, with flat well-layered abyssal plain accumulations. Further north beneath the jet of the East Australian Current, an organized pattern of facies belts occurs eastwards from the continental margin: strong scour against the margin, east to Stillwater sediment drift construction, to mild erosion and then pelagic province. Since all manifestations of current activity lie well above volcanics derived from Queensland Guyot (isotopically dated at 21 ma BP), circulations in the comparatively closed north Tasman basin began only in the Miocene. Re-circulating flows are important due to the basin's cul-de-sac form at depth. They are not associated with severe erosion; the principal example is traced as a southwards flow west of the Dampier Ridge and Lord Howe Rise.

200

TURBIDITE

SEDIMENT DRIFT

CHANNEL

Figures. A. Deep-sea facies in the SE Tasman Sea from seismic data; a path of Oligocene to Recent bottom-water flow is mapped turning NNW from the Macquarie Ridge into the central Tasman basin. [Depth contours are in km; the arrows depict inferred flow directions.] B. Interpretation of a seismic profile through the area, based on line 'Eltanin*

201

15.12

SEABEAM AND REFLECTION-SEISMIC IMAGING OF THE FLANKS AND VOLCANIC APRONS OF TASMANTID SEAMOUNTS C, Jenkins Ocean Sciences Institute, University of Sydney

SeaBeam multibeam echosounders map seafloor topography in swaths 60$ of the water depth wide and c o n s t i t u t e one of a new generation of seabed mappers i n c l u d i n g SeaMARC and GLORIA. In A u s t r a l i a n waters SeaBeam has been used aboard HMAS *Cook' since 1983 and on several occasions has been turned to the study of Tasman seamounts. Features seen i n c l u d e s u b s i d i a r y cones on the flanks, a benched upper construction, fissure-zone lobes and scour-moating about the base. Although many of the seamounts have f l a t tops, in none of those examined i s this due to a summit crater and a l l appear to be true guyots. Reflection seismic coverage of the Tasman Sea is substantial and the seismic stratigraphy of virtually a l l seamounts can be analysed. Those reaching close to the sea surface or which have a f l a t summit possess a volcaniclastic apron of d e b r i s extending up to 70km from the peak. The apron u s u a l l y extends in one main d i r e c t i o n and is r e c o g n i z a b l e in seismic records by (a) being located at and thickening to a seamount, (b) altering acoustic character from chaotic to mounded to d i s t a l l y thin-laminated. The aprons are buried by postv o l c a n i c p e l a g i c / terrigenous sediments and where the seamount is dated isotopically, this provides a stratigraphic datum by which to date palaeo c e a n o g r a p h i c and c o n t i n e n t r e l a t e d d e p o s i t i o n a l events. In agreement with the hotspot-trace hypothesis, aprons for the more r'Footprint* Transmit beam n o r t h e r n s e a m o u n t s a r e more ''.Receive beams deeply buried. I l l u s t r a t i o n of SeaBeam operating geometry and s a m p l e s h i p b o a r d output showing a small subsidiary s u b m a r i n e e r u p t i o n c o n e and possible associated lava-flow l o b e on the W f l a n k o f G a s c o y n e Seamount. E r u p t i o n 5-6 m.a. BP would h a v e o c c u r r e d at s i m i l a r depths to today.

20m ooDtours

Ilea

202

19.1

THE EDIACARAN

REVISITED

R.J.F. Jenkins^, P.W. Haines^ and V.A. Gostin^ ^Department of Geology and Geophysics, University of Adelaide ^Northern Territory Department of Mines and Energy, Alice Springs

Opinion concerning formal subdivision of the time interval represented by terminal Precambrian sedimentary sequences in Australia varies. There are at least four viewpoints; 1) a traditionalist notion of the maintenance of an integral Adelaidean System; 2) continuance of the Adelaidean System sensu stricto in conjunction with the Ediacaran System sensu stricto ^ ; 3) recognition of the overlapping division of the Ediacarian 3 ; or 4) downward extension of the Ediacaran/Ediacarian. Suggestion 2 is advocated here. The Ediacaran s.s. division embraces the Wonoka Formation and Pound Subgroup, Flinders Ranges, while the Ediacarian includes the whole of the Wilpena Group. Preiss"^ prefers usage of the name *Ediacaran* but suggests its downward extension, while M. F. Glaessner (pers. comm.) has recently indicated a moderation of his views concerning the utility of either the Ediacaran or Ediacarian, in the light of a well constrained Rb-Sr isochron giving an age of 621 + 9 M a ^ for the close of the Laplandian Glaciation in the Vendian of the Middle Urals. In northern European settings, rocks with fossil assemblages of Ediacaran aspect commonly postdate the Laplandian (Varangian) Glaciation. A variety of geological studies confirm that the distinctive Ediacaran fossil asemblages of southeastern Newfoundland occur in rocks onlapping volcanic sequences and associated subvolcanic intrusives with U-Pb zircon ages of ca. 621-606 Ma. The U-Pb zircon dating of ca. 800 Ma from a tuff low in the Callana Beds ® enhances the potential utility of an Adelaidean System. A peculiar cyclical aspect of a purported fossil^ described from probably the upper Elatina Formation does not support the claimed biogenicity of this structure®. Jenkins continues to maintain that ' Bunyerichnus', from the mid Brachina Formation, is an accidental marking, perhaps made by arcuate sweeping and rhythmical prodding of a vendotaenid swinging to and fro in the current. It probably is not the imprint of a fossil medusoid®because current lineae cross the structure. Since the Ediacaran is a biostratigraphic concept linked to the occurrence of indications of early Metazoa , there seems no reason for its downward extension. The Ph.D. studies of Haines circumscribed the detailed stratigraphy of the Wonoka Formation and located further indications of metazoa in upper parts of this unit and basal parts of the Bonney Sandstone. The base of the Wonoka Formation is at a thin dolomite marker that can be traced over the greater part of the length and breadth of the Flinders Ranges. In the south and north rocks of the upper Adelaidean s.s. were lithified and received either a coarse fracture cleavage or underwent joining prior to formation of large canyon-like valleys (ca. 800 m deep, ca. 5 km wide) filled by the Wonoka Formation. Regularly sized, small, ovate pellets, a single large structure somewhat resembling a broadening 'leaf, and the abundant occurrence of Palaeopasci chnus in one narrow stratigraphic interval are among the indications of a biota located by Haines in upper parts of the Wonoka Formation. Trace fossils and rare medusoids occur in the Bonney Sandstone.

203

In the far northern Flinders Ranges upper parts of the Wonoka Formation and lower intervals of the Pound Subgroup show a transition into siltstones of the Billy Springs Beds, which reach a thickness of ca. 2.4 km or greater. The latter comprise a lower interval of thin- and rhythmically-bedded khaki siltstones to fine sandstones with occasional storm beds, a middle to upper interval including several prominent carbonate beds and quartzites, and a topmost red-brown siltstone. The recent discovery by Mary Wade (pers. comm.) of elements of the type Ediacara assemblage in association with the Mt. Skinner assemblage of the Central Mount Stuart Beds in the Georgina Basin, central Australia now provides a near correlation with the Rawnsley Quartzite. M. A. Fedonkin located short oblique burrows in the Mt. Skinner assemblage, suggesting it to be marginally younger than the type Ediacara biota, which is defined^ as being late Ediacaran. Oblique burrows (cf. Skolithos) occur with Ediacaran fossil remains at the top of the Redkinian on the Winter Coast of the White Sea, northern USSR Exploration by Haines and Jenkins confirmed the presence of ichnofossils near the base of the Mopunga Group, Elynah Range. Examples of cf. Isopodichnus occur with cf. Bilinichnus, brush marks, rare, short, subhorizontal burrows and 'dimple-marks' in the El3mah Formation; Palaeophycus tubularis and Pianolites montanus were found in the Grant Bluff Formation. Similar forms and small circular structures occur in the Cyclops Member of the Pertatataka Formation, Amadeus Basin. A poorly preserved sea-pen-like marking in the Cyclops Member holds promise for further discoveries. It appears likely that the Central Mount Stuart Formation is partly equivalent to the Mt. Baldwin Formation (restricted sense) and Arumbera Sandstone (restricted). The Ediacaran aspect of fossil remains presently located in the Amadeus and Georgina Basins suggest an overlap with the Pound Subgroup; possible lithological correlations may be made with the topmost Wonoka Formation and Billy Springs Beds. This suggests the possibility that boulders in part of the Billy Springs Beds reflect a glaciation. Haines has also located potentially later Precambrian sandstones in the Georgina Basin. Transitional Ediacaran/Cambrian siltstones and sandstones (Rovnian/ Tommotian) are disconformable on the earlier Ediacaran in all three basins; claims that a continuous succession exists in the Arumbera Sandstone (sensu lato) are evidently based on cursory examination. 1. 2. 3. 4. 5.

Mawson, D. & Sprigg, R.C., 1950. Aust. J. Sci. 13; 69-72. Jenkins, R.J.F., 1981. Trans. R. Soc. S. Aust. 105: 179-194. Cloud, P. & Glaessner, M.F., 1982. Science 217: 783-792. Preiss, W.V., 1987. Geol. Surv. S. Aust. Bull. 53. Krasnobaev, A.A. & Semikhatov, M.A., 1986. In: Metody isotopnoy geologii i geokhronologicheskaya shkala. Acad. Sci. USSR, Vernadsky Inst. Geochemistry and Analytical Chemistry. Nauka, Moscow, pp. 159-183. 6. Fanning, C.M., Ludwig, K.R., Forbes, B.G. & Preiss, W.V., 1986. Geol. Soc. Aust., Abstrs. 15: 71-72. 3. Dyson, I.A., 1985. Nature 318: 283-285. 8. Jenkins, R.J.F., 1986. Nature 323: 472. 9. Glaessner, M.F., 1984. The Dawn of Animal Life: A biohistorical study. Cambridge University Press, Cambridge. 10. Fedonkin, M.A., 1985. In": Vendskaya Sistma, Vol. 1, eds. B.S. Sokolov & A.B. Ivanovskiy. Akad. Nauk, SSSR. Otdel., Geol. Geofiz. Geohem., 112-116.

204

7.9

A S I N G L E T H I C K S H O A L I N G - U P W A R D C Y C L E IN THE NORTHERN GEORGINA BASIN, AUSTRALIA D.

Johnston

Exploration

Johnston

Organisation

Pty

Ltd,

Toowoomba

The Georgina Basin in north central Australia contains a Cambrian to O r d i v i c i a n c a r b o n a t e sequence which outcrops over approximately 500,000 sq. kms. O v e r m u c h of t h e b a s i n o u t c r o p is p o o r . T h e m a n y a n d v a r i e d f o s s i l s p r e s e n t h a v e l e d to a s t r a t i g r a p h i c s u c c e s s i o n b a s e d l a r g e l y on b i o s t r a t i g r a p h y . R e c o g n i t i o n of t h e relationship between the lithofacies present may lead to a 1ithostratigraphic succession. In 1980 and 1981 literature s t u d i e s , m a p p i n g a n d d r i l l i n g w e r e c a r r i e d o u t in t h e n o r t h e r n Georgina Basin by t h e E x p l o r a t i o n Department of t h e B H P Co. Ltd. A s h o a l i n g - u p w a r d c a r b o n a t e s e q u e n c e at l e a s t 3 0 0 m t h i c k w a s f o u n d in d r i l l c o r e . The

succession

contains

from

PIane-1 aminated shaly shale interbeds; Wavy-laminated

shaly

bottom

limestone limestone

top:

with and

thin

recrystal1ised

vuggy

and

thin

oil

marl;

An u p w a r d - c o a r s e n i n g pelletal to s a n d y and calcarenite with a sandstone member; A

chert

pelletal

dolarenite

dolomite.

These lithologies correspond largely with those of the biOStratigraphic units Currant Bush Limestone, V-Creek Formation, Age Creek Formation and Camooweal Dolomite respectively. They occur in a v e r t i c a l and lateral facies a r r a n g e m e n t a n d w e r e d e p o s i t e d in p r o g r e s s i v e l y s h a l l o w e r w a t e r . The depositional environment changed from quiet conditions below s t o r m w a v e b a s e w h e r e t h e b i t u m i n o u s l i m e s t o n e w a s d e p o s i t e d , to g r a d u a l l y shallower e v e n t u a l l y shoal c o n d i t i o n s . The pelletal c a l c a r e n i t e now l a r g e l y d o l o m i t i s e d w a s the shoal and a d o l o m i t e m u d w a s d e p o s i t e d in h y p e r s a l i n e c o n d i t i o n s to l a n d w a r d of t h e shoal. L o n g t e r m l a t e r a l r e g r e s s i o n of a s h o r e - l i n e by g r a d u a l f i l l i n g of a s h a l l o w b a s i n e x p l a i n s t h e s e f a c i e s v a r i a t i o n s . The bituminous limestone has been content and the lower surface of sporadic lead m i n e r a l i s a t i o n .

205

investigated the d o l o m i t e

for mud

oil shale contained

7.23

GLENDONITES

B.C. Jones^,

- OCCURRENCE AND

SIGNIFICANCE

P.F. Carr^ and R. Middleton^

^Department of Geology, University of Wollongong ^Mineralogy Section, Los Angeles County Museum, USA

In 1849 James Dwight Dana, later to become famous for his "System of Mineralogy", described and figured unusual calcareous pseudomorphs from Glendonbrook in the Hunter Valley of New South Wales. These were later named "glendonites" by David et al. (1905) who concluded that the precursor mineral was glauberite (Na2SO^.CaSO^) which formed in cold marine sediments. Subsequent to these early accounts several other parent minerals were suggested and many descriptions and interpretations of the significance of glendonites have been published. A major advance occurred in the last decade with the realisation that the most likely precursor was ikaite (CaCOg.BHgO) which decomposes above 5°C to produce calcite and water. Glendonites are widespread in the Sydney Basin and have been recorded from at least 20 localities. Their stratigraphic distribution appears to indicate that glendonites developed during five major episodes corresponding to the deposition of: (1) the Allandale Formation; (2) the Branxton Formation and equivalent Wandrawandian Siltstone; (3) the Mulbring Siltstone and equivalent Berry Siltstone; (4) the Broughton Formation; and (5) the Kulnura Marine T o n ^ e . The glendonites are found only in Permian marine strata and, in the majority of these occurrences, they are not restricted to a single horizon but normally occur in a number of beds over a stratigraphic interval of up to several tens of metres. The glendonites are associated with bioturbated carbonaceous mudstones or fine-grained sandstones which are essentially free of coarse detritus. This association indicates that the glendonite precursor formed during episodes of slow sedimentation under low energy, reducing conditions. With the exception of the Kulnura Marine Tongue, most glendonitebearing beds are overlain by strata containing abundant ice-rafted erratics. The low temperature (<5°C) required for ikaite formation combined with the slow sedimentation rate, the scarcity of ice-rafted erratics and the low energy level of the shallow marine depositional environment suggests that the glendonite-bearing beds formed under a cover of permanent sea-ice during periods of glacial maxima. Subsequent breakup of the sea-ice resulted in an influx of ice-rafted erratic blocks derived from rivers or coastal areas. The occurrence of ice-rafted clasts in the stratigraphic record coincides with increases in the energy level of the environment, the sand content of the sediment and the faunal diversity. All of these features are consistent with a return to normal, open, wave-influenced, shallow marine sedimentation. Deformed glendonites which commonly occur near the top of glendonitebearing beds probably result from decomposition of the ikaite presursor before consolidation of the surrounding sediment. The occurrence of glendonites in restricted horizons within the five units of the Sydney Basin sequence thus provides evidence of both short-term and long-term palaeoclimate fluctuations during the Permian Gondwana glacial event.

206

7.13 C H A N N E L A V U L S I O N A N D SAND GEOMETRY IN THE GILBERT RIVER FANDELTA, NORTHERN Q U E E N S L A N D B.Go

Jones^, G.C. Nanson^, R.W. Young^, N. Senapati ^ and D.J. B o u r k e ^

^Department of Geology, The University of W o l l o n g o n g ^Department of Geography, The University of W o l l o n g o n g ^Exploration Department, Comalco Aluminium Ltd, Brisbane Fluvial and deltaic deposits have been well dociimented in the literature but very few large monsoonal fandelta systems have been described systematically. The Gilbert River system which enters the Gulf of Carpentaria in north Queensland (Fig. 1) provides an ideal site for investigating the Quaternary and Holocene deposits of an actively prograding fandelta under tropical monsoonal climatic conditions since it is of moderate size and it is not intensively settled or farmed. The Gilbert River represents an ephemeral sand-dominated low-sinuosity river and mud-dominated floodplain and delta system. Discharge in the Gilbert River is controlled by the summer monsoon extending from January to March each year and peak monsoonal floods (maximum instantaneous flows of about 30,000 cumecs and maximum annual discharges of up to 30 M megalitres in the 1973-7A monsoon) inundate much of the floodplain and delta producing widespread overbank deposits. This considerable overbank flood flow becomes concentrated in the topographically lowest portions of the floodplain and eventually results in avulsion of the main channel. The dominance of channel migration by avulsion on a low gradient fandelta system has not commonly been reported but is probably a direct consequence of low gradients, an aggrading surface and extreme monsoonal storms.

142'E

140*E

144* E

i6*a -

Tt?'

GULF OF CARPENTARIA

18* 8 GEORGETOWN

Fig. 1.

Locality map of the Gilbert River fandelta.

207

Numerous avulsion events of the low sinuosity stream system in the upper part of the Gilbert River can be recognized from a study of satellite images and aerial photographs. Channel migration by avulsion results in the production of a network of channel-fill sand bodies separated by clay-rich interfluves. Some- episodes of avulsion and channel filling are too recent to be accurately dated. The Holocene breakout of the present lower Gilbert River from the Smithburne River has resulted in the latter being partially blocked by sand and mud with the last recorded significant flow occurring in the 1973-74 floods. A still younger avulsion of the Gilbert River into Maxwell Creek is still progressing with approximately two-thirds of the flow now being concentrated in Maxwell Creek. The base of the channel in the latter is 2.5 m lower than in the partially plugged main channel of the Gilbert River. However Maxwell Creek itself is breaking out northwards into an unnamed distributary channel of still lower elevation. This latter process has occurred over the past three years and has resulted in scouring and the buildup of a major sand-splay in the northern channel. Channel-fill sequences deposited by the Gilbert River system are generally A-6 m in thickness. The lower part of the vertical profile is dominated by flat bedded and small-scale planar and trough cross-stratified sand. Preserved cross-beds rarely exceed 20 cm in thickness and show very little change in set size, form or abundance upwards through the profile although a gradual upward reduction in mean sand grainsize is evident. This observation is in accordance with the predominance of low amplitude dunes and flat-topped bars in the active channels of the Gilbert River reflecting high flood velocities, rapid sediment transport, and common scouring and erosion. The size and number of interformational clasts decreases downstream in the 100 km reach of the Gilbert River from the junction with the Einasleigh River to Delta Downs. Intraclasts show a more variable size and distribution being especially abundant in areas of recent avulsion. Rare large cross-beds (up to 3 m thick) represent preservation of prograding bars where secondary channels rejoin the main channel. The upper part of the vertical profile consists of laminated overbank silt and fine-grained sand interspersed with lensoidal coarser crevasse splay sand sheets. Maxwell Creek and the middle reaches of the Gilbert River have cut down into Pleistocene (up to 65 Ka) silty overbank and soil deposits which can be traced laterally for more than 60km. Distinct periods of iron-rich and carbonate-rich weathering can be recognized in these older deposits, including calcrete nodule formation at 27-28 Ka during the last glacial episode of lower sea level and channel incision. A drill-hole profile across the midfan region has indicated moderate cross-fan continuity of sand facies although the main channel sands are discontinuous as reflected by variations in porosity and permeability, "pie sedimentary architecture is in accordance with the upper fandelta growing by vertical accretion and periodic channel avulsion. The stratigraphy of these deposits provides evidence of climatic changes and adjustments of flow regimes during the Pleistocene in Australia's tropical monsoon zone. The porous sands exhibit a radial pattern of channel trends which, when deeply buried, could form excellent potential petroleum reservoirs since they are laterally associated with lower delta plain and shallow offshore organic-rich mudstone sequences. The Gilbert River deposits form the basis for a model of tropical monsoonal fandelta facies which has potential application in the exploration for oil and gas in, for example, the Mesozoic Northwest Shelf and Carpentaria Basin sequences.

208

9.4

THE BREAM CREEK E A R T H Q U A K E SWARMS OF

L.E.A. Jones^, M.O. Michael-Leiba^

and D.

1986-1987 Kingston^

^Department of Geology, University of Tasmania ^Bureau of M i n e r a l Resources, Geology and Geophysics, Canberra ^Bream Creek, Tasmania

Since October 1986^ several earthquake swarms have occurred in the vicinity of Bream Creek, Tasmania (42M8.5'S, 147^50'E). The temporal distribution of seismicity, shown in Figure is based mainly on the residents' reports. However, earthquakes of magnitude 1.2 and greater, which were recorded on the Tasmanian State Seismic Net, follow the same pattern, although their numbers are fewer.

30 > < Q DC UJ CL

l2 20 < 3 O DC < LU

fe 10 DC UJ

NOV

FIGURE 1

DEC

JAN

FEB

MAR

APR

MAYI

J-

JUN

BREAM CREEK EARTHQUAKES FELT IN THE VICINITY

The first swarm occurred in November, 1986, followed by several weeks of no activity. The largest swarm, both in terms of number of events per day and total number of events, lasted throughout January and into February 1987. During this swarm, the largest Bream Creek event was recorded by the State Seismic Net on January 19 at 0213UT with a Richter Magnitude ML of 2,5. Other swarms occurred in March and April, with the activity virtually ceasing from May onwards. Twenty events were located from portable field station records during the period from March 18 to April 25, 1987. Their Magnitudes (MD) range from 1.0 to 1.7. Most of the epicentres fall within a two kilometre radius of the centre of Bream Creek. The depths are two kilometres or less. The shallow origin is consistent with the reported characteristics of the earthquakes.

209

They are usually experienced as a s u d d e n j o l t similar to a rifle shot or a small explosion.

accompanied

sound

The cause of the earthquake swarm activity is not known. The geology of the s u r r o u n d i n g a r e a c o n s i s t s of J u r a s s i c dolerite intruded into Triassic sedimentary rocks. Tertiary basalts of 5 8 Ma age a c c o u n t f o r the s t e e p t o p o g r a p h y west of Bream C r e e k , an area that is subject to landslips. Prominent lineaments on Landsat photographs and geologic maps intersect some three kilometres south west of the centre of the seismic activity. It may be significant that the seismicity appeared after a wet winter f o l l o w i n g upon several years of drought.

210

19.6

CARBONIFEROUS BIOSTRATIGRAPHIC CHART FOR AUSTRALIA INTERIM NOTES

P.J. Jones Bureau of Mineral Resources, Geology & Geophysics, Canberra

A biostratigraphic chart of the Carboniferous of Australia, originally prepared for the BMR/APIRA-sponsered Palaeogeographic Map Project, has been updated. The present notes outline the standard stratigraphic, and geochronometric scales, that have been used in its construction. Over the last five Carboniferous Congresses {1971-Krefeld to 1987-Beijing) it has become increasingly apparent that the titular members of the Subcommission on Carboniferous Stratigraphy are favouring a two-fold division of the Carboniferous System. Although not formally ratified, a unified international classification of the Carboniferous System was proposed by Bouroz et al.(1978), combining features of the standard scales from western Europe, the Soviet Union, and North America. Accepting the present uncertainty concerning global correlations about the midCarboniferous boundary (Lane et al., 1985)» it seems premature to follow any international standard scheme, such as Bouroz et al., (1978), that uses both Russian and west European stages in a single scale. Of the three areas (western Europe, Russia and North America) that provide standard stratigraphic scales for the Carboniferous System, western Europe has the scale which is the most appropriate for the Carboniferous of Australia. In the absence of formally designated local stages, the western European scale has been commonly used in Australia, where the cosmopolitan shelly faunas of the Early Carboniferous (i.e., Dinantian) are replaced by the endemic (Gondwanan), poorly-represented faunas of the Late Carboniferous (i.e., Silesian). The scarcity of conodonts and the absence of fusulinid foraminiferids in the late Carboniferous Gondwanan faunas inhibit their correlation with the standard sequences of North America and Russia, where the main biozonations are based on these groups. The geochronometric scale, compiled from more recent sources from both within and outside Australia, brackets the Carboniferous System between 35^ Ma and 295 Ma. Each of these limits is regarded, on balance, as the best compromise of the latest available data. Tl^^ reliable of the available isotopic age determinations are based on Ar^^/Ar^^ dates on sanidines taken from biostratigraphically well dated Silesian tonsteins in West Germany and Czechoslovakia (Lippolt & Hess, 1985)- The dates are: 305 Ma - Westphalian/Stephanian boundary 310 Ma - Westphalian B/Westphalian C boundary 315 Ma - Westphalian A/Namurian boundary 325 Ma - Visean/Namurian boundary. The Paterson Volcanics with an average age of 308 Ma, would be placed on the Lippolt & Hess (I985) scale in Westphalian C. Thus the normally magnetized Paterson Toscanite, at the base of the Kiaman reversed interval, can be correlated with the zone of mixed polarities within British Westphalian C coals. Australian K/Ar age data from"the Hunter Valley (Roberts & Engel, I98O) have been used to estimate ages of the Visean/Namurian boundary (325 Ma), the

211

base of the Brigantian Stage (331 Ma; Martins Creek Ignimbrite), which is compatible with De Souza^s (1982) estimate of 335 Ma for the base of the Asbian Stage, and the Tournaisian/Visean boundary (about 3^2 Ma). The 355±5 Ma proposed by De Souza (1982) for the Tournaisian/Visean boundary in the Midland Valley of Scotland appears to be too old by comparison with the Australian evidence. For the base of the Carboniferous, an estimate of 35^ Ma is preferred, following the 354t.5 Ma figure suggested by McKerrow et al. (1985). This date is reasonably compatible with the 356^6 Ma and 357l6 Ma computations derived by Carr et al (1984), and an age younger than 360 Ma determined from radiometric data from eastern Victoria (Richards & Singleton, I98I; Williams et al, 1982). References BOUROZ, A., EINOR, O.L., GORDON, M., MEYEN, S.V., & WAGNER, R.H., 1978-Proposals for an International Chronostratigraphic Classification of the Carboniferous. Compte rendu 8 eme Congres International de Stratigraphie et de Geologie du Carbonifere, Moskva, 1975> 1: General Problems of the Carboniferous Stratigraphy, Publishing House 'Nauka*, 36-69, Moscow. CARR, P.P., JONES, E.G., QUINN, E.G., & WRIGHT, A.J., 1985--Towards an objective Phanerozoic time scale. Geology 12, 274-277. DE SOUZA, H.A.F., 1982--Age data from Scotland for the Carboniferous time scale. In ODIN, G.S., (Editor) 1982 - NUMERICAL DATING IN STRATIGRAPHY. John Wiley, Chichester, vol.1, 455-460. LANE, H.R., BAESEMANN, J.F., & GROVES, J.R., I985--IS the base of the Reticuloceras - Zone a reliably recognizable biostratigraphic level? In LANE, H.R., & ZIEGLER, W. (Editors) I985,--Toward a boundary in the middle of the Carboniferous: Stratigraphy and Paleontology.Courier Forschungsinstitut Senckenberg 74, 137-148. LIPPOLT, H.J., 8c HESS, J.C., 1985~^°Ar/39Ar dating of sanidines from Upper Carboniferous tonsteins. Compte rendu 10 eme Congres International de Stratigraphie et de Geologie ^ Carbonifere, Madrid, 1983t 4, I75-I8I. McKERROW, W.S., LAMBERT, R. St. J., & COCKS, L.R.M., 1985--The Ordovician, Silurian and Devonian periods. In SNELLING, N.J., (Editor) I985, 73-80. RICHARDS, J.R., & SINGLETON, O.P., 198l--Palaeozoic Victoria, Australia: igneous rocks and their interpretation. Journal of the Geological Society of Australia 28, 394-421. ROBERTS, J., & ENGEL, B.A., 1980--Carboniferous palaeogeography of the Yarrol and New England Orogens, eastern Australia. Journal of the Geological Society of Australia 27. I67-I86. SNELLING, N.J., I985—THE CHRONOLOGY OF THE GEOLOGICAL RECORD. Memoir 10^ The Geological Society, Blackwell, Oxford. WILLIAMS, I.S., TETLEY, N.S., COMPSTON, W., & McDOUGALL, I., I982--A comparison of K-Ar and Rb-Sr ages of rapidly cooled igneous rocks: two points in the Palaeozoic time scale re-evaluated. Journal of the Geological Society of London 139, 557-568.

212

10.3

URBAN GEOLOGY OF THE BALMAIN PENINSULAR AND RECENT GEOTECHNICAL PROBLEMS R.M. Jones Museum of Pioneer Industries, Sydney

At this time, when coal mines are closing down, it may appear that deep coal mining under urban areas would have little to do with the advancement of Australia, This is a time when we stand at the brink of widespread Suburban Redevelopment. If mine subsidence design precautions are taken into account, sterilisation of large areas of coal underlying Sydney would be prevented. Bear in mind that in Germany coal is now such a precious commodity that seams 30 cm thick are being extracted. Consideration of the gaseous nature of the Bulli seam and the difficulty of ventilation and the cost of winning coal in deep mining under Sydney Harbour would seem to negate any possibility of such mining. However the advent of robotics and other technical advances which we can expect by the beginning of the 21st century indicate that such mining may be very possible indeed. What then can a study of the Urban Geology of Balmain (an inner Sydney suburb) bring to bear on this situation? Deep mining of coal at approximately 900m (3,000 ft) occurred under Balmain and the offshore areas from 1901 to 1931. The houses on the surface were predominantly Victorian and Edwardian buildings with solid, double-brick walls cemented with lime mortar. The responses of these buildings and other engineering works (most notably the shallow underground tunnel from Longnose Point, Birchgrove to Mann's Point, Greenwich) to the mining and to the mobilised regional geological structures which cross the area of draw of the mine (see Fig. 1) have been recorded. Joint patterns in the area of draw have been recorded and compared with patterns from North Sydney, Cremorne and Pyrmont. The renewed effects of geological movement, principally in 1987, which are time co-incident with a major Government development at Mort Bay, Balmain have produced additional data and the impetus for this study. The generally accepted angle of draw of 26i''in the coal measures sequence in N = S.W, needs to be modified to take into account the anisotropism introduced by the quasi-vertical zones of regional stress relief which cross the area of draw. The 200m barrier zone +35® angle of draw decision reached by Justice Raymond Reynolds in the Royal Commission into Mining under Stored Water co-incides well with the limit of damage recorded at Balmain. In the urban environment human nature can contribute impediments to geotechnical study. However outcrops newly exposed in parkland development; history of the mining; history of land purchase; demolition and re-building of earlier buildings; the pattern of early roads and geomorphological considerations have all provided additional clues. The subsequent history of the new buildings at Mort Bay may yield further data. These buildings, mostly of full brick (cavity) construction, were designed in the absence of knowledge of the coal mine location. They were built in the belief that all subsidence due to mining and horizontal stress relief on mobilised geological structures would be over and done with in 16 years. They are sited over a known methane source 900m below the surface. The duty of care in design with relation to known methane sources has been put into sharp focus by the judgment in the Abbeystead disaster trial, which is now available in Australia. If the predictive skills of geologists are correctly valued, Australia will step much more economically towards the 21st Century.

213

FIG 1 The assistance of, and supervision by, Associate Professor D. Branagan and Professor A. Hargraves of the field studies which fora the basis of this paper is gratefully acknowledged.

214

1.4 LOOKING AFTER SCIENTIFIC SITES IN AUSTRALIA A DECADE O F W O R K BY T H E GEOLOGICAL SOCIETY O F AUSTRALIA E.B.

Joyce

Department of Geology, University of Melbourne

For more than ten years Divisional subcommittees of the Society have been working in each state and territory identifying, documenting and assessing the significance of geological and geomorphological features. Grants of nearly $250,000 from the Australian Heritage Commission and state govemment departments have allowed hiring of consultants to assist subcommittee members in this work, and also paid for the production of some eighteen volumes of documentation. Each Division has worked separately in its state or territory, developing its own approach to problems of identifying features of significance and promoting their conservation. In 1974 the Society established a Committee for Geological Monuments at the Federal level to work with Divisional subcommittees on their methods of selection and protection of features, reporting regularly on this work to the Society's Council. Beginning in 1977 seminars on geological conservation have been held at the Society's conventions, and in 1980 a review of conservation work in Australia was published (Joyce 1980). A national Workshop was held in Canberra in November 1982 under the sponsorship of the Australian Heritage Commission and representatives of all Divisional Subcommittees met for the first time and discussed their work. In 1983 a major seminar on geological conservation was held at the Sixth Australian Geological Convention in Canberra, with speakers from each Division reporting on their work, and afrillset of abstracts appeared in the Convention volume. Other reviews of work on geological conservation in Australia include the regular reports of the Federal Committee prepar^ for the Society's Council and included in the papers for Council Meetings, for example the meetings at Sydney in 1984, at Adelaide in 1986 and at Brisbane this year (1988). Arising out of a second Workshop held in Canberra in May 1984, a project was begun to prepare a volume summarizing the work in each state and territory, listing features of National and Intemational Significance, evaluating current work and suggesting future activities. Submissions were made by Divisional subcommittees and a prelimin^ report prepared which listed 153 features of National significance and 76 features of Intemational significance (Cochrane and Joyce 1986). A final report with a substantially revised listing and discussion is now in preparation by the Federal Committee's convener. This report will also attempt to evaluate techniques of selection, documentation and assessment used in Australia, making comparisons between the approaches of each Division, and those used in the UK, USA and elsewhere. The philosophy behind such work, including consideration of the criteria used, and thresholds of acceptance into categories of significance, will be discussed.

215

Work in most Divisions has been directed to identification, documentation and the assessment of significance of features. In some Divisions few nominations have been made to the Register of the National Estate, although this was an aim of the Australian Heritage Commission's programme of ^ants. Some Divisions have developed a system of publicising their results, e.g. by distributing their reports widely to libraries and planning and conservation organizations, listing features in state Heritage registers, and sending copies of parts of their reports direct to appropriate state and local government agencies. More work of this type is needed to ensure that geological and geomorphological features are properly considered in the planning process. Threats to geological and geomorphological features have been discussed in several of the Divisional reports. Such features are often robust and resistant to changes by natural processes on the human time scale, and also to some aspects of modem development. However degradation, destruction, burial and removal may occur (for a discussion see Joyce and King 1980 p.29). Current conflicts arising between mining and conservation interests in National Parks and other reserved areas may concem geological features of scientific interest as well as those of biological interest In general, quarrying and some mining activities may enhance geological exposures and hence the value of a site, although a range of changes to features, from degradation through to complete destruction, are possible. It is timely to consider future work in Australia. Most states and territories now have reports covering substantial parts of their areas and further reports are in various stages of preparation, including a report drawing on these studies and assessing the whole of the continent. In the United Kingdom, the Geolo^ and Physiography Section of the Nature Conservancy Council, which carried out such work in Britain, and was in part a model for the initial work in Australia, has recently been disbanded, and future work there seems in some doubt.The Australian Heritage Commission, which has been the main funding source for Australian studies, is anxious to consider the future role of the Society in geological conservation.

References Cochrane, R.M. and Joyce, E.B., 1986. Geological Features of National and International Significance in A u s t ^ a . A report prepared for the Australian Heritage Commission, May, 1986. Federal Committee for Geological Monuments, Geological Society of Australia Inc. Joyce, E.B., 1980. Geological conservation in Australia and overseas, in Joyce and King (1980), Appendix B, pp. 191-202. Joyce, E.B. and King, R.L., 1980. Geological Features of the National Estate in Victoria. An inventory compiled for the Australian Heritage Commission. Victorian Division, Geological Society of Australia Incorporated (x + 208 pp).

216

9.12

DAMAGE RISK FROM BLASTING VIBRATIONS G.D. Just

Department of Mining and Metallurgical University of Queensland

Engineering,

Detonation of explosive charges, usually confined in holes drilled in the rock, releases energy which causes rock fragmentation and rock displacement. A variable proportion of the explosive energy released, depending upon the blasting design, is transmitted through the surrounding rock and air as ground vibration and noise. Perception of these vibrations may cause annoyance at low magnitudes and at higher levels may be potentially damaging to structures. Urban expansion, environmental concerns and changes in blasting technology have increased the general level of interest in blasting vibration phenomena. Availability of reliable, relatively low cost instrumentation also now makes it feasible to obtain a significant volume of field data. These factors have all contributed to the justifications for recent investments in research projects related to blasting vibrations. The Australian Standard AS2187-1983 recommends peak particle velocity limits for different types of buildings. For example historical structures 2 mm/s, houses 10 mm/s and industrial buildings 25 mm/s. In 1985 the Australian Environmental Council (AEC) circulated a draft Technical Basis document limiting vibrations according to human discomfort. The time of day was incorporated in the recommendation so that outside normal working hours the vibration level could not exceed from 1 mm/s to 2 mm/s. Legislation to require the standards to be achieved would have a severe economic impact on many mining operations. Adherence to these standards also would not eliminate the possibility of legal proceedings relating to perceived discomfort or possible damage caused by blasting vibration. Non-technical factors such as improved community relations and public dissemination of information can reduce discomfort levels. Damage risk assessments require monitoring and technical measurements of vibration characteristics. The vector sum of peak particle velocities in three orthogonal directions is the most widely used number to quantitatively characterise blasting vibration. Recent recognition of the existence of vibration frequencies both above and below the normally accepted range of 10-50 Hz for quarry blasts has emphasised the possible significance of frequency. In the lower range 1-5 Hz, as in earth quake events, structural resonance is possible. These levels are below the lower frequency limit of many commercial blast vibration monitoring instruments. Controlled field measurements have demonstrated the fact that the frequency can be changed by varying the blasting design, for example the ratio of charge length to hole diameter. Structural damage is dependent not only on the vibration magnitude but also on the number

217

of vibration "events". Regular blasting from quarrying operations should therefore require limits which are different from those associated with "one-ofF' construction activities such as tunnelling. Insufficient evidence is currently available to allow this fatigue effect to be incorporated in vibration limits. This is a potentially significant factor since future legislation could require operations to increase the number of blasts in order to achieve lower absolute vibration levels. Damage probability analyses will be needed to account for such effects. Reliable prediction of vibration levels requires knowledge related to: (a)

geological in-homogenities,

(b)

location of free-faces, e.g. underground or surface locations,

(c)

different types of explosives,

(d)

variable initiation sensitivity and reliability

(e)

decoupling of explosive charges

(f)

explosive charge geometry

(g)

confinement of the explosive charge.

If precise predictions are to be made such information can only be obtained on a sitespecific basis. A wide choice of design options are available using current blasting technology. It is therefore technically possible to achieve very low blasting vibration levels. This may involve a significant marginal cost depending upon the type of changes that are made. Such costvibration relationships can only be assessed from field tests and detailed cost analyses. Cost-benefit information should be incorporated in any evaluation of damage risks from blasting vibrations. REFERENCES JUST, G.D. and FREE, G.D. (1980). "Coal and overburden blasting vibration measurement and analysis," Environment controls seminar - Aust. Coal Assoc. Sydney, 26 pp. JUST, G.d. and McKENZIE, C.K. (1986), "Blast vibration measurement," Rock excavation engineering short course, Univ. of Qld. 19 pp. McKENZIE, C.K. and JUST, G.D. (1986), "Ground vibration prediction measurement and control," I.E. Aust. Qld.Div. Tech. Papers, Vol.27, No. 16, pp.36-40. JUST, G.D. and CHITOMBO, G.P. (1987), "The economic and operational implications of blast vibration limits," AusIMM., Mining & Environment Confce., Brisbane, July, pp.117-124.

218

3.15

A N A L T E R N A T I V E APWP FOR T H E M I D D L E TO LATE PALAEOZOIC O F AUSTRALIA - IMPLICATIONS FOR T E R R A N E MOVEMENTS IN T H E T A S M A N FOLD BELT C. Klootwijk and J. Giddings Bureau of Mineral Resources, Geology and Geophysics, Canberra

Palaeomagnetic studies on the Carboniferous Texas Beds and Coffs Harbour sequence and on Lower Carboniferous sediments and volcanics from the Rouchel region of the Tamworth Belt have shown a very predominant magnetic overprint of a uniquely reversed polarity, steeply west to southwest dipping, and of post-tectonic and probable Early Permian origin. In the Visean ignimbrites of the Isismurra Formation only, another component could be isolated which showed a very low inclination, north to northwestern declination and mixed polarity. Its implied equatorial palaeolatitude agrees with palaeoenvironment indicators and is therefore taken as the primary magnetization. This late Early Carboniferous result from the Tamworth Belt differs considerably from the Late Palaeozoic APWP for Australia as proposed by Schmidt et al. (1986), whereas latest Carboniferous results from the Tamworth Belt (Irving 1966) show no obvious disagreement. On this basis a large-scale post- or possibly syn-Visean but pre-latest Carboniferous southward displacement of the New England region east of the Mooki Fault Zone relative to (neo-) cratonic Australia was proposed tentatively (Klootwijk 1985). However, new results from the Cowra Trough-Molong High of the Lachlan Fold Belt and from the Ngalia Basin of Central Australia do not support the Schmidt et al. APWP, and suggest an alternative Middle Silurian to Late Carboniferous path (Klootwijk 1987). An extensive study of Middle to Late Silurian volcanics from the Canberra-Yass region shows that the SSE to SE directed low inclination magnetic components, hitherto interpreted as primary, are in fact of post-tectonic origin. Other SSW directed components observed both in the volcanics and also in Early Devonian red beds and carbonates from the Taemas Basin are of pre-tectonic origin and may represent either the primary .magnetization or an Early Devonian overprint. The post-tectonic SSE to SE directed component is undated as yet, but an Early to Middle Carboniferous (Kanimblan) age of acquisition is surmised. An alternative APWP drawn according to these new results agrees well with the pole positions for the Visean Isismuyra Formation, and shows no evidence for large-scale latitudinal displacements in excess of the palaeomagnetically detectable level (about 500 km). A joint study by the BMR and CSIRO-Macquarie University palaeomagnetic groups on the latest Devonian to Late Carboniferous Mount Eclipse Sandstone from the Ngalia Basin failed to conclusively settle the controversy in APWP

219

inlerpretation. Both groups ideiiU t ied a syntectonic overprint (see also Zheng Xiang Li, 1937) whose pole position closely agrees with the post-tectonic and presumably Early Permian overprint observed in the New England region. Pre-tectonic and presumably primary magnetization components, whose pole positions sliow a gradual shift within the alternative APWP from the Givetian-Frasnian Conierong Volcanics of the Lachlan Fold Belt towards the Visean Isismurra Formation of the Tamworth Belt, were identified in BMR data but not by Zheng Xiang Li (1987). Further studies relevant to this controversy are currently being undertaken by BMR on the Early Devonian Boggy Plain Supersuite and results will be presented as available. By virtue of Australia's unique position on the eastern fringe of Gondwana the two opposing Australian APWP'shave important implications for the Middle to Late Palaeozoic configuration of Gondwana and Laurasia. It will be shown that the alternative APWP implies close contact in the Late Devonian between NW Africa and the "Armorica" part of Laurasia (Kent et al. 1984, Young 1987). whereas the Schmidt et al. APWP implies a wide(ning) oceanic gap which r-ay not have closed before the Visean phase of the Alleghenian-Hercynian orogeny (Van der Voo 1983). Irving.E.,1966. Palaeomagnetism of some Carboniferous rocks from New South Wales and its relation to geological events, J. Geol. Res., 71, 6025-6051. Kent,D.v., Dia.O. and Sougy,J.M.A.,1984, Palaeomagnetism of Lower-Middle Devonian and Upper Proterozoic-Cambrian rocks from Mejeria (Mauritania, West Africa), in: Plate Reconstructions from Palaeozoic Palaeomagnetism, R. Van der Voo, C.R. Scotese and N. Bonhommet eds., Geodynamics Series 12, 99-115. Klootwijk,C,T.,1985, Palaeomagnetism of the Tasman Fold Belt: Indications of Mid-Carboniferous large-scale southward displacement of the New England region. Third Circum-Pacific Terrane Conference, Geol. Soc. Australia, Abstracts, 14, 124-127. Klootwijk,C.T.,1987, Displacements within the Tasman Fold Belt: Palaeomagnetic indications ?, Proceedings Pacific Rim Congress 87, 241-243. Van der Voo,R.,1983, Palaeomagnetic constraints on the assembly red Continent, Tectonophysics, 91, 271-283. Young,G.C.,1987, Devonian palaeontological data and the Palaeogeogr, Palaeoclimat., Palaeoecol., 60, 283-304.

of the Old

Armorica problem,

Zheng Xiang Li,1987, New palaeomagnetic results from Late Palaeozoic rocks of Australia and their tectonic significance. Proceedings Pacific Rim Congress 87, 267-271.

220

3.22 PALAEOMAGNETIC CONSTRAINTS ON THE TECTONIC EVOLUTION OF NEW GUINEA - RESULTS FROM THE HIGHLANDS AND NORTH SEPIK REGIONS OF PAPUA NEW GUINEA C. Klootwijk^, J. Giddings^, C. Pigram^, C. Loxton^, H. Davies^, R. Rogerson^ and D. Falvey^ ^Bureau of M i n e r a l R e s o u r c e s , Geology and Geophysics, Canberra ^ G e o l o g i c a l S u r v e y of P a p u a N e w G u i n e a , Port

Moresby

The northern margin of New Guinea has been proposed by Silver and Smith (1983) to be a present-day analogue of Mesozoic terrane tectonics along the North American Cordillera. Pigram and Davies (1987) have identified in the New Guinea Orogen more than thirty terranes of continental, oceanic or composite origin, and have constrained the timing of accretion on the basis of available field data. Palaeomagnetic evidence, however, for terrane movements in this complex zone of interaction between the Pacific, Australian and Southeast Asian plates is sparse (Green and Pitt,1967; Manwaring,1974; Falvey and Pritchard,1984). To improve palaeomagnetic control BMR in cooperation with the Geological Survey of Papua New Guinea has started a palaeomagnetic study which has concentrated so far on the North Sepik region of SEATAR TRANSECT IX and on the Highlands region. NORTH SEPIK The main aim of this study is to obtain palaeolatitudinal control of movement of the Torricelli terrane, a Late Cretaceous to Early Miocene arc complex. Samples from tuffaceous sediments, pelagic carbonate lenses and volcanics from the Palaeocene to Early Miocene Bliri Volcanics show a predominant magnetic overprint acquired at a southern pal aeol atitude of about 15 degrees during Early to Middle Miocene accretion onto the northwards-moving Australian craton. Pelagic carbonate lenses of probable Palaeocene-Eocene age within the accretionary complex of the terrane show a very well-defined primary component with an equatorial palaeolatitude. This demonstrates northfacing of the arc and southwards-directed subduction over at least 1500 km. A possible primary magnetization component in pre-Late Oligocene volcanics of the Bliri Volcanics of the Tring Block shows a palaeolatitude of about 30 degrees south. This suggests that the Torricelli terrane may have been part of an east-west aligned Baining Arc (Cooper and Taylor 1987), because similar 30 and 15 degrees southern palaeolatitudes are observed (Falvey and Pritchard 1984) for Late Eocene and Early to Middle Miocene formations respectively of the Huon Peninsula-New Britain-New Ireland-Manus section of the Baining A r c . All studied localities of both the cover sequence (5; unnamed Late Oligocene reefal limestone near Amanab, and Early to Middle Miocene Puwani Limestone near Imonda and Vanimo) and of the Bliri Volcanics (3) show large-scale counterclockwise rotations between 50 and 100+ degrees, with the exception of large-scale clockwise rotations in the Tring Block. This pattern of rotations is attributed to sinistral displacement along the westward continuation of the Bismarck Fracture Zone in the North Sepik region. HIGHLANDS Samples were studied from two tectonic belts which make up the para-autochthonous northeastern margin of the Australian craton: Triassic to Miocene sediments and volcanics from the Kubor Anticline, Jimi terrane and Yaveufa Syncline (15 localities) in the southern external zone of the New Guinea Orogen; and Middle Eocene to Middle Miocene carbonates and clastics (5 localities) from the adjacent Papuan Fold Belt. Results from the Kubor-Jimi-Yaveufa belt show a very predominant magnetic overprint acquired at a 10 to 20 degrees southern palaeolatitude, which is attributed to overprinting during widespread Middle to Late Miocene igneous activity (Page 1974, Rogerson and Williamson 1985). This overprint and a primary 221

magnetization identified within 6 of the localities show a consistent pattern of large-scale counterclockwise rotations throughout the studied parts of the Kubor Anticline and Jiini terrane, varying between 30+ and 100+ degrees. Earlier preliminary palaeomagnetic results from the Bismarck Intrusive and Yonki Dome Intrusive Complex (Manwaring 1974) and from the Kubor Anticline (Green and Pitt 1967) show comparable counterclockwise rotations. These" rotations reflect reorganization of Australia's northern margin under the influence of the sinistral Tonga-Sulawesi megashear, following Middle Miocene overriding of the southward dipping subduction zone of the Torricelli-Baining Arc by the northwards advancing Australian craton. The regional extent of this pattern of areally confined smeared rotations needs to be defined further. Contrasting clockwise rotations between 20 and 90+ degrees are observed in the southern part of the Yaveufa Syncline. These may indicate a young phase of warping of the N-S trending Yaveufa Syncline-Aure Trough around the northeastern edge of the Australian craton. Results from the Papuan Fold Belt show a different pattern of local clockwise rotations varying from 20 to 50 degrees, largely following the structural trends. The contrasting rotation patterns in the Papuan Fold Belt and of the southern part of the New Guinea Orogen indicate effective decoupling of both belts, probably along crustal penetrating roots of the Papuan Fold Belt detachment zone. Palaeomagnetic data obtained so far from Papua New Guinea and from the Bird's Head region (Thrupp 1986, Giddings et al. this volume) will be interpreted in a speculative, but palaeomagnetically testable model. It is based largely upon Pigram and Davies (1987) terrane tectonics interpretation and upon the hypothesis that the northwards-advancing Australian craton has overriden successively a northwards and a southwards dipping subduction zone between the Australian and the Pacific plate. Co.oper,P. and Taylor,B., 1987, Seismotectonics of New Guinea: A model for Arc reversal following Arc-Continent collision, Tectonophysics in press. Falvey,D.A. and Pritchard,T.,1984, Preliminary results from northern Papua New Guinea: Evidence for large microplate rotations, in: Transactions Third Circum-Pacific Energy and Mineral Resources Conference, S.T.Watson Ed., 593-399. Green,R. and Pitt,R.P.B.,1967, Geomag., Geoelectr., 19, 317-321.

Suggested

Manwaring,E.A.,1974, A palaeomagnetic Highlands, BMR Report 1974/92.

rotation

reconnaissance of

Page,R.W.,1974, Geochronology of igneous and Guinea Highlands, BMR Bulletin, 162.

New

Guinea,

Papua New

Guinea

metamorphic rocks in

the New

Pigram,C. and Davies,H.,1987 Terranes and accretion history Guinea Orogen, BMR Jour. Geol. Geophys., in press.

of the

New

Rogerson,R. and Williamson,A.,1985, Age, petrology and mineralisation associated with two Neogene intrusive types in the Eastern Highlands of Papua New Guinea, GSPNG Report, 85/2. Thrupp,G.A., Silver,E.A. and Prasetyo,H.,1986, Preliminary results of a palaeomagnetic study of Misool, Irian Jaya, Westpac Symposium, Abstract 29. Silver,E.A. and Smith,R.B.,1983, Comparison .of terrane accretion in modern Southeast Asia and the Mesozoic North American Cordillera, Geology, 11, 198-202.

222

2.14

ORIGIN AND EVOLUTION OF THE AMADEUS BASIN, CENTRAL AUSTRALIA R.J. Korsch and J.F. Lindsay Bureau of Mineral Resources, Geology and Geophysics, Canberra

The Amadeus Basin in central Australia is a broad, relatively shallow, crustal depression consisting of three major subbasins connected by shallow troughs along its northern margins. The sub-basins are separated from a much larger platform area to the south and west by the Central Ridge that at times acted as a barrier to sedimentation. The basin evolved in three stages, the first two involving crustal extension and the final one being compressional; thus the Amadeus Basin can be regarded as a complex of three vertically stacked basins. The A m a d e u s Basin developed during an i n i t i a l p e r i o d of e x t e n s i o n in the Late Proterozoic (about 900 Ma) and was subjected to another period of extension just prior to the Cambrian (about 600 Ma) with the greatest subsidence occurring near the northern margin during the second phase. At the same time as the later phase of extension, a major compressional event, the Petermann Ranges Orogeny, was occurring along the s o u t h e r n margin together with the development of a small foreland basin. During the third stage, a foreland b a s i n associated with southward-directed overthrust sheets of the Devonian Alice Springs Orogeny formed in the northern part of the basin. There is a close relationship between the location of the rifting phases of the e x t e n s i o n a l b a s i n s and the l o c a t i o n of the l a t e r t h r u s t s and foreland basins. The extension formed zones of structural weakness t h a t l a t e r nucleated the inversion of the basins during shortening. The style of sedimentation and major sequence boundaries within the basinal succession was controlled to a large degree by basin dynamics. Apparent sealevel rose with the initiation of e a c h e x t e n s i o n a l s t a g e , and t h e n g r a d u a l l y declined as sedimentation commenced and the peripheral bulge appeared, peaked and declined. Following the demise of the peripheral bulge, relative sealevel again appeared to rise as thermal subsidence became the dominant controlling mechanism. As a consequence, a predictable depositional pattern occurred. During the major relative sealevel cycle following initial crustal extension, shallow marine clastics were followed by evaporites as the peripheral bulge developed and the supply of clastic sediments was gradually restricted. The evaporites were t h e n replaced by carbonates which shallowed upward to an erosional surface as the peripheral bulge peaked and began to decline. As thermal recovery began again to dominate, mature shallow marine clastics were deposited over the major sequence boundary. This depositional pattern has occurred twice in the Amadeus Basin, firstly in the Late Proterozoic sequence (preHeavitree Quartzite to base of Arumbera Sandstone) and secondly in the Latest Proterozoitf to Ordovician (Arumbera Sandstone to Carmichael Sandstone). Volcanics in the lower part of the succession are consistent with an extensional origin. Tectonism

223

on the southwestern margin of the basin towards the end of the late Proterozoic introduced clastic sediments to the western end of the basin which were able to bypass the peripheral bulge during the early part of Stage 2, displacing the carbonates and evaporites from.the western sub-basin. Relative sealevel is one of the most imporant p a r a m e t e r s controlling facies distribution and thus the distribution of the source and reservoir rocks within a basin. The shape of the relative sealevel curve in the Amadeus Basin was determined over the longer term by basin dynamics. However, superimposed on the r e l a t i v e s e a l e v e l c u r v e is a e u s t a t i c sealevel component. In general eustatic sealevel appears unrelated to basin dynamics. However, when basins such as the Amadeus result from major continental rifting events major sealevel highs follow extension by about 70 m.y. as the new ocean ridge system develops. In the Amadeus Basin, clastic sediments deposited during early, rapid basin subsidence prior to the rise of the peripheral b u l g e and t h o s e d e p o s i t e d late in b a s i n evolution when s u b s i d e n c e r a t e s are s m a l l are p o t e n t i a l reservoirs. Evaporites, deposited during early subsidence as the peripheral bulge begins to rise and preclude clastic s e d i m e n t s , are potential source rocks. Source rock-reservoir rock associations such as the Horn Valley Siltstone and the Pacoota Sandstone result from the fortuitious interaction between apparently random eustatic sealevel rises and basin dynamics. The thermomechanical model for the evolution of the Amadeus Basin in central Australia accounts for the gross sedimentary facies distribution and changes in facies type within the basin, and explains the close relationship between several deformational events (both extensional and compressional) and development of the basin through time.

224

3.12 M A G N E T I C FABRIC A N D P A L A E O M A G N E T I S M OF THE RHYODACITE A N D C O M A G M A T I C G R A N I T E S OF THE MOONBI NEW E N G L A N D M.A.

DUNDEE SUITE,

Lackie

School of Earth Sciences, Macquarie University, North Ryde

This paper documents both the magnetic fabric and the palaeomagnetism of a number of plutons of the Moonbi Plutonic Suite and the spatially related Dundee Rhyodacite of the northeast New England Fold Belt. The Dundee Rhyodacite is a porphyritic ignimbritic rhyodacite occuring as eight compositionally uniform masses throughout the Glen Innes Tenterfield area of northern New England (Flood et al 1977, McPhie 1986). The Moonbi Plutonic Suite occurs both in the southwest of the New England Batholith near Tamworth and in the north around Glen Innes and Tenterfield. The plutons to the north are spatially related to the Dundee Rhyodacite. The Moonbi Suite Plutons and the Dundee Rhyodacite are mineralogically and chemically similar (Flood et al 1980). The plutons are of Late Permian (-250 Ma) age similar to the Rhyodacite (-247 Ma). The main magnetic mineral of both the Dundee Rhyodacite and the Moonbi Suite Plutons is an ulvospinel-poor magnetite. Low-field susceptibility versus temperature runs for the rhyodacite indicate a CXirie Point near 580 C and the presence of maghemite in some samples. The bulk susceptibility for the Dundee Rhyodacite ranged .from 300 to 2000 uG/Oe with a mean about 1500 uG/Oe. The susceptibility was lower in the lowermost part of the ignimbritic flow(?s), with the bulk of the mass consistent about the mean. The bulk susceptibility of the plutons varies quite considerably, with the Bungulla Porphyritic Adamellite being high, generally above 2000 uG/Oe up occasionally to 3200 uG/Oe, while the Moonbi and Walcha Road plutons range between 100 and 600 uG/Oe. The susceptibility of the Bendemeer Adamellite varies from below 50 up to 300 uG/Oe. The northern plutons of the suite have a significantly higher susceptibility than the southern plutons. The magnetic susceptibility anisotropy for the Dundee mass of the Dundee Rhyodacite varies only slightly from 1.035 and 1.055, emphasizing the homogeneous nature of the ignimbrite. The magnetic foliation of the Dundee Rhyodacite was quite consistent at most of the sites. Along the southern margin of the ignimbrite, where basal units are present, the foliations are parallel to the margin and dip at moderate angles towards the centre of the mass. Within the central region, the foliation directions are all subhorizontal. Based on foliations defined by flattened pumice lapilli McPhie (1986) documented inward dipping foliations in the Dundee unit of the Coombadjha Volcanic Complex. According to McPhie the Dundee Ignimbrite in the Coombadjha cauldron was deposited very rapidly with subsidence occurring soon after eruption. If this type of rapid emplacement also produced the ignimbrite at Dundee, the collapse basinal structure of the ignimbrite may have formed when the basal units were still at very hot temperatures and thus the remanence of the deeper parts could have been acquired after collapse. The northern margin of the rhyodacite at Dundee is more complicated, with some sites dipping moderately towards the centre, while other sites dip away from the centre. More work is currently being undertaken on new sites from this area of complexity. The Walcha Road Adamellite magnetic susceptibility anisotropy is quite strong near the margin of the pluton (1.11 - 1.28), but is much weaker

225

towards the centre of the pluton (<1.10). The other plutons are moire consistent being around 1.10, with the occasional site being less than 1.05. Magnetic fabric determinations for the Walcha Road pluton indicate a shallow almost horizontal foliation in the central part of the pluton and a well defined outward dipping margin parallel foliation at the edge. Analysis of preliminary palaeomagnetic results from the Dundee Rhyodacite reveals either two or three components of magnetization. The softest component (if present) is a viscous component which is eliminated after 200°C of heating, while the next has a blocking temperature in the range 400°C to 500°C. The final component has a higher blocking temperature in the vicinity of 570°C. The directions obtained for the intermediate component have moderate to steep negative inclinations with a mean direction of r)ec=40, Inc=-76 (n=9 a^^ = 22). The high temperature component directions are generally steep and down with a mean direction of Dec=206, lnc=88 (n=7 ag^ = 10). The high temperature component is interpreted as the primary magnetization obtained as the ignimbrite cooled after eruption in the Late Permian. A negative fold test resulted when the magnetic foliation was assumed to represent bedding, this result being consistent with the collapse of the ignimbrite prior to the deeper parts of the flow cooling below the Curie Point of magnetite. The intermediate component possibly records a Cretaceous overprint that resulted from the heating event associated with the initial stages of the opening of the Tasman Sea. From four of the Moonbi Suite plutons, the Moonbi, Bendemeer, Bungulla and Walcha Road Adamellites, only preliminary palaeomagnetic results are available. The Moonbi and Bendemeer Adamellites both produced high temperature directions which like the high temperature component of the Dundee Ignimbritic Rhyodacite, are steep and down. References Flood

R.H., Shaw S.E., Chappell B.W. 1980. Minerlogical and chemical matching of plutonic and associated volcanic units. New England Batholith, Australia. Chem. Mag. 29, 163-170. Flood R.H., Vernon R.H., Shaw S.E., Chappell B.W. 1977. Origin of Pyroxene-Plagioclase Aggregates in a Rhyodacite. Contrib. Mineral. Petrol. 60, 299-309. McPhie J. 1986. Evolution of a non-resurgent cauldron: the Late Permian Coombadjha Volcanic Complex, northeastern New South Wales, Australia. Geol. Mag. 123, 257-277.

226

6.10

Geology

A S Y S T E M OF S T R U C T U R A L A N A L Y S I S IN D R I L L C O R E , A N D H O W TO U S E IT TO F I N D O R E D E P O S I T S

Department,

W.P.

Laing

Cook

University

James

North

Queensland

E x p l o r a t i o n i s t s are only too p a i n f u l l y aware of the lack of a system of analysis of g e o l o g i c a l structures in d r i l l c o r e . In response to this need, perceived personally over a decade of e x p l o r a t i o n experience, the w r i t e r has formulated a c o m p r e h e n s i v e and unified system of structural a n a l y s i s in d r i l l c o r e . The p h i l o s o p h y behind the a p p r o a c h is to make structures in drillcore easily a c c e s s i b l e to the n o n - s t r u c t u r a l s p e c i a l i s t , i e . to the o v e r w h e l m i n g majority of field g e o l o g i s t s . Understanding structures in drillcore c o n s i s t s of u n d e r s t a n d i n g the fundamental g e o m e t r i e s then u n d e r s t a n d i n g how the structures can be u s e f u l l y applied to finding ore deposits. Both a s p e c t s are developed in the a n a l y s i s . Structures can the f o l l o w i n g .

readily

dealt

with

drillcore

once

recognise

(a)

Some structures need to be described in vector terms (lines in s p a c e ) but some can be specified c o m p l e t e l y in scalar terms (values of an a n g l e ) . Scalar q u a n t i t i e s are easily m e a s u r e d .

(b)

The basis of targeting m a n y drillholes (eg. drilling n o r m a l to s t r i k e ) tends to facilitate g e o m e t r i c i n t e r p r e t a t i o n . The g e o m e t r i e s e n c o u n t e r e d are special cases of the g e n e r a l g e o m e t r y , and these special cases lend t h e m s e l v e s to simple v i s u a l i n t e r p r e t a t i o n .

(c)

E u c l i d e a n g e o m e t r y demands t h a t , to u n i q u e l y fix the o r i e n t a t i o n of a line(s) or p l a n e ( s ) , a m i n i m u m amount of specific information is r e q u i r e d . Put s i m p l y , some drill logging situations will yield a unique r e s u l t , and others w o n ' t . Some o r i e n t a t i o n s of drillhole are inherently m o r e s t r u c t u r a l l y informative than o t h e r s .

Once we u n d e r s t a n d these p r i n c i p l e s , we can m a k e of d e c i s i o n s as to how to get the answers we w a n t :

the

following

(i)

what angles and other i n f o r m a t i o n we need to record;

(ii)

whether don't);

(iii)

w h e t h e r we need to incorporate surface i n f o r m a t i o n , and w h a t ;

(iv)

how we m i g h t usefuIness.

need

tailor

use

our

st ereographic

drill

program

net

(in

optimise

many

its

sorts

cases

structural

The f u n d a m e n t a l p r o b l e m , the reason why we need a separate system of structural analysis in d r i l l c o r e , is that drillcore during extraction from the drillhole rotates by an u n k n o w n a m o u n t . The drillcore is said to be a x i a l l y o r i e n t e d . In a x i a l l y oriented drillcore we need to erect an internal g e o m e t r i c reference f r a m e , analogous to the normal m a p p i n g r e f e r e n c e frame p r o v i d e d by the earth's surface and its set of a z i m u t h s . The most c o n v e n i e n t reference f r a m e , erected at each point c o n t a i n i n g a s t r u c t u r a l surface of i n t e r e s t , is intuitively easy to grasp: it c o n s i s t s of the plane normal to the d r i l l c o r e , plus the plane normal to this p l a n e , w h i c h c o n t a i n s the pole to the s u r f a c e . All other structures in this piece of drillcore can then be c h a r a c t e r i s e d in terms of three

227

angles, measured in the original plane or in these two reference these are analogous to strike, dip and pitch in normal mapping.

planes;

Methods are then available to uniquely fix all these structures in the given piece of drillcore: either by relating one of them to a known structural orientation, or by taking measurements of their angles at three or more different orientations of drillhole (either down a single curving drillhole or in different drillholes). The former approach requires some external (surface) information, while the latter approach can be used without resort to any external data. These methods are expressed as simple stereographic recipes. They provide absolute orientations for all the structures measured. This sort of orientational information is the meat of structural analysis in brittle (vein and fracture) terrains, and is important in ductile (folded) terrains. However in the latter it is also important to establish position on a folded surface, the large scale shape of the rock mass, and areas of low and high strain. To establish these the structural concepts of vergence, facing and interlimb angle need to be .translated into drillcore practice. These .rather forbidding concepts are made much more friendly by reducing them to vector, and in some circumstances scalar parameters. The latter, being independent of absolute orientation, are more easily handled than vectors. The interpretation of vergence and facing in practice comprises a straightforward measuring of the cleavage angle plus noting whether the vergence or facing vector, obtained from small folds or bedding/cleavage relationships, is directed uphole or downhole. A truth table then provides the actual direction of vergence or facing. Application of structural analysis in drillcore to finding ore comes via understanding the many ways in which the geometry of deposit styles provides pointers toward ore. Examples follow. ore shoots These may

deposits specific

Many vein deposits have line of two vein sets. orientation of this line.

parallel to the intersection be tested by determining the

Bendigo-style gold deposits occupy saddle reefs in antiformal crests. These can be found by logging vergence (a vector) or logging interlimb angle (a scalar), while extensions of existing reefs can be found by determining the orientation of the fold axis (a vector). Methods are available for determining all these parameters in drillcore in completely covered areas.

Ballarat-style gold deposits occur in crosscutting veins which form preferentially in the east limbs of antiforms. These can be found by logging vergence. Extensions of existing veins can be found by determining the vein orientation, while extensions of ore shoots might be found from the orientation of the vein intersection with gold-localising beds.

Massive sulphide deposits in deformed terrains commonly localise high strain in their adjacent wallrocks. In this situation fold axes rotate more than normally toward the principal stretching direction. Where a stretching lineation is present (the normal situation), high strain is directly expressed as an anomalously small angle between the fold axis and the stretching lineation. A direct vector toward ore is thus provided by the value of this angle.

It is easily seen on the cleavage plane and is easily measured a scalar) by holding a protractor against the cleavage ellipse.

228

(being

6.4 THE STARRA GOLD-COPPER DEPOSIT - SYNDEFORMATIONAL METAMORPHIC MINERALISATION LOCALISED IN A FOLDED EARLY REGIONAL ZONE OF DECOLLEMENT W.P. Laing, M.J. Rubenach and C.K.

Switzer

Geology Department, James Cook University of North Queensland The Starra gold-copper deposit in the Selwyn region, northwest Queensland (7mt at 5g/t Au), displays features typical of an replacement auriferous iron formation deposit. The enclosing host rocks reveal strong structural and metamorphic constraints on the timing and localisation of the host iron formation and the gold-copper mineralisation. The Starra orebody is not a primary syngenetic banded iron formation hosted gold deposit.

On a regional scale, the Starra orebody is located within a major Di zone of high strain (shear zone), which represents a major decollement juxtaposing high grade (upper a m p h i b o l i t e ) metasediments and anatectic granitoids of the Gin Creek block against low grade (lower-upper g r e e n s c h i s t ) calc-silicates and metasiltstones of the younger, overlying Staveley Formation. The shear zone is over one kilometre thick and was shallowly north dipping prior to the main D2 regional north-south fold phase. It contains a strong mineral stretching lineation and s-c planes indicating north-sough directed shear with the upper sheet (Staveley Fm) moving northward, D2 folding was coaxial with Di stretching lineation and folded the decollement about major shallowly north plunging folds on the northern flank of the Gin Creek block. Stratigraphic units above and metamorphic isograds below are parallel to the decollement. The interpreted shear sense, the rapid metamorphic grade decrease, and the stratigraphic relations across the decollement are consistent with a metamorphic core complex. Extensional tectonics on a detachment zone during Di are suggested, perhaps prior to or contemporaneous with north-south directed thrusting in the western Mt Isa block.

The Starra deposit is located on the eastern limb of the Gin Creek Antiform (D2) in the thick (2km) region of the D^ shear zone. The host ironstone, a poorly foliated m a g n e t i t e - q u a r t z - h a e m a t i t e unit lies on the contact between highly altered basic to intermediate volcanics and calc-silicates of the Staveley Formation. Textural studies show that the main mineralising event commenced in early D2 with the development of coarse m a g n e t i t e and synchronous pyrite. C h a l c o p y r i t e infill of open space and small fractures within m a g n e t i t e and pyrite occured later together with the alteration of magnetite to haematite. Alteration in the hanging wall consists of carbonate, actinolite, chlorite, and quartz in veins and replacement in calcsilicates. Gold occurs intimately within magnetite, pyrite and chalcopyrite and bears a direct correlation with magnetite. Brecciation and localisation of gold and copper in the area of D2 fold hinges in D2 or D3 in age. The textural and field evidence indicates that the Starra gold-copper deposit is a syndeformational replacement style of deposit. Evidence supports the hypothesis that mineralisation is contemporaneous with and postdates D2 m e t a m o r p h i s m . Mineralisation is controlled by fluids focussing in shear zones and dilatent fractures respectively at regional and local scales. Trace element (including metals), and mineral stability studies are consistent with a "high salinity metamorphic fluid or a metamorphic fluid with a magmatic component. The source of the fluid is enigmatic as the age and the role of the granitoids in the Gin creek block is not known. A l t h o u g h the Starra iron formation and gold-copper mineralisation are not synsedimentary, iron may have been derived from originally synsedimentary ironstones as exemplified in other areas of the Selwyn region.

229

2,11

THE PERTH BASIN - A POSSIBLE FRAMEWORK FOR ITS FORMATION AND EVOLUTION K. Lambeck Research School of Earth Sciences, Australian National University, Canberra

The Perth Basin of Western Australia is a linear structure containing, in parts, up to 15 km of Phanerozoic sediments.

An examination of geophysical data, including

recently observed teleseismic travel time residuals, and of geological evidence has led to the following preliminary model of the evolution of the basin and the adjacent part of the Yilgarn Block. The starting point is taken to be in middle-to-late Proterozoic time when the western part of the Yilgarn Block was assumed to be a topographic high, formed by some unspecified mechanism. Erosion of the uplifted area provides the bulk of the sediments for the proto-Perth Basin and, as erosion continued, isostatic rebound occurred. If the late Proterozoic highlands were in isostatic equilibrium their elevation need not have exceeded about I km on average in order to produce a total rebound of about 3-4 km. The stresses generated by the sediment loading and erosional unloading exceed several kilobars and can lead to crustal failure, the nature of the stress field and failure being a function of the regional stress fields at the time, as well as of previous zones of weakness.

Subsidence of the basin continues for as long as

sedimentary supply remains sufficient.

Rebound of the region occurs when regional

stress fields are modified. The proposed model offers a possible explanation for an apparently disparate set of observations extending from Late Proterozoic to the present. These include gravity, the present stress-regime, Quaternary vertical movements, present topography, and the travel time residual analysis of the fault geometry and Moho depth variations.

230

2.12

SEISMIC TRAVEL TIME ANOMALIES IN CENTRAL AUSTRALIA AND IMPLICATIONS FOR DEEP CRUSTAL STRUCTURE

K. Lambeck Research School of Earth Sciences, Australian National University, Canberra

Teleseismic

travel

times

have

been

recorded

along

two

lines

instruments

traversing the Amadeus Basin and southern Arunta Block of central Australia for the purpose of features.

examining

the deep crustal and upper mantle structure underlying

these

Pronounced differential travel times, approaching 1.5 seconds, and with major

azimuthal variation, have been recorded.

Most of the useful observations originated

from events to the north (Japan-Marianas) and to the east (Fiji-Tonga) at epicentral distances of 40*' to 65**.

A few observations of southerly azimuth events (Macquarie

Ridge and South Sandwich Islands) were also recorded.

The inferred structure beneath

the basin and block exhibits major lateral variations, the essential aspects of which are: (i) a zone of relatively high velocity material dipping steeply northwards such that the continuation

its

southern

edge

the

surface

coincides

approximately

with

mylonitic shear zone known as the Redbank Deformed Zone, and (ii) a steeply dipping zone of relative low velocity material immediately to the south of this shear zone and partly underlying the high velocity zone.

If the travel times are interpreted in terms

of an undulating Moho, variations in Moho depth of 20 km occur over

horizontal

distances of less than 50 km with relatively shallow Moho depths occurring immediately to the north of the Redbank Deformed Zone.

Such an interpretation is consistent with

the gravity anomalies observed across the geological structures.

The inferred structure

is out of hydrostatic equilibrium and departures from local isostasy approach 150 MPa. These stress-differences must have persisted during the 300 Ma following the Springs Orogeny that led to the present crustal structure.

Alice

Evidence, in the form of

the surface topography and seismicity, suggests that some stress relaxation has and still is occurring but not at the shear zone itself. structure

may

have

been

isostatic

At the time of

equilibrium

but

the orogeny

as erosion

took

the place

complete local rebound did not occur, either because horizontal compressive forces still operated or because the strength of the lithosphere increased with time. 231

7.10 LATE DEVONIAN-EARLY CARBONIFEROUS SEDIMENTATION IN THE BUNDOCK CREEK GROUP OF THE BROKEN RIVER PROVINCE, NORTH QUEENSLAND S.C. Lang Queensland Department of Mines

The Bundock Creek Group comprises the upper 6000 m of Late Devonian to Early Carboniferous clastic sedimentary rocks in the Graveyard Creek Subprovince at the western end of the Broken River Province, north Queensland. The sedimentary evolution of the Group is summarised in Figure 1. The base of the Bundock Creek Group is marked by a thin marine to lower delta plain sequence of probable Frasnian age, and is separated from the underlying Broken River Group by a slight angular unconformity to disconformity. This break was caused by uplift in the Camel Creek Subprovince to the southeast, and the Lolworth-Ravenswood Block to the south. This event can probably be correlated with the NE trending fold event in the Camel Creek Subprovince during the ?Late Givetian to Frasnian. This event heralded a rapidly deposited regressive clastic wedge of sheetflood, braided stream, and alluvial fan sediments. The sequence is characterised by redbeds, commonly with well developed caliche. The dominant transport directions were from the south and southeast, and probably influenced by renewed activity along the Clarke River Fault Zone and the Gray Creek Fault. The Frasnian sea is presumed to have retreated rapidly northward, over the Georgetown Block, to the Hodgkinson Province. Calcirudites, derived locally from uplifted blocks of Broken River Group limestones, interdigitate with the alluvial fan and braided stream facies, as debris flow, and other short-headed alluvial fan facies. During the Famennian, continental redbeds were replaced by marginal and shallow marine elastics as the sea moved westward from the Burdekin and Clarke River Basins. The peak of the transgression was marked by the deposition of thin, dirty limestones, and conodonts recovered from these beds indicate a latest Famennian age (Middle expansa Zone to within the lower part of the Middle praesulcata Zone). This is a slightly older age than previously thought, making all but the very uppermost part of the Bundock Creek Group, of late Devonian age. Non-marine conditions returned during the Tournaisian and ?Visean, enhanced by the uplift of the Georgetown Block which in turn generated a southward directed regressive phase, dominated by fluviatile and lacustrine sedimentation. Minor redbeds occur in this uppermost sequence, but differ from the older redbeds by being derived from the oxidation of volcaniclastic sediments, mainly tuffs and reworked tuffs. The volcanic influence steadily increases upwards through the Bundock Creek Group, but the top is almost entirely volcaniclastic, including rare andesitic lava flows, and a reasonalby proximal source is indicated. Rhyolite and dacite dykes and sills intrude mainly the upper half of the Group, and predate the main fold event. It is possible that the source of the volcaniclastics was indeed related to the Montgomery Range Igneous Complex that intrudes the Graveyard Creek Subprovince, but as yet this has not been firmly established. The plutonic rocks post-date the main Early Carboniferous NE fold event, but were then gently deformed by a N-S compressional phase which may be Mid-Carboniferous in age.

232

CAMEL CREEK SUBPROVINCE

REFERENCE MONTGOMERY RANGE IGNEOUS COMPLEX r ^ ; 1 BOROSTON

FORMATION

TEDDY MOUNT

FORMATION

SUBSIDENCE

TURRETS

EARLY TO 7MIDDLE CARBONIFEROUS LANDSCAPE EASTERN BUNDOCK BASIN

GEORGETOWN BLOCK

FORMATION

BULGERI

FORMATION

MYTTON

FORMATION

WANDOVALE J

SHIELD

SUBGROUP

CREEK

GRAVEYARD

GRAY C R E E K F A U L T

FORMATION

CREEK

GROUP

TONALITE

LATEST FAMENNIAN TO EARLY TOURNASIAN LANDSCAPE EASTERN BUNDOCK BASIN

WAIRUNA

JUDEA

BEDS

DIDO

GRANODIORITE

ALLUVIAL

FORMATION

FANS

FLOOD PLAIN

GRAY CREEK F A U L T

LAKES

PALAEOCURRENT

-y

BRACHIOPODS

VOLCANIC DELTAS,

DIRECTIONS

ACTIVITY SHALLOW

SEA

FAMENNIAN LANDSCAPE, EASTERN BUNDOCK BASIN .

FLUVIATILE

-^UPLIFT

GREENVALE

DEBRIS

ENVIRONMENTS

FLOWS

GASTROPODS FISH GRAY C R E E K

FAULT

C^ SUBSIDENCE

PLANT

?FRASNIAN^PAMENNIAN LANDSCAPE EASTERN BUNDOCK BASIN Figure 1.

BIVALVE

Sedimentary evolution of the Late Devonian to Early Carboniferous Bundock Creek Group,

233

FRAGMENTS

7.7

EARLY TO MIDDLE DEVONIAN CARBONATE-TERRIGENOUS SEDIMENTATION IN THE BROKEN RIVER GROUP, GRAVEYARD CREEK SUBPROVINCE, NORTH QUEENSLAND

S.C. Langl, J.S. Jell^, J. Talent^, R. Mawson^ and I.W. Withnall^ ^Queensland Department of Mines ^Department of Geology and Mineralogy, University of Queensland ^School of Earth Sciences, Macquarie University, North Ryde

The Broken River Group (formerly known as the Broken River Formation) is confined to the Graveyard Creek Subprovince at the western end of Broken River Province. Completion of regional mapping in the area, coupled with ongoing detailed studies, has revealed that the Group is a complex of carbonate and terrigenous lithofacies, represented by fourteen newly proposed stratigraphic units. Sedimentation in the Broken River Group can be interpreted in terms of six major environmental facies associations which are summarised in Table 1. Depositional models have been developed for the Late Emsian-Eifelian, and Givetian respectively, and these are illustrated in Figure 1. The distribution of shoreline facies indicates that the southern margin of the basin was approximately parallel to the Clarke River Fault Zone throughout most of this time. However, the basin margin in the north and west moved gradually westward during a major marine transgression beginning in the Emsian, reaching its peak in the Givetian. At the peak of the transgression the sea was probably connected to the Hodgkinson Province in the north via a shallow sea across the eastern part of the Georgetown Block. This is based on downfaulted blocks of Early to Middle Devonian carbonate and terrigenous rocks (e.g. Blue Rock Creek Beds). The sea was also probably connected to the Burdekin Basin in the east. This is based firstly on the trend of palaeocurrents within the eastern part of the Group, which overall trend eastward from shelf to slope facies respectively parallel to the axis of the Broken River Province. Secondly, the nearest Early to Middle Devonian sediments in the east occur near Blue Range in a fault slice, and then at Mt Podge at the northern end of the Burdekin Basin. Carbonate sedimentation became severely restricted by the end of the early Givetian due to an increase in terrigenous sedimentation. Deposition of the Broken River Group was terminated in the Late Givetian or Frasnian by tilting and erosion, resulting in a slight angular unconformity and disconformity with the overlying Late Devonian . Bundock Creek Group. Tnble I : Summary of environmental facies associuUons Envimninontnl f a d e s nssociations 1.

Interpreted deposit types

SiliciclQstic Shoreline

Well sorted, laterally extensive sandy strandiine deposits.

Mixed Silicioln<;lic/ Carbonate Shoreline

Fnn-doltn conRlomerote'; find course snnristones, Inlertldal to suhtldnl redhods with nssociated limestones (Including minor algnl laminates).

Sillciclastic Shelf

Sandy and congJotneratic shallow shelf deposits laterol to fon-deita deposits. Storm Influenced, wove-domlnated sandy shelf deposits.

Carbonate Shelf

Carbonote barrler/shool complex with limited development of wnve-resistont stromatoporoldcoral Yeefs' - mostly blostromes. Extensive qtjlet carbonate shelf lagoons. Oncolltic/ooiltlc shools.

Miitldy SIJGlf

Richly fosslllforous miidstoncs, and minor nodular llme'^toncs and colcareous sandstones. These probobly ronge frojn intertidnl to deeper muddy shelf deposits. Minor carbonate banks, and locally restricted areas.

Mixed Siliclclnstic/ Carbonate Slope

Conglotneratlc submarine channel deposits, some with nllochthonous limestone blocks. Mudstone, limestone ond aretiite slope fleposits Intorpreted ns lurl)idites, slump blocks and flcl>ris flows.

234

<ONCOLITE/OOLITE SAND S H O A L S AND C A R B O N A T E BUILDUP

MUDDY S H E L F TO T I D A L F L A MINOR G R A V E L L Y CHANNELS

STORM INFLUENCED SHALLOW SHELF N E A R S H O R E SAND B A R S , C H A N N E L S AND B E A C H E S

SUBSIDENCEV

GIVETIAN LANDSCAPE-BROKEN RIVER GROUP CARBONATE SHOALS/ BARRIER COMPLEX OFFSHORE

MUDDY S H E L F TO S L O P E WITH GRAVELLY CHANNELS

CARBONATE S H E L F LAGOON AND P A T C H R E E F S

MUDDY S H E L F / S H E L F LAGOON N E A R S H O R E SAND B A R S , C H A N N E L S AND B E A C H E S

SUBSIDENCE

LATE EMSIAN-EIFELIAN LANDSCAPE, BROKEN RIVER GROUP REFERENCE =

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Figure 1. Depositional models for the Broken River Group

235

11.3

REGMAP - A REGIONAL MAPPING FIELD DATA MANAGEMENT SYSTEM

S.C. Lang, I.W. Withnall, K.G. Grimes and P.R. Murphy Queensland Department of Mines

A regional mapping field data management system (REGMAP) has been designed by the Geological Survey of Queensland, to facilitate rapid manipulation and retrieval of field data for research, report writing and map compilation. REGMAP uses a standardised field notebook for data entry (Figure 1). The first part of each data page has fixed fields for the site description (field number, grid reference, formation name, etc.); followed by several lines for structural data which also has fixed fields (data type, dip, facing etc.). The second part of the page has a more flexible format and consists of a series of records, each containing 3 fields: an optional Vock class' followed by the 'data type' and then the actual 'description' field, which can be in full english or use a set of standard abbreviations. The 'data type' field in this second part of the form is the key to the flexibility of the system, avoiding the restrictions imposed by fixed fields, or 'tick-the-box' forms. The geologist can decide what information needs to be recorded, and is aided by a hand held prompt list of common data types and their 4 letter abbreviations (COLR, GNSZ, COMP, etc., including a REM datatype for miscellaneous remarks). In addition new data types can be added to the system at any time to meet specialised needs. This descriptive part can contain as many lines as is required, even to spreading over several pages of the notebook. The information is structured by the use of a special data type called LITH, with a rock name in the associated description field and an entry also in the associated rock class field. All subsequent data is taken to refer to that lithology until a further LITH is entered. The data is entered into an 1MB PC - based system, using a set of DBASE programs for loading, maintaining, selecting and outputting the data. Direct data entry in the field, using a 'lap top' battery powered microcomputer supplements data entry in the Field Office. The site information provides the main basis for selecting subsets of the data, but data in the free form area can also be searched with the assistance of special programs. Retrieval of these data using either the site information, or selected 'data types' as keys, provides a means by which similar information can be collated and report forms generated, thus speeding up mundane and specialised jobs; such as formation descriptions, palaeocurrent analysis, sample listings, grain sizes, fossil occurences, and numerous combinations. A well as tabulations, the data can be used graphically. Plots at any scale, with or without attached annotations, are produced on plotters attached to the State Governemnt Computer Centre PRIME mainframe or on a small HP compatible ROLAND plotter attached to an IBM PC. Data can also be transferred to the CAD system on the PRIME for incorporation into maps or diagrams. As well as being used by GSQ Regional Mapping staff, the data will be made available to the public as printed tabulations, plots, or as bulk or selected data on magnetic media. The data will be archived for future reference.

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8.13

THE

BULGONUNNA VOLCANICS CAULDRON COMPLEX IN S.R. Queensland

- A CARBONIFEROUS IGNIMBRITIC CENTRAL QUEENSLAND Law

Department

Mines

The Bulgonunna Volcanics are located 80 km southwest of Collinsville in Central Queensland. The southern part of the volcanic complex is exposed in the Mount Coolon 1:250 000 sheet area and was mapped during the 1985-86 field seasons. The northern part of the complex (in the Bowen 1:250 000 sheet area) is yet to be examined in detail. Volcanism c o m m e n c e d with eruption of voluminous ignimbrite sheets from vents located along ring fractures. These ring fractures delineate part of an elliptical 40 km by 24 km cauldron structure centred about Bulgonunna Peak. The seven ignimbrite units identified from field mapping have been assigned to three new stratigraphic unit The Bobby Dazzler Dacite, Locharwood Rhyolite and Pinang Rhyolite M e m b e r - and are included in the Bulgonunna Volcanic Group. The Locharwood Rhyolite is mainly a densely welded moderately crystal rich to crystal rich ( 1 0 % to 3 0 % crystals) rhyolitic ignimbrite with well developed eutaxitic texture and lacks lithic fragments. T w o local variants - a lithic bearing (up to 5 % ) facies and a non-eutaxitic (pumice fragments absent) facies have been delineated in the southeast. A thin, areally restricted andesitic pyroclastic flow deposit occurs at the base of the Locharwood Rhyolite in the southwest (Mount Coolon area). The Pinang Rhyolite M e m b e r forms the upper part of the Locharwood Rhyolite and consists of moderately crystal rich rhyolitic ignimbrite. The youngest unit, the Bobby Dazzler Dacite, is characterised by a crystal rich, moderately lithic rich, lapilli - ash dacitic to rhyolitic ignimbrite. The lithics include nonvesiculated and vesiculated felsic volcanics and scattered andesite fragments. Co-ignimbrite breccias are associated with both the Locharwood Rhyolite and the Bobby Dazzler Dacite. The Bulgonunna Volcanic Group also contains some rhyolite lava flows and is cut by dykes and pods of rhyolite, quartzfeldspar porphyry and feldspar porphyry. The evolution of the Bulgonunna Volcanics involved three eruptive episodes. The first produced an andesitic pyroclastic flow deposit. There is insufficient information to determine its extent or source because of the limited outcrop. The second phase of eruptive activity produced the rhyolitic ignimbrites of the Locharwood Rhyolite and the Pinang Rhyolite M e m b e r . These deposits are preserved within the cauldron structure except in the southern area where poorly exposed remnants extend beyond the southern margin of the cauldron. The third eruptive phase produced the ignimbrite of the Bobby Dazzler Dacite. This widespread unit occurs adjacent to, but outside the cauldron structure. A n outlier has also been mapped within the cauldron. T w o possible source areas for the ignimbrites are located on marginal ring fractures of the cauldron. Field relationships between the lapilli - ash ignimbrite, co-ignimbrite breccias and extrusive rhyolite lava indicate the Wyarra Hills area is one source for the Bobby Dazzler Dacite Uhird eruptive phase). Locating the source of the second phase rhyolite ignimbrite is more difficult. It may have originated from the Wyarra Hills or from a vent located on the southeastern margin of the cauldron, in the Terang area. The lithologies and facies associations in the Terang area are similar to those in the Wyarra Hills, but the units are much thinner. Subsidence which gave rise to the cauldron structure possibly occurred either during, or soon after eruption of the Locharwood Rhyolite. Subsequent activity involved the intrusion of rhyolite and porphyries along the marginal ring fractures and extrusion of rhyolite lava from vents located on the ring fractures. There is no evidence of subsequent major resurgence or structural updoming of the cauldron floor. The last igneous event was the emplacement of the comagmatic Bluegrass Creek Granite (new name) into the Locharwood Rhyolite on the eastern margin of the cauldron. The Bulgonunna Volcanic Group and the adjacent volcanic influenced sequences of the D r u m m o n d Basin are currently being investigated, by several private companies, for potential acid volcanic associated epithermal and hydrothermal gold mineralisation. 238

7.21

SEDIMENT SOURCE FOR THE MARTIAN NORTH POLAR DUNE FIELDS J.H^J. Leach

CSIRO Division of Geochemics,

Indooroopilly

Tlie viking II Orbiter found a vast body of sand dunes surrounding the Martian North Polar Ice Cap (Cutts et al. 1977). The sediment s o u r c e f o r t h e s e d u n e s w a s r e c o g n i s e d as a p r o b l e m . T h e i r proximity and close association with the polar sediments implied tliat they were derived from the ice cap materials. This has been rejected by most authors for two reasons. The first is that the dunes have a much lower albedo than the other polar sediments and this was interpreted as a difference in composition (Cutts et al. 1976, Botts 1979). The second was that the polar sediments were thought to be derived from atmospheric dust (Cutts 1973) and this material would be too fine to form dunes (Bagnold 1941). However, the association of the ice cap and the dune fields is far more complex than mere proximity. The dunes abut against, and in some cases overlie, the p e r i p h e r y of the ice c a p and they extend along the floors of valleys which penetrate into the ice cap. They also exhibit their greatest development opposite those ice c a p f e a t u r e s w h i c h are most o b v i o u s l y the p r o d u c t s of erosion: the large valleys and their associated scarps. Also, the wind d i r e c t i o n over the northern ice cap, indicated largely by the d u n e s t h e m s e l v e s ( L e a c h 1 9 7 9 , T s o a r et a l . 1 9 7 9 ) , is predominately outwards from the ice cap. This is especially true along the floors and at the mouths of the large valleys. That is, t h e d u n e f o r m s d e v e l o p in r e g i o n s w h e r e the d o m i n a n t w i n d direction is outwards from the pole. The s t r u c t u r e of the dune fields also indicates that they are formed from sediment which has a polar source. Each of the dune fields has the same g e n e r a l s t r u c t u r e , a c o r e of transverse dunes, closely associated with one of the large valleys in the ice c a p , w h i c h b r e a k s u p into b a r c h a n s and b a r c h a n - d e r i v e d longitudinal dunes to the south and along the eastern and western p e r i p h e r i e s . A c h a n g e from t r a n s v e r s e to barchan dunes is an indication of a decrease in sediment s u p p l y (Cooke and W a r r e n 1973). T h i s r e l a t i o n s h i p is d e m o n s t r a t e d c l e a r l y w i t h i n the southern most s e c t i o n of the dune field at the m o u t h of the Chasma Boreale. Here a portion of the underlying plains protrudes through the dune field, c a u s i n g a local d e c r e a s e in s e d i m e n t supply. Around the periphery of the obstacle the transverse dunes of the main field break up into small barchan dunes. Thus, the p r o g r e s s i v e c h a n g e in dune m o r p h o l o g y away from the ice cap clearly indicates a decrease in sediment s u p p l y to t h e south. This is consistent with a polar sediment supply. Perhaps the most convincing evidence for a polar source for the dune field sediments, however, is the e x i s t e n c e of small d u n e fields w i t h i n the ice cap itself. These dunes occur along the floor of the Chasma Boreale on within a number of mature ablation troughs (Leach 1984). They are normally composed of irregular and d i s c o n t i n u o u s t r a n s v e r s e or b a r c h a n d u n e s , and t h e y a l w a y s develop downwind from either one or a series of arcuate scarps. Not only are these dunes developed well within the confines of the ice cap, so that if they had a sediment source outside the ice c a p they w o u l d have had to p r o g r e s s s o m e h u n d r e d s of kilometers against the prevailing wind direction, but many of the dunes developed in the ablation troughs occupy closed or s e m i closed depressions with either no access or very limited access to the surrounding plains. All of these f a c t o r s i n d i c a t e that these dunes, at least, were formed from sediment derived from the ice cap itself, and they this indicate that the polar sediment is capable of forming dunes. The lower albedo of the dunes could be the result not of a change in composition, but of a change in morphology. Albedo is not only determined by the composition of the surface material but is also very largely determined by surface r o u g h n e s s . In t h i s respect 239

d u n e fields w i l l always have a lower albedo than smooth plains composed of the same material. In any c a s e , t h e low a l b e d o of these materials is shared by all the polar dune fields, including the intra-cap fields for which it is not reasonable to propose a distant source. In order to. form sand dunes, the fine dust particles that were originally deposited over the ice cap would need to be aggregated in some way to form sand size particles. There are a number of ways in which this could happen. The p a r t i c l e s c o u l d be b o u n d together by soluble salts, a process which is thought to result in duricrust formation at the Viking Lander sites (Baird et al. 1976, Toulmin et al. 1977). Greeley (1979) has demonstrated that electrostatic forces can cause dust particles to aggregate into coliesive s a n d s i z e d p a r t i c l e s w h i c h c o u l d t h e n for e o l i a n bedforms, such as dunes. It is also possible that fragments of the r e m a i n i n g i n t e r s t i t i a l ice c o u l d act as c e m e n t , binding several dust grains together to form a sand sized particle. W h i l e t h e r e are p r o b l e m s with assigning a polar source to the dune f i e l d s e d i m e n t s , t h e s e m u s t be w e i g h e d a g a i n s t the c o n s i d e r a b l e b o d y of e v i d e n c e t h a t e x i s t s in f a v o u r of the hypothesis , and the very real problems that are associated with the alternatives. None of the proposed alternatives can account for the development of the intra-cap dune fields, and a l l m u s t consider the close association of the dune fields and the ice cap as essentially coincidental. REFERENCES Bagnold, R . A . 1941. "The Physics of Blown Sand and Desert Dunes." London: Methuen and C o . , 265pp. Baird, A.K., Toulmin, P., Clark, B.C., Rose, H., Keil, K., C h r i s t e n , R.P. and J.L. Gooding. 1976. "Minerologic and p e t e r o l o g i c implications of Viking geochemical results from Mars: Interim Report." Science 194: 1288-1293. B o t t s , M . 1 9 8 0 . " S t r a t i g r a p h i c S e q u e n c e of V o l c a n i c a n d Sedimentry Units in the N o r t h P o l a r R e g i o n of M a r s . " N A S A T e c h . Memo N o . 78254. C o o k e and Warren 1973. "Geomorphology Batesford Press.

in D e s e r t s . "

London,

C u t t s , J.A. 1973. Nature and Origin of the Layered Deposits of the Martian Polar Regions. J. Geophys. Res. 78: 2431-4249. C u t t s , J . A . , Blasius, K.R. Briggs, G.A., C a r r , M . H . , Greeley, R . and H . Marsursky 1976. North Polar Region of Mars: Imaging Results from Viking 2. Science 194: 1329-1337. G r e e l e y , R . 1 9 7 9 . Silt and c l a y a g g r e g a t e d u n e s o n M a r s . Geophys. R e s . 84: 6248-6254.

L e a c h , J.H.J. 1979. Dune form and patterns of wind circulation in the north polar region of M a r s . P r o c e e d i n g s of t h e S e c o n d International Colloquium on Mars. NASA Conference publication N o . 2075. L e a c h , J.H.J., 1984. "The Geomorphology of the Martian North Polar Region." Ph.D. Thesis, University of Melbourne. T o u l m i n , P . , B a i r d , A . K . , C l a r k , B . C . , Keil, K. , Rose, H.J., C h r i s t i a n , R . P . , Evans, P.H. and W.C. Ke11iher.1977 . G e o c h e m i c a l and m i n e r o l o g i c a l interpretation of the Viking inorganic chemical r ^ u l t s . J. Geophys. Res. 82: 4625-4634. Tsoar, H . , Greeley, R . and A . Peterfreund 1979. M a r s , The north polar sand sea and related wind p a t t e r n s . J . G e o p h y s . R e s . 84: 8167-8180. 240

12.1

TECTONIC RAMIFICATIONS OF ASTEROID IMPACTS J.H.J. Leach

CSIRO Division of Geomechanics,

Indooroopilly

The early history of the solar system is one intense meteorite bombardment. This is seen in all of the ancient surfaces of the Moon, M e r c u r y , Mars, and the outer Jovian Satellites. These surfaces have been saturated with multiple impact craters. It is reasonable to suppose that the Earth, being in the same region of space, also suffered this bombardment, although a dense primitive atmosphere may have protected it to some extent. However, no trace of this early crust is currently preserved on the Earth's surface. The last stage of this early bombardment, which was really the last stage of planetary accretion, was a series of very large impacts as the planets collected the last planitesimals formed in their region. These impacts have left large basins on Mars, Mercury and the outer Jovian satellites and they formed the basins of the lunar maria. They must have effected the primitive terrestrial crust on the same scale. It seems likely that the Earth's first oceans were formed by such an impact or by a series of such impacts. The Moon has over twenty such impacts. Some of which make up the dark lunar plains, or Mare, which are obvious from the Earth. These Mare are lava plains which are a result of the melting caused by the impacts whose basins they flood. The largest of these is the Mare Imbrium which is over 1000 Km in diameter. The Mare Serenitatis, the Mare Tranquillitatis, the Mare Crisium, the Mare O r i e n t a l e , and the Sinus Iridum are all examples of this massive type of impact basin on the moon. In contrast, the ancient lunar highlands record the earlier phase of saturation bombardment. The southern hemisphere of Mars, like the highlands of the Moon, is marked by the saturation impacts of late planetary accretion. Here, however, the evidence is softened by weathering and erosion and mantled by eolian deposition. In this region, two very large basins are found, the Argyre and Hellas basins, 600 and 800 Km diameter respectively. These are evidence that the last stage of major asteroidal impact also occurred on Mars. The s u r f a c e of M e r c u r y a l s o r e c o r d s t h e e a r l y p e r i o d of b o m b a r d m e n t , without even the extensive lava coverage of the lunar maria or the weathering of the southern hemisphere of Mars. The only regions where extensive mare like plains occur on Mercury is on the floor ^of one very large impact basin. This is

241

the Caloris Basin which was only ever half imaged by the Mariner 10 probe. The basin has a diameter of 1300 Km and a range of s m a l l h i l l s s e e m s to h a v e b e e n formed by the reinforcing shockwaves at the basin^s antipodes. That is, the shock of impact was catastrophically sever over the whole planet. A similar story can be seen in the outer Jovian satellites, particularly on Callisto. Most of the surface is densely covered by small impact craters but there is one large, spectacular impact structure. This is the Valhalla basin. It has an internal diameter of about 600 Km and it is surrounded by a remarkable s e r i e s of c o n c e n t r i c ridges, probably formed by the rapid deformation of the icy crust of Callisto, w h i c h extends the topographic effects of the impact out to over 2000 Km. These large scale impact events occurred early in solar system history (about 4 b.y. ago) and no evidence of similar terrestrial events still exists. Although the remains of some large impact craters, such as the 280 - 290 m.y. old Manicouagan structure, have been found in the Pre-cambrian Canadian Shield. However, since this time a steady, if diminishing, rain of rocky debris has been falling on all of the terrestrial planets. This has caused a scattering of smaller, more recent impact craters and numerous terrestrial examples of these more recent impacts are preserved. One example of this is Meteor Crater, Arizona, which was formed about 25000 y.b.p. when a 150,000 metric ton body hit the Earth at 15 K m / s e c . The crater was llOOm wide and 1005m deep. An Australian example is 22Km diameter Goss^s Bluff in the Macdonald Ranges. Goss^s Bluff would have excavated 20 Kms into the crust and covered over 12000 sq Kms with ejecta. There are numerous other Australian examples such as Wolfe Creek and the Connolly Basin. There are still a large number of astronomical objects which cross Earth's orbit and could potentially impact. The expected rate of impact, or meteorite flux, is 3 + 1.5 craters of 10 Kms or more every million years. Some impacts could be very much larger. There are about twenty observed and charted objects whose orbits could one day bring them into collision with the Earth, and more are still being found. They range in size from Apollo (3 Km) and Icarus (2 Km) to 1976UA (200m). If any of these objects were to impact, the results would be catastrophic. Such objects would excavate to the mantle and, as w e l l as w i d e s p r e a d crustal fracturing, they would cause a profound disturbance in the mantle itself. This is particulary true if they impacted, as is probable, over the relatively thin oceanic crust. What role such impacts have p l a y e d in p a s t tectonics is not known, although it could be suggested that they were involved in the formation and subsequent breakup of the continental land masses. More recent impacts could have formed the oceanic mantel hotsppts, such as the Hawaiian Chain.

242

16.3

SATELLITE OBSERVATIONS OF THE MAJOR STRUCTURAL ELEMENTS OF THE BOWEN BASIN J.H.J. Leach

CSIRO Division of Geomechanics, Indooroopilly

The Bowen Basin in Central Queensland is a large region of late Paleozoic to Triassic sedimentation. It consists of the Denison Trough (in the west) and the Taroom Trough (in the east). These are divided by the Comet Ridge, a topographic and structural high. The Basin is bounded to the east by a large thrust structure (the Gogango Thrust) while to the west the basin sediments onlap the Devonian basement rocks. The basin terminates in the north where the Gogango thrust intersects the Devonian basement and disappears under a series of Mesozoic basins to the south. The eastern portion of the Bowen Basin is characterised by shallowly dipping beds which are cross-cut by strike slip faults and displaced by low-angle thrusts, some of which occur along bedding planes. It is in this eastern portion that large reserves of coal are found at shallow depths and extensive mining takes place. The coal is mined in a series of large open cut operations as well as at a number of underground mines. The output from these mines forms a very large part of the coal produced in Australia for export and makes the Bowen Basin the largest coal exporting province in the world. The Bowen Basin coal mines are located between the major cross cutting fault sets and the location of these faults is of major importance to the coal mining operations. Where major fault sets have intersected mine sites, they have produced significant complications and constraints for the mine operation. It was, therefore, decided to use satellite imagery to try and study the location, orientation, and nature of these fault sets. The first stage of this work used hard copy of both Landsat MSS and NOAA-AVHRR imagery. Conventional lineament maps and half rose diagrams were used to find the orientation and distribution of lineaments within the study area. These revealed four lineament groups. These were between 50 and 70 degrees, between 305 and 340 degrees, between 270 and 280 degrees, and between 5 and 40 degrees. Of these, only the first two could be clearly related to major geological structures. Even then, precise location of the lineaments was difficult, if not impossible, and the geological relationships were difficult to determine. In

addition,

the ^rea was one of low relief with extensive

243

surfical cover. This cover included broad area covered by tertiary basalts, thick sequences of Tertiary sediments, as well as an extensive cover of Holocene alluvial cover from the Fitzroy river system. . All of these features made analysis difficult and it was decided that if any further progress were to be made in the identification and geological characterization of the features, then digital processing would be necessary. The digital analysis used was to apply edge enhancement filters across the first principle component of a night time NOAA-AVHRR image. The thermal image was chosen because it was considered to be less susceptible to interference from cultural artifacts than the visible bands. Two types of filter were applied, directional and non-directional. It was found that each of these techniques enhanced different classes of structures. The predominantly linear north east trending features were enhanced by the directional filter more than the curving and anastomosing north west trending features. These north west features were more easily seen on the non-directional filtered image. The images show the basin is crossed, in at least two places, by lineament swarms which define controlling structural zones, or corridors. These are one of the sets of linear, north east trending structures. The other is a more northerly trending set which is established between the corridors. The north east trending features, which have been identified with the large thrusts seen in some mines, curve and anastomose. There seems to be some correlation between the junctions of these features and the position of the structural corridors. The relationships of the four groups of lineaments found in the earlier study were thus found to be controlled by the north east trending structural corridors, these separate the other, more northerly trending, set of north east structures and seem to determine the curving and the locations of the splits and junctions of the north west trending features. ACKNOWLEDGMENTS Thanks are due to Mr. P. Soole who did much of the image processing, and to Dr. R. Hammond for informative discussion.

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2.32

THE ENIGMA OF THE HASTINGS BLOCK, NEW ENGLAND OROGEN P.G. Lennox, J. Roberts and S. Jeffrey Department of Applied Geology University of New South Wales

The stable and little deformed, NW-SE-trending Hastings Block is presently located in a tectonically anomalous position east of deep water sediments of a subduction complex, and south of the multiply deformed and cleaved Nambucca Block. Present models support a latest Carboniferous dextral shearing to generate the Coffs Harbour Megafold, simultaneous translation of the Yarrol Orogen some 500 km southeastwards and, by implication, the 100-150 km dextral rotation northwards between dextral shears of the Hastings Block from its position along strike from forearc sequences in the Tamworth Belt to the south.

The Hastings Block contains two distinct sedimentary successions separated by a disconformity spanning the Late Carboniferous: 1) a shallowing upward succession of Devonian - Early Carboniferous distal turbidites and Middle Carboniferous shallow marine to continental facies, initially derived from a dacitic and basaltic source, with the later addition of granitic, sedimentary and metamorphic constituents; and 2) an Early Permian marine succession of platform limestone, shale, diamictite and turbiditic sediments derived from a predominantly sedimentary source, but with continued influx and volcanic and granitic debris. In the Middle Carboniferous the sedimentary facies of two formations shallow towards the southeast and northeast, and become deeper westwards, suggesting an almost opposite facing direction to that of the Tamworth Belt. This may be due to dextral rotation during emplacement of the Hastings Block into its present position or it may merely reflect the sedimentary facies on an autochthonous Hastings Block.

Both successions are folded by a gently NW-plunging, regional, upright, asymmetrical anticline (A/^ -20 km), verging to the southwest with asymmetrical parasitic folds (X/^ - 2 km) called the Parrabel Anticline (Fig. 1). Unlike rocks in the adjacent Nambucca Block the Parrabel rock sequences lack an axial plane crenulation cleavage. The wide crestal region of the Parrabel Anticline contains evidence of early folding consisting of gently plunging, open to tight, isoclinal, E-W trending folds probably formed during the Late Permian (?)Di deformation which folded and superimposed a slaty cleavage on rocks in the Nambucca Block (Fig.l). The Parrabel Anticline trend corresponds to E-W- to NW-SE-trending folds formed during the Late Permian (?) Dg deformation in the Nambucca Block. The geometry of the Hastings Block was partly modified by movement of bounding faults, and tectonism was terminated in the Triassic by silicic magmatism in the hinge zone and western flanks of the Parrabel Anticline.

Serpentinites in western bounding faults of the Hastings Block should indicate whether the block was emplaced by dominantly dextral movement. In spite of the faunal similarities between the Tamworth Shelf and Hastings Block successions there are significant differences in sedimentary facies in the region of supposed continuity. Current research should resolve the amount of erosion at the Permo-Carboniferous boundary which may reflect the disruption caused by emplacement of the Hastings Block and the change from a convergent to a dextral transform margin.

245

Figure 1: Folding and faulting within and around the Hastings Block

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2.24

A DEEP SEISMIC SURVEY ACROSS THE TUMUT TROUGH, NEW SOUTH W A L E S

J. Leven^, F. Stuart-Smith^, M. Rickard^

and K. Crook^

^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Geology Department, Australian National University,

Canberra

The Tumut Trough is an Early Palaeozoic tectonic and palaeogeographic feature striking NNW-SSE within the Lachlan Fold Belt, (Figure 1). It is bounded to the east by the Mooney Mooney Fault, and to the west by the Gilmore Fault System. The Gilmore Suture is a significant metallogenic province with numerous gold occurrences along the western side of the trough, and copper, tin and chromite deposits elsewhere within the trough. Large bodies of ultramafics, including the Coolac Serpentinite, occur along the Mooney Mooney and Gilmore Faults, and in the centre of the trough. A basement anticline of actinolitic schists of possible Ordovician age is flanked by Silurian basaltic and dacitic volcanics. The trough is bounded to the east and west by the dominantly granitic rocks of the Lachlan Fold Belt: to the east - the Young Granodiorite; and to the west - the Wagga Metamorphic Belt. Several geological models have been proposed to explain the origin of this trough. They include a marginal sea (Ashley et. al. 1979, Schiebner, 1973, 1986); a continent rift (Wyborn, 1977); and a volcanic arc and accretionary prism (Crook, 1980). To investigate which of these models most accurately represents the development of this trough, it is necessary to understand the crustal structure underlying the trough. For this purpose, a deep seismic survey traversing the Tumut Trough was conducted as a joint project by the Bureau of Mineral Resources and the Geology Department of the Australian National University in May, 1987. The target of this seismic survey was the bounding faults of the trough which were thought to dip to the east and have a listric form. It was also postulated that the occurrence of serpentinite along these major faults would provide a marked seismic impedance contrast with the surrounding rocks, and these faults should therefore produce relatively strong reflection signals. In Figure 2, a line drawing of the reflection segffl^ents taken from the brute stack section of the Tumut Trough seismic data shows its general character. Noticeably, there is no indication of the listric bounding faults which were the targets of the survey, even though strong reflectors at 7.2 seconds two-way time (twt) indicate the penetration of seismic energy to depths of 20 km, and the capacity of the technique to image such features. This suggests that these bounding faults do not have a listric form and are probably near-vertical. It requires a reconsideration of previous rift or subduction models for the trough formation. Two possible hypotheses are a transtensional pull-apart basin in a larger scale strike-slip system, or a rift system analogous to the present-day Ethopian Rift.

Beneath the Young Granodiorite there is non-reflective zone to 3 seconds twt, interpreted to be the batholith extending to a depth of around 9 km. Beneath this zone reflections extend down to a particularly strong subhorizontal reflector at 7.2vseconds twt.

247

Within the Tumut Trough there are two regions which display coherent reflections. The mapped fold structure of the Bullawyarra Schist is reflected in th^ short segments of coherent energy east of Brungle. Southwest of Minjary, a set of strong sub-horizontal events are also interpreted to be reflections from within basement.

Figure 1. A simplified geological map of the Tumut Trough region showing the location of the seismic traverse.

C r»rklar»

Figure 2. A line drawing of the coherent energy segments from the brute stack of the Tumut Trough seismic data.

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3.14

PRE A N D S Y N - D E F O R M A T I O N A L R E M A N E N T M A G N E T I Z A T I O N S FROM MID-PALAEOZOIC SEDIMENTARY ROCKS OF AUSTRALIA, A N D THEIR TECTONIC IMPLICATIONS

Z.X. Li^, P.W. Schmidt^, C.McA. Powell^

and B.J.J. Embleton^

^School of Earth Sciences, Macquarie University, North Ryde 2CSIR0 Division of Mineral Physics and Mineralogy, North Ryde

Although the pre-Mesozoic apparent polar wander path (APWP) of Australia has been considered as representative for the whole of Gondwanaland, the mid-Late Palaeozoic part of the path is still poorly defined (Schmidt and Embleton, 1987; Kent and May, 1987). This study yielded some new palaeomagnetic data of this age interval from the Hervey Group of central N. S.W., and the Mount Eclipse Sandstone of Ngalia Basin, Central Australia. The Hervey Group consists mainly of red and white sandstones, and red siltstones. Its age ranges from latest Devonian to earliest Carboniferous (Jones, 1982). A total of 104 samples was collected from 11 sites. In addition to a soft component possibly acquired during recent weathering, a hard component was revealed from the non-cleaved, or only slightly cleaved sites. Although a fold test on this hard component was inconclusive, a pre-deformational origin was suggested by the persistence of predeformational magnetic fabric at these sites, and by the significant different remanence directions revealed from severely cleaved samples. This hard component gives a latest Devonian-earliest Carboniferous palaeomagnetic pole at (54''S, 24°E) with DP=8°, DM=16°. The Mount Eclipse Sandstones is a thick sequence of medium-course grained detrital sediments. Its age ranges from latest Devonian to Early Carboniferous. A total of 373 samples from 31 sites were studied. A consistent hard component was revealed from 16 sites. A fold test on data from a syncline produces a cross-over feature of the site-mean directions, which lead us to interpret this hard component as a syn-def ormational remanent magnetization. This interpretation is consistent with the regional structural analysis and the magneto-mineralogy observations. The best estimated mean direction gives a mid-Late Carboniferous palaeomagnetic pole at (34°S, 121°E) with DP=19° and DM=20°.

Fig. 1. Mid-Late Palaeozoic APWP of Gondwanaland. HGHervey Group; EL-Mount Eclipse Sandstone

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Fig. 2. Paiaeoreconstruction of Gondwanaland and Laurussia during latest Devonian-earliest Carboniferous (a) and mid-Late Carboniferous (b). By comparing the new palaeomagnetic poles with the existing poles from Gondwanaland (Fig. 1), which are either class A or B poles according to the criteria set by Briden and Duff (1981), we concluded that: 1) The Lachland Fold Best has been part of cratonic Australia since at least latest Devonian or earliest Carboniferous. 2) There was possibly during Late Devonian.

rapid

anticlockwise

rotation

Gondwanaland

3) An ocean of about 3000 km wide still existed between the facing boundaries of Gondwanaland and Laurussia during Late Devonian to earliest Carboniferous (Fig. 2a). From then on until mid-Late Carboniferous, Gondwanaland moved rapidly across the southern pole, and finally collided with Laurussia (Fig. 2b). REFERENCES Jones, R.K., The Late Devonian Succession and Faunas of Central and SouthUstern New South Wales, M. Sc. Thesis, University of Sydney, 105pp., 1982. Kent, D.V. and May, S.R., Polar wander and the reference pole controversy, Rev. Geophys., (in press). Briden, J. C. and Duff, B.A., Pre-Carboniferous palaeomagnetism of Europe north of the Alpine Orogenic Belt, in M.W. McElhinny and D.A. Valencio (editors): Paiaeoreconstruction of the Continents, Geodyn. Ser. Vol. 2, AGU, Washington, D. C., 137-149, 1981. Schmidt, P.W. and Embleton, B.J.J., A critique of palaeomagnetic results from Australian Palaeozoic fold belts and displaced terranes, in E.G. Leitch and E. Scheibner XEds.): Terrane Accretion and Orogenic Belts, Geodynamics Series Vol. 19, 21-30, 1987.

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2.16

TIMING THE BREAKUP OF A PROTEROZOIC SUPERCONTINENT EVIDENCE FROM AUSTRALIAN INTRACRATONIC BASINS J.F, Lindsay and R.J. Korsch Bureau of Mineral Resources, Geology and Geophysics, Canberra

In the Late Proterozoic, several broad shallow intracratonic depressions appeared across a vast area of central Australia. The basins, which all contain shallow marine to fluvial successions, appear to have been tenuously interconnected through much of their history. Analysis of their fill by means of tectonic subsidence curves suggests that they are the product of two separate and distinct periods of crustal extension, one at about 900 Ma and a second at about 600 Ma. The basins can be divided into two categories: 1. Those initiated at about 900 Ma (Amadeus, Officer, Georgina and Ngalia basins) and which evolved during two extensional stages, and 2. Those initiated at about 600 Ma (Bonaparte, Ord, Wiso and Warburton basins) and which show evidence of a single evolutionary stage. The Adelaide Geosyncline was initiated at about the same time as the category 1 basins and initially its tectonic subsidence curve follows the typical thermal decay pattern. However from about 750 Ma to 500 Ma subsidence is more rapid than would be expected from the second extensional phase and hence other major tectonic events, that are not detected in the interior basins, involved the Adelaide Geosyncline on the southern margin of the continent. Nevertheless, the Late Proterozoic sequence is very similar to that in the other category 1 basins. The two extensional episodes were probably the result of failed rifting events that almost fragmented the Australian continent during the Late Proterozoic. The second period of extension almost certainly relates to the breakup of a Proterozoic s u p e r c o n t i n e n t . A major sea-level rise in the Cambrian following the second extensional event offers evidence of a global sea-level rise at that time, and supports the argument that the rise relates to the development of a new sea-floor spreading ridge system accompanying and following breakup. The results suggest that sediments preserved in the relatively protected environment of interior basins may provide a more subtle record of major tectonic events than continental margin sequences that were exposed to major continental interactions.

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15.1

EXTENSIONAL HISTORY OF THE MARGINS OF THE TASMAN SEA G,S. Lister^, M.A. Etheridge^ and P.A. Symonds^ ^Department of Earth Sciences, Monash University ^Bureau of Mineral Resources, Geology and Geophysics, Canberra

The margins of the Tasman Sea display a marked complementary asymmetry. The narrow, uplifted Eastern Australian margin contrasts with the broad, submerged margin largely represented by the Lord Howe Rise. Syn-rift faulting is virtually absent on the Australian margin, whereas half-graben and rotated tilt blocks are widespread on Lord Howe Rise. Such complementary asymmetry is widespread on conjugate margin pairs across other ocean basins, and we ascribe it to an inherent asymmetry in the process of continental extension that preceded sea-floor spreading. Several recent models for continental extension involve the operation of one or more major low-angle, normal faults (detachments) that extend through part or all of the continental lithosphere. Operation of these detachments, with or without more uniform pure shear extension at depth, results in asymmetry of the structure, uplift/subsidence pattern and/or thermal history of the extended terrane. Subsequent sea-floor spreading gives rise to conjugate passive continental margins that may be markedly different from one another. One margin will largely comprise the footwall or lower plate to the master detachment (lower plate margin), commonly with highly faulted remnants of the upper plate producing the classical half graben rift structure. The conjugate margin comprises the hanging wall or upper plate of the master detachment (upper plate margin). Lower plate margins tend to be broader, more highly structured and to have undergone more post-rift subsidence. Conversely, upper plate margins tend to be narrower, relatively unstructured and are commonly uplifted. We have interpreted the margins of the Tasman Basin in terms of the detachment model. In this interpretation, we assume that extension was approximately orthogonal, whereas there is considerable evidence for oblique extension, at least during the sea-floor spreading history. It is proposed that the eastern margin of the Basin is a very broad lower plate margin comprising the Lord Howe Rise, the New Caledonia Basin and the Norfolk Ridge. The location of the eastern limit of the extended terrane (i.e., the headwall of the detachment system) is unknown, because their is no emergent continental fragment to the east that is clearly unextended. We surmise that the detachment headwall developed at or very close to the edge of the mid-Cretaceous Australian continent, isolating only a very small, previously thin continental fragment, now represented by the Norfolk Ridge. The detachment is interpreted to have passed beneath the Lord Howe Rise, where it flattened out at mid crustal depths, before ramping down to the crust-mantle boundary or deeper beneath eastern Australia. In this model, the New Caledonia Basin would

252

be thinned continental crust pulled (and/or stretched) from beneath the eastern part of the Lord Howe Rise, which is a relatively unstructured basement plateau. The western half of the plateau contains a number of half-graben and rotated tilt blocks, suggesting that a branch to the detachment outcrops along the centre of the Rise and dips west beneath the rifted portion. The Tasman Basin is bounded on the west by a particularly abrupt continental margin with very little rift structure, and an uplifted hinterland, and is interpreted to be an upper plate margin. The uplift of the hinterland to form the southern and more spectacular portion of the Great D i v i d i n g Range is interpreted to have taken place during continental extension in the (?Early to) Late Cretaceous. Uplift of upper plate margins is predicted by the detachment model to result principally from two separate but related phenomena. 1) Uplift and/or thinning of the mantle lithosphere (with little or no crustal thinning) is greatest beneath upper plate margins, leading to syn-rift uplift as the mantle lithosphere heats and expands. This thermal uplift decays exponentially with a half-life of about 60 to 80 Ma after extension ceases. 2) Uplift of the lower thermal boundary layer of the lithosphere may result in substantial partial melting of the upper mantle, giving rise to basaltic melts which may intrude and underplate the lower crust. Partial melting and underplating results in a reduction of the average density of the lithospheric column, giving rise to an isostatic uplift. This uplift is added to the thermal uplift, but differs from it in that it is essentially permanent, unless the basaltic underplate transforms to eclogite as the margin cools. Seismic refraction data from southeastern Australia suggest crustal thickness of 50km or more, with a lower crustal layer up to 25 km thick which has a P-wave velocity in excess of 7.0 km/sec. This lower crustal layer must be substantially mafic in composition, and is interpreted to be largely underplated mafic granulite. Seismic refraction and igneous geochemical data from elsewhere in Australia suggest that underplating also took place in the Early Proterozoic and the Palaeozoic, so not all of the very thick high velocity layer beneath the Tasman margin need have been underplated during the Cretaceous. One dimensional isostatic modelling of the uplift of the eastern Australian highlands predicts that they may have been as high as 3 to 4km immediately after extension, decaying to the present 1.5 to 2km as the lithosphere cooled. A component of the uplift may also be due to flexure of the elastic lithosphere due to the weight of the cool oceanic lithosphere and sediment at the margin of the Tasman Basin. However, there is no sign of matching uplift on the Lord Howe Rise, as would be predicted by what should be an essentially symmetrical phenomenon. Finally, the Tertiary, apparently h o t s p o t - r e l a t e d igneous activity in the Tasman Basin and its Australian hinterland may result indirectly from the lithospheric extension. We conclude that the hot spots resulted from the rise of mantle diapirs triggered by rapid extension of lithosphere overlying anomolously hot asthenospere at thevdown-dip end of the detachment system, beneath the upper plate margin. The lag between extension and hot spot volcanism reflects the ascent time of the diapirs prior to reaching the solidus.

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6.14

SUPERGENE COBALT MINERALIZATION, NEW CALEDONIA GEOLOGY AND MINERALOGY S. Llorca

CSIRO Division of Mineral Physics and Mineralogy, North Ryde In New Caledonia, ultramafic rocks (essentially harzburgites and dunites which have been variously serpentinized) are covered by a thick weathering mantle. The profile consists of two levels: a lower one with preserved structure in which there is hydrolysis of the silicates (saprolite) and an earthy residual upper one essentially composed of iron hydroxide (limonite). They are separated by a "transition horizon" in which the hydrolysis of the silicates ends abruptly, MgO decreasing from 10% to 1.15%. Depending on the parent rock, structural features, drainage and erosion, profile development and truncation vary. These profiles contain supergene cobalt mineralizations, which have been studied for the first time. Five hundred samples were taken along various profiles, principally located in Poro and Tiebaghi blocks which are considered to be representative of the cobalt mineralization in New Caledonia. Chemical analysis of these samples shows that in all the profiles, cobalt contents increase progressively from the ultramafic rock (150 ppm) to the transition horizon (0.15% to 1.5% depending on the profiles), and then decrease. Everywhere, the enrichments occur in the form of bluish blackgrey products. They can form as spots or coatings especially along the joints of the saprolites, form concretions, impregnate the earthy mass and substitute for roots in pedoturbed profiles, fill karsts, and even form hardpans where the transition horizon outcrops. A detailed mineralogical examination was made of 40 samples. Microscopic examination showed the "black products" consist of cryptocrystalline aggregates up to single crystals 100 ym diameter, and that they can in particular pseudomorph silicates (talc, serpentine). Optical, thermal, chemical, crystallographic and crystallochemical analysis led to the identification of seven minerals or families. Some of them, todorokite, cryptomelane, ramsdellite, are rather rare and contain only traces of cobalt (0.7%). The others, heterogenites (55% Co), asbolans (0.5 to 17% Co), lithiophorite (6% Co) and intermediate phases between asbolans and lithiophorite (4 to 13% Co), are the main cobalt-bearing minerals. They have similar structures (single octahedral layers stacked along c-axis), similar physical properties, and are mostly only distinguishable from each other by the relative proportions of manganese, cobalt, nickel and aluminium. The same analysis also provided more specific information on the structure and composition of these minerals. The heterogenites (Co layers) occur as both 2H and 3R polytypes; this is only the second reported occurrence of the 2H polytype in the world, after Shaba. These phases are here highly nickeliferous (5 to 7% NiO). This is the first time that heterogenites have been reported in weathered ultramafics, other occurrences being

254

supergene enrichments above sulfidic ore deposits also minor nickel. The lithiophorites (Mn and Ai layers) here contain some cobalt and some nickel as well (10% CO2O3 and 2% NiO), incorporated within the crystal lattice. Cobalt, trivalent and octahedral is located within Mn- layer whereas nickel is distributed within Al-layer. Such a structure is perfectly balanced, and needs no Mn^"*" ions in the MnOo layer as it has been supposed for the Postmasburg type-lithiophorite. The asbolans, with cobalt and nickel (Mn and Co-Ni layers), show more varied compositions than typespecies known from the Urals. They form a continuous series with a single structure in which cobalt, trivalent and octahedral, is separate from nickel and constitutes heterogenite-type domains (CoOOH). Finally, phases with intermediate compositions between asbolans and lithiophorite have been found, that are either members of a continuous series or occur as irregular inter-stratifications. Because of the wide range of cobalt contents in the various minerals which constitute the black products, the overall quantity of visible "black products" cannot be directly related to the cobalt content. However, for a same facies and area, the quantity of "black products" and cobalt content are linked. As a matter of fact, in the weathering profile, each mineral family has its own Eh—pH stability field. With increasing acidity and aeration (from the bottom to the top), there is a succession from heterogenites (from the rocky saprolite to the transition horizon), asbolans (from the soft saprolite to the transition horizon), Al-rich asbolans-lithiophorite intermediates (f rom the transition horizon to the base of the limonite), to lithiophorites (in the same horizons as asbolans and intermediates but only when exposed at the surface). In addition, each one of these minerals does not necessarily occur in each profile. The nature of the phases precipitated in a profile depends on the relative concentration in Co, Mn, Al, Ni. Heterogenites and the most cobaltiferous members of the asbolans and the asbolans-lithiophorite intermediates only occur where the concentration of cobalt is particularly high, and lithiophorite where the rock is aluminium-rich. Three mechanisms act to concentrate cobalt during the weathering. Initially, there is residual concentration by leaching of MgO and Si02* Secondly, cobalt in minerals which are unstable above the transition horizon or the base of the limonite, dissolves and percolates into the underlying part of the profile where changes in Eh and pH cause it to reprecipitate. This accounts for the accumulation of cobalt in and near the transition horizon as the weathering continues. If this mechanism was acting alone, an average content of about 0.55% Co would be found in the transition horizon. Thirdly, part of the cobalt is elutriated by the circulations of groundwaters in and near the transition horizon, and accumulates at the bottom of the depressions formed at this level. In these depressions, contents reach 1.30% compared with average values of 0.3% elsewhere, and this causes heterogenite to appear. These morphologic irregularities are linked to the heterogeneities in the underlying parent rock resulting in differential weathering.

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4.6 NUMERICAL SIMULATION OF HIGH-T METAMORPHISM AND COEVAL CRUSTAL THICKENING, WITH REFERENCE TO THE MOUNT ISA INLIER R.J.H.

Loosveld

Geology Department, Australian National University,

Canberra

Although as a direct result of crustal thickening geothermal gradients become shallower, a wide variety of fold belts (e.g. the "Hercynotype" orogenic belts of Zwart, 1967) has undergone high-T/low-P metamorphism during their formation, specially so, it seems, during the Precambrian. The suggestion that this secular trend reflects the cooling of the Earth was refuted by England & Richardson (1977), who attributed this trend to polychronic metamorphism and continued erosion. The extra heat source for the high-T metamorphism would be "radioactive selfheating", i.e. the reaching of high T/P ratios in a collisional zone due to the thickening of the Heat-Producing-Elements-enriched crust (e.g. England & Thompson, 1984). Additional complexities like frictional heating and relaxation of a crust with a heterogeneous vertical thermal conductivity distribution can locally further condense the isotherms. With certain favourable parameterisations, geotherms can thus transect the andalusite-sillimanite field. P-T-t paths resulting from such crustal thickening, however, will be clockwise and can only transect the andalusite/sillimanite field in a retrograde way, unless unrealistic parameters are assumed, e.g. a very low conductivity, high radioactive heat production, high basal heat flow, introduction of "magmatic or hydrothermal heat" and the presence of eclogite- or other highdensity-sinkers. In contrast, in the northern Australian Early to Middle Proterozoic inliers, the high-T metamorphism is prograde and the P-T-t paths anti-clockwise (high-T/mod-P—> modT/mod-P). Three mechanisms are mentioned for the anomalously high paleo-geothermal gradients and anti"Alpinotype"-clockwise P-T-t paths: (1) syn- to slightly pre-metamorphic lithospheric extension (McKenzie, 1978), (2) various magmatic events (Wells, 1980) and (3) various modes of crustmanfle delamination coupled with the upwelling of hot asthenospheric material to the base of the crust (Bird & Baumgardner, 1981; Etheridge et al.. 1987). In this study, the thermal effects of these various situations, and combinations thereof, have been one-dimensionally approximated, using a finite-difference technique. The partial differential heat equation is solv^ by an implicit (Crank-Nicolson) scheme. The computation here is iterative rather than by direct matrix solution and is tackled using compiled Basic. Results were then compared to the P-T-t data of the Mount Isa Inlier. (1) Steep paleo-geothermal gradients, affecting large areas, are often linked to continental rifting, which would result in mantle diapirism (or vice versa), partial melting of the lower crust, emplacement of granodiorites in the middle crust and a condensed series of isograds. Generally, the thermal relaxation of such an event will have a time-span of 50-100 Ma. No extension^ event is directly documented to have affected the Mount Isa Inlier in such a timeinterval before the prograde metamorphism. Recent asymmetric extension models (Lister SLSL, in press), in which the rift area is geographically offset from the area undergoing the highest lithospheric thinning, however, provide an attractive solution. These extensional models can also provide the dynamics for the generation of voluminous granites (2) and crust-mantle delamination (3). (2) Wells (1980) studied the thermal effects of crustal thickening by magmatic accretion, i.e. both under- and over-accretion. Rocks above the intruded material will isobarically or slightly decompressively heat up and, during4:he later stages of thermal relaxation of the magmatically thickened crust, isobarically or decompressively cool. Again the thermal anomalies are short-

256

lived, unless the granites are enriched in HPE, as are the pre-metamorphic fractionated felsic granites of the Mount Isa Inlier, in which the radiogenic heat production is some 70 % higher than in the worldwide-averaged granite. The permanent thermal anomaly around these can easily explain local prograde sillimanite-blastesis. In the inliers, however, high-T metamorpWsm is not local. (3) Relaxation paths after crust-mantle delamination with the upwelling of asthenospheric material to the base of the crust were calculated. Both ongoing convection in the anomalously shallow asthenospheric material, and immediate "freezing" of the asthenospheric material were considered (resp. the "hot mode" and the "cold mode" of Bird & Baumgardner, 1981). Considering the "cold mode" delamination and heat transfer in the crust by conduction only, andalusite-sillimanite conditions are not reached on any level. Peak temperatures in the upper crust are reached between 10 and 20 Ma. after delamination and are shortlived. To obtain larger and/or longer-lasting temperature anomalies in the middle crust, another heat source (or fluid advection) has to be added, either some form of "hot mode" intrusion of the asthenosphere (to reach sill, minimum time of hot mode ±lMa.), or radioactive selfheating in a contemporaneously thickened crust, or immediate differentiation of the magmatic products leading to the HPE enriched upper crustal granites. All three mechanisms (1,2,3) have the problem that, although the initial geothermal gradient could be steep, the P-T-t trajectory from incipient crustal thickening onwards should generally be clockwise. In the Mount Isa Inlier, however, the absence of Flysch and/or molasse deposits indicates an absence of syn- to post-orogenic erosion, so that decompressive cooling failed to materialize and the syn- to post-orogenic part of the P-T-t path could exist of an essentially adiabatic compression followed by an essentially isobaric temperature path, governed by the competing effects of ongoing cooling of the thinned c.q. intruded c.q. underplated crust and heating due to the thickened radiogenic upper crustal layer. REFF.RENCES Bird, P. & Baumgardner, J., 1981. Steady propagation of delamination events. J. Geoph. Res., V. 86, No. B6, 4891-4903. England, P.C. & Richardson, S.W., 1977. The influence of erosion upon the mineral facies of rocks from different metamorphic environments. J. Geol. See. Lend., 134, 201213. England, P.C. & Thompson, A.B., 1984. Pressure-Temperature-Time paths of regional metamorphism I. Heat transfer during the evolution of regions of thickened continental crust. J. Petrol., V.25, part 4, 894-928. Etheridge, M.A.E., Rutland, R.W.R. & Wyborn, L.A.L, in press. Orogenesis and tectonic process in the Early to Middle Proterozoic of northern Australia. Am. Geophys. Union., Geodynamic Series. Lister, G.S., Etheridge, M.A. & Symonds, P.A., in press. Detachment models for the formation of passive continental margins. Tectonics. McKenzie, D.P., 1978. Some remarks on the development of sedimentary basins. Earth and Planetary Science Letters, 40, 25-32. Wells, P.R.A., 1980. Thermal models for the magmatic accretion and subsequent metamorphism of continental crust. Earth and Planetary Science Letters, 46, 253265. Zwart, H.J., 1967. The duality of orogenic belts. Geologie en Mijnbouw, 46, no.8, 283309.

257

11.5.

QUEENSLAND

LAND

INFORMATION

SYSTEM

C.J. Lucey Queensland Department of Mines The genesis of the Queensland Land Information System was a Ministerial study tour of overseas systems in 1978. As a consequence of the study tour report a committee (Land Data Bank Steering Committee) was established that year. This initial committee through a project team investigated the development of a Land Data Bank for Queensland. The report was submitted to Cabinet in 1982. Cabinet decided in part that: development proceed independently within the three streams identified (Administration, Mapping and Resource), in accordance with linkage requirements. Department of Mapping and Survey be responsible for the mapping and cadastral land information to be held as part of the central land information system. the State Government Computer Centre ( S G C C ) proceed with computer system development in the administrative land information stream for functions related to titles, land tax and valuations. a Land Information Steering Committee (LISC) replace the existing Land Data Bank Steering Committee. Membership of the Land Information Steering Committee (LISC) is as follows:Valuer-General (Chairman) Surveyor General Director, State Government Computer Centre Coordinator General Chairman, Land Administration Commission Director-General, Department of Mines Director-General, Department of Works Registrar of Titles Commissioner of Land Tax The Queensland strategy is in the development of discrete distributed databases which are integrated through a H U B . The concept of a central H U B derives from the original study tour and has been retained as the keystone to the Queensland Land Information System. Individual Government departments or authorities are entirely responsible for the development of their own systems with the linking system ( H U B network) being developed by the State Government Computer Centre. The Department of Mines is responsible for the registration of mining tenements and Authorities to Prospect. Computerised data bases (Mining Tenure Data Base) are being established to replace the manual hard copy registers. The paper traces the development of and the future proposals for the Mining Tenure Data Base within the concept of an integrated Land Information System.

258

9.13

TSUNAMIS ALONG THE AUSTRALIAN COASTLINE C. Lynam^, J. Rynn^ and B. Barlow^

^Department of Geology and Mineralogy, University of Queensland ^Bureau of Mineral Resources, Geology and Geophysics, Canberra The risk of a tsunami striking the Australian coast has been virtually ignored in studies of natural hazards for our continent. Part of this problem is that preliminary research, even the compilation of a single catalogue, has not been forthcoming. This situation persists despite documented cases in the literature of tsunami effects on Australia's eastern and western seaboards. At least five tsunamis have been documented reporting effects on the Western Australian coast from Indonesia and local earthquakes and the 1883 Krakatoa eruption and on the eastern coast from South American earthquakes. The situation for the northern and southern shorelines is essentially unknown. Unconfirmed reports exist, however, for tidal variations in Darwin possibly related to Banda Sea earthquakes and in southern Tasmania for possible undersea landslides. Other "sea-level disturbances" have also been reported. Known damage has been confined to the eastern seaboard. Most notably this relates to tsunamis effects from the 22 May 1960 Chile earthquake where fishing boats were moved from their moorings in Brisbane and Sydney and went aground and were damaged at Evans Head and Newcastle. There is a need to examine tide gauge records in conjunction with earthquake catalogues to determine if effects exist related to both distant and local earthquakes. Caution must be exercised to ensure that extraneous events related to meteorological and man-made phenomena are eliminated from any analyses. The effects of Australia's broad continental shelfs and the Great Barrier Reef on possible attenuation of tsunamis is also of interest. Current research programmes include computation of travel-time charts (Griffith University) and modelling of shore effects (University of Adelaide). The Australian Seismological Centre will begin a tsunami risk assessment based on historical data in the near future. This paper does not purport to definitively state the situation regarding tsunami risk in Australia. It merely is to provide an awareness to the potential for the tsunami hazard based on the meagre historical record available.

259

15.11

Nd-Sr ISOTOPE GEOCHEMISTRY OF THE TASMANTID SEAMOUNTS - EVOLUTION OF A HOTSPOT TRACE M.T. McCulloch Research School of Earth Sciences, Australian National University, Canberra

The Tasmantid Seamounts lie between the Lord Howe Rise and the eastern margin of the Australian continent and consist of a chain of submarine volcanic mountains extending over a distance of approximately 1300 km. K-Ar age measurements (McDougall and Duncan, this volume) indicate a systematic decrease in the age of volcanism from north to south consistent with the seamounts being volcanic products of a hotspot trace. These seamount samples therefore provide a unique opportunity to study the evolution of a single hotspot trace over a substantial time interval during the Miocene. Here the results of a Nd-Sr isotopic study of the five major guyots which define the hotspot trace are presented. In Figure 1 the evfd and ^'^Sr/^^Sr ratios are shovra. The seamounts exhibit a relatively large range in isotopic compositions with ^^'Sr/^^Sr ratios ranging from 0.70379±2 to 0.70472±1 and EN^ values from +3.0+0.2 to -3.3+0.2. A remarkable and unexpected feature of this plot is the distintive fields defined by each of die seamounts. The lowest ^' Sr/°^Sr ratio is present in the single sample from Queensland guyot which is the most northerly and therefore oldest seamount. The remaining seamounts form an approximately linear array with the Derwent Hunter seamount having the most evolved compositions. This progression in isotopic compositions does not correlate with age as the you^est seamount (Gascoyne) has intermediate values. This is illustrated in Figure 2 where ° 'Sr/®oSr ratios are plotted versus South Latitude. Although there is no consistent temporal evolution of isotopic compositions it is clear however that each individual seamount has its own characteristic isotopic values. For comparison the isotopic compositions of basalts from Lord Howe Island and Victoria SE Australia are also shown. The basalts from Lord Howe Island are part of a parallel volcanic chain immediately east of the Tasmantid Seamounts. The Lord Howe Island basalts have a limited range in compositions which overlap with those from tiie Taupo seamount of the Tasmantid chain. Tertiary to Recent basalts from southeastern Australia

TASMANTID GUYOTS

Victorian basalts

Lord Howe Rise

-A

Denvent Hunter

Queensland

Gascoyne •2 Britannia

0.7036

0.7038

0.7040

0.7042 87 86

Sr/ Sr

0.7044

0.7046

Figure 1. Nd -Sr isotopic compositions of the Tasmantid seamounts (shaded).

260

0.7048

which are also thought to be hotspot related (Wellman and McDougall, 1974) have Nd-Sr isotopic compositions (McDonough et which in part overlap with the Taupo and Gascoyne seamounts but have lower ^^Srr^Sr ratios and higher e^a values than the Derwent Hunter seamount. It is therefore evident that contamination by the continental crust is not an important factor in controlling the composition of these basalts. The isotope variability observed in the seamount basalts may be due to a number of factors. Firstly it may reflect an intrinsic heterogeneity within the plume or the plume source itself. For example if it is assumed that the rate of ascent of magma within the plume is similar to the horizontal plate velocities then even for deep sources (e.g. sources located at 650 km) each seamount could represent an essentially separate magma pulse. Alternatively the variations may be due to interaction of the of the hotspot with either the surrounding asthenosphere or the sub-oceanic lithosphere. Entrainment of surrounding upper mantle during diapir ascent has been proposed for some islands in the Marquesas chain. However the Tasmantid seamounts do not appear to show the distinctive chemical dichotomy as represented by the alkalic and tholeiitic basalts of either the Marquesas or the more well known Hawaiian chain. 0.7048

BRITANNIA

0.7046 0.7044

DERWENT HUNTER 0 B GASCOYNE

0.7042

laJ TAUPO

0.7040

0.7038 0.7036

QUEENSLAND

SOUTH LATITUDE

Figure 2. Variation of Sr isotopic composition versus latitude. Age progression is from north to south with the oldest volcanic rocks being present in the Queensland guyot. McDonough W.F., McCulloch, M.T.and Sun, S.S. (1985). Isotopic and geochemical systematics in Tertiary-Recent basalts from southeastern Australia and implications for the evolution of the sub-continental lithosphere.Geoc/iim. Cosmochim. Acta, 49, 2051-2067. McDougall, I. & Duncan R.A. (1988). Age progressive volcanism in the Tasmantid Seamount Chain, a hotspot trace. This volume. Wellman P. & McDougall,!. (1974). Cainozoic igneous activity in eastern Australia. Tectonophysics, 23, 49-65.

261

15.7

AGE PROGRESSIVE VOLCANISM IN THE TASMANTID SEAMOUNT CHAIN, A HOTSPOT TRACE I. McDougall^ and R.A. Duncan^

^Research School of Earth Sciences, Australian National University, Canberra 2college of Oceanography, Oregon State University, Corvallis, USA

The Tasmantid Seamounts or Guyots comprise a northeriy-trending chain of generally flat-topped, submarine mountains extending over at least 1300 km in the middle of the Tasman Basin, which lies between the eastern margin of the Australian continent and the submarine Lord Howe Rise of continental structure. The seamounts are built upon oceanic crust which was generated by seafloor spreading processes from north-northwest striking ridge segments in the Late Cretaceous-Early Cenozoic as the marginal basin opened. The seamounts rise from abyssal depths (>4000 m) to near sea level, and clearly are of volcanic origin from their morphology and from seismic profiling data, confirmed by recovery of basalt from some edifices by dredging. The evidence indicates that most of the volcanoes were built above sea level, subsequently erosionally bevelled to sea level, followed by subsidence to depths ranging from 90 m for Gascoyne in the south to more than 400 m for Recorder Guyot in the north. Vogt and Conolly (1971) suggested that the progressively greater submergence to the north indicated that the volcanoes became older in that direction, interpreted as reflecting northward movement of the Indo-Australian lithospheric plate over a fixed mantle source in the mantle. Here we present results of a test of this hypothesis. Basalt samples were successfully dredged from Gascoyne, Taupo, Derwent Hunter and Britannia Seamounts during segments of two cruises (FR3/85 and FR7/86) of the CSIRO oceanographic research vessel FRANKLIN in a joint project between the Australian National University (RSES) and the University of Tasmania (Department of Geology). The dredged samples have been utilized for a variety of petrographic, geochemical and other studies to characterize the lavas and their sources. Most samples contain olivine microphenocrysts with or without plagioclase and less commonly clinopyroxene as phenocrystic phases. Virtually all samples contain glass or poorly ciystallized mesostasis which ranges from quite fresh to markedly altered. Isotopic age measurements using the K-Ar and ^^ArP^Ai total fusion methods on the freshest available samples yield a pattern of progressive and systematic younging along the seamount chain, indicating that the focus of volcanism migrated southward at an average rate of about 70 nWyear. The predicted present position of the volcanic focus derived from linear regression of these data is at ~40.5°S latitude, ~156°E longitude, virtually coincident with the epicentre of a magnitude 6 earthquake that occurred on 25 November 1983 (Denham, 1985). We interpret the results as providing strong confirmatory evidence that the Tasmantid Seamounts represent a hotspot track. The hotspot trail of central volcanoes previously recognized by Wellman and McDougall (1974) in eastern Australia shows a rate of propagation of the volcanism and in a direction consistent with that found for the Tasmantid Seamounts. The overall consistency reinforces the view that these hotspot traces effectively are recording motion of the Indo-Australian plate across the sublithospheric mande source regions for the volcanism over the last 25 to 35 Ma. Comparisons with results from other unequivocal hotspot traces on the same plate and on other plates confirm that hotspots provide a good frame of reference for plate motions. Denham, D. (1985). The Tasman Sea earthquake of 25 November 1983 and stress in the Australian Plate. Tectonophysics, 111, 329-338. Vogt, P.R. & Conolly, J.R. (1971). Tasmantid Guyots, the age of the Tasman Basin, and motion between the Australian plate and the mantle. Geological Society of America Bulletin, 82, 257-2584. Wellman, P. & McDougall, L (1974). Cainozoic igneous activity in eastern Australia. Tectonophysics, 23, 49-65. 262

3.2

THE ORIGIN OF THE GEOMAGNETIC FIELD AND ITS PALAEOMAGNETIC CONSEQUENCES P.L. McFadden Bureau of Mineral Resources, Geology and Geophysics, Canberra

It is now generally accepted that the geomagnetic field is caused by dynamo action taking place in Earth's outer core. This has immediate consequences for tectonic applications of palaeomagnetism. For example, Cowling's theorem shows that dynamo action cannot sustain an axis3niimetric magnetic field, and so the basic palaeomagnetic hypothesis of a geocentric axial dipole (GAD) cannot hold except in the sense of an average. Dynamo action naturally produces a magnetic field that varies with time. At Earth's surface time constants in the range 1 year to 100 million years are recognised, the variations associated with each of these time constants having a different palaeomagnetic consequence. A brief outline will be given of the generation of the field, differences between weak-field, and strong-field dynamos, and the causes and palaeomagnetic consequences of the latitudinal dependence of secular variation, hemispherical and polarity asyimnetries, and excursions and reversals.

263

8.14 PETROLOGICAL AND STRUCTURAL EVOLUTION OF THE PERMOCARBONIFEROUS FEATHERBED VOLCANICS, NORTHEASTERN QUEENSLAND, AND THEIR RELATIONSHIP TO MINERALISATION D.E. Mackenzie Bureau of Mineral Resources, Geology and Geophysics,

Canberra

The Permo-Carboniferous Featherbed Volcanics are located about 100km west of Cairns, near the western margin of the Siluro-Devonian Hodgkinson Basin. They are almost entirely confined to a composite volcano-tectonic depression about 100km long and 30km wide, made up of at least six nested subsidence structures (Fig. 1). The volcanic and associated intrusive rocks may be divided into late Carboniferous I-types and early Permian A-types. 145^00 ua' 30' 'b.Mount M u l l i g a n \

A'j

Au Au

Wolfram Camp (W.Mo.Bt) Jt Au V Dimbutah

Mine Red Dome (Au) W >4Zja/?c/o/7eo/77/n^NCu •Chillagoe

prospect or occurrence

aj^ Cu

Au -^o/cy Bi - bismuth Cu - copper Mo — molybdenum Pb - lead - zinc — silver -tin U - uranium W -tungsten

Sfl-

Ring Dvke''

Margin of caldera-collapse incipient caldera-collapse -">-•- Inferred Dyke Fault

•Sn

or — structure

doubtful 0 '

PERMIAN-TRIASSIC M o u n t M u l l i g a n Coal Measures, Pepper Pot S a n d s t o n e

cV-^-n" s.

~ s V

Sn - S ft^n Sn _ - WCuSn ^Tii , W S"' - yy -Sn ^

LATE CARBONIFEROUS - EARLY PERMIAN Other volcanic

rocks

18/E55/5

LATE CARBONIFEROUS

EARLY PERMIAN

Nychum Volcanics

Acid-intermediate

/ .

I Acid-basic

intrusrves

; Featherbed Volcanics

intrusives

Featherbed V o l c a n i c s

Fig. 1. Generalised Late Palaeozoic geology and mineral deposits of the Featherbed Range - Herberton area. Most of the late Carboniferous rocks are in the southeast, where andesitic to rhyolitic ignimbrites and minor andesite lava form the two main, overlapping sequences preserved in a basin-like sag structure. Late Carboniferous dacitic to rhyolitic ignimbrites are also preserved within a large ring-dyke in the south, as tilted fault slices and remnants of thin outflow sheets in the west, and as thin cauldron-fill(?) remnants in the northeast. Associated with the volcanic rocks are extensive granitic to dioritic and rare gabbroic intrusive rocks. A resurgent-style pluton of porphyritic microgranite cuts the central part of the southeastern structure, but otherwise the structure shares nothing in apmriion witn the conventional model of caldera collapse. The late Carboniferous rocks contain biotite and commonly pyroxene and/or hornblende, are oxidised and metaluminous to mildly peralkaline, and are relatively high in MgO, CaO, Sr, Ni and Cr, and low in Pb. They conform

264

to the I-type concept of White and Chappell (1983)» Mineralisation associated with the late Carboniferous I-type rocks includes W-Sn, W-Mo-Bi, base metals, and Au. In contrast, much of the Sn-dominant mineralisation in the southeast is associated with Carboniferous granites of A-type character. The early Permian volcanics comprise rhyolitic ignimbrite, relatively minor rhyolite lava, and rare dacitic ignimbrite preserved in at least four overlapping caldera-collapse stuctures which together make up the main Featherbed composite cauldron. Associated intrusive rocks include resurgent-type plutons of porphyritic microgranite and microgranodiorite, a discontinuous peripheral ring dyke of porphyritic microgranite, and minor dacite and andesite dykes. Domes, flows, and intrusions of rhyolite crop out around the cauldron margin. These are classic features of the conventional caldera-collapse model, but other features, such as caldera-wall collapse breccias and moat sediments, are absent, and the resurgent features and outflow deposits are poorly developed. The lack of caldera-wall breccias suggests that subsidence and filling may have occurred at about the same rate, so that no unstable topographic scarps formed. The scarcity of preserved outflow deposits may be due to their having been largely destroyed by erosion; alternatively, ash flows may have been essentially restricted to the cauldron because vents were inside the (raised) margins, or because peripheral vents were asymmetric and directed eruption columns inward. The early Permian rocks have low MgO, CaO, V and Cr, and high Ba, Pb, Zr, Nb, Y, rare earths, Zn, Sn, W, Ga and Ga/Al202 relative to the late Carboniferous rocks, features consistent with A-type character (White and Chappell, 1983)- Mineralisation associated with the early Permian rocks is relatively sparse, and includes Pb-Zn-Ag, Sn, Au, U, Sb, Eind W distributed almost entirely around the periphery of the cauldron. The data summarised above suggest that late Carboniferous I-type magma was more mafic, dense and hydrous, but less voluminous, than the later A-type magma, resulting in intersection of the solidus and initial emplacement at relatively deep levels (several km), and sagging of the roof(s) of the magma chamber(s) rather than cauldron collapse. Extended fractionation (from dioritic or gabbroic to granitic compositions) and probable compositional stratification of the magma chamber(s), along with the relatively hydrous, oxidised composition and high load pressure, may have provided favourable conditions for concentration of some metals (e.g. W, Au). However, metals such as Sn would have tended to remain fixed in residual minerals such as hornblende, biotite, and sphene. In the early Permian, a larger volume of more felsic, less dense, and probably relatively fluorine-rich, water-poor, and less viscous magma was emplaced at significantly shallower levels (2km or less), resulting in more dilation and fracturing of the roof and subsequent cauldron collapse. The relatively felsic, anhydrous composition, depleted source composition, and shallow level of emplacement of the A-type magma may have limited scope for concentration of some metals, and may explain the apparently sparse mineralisation. However, the high Sn and W contents (2-4 times normal for felsic rocks) of the A-type volcanics, the scarcity of mineral phases into which Sn is partitioned, and the prospectivity of A-type rocks in the adjacent Herberton tinfield and elsewhere suggest that concentrations of Sn, W, Mo, and Au may remain to be discovered. Favourable environments may include cauldron margins, margins of component collapse structures or incipient collapse structures, major cross-cutting faults, and roof zones of resurgent plutons. Most of these areas in the Featherbed Volcanics coincide with extensive, pervasive, hydrothermal alteration. Reference; White A.J.R. and Chappell B.W. (I983) Granitoid types and their distribution in the Lachlan Fold Belt, southeastern Australia. In CircumPacific Plutonic Tevvains (editor J.C. Roddick), 21-34. Geological Society of America Memoir 159.

265

8.12 THE BOUNDARY BETWEEN THE BROKEN RIVER PROVINCE AND THE LOLWORTH-RAVENSWOOD BLOCK, NORTH QUEENSLAND - A MAJOR CRUSTAL SUTURE, THE CLARKE RIVER FAULT ZONE T.P.T. McLennan Queensland Department of Mines

The Clarke River Fault Zone (CRFZ) is a fundamental regional structure which has significantly influenced the Early to Middle Palaeozoic evolution of the Broken River Province of North Queensland. Published data on the CRFZ have all inferred that this structure had a regional sinistral sense of shear displacing the Broken River Province to the west, eg. Arnold (1975), Harrington (1981), and Henderson (1980, 1987). However, detailed mapping by GSQ geologists (McLennan in Withnall and others 1984) has lead to a significant reinterpretation of the CRFZ and the tectonic evolution of the Broken River Province. The CRFZ forms the easterly trending boundary between the Early to Middle Palaeozoic Broken River Province to the north and the Proterozoic, or Early to Middle Palaeozoic Lolworth-Ravenswood Block to the south. The CRFZ is now interpreted as a composite structure consisting of: 1. a steeply dipping, dextral transcurrent mylonite zone now termed the Clarke River Mylonite Zone (CRMZ) and 2. a brittle fault, here termed the Clarke River Fault (CRF). The CRMZ is at least 1 km wide and 120 km in length. It is well exposed near "Craigie" Outstation where a belt of mylonitized, fine-grained schistose and gneissic rocks retrogressed to green schist facies, is juxtaposed against the northern edge of the Craigie Granodiorite. The CRMZ also contains narrow migmatites, pegmatite veins, and rare, foliated marble, metavolcanics, hematite quartzite, metapyroxenite and serpentinite. Relic garnet porphyroclasts, hornblendes in metabasites and migmatites indicate some of these rocks were originally amphibolite grade. The Craigie Granodiorite is an Early to Middle Palaeozoic, medium to coarse-grained, foliated, I-type hornblende-biotite tonalite (McLennan, 1987). It exhibits a gneissic foliation which is sub-parallel to the trend of the CRMZ and as well is locally sheared and contains thin mylonites. The latter two fabrics reflect movement along the CRMZ. The gneissic foliation may also, be an incipient foliation which has resulted from movement on the CRMZ. Deformation within the CRMZ is variable, ranging from ultramylonite to narrow zones of cataclasis. However, most lithologies exhibit a penetrative mylonitic foliation (Sm) which anastomoses 20® about the easterly trend of the CRMZ and is vertical to steeply dipping. Stretching lineations are weak to strongly developed, and commonly dip shallowly east and west within the plane of the foliation (Sm). A dextral sense of shear on the CRMZ is evident from a number of micro and mesoscopic kinematic indicators including: A. asymmetrical porphyroclasts, B fragmented and rotated quartz veins C. steeply plunging asymmetrical folds D. Type - 1 S-C planes (after Berthe and others, 1979) and E. Type - 2 S-C plane fabrics (after Lister <5c Snoke 1984). Post mylinitic deformation is present in both the CRMZ and Craigie Granodiorite as small scale faults and kinks. Two sets of kinks have been recognised. One set has subvertical northerly plunging axis, the other has subhorizontal to shallow southerly plunging axis. 266

The CRF is a brittle fault which crops out adjacent to the northern edge of the CRMZ. It is up to 30 m wide and separates the rocks of the Broken River Province from the CRMZ. In the Graveyard Creek Subprovince near "Craigie" Outstation the CRF is a 30 m wide zone of brecciated and intensely boundinaged fine-grained quartzose arenite and shale. In this area, the fault separates the CRMZ from the Devonian Broken River Group. The lithology within the CRF is similar to the flyschoid lithologies of the Ordovician Wairuna and Judea Formations. In the Camel Creek Subprovince east of "Craigie", the CRF is recognised in the Clarke River as a 10 m wide zone of brecciated and locally silicified quartzose arenite and shale. Here the CRF separates the Wairuna Formation from the CRMZ. The age of the CRFZ is not known. However, the CRMZ probably developed during the Early Palaeozoic, prior to the deposition of the oldest known sedimentary unit in the Broken River Province (the Ordovician Wairuna Formation). The final major phase of ductile deformation possibly occurred in the Early Silurian during the formation of the Graveyard Creek Subprovince. The last major phase of faulting on the CRF may have occurred during the Middle Carboniferous folding event of the Graveyard Creek Subprovince. REFERENCES ARNOLD,G.O., 1975: A structural and tectonic study of the Broken River Province, north Queensland. Geology Department, James Cook University of North Queensland, Ph.D. Thesis (unpublished). BERTHE,D., CHOUKROUNE,P. & JEGOUZO,P., 1979: Orthogneiss, mylonite, and non coaxial deformation of granites: the example of the South Armorican Shear Zone. Journal of Structural Geology, 1, 31-42. HARRINGTON,H.J., 1981: Big Bend Megafold or Broken River Triple Junction? Journal of the Geological Society of Australia, 28, 501-502. HENDERSON,R.A., 1980: Structural outline and summary geological history for northeastern Australia. In: Henderson,R.A., and Stephenson,P.J. (editors), The Geology and Geophysics of Northeastern Australia. Geological Society of Australia, Queensland Division, Brisbane, pp 1-26. HENDERSON,R.A., 1987: An oblique subduction and transform faulting model for the evolution of the Broken River Province, northern Tasman Orogenic Zone. Australian Journal of Earth Sciences, 34, 237-249. LISTER,G.S. & SNOKE,A.W., 1984: S-C mylonites. Journal of Structural Geology, 671-638. WITHNALL,I.W., LANG,S.C., WARNICK,J.V., SCOTT,M., MCLENNAN,T.P.T., LAW,S.R. & HUTTON,L.J., 1985: Summary of results from the 1984 field season in the Eingalseigh & Clarke River 1:250 000 Sheet areas - RGMP Progress Report. Geological Survey of Queensland, Record 1985/30 (unpublished).

267

8.17

IGNEOUS I N T R U S I O N S AND T H E I R R E L A T I O N S H I P REGIONAL STRUCTURAL FABRIC OF THE C E N T R A L N O R T H E R N BOWEN B A S I N

THE

T.P.T. McLennan Queensland Department of Mines

The central northern Bowen Basin, as covered by the Mt Coolon 1:250 000 Sheet area, is the most highly intruded region of the Bowen Basin (Hamilton, 1985). Intrusives within the Permo-Triassic sedimentary sequence include stocks, sills, dykes, and plugs which range in composition from rhyolitic to gabbroic. The relationship between igneous intrusions in particular sills and dykes, and structure of the region has been investigated during the course of mapping parts of the Mt Coolon 1:250 000 Sheet area. Preliminary interpretations of this study show that the distribution, composition and habit of sills and dykes changes with respect to the structural fabric of the region. Sill morphology and distribution varies across the axis of the basin and appears strongly influenced by northerly trending regional structures such as the Nebo Synclinorium, Collinsville Shelf and major faults. However, the distribution and orientation of dykes appears closely linked to the local tensional regime of an area, with some lithological variation in dyke composition occurring down the axis of the basin. Sills most commonly intrude coal measures of the Black water Group and are rare in the overlying Triassic fluviatile sediments. Sills are most prevalent along the east side of the Nebo Synclinorium where they crop out as sheet-like bodies. Along the western edge they are predominantly exposed as elliptical fingers or "boudins" linked by narrow bridges or as a horizon of disconnected fingers. Dramatic size differences between fingers can occur where they occupy coaly horizons. Seven major lithological sill types have been recognised (Table 1). Medium to coarse grained granodioritic sills are volumetrically the most significant and mainly cropout along the east side of the Nebo Synclinorium. Extremely large sill-like bodies of this type are upto 1 km thick. The distribution and unique size of these intrusives is probably related to a major north-south trending fault in that area. The Crowsfoot Granodiorite and the mustard-coloured intermediate sills are more common along the western than eastern side of the synclinorium. Feldspar porphyry, doleritic and rare acid and gabbroic sills cropout throughout the region. Seven major types of dykes have also been recognised (Table 1). A strong correlation exists between dykes composition, trend and location within the region (Figure 1). In the north, basic and acid dykes trend N N E and N E respectively where as andesitic dykes trend N W . Dykes in the south have a northerly trend. The majority of dykes are parallel to the locally dominant regional fracture trend of the areas. Changes in trend of dykes of similar composition down the axis of the region probably reflects localised changes in the regional stress field due, to discontinuities in the upper crust, such as major faults. However, gross changes in trend between dykes of different composition may be the result of differing gross tectonic regimes. More conclusive age data on various compositional groups may reveal a sequence of emplacement episodes associated with respective tectonic and stress regimes.

REFERENCES H A M I L T O N , L . H . , 1985: Igneous intrusions in the Bowen Basin. In, Bowen Basin Coal Symposium, Geological Society of Australia Abstracts 17, 107-108.

268

Table 1. Sill and Dyke Morphology Dykes

Sills

Width (m)

Thickness (m)

Lithology

20-1000 1-10

Diorite Granodiorite

Qtz Feld. Porphyry

0.2-1

Qtz Feld. Porphyry

0.3-1

Crows foot Granodiorite

1-5

Acid rock

1-3

Acid rock

0.3-1

Basalt Dolerite

1-2

Basalt witli megacrysts

?>3 m

2-4

Gabbro

2-4

Gabbro

0.3-2

Mustard coloured intermediate rock

•2-2

Andesite

1-2

Lithology Diorite Granodiopite

Basalt Dolerite

) )

FIGURE I . DYKE & FRACTURE TRENDS

REGIONAL F R A C T U R E SETS 3iO(iiO) 040(*I0)

295(115) 000(110)

MAFIC DYKES

INTERMEDIATE ACID DYKFn North

Andcsltc

269

10-15

1-2

8.27 GROUND MELTING BY KOMATIITES AT KAMBALDA, WESTERN AUSTRALIA - EVIDENCE FROM U-Th-Pb SYSTEMATICS N.J. McNaughton, K.M. Frost and D.I. Groves Department of Geology, The University of Western Australia

Thermal erosion of wall rocks by high temperature magmas during ascent to the surface is now widely postulated. Both Archaean komatiitic and tholeiitic lavas have isotopic and chemical signatures which unequivocally implicate melt contamination by trace and major elements from wall-rock conduits (Chauvel et aL, 1985; Compston et aL, 1986; Barley, 1986). Komatiites also have the ability to melt their substrate due to their high liquidus temperature, low viscosity and high turbulence during extrusion (Huppert et al, 1984). Field evidence for such ground melting is largely equivocal, although the replacement of interspinifex glass by a thermally conductive sulphide liquid from an overlying flow at Kambalda has been interpreted to indicate limited ground melting (Groves et al, 1986). A major point of discussion is whether "troughs" which host major Fe-Ni-Cu sulphide ores at Kambalda are structurally modified pre-existing topographic depressions, or the result of ground melting by the overlying thick komatiite flows that host the ores. Rare ocellar units within komatiite flows that flank the ore "troughs" have recently been described from Kambalda (Frost & Groves, 1987; Frost et aL, in prep.). On textural, morphological and chemical evidence, these are concluded to have formed by melting of sulphidic sediments which formed the substrate of the komatiite lava channel, with selective preservation of the melted sediment (termed xenomelt) along the quiescent levee position of the "trough". Extensive underground development and drilling at Kambalda allow the sampling of both the "contaminant" sediment and the contaminated ocellar komatiite to test this scenario. Lead isotopic analysis of these rock types should show: 1) a common initial Pb (of the Pb- and S-rich contaminant), and 2) U-Th-Pb systematics compatible with melting of the sediment and separation of an immiscible Fe-rich sulphide liquid from the xenomelt. Liquid immiscibility within the xenomelt to produce felsic ocelli is also tested by U-Th-Pb partitioning between ocelli and matrix via Pb-isotope analysis.

270

The Pb isotope study shows that the initial Pb isotopic composition of interflow sulphidic sediments and the ocellar unit is identical within experimental error, and represents a common, synvolcanic Pb isotopic composition. Further, the ThAJ of the ocellar unit samples (2.8-3.9) and the sediments (mosdy 2.8-3.9) are variable, and tend towards significantly lower values than the restricted Th/U values normally associated with mafic-ultramafic igneous suites, including those atKambalda (3.6-3.9). The Pb isotopic data, calculated U/Pb and ThAJ of each rock type (McNaughton et a/., submitted), and the field, petrographic and chemical observations of Frost & Groves (1987) and Frost et al (in prep.), have been integrated into a model for the formation of the Kambalda ocellar komatiites, as follows: 1) melting of substrate sulphidic sediment by a thick overlying komatiite flow to form a xenomelt which resists mixing with the host komatiite due to viscosity contrast, 2) immiscible sulphide-liquid formation within the xenomelt, and gravitational separation of the sulphide liquid from the silicate liquid, 3) eventual partial assimilation of the xenomelt within the lava channel, 4) convective accumulation of the buoyant residual xenomelt to form a discrete unit within the komatiite flow flanking the ore-bearing trough, 5) cooling-induced liquid immiscibility within the xenomelt to produce immiscible felsic and basic liquids, 6) gravitational accretion of the felsic ocelli into layers at the top of the unit, and 7) solidification to form the ocellar unit. The Pb-isotope data support ground melting as a significant mechanism of lava contamination and ore formation at Kambalda. Acknowledgements Logistical and scientific support from A. Cowden and M. Elias (Kambalda Nickel Operations), mass spectrometry from Curtin University of Technology, and financial support from Kambalda Nickel Operations and The University of Westem Australia are gratefully acknowledged. References Barley M.E., 1986. Geology 14, 947-950. Chauvel C., Dupre B. & Jenner G.A ., 1985. Earth Planet, Sci, Lett. 74, 315-324. Compston W., Williams LS., Campbell LH. & Gresham J.J., 1986. Earth Planet. ScL Lett, 76, 299-311. Frost K.M. & Groves D.I., 1987. Abstr. 5th Magmatic Sulphides Conf., Zimbabwe, 15. Groves D.L, Korkiakoski E.A., McNaughton N.J., Lesher C.M. & Cowden A., 1986. Nature 319, 136-139. Huppert H.E., Sparks R.S.J., Turner J.S. & Arndt N.T., 1984. Nature 309, 19-22. McNaughton N.J,. Frost K.M. & Groves D.L, submitted. GeoL Mag.

271

2.19

ARE ANORTHOSITIC ROCKS BASEMENT TO THE PILBARA CRATON? N.J. McNaughton^, M.D. Green^, W. Compston^ and I.S. Williams^

^Department of Geology, The University of Western Australia ^Research School of Earth Sciences, Australian National University, Canberra

Granodioritic plutonic rocks of the northern Shaw Batholith represent the oldest, precisely dated granitoids of the Pilbara Craton with a Pb-Pb whole-rock age of 3499 ± 22 Ma (Bickle et a/., 1983). Utilising zircon U/Pb geochronology and the sensitive, high-resolution, ion microprobe (SHRIMP) at the Research School of Earth Sciences, ANU, the antiquity of this suite is confirmed with ages of 3493 ± 4 Ma and 3467 ± 6 Ma for two discrete samples. The geological relationship between the greenstone succession and the ca 3.5 Ga granodiorites is unknown due to equivocal contact relationships. However, a granodiorite from the South Daltons area of the Shaw Batholith intrudes the greenstone succession and yields a SHRIMP U/Pb zircon age of 3446 ± 8 Ma, which is synchronous with the U/Pb zircon age of 3452 ±16 Ma (Pidgeon, 1978) for felsic volcanics in the lower greenstone sequence. These data suggest (1) that the granodiorites of the northern Shaw Batholith are part of a composite batholith emplaced between ca 3.50 and 3.45 Ga, and (2) the younger plutonic phases are coeval with felsic volcanism towards the base of the greenstone sequence. The temporal relationship between the base of the greenstone sequence and the ca 3.5 Ga granodiorites is equivocal. A Sm-Nd whole-rock isochron age of 3540 ± 30 Ma on ultramafic to intermediate volcanics from the basal greenstone unit, the North Star Basalt (Hamilton et aL, 1981; 1983), is now considered suspect due to possible rare earth element contamination of high-temperature magmas by older crustal materials (see Cattell et al, 1984, Compston et aL, 1986). Thus 3540 Ma is possibly the minimum age of the basement to the greenstones. The stratigraphic sequence of Hickman (1983) suggests that 4 km of conformable volcanic stratigraphy underlies the 3452 ± 16 Ma felsic volcanics of the Duffer Formation. In the absence of any regional unconformity within this stratigraphy, there are no compelling reasons to consider the North Star Basalt to be significantly older than 3452 Ma, and that the Sm-Nd data reflect a basement older than 3540 Ma underlying the greenstones. Bettenay et al (submitted) have described enclaves of gabbroic anorthosites within the 3446 Ma granodiorite of the South Daltons area, and an imprecise Pb-Pb whole-rock age, and Sm-Nd 272

model ages for these enclaves suggest their formation at ca 3.6 Ga. Zircon U/Pb geochronology using SHRIMP yielded a 3578 ± 4 Ma formation age for one gabbroic anorthosite enclave. Hence these rocks are: (1) significantly older than, and unrelated to, their granodiorite host, and (2) compatible with the >3540 Ma sialic basement required to yield the North Star Basalt Sm-Nd data by contamination of ca 3.45 Ga mantle melts. The gabbroic anorthosite enclaves are highly altered, with metamorphic minerals psuedomorphing the primary silicate phases (Bettenay et al. submitted). Their coarse grain size and rare relic banding indicate a plutonic derivation. Normally quartz-free and comprising 65-85% feldspar, these rocks are considered part of the "anorthosite suite" (cf. Barton et al., 1979), although no anorthosites sensu stricto have been found. The anorthositic enclaves of the South Daltons area of the Shaw Batholith are the oldest rocks knowoi from the Pilbara Craton, and considered fragments of the basement to the greenstone sequence. Thus models of greenstone formation on an early sialic crust are confirmed for the east Pilbara (Barley et al., 1979; Hickman, 1983). Acknowledgements Leigh Bettenay, Mike Bickle and David Groves (UWA), and Ian Campbell (RSES, ANU) are gratefully acknowledge for their field and scientific counsel, as is the Australian National University and University of Western Australia for financial assistance. References Barley M.E., Dunlop J.S.R., Glover J.E. & Groves D.I. (1979). Earth Planet. Sci. Lett. 43, 74-84. Barton J.M. Jnr, Fripp R.E.P., Horrocks P. & McLean N. (1979). Amer. J. Sci. 279, 1108-1134. Bettenay L.F., McNaughton N.J., Bickle M.J., de Laeter J.R., Groves D.I. & Boulter C.A. (submitted). Contrib. Mineral. Petrol. Bickle M.J., Bettenay L.F., Barley M.E., Chapman H.J., Groves D.I., Campbell I.H. & de Laeter J.R. (1983). Contrib. Mineral Petrol. 84,25-35. Cattell A., Krogh T.E. & Amdt N.T. (1984). Earth Planet.Sci. Lett. 70,280-290. Compston W., Williams I.S., Campbell I.H. & Gresham J.J. (1986). Earth Planet. Sci. Lett. 76, 299-311. Hamilton P.J., Evensen N.M., O'Nions R.K., Glikson A.Y. & Hickman A.H. (1981). Spec. Publ. Geol. Soc. Aust. 7, 187-192. Hamilton P.J., O'Nions R.K., Bridgewater D. & Nutman A. (1983). Earth Planet. Sci. Lett. 62, 263-272. Hickman A.H. (1983). Geol. Surv. WA. Bull. 127. Pidgeon R.T. (1978). Earth Planet. Sci. Lett. 37,421-428.

273

2.18

CRATON MARGIN TECTONICS AND THE ORIGIN OF Sn-Ta PEGMATITES IN THE SOUTHWEST YILGARN BLOCK THE Pb ISOTOPE APPROACH

N.J. McNaughton, G.A. Partington, L.H. Seet and D.A. Kepert Department of Geology, The University of Western Australia

Craton margins often undergo episodic tectono-magmatism in response to adjacent younger mobile belt or basin development The SW comer of the Archaean Yilgam Block is bounded to the south by the Proterozoic Albany-Fraser Mobile Zone, and to the west by the Darling Fault, a normal fault system superimposed on an earlier late Proterozoic sinistral shear zone (Blight et ai, 1981). Pegmatites appear to have formed by crustal anatexis during these, as well as earlier, craton margin tectonic episodes. We have studied major pegmatites or pegmatite swarms in the SW Yilgam Block, including the giant Sn-Ta-Li-rich Greenbushes Pegmatite, the sub-economic Femdale Pegmatite, and four unmineralised pegmatites (Peninsula, Maranup Ford, Cundinup and Mullalyup). All intrude into major shear zones at or near regional lithological contacts in amphibolite facies host rocks, and all are deformed. Stmctural studies indicate that all the pegmatites were emplaced into a major sinistral transcurrent shear zone, the Donnybrook-Bridgetown Shear Zone, trending NW-SE and now tmncated by younger craton-margin shear systems (Partington, 1986; in prep.). The NW-SE trend in the S and W Yilgam is a major Archaean feature, now reflected by Magsat crustal magnetic anomalies (Johnson et aL, 1986) and regional lineaments (Partington, in prep.). Available geochronology (Partington et aL, 1986; Compston et aL, 1986) suggests that the Donnybrook-Bridgetown Shear Zone was active from 2.61 to 2.53 Ga, with numerous granitoids-pegmatoids emplaced into, and deformed by, the shear zone during that time. A model of anatectic melting, fluid-melt focussing, and granitoid-pegmatoid emplacement and fractionation within the shear zone during ductile movement has been proposed (Partington, 1986; in prep.) to explain the stmctural and chemical features of the Greenbushes Pegmatite. Lead isotope studies of K-feldspars indicate that four of the pegmatites (Greenbushes, Peninsula, Maranup Ford, Cundinup) were emplaced at 2.53 ± 0.04 Ga into the Donnybrook-Bridgetown Shear Zone. However, all pegmatites show isotopic resetting of their K-feldspars at an age calculated from a Pb-Pb palaeoisochron to be ca 1.06 Ga, which undoubtedly reflects a major reactivation of the shear zone in response to craton-margin tectonicswithin the Albany-Fraser Mobile Zone to the south (Rosman et aL, 1980). The Femdale and Mullalyup pegmatites are the least deformed, and show litde evidence of isotopic resetting. Model Pb-Pb ages on K-feldspars 274

yield 0.73 and 0.80 Ga, respectively, for a two-stage Pb isotope growth model further refined from that presented by Partington et aL (1986). These ages record another reactivation of the Donnybrook-Bridgetown Shear Zone, which within the uncertainty of the model ages, overlaps in time with the breakup of Gondwana, the formation of the Leeuwin Block (Fletcher et aL, 1985), and the initiation of the "proto-Darling Fault" sinistral shear system along the western margin of the Yilgam Craton (Libby & de Laeter, 1979). The Donnybrook-Bridgetown Shear Zone has been a locus for both rare-metal and barren pegmatite emplacement during two discrete episodes, and metamorphic activity during a third. The two younger events are directly correlated to shear zone reactivation due to craton-margin tectonics. Sn-Ta mineralization does not correlate with a particular event Similarly, the U/Pb of the source region for all pegmatites is indistinguishable, suggesting that rare-metal enriched source rocks are not responsible for the Sn-Ta mineralization. Further, all pegmatites were emplaced into similar structural sites and into hosts of similar metamorphic grade. The only positive correlation is that Sn-Ta mineralized pegmatites are hosted, in part, by mafic to ultramafic lithologies. This may reflect either a chemical control on ore deposition, and/or a rheological control of ore fluid access (Bettenay et aL, 1985). Acknowledgements Leigh Bettenay, David Groves and Lyal Harris (UWA), and Roger Thompson, Mike Hatcher and John Davis (Greenbushes Tin N.L.) are gratefully acknowledged for scientific and logistical support. Greenbushes Tin N.L. and The University of Western Australia provided financial support. References Bettenay L.F., Partington G.A. & Groves D.L (1985). W, Aust. Mining PetroL Res. Inst, Rep, No, 13, 56pp. Blight D.F., Compston W. & WUde S.A. (1981). W, Aust, GeoL Surv, Ann, Rep. for 1980, 72-80. Compston W., WilUams I.S. & McCulloch M.T. (1986). Aust, J, Earth ScL 33,193-200. Hetcher LR., Wilde S.A. & Rosman K.J.R. (1985). Aust, J, Earth ScL 32, 73-82. Johnson B.D., Mayhew M.A., O'Reilly S.Y., Griffin W.L., Arnott F. & Wasilewski P.J. (1986). GeoL Soc, Aust, Abstr, No, 16 , 127-129. Libby W.G. & de Laeter J.R. (1979). W, Aust, GeoL Surv, Ann, Rep, for 1978, 79-87. Partington G.A. (in prep.). Univ. West. Aust. PhD thesis. Partington G.A. (1986). Bur, Min, Resourc, Record 1986/10, 53-54. Partington G.A., McNaughton N.J., Kepert D.A., Compston W. & WUliams I.S. (1986). Bur, Min, Resourc, Record 1986/10, 55-56. Rosman K.J.R., Wilde S.A., Libby & de Laeter J.R. (1980). W, Aust, GeoL Surv, Ann, Rep, for 1979, 97-100. 275

8.3

TIN ISOTOPE FRACTIONATION IN CASSITERITE AS A PETROGENETIC INDICATOR N.J. McNaughton^, and K.J.R. Rosman^

^Department of Geology, The University of Western Australia ^School of Physics and Geosciences, Curtin University of Technology, Perth

Stable isotope fractionation of light elements (e.g. H C N O S) has long been known to occur naturally, and now have important petrogenetic applications in many branches of the geosciences. Natural isotopic fractionation of some heavier elements is predicted from the large mass range of stable isotopes for such elements (e.g. McNaughton & Loss, in press). Further, isotopic fractionations have been observed for Ga (Gramlich & Machlan, 1985), Pd (Mermelengas et aL, 1981), Te (Smithers & Krouse, 1968), Se (Krouse & Thode, 1962) and Sn (Rosman & McNaughton, 1986). Although, some of the observed fractionations may be attributed to processing purification of the element, it is important to establish the degree of natural fractionation and hence assess the potential use of isotopic fractionation as an indicator of the petrogenetic processes leading to economic mineralization. Tin possesses ten stable isotopes which span the mass range of 112 to 124, and thus has a similar potential for isotopic fractionation as the light elements, C and O (McNaughton & Loss, in press). Recent improvements in the mass spectrometric analysis of tin (Rosman et al., 1984), together with improved mass spectrometer design, including multicoUection, embodied in the new VG 354 mass spectrometer at Curtin University of Technology, and double spiking to overcome isotopic fractionation during analysis (see Rosman & McNaughton, 1986), now allow tin isotope fractionation to be measured on purified tin to a precision of ±0.01% per mass unit (95% confidence level). This precision is comparable to that of the analysis of gases in dual-inlet comparative mass spectrometers routinely used for light-element isotopic analysis. Fractionation of tin isotopes in nature should occur due to: (1) kinetic fractionation during partial dissolution of tin from a source rock, and partial precipitation of tinfromthe transporting medium into tin minerals, and (2) equilibrium fractionation between coexisting tin oxides and sulphides (McNaughton & Loss, in press). As tin is highly incompatible in nature and preferentially resides in the upper crust (Taylor & McLennan, 1983), upward mobilization of tin in the crust via a fluid or melt phase is required to produce the 10^-10"^ enrichment above crustal concentrations necessary to form an economic ore body. Such high enrichment factors may require repeated tin remobilization, which may further enhance any kinetic isotopic fractionation. Similarly, young tin ore deposits may be the outcome of more crustal reworking than Archaean deposits, and hence their tin isotopes may show more fractionation. Further, coexisting tin oxides and sulphides may show equilibrium tin isotopefractionationand leave the oxide isotopically labelled. 276

Analysis of tin in cassiterite requires dissolution followed by tin separation and purification. The method of fluxing cassiterite with lithium metaborate (McNaughton & Loss, in press) introduces molecular isobaric interferences to the tin spectrum. An alternative method of reducing cassiterite to tin metal with graphite in an induction furnace, followed by ion exchange chomatography (McNaughton & Rosman, in prep.) resulted in acceptably pure tin and is applicable to both cassiterite and stannite analysis. Cassiterite samples have been analysed from environments with a variety of ages and formation processes: these include cassiterites from Archaean pegmatites at Greenbushes and Moolyella (W.A.), Devonian hydrothermal replacement deposits at Mt Bischoff (Tas.), Jurassic alkaline granites from Nigeria, wood tin from Cornwall and Nigeria, and coexisting cassiterite and stannite from north Queensland. Within the analytical uncertainty of each determination (i.e. ±0.01-0.02% per mass unit; 95% confidence level), none of these samples are fractionated with respect to either our laboratory standard, or each other. It is concluded that if these data are representative, then natural stable-isotope fractionation of tin is negligible. Thus, tin isotope fractionation apparently has no apparent value as a petrogenetic indicator. It follows that the mechanism by which tin is scavenged into a transporting fluid, and precipitated from that fluid to form cassiterite, is highly efficient and precludes kinetic isotopic fractionation of tin. Further, equilibrium fractionation of tin isotopes between coexisting oxide and sulphide at mineralization temperatures appears negligible, such that sulphide-associated cassiterite is not isotopically labelled. In view of this, the observed fractionation of isotopes in a highly purified tin sample (Rosman & McNaughton, 1986) is probably an artifact of purification rather than a reflection of a naturallyfractionatedtinore. Acknowledgements This work was supported financially by the ARGS, CUT and UWA. Scientific and technical support from John de Laeter, Bob Loss and Matthew Shields (CUT), and David Groves and Sue Ho (UWA) is gratefully acknowledged. References GramUch J.W. & Machlan L.A., 1985. AnaL Chem. 57,1788-1790. Krouse H.R. & Thode H.G., 1962. Can, J, Chem, 40, 367-375. McNaughton N.J. & Loss R.D., in press. Spec. PubL GeoL Soc. Aust. Mermelengas N., Rosman KJ.R. & de Laeter J.R., 1981. Int. J. Mass Spectrom. Ion Processes 37,1-11. Rosman K.J.R., Loss R.D. & de Laeter J.R., 1984. Int. J. Mass Spectrom. Ion Processes 56, 281-291. Rosman K.J.R. & McNaughton N.J., 1986. Int. J. Mass Spectrom. Ion Processes 75, 91-98. Smithers R.M. & Krouse H.R., 1968.^ Can. J. Chem. 46, 583-591. Taylor S.R. & McLennan S.M., 1983. Chem. GeoL 39, 273-280. 277

8.15

PROXIMAL AND D I S T A L F A C I E S A S S O C I A T I O N S SUBAERIAL S I L I C I C CALDERA VOLCANOES J.

Bureau

of M i n e r a l

Resources,

McPhie Geology

and G e o p h y s i c s ,

Canberra

Subaerial s i l i c i c calderas are large, multiple-vent volcanic centres, c o m p r i s i n g c o m p o s i t i o n a l l y and t e x t u r a l l y d i v e r s e v o l c a n i c and v o l c a n i c l a s t i c rocks. Modern examples provide c r i t e r i a useful in distinguishing between proximal and distal caldera facies associations in ancient, less well preserved or exposed volcanic sequences. Using these c r i t e r i a , the Coombadjha Volcanic Complex of northern NSW has been recognised as the remnant of the most proximal parts of a Late Permian s i l i c i c caldera volcano. There are many other such proximal facies associations among the Palaeozoic volcanic sequences of eastern Australia, each warranting closer examination in order to demonstrate the presence of caldera relics. For comparison. Late Carboniferous ignimbrite-conglomerate sequences elsewhere in NSW (Currabubula Formation and equivalents) exemplify the distal facies associations of caldera volcanoes. Proximal caldera environments typically encompass at least a few hundred square kilometres and involve locally rugged relief, with elevation changes ranging up to about 1000 metres. There are obvious contrasts in the topography of structurally resurgent or domed calderas and those that remain permanent depressions. In both cases however, the intracaldera stratigraphic record is dominated by the products of effusive and explosive eruptions (lava flows and lava breccias; very thick, welded ignimbrites), with a minor contribution from contemporaneous erosion and redeposition of unconsolidated pyroclastic debris (lacustrine and f l u v i a l sedimentary rocks). The sedimentary facies are characterised by mineralogical immaturity (abundant glass, pumice, feldspar and ferromagnesian phases), resulting from the dominance of mechanical over chemical weathering, and from rapid transportation across short distances. Hydraulic equivalence of larger but low density pumice and smaller but denser crystals and lithics commonly results in very poorly size-sorted deposits. Intracaldera ignimbrites genetically related to the host caldera are typically hundreds of metres in thickness (very high aspect r a t i o ) , texturally uniform and thoroughly welded and crystallised. Lithic fragments in these intracaldera ignimbrites may be relatively coarse (lapilli and blocks) and concentrated in discrete l i t h i c breccia i n t e r v a l s , derived from collapse of caldera w a l l s , or else from lag accumulation marking eruptive s i t e s . Other ignimbrites present within the caldera may be outflow sheets from nearby, coeval but separate volcanic centres, and/or locally erupted from minor vents active prior to or following the main caldera-forming ignimbrite eruption. The former may be distinguished as exotic in having d i f f e r e n t compositions, palaeoflow patterns and internal f a c i e s variations when compared with the main intracaldera ignimbrite; the latter are typically small in volume, and interbedded with other pyroclastic deposits, all of which can be related to lava domes or flows. Intracaldera airfall deposits are also r e l a t i v e l y coarse, thick (several metres), and lithic-rich, but being unconsolidated and distributed on topographic highs as well as in depressions, they have low preservation potential. Most examples are byproducts of e f f u s i v e activity at minor vents commonly, though not e x c l u s i v e l y , situated along the caldera margin. The apparent paucity of

278

intracaldera a i r f a l l deposits related to caldera-forming ignimbrite eruptions may be an artefact of non-exposure. In some cases however, the omission i s real and implies that the eruptions of these intracaldera ignimbrites did not involve collapse of plinian-style eruption columns; rather, eruptive fountains of low height seem to have operated from the outset. Proximal intracaldera facies variations and geometry are complex over short distances because subaerial volcanics are constructional, creating positive topography which is continuously modified by additions to the volcanic p i l e , structural disturbances and denudation. The intracaldera volcanic p i l e overlies precursor volcanic rocks, often intermediate-composition lavas and breccias, or else forms the roof to comagmatic high-level intrusions. The proximal sequence may be locally affected by hydrothermal alteration and pervasive fracturing or shearing related to such intrusions and/or to faults. The structure reflects the effects of subsidence of the fault- or hinge-bounded intracaldera region r e l a t i v e to the pre-caldera basement. D i s t a l f a c i e s associations of caldera volcanoes contrast with proximal associations in having greater extent, more regular geometry, and a s i g n i f i c a n t component of e p i c l a s t i c lithologies. The distal environments cover hundreds to thousands of square kilometres. The products and effects of eruptions at the caldera are diluted by the increasing dominance of e p i c l a s t i c processes with distance from the source vents. The record of volcanic activity is biased in that there is preferential preservation of widespread, low to moderate aspect ratio, welded ignimbrite sheets which display systematic l a t e r a l and v e r t i c a l g r a i n s i z e v a r i a t i o n s . Any nonwelded ignimbrites and airfall ash deposits are not sufficiently resistant to escape erosion. Lavas and vent-produced base surges typically do not flow far enough from their sources to be incorporated in distal sequences. The primary volcanic rocks thus record only a small portion of the eruptive a c t i v i t y of the source caldera. The rest i s represented by abundant c r y s t a l s , shards, pumice and volcanic rock fragments in the associated fluvial, lacustrine and/or shallow marine sedimentary rocks. Because large volumes of pyroclasts rapidly accumulate at the earthquake-prone source caldera volcano, mass-flow redeposition occurs frequently, so that the drainage of distal settings is repeatedly choked and modified by the influx of lahars. Although contemporaneous erosion may produce disconformities in the distal sequence, the overall facies geometry is relatively regular over large distances and consists of a stack of ignimbrites interbedded with volcanogenic sedimentary rocks. The d i s t a l facies is in general free of hydrothermal alteration, being remote from the locus of intrusive magmatic activity, although some ignimbrite sheets may be affected by vapour-phase a l t e r a t i o n during degassing a f t e r e m p l a c e m e n t . The d i s t a l f a c i e s association of caldera volcanoes may overlie entirely unrelated sequences which show no signs of premonitory volcanic activity in the source region. Because of the great extent of the distal facies association, relationships with underlying units may vary regionally from unconformable, through disconformable to conformable.

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13.2

THE DEPOSITIONAL SETTING OF THE GERMAN CREEK AND MORANBAH COAL MEASURES, BOWEN BASIN C.W. Mallett^ and N.H.H. Godfrey^ ^CSIRO

Division of Geomechanics, Indooroopilly 2po Box 525, Indooroopilly

Widespread coal measure deposition was initiated in the Bowen Basin in the Late Permian. Peats developed first in the north and for a long time coexisted with open marine deposits in the south of the Basin (Draper 1985). The broad flat stable blocks represented by the Collinsville Shelf and the Comet Platform provided ideal location for extensive marginal marine and wetland conditions suitable for very extensive peat deposits. These coals now outcrop on the northwestern margin of the Bowen Basin from Collinsville to Gregory. The outcrop line of the surface mines displays a progression of depositional environments. In the north the environment is entirely freshwater and ascribed as the Moranbah Coal Measures. However, around Peak Downs the beginnings of a transition to the marine influenced environments which are included in the German Creek Coal Measures can be seen. Regional correlation is provided by tuffaceous marker beds, which can be seen in the mined sections and bore core, but are particularly prominent in the geophysical logs of boreholes (Koppe & Mengel 1973). The continuity and regional recognition of these markers, within a sequence where tuffs are common, indicates that they originated from explosive eruptive events on a very large scale. These will have been associated with significant seismic events which could have affected the sediments. This is evidenced in penecontemporaneous sedimentary deformation, (for example at Goonyella Mine). Ash falls, which were extremely common during the coal measures, have been preserved in a few rare localities in the peat environments. These ash falls provided a continual supply of sediment to the system, supplying trace elements and nutrients evenly distributed across the gigantic peat fields. A rare "island" between the Goonyella Lower and Middle seams in the north of Goonyella Mine was protected from fluvial terrigenous input and has preserved numerous ash falls in a colloidal, carbonaceous mudstone. These tuffs vary from less than a millemetre up to 200mm. The are identified as tuffs by the texture and dessication effects on the organic muds. Sections of the coal measures are exposed in mine highwalls from Gregory to Goonyella, and show a range of depositional environments. Goonyella in the north has entirely freshwater environments with thick coal seams, particularly in the lower part of the coal measures. They are similar to the Rangal Coal Measures in that peat accumulated over the area continuously, and was only ocassionally interupted by fluvial channel systems that rapidly deposited a thick sequence (20-50m) in narrow strips. These channel systems with associated splays migrated laterally with time, and were quickly covered by peat regrowth after deposition. (Godfrey 1985) At Peak Downs the thick coal seams have split into many thinner units. Thick migrating sandstone fluvial channels are still present, but in the north of the mine sedimentation is dominated by interbedded splay deposits with minor feeder sandstones. In addition there are extensive thinly bedded lacustrine units into which splay deposits migrate. About the centre of the mine is the most northerly occurence of thin marine bioturbated beds.

280

A marine trend is continued south into Saraji, where the sedimentary associations are very large massive migrating channel/splay systems with planar interdistributary bay-fill sequences. The channels are the largest that outcrop in the northern part of the Basin, and represent one of the major fluvial inputs on the palaeocoast. The bay-fill deposits are black organic silts which show an increase in sand upsection, from initially <lmm stringers to sand dominated interbedded sand and siltstone at the top of the interseam sections. The'channel/splay systems are invariably underlain by bay-fill deposit, but are significantly eroded in places. At Norwich Park, the section is composed mainly of interdistributary bay-fill deposits, but here they are associated with overlying burrowed marine units and quartzose sands, rather than the fluvial systems at Saraji. It is also here that the coal seam distribution is established that continues with remarkable continuity and consistancy to the south. At the south of Norwich Park, burrowed prodeltaic muds are found. German Creek and Oaky Creek Mines have seams overlain by prograding deltaic tongues which extend to the southeast into heavily burrowed mudstones and quartzose sands. The migration of the sandstone feeder channels can be seen in highwalls, and the delta mouth bars identified in bore records. Distinct peaks of marine influence occur in the section as shown by widespread continuous concentrations of burrowing, and high quartz percentages. Gregory Mine is a little to the west, and sediments associated with the coals are mainly strandline and intertidal. There are heavily bioturbated mudstones, and tidal "bundles" in laminated sequences. The coals are immediately overlain by coarse quartzose and micaceous prograding strand sands. Between mines the coal seams are split by more sanc^ sequences. The thickness and continuity of coal seams in the coal measures relates directly to the depositional setting along the strike. It ranges from thick seams in the north where freshwater swamps grew continuously, and were never affected by sea level changes, to the German Creek Coal Measures where peat accumulation was extremely sensitive to eustatic events. This resulted in the remarkable continuity of German Creek Coal Measure seams whose formation was dependent on the relative sealevel position. A seam was terminated or favourable conditions created synchronously over large areas by small sea level shifts. In intervening areas from Norwich to Peak Downs, there was a interplay of a complex deposition system, as fluvial and marine influences were variously imposed on the deltaic system. Draper,J.J. 1985 Summary of the Permian stratigraphy of the Bowen Basin. Geol. Soc. Aust. Abstracts 17,45-49. Godfrey,N.H.H. Strip mine highwall geology in the Moranbah and German Creek Coal Measures Geol. Soc. T^t. Abstracts 17 75-81. Koppe,W.H. 8c Mengel,D.C. 1973 Correlation of coal seams in the Fair Hill Formation, Northwest Bowen Basin. Qd Govt Min.J. 74 ,3-7.

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6.9

THE INTERPRETATION OF DURCHBEWEGUNG STRUCTURE, PIERCEMENT CUSPS AND PIERCEMENT VEINS B. Marshall Department of Applied Geology, New South Wales Institute of Technology

Durchbewegt ore with its characteristic durchbewegung structure, as discussed by Yokes (e.g., 1973), is also referred to as ball ore, fragmental or fragmentary ore, and breccia ore. The structure typically comprises angular to rounded fra^nts and rafts of disrupted layers of wallrock, vein quartz and ore minerals (such as pyrite and magnetite), in a groundmass of sulfides (such as galena, chalcopyrite and pyrrhotite). Genetic interpretations usually invoke conpetency or ductility contrasts in zones of high strain. Kneading, milling, rotation and rolling (e.g., Gilligan and Marshall, 1987) are used to convey the disoriented and contorted nature of many of the fragments, and to inply large amounts of rotational deformation. Typical mechanistic descriptions of durchbewegt ore formation state that: "During deformation, the sulfides flow ductilely, moving around and through the more conpetent portions of rock, breaking them up, and incorporating brittle fragments within a sulfide matrix. The fragments may range from large blocks of rock down to grain aggregates or individual grains. Depending on the ductility contrasts, some fragments rtay occur as boudins, some as angular fragments. The lack of orientation of relict fabrics in the fragments confirms tliat rotation has taken place" (Maiden et al., 1986). And that: "The transition from banded pyritic ore to fragmental ore is related first, to an increase in strain ... and second, to an increase in corpetency contrast ... In these situations, the amount of boudinage increases to a stage of conplete fragmentation. This is accortpanied by plastic flow of the weaker minerals, resulting in rotation of fragments. It is thus possible to recognise a se(^ence from banded pyritic ore to a stage where fragments of banded rock are set in a matrix of sulfide." (Klemd et al., 1987). Such relationships are illustrated from Joma mine (Norway) and other localities. Durchbewegung structure may be developed; (a) in zones of intense ductile shear where progressive siitple shear is a irajor component of the non-coaxial defornation; (b) in buckle folding, whenever layers undergoing progressive deformation become subject to extensional strains - for exanple, flattened flexuraJ. folding in which (at least for bulk strain) progressive pure shear has dominated the coaxial deformation; and (c) wherever, during deformation, one material (e.g., sulfides) ductilely invades another material (e.g., silicate rock) which is experiencing brittle failure - for exanple, in some boudinage and disruptive ductile piercement. The essential requirements for developing durchbewegung structure are: (a) materials of markedly different mechanical behaviour (i.e., with a substantial ductility or competence contrast) under the particular conditions (e.g., pressure, temperature, strain rate); and (b) time for sufficient deformation to accrue and be expressed in the two materials in distinguishably different ways. Factors pertaining to (a) and (b) are the presence of a strong planar (or linear) anisotropy, and of appropriate boundary conditions on material interfaces; the former favours non-coaxial flow, whereas the latter, when interfaces are unconstrained (or weakly constrained) and support negligible shear stress, favours coaxial strain accumulation. The two opposing factors collectively enhance the partitioning of flow preserved in durchbewegt ores.

282

The velocity fields in a flowing continuum can be partitioned into translation, strain and vorticity (corrprising shear-induced vorticity and spin). In durchbewegt ore, the disoriented and polished clasts irrply a substantial spin corponent, and in general either coaxial or non-coaxial rotational deformation. But because scale of observation influences perception of flow, it is inappropriate to interpret bulk deformation in terms of the behaviour of corrponent elements. Thus, bulk coaxial irrotational deformation may partition into spin and shear-induced vorticity corponents at element scale; and bulk non-coaxial rotational deformation may partition into coaxial spinning in a thin conpetent element and shear-induced vorticity in the inccarpetent matrix. I therefore reaffirm that durcbewegung expresses local partitioning of deformation, and need not mirror bulk deformation characteristics. PTERTEMEJ^ STFUCTURES Cusps, piercement cusps and associated piercement veins comnonly occur at the interface between sulfides and silicate host rocks. Cusps are pinched-in folds, between materials of highly contrasting corrpetence, where layering in the conpetent layer has maintained its integrity; piercement vein is used where a cusp association is not apparent and ductile errplacements of inccxrpetent material form vein-like bodies transgressing layering (Gilligan and Marshall, 1987). Maiden et al. (1986) ascribe cusps and related structures to layerparallel or layer-oblique (shallow angle) ccarpression. They suggest that cusps form where the more carpetent layer is relatively ductile, and veins where it is relatively brittle; however, their exanples are from different deformation events. Piercement cusps at Joma concurrently parallel the hinge surface and short limb of asymmetric folds which have nucleated at the sulfide/ silicate interface. The folds are consistent with layer-oblique corpression, this probably causing layer-parallel, shear-induced vorticity and spin. Gash veins of quartz±carbonate±sulfide are associated with the piercement, but do not necessarily terminate it. Fluid inclusion data from the quartz are consistent with the fluid being of late metamorphic derivation. Piercement cusps result from the ductile injection of sulfide at fold-focussed sites. Injection may be along extension fractures induced by the focussed stress exerted by the flowing sulfide and/ or the pocket of hydrothermal fluid at the tip of the flowing sulfide; or along sites of shear failure (as at Joma) dilated by the flowing sulfide and/ or the metamorphic fluid. Piercement veins, in the absence of a focussing process, are more likely the product of layer-parallel or layer-oblique (shallow angle) extension. As such, they are mechnistically unrelated to cusps and piercement cusps. REEERENCES Gilligan, L.B., and Marshal, B., 1987. Textural evidence for remobillization in netamorphic environments. Ore-Geology Reviews, 2, 205-230. Klemd, R., Maiden, K.J., and Okrusch, M., 1987. The Matchless copper deposit, ^ u t h West Africa/ Namibia: a deformed and metamorphosed massive sulphide deposit. Econ. Geol.; 82, 587-599. Maiden, K.M., Chiirmriba, L.R., and Smalley, T.J., 1986. Cuspate ore-wall rock interfaces, piercement structures and the localization of sane sulphide ore in deformed sulphide deposits. Econ. Geol., 1464-1472. Vokes, F.M., 1973. 95, 403-406.

"Ball texture" in sulphide ores. Geol. Foren.

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Forhand.,

9.3 MACROSEISMIC EFFECTS, LOCATIONS AND MAGNITUDES OF SOME EARLY TASMANIAN EARTHQUAKES M.O. Michael-Leiba Bureau of Mineral Resources, Geology and Geophysics,

Canberra

The Richter magnitude, ML, for historical earthquakes can be obtained from the Modified Mercalli intensity, I, and hypocentral distance, R (km) using the formula: ML = 1.13 InR + 0.6671 - 2.60 A magnitude is calculated from each intensity contour with a mean radius greater than 35km, and the arithmetic mean of these magnitudes is designated, ML(I). It approximates ML, usually to half a magnitude unit or better. For Tasmanian and Victorian events for which the MMIII contour is not included in the magnitude determination, a correction of -0.2 should be applied to ML(I). For New South Wales earthquakes, the correction is -0.1. Using felt reports from contemporary newspapers, I prepared isoseismal maps and, from these, determined the epicentres and magnitudes of what are probably the largest five western Tasmanian earthquakes during the period 1853-1957: Circular Head, 21 November 1859, ML(I)5.^; South West Tasmania, 3 February I88O, ML(I)5.5; Queenstown, 4 May 1908, ML(I)4.8; West Coast, 4 November 1911, ML.(I)4.8; and Zeehan, 1 March 1924, ML(I)5.2. All were felt with a maximum intensity of at least MMVI. The South West Tasmania event, the largest historic western Tasmanian earthquake, may have occurred on the Lake Edgar fault, but its epicentre is not well-constrained. The Tasmanian earthquake swarm of 1883-1892 consisted of around 2000 events felt in the northeastern Tasmanian region. The activity occurred at the rate of at least one event per month from April I883 to May 1887, and there were at least two events each year up to and including 1892 when the series ended. Using contemporary newspaper reports and publications, I drew isoseimal maps and determined epicentres and magnitudes for the three largest events: 26 January 1892, ML(I)6.9; 12 May I885, ML(I)6.8; and 13 July 1884, ML(I)6.4. All three earthquakes were felt over an area extending from southeastern New South Wales in the north to Hobart in the south, and each caused minor damage in Launceston. From the isoseismal maps, their epicentres were ascertained to lie in the Tasman Sea off the northeastern tip of Tasmania. The ML(I)6.9 (±0.4) event of 1892 is the largest historic earthquake recorded in eastern Australia. In 1883, Alfred Barrett Biggs of Launceston, Tasmania built and operated the first seismoscopes used in Australia. From his measurements and my epicentres, I calculated the instrumental Richter magnitudes of the 1884 and 1885 events to be ML6.2 and 6.5 respectively, in good agreement with their intensity-deduced magnitudes, ML(I)6.4 and 6.8.

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15.10

PETROLOGICAL FEATURES OF LAVAS FROM THE DERWENT-HUNTER TABLEMOUNT E.A.K.

Middlemost

Department of Geology & Geophysics,

University of

Sydney

The Tasmantid Seamounts form a north-south chain of volcanoes which transect the Tasman abyssal plain. In the middle of this chain is the Derwent-Hunter Tablemount. Earlier studies have shown that most of the upper surface of the tablemount is covered with limestones that are essentially coiposed of "shallcv^-water biogenic debris cemented together with micrite". Some pebbles of mafic volcanic rocks have also been described fron the Derwent-Hunter Tablemount. In March 1986 a further 22 pebbles of volcanic rock were collected at a depth of 4520 m on the northeastern flanks of the tablemount. Most pebbles corprise a kernel of mafic rock surrounded by, a rind of weathering, and an outer Mn-rich coating. Twenty thin-sections of different pebbles were studied using a petrographic microscope. All the phases in twelve samples were chemically analysed using an electron microprobe; and eight whole rock chemical analyses v^re obtained. All the rocks are mafic and have vitrophyric textures. They generally contain olivine and plagioclase phenocrysts arranged in a groundmass that contains a second generation of plagioclase, possibly seme augite, and a mesostasis of transparent brcwn glass. Many of the rocks are vesicular, and the vesicles are usually surrounded by a yellowsih or reddish, brown material. In sane speclinens patches, that appear to be hcmogeneous glass under the microscope, are found at higher magnification (E.S.M.) to consist of an intricate mosaic of fine, interlocking laths of plagioclase and augite set in glass. Specimens 01 and 20 contain small inclusions and xenocrysts that contain plagioclase and clinopyoxene crystals that have high-Ca compositions that are quite different fron those of the host rocks and all the other rocks studied. If the volcanic rocks are classified, using the chemical classification of Le Bas et al (1986:747), they are found to cluster in an area that overlaps into the fields of the basalts, basaltic andesites and mugearites. The glasses that form the mesostasis of these specimens also plot in these fields. As none of these rocks, or glasses, contain normative nepheline, all specijnens are regarded as being subalkalic (Le Bas et al. 1986:748). All the basaltic rocks contain both normative hypersthene (-^^11.4%) and normative olivine (A/1i.o%), and can thus be classified, more precisely as olivine tholeiites (Yoder & Tilley 1962:352). The most differentiated rocks are mainly basaltic andesites, and they generally contain more normative hypersthene (^#15.7%), and less normative olivine ( ^ 2 . 3 % ) than the basalts. It would be apposite to call such rocks subalkalic basaltic andesites as t h i s term would set them apart from the orogenic (calcalkaline) basaltic andesites and andesites with their higher AI2O3 contents; yet link them with the oceanic andesites with their tholeiitic affinities and lower AI2O3 contents. On the total alkali silica diagram the few mugearite specimens plot close to the basaltic andesite field; and this is regarded as a reminder that the olivine tholeiites may be described as transitional basalts. The glasses that form the mesostasis of these rocks all differ in ccnposition frcm their host rocks in that they are hyperthene and quartz normative. They thus have corpositions that are akin to oversaturated tholeiitic basalts, or oversaturated tholeiitic basaltic andesites.

285

Olivine and chranite were the first minerals to crystallize in these rocks. The chromites occur as small, generally euhedral grains in olivine phenocrysts, or as accessory minerals in the grounciTiass. They differ in chemical composition from the chromites usually found in MOR basalts (Hekinian 1982:43); that is, their CrÔ-j (/v36.0%) values are siinilar, but they are richer in total iron ( ^ 33.2 % of FeO) and Ti02 3.4%), but depleted in MgO (^9.8%) and AI2O3 (^15.8%). These chemical characteristics are interpreted as inclLcating that these spinels probably precipated after the onset of olivine crystallization, as this would enable olivine to dominate the initial partitioning of Mg. The chemical corpositions of the olivine phenocrysts, in both the basalts and basaltic andesites, extends over the same range; that is F077 to Fog^, and their respective arithmetic means are Fog^ 4 and Fog^ • ^ ^^^ specimens contain olivine phenocrysts together vdui a second' generation of smaller olivine crystals. The corpositions of the latter range fron FO74 ^^ It is evident that the rocks that contain these olivines have had a ccgrplex crystallization history, particularly as their mesostasis is quartz normative. Labrandorite is the only plagioclase in the basalts. It ranges in corposition fron An52 to An^Q, with a mean of Or^^ 3 Ab^g p ^62 7 ^^^2^3 0.77%). Most of the plagioclases, in the basaltic anc3esites/mugearites, range in corpsotion between to An^g, with a mean of Or^^-y -^42,8 ^55 5 (Fe203 ^0.76%). Plagioclases that are much richer in Ca'occur in the small :mclusons found in sate specimens. Pyroxenes occur in only sane of the rocks examined. They are all augites, and they are readily separated in to two groups. The carposition of the first group ranges frcm WO42 6 En42.9 ^^14.5 ô W042.6 En.g 4 FS21.O' ^^h a mean of W041 En^^^ Fs^n'Q. tKis group is also realtively enriched in the non--quadrilateraI carponents (TiO^-^^2.7%, Al2'^6.0%, Na20 ^ 0.65%). These augites, with their high abundance of non-quadrilateral carponents, are chemically akin to the clinopyroxenes characteristic of alkali basaltic rocks (Hekinian 1982:40). The carposition of the second group of augites ranges fran ^^40.6 ^48.9 FÎO.S to Wojg^g En4^.i FS22.1. vdth a iân of Wojg^g. ^^45 8 FS3^4 4, and the nean of their mam non-quadrelateral corponents is T1O9 1.2%; AI2O3 2.7% and NanO 0.29%. Calcic salites (WO50.6 ^37.3 occur in the small inclusions found in some specimens. Part or the mesotasis, in all the speciirens studied, has been altered to a material that is reddish, or yellowish, brcwn in colour; and differs frcm the glass of the host rock In being enriched in Ti02, FeO^ and K2 0, and depleted in Si02, AI2O3, CaO and Na20. This cryptocrystalline material probably contains K~phillipsite, nontronite, chlorophaeite and Fe-Ti-Mn oxides . It is concluded that the parental magma, responsible for the rocks studied, was initially olivine and hypersthene normative. With the removal of the early formed minerals (01 + Sp. + PI.) the carpsotion of the magma moved out of the hypersthene normative field, and it became mildly alkalic. Olivine, plagioclase ( ~ An^^) and a relatively small amount of augite (enriched in the non-quadrilateral cotpoenents) formed and \^re rernoved. The magma returned to being hypersthene normative, and precipitated plagioclase AnÔ and augites poor in the non-quadrilateral corponents, and the interstitii liquid became quartz normative. References Hekinian, R. 1982. Petrology of the Ocean Floor. Elsevier Sci. Publ. Co., Amsterdam. Le Bas, M.J. 1986. A cherrcial classification of volcanic rocks based on the total alkali-silica diagram. Journal of Petrology, Vo. 27(3), 745-750.

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2.8

SUBSIDENCE TRENDS IN WESTERN AUSTRALIAN SEDIMENTARY BASINS - IMPLICATIONS FOR FORMATION MECHANISMS M.F. Middleton Geological Survey of Western Australia

Subsidence (S) versus time for sedimentary basins in Western Australia generally follow one or a combination of two trends (i) an exponential decay with time (or square root of time, At^^^^, and (ii) a linear increase with time, At. Data from intracratonic basins, or intracratonic stages of basin development, tend to exhibit linear subsidence (S=At) behaviour. Data from continental margins that are influenced by continental breakup tend to exhibit an exponentially decaying subsidence

Subsidence data from four Western Australian sedimentary basins, the Canning, Officer, Carnarvon and Perth basins are examined in detail. The Palaeozoic of the Canning Basin displays a dominantly linear subsidence with time, although some localities show an exponential decay during the Devonian and Early Carboniferous. Officer Basin data show a linear, or in some cases an exponentially increasing subsidence with time. In general, the Carnarvon Basin underwent linear subsidence during the Palaeozoic and Early Mesozoic (until Jurassic), and then subsided according to an exponential decay process. The Perth Basin experienced a similar subsidence history: essentially linear subsidence with time until continental breakup, then an exponentially decaying subsidence. It is important to note that various tectonic processes induce specific related subsidence rates. These subsidence rates can be expressed as simple mathematical relationships of subsidence to time. The two significant tectonic processes, reported extensively in the literature, are subsidence due to thermal contraction of the lithosphere (S=Ae"^^ and S=At''/^, sleep (1971), and Turcotte and Ahern (1977), respectively) and subsidence due to lateral compression of the lithosphere (S=Ae^^; Lambeck, 1983). The principle tectonic process, to be investigated by this paper, is the linear subsidence with time. Linear subsidence with time, a commonly observed phenomenon, can be demonstrated to occur when the anomalous heat flow at the base of the lithosphere is constant during the period of subsidence. In contrast, the exponential decay subsidence process requires a constant temperature to be maintained at the base of the lithosphere or thermal conductive decay to occur into a semi-infinite medium. A constant anomalous heat flow at the base of the lithosphere can be maintained by a number of processes, three of which are (i) a nett downward convective flow oT the asthenosphere, decoupled from the lithosphere, with a velocity V, such that V= ( z - H ) / ( t ) ( i i ) a

287

redistribution of heat sources/sinks in the deep crust and lithosphere by dynamic and geochemical processes, and (iii) lithospheric extension with with shear stress beneath the basin varying linearly with time • A detailed example of the linear subsidence mechanism is developed in the context of the enigmatic relationship between the complex late Palaeozoic compressional tectonism of central Australia and relatively simple extensional tectonism of the adjacent Canning Basin. The subsidence data and possible interpretations, entailed in the formation of the Canning Basin during the late Palaeozoic, are reviewed in conjunction with seismic reflection data. The data show an extensional trough (Fitzroy Trough) immediately adjacent to the central Australian compressional province (Ngalia Basin, Arunta Block, Amadeus Basin, Musgrave Block). The two tectonic regimes appear to be incompatible if explained by horizontal tectonics. Vertical tectonism (possibly a nett downwelling of the asthenosphere beneath the Fitzroy Trough) and coeval horizontal tectonism in central Australia and the Canning Basin may be due to the same large scale tectonic process. A tectonic model entailing (i) horizontal tension in the Canning Basin, (ii) horizontal compression in central Australia, and (iii) a downward sheet-like convective regime in the mantle beneath western and central Australia is consistent with geological and geophysical data relating to the formation of both tectonic provinces.

REFERENCES Lambeck, K., 1983, Structure and evolution of the intracratonic basins of central Australia, Geophys. J. R. Astr. Soc., 74, 843-886. Sleep, N.H., 1971, Thermal effects of the formation of Atlantic continental margins by continental breakup, Geophys. J. R. Austr. Soc., 45, 125-154. Turcotte, D.J., and Ahern, J.L., 1977, On the thermal and subsidence history of sedimentary basins, J. Geophys. Res., 82, 3762-3766.

SYMBOLS IN EQUATIONS A a H P S t V z

= = = = = = = =

constant exponential time constant thickness of the lithosphere shear stress vertical displacement due to subsidence time after beginning of subsidence vertical velocity of the convecting asthenosphere depth

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9.14

APPLICATION OF S T A T I S T I C S IN GEOTECHNICAL - AN EXAMPLE FROM EARTHQUAKE DATA

ENGINEERING

P.H. Morris University of Queensland

T h e application of empirical and semi-empirical relationships between earthquake parameters is fundamental to earthquake engineering studies. These relationships usually take the form of linear or near-linear correlations between two or more such parameters. However, the data on which such correlations are based often show considerable scatter about the mean relationship. It is also known that the measured values of all such parameters are likely to include errors of comparable value. In these circumstances, the simplified regression analyses in c o m m o n use, which assume at least one parameter to be free of error, can lead to significant errors in the estimated relationships. Consequently, this could result in poor estimates of confidence limits for such relationships, as they are applied to engineering design. In the Australian context, this situation is often exacerbated by the short and frequently poor quality seismological record. A general least-squares solution is available for simple linear relationships which accounts for the errors in both variables. This gives mathematically correct estimates of these relationships. Confidence limits remain restricted to special cases at this time. T h e methodology has been applied to earthquake data to determine functional relationships between selected parameters of each data set. Particular reference is made to relationships between Richter magnitude ( M L ) , intensity values ( M M ) and radius of perceptibility (Rp(III)). R e f : Morris, P . H . , Rynn, J . M . W . and Williams, D . J . , "Engineering Implications of the Linear Regression of Geotechnical Data", Civ. Engg. Trans., L E . Aust., Vol. C E 29, No. 4, 1987, p. 239-247.

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5.3 RADIOCARBON AND AMINO ACID RACEMISATION TIME FRAMEWORKS FOR LATE PLEISTOCENE AND HOLOCENE SEDIMENTATION, GULF ST VINCENT, SOUTH AUSTRALIA C.V. Murray-Wallace^ and A.P. Belperio^ ^The N.W.G. Macintosh Centre of Quaternary Dating, The University of Sydney ^South Australian Department of Mines and Energy

Gulf St Vincent is a shallow elongate marine incursion on the continental shelf of southern Australia. It is approximately 7000 km^ in area with a maximum water depth of some 40 m. Detailed radiocarbon and amino acid racemisation analyses were undertaken on molluscan fossils from two submarine vibrocores SV-4 and SV-5. The cores recovered 2.77 and 3.90 m of sediment respectively in water depths exceeding 36 m. Species sampled for dating included Katelysia rhytiphora, K. scalarina Fulvia tenuicostata, Callucina lacteola, Anapella cycladea, Brachidontes erosus, Circumphalus disjecta and Ostrea angasi. These species frequent subtidal to lower intertidal habitats. The molluscan fossils were generally well preserved, unabraded with articulated and disarticulated individuals represented. Two distinctive suites of ages were obtained for the two lithologically distinct marine units identified within the cores. Holocene ages characterised the upper 0.5 m of each core and Late Pleistocene ages for the rest of each core. A spread of finite ages ranging between 30,500 +650,-600 to 45,100 +5100, -3100 were obtained on the molluscs studied. Despite earlier suggestions to the contrary, recent investigations have provided stratigraphic evidence supporting the validity of radiocarbon ages for interstadial marine sequences. The integrity of radiocarbon dates reported in this study was supported by 13Q/12Q analyses. X-ray diffraction and the good concordance of ages for replicate samples. Amino acid analyses were undertaken for the 'total acid hydrolysate' and free amino acids. Results were obtained for a range of amino acids and include alanine (ALA), . isoleucihe (ALLO/ISO), leucine (LEU), valine (VAL) and aspartic acid (ASP). The relative extent of racemisation for different amino acids in the samples studied is in accord with that for molluscan fossils of equivalent age studied from the northern hemisphere. By analogy with the calibration samples, the sediments representing the majority of cores SV-4 and SV-5 are younger than Last Interglacial but older than Holocene (Fig 1). Systematic downcore increase in amino acid D/L ratios, consistent with increasing age was not evident within the Pleistocene sediments in cores SV-4 and SV-5. In contrast, radiocarbon dating delineated two distinct chronostratigraphic components which relate to the 30,000 and 40,000 yr, BP sea level highs of Oxygen isotope stage 3. These data are corroborated by micropalaeontological evidence. The results are significant in a global context, for few studies have successfully identified interstadial marine strata (Oxygen isotope Stage 3), from settings that have experienced a comparatively quiescent tectonic history.

290

IZ 09 08 07 0-6

• Anadara trapezia ^Katelysia rhytiphora °Katelysia scalarina ^Fulvia tenulcostata ^Katetysia peron'n ^Anapella cycladea ^ Donax(Plebidonax) delto/des

Older Pleistocene marine beds (Stage 7)

Glanville aminozone (Stage Be)

o 05 r! < OA —I

interstadial marine strata (StageS)

^0-3 02 01

St Kiida aminozone (StageV 0-1 02

1 1 0-^ 05

I I 06 07

D/L ASPARTIC ACID

09 VO

Figure 1 Scatter diagram illustrating the extent of aspartic acid racemisation against isoleucine epimerisation in molluscan fossils from Quaternary coastal and marine sediments in South Australia. An interstadial age is clearly evident for the molluscs from Gulf St. Vincent vibrocores SV-4 and SV-5.

291

5.2

QUATERNARY MARINE AMINOSTRATIGRAPHY PERTH BASIN, WESTERN AUSTRALIA

C.V. Murray-Wallace^ and R.W.L. Kimber^ ^The N.W.G. Macintosh Centre for Quaternary Dating, The University of Sydney ^CSIRO Division of Soils, Glen Osmond

2 The Perth Basin is a narrow, elongate structure with an area of some 55,000 km . The basin straddles the Western Australian coastline for some 900 kms starting near Northcliffe, south of Perth, and terminates inland from Shark Bay. The Darling Range defines the eastern boundary of the basin. The sedimentary fill ranges in age from Silurian to Modern. Quaternary sediments within the basin attain a maximum thickness of 150 m, although in most areas they are less than 20 m. Historically, studies of the Quaternary chronostratigraphy of the Perth Basin have been frustrated by complex lithostratigraphic relationships, homotaxis, absence of fossils suitable for dating in the terrestrial lithofacies and the irregular occurrence of the marine sediments. Furthermore, the widespread distribution of Holocene and Pleistocene aeolian dunes which effectively blanket the marine facies presents an additional complication in unravelling the Quaternary history of the basin fill. Numerous specimens of fossil molluscs were obtained for amino acid racemisation dating and include Katelysia rhytiphora, K. scalarina, Anadara trapezia and Bassina pachyphylla. The fossil bivalves were collected from several marine units including the Jandakot Member of the Yoganup Formation (Plio/Pleistocene), Peppermint Grove Member (Oxygen isotope stage 7) and Minim Cove Member (Oxygen isotope stage 5e), of the Tamala Formation and the Holocene Rottnest Limestone (Oxygen isotope Stage 1) from Rottnest Island. The Plio-Pleistocene Roe Calcarenite, sampled from Madura on the Roe Plains provided a basis of comparison with results for the Jandakot specimens. The ages of these deposits have been independently established based on radiocarbon and uranium series disequilibrium dating, electron spin resonance and macro- and micropalaeontological evidence. Where possible the hinge region in replicate samples was analysed. The extent of racemisation for the range of amino acids in the fossils studied varied systematically with increasing fossil age, as reflected with valine (Fig. 1). The extent of racemisation in the Last Interglacial and Penultimate Interglacial mollusc specimens compare favourably with the same species from coastal deposits of equivalent age in South Australia, that have experienced similar ^Effective Quaternary Temperature* histories. Results from this study will provide a valuable benchmark for subsequent amino acid racemisation studies as well as a useful adjunct to other geochronological investigations. The results also have implications concerning the nature of molluscan racemisation kinetics.

292

"u 09 < O) J:: 08 §07 X Q ^

0«

o Katelysia

rhytiphora

• Katelysid

scalar

DAnadara

trapezia

A Bassina

pachyphylla

PI 10/Pleistocene

///////////A

DEPOSITIONAL HIATUS

# •

c O

Penultimate Interglociol

y//////////A Lost

gOi

Interglociol

io3 ^0-2 —J o 01

•••

MINIM COVE MEMBER

MODERN HOLOCENE

120koBP

•o

PEPPERMINT JANDAKOT Holocene ROE GROVE CALCARENITE MEMBER MEMBER 225koBP

( P L I O / PLEISTOCENE)

Figure 1 Summary plot of valine D/L ratio data for Quaternary molluscan fossils from Western Australia. Data are for the 'total acid hydrolysate'.

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3.19

PALAEOMAGNETISM AND TECTONIC HISTORY OF THE SOLOMON ISLANDS ARC R.J. Musgrave Research School of Earth Sciences, Victoria University of Wellington, New Zealand

A palaeomagnetic survey of the eastern Solomon Islands included sampling in Malaita, Guadalcanal and the Florida Group. Positions of palaeomagnetic poles from Malaita indicate an Australian plate origin for this island, continuing the notion of arc polarity reversal, but refuting the conventional view of the "Pacific Province" as the uplifted/overthrust edge of the Ontong Java Plateau. Palaeolatitude studies on the Ontong Java Plateau have introduced ambiguity regarding its relation to the central parts of the Pacific Plate over the last 30 Ma; comparison with Malaitan palaeolatitudes, however, confirms the distinct origin of the plateau and the Pacific Province. Malaita is seen, instead, to have evolved in a fore-arc environment; an accretionary prism was either never developed or tectonically eroded. Magnetostratigraphy from Malaita suggests cessation of Pacific subduction at 5 Ma. Large tectonic rotations are required to explain results from Guadalcanal and the Florida Group. In contrast to expectations, the sense of these rotations is predominantly clockwise, although an earlier, anticlockwise phase of rotations is also indicated in Guadalcanal. The clockwise phase of rotation is associated with a latest Miocene displacement of Guadalcanal and San Cristobal westwards with respect to the rest of the arc. Replacement of these two islands to their former positions restores the arc to a single chain, and aligns a suite of ultramafic bodies. It is suggested that the ultramafics were emplaced in Late Eocene-Oligocene time along a medial fault taking up part of the arc-parallel component of convergence. An analogous system existing today is Sumatra. Break-down of motion on this fault system may have been the cause of the anticlockwise phase of rotation. The complex tectonics of the Late Miocene are thought to have been the result of a period of convergent subduction preceding the end of Pacific subduction at 5 Ma. Resistance to the consumption of the Ontong Java Plateau caused the Solomons Arc to take up part of the Pacific-Australia convergence, beginning at about 10 Ma, and so initiate subduction of the Australian plate. Detachment of the buoyant Ontong Java lithosphere from the lower parts of the downgoing Pacific plate allowed the Ontong Java slab to rise higher. Interaction between the Ontong Java slab and the thicker parts of the arc lithosphere produced the displacement of Guadalcanal and San Cristobal. Subduction of the Pacific plate ceased when the Ontong Java slab collided with, and deformed, the downgoing Australian slab. Current seismicity shows relict features from these events.

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6.3 OXYGEN AND HYDROGEN ISOTOPE STUDIES OF TIN DEPOSITS ASSOCIATED WITH A TOURMALINIFEROUS GRANITE AT COOKTOWN, NORTH QUEENSLAND I.A. Myers^, S.D. Golding^ and R.G, Taylor^ ^Geology Department, James Cook University of North Queensland ^Department of Geology and Mineralogy, University of Queensland

The two tin deposits at the Collingwood prospect are, respectively, an anastamosing, sheeted vein system, and a relatively massive greisen. The deposits are hosted by the Permian, peraluminous (two mica), boron-rich Finlayson granite which intrudes early Palaeozoic lower greenschist-facies metamorphosed sediments. Both the vein system and the greisen are wholly contained within the host granite with little alteration/mineralization of the surrounding metasediments. The greisen, composed of muscovite, quartz, tourmaline, topaz, cassiterite, and minor fluorite, chalcopyrite, and arsenopyrite is hosted by a porphyritic phase of granite and confined to cupolas in apical regions of the pluton. K-feldspathization and minor albitization of granite immediately surrounding the greisen is common. Six phases of vein mineralization have been identified. They are: 1) an early siliceous phase, 2) an albite phase, 3) a quartz-tourmaline phase, 4) an alkali feldspar phase, 5) a quartz-sericite phase, and 6) a tourmaline phase. Although cassiterite accompanies all six phases the early siliceous phase was the main mineralizing event. Late-stage non-mineralized clay veins cross-cut all previous phases. Fluid inclusion studies suggest the mineralizing fluids for both deposits were weakly to moderately saline (2 to 15 wt 7o ^ ^ ^ ^ equivalent), with temperatures of 400 C for greisen formation, and 300 C to 330^C for the vein system. Temperature of granite formation was approximately 650 C. Constraints imposed by andalusite formation in the contact aureole and the presence of primary muscovite suggest the Finalyson granite was emplaced at a level of 3.5 to 7.5 kilometres, at 1 - 2 kb pressures. Oxygen isotope systematics of mineral pairs from granite and greisen display the following features indicative of isotopic exchange under open system conditions, namely: 1) mineral-mineral fractionations are not consistent with geologically reasonable temperatures of equilibration, 2) several quartz-feldspar and quartz-sericite pairs exhibit reversed A values, and 3) the mineral pairs exhibit steep positive-sloped arrays in 6 - 6 plots which require local fluid rock-ratios 0.35 (Gregory, 1987). The cal^c^lated composition of the fluid depositing greisen and vein minerals (6 0 = 2 to 5 /oo, 6D = -40 to -80°/oo) is consistent with the spread of 6 0 values along the disequilibrium arrays. The moderately depleted oxygen isotope composition of this fluid and its 'magmatic' hydrogen isotope composition probably reflect interaction between the granite and a magmatic fluid phase prior to fracture-controlled deposition of tin and gangue minerals in greisen and veins near the roof of the pluton. Gregory, R.T., 1987, Mineral pairs as stable isotopic monitors of fluid-rock interaction"in the lithosphere: Spec. Pubis geol. Soc Aust., V. 13, in press.

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12.4 UPPER DEVONIAN IRIDIUM ANOMALIES AND THE FRASNIAN-FAMENNIAN BOUNDARY IN THE CANNING BASIN, WESTERN AUSTRALIA R.S. Nicoll^ and P.E. Playford^ ^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Geological Survey of Western Australia The initial report of an iridium anomaly in the Upper Devonian carbonate sequence of the Canning Basin of Western Australia placed the anomaly at the base of the Upper Palmatolepis triangularis Subzone and associated it with a bed composed of the cyanobacterium Frutexites. Examination of conodont faunas from five detailed sections measured across the Frutexites interval now place this bed at the base of the Palmatolepis crepida Zone which is younger than the suggested Frasnian - Famennian boundary. A second association of Frutexites and an iridium anomaly has been found in the Napier Range and has been dated using conodonts as representing the Ancyrognathus triangularis to Upper Palmatolepis gjgas Zones. From these results and others it seems probable that the iridium anomalies in the Canning Basin are related to organic concentration of the iridium by the cyanobacterium Frutexites and not to a meteoroid impact. Moreover, the age range of the anomalies shows that they cannot be directly correlated with a single extinction event at or about the Frasnian - Famennian boundary.

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6.5

C-O-S-H FLUIDS - REDOX REACTIONS AS A MINERALIZATION MECHANISM IN LAYERED MAFIC INTRUSIONS N.W.A. Odling Department of Geology, University of Tasmania

The Merenskv Reef As the host of the Merensky Reef the Bushveld complex is the single largest source of platinum group e l e m e n t s ( P G E ) in the world and as such has attracted much attention. The reef lies at the top of the 'Critical Zone' where it passes upward i n t o the b a s e of the ' U p p e r Z o n e ' . It consists of a conformable pegmatitic feldspar pyroxenite orthocumulate which is f l a n k e d top and bottom by chromite s t r i n g e r s . In a d d i t i o n to the main p y r o x e n e and feldspar mineralogy, a complex variety of interstitial, post-cumulus phases have been recognised. In the S . E . section of the Bushveld complex the dominantly conformable nature of the Merensky reef is disrupted by pothole s t r u c t u r e s where the r e e f cuts i n t o i t s footwall. The most unusual feature of the pothole structures is the association and abundance of g r a p h i t e , hydrous silicates and PGE minerals in their centers. Redox Indicators The chemistry of chromites (Buntin et a l . , 1 9 8 5 ) , the presence of carbon and native metals in and below the reef (Elliott et al.,1982) and high pressure CH. and H^S gas pockets indicate relatively reducing conditions at the time of reef f o ^ a t i o n . However, the presence of C0.-H20-NaCl i n c l u s i o n s in q u a r t z ( B a l l h a u s and Stumpf 1, 1986) and magn^ite in the strata above the reef would indicate that the reef occurs at thegboundary between domains of d i f f e r i n g redox s t a t e . On the b a s i s of Sr/ Sr ratios, Kruger and Marsh(1982) proposed that the reef marked the incursion of a new batch of magma i n t o the magma chamber. Thus it is proposed that this redox contrast is of critical importance in the petrogenesis of the Merensky Reef and that a COSH fluid phase is the major vehicle of PGE transport. Fluid Equilibria in the System C-O-S-H A model has been developed based on the method of French(1966) but augmented by the addition of factors- to account for the non-ideal nature of fluids at h i g h t e m p e r a t u r e and pressure. The results are plotted in an isobaric, isothermal diagram developed by F r o s t ( 1 97 9 ) , in which the e f f e c t s of o x i d a t i o n , r e d u c t i o n and carbon saturation on the fluid composition are clearly illustrated. Fig.l shows a 3-D representation in log(f02)-log(f$2)XC space of the carbon saturation surface for 5kb and 1250C i . e . an estimate for the P and T of formation of the Merensky r e e f . The v a r i a b l e XC is d e f i n e d as ( C / C + H2) . The diagram is divided into two regions by a carbon saturation surface 1. A pseudo-divariant, fluid-only region above and to the front of the carbon saturation surface. 2 . A ' f o r b i d d e n ' f i e l d where f l u i d compositions are metastable and must precipitate carbon to achieve equilibrium. With i n c r e a s i n g s u l p h u r fugacity the carbon saturation surface moves to lower values of XC in the reduced portion of the diagram, where H2S j o i n s the CH. and H^ as a component of the fluid. In the oxidized region of the diagram sulphur-i)earing components make up less than 0.1 mol% of the f l u i d and sulphur is present in condensed phases. Fluid Evolution at the Merenskv Reef In f i g . 1 point A represents an initial reduced fluid composition composed mainly of CH^ , H2 and minor H2S. At this point p l a t i n o i d s may have been

297

present as chloride complexes as Cl partitioned into the evolved fluid/melt. As crystallization of the footwall cumulates proceeded the r e s i d u a l melt would have become increasingly enriched in volatile components and as a consequence the sulphur fugacity rose though not enough to stabilize either b a s e m e t a l or PGE sulphides (point B) . Once the fluid mixed with the overlying oxidized magma it will quickly oxidized and saturated in carbon (points C-D) . With this oxidation the fluid chemistry changed dramatically from being CH.- H^- H2S dominated to a mixture of H2O and CO2. This process has several important consequences: 1. In the reduced region sulphur is present as H2S but on o x i d a t i o n the ability of the fluid to carry sulphur decreases rapidly even at high sulphur fugacities. 2. The high water activities achieved through oxidation would have resulted in a hiatus in crystallization and perhaps resorption of the cumulus pile. At locations where fluid flow was high resorption was enhanced leading to localized depressions in the crystal pile i.e. p o t h o l e structures. Carbon precipitation will be high and hydrous silicates stabilized. 3. PGE and base metals, initially transported as chloride complexes, would condense out as sulphides as the H2S component of the fluid decreases. General Applications The most important factor for this process to operate is the development of a redox contrast at a cumulate-pile/magma interface. It might seem from the above discussion that carbon saturation is a necessary consequence of these redox reactions, however if the fluid is initially carbon poor oxidation may lead to sulphide precipitation and high water activities without graphite precipitation. Thus this process could operate in a variety of c o n d i t i o n s and would result in thin pegmatoidal units in which sulphide concentrations are associated with hydrous silicates. Fluid inclusions should form during periods of fluid saturation and could prove a valuable exploration tool. As only the four component system C-O-S-H has been studied, the inclusion of Cl to the system would be a useful addition for future work. REFERENCES Ballhaus,C.G. & Stumpf1,E.F.,1985, Earth Planet Sci. Lett..74. 58-68. Ballhaus,C.G. & Stumpfl,E.F.,1986, Contrib. Miner. Petr., 94, 193-204. Buntin,T. et al.,1985, Econ. Geol.. 80, 975-987. Elliott,W.C. et al.,1982, Econ. Geol.. 77, 1493-1510. French.B.M.,Rev. Geophvs..4.223-253. Frost.B.R..Am. J. Sci..279.1033-1059. Krueger,F.J. & Marsh,J.S.,1982, Nature. 298, 53-55. Figure 1. 00

C-O-S-H FLUIDS P=5KB,T=1250C

25 -5 O

L0GfS2 298

8.26

SYNTHETIC HIGH TEMPERATURE, HIGH PRESSURE FLUID INCLUSIONS - A NEW TECHNIQUE N.W.A.

Department

Odling

of Geology, University of

Tasmania

The topic of mantle metasomatism has received much attention as an important precursor of magmatism in a variety of tectonic settings. Early speculations on the composition of the metasomatic fluid have centred on water-rich fluids as evidenced by the occurrence of amphibole in mantle xenoliths and more recently suggestions have been made that reduced fl^ids^ too, may be important in magma genesis (Taylor and Green,1988). The ubiquitous occurrence of high density pure CO2 inclusions in xenolithic material has long been a puzzle in that a wide range of fluid compositions might be expected if current estimates of the range of mantle fO^ are correct (J.D.Pasteris, 1 987 ) . As a part of a study of the phase relations of a model pyrolite-C-O-S-H system a technique has been developed for the preparation of high pressure synthetic fluid inclusions. Experimental Technique Plugs cut from inclusion-free, natural mineral crystals (olivine and quartz) were checked for absence of volatiles by infra-red spectroscopy then fractured by thermal stressing. The plugs were then loaded into precious metal capsules along with a buffer assemblage and organic fluid source. Mixed fluid compositions (CH.+ H^O dominated) were buffered by a WC/W02/W^/C mixture (log(fO^) = approx fw+l, log(fS2) = approx Fe/FeS+1) while pure Cll^ fluids were buffered by an Al^C2/Al26^ assemblage. Fluid compositions we: were analysed lercing/mass spectrometric technique: after quenching by a capsule pie P(kbar) 15 15 25 35 15

T(°C) 1175 1075 1175 1250 1200

Buffer wc/wo./ws./c WC/WO^/WS^/C WC/WO^/WS^/C WC/WO^/WS^/C Al^C37Al263

Duration(hrs) Host 24 Olivine 24 Olivine 24 Olivine 24 Olivine 72 Quartz

100[CH,/CH.+H.0] 37:6 ^ ^ 76.2 51.1 26.0 100.0

After removal from the capsule the mineral plugs were doubly polished optical, microthermometric and infra-red analysis.

for

Results After quenching, planes of inclusions are found where cracks had existed before the run. Inclusions vary in size from <l|im to 4 0 M.m in length with large,negative crystal shaped inclusions being more common at higher temperatures. At room temperature, inclusions hosted in quartz are single phase but all of the mixed fluid experiments in olivine produced multi-phase inclusions. In all of the olivine hosted inclusions a gas bubble is contained in a liquid in which there is also two solid daughter phases. The first is a large, high relief, deep red coloured crystal which microprobe analysis has shown to be dominated by W, thus the phase is thought to be WO2. The second consists of small, highly reflective spheres which adhere in a group to the gas bubble. Microprobe analysis of this phase was impossible but it is likely to be graphite and/or a sulphide phase. Microthermometry indicates that the liquid compohent is dominated by H^O and that the gaseous phase has significant CO^ and CH, component. Preliminary laser Raman spectroscopy of an inclusion from the l5kb/1175 run shows the vapour bubble to contain S 8 mol% CH^ and 12 mol% CO2. The presence of CO2 in the inclusion contrasts with the fluid composition as analysed by mass spectrometry.'

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Infra-red Spectroscopy I n f r a - r e d s p e c t r o s c o p y was c a r r i e d out on a Fourier transform infra-red spectrometer fitted with a microscope attachment. R e p r e s e n t a t i v e spectra are shown in- F i g . l . The spectra show strong absorbance^at wavenumbers less than -2500 cm"" due to an intense harmonic at 1 8 0 0cm" host o l i v i n e s p e c t r u m . S p e c t r a show a broad feature between 3650crn- and 3100cm with sharp bands at -SeiOcm" ,-32^0cm" , - 3 3 4 0 c m " ,'-3300cm" . A weak band is somtimes present at -2360cm . Discussion The group of bands between 3 6 5 0 c m " and 3100cm has been observed in a variety of olivine samples of mantle origin (Freund & Oberheuser^1980, Beran & p u t n i s , 1 9 8 3 , Kitamura et al.,1987) and has been ascribed to OH both nonspecific and structurally bound on defect sites in the olivine lattice. Tt is p r o p o s e d that fluid as analysed by mass spectrometry permeated the fractures in the mineral host. However, as soon as portions of the cracks were s e a l e d the i n f l u e n c e of the b u f f e r on.the included fluid was lost. In annealing, recrystallized material rich in OH" is deposited to g a i n minimum s u r f a c e energy. As oxygen is already present in the olivine lattice the included fluid is progressively depleted in H and e n r i c h e d in C. I f g r a p h i t e f a i l s to n u c l e a t e or equilibrate with the fluid a metastable mixed H20-CH^-C02 fluid will result. Thus the inclusion is gradually oxidized and as hydrogen has been shown to d i f f u s e s e v e r a l orders of magnitude faster than any of the other lattice components (Makwell,1985), the process described above may c o n t i n u e a f t e r the inclusion has gained a stable form. The inclusions fopied in quartz from a pure methane fluid show a very intense IR band at 2360cm indicating that COp is abundant. Since the duration of this run was 72hrs it may be that the process described above has continued until little if any CH^ i s l e f t in the inclusions. As CO^ is the end point of the above process it is concluded that the fluids found in inclusions in mantle xenoliths may not n e c e s s a r i l y represent pristine samples of mantle fluids. References. Beran,A. & Putnis,A.,1983, Phvs. Chem. Minerals, 9, 57-60. Freund,F & Oberheuser,G,1986, J. Geophvs. Res.. 91, 745-761. Kitamura,M. et a l . , 1 9 8 7 , Nature. 328, 143-145. Makwell,S,J, et a l . , 1 9 8 5 , J. Geophvs. Res.. 90, 11319-11333. Pasteris,J.D.,1987,In : Mantle Xenoliths ed. P.Nixon. Wiley Taylor,W.R. & Green,W.R. , in p r e s s . In : K i m b e r l i t e s and R e l a t e d R o c k s , Geological Society of Australia Special Publication. Figure 1.

3000 Wave NuirLer

300

2000

-1 (cm

)

8.36 bo AND ILLITE CRYSTALLINITY STUDIES OF K-WHITE MICAS IN ROCKS FROM FOREARC BASIN AND ACCRETIONARY COMPLEX SEQUENCES, SOUTHERN NEW ENGLAND FOLD BELT, NEW SOUTH WALES R. O f f l e r , M. Hand and R. Bale Department of Geology, The U n i v e r s i t y of Newcastle

In the Southern New England Fold Belt, N.S.W,, b studies of K-white micas have shown that the forearc sequences of the Tainworth Belt and accretionary conplex sequences of the Tablelands Coirplex, have distinctive metamorphic iirprints. The white micas in the metamorphosed sediments of the forearc basin record low pressure conditions, whilst those in the accretionary conplex sequences show an intermediate to high pressure character. These results substantiate the plate tectonic models proposed for the Southern New England Fold Belt. I l l i t e c r y s t a l l i n i t y analyses have revealed that rocks of diagenetic to lower anchimetamorphic grade are developed in the forearc succession, and diagenetic to epimetamorphic grade in the accretionary complex; abrupt changes in grade occur across f a u l t s in the accretionary carplex indicating a highly disrupted t e r r a i n . The analyses also indicate that the Peel Fault System i s not a major zone of thrusting.

301

8.31

EXTRA HIGH GRADE (?) METAMORPHISM IN THE NORTHWEST GAWLER CRATON, SOUTH AUSTRALIA R.L. Oliver, A.J. Purvis and M.J. Taylor

Department of Geology and Geophysics, University of Adelaide

Gneisses fron drill holes OR? 1 and Ooldea 2 can be regarded as part of the northwestern fringe of the Gawler Craton. The nearest outcrops, 40 km east, and 50 km southeast margining Lake Ifould, are mapped as 2300-2500 Ma Mulgathing Carplex and 1670-1721 Ma Lincoln Caiplex, respectively. The rocks from ORP 1 and Ooldea 2 contain varying proportions of quartz, magnetite, hematite (with ilmenite exsolution lamellae), sillimanite, hypersthene, garnet, sapphirine, cordierite, biotite, spinel, plagioclase and K feldspar. Quartz, iron oxides and sillimanite are the most abundant phases. The minerals display a great diversity of association, reflecting the COTplex textural evolution of the rocks. Based on ccaisideration of the Fe-Mg-Al-Si-O petrogenetic grid by Hansen (1986) and on further consideration plus petrographic data by Powell and Sandiford (in press), the assenblages sillimanite-hypersthene-quartz and sapphirine-quartz, plus coexisting magnetite and hematite and abundant spinel, are indicative of maxiirum crystallisation tenperatures of ca 1000°C at pressures in the vicinity of 7-9 kbars and a(02) near the magnetite-hematite buffer. Reaction textures suggest that the sillimanite-hypersthene and sapphirine have grown at the expense of prograde magnetite and, perhaps, spinel and cordierite. Using the petrogenetic grid referred to above, such reactions can be shown to infer cooling at constant pressure at a depth of 40-45 km in the crust. The tectonic significance of this is discussed. Hensen, B.J. 1986. Theoretical phase relations involving cordierite and garnet revisited: the influence of oxygen fugacity on the stability of sapphirine and spinel in the system Mg-Fe-Al-Si-O. Contributions to Mineralogy and Petrology, 92, 362-367. Powell, R. and Sandiford, H. (in press). Sapphirine and spinel phase relationships in the system Fe0-Mg0-Al203-Si02-Ti02-02 in the presence of quartz and hypersthene.

302

17.4

HIGHLIGHTS OF A MANTLE PERSPECTIVE ON EASTERN AUSTRALIA S.Y. O'Reilly^and W.L.

^CSIRO

PHANEROZOIC

Griffin^

^ S c h o o l of E a r t h S c i e n c e s , Macquarie University, North Ryde D i v i s i o n of M i n e r a l P h y s i c s a n d M i n e r a l o g y , North Ryde

The combination of petrologic, geochemical and petrophysical information on xenoliths in basaltic rocks, geothermobarometry techniques, and geophysical data (seismic reflection and refraction, and MAGS AT data) has resulted in a detailed and integrated model for the lithology and geometry of the lower crust, upper mantle, crust/mantle boundary (CMB) and Moho in eastern Australia (e.g. Griffin & O'Reilly, 1987). Geochemical and geochronological data on these xenoliths provide a basis for interpreting the Phanerozoic tectonic evolution of eastern Australia. The CMB in eastern Australia occurs at depths ranging from 25 to 40 km in different regions and is defined by the depth at which ultramafic rocks (spinel Iherzolites) become volumetrically significant. The lower crust wall rocks are mafic to felsic granulites. There is a zone from about 15-40 km depth with abundant mafic lens-like intrusions (some re-equlibrated to granulites) which correspond to horizontal reflectors seen on seismic reflection profiles. The CMB lies well within this zone, but the Moho (as defined by seismic refraction data) is commonly located at the base of this zone. Reversed seismic profiles in southeastern Australia demonstrate that the refraction Moho lies about 55 km. Petrologic and P/T data show that this does not coincide with the CMB but probably represents the spinel - to garnet-lherzolite transition. Calculated and measured Vp*s for samples of mantle rock types from eastern Australia are consistent with this interpretation. The demonstrated significant lowering of Vp (due to modal pyroxene in mantle rocks coupled with the high geothermal gradient indicated by P/T calculations), invalidates the practice of using dunite's physical properties in modelling the seismic character of this mantle. Mafic granulites occur as xenoliths in host rocks from Recent to Jurassic in age. These granulites were derived mainly from levels between 15-35 km, but some come from depths up to 75 km. Therefore, some lie within the mantle. The protoliths of the mafic granulites were derived by fractionation and/or freezing of basaltic melts. Sr and Nd isotopic data suggest at least 3 different petrogenetic histories for granulite xenoliths of different ages and from different regions. Some are formed by the simple process of basaltic intrusion around the CMB and are closely related to the Jurassic to Recent volcanism. Some show evidence of mixing with a crustal-type component. Others (confined to the Jurassic host rocks) suggest a complex history involving a subduction component. Complex histories with regional variations also characterize upper mantle volumes. The western Victorian mantle samples carry geochemical signatures of at least 3 metasomatic episodes. The oldest event (300-500 Ma) involves addition of material which can be identified geochemically as a subduction component. Another is clearly associated with Tertiary-Recent basaltic activity. The remainder may represent degassing from a reservoir which has retained primordial characteristics. The lithospheric mantle beneath Queensland is geochemically different from that in western Victoria. Preliminary data suggest that similar events have affected the Queensland mantle, but the relative importance, timing and origin of the metasomatism there appear to be distinct. Both geochemical data and evidence for fluid activity in the mantle give information on the mechanisms of metasomatism. The metasomatism is attributed to the infiltration of C02-rich fluids (some sulphide-bearing) which are observed in fluid inclusions and inferred to have filled large vugs constituting up to 3% by volume of some mantle rocks. Most of the fluid was probably introduced during the intrusion and

303

crystallization (in the mantle) of basaltic magmas, now represented by veins and lenses of pyroxenite. Contrary to conventional lore, the metasomatism does not generally result in higher K and Ti. The distribution of trace elements in the Iherzolite wallrocks is controlled by the crystal chemistry of clinopyroxene and the metasomatic amphiboles, micas and apatites. The variable distribution of these volatile-bearing phases in space and time results in a decoupling of major, minor and true elements during metasomatism, mainly reflecting crystal/fluid partitioning. Griffin, W.L. & O'Reilly, S.Y. (1987) Is the continental Moho the crust-mantle boundary? Geology, 15, 241-244.

304

6.15 RECENT MINERALOGICAL INVESTIGATIONS ON TERRESTRIAL AND MARINE MANGANESE OXIDES OF THE AUSTRALIAN REGION J. Ostwald The Broken Hill Pty Co. Ltd Central Research Laboratories, Wallsend

Weathering of manganese-containing rocks on land and precipitation from sea, lake and river water under appropriate redox conditions produces an assemblage of essentially tetravalent manganese oxides of complex and variable chemistry and, quite commonly, of poorly-understood crystal structure and genesis. Current research on terrestrial and marine manganese oxides from the Australian region has added significantly to the understanding of this field of mineralogy. Manganese oxide minerals are based on [Mn^"^05] octahedra, which are linked by corner and edge-sharing to produce a variety of densely-packed parallel chains; chains with internal voids (tunnel structures), and sheet structures (phyllomanganates). "Pure structures" are the exception rather than the norm, and most natural examples exhibit "coherent-lattice intergrowth" or "hybrid structure" ie structurally-aligned crystal domains which differ in chemical composition and structure. Hybrid structure in the manganese oxides was first recognized three decades ago, when the battery-active manganese oxide nsutite (gamma Mn02) was shown to be an atomic scale intergrowth of pyrolusite (tetragonal Mn02) and ramsdellite (orthorhombic Mn02). Although extensive deposits of ferromanganese nodules on the ocean floor have been known since the voyage of the "Challenger" (1872-1876), the mineralogy of these nodules is still not completely resolved, largely because of their poor crystallinity. X-ray Diffraction studies in 1956 by the Swiss chemist W. Buser indicated that these contained two major phases, "lOA manganate" and "6Mn02". Since this date there has been continued debate as to whether marine " l O A manganate" is equivalent to the terrestrial tunnel structure todorokite (Ca, Na, K, Ba)(Mg, Mn2+)Mn50i2 .XH2O or to a group of synthetic manganese oxide sheet structures, termed synthetic buserites (after the chemist). Recent studies on a "lOA manganate" from the sea floor near Manus Island, S.W. Pacific, indicate that this is a sheet-structured, natural buserite, formed by submarine volcanic activity. Most examples of marine nodule "lOA manganate" are hydrogenous precipitates of finely-crystalline todorokite. Determination of the crystal structure of marine (Tasman Sea) and terrestrial (Groote Eylandt, N.T) 6Mn02 (vernadite) by X-ray Diffraction (XRD) has so far proved impossible, but studies by Extended X-ray Absorption Fine Structure (EXAFS) methods have shown it to contain only edge-shared octahedra, and thus to be a sheet structure (phyllomanganate).

305

The nature of the terrestrial phyllomanganates birnessite (Na, Ca, K)(Mg, Mn) Mn60i4.5H20, lithiophorite (Al, Li)Mn02.(0H)2, chalcoohanite (Zn, Fe. Mg, Mn) Mn307.3H20, asbolane [Mn^+02-x(0H)x]^'^[Rf±y(0H)2-2y+x]^~ where R^"^ is Ni, Co, Ca; x and y < 1 and vernadite Mn02.(R20, RO, R203)x«yH20, where R is typically K, Na, Mg, Ca, Ba, Fe, has also been investigated, and these have been shown to be non-stoichiometric, often highly variable in their chemical composition and, in most cases, to occur as complex hybrid structures. Recent discoveries of magnesium-replaced zinc chalcophanite (from the Balfour Shale in the Manganese Subgroup of the Proterozoic Bangemall Basin, Western Australia) and a heterogenite (CoO.OH) -like component in lithiophorite from Western Queensland confirm the above concepts. Terrestrial examples of the marine phyllomanganate buserite Na4 Mni4 O27.2IH2O have not been discovered, possibly because they are unstable and readily dehydrate in air to birnessite. A specimen of manganese oxide, from a Manganese Subgroup Bangemall Basin dolomite, appears to be a magnesian buserite partially transformed to birnessite.

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8.16 LATE P A L A E O Z O I C MAGMATISM I N NORTHEASTERN QUEENSLAND I N C I P I E N T R I F T I N G A T A P A S S I V E M A R G I N O F GONDWANALAND?

B. Oversby Bureau of Mineral Resources, Geology and Geophysics, Canberra

Voluminous mid to late Carboniferous "transitional" magmatism of the Coastal Ranges Igneous (or North Queensland Volcanic and Plutonic) Province between approximately Broad Sound and Torres Strait (including southernmost Papua) overprinted several previously cratonised tectonic elements. Dominant products were broadly cogenetic granitoids and subaerial dacitic to rhyolitic ignimbrites, associated with single to composite subsidence structures. Predominantly continental late Devonian to early Carboniferous mafic to (mainly) intermediate igneous rocks which might represent, or be equivalent to, a premonitory episode of stratovolcano buildup, as in southwestern North America, are common only in the Drummond Basin and Connors Arch areas. Succeeding early to mid(?) Permian magmatism was typically less voluminous than that of the Carboniferous, except in northeastern areas (previous Hodgkinson Province), where major loci of activity apparently extended farther east than earlier intrusiveextrusive centres, although only plutonic rocks are seen at the present level of exposure. Many preserved extrusive sequences contain products of diverse eruptive processes with only subsidiary to minor small-volume ignimbrites. Sequences also tend to be more markedly bimodal in composition than Carboniferous ones. Concomitant subsidence structures, where present, are single and simple. A possible significant exception to these generalisations is the youngest preserved (northwestern) sequence of ignimbrite-dominated Featherbed Volcanics which might be younger than adjacent more heterogeneous, probable early Permian, Nychum Volcanics. Broadly comparable magmatism continued intermittently through Triassic and Cretaceous times in the southeast, and similar Mesozoic rocks might be common in areas of submerged continental basement (mainly Marion and Queensland Plateaus) offshore. Overall orientations of Carboniferous and Permian intrusive-extrusive features in northeastern Queensland suggest regional horizontal minimum principal stresses trending approximately east-west and northeast-southwest respectively. Local to semi-regional perturbations in orientations and magnitudes of one or more principal stresses, and to some degree in types and volumes of magmatic products, can be attributed at least in part to variable responses of upper crustal lithologies and structures within and/or between older provinces to the imposed late Palaeozoic tectono-magmatic activity. Perturbations tend to be most marked in the vicinity of major dislocations such as those represented by the present Palmerville and Millaroo Fault zones. Regionally, bulk crustal thickness and/or density apparently remained essentially constant throughout late Palaeozoic time at approximately current value/s. By themselves, geochemical data from late Palaeozoic intrusive-extrusive rocks in northeastern Queensland apparently do no, and probably should not be expected to, unambiguously discriminate between possible tectonic settings because of ubiquitous sialic crustal contamination signatures. Such contamination effects would be likely to occur in even the most mafic material (generally andesite-diorite) available in sufficiently wide distribution and large quantity to provide statistical reliability. However, the widespread development of a broadly cogenetic association of voluminous granitoids and ignimbrites with significant subsidence structures throughout late Palaeozoic time, and a lack of evidence for contemporaneous (or subsequent) region-wide compressional deforTnation, are together suggestive of an extensional tectonic regime. Apparent compressional (transpressional?) folding and faulting in areas such as the Bundock and Clarke River Basins, and possible strike-slip movement

307

on components of the Palmerville Fault system, were probably reflections of extreme though only local and transient/intermittent upper crustal stress perturbations within the framework of overall extension. Trends of extensional features, especially dyke swarms and subsidence structures, are essentially parallel to the present coastline and continental margin, and to the most likely trend of any late Palaeozoic subduction zone. These relationships, plus inferred lack of appreciable crustal thickening and/or underplating, are not consistent with an interpretation of setting as Andean intra-arc. Location in an extending back-arc environment is likewise not supported because of complete lack of evidence for any other subduction-related elements of appropriate age within or east of the region, even though a good case has recently been made for contemporaneous subduction in the New England fold belt farther south. Currently available data are consistent with an interpretation of the site of northeastern Queensland as part of a moderately extending passive continental margin during late Palaeozoic time. Even though no single data set provides "proof" of this interpretation, none at present demonstrably compromises it. During the Carboniferous and Permian, Australia and its offshore continental extensions lay at the trailing edge of Gondwanaland as it changed longitude rapidly towards and over south rotational and magnetic poles. Margins of the supercontinent would have been subjected to tensional stresses as part of an adjustment to decreased surficial curvature of a non-spherical Earth at high latitudes. While such tensions need not have had any major consequences in their own right, even at passive margins, the inherent instability and tendency towards extension introduced in consequence of them might have initiated and/or been exacerbated by ascent of thermal and asthenospheric plumes into the lower crust. Such "hot-spots" are believed to have been responsible for the observed active-type subsidence structures which mostly propogated northwards in sympathy with the southward movement of Gondwanaland. Northward propagation of the structures mimicked late Palaeozoic migration of glacial centres across western Gondwanaland. These and other direct and indirect consequences of extensional "membrane tectonics" would probably have been largely masked in environments of active plate consumption by the more conspicuous manifestations of subduction; such could have been the situation in much of eastern Australia to the south of northeastern Queensland. Assuming the validity of recent analyses of New England late Palaeozoic tectonics, the scenario developed above implies separation of the site of northeastern Queensland from the remainder of eastern Australia by a major transform zone during that interval of time, and possibly subsequently. The late Palaeozoic magmatism in the region is envisaged as the principal expression of a very early and tentative episode of passive margin extension which discontinuously but progressively intensified and focussed eastwards then and subsequently. A climax was reached during the early Tertiary with rifting and initiation of seafloor spreading in what was to become the Coral Sea. The rather enigmatic occurrence of high-grade Barnard Metamorphics in the eastern Hodgkinson Province might represent a metamorphic core complex which developed concurrently with uplift and exposure of exclusively batholithic late Palaeozoic levels in immediately adjacent areas as extensional tectonism moved towards this climax.

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4.3 GEOCHRONOLOGY OF A RAPID 1.85-1.86 Ga TECTONIC TRANSITION - HALLS CREEK OROGEN, NORTHERN AUSTRALIA R.W. Page^ and S.L. Hancock^ ^Bureau of Mineral Resources, Geology and Geophysics, ^Western Mining Corporation, Darwin

Canberra

Early Proterozoic erogenic rocks of the Halls Creek orogen of Western Australia are characterised by widespread, ensialic orogeny and magmatism, and linear, post-tectonic igneous complexes several hundred kilometres long. Conventional U-Pb zircon techniques are applied to erect a precise chronological framework quantifying the tectonic evolution of ^is fold belt. Volcanism associated with early rifting (Ding Dong Downs Volcanics, lowermost Halls Creek Group) whose age has not yet been determined, is followed by a quartz-rich, arenaceous formation, and in turn by fine-grained, variably volcanogenic, phyllitic deposits with abundant carbonates (Biscay Formation, --middle Halls Creek Group). The felsic to intermediate composition tuffs occuring in the Biscay Formation are considered to be coeval with stratabound, high-level "rhyolitic" sills in the same parts of the sequence, and if so, the sill's U-Pb zircon age of 1856 ± 5 Ma closely dates the time of deposition. However, confirmation of this interpretation depends on substantiation of this inferred coeval link between the dated sill and the Biscay Formation tuffs, and until such, the 1856 ± 5 Ma result remains a minimum for the age of the Halls Creek Group. The felsic tuffs, themselves, contain a zircon population dominated by brown euhedral morphologies whose U-Pb age would normally be interpreted as dating their igneous crystallization. However, much of the population includes grains centred with minute, older zircon cores. This is evident in the U-Pb data which show extreme inheritance (>2.0 Ga) contributed from the relic zircon components, and precluding the determination of a meaningful stratigraphic age by conventional methods. This resurgence of igneous activity (1856 ± 5 Ma) evidenced in the upper-middle Biscay Formation is associated with renewed thermal rifting, following the relatively quiescent phase of carbonate and chert deposition (associated with minor stratabound volcanogenic base-metal mineralization in the lower-middle Biscay Formation). Both the tuffs and sills have distinct incompatible trace element compositions, with large enrichments in Zr, Nb and Y, and are regarded as second stage A-type melts derived from a Rb-depleted granulite source. This igneous activity is also concomitant with emplacement of large volumes of MORB-style tholeiitic Woodward Dolerite, and the onset of quartzo-feldspathic turbidite deposition, which begins in the upper part of the Biscay Formation and continues into the overlying Olympio Formation. Subsequent compressional orogeny of the basinal trough sediments was accompanied by deformation, high-temperature polymetamorphism, and syn-tectonic plutonism. Anatectic pegmatite, believed to have formed as a melt product of granulite-facies metamorphism, has a U-Pb zircon age of 1854 ± 6 Ma. This age for high grade metamorphism is in agreement with reassessed Rb-Sr whole-rock ages measured previously. It closely controls the timing of the Barramundi orogeny as being somewhat younger in this subprovince than elsewhere in northern Australia, and implies a relatively short interval, of only several million years or less, between supracrustal deposition and deep crustal orogenesis. A better constrained and even more rapid tectonic transition is evident from the Whitewater Volcanics - late-tectonic, felsic volcanism dated at 1850 ± 5 Ma. The onset of this unconformably younger volcanism is marked by a concomitant change in tectonic style. This changeover from a pre-cratonic to cratonic setting took place within a few million years, a time scale comparable to that of Tertiary and Quaternary tectonic settings.

309

8.18

THE INTRUSIVE HISTORY OF THE NORTHERN BOWEN BASIN TECTONIC IMPLICATIONS C.I. Pattison^, L.H. Hamilton^, R.L. Hammond^ and C.W. Mallett^ ^"Queensland Institute of Technology ^CSIRO Division of Geomechanics, Indooroopilly

Located in Eastern Central Queensland, the B o w e n Basin hosts l o c a l l y t h i c k a c c u m u l a t i o n s of E a r l y P e r m i a n to T r i a s s i c sediments. The basin broadens and deepens to the south along a NNW-SSE trending axis and tapers to an apex in the north. It is continuous, beneath cover of the Surat Basin, with the Gunnedah and Sydney basins. Recent interpretations of the structuralstratigraphic evolution of the region are outlined by Hammond & Mallett (this volume). The pattern of intrusion is dominated by the presence of sills w h i c h p r e f e r e n t i a l l y intruded coal seams at all stratigraphic levels. Their distribution is confined to two major zones (fig. 1), of which the most extensive stretches from the basin apex to Moranbah. Concentration of intrusives, which range from basic to acid in c o m p o s i t i o n , is highest w i t h i n this n o r t h e r n zone and progressively decrease to the south. The relatively shallow level strata occupying the Collinsville Shelf are less intruded than the thicker sequences of the Nebo Synclinorium. Structures (e.g. t h r u s t s ) w i t h N / N N E o r i e n t a t i o n s are c o n s i d e r e d to fundamentally control the distribution of sills. W i t h i n the central zone, extending from N o r w i c h Park to G e r m a n Creek, sill intensity increases toward the south and compositions are generally syenitic. This zone is characterised by abundant m i n o r faulting. The form and distribution of sills and their relationship to structure indicate that magma migrated westward from the N N E trending Jellinbah and Foxleigh Faults. Apatite fission track age determinations on sills and host sediments in the region indicate a minimum Early Cretaceous emplacement age (Marshalsea & others, 1985). An a n o m o l o u s s w a r m of Early C r e t a c e o u s t e s c h e n i t e d y k e s , o r i e n t a t e d p a r a l l e l to m i l d c o m p r e s s i o n of similar age, deliniates the southern extent of silling. Excluding some rare occurrences, no such s w a r m is encountered until Eastern Creek in the north. Isolated plutonic bodies occur sporadically in the northern part of the basin. Most are granodiorites and those dated yield Early Cretaceous K-Ar and fission track ages (Webb & M c D o u g a l l , 1968; S.Marshalsea, pers. comm.). These intrusives and P o s t - P e r m i a n Plutonic bodies east of the Connors Arch display an alignment with major regional morphological boundaries w i t h i n the basin. These features correspond with the porphyry-Cu truncation zones of Horton (1978). These deep seated structu];es were fundamental in controlling the t e c t o n o - t h e r m a l evolution of the basin. It is likely that such features originated during an Early P e r m i a n extensional phase (Zoilkowski & Taylor, 1985; Hammond & Mallett, this volume). The envisaged d e v e l o p m e n t of half-graben c o m l e x e s w a s confined to 310

distinct domains deliniated by transfer faults (Gibbs, 1984) which subsequently became corridors of concentrated structure and intrusive activity. Subsequent reactivation of some half grabens was initiated during a Late Permian to Mid Triassic compression, culminating in a m a j o r t h r u s t event. G r a n i t o i d and sill e m p l a c e m e n t w a s associated with reactivation of earlier extension and thrust derived structures during mild compression occuring in the Early Cretaceous. This event was arguably associated with much of the intrusive activity, and the emplacement of ENE trending dykes controlled by it. Late Cretaceous and Tertiary extension, culminating in the opening of the Coral Sea, appears to have initiated the extrusives of the Nebo, Clermont and Springsure basalt Provinces.

References Gibbs, A.D., 1984, Structural evolution of extensional margins. J. Geol. Soc. Lond., 141, 609-620.

basin

Horton, D.J., 1978, Porphyry-type Cu-Mo mineralisation belts in Eastern Queensland. Qld. Govt.Min. J., 79,474-489. Marshalsea, S.J., Green, P.F., Buddy, I.R. & Gleadow, A.J.W., 1985, The thermal history of the southern Bowen Basin. Geol. Soc. Aust., Abs., 17, 109-113. Murray, C.G., 1985, Tectonic setting of the Bowen Basin. Geol. Soc. Aust., TUDS. , 17, 5-16. Webb, A.W. & McDougall, I., 1968 , Geochronology of the igneous rocks of Eastern Qld.," J. Geol. Soc. Aust., 15(2), 313-346. Ziolkowski, V. & Taylor, R., 1985, Regional structure of the north Denison Trough. Geol. Soc. Aust., Abs., 17, 129-137. 311

6.16

ZEOLITE DEPOSITS IN NEW SOUTH WALES COMMERCIAL PROSPECTIVITY S.R. Pecover

Geological Survey of New South Wales Zeolite minerals found in New South Wales occur in rocks of diverse age, type, and geological setting. Three separate geological environments in which potential economic deposits of zeolite minerals may occur have been recognized in New South Wales. These include: 1 2

Zeolite-bearing acid tuffs of the Carboniferous Tamworth Belt. Zeolite-bearing intermediate to basic tuffs associated with Tertiary lacustrine diatomite deposits occurring in the Nandewar and Warrumbungle Ranges. Zeolite-bearing basic tuffs and lavas associated with Triassic, Jurassic, and Tertiary volcanics occurring in the Warrumbungle and Liverpool Ranges.

The most widespread occurrence of zeolite minerals in the settings listed above is associated with dacitic and rhyodacitic ignimbrites and their epiclastic equivalents, within the continental Carboniferous Tamworth Belt. These rocks are also considered to be the most prospective for the discovery of economic concentrations of zeolite minerals in New South Wales and hence will be the focus of this abstract. The Tamworth Belt forms the western part of the New England Orogen. It is considered to have developed during the Devonian and Carboniferous as a region of shelfal deposition, between a volcanic arc in the west and a deepwater accretionary wedge of sediment in the east. There is widespread evidence that the volcanic arc was extremely active throughout most of the Carboniferous and supplied large amounts of volcaniclastic materials to both marine and non-marine depositional environments. The volcanism changed in composition over time from andesitic during the Devonian to dacitic in the Early Carboniferous and then to mainly rhyolitic in the Late Carboniferous. As deposition proceeded, outflow ignimbrite sheets and associated ash-fall pyroclastic and epiclastic deposits were progressively interbedded with various types of near-shore marine and terrestrial sediments. The Tamworth Belt can be divided into two main geographic northern Tamworth Belt and the southern Tamworth Belt.

parts,

the

In the northern Tamworth Belt, highly prospective zeolite-bearing Carboniferous strata crop out within two major coaxial synclines. These include the Rocky Creek Syncline and the Werrie Syncline. Formations or units in the Rocky Creek Syncline in which zeolite minerals have been widely recorded include the Caroda Formation, Clifden Formation, Rocky Creek Conglomerate, and the Lark Hill Formation. In the Werrie Syncline, the Currabubula Formation appears to be the most prospective formation for economic concentrations of zeolite minerals. Highly prospective zeolite-bearing rocks occurring in the southern Tamworth Belt have been extensively faulted into three major structural blocks. 312

These blocks comprise the Rouchel Block, the Gresford Block, and the Myall Block. Formations or units within these structural blocks in which zeolite minerals have been widely recorded, and which are considered to be highly prospective, are shown below: Rouchel Block

Gresford Block

Myall Block

Seaham Formation Paterson Volcanics Mt Johnstone Formation Chichester Formation Isismurra Formation

Seaham Formation Paterson Volcanics Mt Johnstone Formation Chichester Formation Gilmore Volcanic Group Isismurra Formation

Nerong Volcanics

Throughout the Tamworth Belt, zeolite minerals are widely developed as cementing agents and as replacements of vitric clasts in both terrestrial and shallow marine sedimentary rocks. However, the highest concentration of zeolite minerals are generally in the acid to intermediate pyroclastic and epiclastic rocks that are interbedded with these sediments. Acid to intermediate pyroclastic rocks within the Tamworth Belt consist of three main types. They include welded and non-welded ignimbrite ash-flow units and ash-fall tuffs. All three types of pyroclastic deposits show evidence of having contained high vitric contents at the time of their deposition. However, alteration of these vitric components to zeolite minerals appears to have occurred mainly in the non-welded ash-flow pyroclastic units and in the ash-fall tuffs. Vitric components in the welded ignimbrites generally show evidence of devitrification to quartzo-feldspathic assemblages. Two major inter-related factors appear to have influenced the formation of zeolite minerals in these rocks. They include the porosity of the original host rocks and the availability of water both during and after emplacement. A further indication that water has played a major role in the zeolitic alteration of pyroclastic rocks within the Tamworth Belt is the typical red to reddish orange and pink colour of zeolite-rich rocks. This colouration is due to a high ferric iron content in these rocks, which suggests that the alteration of silicic glass to zeolites probably took place under conditions of diffusion-controlled hydration and alkali ion exchange in an oxidizing, water-saturated environment. Typical zeolite phases in these rocks include mordenite, and laumontite.

clinoptilolite/heulandite,

The highest concentrations of zeolite minerals occur in ash-fall tuffs that appear to have been laid down in lakes, fluvial overbank environments or in shallow marine embayments. Clinoptilolite contents in these rocks can be as high as 9056 with corresponding ammonium exchange capacities of up to 140 meq/100g. While these values are not as high as some western USA zeolitic rocks, they are comparable to Japanese and Hungarian rocks that are presently being commercially exploited. However, New South Wales Carboniferous zeolitic rocks examined so far are harder and less porous than material being mined overseas. This results in a lower specific area available for reaction and indicates that fine grinding may be required. Finely ground zeolite may be well suited for such uses as pelletized stock feeds, soil conditioners, odour control reagents, pesticide and herbicide carriers, and in anticaking applications. 313

17.15

CAINOZOIC MAAR VOLCANISM AND THE ORIGIN OF SAPPHIRE AND POSSIBLY DIAMOND IN EASTERN AUSTRALIA S.R. Pecover Geological Survey of New South Wales

Gem-quality corundum megacrysts (and rare diamond) are associated with many Cainozoic basalt terrains in eastern Australia. Similar occurrences of gem corundum are associated with basaltic terrains in southeast Asia, southeast China, and Nigeria. In the past, slightly to moderately evolved basanites and alkali olivine basalts containing rare corundum megacrysts have been described as the probable major source rocks for the alluvial corundums in these regions. However, while corundum has occasionally been observed as xenocrysts in basaltic boulders in these regions, it has rarely been observed as an in situ component of the basaltic lavas. This is despite the fact that many sapphire-bearing streams draining these rocks have exceptionally high concentrations of corundum, zircon, pleonaste, and ilmenite at various localities along their routes. Recently, extensive deposits of deeply weathered "basaltic" volcaniclastic rocks containing corundum, zircon, pleonaste, ilmenite, quartz, olivine, and pyroxene have been recognized near the base of the volcanic pile in the Inverell-Glen Innes-Guyra region of northeastern New South Wales. These rocks comprise several broad lithotypes including: 1 2 3

Thickly bedded, brick-red pyroclastic units of variable grain-size, composed of ferruginous kaolinite with minor carbonate. Thinly bedded, grey, fine to medium-grained tuffaceous claystone units. Thinly to thickly bedded, white, very coarse-grained (+ 5 mm to 10 mmj, tuffaceous claystone units.

The red pyroclastics appear to be the most widespread of the three lithotypes described, and occur at or towards the base of the volcanic pile, commonly mantling basement rocks. Furthermore, they are typically associated with extensive deposits of ferricrete (laterites and bauxites), which appear to be the weathered equivalents of the red pyroclastic rocks in various stages of iron oxide replacement. Abundant sapphire is often "specked" by fossickers from these ferricretes after rain, whilst ferricrete nodules are a very common associate in concentrates from sapphire-washing plants. The other two lithotypes described are generally interbedded with basaltic lava flows near the base of the volcanic pile and appear to represent bleached, slightlv reworked, basal red pyroclastic rocks. These rocks are well developed at the Braemar sapphire mine and on the Kings Plains sapphire field near Inverell, where they host rich lag deposits of corundum. The corundum, zircon, pleonas'te, and ilmenite megacrysts in all these rocks are commonly smooth and glossy with rounded edges and embayments. Such features are characteristic of resorption under conditions of disequilibrium, indicating a xenocryst origin for these minerals. Furthermore, these crystals commonly show little or no evidence of stream abrasion. 314

Air photo investigation and geological mapping by the author over areas known to contain sapphire-bearing pyroclastic rocks and ferricretes has revealed the existence of numerous eroded maar volcanoes. These maars are generally located on prominent northeast-southwest, northwest-southeast and east-west, lineament-controlled valley drainage systems, that have yielded tens of millions of dollars worth of sapphire (eg Kings Plains Creek and Reddestone Creek). All of the maars recognized have shallow bowl-shaped craters that are wide relative to their rim heights. Their rims are primarily composed of ferricretes which commonly grade at depth into bedded deposits of red lapilli tuffs and agglomerates, with some minor basalt. They are either circular, oval, or irregular in shape. The majority of the maars examined consist of craters cut into pre-eruption surfaces and range in size from less than 4 ha to over 100 ha. Confirmation that these structures possess underlying diatreme "roots" has come from recent geophysical studies. Diamond of eclogitic origin is a rare and enigmatic associate of sapphire in Tertiary deep lead deposits at Bingara and Copeton. Furthermore, diamonds have recently been recovered from sapphire concentrates derived from alluvial, epiclastic, and pyroclastic rocks on the flanks of eroded maar volcanoes at the Kings Plains sapphire field. In other parts of the world, there appears to be a strong genetic (eclogitic) link between the formation of corundum and the formation of diamond. Crater facies "kimberlitic" tuffs rich in eclogitic diamond, associated with corundum, have been found in southeastern Africa (eg, the Orapa diamond pipe) and in Western Australia (eg, the Argyle diamond pipe), while diamond and corundum-bearing peraluminous eclogites have been found in diatreme facies rocks from many other localities in southern Africa and Russia. Corundum pyroxenites (resembling grospydite xenoliths in kimberlites) containing graphite octahedral pseudomorphs of diamond (up to 7 mm across) are also known to be present in the Beni Bousera Complex in Morocco. Furthermore, in Sierra Leone and Liberia corundum has been used as a search indicator mineral for diamond. Eclogitic diamonds commonly occur as rounded, resorbed dodecahedra and many of the diamonds found spatially associated with corundum in Cainozoic basalt terrains in eastern Australia exhibit this morphology. Furthermore, these types of diamonds can be remarkably similar to the glossy, rounded, colourless zircons which are commonly found associated with corundums in eastern Australia and therefore may be overlooked during prospecting and mining operations. As corundum and diamond may occur together in the volcaniclastic rocks described, these rocks and the maar-diatreme structures which produced them should be investigated as potential economic diamond sources. Furthermore, as sapphires are an important commodity in eastern Australia and southeast Asia, the recognition of these volcaniclastic rocks as a potential primary host for gem-quality corundum could lead to a major expansion of Australia's and southeast Asia's sapphire (and ruby) mining industries in the future.

315

3.16 REVIEW OF TERRANE TECTONICS IN NEW GUINEA AND EASTERN INDONESIA, IN RELATION TO PALAEOMAGNETIC PROBLEMS C.J. Pigram Bureau of Mineral Resources, Geology and Geophysics, Canberra

Since the development of the plate tectonic theory the evolution of the New Guinea Orogen has been interpreted in terms of a continent/island arc collision. Basically this interpretation did not change over the next 15 years although in recent times the development of the orogen has been seen as a consequence of multiple island arc collisions.

All of the reconstructions that were attempted

during this period however have a fundamental flaw : they are all based on the assumption that the elements that are interpreted to be parts of island arc complexes (volcanic arcs, forearc assemblages etc) had, in the past, occupied the same positions relative to one another and to the autochthonous craton that they do now.

Palaeomagnetic results from the terranes of the North American

Cordillera which show pre and post docking movements in the order of 1000's of kilometres, demonstrate just how erroneous such an assxamption can be. In an attempt to avoid this problem and to obtain an understanding of the accretion history of the New Guinea Orogen Pigram and Davies (1987) carried out a tectonostratigraphic terrane analysis.

But even this approach only provides

the bare essentials of the timing of events in the orogen.

The analysis only

provides data on when a terrane became geographically fixed in relation to its immediate neighbours.

It does not provide any information on where the terranes

originated, at what rate and in which direction they have moved prior to and after docking.

Without such data meaningful reconstructions are impossible.

Eastern Indonesia consists of a complex montage of island arcs and microcontinents separated by deep ocean basins and troughs.

An understanding of

the tectonic evolution of this region is intimately linked to an understanding of the evolution of the New Gu'inea Orogen.

One of the fundamental problems of

the region and one that links it to events in New Guinea is the origin and movement history of the microcontinents that are scattered throughout east Indonesia.

They are generally assumed to have been detached from the northern 316

edge of the Australian craton so an understanding of the evolution of the New Guinea Orogen is essential if we are to establish from where the microcontinents were detached and how and when this took place.

But the basic assumption that

the microcontinents are derived from Australia has yet to be proven.

Clearly if

some of them were shown to have been derived from other sites it would require a major reassessment of our ideas on the tectonic processes operating in the region.

Other examples of problems that can best be investigated using

palaeomagnetic methods include: Guinea Orogen?

How much overthrusting has occurred in the New

Are the Aure and Lengguru Fold Belts primary or secondary

features of the foreland fold belt?

When was the Papuan Ophiolite emplaced?

western Irian Jaya part of the Australian craton, a microcontinent or a composite terrane? problems.

There are many others but not all of them are tectonic

New Guinea contains a much more complete and accessible autochthonous

Mesozoic and Cainozoic rock record than Australia and it is from those sequences that a far more complete and precise Australian Polar Wander Path could be compiled.

The region has generally been ignored by palaeomagnetic researchers

and I suspect that one of the main reasons why is due to the perception that the movements that we are trying to document have largely been east-west (along latitude) and consequently not suitable for palaeomagnetic investigation.

will attempt to show that this is not necessarily the case by examining 3 hypotheses for the movement of one of the microcontinents (the Sula Platform) all of which are amenable to a palaeomagnetic test.

The problems of the region

that can be addressed by palaeomagnetic analysis are not of a trivial or local nature.

As Silver and Smith (1983) have pointed out the region is an excellent

modern analog for older mountain belts.

By working toward an understanding of

the way this region has arrived at its present day configuration we can and are providing essential models/scenarios for the interpretation and reconstruction of events in older orogenic belts.

REFERENCES

PIGRAM C.J. & DAVIES, H.L. 1987 - Terranes and the accretion history of the New Guinea orogen.

BMR Journal of Australian Geology and Geophysics Vol.

11.

SILVER E.A. 6c SMITH R.B. 1983 - A comparison of terrane accretion in modern southeast Asia and the Mesozoic north American Cordillera. 198-202.

317

Geology 11,

2.10

DEVELOPMENT OF THE MORESBY TROUGH REGION PASSIVE MARGIN TO FORELAND BASIN C.J. Pigram and P.A. Symonds Bureau of Mineral Resources, Geology and Geophysics, Canberra

The Moresby Trough is a gently arcuate assymetric depocentre that separates the Papuan Peninsula from the Eastern Plateau and part of the Papuan Plateau (Fig. 1).

The trough is bound on its northeasterly side by a fold and thrust

belt and onlaps the sag phase sediments of the plateau along its southeasterly margin.

The sedimentary fill across the plateaus and within the trough consists

of three megasequences (Fig. 2): Megasequence A consists of Late Cretaceous to Paleocene rift and pre-rift sediments up to 3 km thick; Megasequence B consists of Eocene to Recent sag phase sediments up to 2 km thick and Megasequence C consists of Miocene to Recent foreland basin fill up to 10 km thick in the Moresby Trough. The Moresby Trough is a foreland basin that has developed on continental marginal plateaux (crustal thickness 20 km) due to loading by collisional processes along the northern edge of the plateaus.

The deformation front

associated with the fold and thrust belt formed by the collisional processes that cause the loading parallels the coastline of the Papuan Peninsula and the south side of the Louisiade Archipelago to at least 152^E.

The sediments of

foreland basin (megasequence C) however only extend as far east as 149°E about halfway along the Papua Plateau and become less extensive in width and thickness east of the structural lineament separating the Eastern and Papua Plateaus. This easterly limit of the basin corresponds to the westerly limit of extension (Dayman Dome area) associated with the opening of the Woodlark Basin. Variations in the form of the foreland basin correspond with the orientation of the rift- related structural provinces and lineaments of the old Cretaceous/Paleocene passive margin.

Further east, Late Miocene to Recent

extension has modified the effects of the collisional processes and may have unloaded this part of the Papuan Peninsula. The effects of foreland loading on pre-existing extensional structures include fault reactivation, reversal of fault movements, incorporation of sag phase sediments into fault blocks and deformation of fault blocks. 318

It also appears that the Eastern Plateau has been thrust southwestward. The Eastern Plateau is separated from the Fly Platform by the southwesterly trending Pandora Trough which merges with in the north Moresby Trough.

Although

the Pandora Trough is almost perpendicular to the trend of the Moresby Trough it also has features characteristic of foreland basins also i.e.

a marked

assymetry in the depocentre, updip migration of the depocentre and an unstructured homoclinally dipping basement surface. Regional structural trends both on the Papuan Peninsula and to the south and north of it suggest that the Pandora Trough also formed as a result of southwesterly convergence.

Figure 1.

Locality map.

s w

NE PNG

PAPUAN

PLATEAU

SLOPE

2200m

MORESBY TROUGH

deformation front/ ""

- 4

—

Figure 2.

S c h e m a t i c s e c t i o n across the eastern end of the M o r e s b y T r o u g h .

319

4.2

METALLOGENY AND TECTONICS OF THE MID-PROTEROZOIC OF NORTHERN AUSTRALIA, WITH PARTICULAR REFERENCE TO THE McARTHUR BASIN K.A. Plumb

Bureau of Mineral Resources, Geology and Geophysics, Canberra

Major lead-zinc and copper bodies were deposited into similar rock types during a narrow time span of the mid Proterozoic in northern Australia. The largest tonnages of lead-zinc^ in the shale-hosted deposits at Mount Isa^ Hilton and McArthur River (H.Y.C.), are confined to the stratigraphicallyequivalent carbonate sequences of the Mount Isa and McArthur Groups, and yield identical U-Pb isotopic ages about 1670 Ma; Lady Loretta (McNamara Group) is of similar or marginally younger age. Dugald River, hosted by the Corella Formation, is perhaps 70 Ma older. The major epigenetic copper deposit at Mount Isa p o s t - d a t e s the lead-zinc m i n e r a l i s a t i o n by a significant interval. Numerous subeconomic discordant deposits of these same minerals are also scattered through the same sequences. Regional stratigraphy has provided the main guide to exploration, but new understanding of extensional terranes in the region, and of the deposits themselves, reveals an ultimate regional tectonic c o n t r o l for this stratigraphic mineralisation. The interval about 1650 Ma ago was a period of regionally high heat flow, extension, and rifting throughout northern Australia. Hypersaline carbonate and evaporite deposits, tuffs, restricted playas and lakes, and hot springs are common in rifts. These features all assist metalliferous brine movements and the structural development of favourable sediment traps, at many localities at the same time. The relatively pristine state of the McArthur Basin makes the models developed therein significant for the interpretation of more complex terranes and deposits at Mount Isa and elsewhere. The structure of the McArthur Basin may be modelled in terms of several northerly-trending rifts, 30-80 km wide by 10a->300 km long, separated by northwesterly-trending transfer faults and of a pattern and scale identical to modern extensional basins. The various rift segments have significantly different histories. The "original Batten Trough" is separated into the Walker (north) and Batten (south) Troughs by the Urapunga Tectonic Ridge. Concealed troughs beneath the Georgina Basin, south of the Kilgour and Tanumbirini Uplifts, are continuous with outcropping Tomkinson Creek beds north of Tennant Creek (Plumb & Wellman, BMR J. Aust. Geol. & Geophys., 10, 243-251, 1987). Extensional tilt blocks within the Batten and Walker Troughs were elevated throughout much of the basin's evolution. The Emu Fault probably represents the surface relict of the major detachment beneath the B a t t e n Trough, but a m o r e c o m p l e x history and s t r u c t u r e precludes identification of the equivalent detachment beneath the Walker Trough. Various shallow-marine and continental environments have been interpreted from the sequences in the southern part of the basin (Jackson & others,Bur. Miner. Resour.Aust. Bull, 220, 1987), and may be inferred for less well known areas in the north, but revised correlations significantly modify basin-wide palaeogeographic models (Plumb, Precambrian Res., 29, 303-329, 1985; cf. Plumb & others, in, THE GEOLOGY AND GEOPHYSICS OF NORTHEASTERN AUSTRALIA. Geol Soc. Aust, pp.Jl-SB, 1980).

320

The McArthur Basin succession comprises four main sequences^ separated by regional unconformities. The Tawallah Group (oldest) displays generally uniform thicknesses of shallow-marine, lacustrine, and alluvial sandstone and volcanic-lutite-carbonate facies throughout the southern McArthur Basin; in contrast, the equivalent Parsons Range Group in the north has no equivalents outside the Walker Trough. The succeeding hypersaline paralic to lacustrine carbonate units of the McArthur Group, in the south, are confined to the Batten Trough or adjacent areas. Extension is most strikingly demonstrated by the Batten Subgroup and the underlying Barney Creek Formation (host to the McArthur (H.Y.C.) deposits). These show hypersaline lacustrine sequences with abundant tuffaceous volcanism, deposited in a half-graben; striking stratigraphic changes between fault blocks; and demonstrable growth faulting and talus breccias along the Emu and related fault zones. To the north, the McArthur Group in the Walker Trough displays similar structure and facies to that of the Batten Trough, but the equivalent Katherine River Group on the Arnhem Shelf comprises mostly fluvial sands alternating with volcanics, carbonates, and lutites. The lacustrine and paralic carbonates of the succeeding Nathan (south), and Mount Rigg Groups (north), may represent a sag phase. The much younger shallow marine blanket sands and intervening lutites of the Roper Group show a marked shift in depositional pattern: a gradual southwest thickening resembles a sag phase above the Batten Trough, but the geophysical signature suggests faulted (rifted) margins to the Beetaloo Sub-basin farther west. The shale-hosted McArthur lead-zinc deposits are confined to a small district within the Bulburra Depression. The lacustrine or lagoonal facies which host the deposits are clearly localised by growth faulting on the adjacent (listric?) Emu Fault. The restricted lake sediments acted as a hydrologic trap for diagenetic brines, which were demonstrably derived from the nearby fault zone (Walker & others, J. geoi. Soc, Aust., 24, 365-280, 1978; Williams, Econ. Geol., 73, 1005-1035, 1978; Logan & Williams, Geol. Soc. Aust. Abstr., 12, 339-340, 1984). The Glyde Sub-basin has an almost identical structural setting and stratigraphy, but lacks lead-zinc mineralisation (Dashlooty & Davidson, Geol. Soc. Aust. Abstr., 15, p. 54, 1986). Not surprisingly, the main constraint on mineralisation is the local source of mineralising brines. Tuffs, abundant in both sub-basins and common through the sequence as a whole, simply reflect the overall rift setting, rather than indicate volcanogenic mineralisation. A new class of widespread small lead-zinc and copper deposits in the McArthur Basin comprises discordant karstic and unconformity-related deposits (Jackson & others, 1987) . These deposits are also clearly localised along major fault zones of the rift systems, probably because unconformities are best developed on the uplifted tilt blocks, and the mineralising brines are demonstrably derived from a similar source to those of the H.Y.C. (Williams, Econ. Geol., 73, 1978; Walker & others, Econ. Geol., 78, 1983; Muir & others, Aust. J. Earth. Scl., 32, 239-260, 1985). Clearly extension, rifting, and their associated features have provided a common regional tectonic control for both shale-hosted and discordant deposits in the McArthur Basin. Similar controls to the H.Y.C. are readily apparent for equivalent deposits of the Mount Isa region, and the many discordant deposits of the Lawn Hill Platform may be karstic or unconformity-related deposits. The Birrindudu and Victoria River Basins, punctuated as they are by numerous unconformities, may have a potential for karstic deposits but, in the absence of rifts, have no evidence of any shale-hosted mineralisation. The rifted Walker Trough and Tomkinson Creek beds probably represent the only little-explored terranes with potential for significant new shale-hosted lead-zinc mineralisation.

321

5.5

SUBDIVISION OF THE PRECAMBRIAN - PROPOSALS BY THE SUBCOMMISSION ON PRECAMBRIAN STRATIGRAPHY A N D COMPARISON WITH PHANEROZOIC CHRONOSTRATIGRAPHY K.A. Plumb

Bureau of Mineral Resources, Geology and Geophysics,

Canberra

The Subcommission on Precambrian Stratigraphy has addressed, for two decades, the problem of developing a worldwide subdivision of the Precambrian. While accepting that no absolute necessity exists for such a subdivision - the age of individual rock bodies may be expressed simply by reference to their isotopic ages - named time subdivisions are traditional and deeply ingrained, they are particularly useful for small-scale interregional maps, and consistent usage will aid international scientific communication. Several types of subdivision have been considered: (1) Subdivision by equal-unit numerical division of time; (2) Subdivision based on major magmatic-tectonic cycles or orogenies; (3) Subdivision keyed to stratotypes; (4) Subdivision based on breaks in geologic (isotopic age) record; (5) Subdivision based on concepts of earth evolution; The composite geologic record of the Precambrian has also been assessed from comprehensive time-rock charts of all the major shields of the world. In establishing a time subdivision, "three kinds of decision or agreement need to be made, and by a single authority (i.e. lUGS) : (1) a scheme of divisions with appropriate classification together with (2) agreed names for each division that shall correspond to the time spans between the boundaries and (3) agreed standardisation of boundaries" (Harland et ai., Geological Time Scaler 1982) . Furthermore, such a subdivision should: (1) be simple; 2) reflect major events in Earth history; (3) be acceptable; (4) not be closely identified with one particular region; and (5) have clear operational criteria (James, Precambrian Res,^ 7, 193-204, 1978). The Subcommission proposes a chronometric subdivision of Precambrian time, in which time boundaries have been selected so as to enclose or delimit principal cycles of sedimentation, orogeny, and magmatism, but the boundaries are defined in years, without specific reference to any bodies of rock (Plumb and James, Precambrian Res., 32, 65-92, 1986) (Table 1). This proposal satisfies requirements (1) and (2) of Harland (above), and all the criteria of James (1978). However, the nomenclature (Table 1) is interim and still being developed (Plumb and Gee, {Precambrian Res., 36, 185-187, 1987). Comment or suggestions, on both the subdivision and the nomenclature, are invited from the geological community at large before a final proposal is developed for approval by the Commission on Stratigraphy. It is instructive to look to the Phanerozoic example in assessing the break from the traditional chronostratigraphic approach for the Precambrian. The most precise method of "dating" Phanerozoic rocks remains correlation of fossil assemblages. These assemblages can only be unambiguously defined by reference to rocks. The Phanerozoic scale was originally built up from a series of classic type sections or stratotypes, which were thought, to be representative worldwide. However, as geological studies expanded, particularly into the antipodes, these stratotypes were found to be less and less typical and correlation became more and more difficult. And so, the more rigorous Boundary Stratotype principle arose.

322

Table

1. Present proposal for subdivision of Precambrian time,

EON

PERIOD

ERA -<Base of Cambrian)

PROTEROZOIC III 900 Ma PROTEROZOIC II

(H) -700 Ma •

(G) (F)

-1200 M a -

(E)

—1400 Ma-

PROTEROZOIC

•1600 Ma

(D)

PROTEROZOIC

(B) •2100 Ma.

(A)

ARCHAEAN

-2500 Ma-

But what are Boundary Stratotypes? Simply, they are selected horizons in continuous sequences which, together, form a net of independent reference or c a l i b r a t i o n points, to establish ages by correlation; the biostratigraphic equivalent of isotopic ages. They no longer record earth history. Many are now defined far from the original stratotypes, and subjacent (in time) Boundary Stratotypes may widely separated in space. Isotopic ages will remain, for some time at least, the most universally precise means for dating and inter-regional correlation of Precambrian rocks. If Boundary Stratotypes refer to rocks only because it is the only way that they may be unambiguously defined, it does not follow that there is any need to refer to rocks to define a subdivision of "absolute" (isotopic) ages. It may be appropriate to use the rock record to select boundaries but, once selected, the least ambiguous definition becomes a simple number (age). The Periods of the Phanerozoic scale were originally named for their stratotypes. However, now that many Boundary Stratotypes are situated far from the original stratotypes, there is no spatial connection between the original (name) stratotype and the new Boundary Stratotype. The names remain simply as convenient, traditional labels. The Subcommission's avoidance of regional names for the Precambrian units may be unnecessary, so long as stratotype connotations are clearly avoided.

323

3.10

PALAEOZOIC TECTONICS OF EASTERN AND CENTRAL AUSTRALIA IMPLICATIONS FOR PALAEOMAGNETIC STUDIES C.McA. Powell School of Earth Sciences, Macquarie University, North Ryde

The Tasman Fold Belt in Eastern Australia was part of the Pacific-facing Gondwanan margin throughout the Palaeozoic, It is notable because of its breadth, in places over 1000 km wide, and because much of its history is different from that of Palaeozoic fold belts better known to geologists from the Northern Hemisphere. The Tasman Fold Belt is distinctively Gondwanan, and fits no more into the models of the Palaeozoic Appalachians or Caledonides than the modern west Pacific margins fit into the moulds of the Cenozoic Andes or European Alps. Five of the features that make the Palaeozoic Tasman Fold Belt different from the North American and European fold belts are: 1. Absence of a well-developed sedimentary wedge connecting the platform deposits of Central Australia to the generally deep-water deposits of the Tasman Fold Belt (cf. the "miogeocline" of North America). 2. Absence of any well-developed craton-directed fold-and-thrust belts (cf. Valley and Ridge in the Appalachians, Juras in the Alps). 3. Presence of large volumes of mineralogically-mature quartzose clastics recycled from the Gondwanan craton in the western 3/4 of the Tasman Fold Belt. 4. Sudden appearance in the mid-Silurian of enormous volumes of silicic volcanics and comagmatic plutons after a long interval of relatively quiescent deposition of highly mature quartzose turbidites. 5. Absence of any basement exposures capable of generating the volume of silicic magmatic products of the right composition. The postulated "basement" is commonly thought to be Precambrian on geochemical grounds. Geologically, the Palaeozeric Tasman Fold Belt can be subdivided into three meridional tectonic realms, each of which has a history overlapping the one adjacent, but with varying events in each at common times. The westernmost realm, the Kanmantoo Orogen, is represented by rocks outcropping in Eastern South Australia, and western Victoria and NSW. It had a tectonically active history from the late Precambrian (-650-600 Ma) until the Early Ordovician (--500 Ma). The central realm, the LachIan-Thompson Orogen, is the widest, and was tectonically active from some time in the Cambrian ('-550 Ma) until the mid-Carboniferous (--330 Ma). The eastern realm, the New England Orogen, has an uncertain Early Palaeozoic beginning, but was known to be active by the Silurian (--430 Ma) and continued to be tectonically active until the mid-Cretaceous (-100 Ma). The Kanmantoo orogen comprises late Precambrian to mid-Cambrian quartzose clastics deposited mainly in turbiditic facies. It was uplifted, deformed, and intruded by granites in the Middle Cambrian to Early Ordovician interval (530 to 490 Ma). It could well have been a marginal sea formed adjacent to Australia during the late Precambrian continental breakup. The event which terminated the c^rogenic history of the Kanmantoo realm, the Delamerian Orogeny, was felt in many parts of the Tasman Fold Belt and its extension in northern Gondwanaland.

324

The Lachlan-Thompson Orogen was a deep-water oceanic realm in the Cambrian, its history imperfectly known from the few, scattered outcrops available. Mafic volcanics, cherts and distal turbidites suggest marginal seas or back-arc basins of the west Pacific type. The Ordovician history is far better known. Widespread quartzose clastics of turbidite facies have remarkably consistent palaeocurrents indicating provenance to the west and south. The Early to Middle Ordovician appears dominated by a turbidite apron marginal to the uplifted Delamerian Highlands to the west, but by the Late Ordovician a mafic volcanic arc had formed in the east. The onset of widespread slicic magmatism in the mid-Silurian (--430 Ma) followed deformation known locally as the Benambran and Quidong Orogenies, and accompanied a change to an extensional, horst-and-graben tectonic phase. Late Silurian and Early Devonian volcanics occur throughout the Lachlan-Thompson Orogen. Earth movements varying from intense (central and eastern Victoria and Anakie Inlier, Queensland) to mild (central New South Wales) terminated the silicic volcanic interval and allowed the mineralogically mature, quartzose clastics of continental facies to prograde eastwards. The entire orogen, as well as the cratonic basins of central Australia, was deformed in the mid-Carboniferous (-330 Ma) by a compressive event that terminated the orogenic history of Eastern Australia west of the New England realm. The New England Orogen has a long and complex history that provides some evidence for accretion of exotic terranes, and also for large-scale longitudinal movement forming megafolds or oroclines. The pre-Devonian history is poorly known, but consistent with having lain oceanward of coeval environments in the Lachlan-Thompson Orogen. In central Queensland there is a tectonostratigraphic assemblage (the Calliope "terrane*^) which could have been accreted to the Tasman Fold Belt as an exotic terrane in the midDevonian. Further south, in NSW, there is evidence of a subduction-related terrane lying east of the inferred fore-arc Tamworth Basin. Evidence for connections between the Lachlan and New England Orogens prior to the Late Carboniferous is tenuous, and several possible tectonic reconstructions exist. By Late Carboniferous, the New England Orogen was dominated by a continental, Andean arc, at the edge of the now-extended Gondwanan continent, facing east into a subduction complex. Major dextral shear along the New England Orogen could have formed the Yarrol-New England megafold in the Late Carboniferous or earliest Permian, prior to the massive Permian intrusion of granitoids throughout northeastern NSW and adjacent Queensland. The Sydney-Bowen Basin developed as a foreland basin to the New England Orogen, and forms the overlap assemblage between the New England and Lachlan-Thompson Orogens. Implications of this tectonic history for palaeomagnetic studies are: 1. The Kanmantoo Orogen has probably been welded craton since the Early Ordovician (500 Ma).

the Australian

2. The connection between the Lachlan-Thompson Orogen and the Australian craton prior to mid-Devonian is speculative and uncertain. 3. The Lachlan-Thompson Orogen has been craton since the Middle Devonian (370 Ma).

connected

to the Australian

4. The relationship between the New England Orogen and the rest of the Tasman Fold Belt is speculative prior to the Late Carboniferous (330 Ma). 5.

There could be some exoj^ic terranes in the New England Orogen.

6. There is good evidence for a Late Carboniferous Permian megafold (orocline) in the New England Orogen.

325

- earliest

Late

5.7

PRINCIPLES AND PROBLEMS OF LITHOSTRATIGRAPHIC CORRELATION AND SYNTHESIS IN A FOLDED PROTEROZOIC BASIN A CASE HISTORY OF THE ADELAIDE GEOSYNCLINE W.V. Preiss South Australian Department of Mines and Energy

The goal of stratigraphic studies in sedimentary basins is the reconstruction of evolving palaeogeography. This requires recognition of specific time intervals in the rock record^ over a variety of synchronously deposited facies. While close biostratigraphic zonation provides the time control in many fossiliferous Phanerozoic basins, others are sparsely fossiliferous and Precambrian basins lack all except the crudest biostatigraphic framework. Basin analysis in such sequences has always relied heavily on mapping of lithostratigraphic units and on interpretation of the relationships between units. The late Precambrian stratigraphy of the Adelaide Geosyncliner which exemplifies many of the problems arising, has been subject to serious study since the pioneering work of Howchin at the turn of the century. Howchin's discovery of Sturtian tillite provided a mappable marker bed throughout the basin. The early geologists of South Australia measured numerous isolated sections, but little attempt was made at synthesis. Systematic mapping began only after World War II; in the 1980s, with the first round of mapping almost completed, basin analysis became possible. The earliest subdivision in 1950 involved three chronostratigraphic units, the Torrensian, Sturtian and Marinoan Series, each comprising several formations. The addition of the Willouran Series at the base and upward extension of the Marinoan to the top of the Pound Subgroup completed the scheme in use by 1960. The lithostratigraphic subdivision introduced in 1964 put greater emphasis on glaciogenic units as markers; boundaries of the newly defined groups were based on more widely recognisable stratigraphic features, with minor modifications since. Because of the lithological monotony and repetitive nature of sedimentation in much of the Adelaidean, care is needed in assigning a stratigraphic position to isolated outcrops, some of which are still very uncertain. Much depends on recognition of distinctive vertical assemblages of facies. For example, laminated siltstone is a ubiquitous component of the Adelaidean, but the association of extremely thinly laminated, carbonaceous siltstone with interbedded dolomite, overlying diamictite of inferred glacial origin, is indicative of the base of the Tapley Hill Formation, which may be recognised not only throughout the Adelaide Geosyncline but also in coeval basins in central Australia. This contact is a sharp, unique boundary, probably reflecting an extremely rapid transgressive event; as such it may be as close an approximation to a time-line as possible. In very rare circumstances, '' unique events may provide a more absolute time-line, e.g. the meteorite-impact debris layer.

326

The Sturtian-Marinoan boundary was defined by the change from Brighton Limestone to redbeds of the Willochra Subgroup, south of Adelaide. Although a similar change can be recognised for up to 400 km northwards along the depositional strike, the passage between these units is part of an upward-shallowing sequence from basinal Tapley Hill Formation to exposed-mudflat Willochra Subgroup. By application of Walther's Law, such a vertical succession must reflect laterally juxtaposed facies belts. To the east, the Brighton Limestone lenses out and redbeds are replaced by unoxidised strata lacking evidence of desiccation. The point chosen by the pioneer geologists for the SturtianMarinoan boundary has precise time significance only in the type area. Elsewhere, the time-rock boundary will occur somewhere within the transitional interval, but cannot be located precisely. However, recognition of this regressive phase, the first after the Tapley Hill transgression, means that at least all Adelaidean rocks younger than this regression are Marinoan. Thus, evidence of cold climates, appearing somewhat higher, is Marinoan and provides the next basin-wide, broad time-marker. Because lateral facies changes do not all occur in the same place, lateral transitions between formations or facies can be mapped where bracketed by through-going markers, even if these may eventually lens out. Tentative correlation of mapped units is thus possible. The scheme proposed, based on over 40 years mapping by many geologists, is not considered final, but an interim stage in deciphering the history of this basin. It allows designation of time "slices" within which lateral facies distributions may be mapped and palaeogeography reconstructed. Stratigraphic nomenclature in the Adelaide Geosyncline has evolved over this period. Early workers mapped isolated areas and applied locally-based nomenclature. Often, names were used informally at first but gradually came to be applied by subsequent authors in a formal sense, sometimes without formal definition. Type sections were not always specified. Comprehensive correlations between regions were first attempted in 1964, but some have since been revised. Had a single set of names been applied to all units then thought to be equivalent, such revision would have created nomenclatorial problems. The result is a complex, at times confusing, historically based stratigraphic nomenclature. It cannot be discarded; the names and concepts behind them are in the literature and are part of the history of geology. The burden can be relieved only by explanation and clarification in the light of present understanding. Some sedimentary units in the Adelaide Geosyncline lack information necessary for definition as formations. They are lithologically recognisable and mappable, but lack stratigraphic boundary relationships, commonly because of tectonic disruption. To name them is useful for maps and communication; to name them informally confers no status and does not preclude confusing re-use of thfe names subsequently for other units. Early Australian codes of stratigraphic nomenclature recognised this problem and wisely allowed for the use of "Beds" for formally defined but incompletely known sequences. 327

6.13

PRIMARY

CHROMITE C.

Azimate

DEPOSITS OF NEW

CALEDONIA

Premoli

S.A.R.L., Noumea, New

Caledonia

New Caledonia is the only chromite producer in the South Pacific. Its important resources are of ophiolitic affinity and therefore of the "podform" typ^ These deposits are spatially scattered, of comparatively small size and complex shape. A number of classifications have been proposed (Cassard, 1981; Christiansen, 1986) all of them related to the tectonic setting of such deposits. Recent geological considerations have somewhat altered the conventional thinking about the New Caledonia chromite deposits and offered new insights. THE TIEBAGHI MASSIF The Tiebaghi ultramafic massif of the North East of N.C. has been the main chromite producer of the island with a recorded output near to 2 Mt of chromite, or more than 90% of all reccfrded N.C. production. Remarkably, the 20 x 8 km Tiebaghi Massif represents less than 2% of the ultrabasic cover of New Caledonia. The present Tiebaghi production is about 80,000 t/y of good grade chromite (50-55,7 % Cr203). The Tiebaghi Massif has been emplaced probably in the Lower Eocene and has since suffered a complex tectonic history which has resulted in folds ranging in style from isoclinal to open. The various ultramafic facies range from dunite to plagioclase Iherzolite in an apparently coherent lithostratigraphic succession. The distinctive lithologies of the Tiebaghi and Southern Massif strongly suggest that the spreading of the ultrabasic overthrust took place from a trench parallel to the axis of the island and to its west. This resulted I n litholpgically different ultrabasic blocks perpendicular to the island axis (fig.l). The authors have TIEBAGHI found no evidence, geological, petrograMASSIF phic or structural, that these blocks Iherzolite, | have been cut by a maj or, late—stage NW— cpx-harzburg. harzburglte SE fault with a large lateral strike dip Few.^large traversing the length of the island (Brothers, 1987). Cr deposits

CENTRAL MASSIFS dunite, harzburgite very few Cr deposits

SOUTHERN MASSIF dun i te,harzburgi te,gabbros oceanic plagiogranites numerous, small (<5x10 t), chromite deposits

Fig.l Ult_rabasic Massifs of N.C.

Tiebaghi's chromite occurrences tend to cluster in three stratigraphic levels of which two are of economic significance. The three largest deposits, Tiebaghi, Chagrin and Fantoche, occur along strike and display typical "concordant" geometry: subvertical, lensoidal and elongated, parallel to the country rock (fig.2). Surface expression can be poor particularly considering the thick (3-10 m) laterite capping of the massif. In the absence of reliable geochemical or geophysical guides, exploration must proceed essentially by expensive, subvertical drilling^ this task can be optimized by modern, computer-assisted, geological modelling. The ideal form of exploitation of Tiebaghi chromite deposits is by underground trackJess mining (as the current INCO exploita^

328

Chromitite (showing dunite envelope) Peridotite (showing lineation

Discordant deposits (podform)

Subconcordant deposits Concordant deposits t lens-shaped ^

Fig.2 Structure and geometry of New Caledonia chromite deposits tion of the main orebodies cluster) which permits at the same time well targetted, cost-efficient underground exploration development. Only orebodies in excess of 0.5 Mt of chromite justify this type of exploitation.

THE SOUTHERN MASSIF The Southern Massif occupies the southern third of the island (fig.l) and consists of dunites, harzburgites, gabbros and oceanic plagiogranites (Mouriange granites). These lithologies have been geochronologically dated at 55 MY 1 1 or about 20 MY older than previously thought. The serie is very thick and tightly folded; all chromite concentrations are probably traceable within a 1.5 km thick harzburgite-dunite zone. Chromite deposits tend to be small (<50,000 t), and widely spaced among themselves. Chromite is far more aluminuous (Cr/(CrtAl)<0.73) then the Tiebaghi Massif. Three main types of deposits have been recognized: discordant, subconcordant and concordant. The first are typically podform, very irregular in shape and cross-cutting the foliation of the enclosing peridotite. Subconcordant and concordant deposits have a more regular, lensoidal geometry (a^br^lOc); the former lie between 12° and 25° within the foliation plane, and the latter are essentially parallel to it (fig.2). The increasing elongation and parallelism to the country rock structure of the concordant deposits reflects the increasing deformation of the dunite-harzburgite complex. No clues to the plunge direction for a series of concordant, closely packed, chromite lenses (as for the Tiebaghi main deposit) have yet been found, although this remains a major exploration parameter. None of the deposits found so far in the Southern Massif have warranted underground explotation, which further limits data avalability. Exploration guides remain poor: no geochemical and only few and uncertain geological and tectonic ones. Geophysics is hardly applicable. The deposits do not have LANDSAT or SPOT expression. A statistically-oriented, computersupported approach to the search of blind orebodies offers some scope. A six-variables model has been elaborated. REFERENCES Brothers, R.N., 1987. Regional geology of N.C. and the Northern Island, N.Z. Pacific Rim Congress 87. Aust. I.M.M. pp 61-63. Cassard, D . et al., 1981. Structural classification of chromite deposits of New Caledonia. Economic Geolq^y, Vol.76, pp 805-831. Christiansen, F.G., 1985. Structural classification of ophiolitic chromite deposits in'Metallogeny of basic and ultrabasic rocks'. I.M.M. pp 279-289.

329

17.8

GEOCHEMISTRY OF PLAINS BASALTS OF THE WESTERN DISTRICTS VOLCANICS PROVINCE OF VICTORIA R.C. Pricel, C.M. Grayl and F.A. Frey^

^Department of Geology, La Trobe University, Bundoora ^Department of Earth, Planetary, and Atmospheric Sciences, Massachusetts Institute of Technology, Cambridge, USA Cainozoic volcanism in Victoria is related to the tectonic adjustments that affected the south-eastern Australian continental margin during separation from Antarctica and the Lord Howe Rise. Two major periods of volcanism have traditionally been recognized but this simple two-fold subdivision, into "Older" and "Newer" volcanics, is arbitrary and artificial. Volcanic activity has been almost continuous through the Cainozoic with volumetric peaks at 42-57 Ma and 0-5 Ma. On the basis of geomorphology and composition the volcanics with ages less than 5 Ma. can be subdivided into plains basalts , which form a thin veneer (<50 m.) covering the western plains of Victoria, and cones basalts forming the younger constructional features (scoria cones, maars, tuff rings and lava shields) of the province. The plains basalts are younger than 4.6 Ma with a peak in the number of ages at 2.4 Ma and eruptions continued to virtually the present day. A survey of 400 new analyses shows that tholeiitic rock types dominate the plains; 57% of analysed plains basalts are tholeiites or quartz tholeiites and mildly alkalic hawaiites make up 34% of the sample population. Moderately and strongly undersaturated lavas represent only 2% of the plains samples and some of these samples are from aprons around younger cones. 7% of analyzed plains basalts are basaltic icelandites. Initial 87Sr/86Sr ratios vary greatly within the province (0.7037-0.7058 in 400 samples) and there is a general correlation of increasing isotopic ratio with Si02 saturation whereby the hawaiites have the lowest and the basaltic icelandites the highest ratios. All the relatively radiogenic tholeiites and basaltic icelandites (87Sr/86Sr > 0.7048) are 2 Ma or older and the less radiogenic hawaiites are younger than 2 Ma. With time the eruptives became progressively more alkalic and showed less scattered 87Sr/86Sr isotopic ratios. On a regional scale a north-south boundary through the town of Mortlake separates an eastern radiogenic sector (0.7040-0.7058; mean= 0.7046) from a western sector (0.7037-0.7047). Locally, isotopic mapping coupled with radiometric and whole rock geochemical data has identified domains that represent either single flows or groups of related flows within the plains. In the Melbourne area six distinctive petrographic and geochemical domains have been defined. Interpretation of the geochemistry of the plains basalts is complicated by the effects of alteration. Rocks showing relatively low degrees of alteration, in the form of partial replacement of olivine by iddingsite, show in some instances anomalously high Ba abundance and in other cases unusually high concentrations of rare earth elements and Y. These anomalies arise from the presence of fine grains of BaS04 and fine grains of hydrated rare earth phosphates in the alteration products and are not primary features of the geochemistry. The bulk of the plains basalts represent derivative magmas and have undergone crystal fractionation, involving principally olivine, during ascent. 87Sr/86Sr ratios do not correlate with abundances of some of the elements that one might expect to have relatively high concentrations in normal crustal rocks (eg. K, Na, Al) and this feature and the coherence of the isotopic domains appear to preclude crustal contamination as an important process affecting geochemical variation among the plains basalts. The total range of geochemical variation obsen/ed in the post 5 Ma basalts of the western districts volcanic province reflects mixing of multiple, heterogeneous, mantle sources and crystal fractionation at various levels in the crust and upper mantle. The plains basalts are believed to have been produced by partial melting of a lithospheric mantle source characterized by extensive and variable short range heterogeneity while ultra- alkalic rocks could be derived by partial melting of a more homogeneous , deeper mantle source. 330

15.8

BIOSTRATIGRAPHY OF SEAMOUNTS IN THE TASMAN SEA P.G. Quilty

Antarctic Division, Department of Science,

Kingston

Dredged calcareous sediments associated with the volcanic edifices of Gascoyne, Taupo, Derwent Hunter, Britannia, Stradbroke and Moreton Seamounts have yielded a diverse biota including abundant foraminiferids and calcareous algae. Fossils contained in sediments provide limits on the environmental range within which the volcanic edifice formed and also data on oceanographic conditions in a time interval for which such data are uncommon off eastern Australia. Gascoyne Seamount incorporates a tropical to subtropical, very shallow water calcareous algal/encrusting foraminiferid biota suggesting deposition in less than 15-20 m water depth. Age diagnostic forms have not yet been recovered. Taupo Seamount contains a Middle Miocene (N12) biota including the calcareous algae Lithophyllum or Mesophyllum in addition to a characteristic planktonic foraminiferid fauna. Lepi^cyclina is absent. Again water was shallow and tropical to subtropical. Derwent Hunter Seamount contains a fauna similar in appearance to that from Taupo Seamount but of latest Early Miocene age with Lepidocyclina howchini in addition to a diverse planktonic foraminiferid and calcareous algal biota. Britannia Seamount contains latest Early Miocene (N9) L. howchini, planktonic and encrusting foraminiferids and calcareous algae suggesting deposition in shallower than 12 m water depth. Again, tropical to subtropical conditions applied. Stradbroke Seamount yielded lithified calcareous ooze of Middle Miocene age but probably represents a post volcanic deposit. In addition to the above age/environment data, several younger sediment samples were recovered and yield data on the interval after the initial phase of seamount formation. Older seamounts on the East Tasmania Plateau also have yielded characteristic planktonic foraminiferid faunas.

331

14.5

THE PERMIAN FLORA OF THE NYCHUM VOLCANICS, NORTH QUEENSLAND J.F. Rigby Queensland Department of Mines

Plant remains were collected many years ago from the Nychum Volcanics by parties from the Bureau of Mineral Resources. During 1986-1987 the area has been mapped by parties from the Geological Survey of Queensland, but no additional plant fossils were found. Identifications originally made on that collection posed some problems as names used suggested relationships with both Permian floras of Gondwanaland and carboniferous floras of Laurasia. The present report covers a re-examination of the original BMR collection. Redeterminations are: Paracalamites australis, Raniganjia sp., Neomariopteris lobifolia, Botrychiopsis sp. nov., Glossopteris sp. A nov., Glossopteris sp. B nov. The presence of Botrychiopsis suggests a very late Carboniferous to Early Permian age; other species belong to genera that are restricted to the Permian. As the species present are either new, or long ranging, precise correlation is not possible with other occurrences of the Glossopteris flora. An Early Permian age is most probable. Stellotheca sp. closely resembles some species of the Cathaysian moist-tropical genus Lobatannularia. This suggests the climate at the time of deposition of the fossil horizons was moist, warm temperate to sub-tropical, but because of the presence of Glossopteris, was not tropical.

332

1.2

COPPER MINING, BRICKMAKING AND FURNACE BUILDING AT KARIBOE, DAWES RANGE, CENTRAL QUEENSLAND A.D. Robertson Queensland Department of Mines

Copper mineralisation was discovered towards the headwaters of South Kariboe Creek in what is now known as Portion 25, Parish of Kroombit early in 1869 by Robert Buffet Ridler of Yarrol Station. On the 24th July, Messrs Cooper, Ridler, Trezise and Clayton applied for Mineral Selection (MS) 50 covering an area of 150 acres. The proposed mine was to be called the Blackall Mine in honour of the Governor of Queensland, Colonel Blackall. Difficulty was encountered in raising capital for the new mining venture and in February 1870 the discovery of gold in the alluvials of South Kariboe Creek diverted attention from Ridler^s discovery. Although Messrs Cooper and Partners were experiencing difficulties, interest in the new prospect in some quarters had not waned. Mr Francis Flanagan applied for Mineral Selections 175 and 182 on 20th July and 5th September 1870, respectively. Cooper and Partners forfeited MS 50 for non-payment of money to the Crown. Thomas Perkins, brewer of Toowoomba, applied for MS 50 and by paying the balance of the money owing was granted the Mineral Selection, now known as MS 183 on the 23rd September 1870. Perkins wasted little time developing the Blackall Mine. Between October 1870 and August 1871, both the Blackall Mine and Flanagan^s properties were steadily developed. In early 1871, Flanagan commissioned Mr C. Betteridge, a furnace builder of 30 years' experience in England, to build a reverberatory furnace capable of producing "3 tons of pure copper every 24 hours". Betteridge's furnace was known as a "Speedwell" furnace. All the bricks for furnace construction were made on site. By September, Flanagan's furnace had failed on three separate occasions. The intense heat had destroyed the crowns in the furnace and the outer bricks had failed because of their inferior quality. The local material proved unsuitable for fire brick construction. The bricks had been made by the "hand-throw" process using wooden moulds and fired in a modified version of an "updraft bottle kiln". This crude kiln was constructed using diorite boulders cemented with a similar sand-clay mix of which the bricks were made. The repeated failure of the bricks prompted a search in the surrounding district for suitable fire clay deposits. A deposit was discovered adjacent to South Kariboe Creek approximately 7 miles downstream of the Blackall Mine site. A mixture of clay and pure greyish-white medium-grained sand, derived from friable Tertiary sandstone, proved to be most suitable for making bricks for furnace construction. Perkins encountered difficulties by the end of 1871 in making the Blackall Mine a viable commercial proposition. Unlike Flanagan, Perkins did not have access to furnace facilities and was attempting to transport the ore by dray to Gladstone. His lack of mining expertise and the distance to port facilities weighted heavily against him. During an attempt to raise capital to expand mining operations and install smelting facilities, Perkins attracted the attention of Ebenezer Vickery, a Sydney Merchant who had acquired extensive interests in a number of mining companies in the Mount Perry area. By 31st August 1872 Vickery had acquired the Blackall Mine and on 3rd September of that year, the Great Blackall Copper Mining Company, Kroombit run was registered with a working capital of 100 000 English pounds.

333

An abnormal wet season in early 1873 left many of the mines on the Kariboe field flooded and in May of the same year the price of copper on the Europe Market collapsed. The Great Blackall Copper Mining Company began to exercise a rigid control on expenditure and postponed the construction of furnaces. Flanagan closed down his mining operations. By August 1974, the Blackall Mine had been forced to close. Little ore had been shipped from the site since late 1872. Two reverberatory furnaces were constructed after the closure of the Blackall Mine but were never fired. There is no record of what happened to the stockpile of ore and by 1875 the mines at Kariboe had been completely abandoned.

334

1.3

HISTORY

SAPPHIRE

MINING

I N CENTRAL

QUEENSLAND

A,D. Robertson Queensland Department of Mines

A precise data cannot be given for the discovery of sapphire and the commencement of commercial mining of sapphire in Central Queensland, but available literature suggests that A.J. Richardson discovered sapphire on the north bank of Retreat Creek in the area now known as Reward-Freehold somewhere between 1873 and 1878. Rees R . Jones may have been the first to commence mining on Mineral Selection 3022 during 1881. By 1891, Richardson and Fischer were mining sapphire in the RewardFreehold area. Between 1893 and 1899, little or no market existed for Central Queensland sapphire and a number of mining tenures were sold. The discovery of light blue sapphire in Kashmir (1881), together with the popular acceptance of Ceylonese and Southeast Asian stone and the difficulty experienced in cutting dark blue sapphire with a bluegreen cross-table, had an adverse effect on marketing conditions. A generally depressed market continued through until 1909 when Germany and the Imperial Russian Court entered into the market for sapphires. This improvement continued until 1914. The outbreak of war, the revolutionary upheavel in Russia (1917) and the economic instability in Europe after 1918, resulted in depressed market conditions that caused hardship for the miners. Between 1921 and 1928, the Queensland Government provided marketing assistance through the operation of a " G e m Pool Scheme". This scheme provided a guaranteed source of income for miners but provided disastrous for the Queensland Government. As a consequence, a commission of inquiry was established in 1928 to inquire into the operations of the " G e m Pool Scheme". From 1929 to 1960, the sapphire industry struggled for survival with few periods of favourable market. By 1968, the Central Queensland sapphire fields had become a popular tourist attraction and, with an increasing demand for sapphire on the world market, mechanised mining had made its appearance. A rift rapidly developed between the hand miners and the machine miners. The Queensland Government through the Mines Department placed certain restrictions on the mining field, but by 1970 the machine miners had forced the granting of a 283.3 ha area in the Scrub Lead for the purposes of machine mining. The entry of the Thai buyers to the field resulted in an unprecedented boom in sapphire mining and in the spread of machine mining. Illegal mining became rampant and the Mines Dep;artment began to lose administrative control of the fields. The rift between the mining factions widened still further and threats of violence increased. Numerous Government proclamations under the Mining Act were issued in an attempt to maintain orderly mining operations. Detailed geological investigations were instituted during 1973 to acquire knowledge of the origin and distribution of the sapphires. The program was abandoned with only one of four 1:100 000 Sheet areas geologically mapped. The Thai buyers used the conflict between the mining factions to gain economic control of the field through their buying methods. By 1976 they could control the prosperity of the fields and when Sri Lanka (Ceylon) re-opened their gem fields to the Thai's, they abandoned the Central Queensland sapphire fields. As a result, the sapphire industry in Queensland collapsed.

335

17.12

C A I N O Z O I C VOLCANISM ALONG THE NORTHWESTERN M A R G I N O F T H E MARYBOROUGH B A S I N

A.D. Robertson Q u e e n s l a n d D e p a r t m e n t of M i n e s

From the end of the Cretaceous to late Pleistocene, six periods of short-lived, predominantly basic volcanics have been recorded adjacent to the western edge of the Maryborough Basin in the Bundaberg-Gin Gin area of Queensland. The volcanics were extruded from a series of vents located either within the late Triassic to midCretaceous basin or within the Carboniferous to Lower Triassic sediments in the Gympie Block. In the vicinity of Gin Gin, the controlling influence of the Electra, Bullyard and associated faults on the emplacement of the volcanics is most pronounced. With the exception of the Pleistocene Hummock Basalt, the various volcanic outpourings are now represented by outcrops of restricted areal extent and thickness. Outpourings of quartz and olivine tholeiitic basalt of the Gin Gin basalt and the Pemberton Grange Basalt occurred in the Paleocene to Early Eocene. The Pemberton Grange Basalt is a subsurface unit known only from the base of the Bundaberg Trough and below the Elliott Formation in the Coonarr Creek area. The Gin Gin Basalt is confined mainly to the Tertiary Wallaville Basin where it is partly overlain by lateritised Elliott Formation and other Tertiary volcanics. Near Maroondan, the Gin Gin Basalt shows the mixing of basic and acidic (rhyolitic) lava. During the mid- to late Eocene, transitional basalt was extruded and intercalated with a sequence of sandstone, mudstone and siltstone in the northern part of the Wallaville Basin. Lateritised and deeply weathered basic volcanics crop out in the vicinity of Childers and Goodwood. In most instances outcrop is poor and fresh rock is almost nonexistent. The volcanics are feldspar phyric with tholeiitic affinities. A tentative age for these volcanics has been given as Oligocene to early Miocene. Nepheline-rich lava has been erupted, sometimes violently, from eight vents in an area extending from Lake Monduran in the north, to Tirroan west of Gin Gin in the south, and to Kolan South in the east during the late Miocene and Pliocene. The outpourings from the Tararan vent and a vent at Stony Range west of Lake Monduran are the largest. The Tararan and Maroondan vents are tuff-lava cones, the Tararan vent being the best developed. The remaining six vents can be classed as either fissure vents or low lava cones. Classification of the lavas based on a revised classification scheme proposed by Coombs and Wilkinson indicates that the lavas are mainly ne-hawaiite and basanite with little nephelinite, although many of the rock types contain no feldspar and have clinopyroxene as a major constituent. Xenoliths and megacrysts are common in the tuffs and early flows originating from the Tararan vent and in the flows emanating from the fissure vent at Stony Range. Xenoliths and megacrysts generally appear to be absent in the other lavas. During the Pleistocene (1.1 to 0.9 Ma), at least three flows of basaltic lava (Hummock Basalt) ranging in composition from me-mugearite to olivine tholeiite were extruded from a vent at Sloping Hummock east of Bundaberg. The basalt is weU exposed in coastal headlands between Burnett Heads and Elliott Heads and has been recorded as remnants on the sea floor within 3 km of Burnett Heads.

336

The Berrembea Basalt occurs as a sequence of volcanic flows emanating from a single ill-defined vent in the vicinity of Berrembea- South of the Burnett River in the Wallaville area, a low conical hill adjacent to the Bruce Highway may be a point of extrusion of basalt of equivalent age to that of the Berrembea Basalt. A similar structure has been mapped on Burnett Camp Creek in the same general area. The composition of the basalt (Berrembea Basalt and equivalents) based on the modified Coombs and Wilkinson classification ranges from hawaiite to basanite. The Berrembea Basalt is considered to be younger than the Hummock Basalt and may be as young as the Barambah Basalt (0.6 Ma) extruded in the Coalstoun Lakes area further to the southwest.

337

6.12. WHICH ROCKS REALLY CONTAIN DIAMONDS? N.M.S. Rock Department of Geology, University of Western

Australia

For almost a century, until the late 1970's, kimberlites were thought to be the only primary source of diamonds. The face of diamond exploration changed forever when rich diamond deposits were recognised in lamproites, notably at Argyle in NW Australia (Jaques et aL 1986), but also in the USA, Zambia, and India. At the 4th International Kimberlite Conference in Perth (August, 1986), it became apparent that other closely related rock-types can also be diamondiferous. The Wandagee (NW Australia) diamondiferous rocks, previously described as 'kimberlites' and 'picrites', are now redescribed as monchiquites (alkaline lamprophyres: Jaques et al 1988). Alnoites and related ultramafic lamprophyres have also yielded rare but certified diamonds in S.Africa and Australia, with further possible diamonds in Swedish and Canadian examples (Rock 1987,1988). At present, Australia is the only country known to host diamondiferous kimberlites, lamproites and other lamprophyres, yet none of the diamonds reputedly associated with Australian basaltic rocks (notably in NSW) has been certified as coming from the basalts themselves (MacNevin 1977). Therefore, the range of undoubted diamondiferous rocks can be expanded from kimberlites and lamproites alone, to cover nearly all varieties of lamprophyres, but as yet no further. The century-old maxim: "where there's diamond there's (been) kimberlite", is thus best replaced nowadays with a revised maxim: "where there's diamond there's (been) lamprophyre". The only lamprophyres not yet known to carry diamonds are minettes, spessartites and related varieties (calc-alkaline lamprophyres: Rock 1984). Most such rocks occur in orogenic or arc environments associated with granitic plutonism — an alien environment for diamonds. However, a few examples are known from non-orogenic areas (e.g. Navajo, USA), and here they may carry mande xenoliths indicating depths of origin (> 130 km) not far short of the diamond stability limit in the mantle (150 km). Furthermore, the contemporaneous emplacement of lamproites with calc-alkaline lamprophyres has recently become evident in India (Gondwana coalfields), England (Hercynides) and the USA (Ouachitas). Calc-alkaline lamprophyres should not therefore be entirely disregarded in the search for diamonds — or indeed for other precious stones, which they do carry in significant quantities locally (e.g. Meyer & Mitchell 1985). Rich diamond deposits are still only known from kimberlites or lamproites emplaced in ancient cratons (or craton margins). However, an increasing number of stable cratons and environs are proving to be the sites of repeated and varied lamprophyric-alkaline magmatism over long periods of geological time. For example, around the Kimberley craton (NW Australia), kimberlites (Skerring pipe etc), lamproites (Argyle and W.Kimberley fields) and ultramafic lamprophyres (Bow Hill dykes) were variously emplaced in the mid-Proterozoic, late Proterozoic and Miocene. Overall, the occurrence (in a suitable tectonic environment) of any lamprophyres on a large enough scale may in future prove to be sufficient grounds for a diamond exploration programme. The large number of varied lamprophyre discoveries in Western Australia in the 1980's (Jaques et al. 1986), which shows every sign of multiplying still further (Bettenay et al. 1988; Rock & Barley 1988; Rock et al. 1988; Robey et aL 1988) means that Australia will be a major world focus of both theoretical and exploration models for diamond into the forseeable future. An increasingly apparent connection between long-lived cratons, diamond pipes and major platinum-group element deposits (Groves et aL 1987), and a possible role for calc-alkaline lamprophyres in the genesis of gold deposits (Rock et aL 1987) may further stimulate such exploration. 338

In the past, 'true' kimberlites have been repeatedly and globally confused with alnoites and other ultramafic lamprophyres, not least because no reliable criteria for distinguishing the indicator minerals of kimberlites and alnoites have been available for routine heavy mineral exploration. Thus alnoites can contain pyrope garnets and picroilmenites, which extensively overlap the compositional fields of traditional kimberlite indicators. Insufficient is yet known about the petrogenesis of alnoites (notably their depth of origin) to suggest whether they may ever yield commercial quantities of diamonds. As a result, much recent exploration effort in Australia has gone into rocks which may be intrinsically less productive than classical kimberlites. There is consequently an urgent and immediate need for criteria not only to identify potentially diamondiferous rocks at the earliest stages of exploration in specific areas, but also to delineate the field of diamondiferous rocks in general more closely. Using very large databases of over 20,000 whole-rock and mineral analyses, these problems are being addressed via the use of multivariate and nonparametric statistical techniques (e.g. multigroup discriminant analysis, multidimensional scaling). Results so far indicate that the whole-rock compositions of alnoites, monchiquites and laniproites are readily distinguishable. Whole-rock discrimination between alnoites and kimberlites is also possible, though less reliable because of the difficulty of separating magmatic from contaminant phases. More reliable discrimination between kimberlites and alnoites is, however, achievable via cognate ilmenite, phlogopite and spinel compositions. Most interestingly for exploration, discrimination can still be achieved using stream sediment mineral compositions — despite their complex admixture of cognate and xenocryst phases from a variety of source rocks. An expert computer system is ultimately envisaged, which will incorporate all potentially discriminatory information on diamondiferous rocks (compositions, petrography, geological setting, etc.). After suitable iterative refinement, the geologist will be able to enter all available information on his source area into this system and be presented with an indication not only as to what the source rock-type is (petrologically), but also whether it has theoretical potential to yield large, small or negligible quantities of diamonds. REFERENCES BETTENAY L.F., ROCK N.M.S. & MATHER P. 1987. Ultramafic dykes of lamprophyric affinity from the Shaw Batholith area, Pilbara Block, Western Australia (submitted). GROVES D.I., HO S.E., ROCK N.M.S., BARLEY M.E. & MUGGERIDGE M. 1987. Archean cratons, platinum and diamonds: evidence for long-lived, coupled crust-mantle systems. Geology (in press). HALLBERG J.A. 1985. Geology and mineral deposits of the Leonora-Laverton area, northeastern Yilgarn block, Western Australia. Hesperian Press, Perth, 140 pp. JAQUES A.L., CREASER R.A., FERGUSON J. & SMITH C.B. 1985. A review of the alkaline rocks of Australia. Trans, geol. Soc. S.Afr. 88, 311-335. JAQUES A.L., LEWIS J.D. & SMITH C.B. 1986. The kimberlites and lamproites of Western AustraUa. Bull. Geol. Surv. W.Aust. 132, 268pp. JAQUES A.L., KERR I.D., LUCAS H., SUN S-S. & CHAPPELL B.W. 1988. Mineralogy and petrology of picritic monchiquites from Wandagee, Carnarvon Basin, Western Australia. Spec.Publ.Geol.Soc.Aust.(in press). MACNEVIN, A.A. 1977. Diamonds in New South Wales. Geol.Surv.NSW Miner.ResourRept. 42,125pp. MEYER H.A.O. & MITCHELL R.H. 1985. Sapphire-bearing lamprophyres from Yogo Gulch, Montata. Geol.Soc.Can.AnnMeeting, Fredericton (New Brunswick), Abstr.Progr.A39. (abstr.) ROBEY J.V.A., BRISTOW J.W., MARX M.R., JOYCE J., DANCHIN R.V. & ARNOTT F. 1988. Alkalic ultrabasic dykes of the south-east Yilgarn margin, WA. Spec.Publ.Geol.Soc.Aust.(in press). ROCK N.M.S. 1984. Nature and origin of calc-alkaline lamprophyres: minettes, vogesites, kersantites and spessarutes. Trans. R. Soc. Edinburgh: Earth Sciences 74 iP3-227. ROCK N.M.S. 1987. The nature and origin of lamprophyres: an overview. In Fitton, J.G. and Upton, B.G.J. (eds). Alkaline Igneous Rocks. Spec. Publ. Geol. Soc. London 30, 191-226. ROCK N.M.S. 1988. Kimberlites as varieties of lamprophyres: implications for geological mapping, petrological research and mineral exploration. Spec.PublGeol.SocAust.(m press). ROCK N.M.S. & BARLEY M.E. 1987. Lamprophyres from the Pilbara Block, Western Australia (submitted). ROCK N.M.S., DULLER P., HASZELDINE R.S. & GROVES D.I. 1987. Lamprophyres as potential gold exploration targets: some preliminary observations and speculations. Univ.WAust.Geol Dept. & Extension PwN.ll,271-286. ROCK N.M.S., HALLBERG J.A., GOLDING S.D. & MATHER P.J. 1988. Lamprophyres in the Yilgarn Block, Western Australia (submitted).

339

18.5 GEOLOGICAL NUMERACY AND COMPUTERACY IN AUSTRALIA - WHERE DO WE GO FROM HERE? N.M.S. Rock Department of Geology, University of Western

Australia

All recent predictions by politicians, economists and scientists are that the mining industry will continue to bear much of the strain for bringing Australia out of her economic doldrums. As mineral deposits can only get harder to find, use of more and more sophisticated techniques, involving computers, numerical and statistical methods, must inexorably increase, if the industry is even to maintain its competitiveness. Unfortunately, the recent monumental AMIRA (1987) study confirms a yawning gap between future expertise requirements, and available numerical/computing capabilities of professional geologists, middle managers and recent graduates alike. Australian geologists, on average, are still taught rather less about such methods than their colleagues in the USA, Canada, the UK and Japan, despite the efforts of several training programmes. The general scientific problem of Australia's remoteness (Anderson 1985) has significant drawbacks here, such as the restricted or belated availability in Australia of the latest hardware and software, spectacular mark-ups in price, and inaccessibility to the expertise and advice of large software houses. Given the difficulties now being experienced in attracting experienced geologists from abroad, the only medium-term solution is for increased exposure of resident geology students and professionals to teaching courses in the applications of computers and numerical methods to real geological data. In the geological professions, most organisations (commercial, educational and govemmental alike) routinely produce large volumes of geological data (field measurements, sample locations, remote sensing data, rock and mineral analyses, fossil assemblages, age determinations, borehole logs, etc.), which cry out for systematic computer storage and rigorous interpretation. Although a number of organisations (reviewed by Merriam 1981) have made great strides in the implementation of databases and data-systems, in all too many others, data are no more than filed in bottom drawers as piles of ageing paper, or at worst discarded altogether. Organisations which interpret accumulated data in an informed way, using advanced numerical techniques via proven statistical packages and modem computer networks, are probably too few to support the medium-term economic need. The situation is exacerbated by the notorious 'generation gap' as regards computers, in which geologists trained prior to the late-70's will have had littie exposure to computers even as postgraduates, whereas recent graduates may have had some exposure as undergraduates and possibly even at school, and those of tomorrow will be familiar with them from childhood. This gap is unlikely to disappear until at least the end of the present century. Most Australian geological organisations seem to fall into a blend of the following categories: 1) no computing is done at all; all interpretation and data storage is manual; 2) senior management is prepared to purchase computers, often at considerable cost, but neither they nor most other employees know how to use them; most real data interpretation and storage consequently remains manual methods; 3) computers are used regularly by a minority of young employees, who in some cases face an uphill struggle trying to persuade their colleagues and line management of their value. The larger the organisation, the higher in general the category seems to be, although there are many exceptions. All these scenarios, however, are already cost-inefficient and will become more so as acquiring new data becomes more expensive (salary + analysis costs) than making fuller use of existing data (computer costs). In tertiary education, obstacles to improving numeracy are equally formidable: geology students tend to have a much less mathematical background than those in other sciences, and indeed often still enter geology courses because they believe them to be free of numbers and computers. The datum in the haul up the numeracy learning-curve is thus exceptionally low-lying. A problem common to both students and professionals is that geological data are among the most difficult of scientific data to interpret. This reflects: (1) poor laboratory control over data acquisition, and difficulty of experimental design (non-random sampling, etc.); (2) non-normal data distributions; (3) multivariate complexity; (4) small, unbalanced data-sets; (5) closure (as in 340

percentages and all other compositional data); (6) randomly missing data-values; (7) high internal correlations between variables leading to ill-constrained matrices; and (8) the enormous theoretical difficulty in simultaneously treating very common multivariate geological data where variables differ by several orders of magnitude (e.g. major and trace element an^yses of rocks). Geological data require the most advanced computing, mathematical and statistical skills, but it is now increasingly difficult for a professional to keep up with his own geolo^cal speciality, let alone the gargantuan and rapidly-expanding fields of computers and statistics in addition. As a result, some current geological data analysis might be deprecated by mathematicians and statisticians as misguided, 'black box' application of inappropriate techniques. There is still a widespread tendency to regard computers as magic boxes, capable of producing stupendous results from very few data and no program. This is particularly true in the 'toying' of the mining industry with geostatistics — an advanced and complex technique which is still highly contentious even among professional geomathematicians, and has been widely described as an "art" rather than science. Geologists cannot expect 'correct' answers from geostatistics if they do not understand the basic statistics behind the technique (and their limitations). Overall, there is no substitute to the gradual dissemination of numeracy and computeracy knowledge at all levels of the geological profession. Experience with new courses at the University of WA suggests that most geologists, however limited their mathematical and computing background, can still quickly appreciate the potential of computers in their work, whether this be palaeontology, petrology, primary survey, exploration or mining. Exposure even to the basic software of word-processors, spelling-checkers and graphics programs can overcome any psychological barrier to computers. Computer-aided instruction using systems such as PLATO (Mann 1976) can be a considerable further help, and can be a particularly good introduction to computers for undergraduates with no previous experience. PLATO now has a basic selection of specifically geological lessons (e.g. naming minerals from the physical properties) as well as a wider range of statistical and computing lessons. The advent of powerful, user-friendly micros such as the Macintosh™ is particularly timely since, without studying the daunting and impenetrable multi-volume manuals for mainframe computers, students can now quickly produce high-quality text and maps, allowing them to concentrate their effort on the scientific rather than production aspects of their work. Similarly, the power and simplicity of recentiy-released statistical packages allows students to experiment with different approaches to their data, and get a far better 'feel' for its behaviour than formerly (even though the dangers of cookery book statistics cannot be over-emphasised). Most students prove as willing to increase their numeracy as their computeracy, even though some simply do not have the background mathematics to cope with any but the simplest statistical techniques. Just as the national move is towards 'adding-on' value to raw mineral exports, students also quickly appreciate the 'add-on' value that accrues to themselves when they acquire numeracy and computeracy. Apart from being better able to produce coherent, grammatical and well-presented reports in a shorter time (and thus at less cost to their organisation), their enhanced skills give them ^eatly increased versatility. Thus computerate geologist5can assist field geologists, palaeontogists, petrologists, structural geologists and geophysicists alike, given that many statistical methods (e.g. discriminant analysis) have now found widespread application in all these specialities. Should their own geological speciality suffer a retrenchment, their computer skills can always be put to use in another field under less strain. In business, finance and many other professions, the maxim 'information is power' has been a hallmark of the InfoTech 1980s. This has perhaps only filtered down to geologists so far in their stock market speculations (!), but the next few years will surely bring the realization to all established organisations that their accumulated data (in some cases representing more than a century, and thousands of man-years, of effort) represent an information resource of prodigious value — monetary as well as scientific. This revelation has even reached the politicians in some countries, leading to the funding of national geoscience data-centres (e.g. in the UK). Once such programmes are begun, computerate geologists will be busy well into the 21st century REFERENCES AMIRA (Australian Mineral Industry Research Association).1987. Computer Applications in the Australian Mining Industry . AMIRA, Melbourne. Anderson, A. 1985. Science at a very great distance. Nature 316,185-201. Mann, C.J. 1976. The PLATO system: its language, assets and disadvantages in geological education. Computers & Geosciences 2,41-50. Merriam, D.F. 1981.Use of computers by geologists in Australia. Computers & Geosciences 7,323-326. 341

15.4 THE MARINE EVIDENCE FOR SUBDUCTION OF TASMAN SEA FLOOR BENEATH AUSTRALIA AND A PRE-TASMAN CONTINENTAL RECONSTRUCTION WHICH INCLUDES THE LOST AREA W.D. Roots School of Earth Sciences, Macquarie University, North Ryde

A summary of marine magnetic anomaly patterns from oceanic margins where, as in the west Tasman Sea, the COB is not orthogonal to the adjacent fracture zones, indicates that the west Tasman margin has unique geometry for a passive margin. All oblique-to-spreading margins, other than the Australian-Tasman margin, are flanked by marine magnetic quiet zones, the result of the constraints placed on MOR-spreading-segment length by the close proximity and relative obliquity of the margins as spreading began. The (oblique) west- and (orthogonal) east-Tasman margin geometries are not a matching pair, and so pre-Tasman continental reconstructions using the present sea-floor area and made by reversing opening-pole rotations, all have over-laps to the north and under-laps to the south. In addition, there is a contrast between east-Tasman and west-Tasman ratios of FZ lengths to FZ opening-pole latitudes, presenting rigorous proof that both sides of the Tasman Sea developed between a pair of matching orthogonal margins, before the west Tasman Sea lost area beneath Australia by subduction, to reach its present non-orthogonal geometry. A pre-Tasman Sea reconstruction is presented after first replacing the subduction-consumed area of the west Tasman Sea floor so as to match the sea floor magnetic and structural patterns across the MOR.

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8.35

PETROGENESIS OF SKARNS FROM THE CHILLAGOE AREA OF NORTHEAST QUEENSLAND M.J. Rubenach and G.W. Morrison Department of Geology, James Cook University of North Queensland

The petrogenesis and sources of metals are the principal aims of a research project on the skarns of the Chillagoe area. Besides a study of the Redcap Creek "magmatic" skarns, representative of the higher temperature skarn processes, work is currently concentrating on the mineralized skarns, which form two principal metal associations. Group A is characterized by Pb, Zn and Ag, with additional Cu,, Sn, W, Mo and Bi, while Group B has the association Cu-Au, with additional Zn, Pb, Bi and Ag. The Group A skarns are typically andadite-rich, with or without magnetite, clinopyroxene, vesuvianite and wollastonite. They have formed at or near contacts between the Chillagoe Formation (Siluro-Devonian) and granodiorite or adamellite plutons of the Tate Batholith (late Carboniferous). Irregular lobate contacts, roof pendants and faults are the most highly favoured sites for skarn formation. The massive garnet skarns in many cases have replaced granitoid as well as, or in addition to, limestone. Other endoskarn tjrpes include veins in granitoid and extensive areas of "bleaching" where hornblende, biotite and magnetite of the granitoid, have been replaced by very light green clinopyroxene and sphene. The patterns of distribution of the exoskarns and various endoskarn types are providing insights into the pathways of fluid circulation and the factors controlling this circulation. The Group B skarns, represented by the Red Dome deposit, appear to be localised along a southeast trending linear zone extending from Red Hill to Fluorspar. The zone is locally occupied by late Carboniferous subvolcanic intrusions and volcanic rocks and is interpreted as the southwestern boundary to the zone of cauldron subsidence. The Red Dome deposit is an unusual combination of calcsilicate and quartz stockworks, both endo- and exoskarns formed at the contact of a zoned rhyolite porphyry plug with limestone, mafic/intermediate volcanics and clastic sediments. Compared with Group A skarns, the Red Dome skarns and porphyry clearly formed in a fluorine-rich system. Wollastonite-garnet-(cuspidine-ferrobustumite), garnet-magnetite and clinopyroxene skarns are cut by green andradite-rich garnet skarn. Free gold and tellurides are associated with bornite, chalcocite and sphalerite. The Redcap Creek skarns are related to multiple dykes and irregular tongueshaped and lobate bodies of diorite occurring at the southern contact of the "Belgravia Granodiorite" (an outlying pluton related to the Tate Batholith) and the Chillagoe formation. The diorite has been largely replaced by garnet-diopside-wollastonite endoskarns. The limestone localized between sets of dykes or tongues was replaced by early stage course-grained massive tilleyite, in turn overprinted by second-stage skarns consisting of an inner melilite-dominated zone (with garnet, and less common tilleyite, wollastonite and hydroxyl-ellestadite), and an outer zone of wollastonite-vesuvianite-melanite garnet. Higher temperature alteration included breakdown of the ackermanite component of melilite to spurrite, monticellite, spinel and xanthophyllite, and minor lower temperature alteration includes vesuvianite, rustumite, bicchulite, dellaite, prehnite, serpentine and chlorite. The first and second stage

343

skarns probably formed at 2 Kb, around 600-800^ C. The second stage assemblages imply extremely low XCO2, and are believed to have formed during a focussed single pass flux of magmatic fluids from the granodiorite, transporting components from already solidified diorite to the previously-formed tilleyite and calcium to the diorite. The geometry of granodiorite-diorite-limestone contacts was initial to the formation of these skarns, and the proposed model is believed to apply to other "magmatic" skarns such as Crestmore (California) and Tombstone (Arizona).

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8.22

MAGMA EVOLUTION ON THE EASTERN MANAGALASE PLATEAU, PAPUA NEW GUINEA B.P. Ruxton Canberra College of Advanced

Education

The Managalase Plateau lies above the Papuan ultrabasic belt in north-east Papua which forms the south-western margin of the Solomon plate. It is sandwiched between two large volcanoes; the Pliocene shoshonite Sesaro volcano to the south and the Pleistocene calc-alkali Hydrographers Range to the north. Mount Lamington lies close by to the north-west. The Papuan ultrabasic belt is believed to have overridden the Owen Stanley Metamorphics in early Tertiary time. Johnson (1979) postulates a choked former arc trench system with sleeper magma released by uplift and normal faulting in the late Cainozoic. Seismicity in eastern Papua is weak and shallow and there is no Benioff zone. The Managalase Plateau is a structural north-east facing cusp of Cretaceous metabasalt and limestone (Lokanu Beds) veneered by Pleistocene to Recent volcanics between two terranes; the Papuan ultrabasic belt and the volcanoes and basins adjoining the Solomon Sea. Some forty eruption centres occur on the eastern Managalase Plateau (Ruxton 1966) in a pattern suggesting faulting and a possible ring structure. From west to east across the eastern Managalase Plateau alkali basalt gives way to rhyodacite and then shoshonite. Minor andesite and hornblende dacite occur in the centre. The plot of potash versus silica clearly distinguishes the shoshonites with negative gradient from the regular calc-alkaline series with positive gradient. ^Dating (Ruxton and McDougall 1967) shows the alkali basalt activity began some 440,000 years ago and has continued to modern times. The rhyodacite and shoshonite activity is at least 80,000 years old and has continued up to 500 years ago. Andesite lava was erupted 500 years ago. The alkali basalt and rhyodacite have low strontium 87/86 ratios. It is possible that one zoned complex magma chamber is tapped at different levels by different faults. Recent work centres on the intermediate and acid rocks as the banakite converges on the calc-alkali andesite compositions. A 500 year lava flow of andesite has resorbed olivine and quartz in the same thin sections and its composition can be obtained stoichiometrically by mixing basalt and rhyodacite. It is a mixed magma analagous with the Taos Plateau, New Mexico (McMillan and Dungan 1986). The dacite lavas have phenocrysts of plagioclase with oligoclase cores and andesine rims. This suggests zoning in the magma chamber. Mixing of magmas is also supported by the presence of resorbed hornblende and biotite in the basalts of both the calc-alkali and shoshonite series. Recent tephra shows a 9150 year rhyodacite coarse ash, overlain by 6450 year dacite pumice lapilli and capped by 500 year mixed deposits. The pumice is compositionally zoned dacite and rhyodacite with three main amphibole members, pale, green and brown in different layers. Alternations of micropumice and massive glass in the fine ash fraction probably reflect slight fluctuations in the intensity of the eruption pulses as at Mount Saint Helens. The pumice fragments are rich in iron (5.9 per cent) probably due to a content of magnetite and this corresponds with a high water content. The upper mixed tephra is composed of basalt ash, rhyodacite ash and explosion pit debris and in places these eruptions were synchronous.

345

Synchroneity of basalt, rhyodacite and explosion pit activity might mean a sill of magma being emplaced under the Managalase Plateau or an active zoned complex magma chamber. It is not clear yet whether the activity of the Managalase Plateau represents the dying stages of offshoots of the Pleistocene Hydrographers volcano or the embryonic birth stages of a new large strato volcano in Papua New Guinea. Both the shoshonite series and the calc-alkali series are now producing andesite by differentiation and mixing respectively. In the rhyodacites the average plagioclase (An36) and green hornblende has the same composition as those of the Mount Lamington andesite ash but the biotite (Arculus et al 1983) has lower Mg/ (Mg + Fe) ratios (0.32-0.38). Biotite is uncommon in Mount Lamington ash but very common in the rhyodacite ash and lavas. Analysis of glass show a trend to more acid from the andesites to the rhyodacites. Evidence so far indicates that phenocrysts of similar composition are shared by a wide variety of magmas and in the more basic ones they are partly or nearly completely resorbed. The biotite and acid glass content distinguish the rhyodacite from the andesite. REFERENCES Arculus, R.J., Johnson, R.W., Chappell, B.W., McKee, C.O., 1983. Jour. Volcanol. Geothermal Res. 18, 215-247. Johnson, R.W. 1979. 181-207.

Sakai,

Bureau of Mineral Resources Geol. & Geophys. Jour., 4,

McMillan, N.J. and Dungan, M.A. 1986. Ruxton, B.P. 1966.

and

Jour. Geophys. Res., 91, 6029-6045.

Bull. Volcanol., 29, 347-374.

Ruxton, B.P. and McDougall, I. 1967.

Amer. Jour. Sci. 265, 545-561.

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10.4

COLLUVIUM AND SLOPE STABILITY IN HONG KONG B.P. Ruxton Canberra College of Advanced

Education

Hong Kong is in the humid mountainous tropics with a mean annual temperature of 21.6^0 and a mean annual rainfall of 2134mm per annum. There is rapid and deep weathering and mass movement of the thick regolith is common especially in torrential rain storms. Seismic activity is minimal. Regolith failures on the ridges and scarps cause a blanketing of the side slopes with boulder colluvium which may re~slip and re-deposit several times on its journey towards the sea. In the past mass movements were mainly debris flows and most of the colluvial blanket is fossil. Today torrential rain causes groups of debris avalanches. The colluvium and underlying weathering profiles are roughly layered parallel with the hillslopes and most of the slips are planar. Many of the present slips occur on the side of alcoves especially if adjacent to previous slips. The slip debris may reach the harbour and become interbedded with the alluvial succession. In some cases it becomes run-out facies as at Taitam Bay. Thin colluvium acts as a sponge causing excessive weathering in bedrock. Intermediate thickness colluvium is often rafted off in landslides. Thick colluvium tends to armour the slope often being very stable and in places there is an inversion of relief with thick V-shaped colluvium mantling ridge crests. Debris slopes up to 38° mantled by boulder colluvium are more stable than laboratory shear strength testing indicates (up to 35 ). The discrepancy may be due to lack of in situ shear strength testing and failure to take into account the positive or negative effect of high percentages of boulders. Other suggestions are the effect of suction, pipe networks, and cementation (Ruxton 1987). Research has concentrated on Victoria Peak, Hong Kong Island (Geotechnical Control Office 1982) where rocky cUffs provided the source of debris flows mantling the debris slope (up to 38°) and ramp slope (15-25°) which is now a built up area. The boulder colluvium has mostly acid volcanic boulders and rests on either decompo^d acid volcanic or granite bedrock. Re-slip of this regolith has posed a problem for many years and a disastrous slide of colluvium caused many deaths in 1972 at Po Shan Road. This slide was regressive and time dependent (Howat 1985a) caused by excavation at the toe and is in contrast with the rain dependent slides so common in cloudbursts. Blocked pipes could be an important factor in causing some landslides in intense rain periods. Most slips occur on slopes of 27-45 with an average of 35°. On the slopes of Victoria Peak the residence time of the colluvium may be up to or over a hundred thousand years and locally pockets of colluvium are up to 32m in thickness. For the most part the colluvium is well graded and matrix supported. The colluvium was deposited in 2-4m thick overlapping lobes each of which spent varying time periods at the surface undergoing soil formation before being buried. The colluvium is made up of fresh and weathered boulders in a variably weathered matrix and continued weathering

347

after deposition can produce up to 40 per cent clay. Reworked colluvium with infilling between boulders also occurs. Bulk density varies from 1.3 to 1.7 and low values are often younger colluviunK^ Porosity is 30-40 per cent and the permeability ranges from 30-350 x 10 m/s in older colluvium. In 1980 (GCO 1982) it was suggested that some colluvium was cemented and it has now been found that cementation takes place in three stages. Firstly during soil formation the soil fauna produce water stable aggregates of sand and silt size kaolin-quartz-goethite. Secondly neoformation of halloysite struts connect up the water stable aggregates. Thirdly aluminium goethite connects up the framework. These three phases of cementation may be reinforcing. The first two occur on the debris slope and all three occur on the ramp slope. Neoformation and hamrolytic alteration thus overlap with pedogenesis. Cementation prevents compaction and the colluvium may be sensitive. It can take higher stresses due to cohesion but then collapses to a residual shear strength. Cementation may take place in as little as 8500 years (Howat 1985b). At and below the mid-slope concave break of slope in many areas of the tropics is the locus of iron seepage and ironpan formation. It is in this area that Leach and Herbert (1982) postulated a low pemeability layer at the base of the colluvium. On Victoria Peak the iron cementation is most pronounced on the upper ramp slope and So (1971) has shown a dearth of landslides on slopes of 18-26° corresponding with this zone. Below 18 washouts and blow outs are common in rainstorms. Slope dynamics are speculative but slope retreat of the rocky cliffs on Victoria Peak is expected to be about 40-50cm/1000 years and all the. colluvium could have been produced in about 200,000 years. Glass in airfall ash about half way up the colluvium sequence has very thick hydration rims and preliminary dating results would agree with this age. Rough estimates indicate that if debris flows are equally spaced in time and we allow for slip, res lip and res lip then there would be an event of the magnitude of Po Shan Road (38,000m ) every 250 years. This figure is similar to the frequency of extreme events caused by abnormal rainfall in other area. REFERENCES Geotechnical Control Office, 1982. Hong Kong. Howat, M.D. 1985a. 399. Howat, M.D. 1985b.

So, C.L. 1971.

Public Works Dept.

Proc. 1st Inst. Conf. Geomechanics, Brasilia, 3, 395Geol. Soc. Hong Kong Newsletter, 3(6), 6-12.

Leach, B . & Herbert, R . 1982. Ruxton, B.P. 1987.

Mid-levels study.

Quat. Journ. Eng. Geol. 15, 243-259.

Geol. Soc. Hong Kong Bulletin 3, in press.

Trans. Inst. Brit. Geogr. 53, 55-65.

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9.7

STUDIES OF EARTHQUAKE RISK - A MULTIDISCIPLINARY APPROACH J.M.W. Rynn

Department of Geology and Mineralogy, University of Queensland During 1987, revised earthquake risk maps for Australia were produced based on the most up-to-date earthquake catalogue for Australian earthquakes through 1984 (Gaull et al., 1987). The compilation of these probabalistic risk maps was based on the Corne11-McGuire method in collaborative studies by the Australian Seismological Centre and the University of Queensland. In the companion study for Northeastern Australia (Rynn, 1986), the risk study was undertaken through a multidisciplinary approach. That is, although the earthquake history was compiled in terms of seismology, geology and tectonics, the second phase of research was performed in conjunction with civil engineering and insurance requirements. As a result of this more practical earthquake engineering approach, an appreciation of the uncertainties in both the basic analyses and the dissemination of the results into design criteria was forthcoming. Earthquake occurrences, particularly in those areas of urbanisation which are the continental or intra-plate regimes, are low probability events. This in itself presents a problem in interpreting an extrapolated result from a relatively small data sample. However, such numerical (i.e. probabilistic) solutions must be attempted as engineers seek quantitative solutions to integrate into their design criteria. This naturally must include some quantitative information on uncertainties for relating to the required factors of safety (Rynn and Boyce, 1987). The multidisciplinary approach covers the disciplines of seismology, geology, tectonics, remote sensing, civil engineering, insurance and sociology. This necessitates a link from fundamental to applied research. The initial criteria in risk studies concerns the limitations to the earthquake data, both instrumental and macroseismic (from felt reports). These must be taken into account when using earthquake catalogues. In subsequent analyses of the data, many pitfalls are encountered. These include determination of the required earthquake parameters, relationships between the parameters and application of statistical methods of regression. The application of these seismological results is thus critical to engineering considerations. The uncertainties and realistic variations of critical parameters are precisely the factors which influence the risk assessments applicable to engineering design. There must be an understanding of strong-ground motions, usually in respect of peak accelerations. These recorded data, and, in their absence macroseismic data (as is the typical situation currently for Australia), are vital in attempting to define the attenuation of seismic energy relationships. This is now the prime concern in earthquake engineering. As there is a worldwide trend towards Limit State Design, information on the design earthquake (particularly values for the maximum magnitude expected and

349

maximum possible vertical acceleration possible) is paramount. The close collaboration between the seismologist and design engineer is thus mandatory. GAULL, B.A., MICHAEL-LEIBA, M.O. and RYNN, J.M.W., 1987. Probabilistic earthquake "risk maps for Australia. Bureau of Mineral Resources Journal of Australian Geology and Geophysics (in press). RYNN, J.M.W., 1986. Queensland Seismic Risk Study. Final report to the State Government of Queensland, University of Queensland, Department of Geology and Mineralogy Report, 191 pp. RYNN, J.M.W. and BOYGE, W.H., 1987. Earthquake risk assessment and engineering design - a vital consideration for the Pacific Rim. In "Proceedings of the Pacific Rim Congress 1987", The Australasian Institute of Mining and Metallurgy, 729-735.

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13.3

MACERAL AND MINERAL MATTER ASSOCIATIONS OF COLLIE COAL, COLLIE BASIN, WESTERN AUSTRALIA K.K. Sappal Department of Geology and Geophysics, Curtin University of Technology, Perth

Western Australia's commercial or potentially commercial coal resources occur in sediments ranging in age from Permian to Tertiary, At present, the Collie Basin of Permian age situated approximately 150 km SSE of Perth and 27 km east of the Darling Fault, is the only commercial field providing coal for local consumption and power generation. The Collie Basin is subdivided into three sub-basins; Cardiff, Muja and Shotts. The three sub-basins cover an area of approximately 230 km^ within a basement complex of Archaean granite and gneiss. In broad terms the stratigraphy of the Collie Basin is simple and consits of three sedimentary formations overlying Archaean basement. At the base the earliest Permian (Sakmarian) Stockton Formation, is overlain conformably by the Early to Late Permian Collie Coal Measures, which is overlain unconformably by the Nakina Formation of Cretaceous to Tertiary age. The sequence is similar in all three sii>-basins, except that uppermost Permian Coal Measures are absent from the Shotts Sub-basin. Maceral and Mineral matter associations of coal samples from the nine seams (Ate, Bellona, Ceres, Diana, Eos, Flora, Galatea, Hebe and lona) being mined at the Muja O p ^ Cut Mine, Muja Sub-basin are discussed in this paper. The coal is non-coking, low ash, moderately high in moisture and is classified as sub-bituminous. Proximate analyses show that the moisture content is between 25.94% and 31.33%, ash is between 2.54% and 7.46%, volatile matter ranges between 22.07% and 27.11%, fixed carbon between 37.80% and 43.56%, and the sulphur content is low and varies between 0.29% to 0.69%. The specific energy is between 18.00 and 20.52 MJ/kg, and thus the coal is suitable for steam raising. The Collie Coal is finely banded and the dominant lithotypes of the coal are dull, dull banded, banded types, with minor bright and bright banded types. Hie maceral composition of the individual seams is variable, and the dominant maceral groups are vitrinite and inertinite, and the exinite and mineral matter contents are low. The maximum relfectance (R0% MAX) of vitrinite lies between 0.43% to 0.49% which is within the range of sub-bituminous rank of coal. The relative proportions of minerals associated with maceral groups in the nine coal seams is shown in Fig.l. The major mineral groups identified in the coal are; clays, pyrite, carbonates and quartz. The clay minerals are the most impoi±ant group of minerals in Collie Coal, they range from 1.1% to 6.1%, and occur mostly in association with inertinite and vitrinite, either as infillings in plant cells or as finely dispersed inclusions in vitrinite. The clay minerals are probably products of chemical and biochemical processes active in the coal swamp. The pyrite content varies between traces to 0.8%, and it occurs as infillings in the cell cavities of fusinite and semi-fusinite. The association of pyrite with vitrinite and exinite is rare in this coal. The carbonate minerals

351

siderite, calcite and dolomite range from traces to only 0.3%, and siderite is common amongst the carbonates and it occurs as infillings in cell cavities of'^ fusinite and semi fusinite. The possible origin of siderite may be due to interaction between iron in the ^t's pore water and carbon dioxide produced by formation of the organic matter. The quartz is present in all nine seams and it varies between traces to 1.0%. Most of the quartz is associated with inertinite and vitrinite. The distribution of macerals and mineral matter in size fractions of pulverised coal shows the similarity of trends in all nine coal seams of the Muja Sub-basin. VITRINITE ^-EXIMITl. INERTINITE, MINERAL MATTER

35"

IJo

ATE A T I LOWER KUONA CCRitl

•ALATlAl

•

TOTAL • •

VITRINITE • EXfUTE

SULPHUR %

INERTINITE

(OAF)

O MINERAL MATTER

SULPHUR CONTENT

Figure 1.

Mineral mtter and iiacerals

352

3.11

CURRENT PALAEOMAGNETIC CONSTRAINTS ON T A S M A N T E R R A N E ANLAYSIS P.W.

Schmidt

CSIRO Division of Mineral Physics Mineralogy, North Ryde

and

Palaeomagnetic data offer one of the few methods, and probably the most quantitative method, for identifying and tracking tectonic terranes now assumed to comprise many ancient foldbeits. However, recent evidence from the Lachlan Foldbelt and other foldbeits (both within Australia and overseas) emphasises the ubiquity of secondary magnetisations acquired at the time of deformation, i.e. syn-deformational remagnetisation. There is a clear need for existing palaeomagnetic data to be scrutinised with this in mind, because not only must the ages of many magnetisations present in rocks of the Lachlan foldbelt now be considered as uncertain or unknown, but the appropriate palaeohorizontals are also in doubt. Only when the palaeohorizontal of a particular magnetisation can be established is it justifiable to calculate a palaeomagnetic pole position, and in folded terranes this can only be established with confidence through the application of the fold test. The indiscriminate use of palaeomagnetic data has serious geological consequences and the stringent application of selection criteria is mandatory before such data may be used to constrain tectonic models. The number of Silurian and Devonian palaeomagnetic results from Australia that survive these criteria is insufficient to determine, with confidence, whether or not the Lachlan Foldbelt (or parts thereof) has undergone large motions with respect to the palaeocraton. Fig. 1 depicts an apparent polar wander path based on the published data that pass the selection criteria, i.e. data for which a palaeohorizontal and/or age has been established. The path presented has the attributes of accommodating all the Lachlan Foldbelt data without violating known age (-of-magnetisation) constraints. Thus, at present all data from the Lachlan Foldbelt can be considered to fall on a single path and there is no requirement, from palaeomagnetic considerations alone, to invoke largescale tectonic motions within the Lachlan Foldbelt. On the other hand, preliminary data from the New England Foldbelt (specifically the Lower Carboniferous Merlewood Formation) can not be incorporated in the pole path of Fig. 1 and suggest large relative motion of New England with respect to the craton. The palaeomagnetic directions are similar to those reported by Klootwijk (1985) who also suggested relative motion. The timing of this motion is not clear but from a negative fold test it would seem to have been after at least some deformation in the Tamworth Trough, but before New England had reached high latitudes at the end of the Palaeozoic. Since the Australian palaeocraton is thought to have reached high latitutdes by the Permo-Carboniferous, this implies that New England was a separate entity prior to accretion, presumably in the Late Permian. Reference Klootwijk, Chris, 1985. Palaeomagnetism of the Tasman Fold Belt: Indication for mid-Carboniferous large-scale southward displacement of the New England region, in Abstracts of the Third Circum-Pacific Terrane Conference, Geol. Soc. Aust. No.14 (ed. Evan Leitch) ppl24127. 353

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6.11

MINERALOGICAL M A P P I N G OF ALTERATION IN OUTCROP AT MOUNT LEYSHON, NORTHEAST QUEENSLAND, USING X-RAY DIFFRACTOMETRY K.M.

Scott

CSIRO Division of Mineral Physics and Mineralogy, North

Ryde

Mineralization at Mt Leyshon, 24 km south of Charters Towers, occurs in a brecciated and altered Permo-Carboniferous volcanic complex on the boundary between the Ordovician Ravenswood Granodiorite and the Cambrian Puddler Creek Formation (Henderson, 1986). The deposit was discovered in 1872 and worked from 1887-1916 (Levingston, 1972). Prior to the recommencement of mining in 1986, a suite of 316 samples was collected from the original surface. The mineralogy of these samples was determined by X-ray diffractometry, with the exact compositions of alunite-jarosite family minerals being determined by electron microprobe. These methods enabled a 1 km diameter zone characterized by jarosite, abundant muscovite/illite and Fe oxides (mainly hematite) to be identified. Within that zone, a smaller 500 x 300 m area centred on Mt Leyshon was defined by the development of alunite and kaolinite and the absence of K-feldspar. Other alunite sub-group minerals e.g. plumbogummite and hinsdalite, also occur exclusively within this zone, but the distribution of natroalunite extends out into the surrounding larger zone, south east of Mt Leyshon. Of these diagnostic minerals, Fe oxides, jarosite, plumbogummite and hinsdalite are secondary, derived from sulfides. Natroalunite formation requires solutions with Na/K (atomic) > 10 (Parker, 1962) and hence it is unlikely to form during weathering. The occurrence of jarosite replacing alunite as adjacent pyrite weathers also implies that at least some of the alunite may be hypogene. However, Bird et al. (1987) dated alunite and natroalunite at Mt Leyshon at 2-3 million years using K/Ar methods. Thus the origin of alunite and natroalunite is unclear. Similarly kaolinite could be either a weathering or hydrothermal alteration product. Nevertheless the development of consistent alunite and kaolinite zones which host Au mineralization suggests, by comparison with acid-sulfate epithermal systems,^ that these minerals are substantially hypogene. Because the yellow coloration of jarosite or Fe oxides may discolour white alunite, alunite cannot be confidently identified visually in outcrop. Thus laboratory-based techniques (like X-ray diffraction) are needed to supplement field mapping of the distribution of key minerals associated with epithermal gold deposits. References

Bird, M.I., Chivas, A.R. and McDougall, I., 1987. Isotopic studies of surficial alunite in Australia. Research School of Earth Sciences A.N.U. Annual Report 1986. 107-108. Henderson, R.A., 1986. Geology of the Mt Windsor Subprovince - a Lower Palaeozoic volcano-sedimentary terrane in the northern Tasman Orogenic Zone. Aust. J. Ea. Sci. 33:343-364. Levingston, K.R., 1972. Ore deposits and mines of the Charters Towers 1: 250,000 sheet area, North Queensland. Geol. Surv. Qld. Report 57. Parker,

R.L., 1962. Isomorphous substitution alunite. Amer. Miner. 47: 127-136. 355

in natural

and

synthetic

2.2 6

STRATIGRAPHY OF THE CLARKE RIVER BASIN, NORTH QUEENSLAND M. Scott Queensland Department of Mines

This paper will consider the stratigraphy, petrography and structure of the Clarke River Basin, North Queensland. The relationship between the "Oweenee Granite* and the Meath Rhyolite Member of the Clarke River Basin will also be considered. The Clarke River Basin is one of four Late Devonian to Carboniferous sedimentary basins (the Burdekin, Clarke River, Bundock, and Gilberton basins) developed in the Townsville hinterland. Field work by GSQ officers in 1984 resulted in the following revision of the stratigraphy: 1. subdivison and raising of the Clarke River Formation to group status; 2. definition of Venetia and Lyall Formations; and 3. recognition of the Meath Rhyolite Member and Furry Hoopiion of the Meath Rhyolite Member and Furry Hoop Member in the Lyall Formation. Venetia Formation. The Clarke River Formation maybe divided into two regionally mappable units. The Venetia is the lowermost of these, being distinguished by the absence of quartz-porphyry/volcanic fragments in its lithofeldspathic sandstone. The dominant lithology is a white, buff, coarse micaceous lithofeldspathic sandstone with stringers of pebbly sandstone. These are interbedded with siltstone and minor tuff. The basal part of the formation is a fossiliferous/calcareous siltstone. The lower part of the Formation is interpreted partly as a marginal low energy marine environment, the non-marine position is interpreted as representing an piedmont/braided-stream, fluvial, (lacustrine?), and deltaic environments. Lyall Formation. Forming the upper part of what was previously the Clarke River Formation, the Lyall Formation now includes two named members, the Furry Hoop Member and the Meath Rhyolite member. The Lyall Formation comprises a lower and an upper subunit seperated by the Meath Rhyolite Member. The lower subunit is composed largely volcanoclastic sandstones, pebble to boulder conglomerates and tuffs. The Furry Hoop Member, consisting mainly of red-green siltstones/fine sandstones, is part of this lower subunit sequence. The Meath Rhyolite Member consists of tuffaceous sandstones, altered ignimbrite and crystal tuff. It is believed to be a distal equivalent of volcanics which were previously mapped as 'Oweenee Granite^ and form the eastern boundary of the Clarke River Basin. The upper subunit of the Lyall Formation has calcareous siltstone, tuffaceous sandstones and limestones near the base which are overlain by coarse volcanolithic sandstone, boulder-cobble conglomerate. 356

The pebbly eongloTiierates of the lower and upper subunits of the Lyall Formation are poorly sorted and contain local and laterally extensive tough cross-stratification; these features indicated a torrential, braided environment. In contrast, the largly argillaceous, calcareous and well-laminated beds of both the Furry Hoop Member and the basal siltstones of the upper subunit of the Lyall Formation suggest quiet shallow waters. Structure, basement rocks, mainly flysch, are strongly deformed (probably two deformations. By contrast, the Clarke River Group is generally only weakly deformed, dips being mostly less than 30 degrees. Faulting in the basin is mostly vertical and has a marked e f f e c t with: faulting of underlying basement rocks resulting in monoclinal structures; both western and northern edges of the basin being, at least in part, fault bounded; and, the control of sediment deposition locally within the basin.

TERTIARY LATERITE ALKALI BASALT RHYOLITE INTRUSIVES UPPER SUBUNIT

]

MEATH RHYOLITE

] LYALL FORMATION

LOWER SUBUNIT

]

VENETIA FORMATION BASEMENT

G e n e r a l i s e d g e o l o g y of the c e n t r a l p a r t of main o u t c r o p a r e a of C l a r k e R i v e r Group.

357

the

7.20

E N V I R O N M E N T S O F D E P O S I T I O N IN NORTHEASTERN CARPENTARIA BASIN N . Senapati and D.J.

THE

Bourke

Exploration Department, Comalco Aluminium Ltd, Brisbane

A refined stratigraphy for the northeastern Carpentaria Basin has been established (Table 1). The basal units are dominantly fluvial in origin and there is an increasing marine influence on the deposition of the sediments with a rise in the stratigraphic column. The climate throughout the Carpentaria Basin sedimentation was tropical, probably with seasonal rainfall and flooding. The pre-Mesozoic surface of the northeastern Carpentaria Basin was a mature topography with gentle relief. There were highlands in the east from which extended valleys and ridges that tended in a northeast direction and were a result of basement geology and structure. The geology of the valleys consisted of sediments and volcanics, with some of the valleys containing relatively undisturbed Permian sediments. The ridges were composed of Sefton Metamorphics and Carboniferous granites. At the onset of Mesozoic sedimentation moderately sinuous fluvial systems occupied the valley floors and deposited mainly quartzose sands of the Wenlock Member. The sediments came from the highlands in the east and had a unimodal transport direction. Each valley's fluvial system was similar but probably unconnected (Figure 1). There was considerable vegetation and the water table was high facilitating the development of discontinuous coal seams. The fluvial systems in the valleys and the eroded ridges were then transgressed by a shallow marine or open lacustrine environment, which left the highlands in the east exposed. On the available evidence, the environment was localised, not extending much further south than the Pascoe River area. Ooliths, chamositic rich sandstone and mudstone were deposited forming the Bromley Member. Bottom water temperatures were >20° C . The sea or open lake then regressed and left a topography of extensive low relief plain. As the climate was tropical with seasonal rainfall and flooding, sediments were carried from the eastern highlands. They were deposited in bed-load dominated, braided, fluvial systems (Figure 2). The braided rivers migrated laterally depositing extensive sheet sands which became the Wreath Member. The environment was probably similar to the present day Gilbert River system in the southern Gulf of Carpentaria. The history from this period o n , was that of a marine transgression. The encroachment of the sea being towards the east due either to subsidence and/or eustatic changes in sea level. The transgression took place in the form of pulses with progradation of sediments between each pulse. The transgressive sequence started with the deposition of the Batavia Member in a distal meandering river environment which was transitional between a braided river and a near shore marine system (Figure 3). There was a tidal influence on the deposition of the sediments with the dominant palaeocurrent direction orientated towards the east. The next two transgressive pulses deposited the Glennie and Briscoe Members. The coarsening upward cycles of these members have bioturbated laminated mudstone at their bases overlain by marine sandstone with 358

Skolithos burrows. They formed in a wave and tide dominated deltaic to shallow marine environment (Figure 4) where tidal reversals along with longshore currents played a role in their deposition. The environment was probably similar to the present day coast of the southern Gulf of Carpentaria. After the deposition of the Briscoe Member another sea level change took place that caused a further transgression and the deposition of the basal part of the Rolling Downs Group. This sequence of glauconitic mudstone and sandstone was possibly deposited in a shallow marine environment, below wave base and in waters cooler than 15°C. However, the glauconite is not mineralogical glauconite but chloritic, which may suggest a warmer temperature, more consistent with the tropical climate of the period. Basinward, in Z.C.L. Weipa 1, there is up to 650 m of Rolling Downs Group suggesting the marine conditions of deposition for the glauconitic mudstone and sandstone continued for a long period and possibly into the Tertiary. Table

itolling Downs Croup

XlbUn

Afti*n to Early Briscoa M««b«r Gilbert Rivtr CUrxnia Mambar |i«ocaal*A

Forsation BatavU Hanbar

Kidil. to Utt Jurassic

Xraath Kaobar Carraway Sandstona

Broolay Maabar «MM0vt* rm

Early Jurassic

Wanloclc Mambar

DISTAL

MODERATE SINUOSITY FLUVIAL SYSTEM WENLOCK

MEMBER

MEANDERING

RIVER

BATAVIA MEMBER

^^^ ^

Fig

•MtCT M M T M M

«rta VtMTATM

COftTTM. MaMCII »MM

BRAIDED FLUVIAL

SYSTEM

WREATH MEMBER

WAVE - TIDE DOMINATED DELTA SYSTEM GLENNIE a BRISCOE MEMBER

359

Fig 4

2.34

SEISMIC REFLECTION PROFILING ON BEENLEIGH BLOCK, QUEENSLAND

M.J, Sexton, K.D. Wake-Dyster

and D.W.

THE

Johnstone

B u r e a u of M i n e r a l R e s o u r c e s , Geology and Geophysics, Canberra

The Beenleigh Block lies along the Queensland seaboard between Brisbane and the N.S.W. border (Fig. 1). It consists largely of intensely deformed, Devonian-Carboniferous deep-water marine trench fill turbidites with minor ocean floor basalts and chert (Day et al., 1983). Recent work interprets the block as an eastward younging accretionary wedge formed in a subduction complex (Murray et al., 1987; Korsch and Harrington, 1987). However, its relationship to the other basement blocks (i.e. South D'Aguilar, Yarraman and Coff's Harbour) in the region has led to divergent tectonic models for the block. Murray et al. (1987) consider it to be a normal component of the New England orogen whereas Korsch and Harrington (1987) consider it to be an exotic terrane formed along strike to the subduction complex and subsequently accreted to the orogen. Seismic data collected by the Bureau of Mineral Resources (BMR) west of the Beenleigh Block shows a thick, deeply buried, layered sequence overlain by a non-reflecting wedge of material (Fig. 2, Line 16). This led Korsch et al. (1986) to propose that the Beenleigh Block is a thin skinned feature thrust over a deep, layered sequence which in turn was formed by thrust faulting of a sedimentary wedge into a 20 km. thick stack. Both were then accreted to the New England orogen by underthrusting. Further seismic work over the Beenleigh Block during 1985 (Fig. 2, Lines 1 and 2), reveals the same non-reflective zone near the surface overlying strongly coherent relectors that show evidence of gentle arching. The general character of the events can be correlated confidently with those from the previous seismic work; although direct correlations are impossible. Extrapolating the base of the non-reflective zone east and north, it intersects the surface near the eastern side of Stradbroke Island and slightly south of Brisbane respectively. These positions correspond very well with the mapped margins of the Beenleigh Block in these directions. Detailed gravity surveys by petroleum companies and along the BMR seismic lines have allowed a higher resolution gravity map of the region to be produced (Fig. 3). Evident on this map is a strong gravity gradient along the western side of the South D'Aguilar Block. South of this block, the gravity gradient bifurcates with a western limb corresponding to the position of the West Ipswich Fault and the eastern branch to the western margin of the non-reflective zone seen on Line 16. This zone is interpretted to represent the Beenleigh Block. It is therefore a thin-skinned feature approximately 5 km. thick. Noting the basically similar geology, the gravity trends and the structural interpretation of the Esk Trough-South D'Aguilar region by Leven (1977), it is felt that the South D'Aguilar and Beenleigh Blocks are part of the same structural entity. What then of the layered sequence? It is felt that this represents part of the accretionary wedge of the Devonian-Carboniferous subduction complex. The layering is accentuated by low angle thrust faults. To the west, the

360

Fig. 1: Main structural elements and BMR seismic traverse locations west ESK TROUGH-LAIDLEY SYNCUNE 0 -

WEST IPSWICH FAULT

SOUTH MORETON ANTICUNE

CLARENCE LOGAN SYNCUNE

BEENLEIGH BLOCK

Non-ren®ctive zone

- 4

Layered sequence

•

-8

12-

-12

Line 1 1610 —

1-16 2 0 km

Fig. 2: Seismic sections of BMR lines 16 (eastern part), 1 and 2.

361

layering vanishes as the sequence has been intruded by numerous post-orogenic plutons of late Permian and Triassic age. These are exposed to the north in the D'Aguilar Block but are covered by sediments of the Ipswich and CIarence-Moreton Basins in the area of the seismic line. A broad, high amplitude magnetic anomaly over the seismic line further supports the possibility of basic intrusives at depth. The Beenleigh/South D'Aguilar Block is thought to represent the uppermost part of the accretionary wedge that decoupled from the material below during the accretion of the Gympie Terrane during the mid-Triassic (Harrington and Korsch, 1985). It underwent more intense deformation than the underlying layered sequence, and as a result became non-reflective. The South D'Aguilar Block thrust over part of the Esk Trough producing deformation of the rocks in that area. Further south, the Esk Trough sediments remain virtually undeformed. The movement of the Beenleigh Block along a strike-slip fault led to the rifting which produced the Ipswich Basin (Korsch et al, in prep). References: Day R.W., Whitaker W.G., Murray C.G., Wilson I.H. and Grimes K.G. 1983. Queensland geology. Geological Survey of Queensland Publication 383, 1-194. Harrington H.J. and Korsch R.J. 1985. Deformation associated with the accretion of the Gympie Terrane in eastern Australia. In_ Leitch E.C. ed. Third Circum-Pacific Terrane Conference Extended Abstracts. Geological Society of Australia Abstracts 14, 104-108. Korsch R.J. and Harrington H.J. 1987. Oroclinal Bending, Fragmentation and deformation of terranes in the New England Orogen, eastern Australia. In Leitch e.c. and Scheiber E. eds. Teranne accretion and orogenic belts. American Geophysical Union Geodynamics Series 19, 129-139. Korsch R.J., Lindsay J.F., O'Brien P.B., Sexton M.J. and Wake-Dyster K.D. 1986. Deep crustal seismic reflection profiling. New England Orogen, eastern Australia: Telescoping of the crust and a hidden deep layered sedimentary sequence. Geology 14, 982-985. Korsch R.J., O'Brien P.E., Sexton M.J., Wake-Dyster K.D. and Wells A.T. (in prep). Development of Mesozoic transtensional basins in easternmost Australia. Leven J.H. 1977. A gravity survey of the southern Esk Trough, southeast Queensland: Department of Geology, University of Queensland Papers 8, 25-36. Murray C.G., Fergusson C.L., Flood P.G., Whitaker W.G. and Korsch R.J. 1987. Plate tectonic model for the Carboniferous evolution of the New England Fold Belt. Australian Journal of Earth Sciences 34, 213-236.

362

11.2

INTEGRATION OF GEOSCIENCE DATABASES IN THE BUREAU OF MINERAL RESOURCES E.P,

Shelley

Bureau of Mineral Resources, Geology and Geophysics,

Canberra

One of the key inputs to geoscience research and exploration is the body of data and information from past research and exploration activities. This data and information is published in technical journals and is also available in the public domain from government research and organisations. Increasingly researchers and explorationists are requiring the raw data on which maps and publications are based in order to carry out their own interpretation. There is also a demand for a growing range of data types together with the computing power and facilities to integrate, compare and correlate data sets and to convert data into image and graphical forms. In the Bureau of Mineral Resources our strategy for database integration has two key components: the development of geoscience databases in relational form; and the upgrading of our image processing and digital cartography facilities. Relational databases are being developed using the ORACLE database management system on BMR's Data General MV/20000 computer. The ability to integrate data by relating different data tables to each other avoids the need to duplicate databases and thus results in improved productivity. A good example is a table of map sheet names and numbers - data which are commonly used in many geoscience databases. In a relational database system this table needs to be created only once. All users can then link their own data tables to it. As a prerequisite for the development of many geoscience databases in BMR a place names database was created in relational form using data from the National Mapping Gazetteer digital file. This database contains data on almost a quarter of a million place names used on the Australian 1: 250 000 series of topographic maps. The place names database is currently providing geographic data for the GEODX bibliographic database which contains references to Australian stratigraphic names, BMR publications and Australian geoscience generally. The database currently contains some 18 000 references. It is of interest that GEODX is one of the first bibliographic databases to be created using the relational model. In its turn GEODX provides the bibliographic source of the data in MNDEP a database which contains comprehensive mineral deposit data. The portrayal of geoscience data in image and graphical form results in the data being much more easily understood and interpreted. Technology now allows us to easily combine and correlate image and graphical data which were originally at different scales and projections. BMR has been actively involved in digital cartography and image processing for many years. Steps are now being taken to integrate these with the geoscience database environment which will also allow us to produce maps as an end product. W e expect the first part of a new system to be in place within the next twelve months.

363

17.6 CHEMICAL A N D ISOTOPE SYSTEMATICS OF INTRAPLATE BASALTS - IMPLICATIONS FOR M A G M A GENESIS OF CENOZOIC BASLATS IN EASTERN AUSTRALIA A N D NEW ZEALAND S. Shen-Su Bureau of Mineral Resources, Geology and Geophysics,

Canberra

A variety of mantle reservoirs with different chemical and isotopic characteristics may be generated through geological time by differing sequences of physio-chemical processes. The major processes include subduction of altered oceanic lithosphere, dehydration and partial melting of the slab and re-mixing into the convecting mantle, upward migration of silica-undersaturated melts from the asthenosphere into the lithosphere, thermal erosion and delamination of the continental lithosphere and mixing into the convecting mantle and mixing of deep mantle, plume with * plumpudding* type asthenosphere. Interaction of different mantle reservoirs gives rise to the spectrum of geochemical characteristics observed in basaltic rocks. In the oceanic environment, different degree of partial melting of a * plum-pudding' type asthenospheric mantle (MORE source) may generate the chemical and isotopic heterogeneities observed in seamount basalts (Zindler et al. 1984). With small degrees of melting, fertile * plums' or veins will be preferentially melted to generate alkali basalts whereas under larger degrees of melting of both fertile *plum* and the more refractory mag^ix g ^ H contribute to the melt thereby generating tholeiites with lower Sr/ Sr but higher eNd values similar to MORE. This is in contrast to the mantle plume related volcanism such as the Hawaiian volcanism. It involves differing relative contributions of the mantle plume, asthenospheric MORE source, and oceanic lithosphere. Sh^^ld ^gilding tholeiites are mainly derived from the plume and have higher Sr/ Sr and lower eNd than the overlying post-erosional alkali basalts. These alkali basalts were generated after the island was removed from the hot spot and their isotopic and trace element characteristics bear a strong signature of the depleted MORE-type source. Furthermore thermal budget considerations require that high temperature tholeiites formed by large degrees of partial melting are derived mainly from a deep convecting mantle source whereas lower temperature alkali basalts could have been derived from thermally activated lithosphere or its underlying asthenosphere. Intraplate alkali basalts from both continental and oceanic environments generally share the same chemical and isotopic characteristics whereas the tholeiig^s gge commonly different. Continental tholeiites commonly have higher Sr/ Sr and lower eNd than the coexisting alkali basalts, and they often show chemical and isotopic signatures of subduction zone processes. Neither the seamount nor Hawaiian model can explain such differences. A major factor responsible for such contrasts between the oceanic and continental intraplate volcanism is the lithosphere. The oceanic lithosphere is relatively young (_< 200 Ma) and has been formed through relatively simple processes whereas the continental lithosphere has been continuously formed since early history of the Earth by different tectonic processes resulting in a variety of chemical and isotopic characteristics. The extent of lithosphere involvement in intraplate volcanism depends on the tectonic development and thermal condition of the lithosphere below the studied area. In a stable craton, small amounts of silica-undersaturated melt may be generated within the lithosphere by thermal perturbations whereas in areas where rifting and thinning of the lithosphere is well developed, the convective asthenosphere is the main magma source (e.g. Perry et al. 1987). Since tholeiitic magmas are generated by magma

364

separation at shallower depths and ascend more slowly than the alkali basalts, continental lithosphere may have a greater influence on the trace element and isotope geochemistry of the tholeiites. Much intraplate volcanism is induced by lithosphere subduction (e.g. western U.S. and Japan Sea-NE China). Dehydration and partial melting of the subducted oceanic crust may cause melting of the refractory wedge material and contribute to the chemical and isotopic characteristics of basalts generated near the subduction zone. This process could also modify the overlying continental lithosphere. After the subduction process is terminated, further growth of the lithosphere and upward migration of silica-undersaturated melt from the asthenosphere may impose an "oceanic" intraplate signature on the base of the continental lithosphere. Chemical and isotopic composition of the mid-Jurassic Tasmanian dolerites and similar tholeiites from Kangaroo Island, S. Australia, show strong subduction zone influence (Hergt, 1987). They are most likely back arc volcanics related to the Mesozoic subduction at the margins of Gondwanaland. It is reasonable to suggest that the continental lithosphere of south eastern Australia has also been affected. Involvement of such lithosphere in magma generation could explain some distinctive chemical (e.g. Nb/La Ce/Pb %10) and isotopic characteristics of the Newer Volcanics in W. Victoria and basalts in southern N.S.W. Basalts of 'Central'-volcano type (hot spot related) in E.Australia have been affected by crustal contamination to various extents. (Ewart, 1982). However, the least fractionated basalts from the Clermont, Springsure, Main Range, Tweed. Comboyne and Warrumbungle provinces have a cut-off lower limit of ST/ Sr ^>0.7036 (plume source character?). This value is considerably higher than values for nearby lava field basalts (0.70270.7035, mainly 0.7030-33). Higher Sr/ Sr values (up to 0.7043?) are observed in lava field basalts of north Queensland. This is similar to the Newer Volcanics in Victoria. Available seismic and magneto-telluric data suggest that a deep melting zone exists under SE Australia near Tasmania. Major asthenospheric contribution, at least for the hot-spot related volcanism can be expected. Occurrence of both HIMU (high y)and Dupal type isotopic character in some Cenozoic basalts of eastern Australia and New Zealand is unusual and deserves some comments. Some lava field type basalts from central Queensland, New England, Tasmania, South Island and the ^ggpbe^J^Plateau (New Zealand) and I^fgie Byrd Land of Antactica have higg^ ggb/ Pb '\.20 (hj.gh y=high U/ Pb in the mantle source) and low Sr/ Sr (0.702630). Before breakup of Gondwanaland SE Australia, New Zealand and Marie Byrd Land were closely juxtaposed. It is tempting to suggest that this HIMU character results from intraplate CO -rich metasomatic processes originating from the asthenosphere about 200 Ma ago (inferred from widespread alkaline volcanic activities at that time). However, in Tasmania this process would have had to have taken place after the Tasmanian dolerite event (%185 Ma) and to have wiped out the earlier memory in the lower continental lithosphere. Alternatively, some of these HIMU reservoirs (at least for Tasmania and Ross Island of Antactica, where a similar situation occurs) are located within the convecting mantle. Enriched Mantle Type 1 isotODic characteristics of the Dupal anomaly (e.g. Kerguelen Islands with Sr/ Sr ^.705, eNd-2) are observed in lithosphere-derived leucitites of S. Queensland and western NSW, whereas Enriched Mantle Type 2 (e.g. Society Islands Sr/ Sr ^.706, eNd+2) shares chemical and isotopic characteristics of the Tasmanian dolerites. It can be predicted that some Newer Volcanics of W.Victoria also have the EM2 characteristics. 365

19.2

AUSTRALIAN CAMBRIAN BIOCHRONOLOGY J.H. Shergold

(compiler)

Bureau of Mineral Resources, Geology & Geophysics, Canberra

Progress towards a highly resolved biochronological scale for the Cambrian of Australia has been slow and intermittent.

Historically,

it has

developed from the work of three palaeontologist-stratigraphers: R. Etheridge Jr (I88O-I92O), F.W. Whitehouse (1927-19^5) and A.A, C^ik (I9561982)»

Essentially the first period was descriptive being a time when

exploration expanded and a gross stratigraphy was developed. But by the early thirties, Whitehouse was in a position for the first time to classify the three Cambrian epochs using successions of trilobite faunas, on a regional basis (Georgina Basin, western Queensland).

The faunal stages

erected by Whitehouse were accepted and widely disseminated in such important texts as the "Geology of the Commonwealth of Australia" which permitted their concepts to persist well into the fifties.

Whitehouse was

also the first to compare the Cambrian trilobite sequences of Queensland with the classic biostratigraphy of northern Europe.

Opik, through

intensive collecting and taxonomic palaeontology, refined the biochronology founded by Whitehouse, and was the first to produce a highly resolved Zonal Scale for the Middle and Early Upper Cambrian of northern Australia. Subsequently, Stages have been defined on the basis of these zones, which have now become a standard scheme,

albeit with problems.

biostratigraphy was to a large extent based on trilobites.

Opik's

This practice

has been followed by others, so that zonal schemes based on trilobites are also available for the Late Cambrian.

Additionally, conodonts among other

fossil groups have proven especially useful for the biological subdivision of the latest Cambrian and initial Ordovician.

Fossiliferous Early Cambrian sediments are characteristic of southeastern Australia, but a rigorous biochronological scheme is only now beginning to appear. Traditionally, these rocks have been classified on the basis of archaeocyathan and "small shelly fossil" successions, but these are now becoming implemented by zonations, of various types and at various levels of resolution, based on trilobites, brachiopods, and early molluscs.

366

An Australian biochronological chart has been compiled from all available data.

The Early Cambrian biochronological scheme is provisional:

important taxa of ichnofossils from central Australia,

selected

and archaeocyathans

from South Australia are shown in relation to a series of informal assemblages originally proposed in 1956. based mainly on archaeocyathans

faunal

Soviet biostratigraphical schemes

are shown for reference since

Soviet

terminology is frequently applied to such sequences in Australia.

The Middle Cambrian is zoned by trilobites which are assigned to five local stages,

one based on concepts derived from the Ord Basin and the remainder

from the Georgina Basin.

Both agnostoid and polymeroid t r i l o b i t e s have

been used for the 10 zones on which the biochronology is based, but the former

are preferred.

Ranges of selected i n a r t i c u l a t e brachiopods and

bradoriid ostracodes from the Georgina Basin are also shown since they have biostratigraphical potential.

T r i l o b i t e sequences from the Georgina Basin have also been used for the zonation of the Late Cambrian.

Nineteen assemblage-zones are distributed

among three formally named local stages,

and there is an informal interval

between the Idamean and Payntonian which is yet to be f i n a l l y diagnosed. Conodont assemblages become s t r a t i g r a p h i c a l l y useful during the latest C -^brian, and serve to define the beginning of the Ordovician.

The biochronological scale for the Cambrian of Australia is not adequately supported by geochronometry or magnetic stratigraphy.

Nevertheless,

the

chart includes an attempt to show an absolute time s c a l e , but i t s l i m i t s are to a large extent based on age determinations made on Chinese samples from

both

the

Accordingly,

beginning

(58O-9

m.a.)

and end

a Chinese biochronological

correlation adjunct.

367

scale

(500-5)

'.ncluded

also

the

Period. as a

7.18

SUBAQUEOUS AND SUBAERIAL VOLCANIC FACIES IN THE EARLY DEVONIAN TANGERANG FORMATION, SOUTHEASTERN NEW SOUTH WALES C. Simpson

Department of Geology, University of Wollongong

The Early Devonian Tangerang Formation consists of a sequence of subaqueously to subaerially deposited silicic pyroclastics and volcaniclastics interbedded with tuffaceous and quartzose epiclastics and minor limestone. It was deposited on the eastern margin of the Wollondilly Tract, a graben which formed by dextral shear during the Middle Silurian (Powell, 1983), and it crops out as a narrow belt approximately 40 km long between Marulan and Windellema in southeastern NSW. In this area the Tangerang Formation forms the basal unit of the Bindook Volcanic Complex, a subaerial cauldron-fill associated with extensive outflow ignimbrite sheets. Mapping in the northern part of the Tangerang Formation has delineated several thin flows of primary non-welded pyroclastic debris interbedded with tuffaceous sandstone and ash containing a shallow marine fauna of brachiopods, crinoid stems, corals and rare trilobites. These subaqueously deposited, probable pyroclastic flows consist of broken crystal fragments, relict pumice clasts and a variety of lithic fragments set in a matrix of fine-grained devitrified ash and relict glass s h a r d s . Upsequence and lateral reworking of the flows occurs but they can be traced for 8 km as continuous mappable units. Thickness variation, lithic clast size and the degree of lateral reworking indicate that at least two volcanic sources contributed detritus to the subaqueous deposits. The interbedded tuffaceous sandstone and reworked ash also display rapid lateral and vertical facies changes but Uthological associations can be defined on the basis of mineralogy, grainsize and palaeocurrent data. These sediments consist of variable amounts of volcanic detritus and quartz-rich material derived from the uplifted Ordovician horst along the eastern margin of the graben. In rocks of the northern Tangerang Formation, palaeocurrent directions in the tuffaceous sandstone sequence indicate transport to the north and south, parallel to the axis of the graben. The transition from a shallow marine to a subaerial environment is not clearly marked in the Tangerang Formation. A sequence of ash, reworked ash and fine-grained sandstone, deposited in a very low energy environment such as a lake or lagoon, gradually becomes coarser upwards with an increasing input of coarser-grained primary and reworked volcaniclastic detritus. These rocks are overlain by a succession of subaerial pyroclastic flow, fall and surge deposits approximately 950 m thick. Within this succession the basal 300 m consists of quartz and feldspar phyric pyroclastic flows which show evidence of a highly explosive and, at least in p a r t , a phreatomagmatic eruptive origin. Units consisting of single or multiple ignimbrites have been delineated but cannot be traced any significant distance north or south due to cover b y the Bindook Volcanic Complex or intrusion b y Early Devonian granitoids. Lithologies include lithic-rich and lithic-poor, welded and non-welded ignimbrites, accretionary lapilU-bearing ignimbrites, co-ignimbrite lag breccias and thin accretionary lapilli-bearing ash-fall and base surge

368

deposits. These quartz and feldspar phyric rocks are overlain by a sequence of feldspar phyric pyroclastics up t o 650 m thick. The latter sequence is dominated by welded ignimbrites with very rare ash units and no evidence of a 'wet' eruptive style. These feldspathic rocks can be correlated with a sequence of ignimbrites underljring the Bindook Volcanic Complex approximately 25 km to the north of the study area. A slight erosional break occurs between the Tangerang Formation and the extensive densely welded basal ignimbrite of the Bindook Volcanic Complex. Reference Powell, C. McA., 1983: Geology of NSW South Coast. S.G.T.S.G. Field Guide #1 Geol. Soc. Aust.

369

8 1 EASTERN ARUNTA ORTHOGNEISS SUITES ~ IMPLICATIONS FOR A REPEATED TRANSITION FROM RIFT TO SUBDUCTION-RELATED MAGMATISM DURING SUCCESSIVE PROTEROZOIC ENSIALIC OROGENIES W. Sivell School of Science and Technology, Nepean of Advanced Education

College

Four magmatic stages of contrasting geochemical character accompanied the Early to Middle Proterozoic crustal evolution of the eastern Arunta Inlier, central Australia. They reflect episodes of sequential extension and compression during each of two successive orogenies involving basement and cover sequences. Differences in trace element abundances and ratios between the successive magmatic phases reveal a mutistage geochemical evolution for the attendant Proterozoic sub-continental mantle. In the eastern Harts Range area of the Arunta Inlier, cover rocks of the Harts Range Metaigneous Complex and Irindina Supracrustal Assemblage structurally overlie Early Proterozoic basement sequences of the Strangways Metamorphic Complex, including both the Oonagalabi and Entia gneiss complexes. Two types of lithological association comprise the basement complexes. These are (1) early-formed, highly deformed supracrustal sequences of ortho- and paragneisses; and (2) younger (intrusive) amphibolite-felsic orthogneiss associations. The supracrustal suites include mafic granulites and amphibolites, calc-silicates, thin metapelitequartzite layers, banded iron formations and ultramafic rocks. These may form only a very minor portion of the basement terrains (e.g. in the Entia gneiss complex) due to gross plutonic invasion by felsic (and associated mafic) magmas. The orthoamphibolites and basic orthogneisses that form the metaigneous component of the early-formed basement supracrustal sequences show the geochemical features of rift-generated magmas of continental affinity. These include slight to moderate LREE-enrichment ((Ce/Yb)^ = 1.8 - 3.9), unfractionated HREE, and inferred low-pressure fractionation in the tholeiitic trend, dominated by removal of calcic-plagioclase and clinopyroxene. Gross plutonic invasion of these supracrustals by basic-ultrabasic and related granitoid intrusives occurred prior to and during a subsequent episode of compressional tectonism. Within the Entia gneiss complex, massive amphibolites of the intrusive series comprise .5 - 100 m thick layers up to 3 km in strike length, interlayered with quartzofeldspathic gneiss, and represent a suite of deformed and metamorphosed maficultramafic sills. The basic intrusives show geochemical signatures including strong LREE-enrichment ((Ce/Yb)^ = 3.0 - 12.3), and HREE levels that are both fractionated and depleted. Inferred moderate - to high pressure crystal fractionation was controlled, at least in the early stages, by removal of orthopyroxene-dominated crystal extracts. The pronounced decoupling between high field strength elements and highly incompatible LIL-elements and LREE (indicated by high Ba/Nb, K/Nb and La/Nb ratios) is a feature characteristic of convergent plate margin basaltic magmas, and may reflect metasomatism of the mantle source regions for these rocks by subduction-related fluids. Thus the basement intrusive suites are inferred to have formed in a convergent plate margin setting.

370

Chondritic Zr/Nb ratios (Zr/Nb = metabasites reflect an essentially early-generated magmatic suites.

16) for all Harts Range basement undepleted mantle source for these

In the Harts Range cover sequence, the Harts Range Metaigneous Complex comprises a number of concordant amphibolite bodies and associated anorthositic gneisses and ultramafic rocks, interlayered with pelitic, calcareous and quartzose schists and gneisses of the Irindina Supracrustal Assemblage. Within the Harts Range Metaigneous Complex in the Mt Mabel area, massive amphibolites and banded amphibolites comprise lower and upper units, respectively. Layering of the upper amphibolite sequence, on a variable but chiefly centimetre-scale, is defined by alternating melanocratic (basaltic) and leucocratic (tonalitic) layers concordant with the strong, regionally pervasive foliation. LREE-depleted ((Ce/Yb)jg = 0.8 - 1.0), Fe, Ti-rich tholeiites comprise the earliest-formed, rift-generated metabasites (lower amphibolites) from the cover sequence. Magmas with identical trace element contents are rare (though not unknown) among continental tholeiites world-wide. The cover tholeiites differ from those of the basement in being derived from a subcontinental mantle source that had undergone previous extraction of a basaltic melt component (giving rise to high Zr/Nb ratios (30-65) for all cover sequence tholeiites). Later-formed metabasites from the cover sequence (upper amphibolites), whilst displaying overall rift-related geochemical characteristics (e.g. slight LREE-enrichment ((Ce/Yb)^ = 2.5), unfractionated HREE, low-pressure tholeiitic differentiation trend), also show a pronounced arc-signature (i.e. high La/Nb, Ba/Zr, K/Zr etc.) attributed to the influence of a mantle-metasomatising subduction-related component. In summary, early-formed metatholeiite suites in both the basement and cover successions in the Harts Range show the geochemical characteristics of continental tholeiites. They comprise the metaigneous component of rift-generated supracrustal volcano-sedimentary associations. Later-formed metabasites in the basement and in the cover show some trace element characteristics resembling those of magmas from convergent plate margin settings. Incorporation of a subduction-related geochemical component within the inferred mantle sources for these rocks is required to account for their incompatible element contents and ratios. Thus, the geochemistry of the eastern Arunta basic orthogneiss suites indicates t^at the transition from (early) extensional to (later) convergent plate margin magmatism is a characteristic feature of each of the successive Proterozoic ensialic orogens involving the basement and cover sequences in the Harts Range area. The inferred magmatic evolution parallels in a classic fashion the distinctive style of Proterozoic tectonic evolution of the eastern Arunta deduced from structural analysis. Moreover, the basement and cover magmatic suites were derived from relatively undepleted and depleted mantle sources, respectively, necessitating large-scale compositional heterogeneity within the Proterozoic sub-continental mantle, and possibly resulting from a major mantle depletion event (extraction of a basaltic melt component) during Early Proterozoic time.

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17.7

INTRAPLATE BASALTIC VOLCANISM IN NEW ZEALAND

I.E.M. Smithl, S.D. Weaver^ and J.A. Gamble^ ^University of Auckland, New Zealand ^Canterbury University, New Zealand ^Victoria University of Wellington, New Zealand

Although better known for late Cainozoic volcanism associated with convergence of the Pacific and Australian tectonic plates, New Zealand also hosts significant areas of essentially basaltic volcanic rocks of intraplate character. These intraplate volcanic provinces fall into two geographically separate groups, namely the North Island provinces which lie on the Australian lithospheric plate, and the South Island and sub-Antarctic Islands provinces which lie on the Pacific lithospheric plate. New Zealand intraplate basaltic provinces can be further grouped into two categories according to the form and nature of their eruptive products as follows: 1. Mainly basaltic lava-tephra fields comprising relatively thin successions fed from monogenetic eruptive centres. Typically these consist of early pyroclastic cones made up of phreatomagmatic and/or magmatic deposits surrounded by later lava flows. Lava-tephra fields show the widest variety of mafic lithologies; the total spectrum ranges from nephelinite to quartz tholeiite and rarely to alkaline and peralkaline rhyolite although in any one field the rocks are either essentially undersaturated mafic types or transitional to saturated mafic types with rare intermediate and felsic rocks. 2. Central provinces comprising large composite shield volcanoes characterised by mafic lava of mildly undersaturated to transitional composition together with associated felsic derivatives. Central volcanoes typically show a relatively restricted compositional range from nephelinite to transitional basalt; intermediate and felsic rock types are only volumetrically significant in the large central volcano complexes. Hawaiite, either normatively saturated or undersaturated, is the most abundant rock type in both categories. The Northland and Auckland provinces in the far north of North Island are Pliocene and Quaternary lava-tephra fields each showing comparable eruptive styles but differing in their distribution of rock types and fractionation trends. The South and sub-Antarctic Island provinces range in age from Paleocene to Quaternary. Large central volcanoes form Banks Peninsula and Otago Peninsula and, to the south, the islands of the Campbell Plateau. Lava-tephra fields occur in North Otago, Canterbury and Marlborough. The volcanics of the Chatham Islands and south Westland, and the Alpine Dyke swarm of the Southern Alps, are also considered as lava-tephra fields. The South Island and sub-Antarctic Island provinces encompass a great variety of petrologic lineages derived from parental mafic magmas ranging from basanites through alkali basalts to tholeiites. No strongly undersaturated potassic rocks are known, nor are melilitites; carbonitite occurs only as a volumetrically minor component of the Alpine Dyke swarm. The variety of eruptive styles and fractionation lineages shown by the New Zealand intraplate basaltic provinces is attributed in part to differing crustal settings but is probably also controlled by different mantle thermal regimes as well as by mantle heterogeneity inherited from earlier magmato-tectonic events. 372

7.6

THE PALAEOGEOGRAPHY OF THE MURRAY BASIN, SOUTHEASTERN AUSTRALIA A.E. Stephenson and C.M. Brown Bureau of Mineral Resources, Geology and Geophysics, Canberra

T h e Murray Basin of southeastern Australia is a roughly circular feature, covering some 300,000 km^ of New South Wales, Victoria, and South Australia. It consists of a maximum of 600 m of Cainozoic sediments, overlying several disconnected infrabasins of Devonian, Permian, and Cretaceous rocks. Cainozoic subsidence was initiated shortly after the breakup of Australia and Antarctica in the late Cretaceous, and may be related to that event. A comprehensive study of Murray Basin surface geology and subsurface stratigraphy has been undertaken in BMR, as part of a joint Commonwealth-States Murray Basin Hydrogeological Project. Interpretation of these data has made possible a reconstruction of the Cainozoic palaeogeography of the basin. Tectonic subsidence has played a primary role in the development of the basinal succession, but the interplay between secondary tectonic, eustatic, and palaeoclimatic influences has largely determined sedimentation patterns within the succession. The low-lying nature of the Murray Basin has made southwestern and western areas particularly susceptible to partial flooding by epicontinental seas, and sedimentation patterns in eastern and northern areas partly reflect consequent fluctuations in the erosive and depositional potential of the fluvial systems that traverse the basin. A number of depositional sequences, each consisting of a package of genetically related formations have been identified within the sedimentary record of the basin. Thickness variations and the distribution of marine sediments within these packages appear to have been particularly sensitive to the interplay between tectonic and eustatic influences, while variations in Cainozoic palaeoclimates have also contributed to variation in sediment input and type between the packages. Cainozoic sedimentation commenced in the Palaeocene to Early E o c e n e with the deposition of fluvial sands in the central parts of the Murray Basin. By the Middle Eocene, carbonaceous sand, silt, clay, and peaty coal were being deposited over most of the basin. In the east, deposition of these fluvio-lacustrine sediments was to continue (with a possible mid-Oligocene hiatus) until the Middle Miocene, but in the western Murray Basin the first of a series of marine incursions took place in the Late Eocene, leading to initial deposition of shallow-water sediments in the Buccleuch Embayment. A major relative rise in sea level in the early Late Oligocene (or possibly late Early Oligocene) led to flooding of the western Murray Basin by a shallow epicontinental sea, and deposition of the shallow-marine to marginal-marine Murray G r o u p sequence. Palaeogeographic reconstructions suggest that a shallow-marine platform in the southwest was flanked to the east and north by a narrow zone of restricted-marine and lagoonal environments. These were in turn bordered by a marginal-marine zone, including extensive interdistributary bays and tidal flats, which were further flanked by peat-forming swamps and deltaic and fluvial environments. Deposition of the Murray G r o u p sequence was terminated in the Middle Miocene by a major marine regression.

373

Initiation of the last major Tertiary depositional sequence of the basin can be correlated with a further short-lived marine transgressive-regressive cycle in the Late Miocene to Early Pliocene. Palaeogeographic reconstructions suggest that a fluvial flood plain in the east and north was flanked to the west and south by an extensive strand-plain environment of prograding beach ridges separated by inter-ridge fluvial and estuarine quartz sand deposits. In the west the flood plain was connected to the Southern Ocean by a zone of fluvial and estuarine environments that roughly coincided with the present-day course of the River Murray. In the east and north of the Murray basin, aggradational fluviolacustrine sedimentation continued into the Quaternary, resulting in the development of the flat-lying Riverine Plain. In the west, tectonic damming of the ancient Murray River led to the formation of Lake Bungunnia, a fresh-water megalake which persisted for some two million years. The demise of Lake Bungunnia was followed by the onset of semi-arid conditions in the western Murray Basin, and the development of modern landforms of the Mallee. In the far southwest of the basin, glacio-eustatic sea level changes caused a number of short-lived marine transgressions. References: Brown, CM., 1983. Discussion: a Cainozoic history of Australia's Southeast Highlands. Journal of the Geological Society of Australia, 30, 483-486. Brown, C.M., 1985. Murray Basin, southeastern Australia: stratigraphy and resource potential. Bureau of Mineral Resources, Australia, Report 264. Brown, CM., & Stephenson, A.E., 1985. Murray Basin 1:1 000 000 scale geological map, preliminary edition. Bureau of Mineral Resources, Australia. Brown, C.M., & Stephenson, A.E., 1986. Murray Basin, southeastern Australia: subsurface stratigraphic database. Bureau of Mineral Resources, Australia, Report 262. Stephenson, A.E., 1986. Lake Bungunnia - a Plio-Pleistocene megalake in southern AuslraVvd. Palaeogeography, Palaeoclimatology, Palaeoecology, 57, 137-156.

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17.9

THE EASTERN A U S T R A L I A N CAINOZOIC VOLCANIC NORTHERN P.J.

ZONE

QUEENSLAND

Stephenson

James Cook University of North

Queensland

Discontinuous areas of Cainozoic basaltic rocks occur over nearly 1200 km from west of Townsville to Torres Strait. The total volcanic area is about 23,000 km . Summary details are given in Table 1 for various nominated geographic provinces from Maer in the north to Sturgeon, Pentland and Mingela in the south. The range of radiometric age determinations (K:Ar) is Eocene to Holocene. Each province shows an age range over less than 10 Ma. Differences in age are reflected by contrasts in erosion, from remnant plugs without preserved flows, to lavas and lava fields. Volcanoes vary from young cones with craters in excellent preservation, to older progressively denuded features. The physiographic relationships and ages of the basalts provide powerful insights into the Cainozoic record of uplifts and erosion. In most places, circumstances favour the recognition of the numerous young volcanoes, but presumably others are buried. The vent provinces are rel atively compact and there is usually a central area defined by grouped vents, with fewer isolated volcanoes. In many cases the vent distributions can be circumscribed within oval areas 60 — 80 km long and 35 45 km across. The outlines of several younger vent provinces trend NE (McBride, Chudleigh, Sturgeon, Nulla) wheras the older Mingela Province is much more elongate, NW. The provinces are situated in a variety of regional geological settings, and structural circumstances localising the vent provinces are obscure. Deeper hidden structures in the deeper crust and upper mantle may have been respon sible. It is noteworthy that very little activity impinges on the coast, and off-shore provinces have not been recognised. Although numerous vent alignments indicate structural localisation, in only a few cases have local bedrock structures been confirmed. More often, deeper unexposed structures seem more likely. Various alignment directions occur, especially around NE though this seems absent in the Mingela and Atherton Provinces. Trends around NW can be seen in the McLean and Mingela Provinces. The styles of volcanism involved low lava volume centres which were short lived. No thick accumulations of flows are known, except in some coastal valleys in the Atherton Province where gorges were filled with nearly 300m (eg. Johnstone River gorge). Basalt thickness across most flow provinces is estimated around 30 m, and the total volume in Northern Queensland only a modest 700 km^. Pyroclastic activity was relatively minor, represented locally, especially in Atherton, McBride and Piebald Provinces. Topographic, climatic and sustained effusion conditions are believed to have been the main factors influencing some very long flows (up to 160 km) with low slopes ( 0.3^) Six provinces are located on or near the Great Divide, the watershed for east coast drainage. These are McLean, Atherton, McBride, Chudleigh, Sturgeon, Nulla). Various workers since Taylor (1911) have proposed regional uplifts with associated volcanism. Regional topographic dome areas appear to be associated with Atherton, McBride and the Chudleigh - Sturgeon - Nulla group. These broad domes are variously dissected by radial drainage, and are from 50 to 100 km across, with up to 600 m relief. In the case of the southern flanks of the CSN dome, detailed studies indicate uplifts as young as 3 Ma ago stimulated the sharp incision which characterises the Flinders River system

375

headwaters. However, other parts of this dome show erosion features which indicate much earlier uplift. It might be conjectured that the plug provinces (eg. Mingela) were also associated with regional uplifts. Chemically, the basaltic rocks range from nephelinite to tholeiitic basalt (new 45 specimen data set, B.W. Chappell; 400 previous data set). Mugearites occur in several provinces, and rare mafic phonolite at one centre in McBride. Table 1 emphasises real contrasts between some of the provinces, such as percentage of potassic rocks and individual basaltic types. With time, some provinces such as Nulla show progressive increase in K in the younger long lava flows, which is not apparent in basalts collected at the volcanoes. Mingela is unusual in having several composite plugs which contain tholeiitic dolerites which form the core, unchilled against much more alkaline rim basaltic rocks. There are numerous megacryst and varied xenolith localities in basalts with alkaline affiliation however, they are almost absent in Sturgeon and Nulla,

TABLE 1 : SOME PROVINCE CHARACTERISTICS

MAER PIEBALD McLEAN ATHERTON McBRIDE WALLAROO CHUDLEIGH STURGEON NULLA BURDEKIN VALLEY PENTLAND MINGELA Key :

n K 1 2 3 4 5 6 7 C A 20 85 5 0 0 55 20 5 15 3 V/F 30 15 20 0 0 6 27 27 20 0 100 14 V/F 23 48 17 31 9 17 22 0 0 300 15 V/F 85 44 0 23 14 5 28 0 14 1800 52 V/F 5500 164 V/F/P 108 35 1 7 2 43 33 6 5 4 V/F 600 41 49 7 12 0 51 20 0 5 2000 46 V/F 60 18 0 7 0 32 44 0 17 5200 50 V/F 73 33 0 12 1 33 36 0 14 7500 46 V/F — — -

6 V/F/P P 6 P 30

3 (1) (2) 3 (1) 36 6 0 8 6 8 36

0 25

5 0 3

Ages 3 - 1 Ma 2 - 1 6 - 1 7; 3 - 0 8 - 7; 3 - 0 7 - 5 9 - 8; 6 - 0 6 - 0.7 5 - 0

0 17

27 - 18 27 - 22 44 -41; 34 Ma

8 9 0 0 0 20 0 4 8 7 0 1 0 0 1

A area in km^. C : known centres of eruption V/F volcanoes with preserved flows; P plugs, n : number of geochemical data K : % of analyses which are potassic (K2O > 0 . 5 Na20) 1 % Nephelinite (high n e , an%, a b < 5 ) 2 Basanite (An%> 50, ne > 5) 3 Alkali basalt ( A n % > 5 0 , n e < 5) 4 ne Hawaiite ( A n % < 5 0 , ne > 5) 5 Hawaiite (An%< 50, n e < 5) 6 ne Mugearite (DI< 30) 7 transitional hawaiite ( A n % < 5 0 , h y < 1 0 ) 8 transitional basalt ( A n % > 5 0 , h y < 1 0 ) 9 tholeiitic basalt (hy > 10).

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8.8

THE GEOLOGY AND PETROLOGY OF THE PALM ISLANDS, NORTH QUEENSLAND

P.J. Stephenson^ and B.W. Chappell^ ^Geology Department, James Cook University of North Queensland ^Geology Department, Australian National University, Canberra

The Palm Islands lie up to 35 km off-shore. There are 5 larger islands, extending 35 km south from Pelorous Island to Great Palm. They are composed of a variety of volcanic, dyke and granitic rocks. Volcanics include the oldest rocks recognised. Extrusive, bedded tuffs and ignimbrites occur on Pelorous and show relatively gentle northern dips. At the south end of this island and continuing south on Orpheus, Fantome and Curacoa Islands, similar - looking volcanics are more massive in character and display broad lithological banding which is steeply dipping. This banding shows systematic directions which indicate and arcuate belt, trending generally SE to E and concave to the east. These dispositions appear to demarcate a pyroclastic ring dyke feeder system only part of which is now visible on these islands. The rocks are porphyritic in quartz, two feldspars and altered mafic minerals; many were probably originally glassy. They all show considerable metamorphism and deformation, with the local development of closely spaced, steep NW trending cleavage which has several successively developed directions at some localities. Clasts are commonly similar acid prophyries but at some places intermediate lithologies also occur. The age of these volcanics is not known. The volcanics are cut by numerous NW dykes some features of which are summarised elsewhere (Abstracts, Pac Rim Congress 87, 817 - 22). They are members of a regional swarm, which possibly extends for over 600 km in the hinterland from Cairns to Mackay. The members of the swarm in the Palm Islands show a wide span of ages in theit relationships - some are cut by later granites, but these are in turn intruded by similar NW trending dykes. Successive intrusion can be demonstrated, and the oldest dykes show most extensive signs of alteration. However, no dykes display closely developed cleavage and at only a few localities was very weak NW cleavage observed. Some therefore appear to have intruded soon after deformation of their host volcanics, and emplacement continued beyong a period of passive granite emplacement. Analyses (K:Ar, hornblende or augite) provided ages of 272, 265 and 229 Ma for three post-granitic mafic dykes. The dykes vary from less than 50 cms to over 13 m thick and compositions range from mafic to acid. There are also several well-developed composite dykes with mafic margins and acid porphyry centres which display typical simultaneous melt phenomena. Felsic dykes have lower frequency, of the order of 10%. The mafic rocks include basalts and basaltic andesites, with fewer andesites. The mafic compositions are hypersthene normative and appear to be tholeiitic to calcalkaline. On diagrams using Ti, Zr and Y, the limited data show most of the specimens analysed fall in the calc-alkaline field, with one nearby in the within - plate area (after Pearce and Cann). The granitic rocks in the Palm Islands are varied and constitute a number of different, successive plutons. A common, light coloured type carries aoned plagioclas, hornblende, biotite and sphene. It resembles granites well developed further south in the Townsville region (Magnetic Id., C. Cleveland and Mt Elliott). They all belong to the same belt of late Palaeozoic granitoids (Queensland Geology, 1978), 280 - 260 Ma, which extends at least 250 km from south of Townsville to beyond Tully. K-Ar (biotite) age determinations for four different Palms granites gave 281 - 274 Ma. More felsic pale yellow to brown granites are biotite granites which locally show weak mineral alignment. Locally some Palms granites contain mafic

377

xenoliths, up'to several metres long and tending to occur in swarms• Geochemically, the Palms granites and the older volcanics (unknown age, but distinctly older from relationships) show similarities, but also some differences. The granites form somewhat dispersed trends which resemble those of the N. Queensland Ootann Suite of Richards (1980, Geol. Geophys., N.E. Australia, 229 - 46), which may prove to have great regional extent.

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8.9

ASPECTS OF THE GEOLOGY A N D PETROLOGY OF ARFVEDSONITE GRANITES, HINCHINBROOK ISLAND, NORTH QUEENSLAND P.J. Stephenson^,

B.W. Chappell^ and M.T.

Frost^

^Geology Department, James Cook University of North Queensland ^Geology Department, Australian National University, Canberra ^CSIRO Division of Mineral Chemistry, Port Melbourne

Hinchinbrook is noteworthy for the development of arfvedsonite granites for over at least 16 km along its main mountain range, and for its precipitous terrain with up to 1100 m of relief. Extensive field work has confirmed that these unusual granites are virtually confined to the eastern batholith of the island, where they are associated with other hypersolvus granites and with minor 2-feldspar granites. Age determinations (BMR) gave 271 260 Ma (K:Ar, arfvedsonite, biotite). The eastern batholith is 20 km long and over 10 km across. It has been intruded into an igneous complex which contains bedded ignimbritic volcanics and felsic granite to xenolithic hornblende granodiorite intrusives, cut by mafic dyke swarms. The batholith also intersects a felsic ring dyke up to 500m wide which is preserved as a near—semicircle approximately 18 km in diameter. The eastern batholith is itself cut by a west-dipping felsic dyke swarm in the south east and by relatively rare altered basic dykes. The main eastern mountain range on Hinchinbrook is dominated by a chain of high peaks around 1000m, only 5 km from the coasts All except the south peak, Mt Straloch, have extensive developments of arfvedsonite granite. No evidence has been found for separate plutons among the peaks north of Straloch, and there is some indication of structural continuity. Straloch may constitute a separate pluton of unknown time relationship, but it is composed of otherwise similar hypersolvus granites. Elsewhere, adjacent developments of contrasting felsic granite may indicate separate smaller plutons but relationships are not exposed. The granites composing the batholith are all dominantly hypersolvus with fine to coarse quartz and perthitic, variously exsolved alkali feldspar, with common granophyric intergrowths. Other minerals are arfvedsonite (rare aegirine) up to several %, biotite, magnetite, Mn ilmenite, accessory fluorite, and a variety of unusual accessory minerals. Perhaps half the ^ granite volume contains arfvedsonite, with or without biotite, and elsewhere biotite only. At lower altitudes along the central east coast there are pink 2-feldspar granites (oligoclase and perthitic feldspar) but their relationships to the hypersolvus granites is obscured. A widely-developed structural feature involves near-horizontal banding and related structures which are generally most prominent in the upper third of the mountains but is present at lower levels, even to sea level. The banding varies in scale from apparantly rhythmic banding on a centimetre scale in some microgranites, to much broader and relatively more subtle layers on metre or wider scale. Banding can be recognised by textural variations or because of varying mafic mineral abundances. At a number of localities, the structures display remarkable crescumulate details, with downward-facing growth. No reverse directions have been confirmed, but at most localities it is difficult to interpret growth directions. Pegmatites are developed at high levels in some places, and some contain coarse, virtually pure fayalite. Quartz veins are numerous and rarely contain wolframite. Chloritic alteration zones are relatively common, and some are manganiferous and uncommonly spahaleritic.

379

Texturally, the granites display a clear-cut primary crystallisation paragenesis. This involves early alkali feldspar crystallisation, joined by quartz in common intergrowths, coarsening to miarolitic cavities. Arfvedsonite and biotite are late, apparently forming in these cavities which also contain late "hydrothermal" albite, K feldspar, quartz and chlorite. Texturally, the pegmatites appear to show a paragenesis from relatively earlier arfvedsonite to later fayalite. Ewart (1978, Qd.Nat., 22, 25-30) noted the apparent existence of vertical variation in the eastern batholith from the summit of Mt Bowen down to the sea, progressing from more strongly arfvedsonit granite down to 2-feldspar granite. However, arfvedsonite granite does occur elsewhere at sea level. The chemical compositions of specimens from 4 detailed vertical profiles on different peaks , summit to sea level, have not confirmed any systematic vertical fractionation, inspite of the structural banding described above. However the chemical data do- indicate some close similarities between the various arfvedsonite and non-arfvedsonite hypersolvus granites, and only very slight differences with the 2-feldspar felsic granites, the ring dyke felsites and the late felsic dyke swarm. In a few places, large oval intermediate xenoliths occur in the 2-feldspar granited, with finer grained margins suggesting simultaneous melts. The ring dyke felsites are closely xenolithic, with small clasts of intermediate composition which in some cases contain quartz xenocrysts. All these rocks appear to provide a suite of magmatic rocks, actively associated in time. Chemically, the arfvedsonite granites are not strictly peralkaline since their agpaitic index (atomic A1 vs. Na + K) is slightly peraluminous. Development of genuinely peralkaline granites is controversial, and so too are these unusual developments of similar granites on Hinchinbrook. They are found in this localised area within the varied regional granite context of North Queensland where only very few other examples are known. The Hinchinbrook granites contrast with their surroundings - including the older granite-granodiorite (and volcanics) on the island and extensive granites and older volcanics in the Palm Islands to the south, and on the adjacent mainland. The Hinchinbrook suite is A-type in nature (as in Whelan et al. 1987 Contrib. Mineral. Petrol. 95, 407-19), but with strongly fractionated charactersitics (eg. very low Ca, Sr). However, its trace elements seem less extreme for arfvedsonite bearing types. All the textural details in rocks suggest primary crystallisation features, yet the causes for the considerable trace element variation present at Hinchinbrook (as in other examples) are as yet unresolved. Elsewhere, metasomatic processes have been invoked. The late arfvedsonite - biotite paragenesis in miarolitic cavities raises some important questions for melt-fluid evolution in these melts, and for trace element behaviour. Mineralogically, these unusual rocks contain a range of interesting features. In most cases, the arfvedsonites display only very slight chemical zoning, even over the range of altitude. Some coarser crystals display edge zones apparently replacing the earlier amphibole, and these show diminished F and Na contents. Zircons show a remarkable differences in birefringence, and some are isotropic. These are hydroxy-zircons, some with marked oscillator zoning in both (OH) and trace element contents. Ilmenites are unusually rich in Mn, rare examples ranging to pyrophanite.

380

4.7

EARLY E X T E N S I O N IN T H E M O U N T ISA INLIER, A N D A SOLUTION TO THE PROBLEM OF THE D E I G H T O N Q U A R T Z I T E O U T L I E R S A . J . Stewart

Bureau of M i n e r a l Resources,

and P.R.

Williams

G e o l o g y and G e o p h y s i c s ,

Canberra

The importance of major low-angle faulting accompanied by large-scale subhorizontal movement of rock masses has become increasingly recognized in the Mount Isa Inlier since Bell (1983: Nature, 304, 493-^97) suggested the possibility of major D. (1620 Ma) low-angle thrust-faulting. Recently, Loosveld & Schreurs {19'o7: Australian Journal of Earth Sciences, 34» 38?" 402) have presented evidence for thrust nappes in the Inlier, and Passchier (I986: Geology, l4, 1008-1011) and Williams & Etheridge (BMR Research Newsletter 6) have documented pre-D^ low-angle normal faulting. We report further evidence of early low-angle normal faulting in the central Mount Isa Inlier (Fig. 1), and re-interpret Loosveld and Schreurs' work in the Deighton area. The normal faulting described here is regarded as pre-Dj^ because metadolerite dykes in normal faults to the south are cut by S^. Loosveld L Schreurs showed that the Deighton Quartzite forming the synformal Deighton Klippe (Fig. 1) is in tectonic contact with the underlying Corella Formation, suggested that the Deighton was emplaced as a nappe on top of the Corella, and that the Deighton is stratigraphically equivalent to the Ballara Quartzite, which normally underlies the Corella. Recent mapping away from the Deighton Klippe has shown that over large areas the Deighton Quartzite rests conformably on the Corella Formation. The evidence includes a sequence of passage beds between the two formations, comprising alternating beds of Deighton Quartzite rock types and Corella rock types, and pebbles of Corella rock types in the lowermost Deighton Quartzite. The mapping has confirmed that the two units are in tectonic contact right around the Deighton Klippe, and that restricted and minor thrust-faulting placed Deighton over Deighton on the western limb of the Deighton Klippe. The following evidence in the Deighton Klippe suggests that the DeightonCorella contact around the Deighton Klippe is a bedding-parallel extensional fault: * Strong fracturing and extensive fault breccia in the Deighton Quartzite; the brittle structures are strongest at the contact with the Corella Formation, weaken away from the contact, and in many places are deformed by a steep, northerly-striking foliation (S2). * Boudinage of sandstone at the base of the Deighton Quartzite. * Mesoscopic normal faults cut by S2 in the Deighton Quartzite, and filled with marble (massive, and so recrystallized during D2) in the Corella Formation. * Extensive masses of brecciated tourmaline rock along the northeastern limb of the Deighton Klippe (Fig. 1); the breccia is deformed by S2. * Extensive breccia in the Corella Formation below the Deighton Quartzite in the southeast; the breccia is deformed by S2. * The Corella Formation along the entire eastern side is in fault contact with the Leichhardt Volcanics; the formations which normally lie between the Leichhardt Volcanics and Corella Formations are missing.

381

The evidence indicates west-directed extensional f a u l t i n g and brecciation put younger rocks over older, i.e., Deighton over Corella, and Corella over Leichhardt. The bedding-parallel fault beneath the Deighton Klippe was originally a flat that formed part of a system of pre-Dj^ west-dipping normal faults. Extension 'was followed by compression (D^) which resulted in minor westerly-directed thrust-faulting and folding on the western side of the Deighton Klippe, and in the formation of some east-northeast-trending folds. This was followed by major pervasive compression (D2; Fig. 2) which produced northerly-trending upright folds (such as the synform forming the Deighton Klippe), and a steep northerly-striking axial-plane foliation (S2). Of the other Deighton Quartzite outliers (Fig. 1), the Charley Creek outlier has a f a u l t right around i t , like the Deighton Klippe. In contrast, the Campbell, Scorpion, and Maylene outliers are in conformable sequence (including the passage bed"interval) with the underlying Corella Formation on their western sides, and in steep fault contact with older rocks on their eastern sides. At the Scorpion o u t l i e r , the fault i s marked by prominent breccia which is cut by, and therefore predates, S2 foliation. The eastern bounding f a u l t s at the Campbell and Maylene o u t l i e r s are syn or post-S2; Maylene has a steeply east-dipping thrust-fault combined with s i n i s t r a l strike-slip, and Campbell a vertical to steeply west-dipping normal fault. The Maylene and Scorpion o u t l i e r s are projected on to the cross-section shown in Fig. 2, which depicts a system of early normal faults subsequently folded by D2. The folding rotated the normal faults, thereby changing them into thrust faults. On the eastern side of the Campbell outlier, undeformed fault breccias cut S2, separate different orientations of S2 on each side of the f a u l t , and in places contain clasts of S2-foliated rock in random orientations; these indicate post-D2 reactivation of the early f a u l t , and similarly for the combined thrust and strike-slip fault on the eastern side of the Maylene outlier.

WB I Wl..JeB)owFor«<a+.on y j ^ Deia^on Qoar^-z.ffc

KalkaeioonLe.ciikirclf Block

Corella Format.

BrtcCiaVed Iworwaline Lticlikarotf

Volcawics

20-30-

<\v\ c V rnROioN

Fig. 1: Geological map of central part of Mount Isa Inlier showing Deighton Quartzite outliers. Fig. 2: Schematic cross-section through Fig.l showing normal faults folded by D2, Deighton Klippe, and Scorpion and Maylene outliers projected on to line of section.

382

13.7

ORGANIC AND SEDIMENTARY FACIES OF THE EARLY CRETACEOUS OTWAY GROUP, OTWAY BASIN, SOUTHERN AUSTRALIA H. Struckmeyer and E.A.

Felton

Department of Geology, University of Wollongong

The organic fades of a sedimentary sequence are described by the type, abundance and composition of the contained organic matter. Variations in organic matter assemblages can relate to changes in depositional environments. Examination of the organic matter within a sedimentary sequence can thus provide a useful tool, together with sedimentological analyses, for better assessment of those environments. The greater part of the organic matter in the Otway Group is derived from higher plants and occurs as both coal and dispersed organic matter (dom). Coal-rich sedimentary rocks are well developed in the lowest part of the Eumeralla Formation although thin carbonaceous bands are present throughout. Minor coal also occurs in the Pretty Hill Formation. Four coal types, designated A to D, are distinguished, associated with distinct dom assemblages. Humic material is dominant in the liptinite-poor clarites and vitrites of the Type A coal facies, indicating derivation from predominantly woody tissue. Type A coals are associated with sparse to common dom typically containing less than 50% liptinite. Type B and Type C coals are characterized by a predominance of fine-grained detrovitrinite forming the groundmass for layers of well-preserved sporinite, cutinite and suberinite. Type C coals are distinguished from Type B coals by their higher inertinite content. Both types occur in association with abundant liptinite rich dom. The maceral composition and attrital nature of Type B and C coals, together with distinct micro-layering suggests derivation from a dominantly herbaceous flora in subaquatic environments. Type D coals consist of sporinite rich, well banded durites and clarodurites, and are associated with abundant liptinite and inertinite-rich dom. The organic facies recognised in the Otway Group are closely associated with changes in sedimentary facies interpreted from wireline log analysis and core and outcrop examination (Table 1). Type A coals occur in the late Neocomian and Albian in association with high energy depositional systems (fluvial and fluvio-deltaic). Type B and C coals are found in low gradient fluvial to lacustrine environments which prevailed in the early Neocomian and Aptian; they were probably deposited in swamps and shallow lakes on inundated floodplains with varying oxidation conditions. Oil shale, probably indicating brackish conditions, is also present locally in the early Neocomian (Struckmeyer, 1986). Well-oxygenated and somewhat deeper lacustrine conditions are indicated by the dominance of Type D coals in the Early Albian. References DETTMANN, M.E. & DOUGLAS, J.G. 1976: Mesozoic palaeontology. In: Douglas, J.G. & Ferguson, J.A. (eds). Geology of Victoria. Special Publication of the Geological Society of Australia, 5, 16A-169. DOUGLAS, J.G. 1969: The Mesozoic Floras of Victoria, Pts. 1 & 2. Geological Survey of Victoria, Memoir, 28, 110pp.

383

HARLAND, W.B., COX, A.V., Llewellyn, P.G- Pickton, C.A.G., Smith, A.G. & Walters, R. 1982: A geologic time scale. Cambridge Earth Science Series, Cambridge University Press, Cambridge, 131pp. HELBY, R.J., MORGAN, R.P. & PARTRIDGE, A.D. 1987: A palynological zonation of the Australian Mesozoic. Ini Jell, P.A. (ed), Studies in Australian Mesozoic Palynology. Association of Australasian Palaeontologists, Memoir 4, 1-81. STRUCKMEYER, H. 1986: Organic petrology of the sedimentary sequence at Robertson No.l. SADME Open File Env. 5876 v.5 (unpubl.)

Table 1. SCALE Ma Harland, "3l al.!9 0 2

MICnOFLOHAL ZONATION (Helby el at.,1987; after Dcttmann & Douglas.1969 Douglas,1976)

MEGAFLORAL AGE

FORMATION

ZONATION

ORGANIC

DOMINANT

COAL TYPE MATTER ABUNDANCE ASSEMBLAGE

CEN0MAN1AN P. pannosus

FLUVIO-DELTAIC

common

lo ZONE D ALBIAN

C. paradoxa

uppor

Low lo high gradient

abundant

FLUVIAL D.B

lower

LACUSTRINE

C. strlalus

D.C

O.C

Low n'^fdiont FLUVIALLACUSTRINE

major C. hughosi

BARREMIAN

F. wonthagglensis

Pretty HIM

Formation

High ZONE B

gradient

abundant FLUVIAL

130-J

Valanglnlan

Bsrriasian

Caster ton Formation

C. australiensis ZONE A

common

7 B.C

LACUSTRINE

abundant

(7 brackish)

Late JURASSIC AF/DMM/G01156

384

19.4

TIME SCALES FOR THE SILURIAN AND DEVONIAN IN AUSTRALIA D.L. Strusz and G.C. Young

Bureau of Mineral Resources, Geology & Geophysics, Canberra

Palaeontologists of the Bureau of Mineral Resources, in conjunction with colleagues from the State Surveys and University Departments, have recently been working on a series of biostratigraphic charts for the Phanerozoic in Australia, taking account of advances made since previous major continentwide reviews (at the 1972 Society meeting in Brisbane for the Devonian, by Talent, Berry and Boucot in 1975 for the Silurian). Preliminary charts summarising the biostratigraphic distribution of major groups, set against a standard time scale, will be presented for discussion. For both periods there have been major international decisions, defining the Period and many of the Series and Stage boundaries. These, and the biostratigraphic arguments which led to them, have had an immediate effect on correlation of Australian sequences. In the Australian Silurian to Early Devonian, there have been significant advances in the use of conodonts, including the setting up of a zonation for most of the Early Silurian, and in recognition (seldom in sequence) of most of the European graptolite zones. A coral zonation set up by Philip L Pedder in 1967 for the Early to Middle Devonian has proved useful, and has recently been supplemented by work on Silurian corals (in sequences with graptolite control), and Late Silurian to Early Devonian brachiopods. Recent work on Silurian trilobites in the Canberra - Yass area holds promise, as does work on Devonian vertebrates. In the later Devonian, the standard ammonoid zones are now better correlated with conodonts, as are the brachiopod zones reported on by Roberts et al. at the 1972 meeting. For plants, the matter of most interest, although not likely to have a major impact on biostratigraphy, is the settling of the controversy about the earliest Baragwanathia flora, now shown conclusively to be Late Silurian. As part of the study, an analysis has been made of recent radiometric dating work. From this it has been concluded that the base of the Silurian (as now defined at the base of the Zone of Parakidograptus acuminatus) can be fairly confidently set at k^k Ma. There is a reliable age of 420 Ma for the beginning of the Ludlow based on the Yass sequence, and the Silurian/Devonian boundary lies at approximately 408 Ma (Australian data would suggest 4lO Ma may be a closer approximation). Within the Devonian, Australian data suggest a date of about 370 Ma for the Givetian/Frasnian boundary, and support the most recent suggestion, by McKerrow et al. in 1985, of about 35k Ma for the boundary with the Carboniferous.

385

2.23

EXTENSIONAL TECTONICS AND DEVELOPMENT OF THE TUMUT TROUGH, NEW SOUTH WALES P.G. Stuart-Smith Bureau of Mineral Resources, Geology and Geophysics, Canberra

The tectonic setting of the Tumut Trough has been regarded as either a zone of fore-arc collision with a continental margin, or a small ocean-floored rift in a back-arc or marginal sea environment. Recent structural studies in the Brungle area have revealed the presence of major extensional features which characterise Cordilleran detachment terranes and metamorphic core complexes consistent with rift models proposed for the area. Rocks in the Brungle area of the Tumut Trough form two distinct domains: an ?Ordovician basement (Bullawyarra Schist) and a Silurian sedimentary and volcanic cover sequence. These two domains are separated by a sharp discontinuity marking an abrupt change in rock type, structure, metamorphic grade and deformation style. The cover sequence has undergone only one major deformation during the Early Devonian, involving lower greenschist facies metamorphism and upright folding, as a result of east-west shortening. By comparison, the basement underwent at least two additional older deformations at upper greeenschist facies, and has distinct high strain zones subconcordant with the basement/cover contact. The high strain zones, characterised by a ubiquitous mineral elongation lineation, record a progressive discontinuous history of ductile to brittle behaviour consistent with an extensional origin. The structural and metamorphic discontinuity separating the Ordovician basement from the overlying Brungle Creek Metabasalt is a previously unrecognised, major, originally subhorizontal fault zone characterised by comminution of adjacent rocks to massive breccias and cataclasites and extensive chlorite and carbonate alteration. The zone is interpreted as the major detachment associated with extension and the development of the Tumut Trough in the Early to Middle Silurian. Serpentinite was emplaced in basement high strain zones, which were reactivated during this extension, following intrusion of the Blacks Flat Diorite into the basement (Bullawyarra Schist). Major movement on the detachment took place prior to deposition of the Middle Silurian Wyangle Formation which unconformably overlies both the basement and the Brungle Creek Metabasalt. The Brungle Creek Metabasalt therefore represents an allochthonous sequence, possibly of Ordoviciein age, which has probably been considerably attenuated. Basal Silurian trough sediments (Wyangle Formation), derived from nonbasement sources, exhibit chaotic characteristics and rapid facies variations, and onlap the basement and Brungle Creek Metabasalt. The latter probably formed steep-sided fault blocks sitting on the detachment and separating intervening troughs of Wyangle Formation sediment. Felsic volcanics of the Blowering Formation later filled the troughs and covered the intervening tilt blocks of metabasalt.

386

17.13 THE BOYNE BASALTS, SOUTHERN Q U E E N S L A N D - THEIR RELEVANCE TO THE LITHOSPHERIC STATE F.L. Sutherland^,

J.D. Hollis^ and A.D.

Robertson^

^The Australian Museum, Sydney ^Queensland Department of Mines

The Boyne Basalts issued from scattered mid-Wiocene alkaline volcanic centres around Boyne River near Proston. They lie west of the mid-Tertiary 'hot spot' voluminous volcanic sequences of the northern Main Range (22—29 Ma) and also include an extension of the restricted late Pleistocene basalts of the Gayndah-Coulston Lakes area to the north east. The main cluster of centres dates between 16-18 Ma and a young breccia phase dates between 0.3-0.5 Ma (K-Ar ages on amphibole and anorthoclase megacrysts). The basalts form an extended fractionation series and commonly contain a variety of phenocrysts, megacrysts and mantle xenoliths. The inclusions yield gem quality examples and breccias are worked for gem pyrope-almandine garnet. The basalts have a relevance beyond their small volumes as the xenoliths reveal the metasomatic and thermal nature of the underlying mid-Miocene to Quaternary mantle. Two studied centres (Brigooda and Ballogie) contrast in eruptive style and host petrology. Flows extended south from the Miocene Brigooda centre from a plug and show K-rich alkaline affinities. Largely ne normative lavas range from more magnesian alkali basalts to nepheline mugearite. The sequence is olivine-augite ne hawaiite, anorthoclase and anorthoclase-augite hawaiites, ankarmitic alkali basalt and ne hawaiite, with ne mugearite and amphibole-rich ne mugearite as loose blocks of uncertain relationship. The Ballogie Miocene breccia pipe contains amphibole, anorthoclase, clino and orthopyroxene, garnet and Fe-Ti oxide megacrysts and minor late-stage intrusions of sodic, megacryst-bearing ne hawaiite. The Brigooda centre also shows a Quaternary maar-like breccia vent in sheared granite, filled with pieces of local basalts including the megacryst/xenolith rich ne mugearite. Dating of amphibole in this rock suggests it is Miocene and accidental in the breccia, but is the source of abundant megacrysts in the breccia. Fluid inclusions in the megacrysts suggest CO2 as a major volatile in the young vent eruption. Pale low U zircon and rare record of diamond at the vent are possible associates of eruption of deep-volatile rich magma. The most magnesian Ballogie and Brigooda basalts have high compatible element contents (Ni 343-469, Cr 435-257, V 117-158 ppm) and relatively lower incompatible and rare earth element contents (Zr 252-225, Sr 915-1033, Rb 29-47, Nb 70-64, Y 29-26, La 53-51, Ce 103-86, Nd 56-55 ppm), compared to the most evolved amphibole-rich ne mugearite (Ni 157, Cr 189, V 63, Zr 538, Sr 1538, Rb 67, Nb 114, Y 36, La 99, Ce 183, Nd 85 ppm). High pressure xenoliths were studied in detail from the Brigooda breccias. Spinel Iherzolites are commonly altered, pyroxenites are mostly garnet (+ spinel, ilmenite, scapolite) clinopyroxenites, variably replaced by Ti amphibole, but also garnet (hh spinel, magnetite) websterites and the suite is notable for coarse garnetites. Garnet compositions lie in the range (Mg/Mg + Fe 0.5-0.7), clinopyroxenes are sodic A1 augites (Ca 40-42, Mg 41-46, Fe 14-16; A1 0 8-10%, Na^O 2-3%) and orthopyroxenes are bronzites (Mg 77-78, Fe 21-22, Ca 2-4; 5-9%). Garnet clinopyroxenites show lower SiO^ (40-45%) than garnet websterites (51-52%) and norms indicate they both represent re-equilibrated olivine poor to olivine rich tholeiites. Trace elements for garnet websterites show high compatible (Ni 499, Cr 527, V 263 ppm) and low incompatible element values (Zr 31, Sr 86, Rb 2, Nb 2, Y 14, La 5, Ce 23, Nd 12 ppm) typical of cumulates. Modifications in some values in garnet

387

clinopyroxenites (Ni 210, Cr 5, Zr 78 ppm) suggest formation from partly fractionated magmas by settling of magnesian phases. Rare apatite-rich, two pyroxene granulites show lower Al, less sodic pyroxenes compared to the mantle assemblages and mg values (<60) and high Ti, P, Zr and Y suggest former factionated basaltic melts re-equilibrated in the lower crust. Pressure-temperature plots for Brigooda garnet-two pyroxene assemblages are related to the hot South East Australian geotherm defined by O'Reilly and Griffin (1985) and to Tertiary Queensland suites. They suggest a similar or even more elevated geotherm in the Quaternary in this area. The Boyne Basalt xenoliths can be compared with suites found in young volcanics to the north east. They suggest a metasomatised and thermally active mantle profile in the region, capable of further eruption of alkaline volatile magmas. Bibliography GRIFFIN, W.L., SUTHERLAND, F.L. and MOLLIS, J.D., 1987. Geotherm. Res. 31, 177-203.

J. Volanol.

HOLLIS, J.D., SUTHERLAND, F.L. and POGSON, R.E., 1983. 181-194.

Rec. Aust. Mus. 35,

O'REILLY, S.Y. and GRIFFIN, W.L,, 1985.

Tectonophysics 111, 41-63.

ROBERTSON, A.D., SUTHERLAND, F.L. and HOLLIS, J.D., 1985. Univ. 11(3), 58-71.

sea

388

geotheri

Pap. Dep. Geol.

17.11

THE ROCKHAMPTON PROVINCE « A CRETACEOUS CENTRAL V O L C A N O MIGRATION

F.L. Sutherland^, A.D. Robertson^ and J.D.

Hollis^

^The Australian Museum, Sydney ^Queensland Department of Mines

Felsic plugs, flows and pyroclastic deposits, associated with basalt lavas form some spectacular peaks and plateau remnants around Rockhampton. Mapped in outline, the province has been briefly described and sparsely dated to the late Cretaceous, with little published chemistry. A detailed study is in progress to compare these Rockhampton rocks with the better known Tertiary 'hot spot' central volcano provinces to the north (Hillsborough-Nebo provinces), west (Clermont-Springsure-Buckland) and south (Fraser IslandGlass House Mountains-^ain Range-Tweed Shield). A trachytic plug east of Rockhampton is dated to 71 Ma and basalt to the west to 68 Ma, but the latter flow shows alteration and probably gives a minimum age. The western most intrusion of nepheline syenite at Mt. Ramsay is dated at 72 Ma. The volcanics lie along a WSW trending zone from off the Yeppoon coast to Baralaba 120 km inland. A 'banana' or 'boomerang' shaped field with the central apex pointing NNW enclose several separate sub-fields differing in volume and area. At Yeppoon there are a few trachytic plugs and inclined rhyolitic sheets and east of Rockhampton plugs and minor flows of similar rocks are associated with minor basalt fields. Mt. Wheeler the largest intrusion lies within one of the main gold fields of the district but its relationship to the mineralization in uncertain. West of Rockhampton, in the largest exposure of volcanics, lower basalt fills are overlapped by widespread trachytic pyroclastics, flows and intrusions. These form a plateau fringed by small centres including a basalt-rhyolite extrusion at Stanwell. A smaller basalt field to the south contains sporadic felsic extrusions. At Mt. Hay, spherulitic rhyolites are cut by a basalt dyke, but may represent early rather than late Cretaceous activity. At the southern end Mt. Sebastopol forms an isolated trachyte body separated by considerable distance from the terminating Mt. Ramsay nepheline syenite intrusion. In petrology, the basalts range from olivine-phyric through pyroxene and plagiodlase-phyric to more coarsely porphyritic plagioclase-phyric types, but include some aphyric basalts. Plugs form dolerites. The trachytic intrusives range from fine grained types into porphyritic and glomeroporphyritic types containing abundant sanidine (+ aegerine) phenocrysts. Rhyolites are commonly fine grained or microporphyritic rocks showing flow banding. The Mt. Hay rocks include spherulitic perlites and flow banded rhyolites studded with expanded spherulites filled with quartz and agate fillings (thunder-eggs). Trachytic centres can include a variety of pyroclastic deposits ranging through coarse agglomerates to ignimbrites and tuffbeds. Geochemically, most basalts are members of the alkali basalt lineage, typically fractionated to hawaiite. The 'trachytes' are strictly trachyrhyolites and rhyolites, typically contain 10-25% normative qtz and are strongly fractionated rocks. The rhyolites differ in chemistry, contain 30-40% normative qtz, an, mt, c and no di, ac or ns and may have less fractionated, tholeiite affinities. The Mt. Ramsay nepheline syenite is the most fractionated rock in the province and shows marked enrichment (ppm) in Zr 1485, Y 125, Th 30, U 11, La 141, Ce 270, Nb 210, Nd 94, Sm 17, Pr 32, E^ 17, Hf 18, Rb 195, Ga 41, Gd 16, Pb 27 and extreme depletions in Ni <10, Cr <10, V 2, Sr 12.

389

The Rockhampton province geochemical spectrum established so far lies within and closely matches the mafic-felsic range of a typical Tertiary central volcano province (see Table). The 'boomerang' shape of the province is also typical of several Tertiary migratory central volcano chains. Further age dating, geochemistry and isotope work is needed to confirm its assignation to a migratory 'hot spot' intraplate origin. B-1 B-2 B-3 MH-1 ER-4 ER-3 ER-2 MR-1 Analysis ER-1 70.7 75. 1 47.7 71.7 72.4 71.2 69. 1 61.8 SIO 52.0 0.12 0.09 1.82 0.06 0.11 0.11 0. 16 0.12 1.45 TiO^ 13.8 14.3 12.7 14.9 13.2 13.0 18.2 15.1 17.0 Al-00.2 1.5 1.9 1.0 2.5 3.5 2.0 3. 1 1.6 Fe'o^, 0.8 1.5 9.4 0.3 0.4 0.6 1.3 1. 1 7. 1 FeS ^ 0.03 0.05 0.16 0.04 0.05 0.08 0.07 0.14 MnO 0. 17 0. 1 0.2 9.2 0.3 0.2 0.2 0.2 0. 1 MgO 5. 1 0.2 0.3 8.8 0.5 0. 1 0.2 0.3 0.5 6.9 CaO 4.7 6.3 3.4 3.0 5.7 6.2 6.8 8.3 Na-0 5. 1 4.3 1.2 4.5 4.2 3.9 4.2 4.6 4.9 1.8 Ki 0.01 0. 54 0.01 0.47 0.99 0.28 0.47 6.7 1.09 0.93 0.66 1.96 0.97 99. 95 100.20 99. 52 99. 75 100.32 99.66 100.22 100.30 100.22 Total 12.84 33.05 63. 52 12.48 17. 62 9.45 11.48 4.85 55.63 MgN^ 16. 45 30. 62 22. 63 39.56 20. 23 11.65 Q 7.16 26.73 25.55 25. 10 24.65 24.99 27.28 10.78 29.43 Or 40. 00 48.76 24.98 27. 15 45.01 43.78 52.25 37.76 54.31 Ab 0. 30 2.65 22.06 18.41 An 2.20 7. 10 3.23 Ne 1.31 14.99 0.44 0.88 1.3 1.61 10. 80 Di 1.45 1.41 1.99 1.6 2.06 Ac 1.56 4.32 2.71 4.65 6.31 4.75 Hy 21.05 4. 60 12.97 01 0. 24 2.76 0. 34 2.13 Mt 0. 17 0. 23 3. 49 0. 12 0.21 0.21 0. 31 0. 23 2.79 11 1.29 0.02 1.13 Ap 0.02 0. 02 0. 02 0. 02 0. 02 0.02 Py 1.31 2.28 C 0. 74 0. 50 1.58 0.86 0. 38 Ns ER: East Rockhampton 1 'basalt' 2-4 'trachytes', MR: M t . Ramsay ne syenite, MH: Mt.Hay perlitic rhyolite, B: Buckland 1 'basalt' 2'trachyte' 3 rhyolite. Analyses Dept, Minerals & Energy Labs, Brisbane; CIPW norms per R.E. Pogson.

390

18.4

A RECIPE FOR GRADUATING AN

ECONOMIC GEOLOGIST

D. Svenson ERM Consultants Pty Ltd, Brisbane

The prcx>f of the pudding is in the eating, as the saying goes. Ifebody eats a badly made pudding except in times of famine. No employer will acx:ept an inadequately prepared graduate except in times of high demand when there are not enough well made graduates to satisfy the Nation's needs. The preparaticn should start in the secondary sciiool where the Geological Society and the AusIMM should encourage, with the co-operation of the education authorities, the brighter of our 15 and 16 year olds to consider the advantages of tackling the supposedly more rigorous disciplines involved in maths, physics and chemistry v\îcli permit entry to the technical professicos. This encouragement needs to be industry activated in the form of an honest, informative, inspiraticnal or at least motivating, in sciiool public relations exercise, if the profession is to gsdn a share of the cream of our young people. A thorough grounding in hiêr mathematics, physics and chemistry at senior secondary sciiool will be c±)viously beneficial to both undergraduates and their mentors at tertiary level. Where geology is an optional subject at secondary school, its teaching must be of a high order, if only to motivate rather than discourage. Intelligence, self reliance, initiative and personal integrity are the prime requirements of a graduate in any professional field, but without the motivation of a profound interest in his profession no amount of vocational training will produce self fulfilment for a graduate and his or her value to the community would be better achieved elsewhere. The graduate's employer is entitled to e^^êct full professional value from someone v>aio has been professiaially educated at great expense to the Nation. Industry managemoit is disadvantaged if, through imdequate academic education, the graduate's work has to be ataiomally restricted if he does not possess the basic professional knowledge and skill required to efficiently perform basic professional tasks, in the particular industrial environmoit he has entered. Obviously, he can be trained 'in house' and, if well motivated he will train himself to accept progressively increasing responsibilities, but at a cost which need not have occurred and at inconvenience to his professional colleagues. From the viewpoint which, although not that of an official industry spokesman, is of some thirty-six years experience in the mining and civil (geological) aigineering industries, the necessary and rigorous academic training of an eooncmic geologist cannot be achieved in less than 4 years, (without risk of serious damage to the psychological and physical health of the graduate and his tutor si) From the geologist's vievînt, his professional status should be maintained at the same level as an engineer's. This status is, of course, largely a matter of individual value judgement, but the status of geologists generally will remain handicapped if their academic standing is at a lower level.

391

A four year pass or honours degree, or even a masters degree or doctorate, is no guarantee of professicnal value to industry, if it is too specialised and deficient in sane of the essentials of professional practice. Economic or applied geology encoirpasses a multitude of specialities including - engineering geology, mining geology and petroleum geology. We may also refer ^to specialities such as (a) exploraticn geology, (b) coal geology, (c) metalliferous geology and aivironmental geology. Being a ccxiservative traditionalist, I prefer the term 'economic' and coisider (a), (b) and (c) as branches of mining geology. I have not yet seen a satisfactory definiticn of environmental geology. To ray mind, all g ^ l o g y can be applied universally, ecoionic geology must be applied teleologically (ie with an economic aim) and with a soisible, sensitive afproach to the environment. The somewhat irregular cyclic progression of the intematicnal econony inhibits predicticn of the likely requirement of graduates in any one of the above fields, thus it is prudent for an economic geologist not to become over specialised. Any particular arm of industry should be very ccntent with a graduate eccnomic geologist (eg a mining and engineering geologist) who, apart fron being intelligent, self reliant and motivated, has been iiribu^ with an ability for effective written conmunicaticn, well grounded in basic sciences - maths, physics, chemistry, statistics and conputer science, thoroughly trained in the basic geological sciaices - mineralogy and petrology, sedimentology ani stratigrafSiy (including basic palaeontology), and structural geology. Eoonomic geology courses should include: ore mineralogy, coal geology, petroleum geology, the elements of geofhysics ^ geociiemistry, mining geology and engineering geology. In my cpinicxi, mining and engineering geology are virtually inseparable and diverge only vAiere career paths remain in a single arm of industry. The mining and engineering geological practice courses should include essentially: geological mapping and the e l e m ^ t s of topograjiiical surveying, (including fhotogrammetry, aerial and satellite photogeology) exploratory drilling, sait4>l:mg and logging techniques, gecanechanics - ooirprising rock and soil mechanics, the evaluation of industrial minerals and earth materials for constructicn, and the elements of mining & metallurgy, of m a n a g e m e n t and industrial and community relations, of environmental science and of economic evaluation, and report writing. Optional specialised courses would include as available, petroleum geology and producticxi technology, coal geology and technology and metalliferous raining geology. The undergraduate needs to s p ^ d at least two of his long v a c a t i c ^ or the equivalent in field work with geologists or mining engineers to obtain and report cn experience either in industry or as a course organised project. Econonic geology is an analytical, service profession. The undergraduate should be able to think clearly and objectively, separate fact from deduction and write clear, concise reports, accordingly. To 'hDne' this ability and maintain his humanity, a leisurely course in literary appreciatiai would be beneficial. It is easy to write a 'oordoi bleu' recipe, its another matter to make the pudding 1 At the risk of being flour bonbed, I suggest that our economic geology departments are, wherever possible, incorporated in schools of mines, vAiere the essential close relations between miners, geological engineers, metallurgists and economic geologists can b e fostered and maintained. Such an institution renamed 'School of Geoscience & Geotechnology' should overcome academic arguments that all geologists are not tied to the mining industry. More objectively, it w u l d facilitate organisation of courses common to all of the allied disciplines.

392

15.3

D A M P I E R RIDGE IN THE T A S M A N SEA - A CONTINENTAL FRAGMENT

P . A . Symonds

, J . B . W i l l c o x ^ and H . R . K u d r a s s ^

^Bureau of M i n e r a l R e s o u r c e s , G e o l o g y and G e o p h y s i c s , C a n b e r r a ^ B u n d e s a n s t a l t fur G e o w i s s e n s c h a f t e n und Rohstoff, H a n n o v e r , F e d e r a l R e p u b l i c of G e r m a n y The Dampier R i d g e is a 800 km long n o r t h - s o u t h t r e n d i n g r i d g e which lies to the w e s t of the L o r d H o w e R i s e (LHR) a n d is s e p a r a t e d f r o m it by the M i d d l e t o n and L o r d H o w e B a s i n s ( F i g u r e ) . Its o r i g i n has long b e e n a p r o b l e m in r e c o n s t r u c t i o n s of A u s t r a l i a and L H R , s i n c e f u l l c l o s u r e of the Tasman Basin, particulary to the s o u t h , p r o d u c e s o v e r l a p of mainland Australia and the Dampier Ridge. S e v e r a l a u t h o r s h a v e a d d r e s s e d the n a t u r e of the D a m p i e r

Ridge:

Hayes & Ringis (1973) i d e n t i f i e d w e s t - n o r t h w e s t trending magnetic anomalies in the T a s m a n b a s i n a n d c o n c l u d e d t h a t it w a s f o r m e d by s e a f l o o r s p r e a d i n g a b o u t 80 to 60 m . y . B . P . W e i s s e l & H a y e s (1977) s u p p o r t e d a s u g g e s t i o n by R i n g i s t h a t the oldest magnetic a n o m a l i e s of the T a s m a n S e a , e . g . a n o m a l y 32 a n d 3 3 , should e x i s t w i t h i n the M i d d l e t o n and L o r d H o w e B a s i n s , a n d t h a t a r i d g e jump took p l a c e a f t e r a n o m a l y 32 t i m e , p r o b a b l y l e a v i n g the D a m p i e r R i d g e as a m i c r o c o n t i n e n t a l s p l i n t e r to the w e s t of t h e L o r d H o w e p l a t e . Jongsma & M u t t e r (1978) s u g g e s t e d t h a t the w e s t e r n half of the L H R and probably the w h o l e p a r t of the D a m p i e r R i d g e c o m p r i s e a c o m p l e x Early C r e t a c e o u s r i f t s y s t e m f o r m e d p r i o r to s e a f l o o r s p r e a d i n g in the Tasman Sea. Willcox & o t h e r s (1980) c o n c l u d e d on the b a s i s of the c h a r a c t e r of the acoustic basement t h a t the D a m p i e r R i d g e i s , at l e a s t in p a r t , of continental origin. A p r e - d r i f t r e c o n s t r u c t i o n of the a r e a i n d i c a t e s t h a t a (?)volcanic r i d g e in the d e e p w a t e r G i p p s l a n d B a s i n (Jones & V e e v e r s , 1 9 8 3 ) a n d t h e (?) v o l c a n i c s o u t h e r n p o s i t i o n of the D a m p i e r R i d g e , lay on o p p o s i t e s i d e s of a major f a u l t - t h e B a s s F r a c t u r e Zone ( W i l l c o x , 1 9 8 4 ) . Jones & Veevers f u r t h e r s u g g e s t t h a t d u r i n g the l a t e s t J u r a s s i c a n d E a r l y C r e t a c e o u s m i l d l y alkaline, intermediate volcaniclastic sediments were shed westward from this v o l c a n i c r i d g e a n d w e r e d e p o s i t e d in b o t h the O t w a y a n d Gippsland Basins. T h e y a l s o s u g g e s t t h a t the v o l c a n i c r i d g e f o r m e d in an oblique transcurrent zone, presumably during extension and rifting between Lord H o w e R i s e and A u s t r a l i a . L i s t e r , E t h e r i d g e & S y m o n d s (in p r e s s ) h a v e r e c e n t l y i n t e r p r e t e d the L H R as a 'lower p l a t e m a r g i n ' . W e c o n s i d e r it p r o b a b l e t h a t the Dampier Ridge, Middleton/Lord H o w e B a s i n s , and L H R , f o r m e d b e t w e e n a s e r i e s of westerly dipping branching detachment 'faults', with extension being confined largely to the M i d d l e t o n a n d L o r d H o w e B a s i n s a n d the western r i f t - z o n e on L H R . T h e b a s i n s w o u l d h e n c e be f l o o r e d by c o n t i n e n t a l c r u s t . In A p r i l 1 9 8 5 , d u r i n g o p e r a t i o n s w i t h the BGR (West G e r m a n ) R/V 'Sonne', an o p p o r t u n i t y a r o s e to d i r e c t l y s a m p l e r o c k s f r o m the Dampier Ridge (Figure ; Roeser & others, 1985). S a m p l e s from the w e s t e r n f l a n k (157^22.69'E, 31°51.87'S) y i e l d e d f r a g m e n t s of s l i g h t l y metamorphosed g r a n i t e and ( ? ) m i c r o d i o r i t e / a n d e s i t e , together with feldspathic sandstone. This f i n d i n g c o n f i r m s for the f i r s t t i m e t h a t the D a m p i e r R i d g e is a longlost p a r t of m a i n l a i n d A u s t r a l i a . Abbreviated references Hayes, D.E., & Ringis, Nature, 243, 454-458.

J.,

1973 - Sea floor s p r e a d i n g in the T a s m a n

393

Sea.

Jongsma, D . , & M u t t e r , J.C., 1978 - N o n - a x i a l breaching of a rift valley Earth P l a n . S c i . L e t t . , 39, 226-234. Jones, J.G., & V e e v e r s , J.J., 1983 - Mesozoic origins of Australia's eastern h i g h l a n d s . J . G e o l . Soc., 30, 305-322. Roeser, H . A . , & o t h e r s , 1985 - BGR Cruise Report, SO - 36(2). S h a w , R.D., 1978 - ....LHR reconstruction. A u s t . S o c . E x p l . G e o p h y s . B u l l . , 9, 75-87. W e i s s e l , J.K., & H a y e s , D . E . , 1977 - Evolution of Tasman Sea r e a p p r a i s e d . Earth P l a n . S c i . L e t t . , 36, 77-84. W i l l c o x , J . B . , S y m o n d s , P . A . , H i n z , K . , & Bennett, D . , 1980 - L o r d Howe Rise. BMR J., 5, 2 2 5 - 2 3 6 . W i l l c o x , J.B., 1984 - Deepwater Gippsland B a s i n . G e o l . S o c . A u s t . A b s . , 12, 5 5 1 .

Rig Seismic

Sonne dredge site

cruise

Sonne coring site

Sonne cruise —3—

Bathymetric

contour (km)

F i g u r e . Bathymetry of D a m p i e r Ridge/Lord Howe Rise region (km), showing tracks of 1985 BGR 'Sonne* c r u i s e ( n o r t h ) and 1985 BMR 'Rig Seismic' cruise (south); also 'Sonne' sampling s i t e s .

394

3.9

PALEOMAGNETISM OF PRECAMBRIAN MAFIC DYKES AND INTRUSIVES AROUND MOUNT ISA

H. Tanaka^ and M. Idnurm^ ^Tokyo Institute of Technology, Japan ^Bureau of Mineral Resources, Geology and Geophysics, Canberra

Paleomagnetic study of one year project was carried out on mainly dolerite dykes and bodies from the Proterozoic Mount Isa Inlier. We collected 265 cores from 47 sites for reconnaissance study during 3/4/87-4/5/87. 18 dykes are exactly the same ones which have geochemical studies by Ellis and Wyborn (1984)- Sampling localities cover quite a wide area of western and eastern successions including basementMost sites suffered from metamorphism to some extent, but 9 sites are expected to be younge without metamorphismAt 12 sites we also collected surrounding rocks of granite, metabasalt, quartzite etc. for contact test. Most of dyke outcrops are poor and we spent much time to reach the pin point sites. Although samples from almost 30 % localities gave unsuccessful results due to block rotation, lightnig, and unstable NRM, others retain original NRM's or those of metamorphism origin. O n l y t w o to f o u r s p e c i m e n s f r o m e a c h s i t e are t r e a t e d so far by AF demagnetization, yet the results show significant groupings for paleomagnetic direction. In the sense of origin, there are two kinds in NRM's; original NRM of young age which was got by initial formation and secondary TRM acquired by post heating by metamorphism. Those are ascertained at several sites by doing contact test. Most of metamorphism paleomagnetic directions are a little counterclockwised forming antipodal groups. These looks to agree with that of Lunch Creek Gabbro by Duff and Embleton (1976). All unmetamorphosed dykes show the NRM directions of steep positive inclination. This again is a good agreement with their result from Lakeview Dolerite of 1116 12 Ma. There seem to be another direction of NW and almost horizontal. Some of them are obtained from the dykes which are supposed to be young. For the second field trip which is planned from 15/9/87-15/10/87, we are going (1) to c o l l e c t a few m o r e c o r e s from some of the above sites to refine statistics, (2) to study new sites especially of unmetamorphosed young dykes, (3) to collect samples for K-Ar dating from some of the important sites.

395

8.25

MELTING BEHAVIOUR OF P E R I D O T I T E I N THE PRESENCE REDUCED C-O-H F L U I D S - I M P L I C A T I O N S FOR REDOX MELTING OF THE MANTLE W.R. Geology

Department,

Taylor University

Tasmania

Methane-bearing C-O-H fluids are thought to be important carriers of carbon and h y d r o g e n in the Earth's deeper mantle ' . It has been proposed that such fluids play a key role in the. genesis of kimberlitic magmas and diamond in the s u b c o n t i n e n t a l mantle . B e c a u s e the solidus in the system peridotite-C-O-H has not been a c c u r a t e l y l o c a t e d a^ a f u n c t i o n of p r e s s u r e ( P ) , t e m p e r a t u r e (T) and oxygen fugacity(/(92) f it has not been possible to assess the validity of reduced fluid ' i n f i l t r a t i o n ' models of this kind. We report here advances in high pressure fluid b u f f e r i n g t e c h n i q u e s t h a t have a l l o w e d the C-O-H fluid-saturated solidus of a fertile peridotite (Hawaiian pyrolite) to be determined to 35 k b a r . Oxygen f u g a c i t y is c o n t r o l l e d by the assemblage WC-WO^-graphite at -1 log unit above that of the iron-wustite(IW) buffer. Under experimental conditions C-O-H fluids are d o m i n a n t l y H20-CH^ m i x t u r e s w i t h minor R^' ^^^ on temperature. Experimental results for the system Hawaiian pyrolite-C-O-H are presented in Fig.l. The subsolidus region is dominated by a large amphibole s t a b i l i t y f i e l d t h a t i s truncated by the solidus at pressures less than 20kbar. At P<20 kbar amphibole persists to 1 5 - 2 0 a b o v e the solidus. Compositionally, a m p h i b o l e s are p a r g a s i t e s w i t h 15-30 mol% K - r i c h t e r i t e component. Phlogopite is the dominant hydrous phase at P>30 kbar and T>1175'^C but i s s t a b l e to no more than 5-10®C above the solidus. Other minor subsolidus phases include titanoclinohumite and the o x i d e s i l m e n i t e and chromian rutile. O p t i c a l and electron microprobe identification of quenched melt and/or quench crystals and the absence of titanium oxides were u s e d as the main c r i t e r i a to define the solidus. Carbonate has not been identified in any run product. W a t e r a c t i v i t y (aHÔ) has been determined at each point along the solidus from the HÔ/CH. ratio of capsule fluids analysed by mass spectrometry. This a l l o w s the solidus surface of pyrolite to be mapped in ^-T-aRÔ space. The results show that the peridotite-C-O-H solidus is r a i s e d to t e m p e r a t u r e s w e l l above s t a b l e c o n t i n e n t a l geotherms by the presence of only small amounts of CH. in the fluid. This implies that C-O-H fluid induced m e l t i n g b e n e a t h c o n t i n e n t s will be restricted to regions where CH^ can be largely eliminated from the fluid by oxidation ( ' r e d o x ' m e l t i n g ) or where the geotherm i s a b n o r m a l l y high ('hot-spot' melting) or some combination of these processes.

1. Woermann.E. & Rosenhauer.M. Fortschr. Miner. 2.

Taylor,W.R.

& Green,D.H.

Kimberlites

63, 263-349 and

(Geological Society of Australia, in press). 3. Haggerty,S.E. Nature 320, 34-37 (1986). 4. W y l l i e , P . J . ixi Kimberlites and Related Rocks of Australia, in press). 5. Foley, S.F. in Kimberlites and Related Rocks Vol.1 Australia, in press).

396

(1985). Rocks

Vol.1

(Geological Society

(Geological Society of

1000

1300

1100 1200 TEMPERATURE (»C)

Fig.l Experimental results defining the H a w a i i a n (heavy

line)

pyrolite-C-O-H

at/C»2= WCWO (~IW+1 log unit).

amphibole stability field.

Light dashed l i n e

Heavy dashed line is the is

solidus and accompanying amphibole stability field

397

solidus

the

H2O

saturated

8.24

MINERALOGY AND GEOCHEMISTRY OF MELILITE NEPHELINITE AND IJOLITE FROM SHANNON TIER, CENTRAL TASMANIA W.R. Taylor, A,J. Stolz and J.D. Adam Geology Department, University of Tasmania

Several plugs/flows of olivine-melilite nephelinite of Tertiary age occur at Shannon Tier in central Tasmania. They are associated with a small plug of olivine-melilite ijolite (previously called melilite 'fassinite'). The fine grained nephelinites contain olivine phenocrysts (zoned from mg# 87 to mg# 82 with occasional xenocrystic? cores of mg# 90) set in a groundmass of olivine (mg# 82-80), melilite (mg# 80, 4 wt% Na2P), nepheline {lUe^.Ks^^) r titanomagnetite and perovskite. The ijolite is miner a logically complex. Major phases include augite (mg# 80, 1-2 wt% TiO^, 0.5-0.7 Na.O), nepheline (Ne^-Ks.J, melilite (mg# 79, 4wt% Na.O), olivine^ (mg# 79), titanomagnetite and apitite. Nepheline is extensively replaced by zeolites (principally thomsonite) and a little carbonate. Rimming of pyroxene by aegirine-augite, common in many alkaline rocks, was not observed indicating that relatively low and constant fO^ conditions prevailed during ijolite crystallization. Melilite and olivine are both commonly embayed and enclosed by augite. Titanomagnetite contains an array of silicate inclusions: K-richterite, hornblende, natrolite, phlogopite and apatite. Minor phases include monticellite (mg# 49), niobian perovskite, phlogopite and the unusual mineral barian lamprophyllite (first described from the large alkaline intrusions of the Kola Peninsula). The olivine-melilite nephelinites have very similar major and trace element compositions to an olivine-melilite nephelinite and associated ijolitic pegmatoids from Moiliili, Oahu although late stage zeolitization of the Shannon Tier ijolite has resulted in a significant lowering of alkalis (see Table 1). Nevertheless, strong depletion of Cr and Ni and enrichment of Zr and Ba in the ijolite relative to the melilite nephelinite are consistent with an origin by low-P fractionation of olivine, Cr-spinel and melilite from an olivine-melilite nephelinite parent.

Table 1. SiO. TiO^ Al.O^ FeOtot MnO MgO CaO Na^O

K^O

lIsI Total Cr Ni Sr Zr Nb Ba

STMl 36.05 2.67 10.05 14.11 0.26 11.32 14.90 5.06 1.78 1.49 0.57 98.26 209 180 1340 310 182 776

Mo 6 34.60 2.64 10.64 16.15 0.22 11.12 12.88 5.25 1.83 0.93 2.85 99.11 251 245 1458 287 78 1114

STM4^ 35.97 2.79 14.42 11.16 0,21 5.72 15.26 4.63 0.30 2.62 6.28 99.36 9 47 1414 445 203 1170

Mo23 38.79 2.73 12.00 12.94 0.24 5.70 13.25 6.01 2.44 1.68 3.09 98.87 3 22 1915 554 169 2911

Analyses 1 & 2 olivine-melilite-nephelinites. Analyses 3 & 4 ijolites. Analyses 2 & 4 are of rocks from Moiliili (Wilkinson & Stolz, 1983) .

398

3.13

PRELIMINARY PALEOMAGNETIC RESULTS FROM MID-PALEOZOIC ROCKS OF THE TASMAN OROGEN, VICTORIA

G. Thrupp School of Earth Sciences, Macquarie University, North Ryde

Apparent discordance between paleomagnetic results from mid - Palaeozoic rocks of the Tasman orogen in southeastern Australia and results from rocks of the same age from cratonic parts of Australia, was interpreted as evidence for gross displacement during the Paleozoic of tectonic elements of the Tasman orogen relative to the rest of Australia (e.g. Embleton et al., 1974). Indeed numerous tectonostratigraphic terranes have been delineated within the Tasman fold belt system (Scheibner, 1985). Displacement of these terranes may have occurred during an Early Silurian to Middle Devonian tectonic regime of an evolving dextral transform margin (Powell, 1984a; Fergusson et al., 1986). Large-scale displacement of Tasman orogen terranes relative to cratonic Australia, however, is not required by new results from Early and Late Devonian volcanic rocks and latest Devonian - Early Carboniferous sediments, and reinterpretation of pre-existing paleomagnetic data (Schmidt et al., 1986, 1987; Li, 1987). Many of the earlier results are plagued by overprints that were not recognised. For both of the new r e s u l t s from v o l c a n i c r o c k s t h e r e is s t r o n g e v i d e n c e t h a t the magnetisation predates folding which occurred soon after extrusion of the volcanics. T h e n e w p a l e o m a g n e t i c d a t a a r e c o n s i s t e n t w i t h the interpretation that the Late Devonian to Early Carbonifierous Lambian Facies is an extensive overlap sequence deposited when the terranes of the Tasman orogen in southeastern Australia were essentially in their present position with respect to the craton (Powell, 1984b). Additional high quality mid-Paleozoic paleomagnetic poles are needed to better define the Gondwana APWP. In the Grampians, Victoria, 185 samples of Silurian - Early Devonian, predominantly reddish, quartzarenites were collected at 28 sites, and 57 samples of late Early Devonian igneous rocks were collected at 10 sites. In the Mansfield Basin 47 samples of Early Carboniferous red sandstones were collected at 7 sites. Detailed pilot thermal demagnetisation experiments have been completed on over one third of the samples. A reasonably well-defined stable component is present in less than 20 percent of the Grampians sandstone samples; all of these are the same polarity. Clustering of the directions from nine samples of one formation improves with unfolding, suggesting that the magnetisation predates the Carboniferous deformation. Work on additional samples from this formation may improve the resolution of this poorly defined north and moderately upward direction. A stable component, also with a northward and up direction, is present in many of the nearly flat-lying Lambian Facies sandstones of the Mansfield Basin. A negative fold test is provided by results from steeply dipping overturned strata at the east margin of the synclinorium. It is possible, however, that the subhorizontal redbeds in the interior of the basin

399

escaped r e m a g n e t i s a t i o n that probably was associated Carboniferous deformation that is evident at the margin.

with

mid-

Thermal or AF ingremental demagnetisation of the majority of samples of the igneous rocks sampled in the Grampians clearly defines a stable component. These results demonstrate that the intrusions have not caused widespread remagnetisation of the Grampians sandstones, but the scatter of directions within and between the igneous sites is very large. The chaotic results from prominent exposures probably are a consequence of lightning strikes. Uncertainty of paleohorizontal is also a major problem for most of the hypabyssal intrusions. The preliminary paleomagnetic results from the Grampians and the Mansfield region show little promise for new high quality paleomagnetic poles. Additional collections have been made of Early Devonian limestones and sandstones at Waratah Bay, of Early Devonian limestone at the Lilydale quarry, and of Late Devonian, felsic, mainly ignibritic, volcanic rocks in six quarrys and three road cuttings in central Victoria.

References Cited Embleton, B.J.J., McElhinny, M.W., Crawford, A.R., and Luck, G.R., 1974, Palaeomagnetism and the tectonic evolution of the Tasman Orogenic Zone: J. Geol. Soc. Aust., vol. 21, p. 187-194. Fergusson, C.L., Gray, D.R., Morand, V.J., 1986, Tectonostratigraphic terrances, fold-thrust zones and regional metamorphics in the Palaeozoic of central-eastern Victoria: Fieldtrip guidebook for the Specialist Group in Tectonics and Structural Geology, International Conference on Deformation of Crustal Rocks, 50 p. Li, Z., 1987, New paleomagnetic results from Late Palaeozoic rocks of Australia and their tectonic significance: p. 267-271, in Pacific Rim Congress 87, Australasian Institute of Mining and metallurgy, Parkville, Vic, Australia, 949 p. Powell, C.McA., 1984a, Silurian to mid-Devonian - dextral transtensional margin: p. 309-329, in Phanerozoic Earth History of Australia, edited by J.J. Veevers, Oxford University Press, New York, 418 p. Powell, C.McA., 1984b, Late Devonian and Early Carboniferous: Continental magmatic arc along the eastern edge of the Lachland Fold Belt: p. 329340, in Phanerozoic Earth History of Australia, edited by J.J. Veevers, Oxford University Press, New York, 418 p. Schmidt, P.W., Embleton, B.J.J., Cudahy, T.J., and Powell, C.McA., 1986, Prefolding and premegakinking magnetizations from the Devonian Comerong Volcanics, New South Wales, Australia, and their bearing on the Gondwana Pole Path: Tectonics, vol. 5, p. 135-150. Schmidt, P.W., Embleton, B.J.J., and Palmer, H.C., 1987, Pre- and postfolding magnetizations from the Devonian Snowy River Volcanics and Buchan Caves Limestones, Victoria: Geophys. J. Roy. Astr. Soc., in press.

400

3.20

PALEOMAGNETISM OF LATE CRETACEOUS CALCAREOUS SEDIMENTS FROM THE MISOOL ARCHIPELAGO, IRIAN JAYA

G.A. Thruppl, W.V. Sliter^, E.A. Silver"^, C.J. Pigram^, H. Prasetyo^ and R.S. Coe"^ ^School of Earth Sciences, Macquarie University, North Ryde 2uS Geological Survey, Menlo Park, USA ^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Department of Earth Sciences, University of California, Santa Cruz, USA

Numerous allochthonous tectonostratigraphic terranes are recognised in New Guinea and Irian Jaya but their displacement histories are poorly known (Pigram and Davies, 1987). During the early stages of the opening of the Indian Ocean in the Mesozoic, portions of the Australian northern margin may have been rifted away. As the northward drifting Australian craton impinged upon the subduction zone in late Tertiary time, the dismembered continental fragments that arrived at the trench earlier would have accreted to the leading margin of Australia (Pigram and Panggabean, 1984). Indonesian islands around the Banda Sea, and ridges within this sea, have continental basements of Australian affinity (Silver et al. , 1985). Theb:, may be some of the rifted fragments that were rafted northwestward by Mesozoic sea-floor spreading (Pigram and Panggabean, 1984). Alternatively, they may have been displaced from the northern margin of New Guinea and Irian Jaya more recently by Neogene transcurrent faulting (Sliver et al., 1985). The island of Misool is just west of the "bird's head" of Irian Jaya. Excellent coastal exposures of a Mesozoic through Quaternary sequence of carbonate and clastic rocks are well suited for a paleomagnetic investigation of the paleogeographic relationships of Misool, Irian Jaya, and Australia. In addition, the results may provide a framework in which to evaluate the displacement of the continental rocks in the Banda Sea region. We collected oriented cores in the Misool archipelago from 14 formations ranging in age from Paleozoic to Quaternary. Although the magnetisation of most of the formations is either unstable or too weak to accurately measure, thermal demagnetisation of a few sites in the Late Cretaceous Waaf formation reveals two well-defined components of magnetisation. The useful results comprise only five sites (28 cores). The rocks are rythmically bedded, bioturbated, foraminiferal biomicrites. Reddish pigment concentrated in incipient stylolites, subparallel to bedding, give most of the rocks a pale maroon tint. Rare benthic forams are indicative of a bathyal environment of deposition. Abundant planktonic forams are diagonostic of a Santonian age for four sites, and a Turonian age for one site.

401

A post-folding component is removed by 300 C. It is close to the present field, but the same direction could be produced by a late Tertiary overprint. The characteristic component is stable from 400 to 660 C; it predates the Oligocene - Miocene deformation. High coercivity and the blocking temperature spectra indicate that much of this component is carried by hematite. There is only one polarity present as is expected since deposition occurred during the Cretaceous normal superchron. If the characteristic component is essentially primary (nearly the same age as deposition), the mean result indicates ~33 + of anticlockwise rotation relative to the Australian craton. If it is a Tertiary prefolding overprint, the rotation is "10 greater. In either case, at high confidence levels, the indicated latitudinal displacement relative to Australia is insignificant. The timing and regional extent of the rotation are poorly constrained. The secondary component is not rotated, but it is not known if this is a recent viscous component or a partial overprint associated with the late Tertiary deformation. The rotation may have accompanied displacment of portions of the northern margin of New Guinea and Irian Jaya by Neogene sinistral shear. It is also possible that rotation of an allochthonous continental fragment occurred after it was rifted from the Australian margin during the Mesozoic. Although it could be fortuitous, the one slightly older site seems to be the most rotated, suggesting that the rotation was contemporaneous with the Late Cretaceous deposition. Any useful results from the very weakly magnetic Tertiary limestones will help constrain the timing, and results from additional sites in the Turonian limestone would determine if the apparent syn-depositional rotation is real.

References Cited

Pigram, C.J., and Davies, H.J., 1987, Terranes and the accretion history of the New Guinea Orogen: BMR Journal of Geology, in press. Pigram, C.J. and Pangabbean, H., 1984, Rifting of the northern margin of the Australian continent and the origin of some microcontients in eastern Indonesia: Tectonophysics, v. 107, p. 331-353. Silver, E.A., Gill, J.B., Schwartz, D., Prasetyo, H., Ducan, R.A., 1985, Evidence for a submerged and displaced continental borderland, north Banda Sea, Indonesia: Geology, v. 13, p. 687-691.

402

19.10

A CAINOZOIC TIME SCALE FOR AUSTRALIA E.M. Truswell

(compiler)

Bureau of Mineral Resources, Geology & Geophysics,

Canberra

A preliminary biostratigraphic chart is presented, designed to show interrelationships between zones based on marine microfossils (foraminifera, calcareous nannofossils and organic-walled dinoflagellates), marine macrofossils represented by molluscs, land faunas by fossil mammals, and zones based on pollen and spores, which reflect changes in the terrestrial vegetation. These zonations are calibrated, as far as possible, against the timescales for the Palaeogene and Neogene compiled by Berggren and his co-workers in 1985. Shown on the chart are the local stages recognized in sequences in Victoria and South Australia; uncertainties remain concerning their correlation with standard European series and stages. Usage of these by different authors has been variable and has been extended to cover broader intervals of time than those represented by the stratotypes. Analysis of foraminiferal faunas shows the stratotypes of Bairnsdalean, Cheltenhamian and Mitchellian to be overlapping. Foraminiferal zonations are presented in historical order. They include the faunal units published by Carter in 1958 and 1964, the scheme of Taylor (1966), zonations described by Ludbrook and Lindsay in 1969 and 1971, and the succession of biostratigraphic events presented by McGowran in 1978 and subsequently calibrated, as a 'best estimate* against the P and N zones of the international zonal standard. Included too is the zonation for the Early Miocene to Pleistocene of Victoria produced by Mallett in 1977/78, and a sequence of events which Chaproniere (1984) considered to be important in the Australian and New Zealand region; this sequence includes the larger foraminiferal datum of the extinction of Lepidocyclina (Eulepidina) badjirraensis. Most of the foraminiferal zonations relate to southern margin sequences, but included too is the sequence of zones identified in northwest shelf sediments by Wright in 1973, and developed in subsequent work by Apthorpe and Heath. These zones have been correlated with standard tropical zonations but there are some problems in that certain zonal indices appear to have different ranges in the Indian Ocean from those in the Atlantic and Pacific. A developing Australian nannofossil stratigraphy is related to the lowlatitude zones of Bukry (1973, 1975) and Martini (1971) as standards. The Australian biostratigraphy, described in papers presented by Shafik between 1978 and 1985, includes recognition of two of the Martini zones in the Late Palaeocene, and a series of biostratigraphic events based on first and last appearance datums through the Early Eocene to Early Miocene interval. Palynologically-based biostratigraphy includes assemblage zones described for the Gippsland Basin by Stover and Evans, and Stover and Partridge in 1973, and related by Partridge in 1976 to the foraminiferal zonal standard. Assemblage zones for the Otway Basin were defined by Harris in 1971; the Eocene units identified in this and other southern basins were

403

related to the P zones in 1985, thus effecting a correlation with the Berggren timescale. Relationship of dinocyst assemblage zones described for the Eocene of the Otway and St. Vincent Basins was similarly established. Dinocyst zones for the Palaeocene to Oligocene of the Gippsland Basin remain published in name only. Additional palynological information shown includes the Late Oligocene first appearance of Acacia pollen, based on western Murray Basin sequences, and the 'phases* based on relative abundances of pollen in eastern Murray Basin sequences, which Martin described in 1973. The sequence of land-mammal faunal assemblages defined by Woodburne and his co-authors in 1985 has been condensed into two columns. The chronological ordering of the faunas is based on the stage of evolution of selected taxa, on stratigraphic superposition, 'and, less commonly, on relationships to marine sequences or dated volcanics. Local faunas dated by their relation to marine rocks include those from Wynyard, Beaumaris, Forsyths Bank, Lake Tyers and Hamilton. Faunas with age constraints established by proximity to dated volcanics include these from Hamilton, Geilston Bay and Bluff Downs. Molluscan assemblage zones from southeastern Australia were described by Darragh in 1985, based on localities from the Murray, Otway, Bass and Gippsland Basins. They have been related to standard Tertiary timescales by means of the foraminiferal zonations of Taylor, Carter and Mallett, referred to above. The main problems remaining with the Australian Tertiary timescale include chronological placement of some of the Australian stages, firmer establishment of foraminiferal and nannofossil events with standard timescales, development of effective dinocyst zonations, description of biotic events at the Cretaceous/Tertiary boundary, and the development of adequate ways of dating sedimentary sequences in inland basins. Opportunities for relating biostratigraphic units to radiometrically dated volcanics are present in the eastern highlands, where palynological assemblages are being recovered from sediments associated with the volcanic rocks. Similar opportunities exist for calibrating biotic events against magnetic reversal scales; preliminary calibrations are currently being tested in the Lake Eyre Basin and in Lake George in New South Wales.

404

14.8

PALAEOZOIC

FISH

HELP S.

Queensland

TO DATE

AUSTRALIAN

ROCKS

Turner

Museum,

South

Brisbane

New discoveries of macro- and microvertebrate remains found in the last decade are helping to refine dating of Palaeozoic rocks in Australia. Microvertebrates include scales, teeth and spines of various fish and are sometimes dismissed without identification by the term "ichthyoliths" in the literature. Recognition of Devonian microvertebrates in certain sediments, for instance, has enabled rocks to be dated accurately for the first time. Examples include Chillagoe limestones (Qd) and the Tumblong oolite (NSW). A series of useful "zone" fossils is now being recognised for most stages of the geological column. These fossils aid international correlation with North America, Europe, China, South America and Antarctica. Most information is revealed when macro- and microfossils are studied in conjunction. Microvertebrates especially help in dating rocks from small or core samples for example, from the Devonian of South Australia.

405

2.27

SEISMIC FEATURES OF THE THOMSON FOLD BELT UNDER THE W E S T E R N SURAT BASIN

K.D, Wake-Oyster, M.J. Sexton, D.W. and D.M. Finlayson

Johnstone

Bureau of Mineral Resources, Geology and Geophysics,

Canberra

Deep seismic reflection profiling conducted by the Bureau of Mineral Resources (BMR) and Geological Survey of Queensland between 1980 and 1986 across southern Queensland (Wake-Dyster et al, 1985) revealed seismic events from within the crust down to the crust/mantle boundary . The main objective of the seismic surveys was to assist in the understanding of the tectonic development and evolution of eastern Australia. In the region east of the Nebine Ridge beneath the western margin of the Surat Basin, prominent seismic events have been recognised aU^upper, middle and lower crustal levels. They are interpreted as the signatures of petrological differences within the eastern margin of the Thomson Orogen. The seismic profiles (Fig 1) transect the central parts of the Thomson Orogen (Fold Belt) and northern part of the New England Orogen of the Tasman Orogenic Zone (Murray & Kirkegaard, 1978). An example of the deep seismic events is shown in Fig 2 along Traverse 18 west of Roma. The layered seismic reflection events in the first second relate to sediments of the Surat Basin. Beneath top of basement to 3 seconds is a non-reflective zone. Between 3 and 5 seconds a strong mid-crustal seismic event is related from refraction seismic studies to an increase in velocity from 6.0 to 6.2 km/s. Lower down in the section another non-reflective zone occurs at between 12 and 13 seconds, and is interpreted as the crust/mantle boundary. East of the section in Fig 2, along Traverse 14, several of the deep seismic events in the upper crust rise to the top of basement where the sedimentary cover of the Surat/Bowen Basins masks the basement geology. Correlation between seismic events and basement geology is therefore only possible from petroleum exploration wells which have penetrated basement. The description and analysis of the basement rock units by petroleum exploration companies lacks detail because the basement rock units are regarded as non-economic. Despite this, the distribution of basement rock types allows regional maps of basement to be made (Exon, 1976). The deep seismic events map boundaries between the different basement rock units. Beneath the western Surat Basin, rock units occurring at top of basement include the Timbury Hills Formation, Roma Granite, 'Kuttung Formation* and units consisting of schists and gneisses. In Fig 2, the rock type occurring at top of basement is Timbury Hills Formation, with a non-reflective character due to steeply dipping siltstones within the formation- Roma Granite intrudes the Timbury Hills Formation and the seismic events at between 3 and 5 seconds are interpreted to be the top of plutons of the intruding Roma Granite. References: Exon, N.F.,1976. Geology of the Surat Basin in Queensland. Aust. Bur. Miner. Resour., Geol. Geophys., Bull., 166. Murray, C.G. and Kirkegaard, A.G.,1978. The Thomson Orogen of the Tasman Orogenic Zone. Tectonophysics, 48, 299-325Wake-Dyster, K.D., Sexton, M.J. and Johnstone, D.W.,1985. Lithospheric transect study of southeastern Queensland. Exploration Geophysics, 16, 2/3, 312-317.

406

144°

EROMANOA

"TiindS

T BASIN

NEW SOUTH WAL^

P Fauit

-f— Anticline —

Expanding-spread locations

— - 5/W/? se/sm/c traverses

Syncline

" fi/W/? seismic traverses

(1980-84) (1980-821

(1984*&b)

/Vtfiv England-Yarrol Fold Belt ^^^

Outcrop of Permo-Triassic

sediments

Concealed margin of Permo- Triassic sediments

Fig 1. Location map showing BMR seismic traverses and geological setting in southern Queensland.

Fig 2. Deep seismic section of Traverse 18. Prominent seismic events occur between 3-5 seconds (9-15 km). »Moho» interpreted at 12-13 seconds.

407

8.23 ECLOGITE-BEARING, DOMED, LAYERED METAMORPHIC COMPLEXES ("CORE COMPLEXES") IN THE D'ENTRECASTEAUX ISLANDS, PAPUA NEW GUINEA

R.G, Warren and H.L. Davies Bureau of Mineral Resources, Geology and Geophysics, Canberra

Compositionally-layered metamorphic rocks of the D^Entrecasteaux Islands, Papua New Guinea, are folded into domes and anticlines, cored by granodiorite intrusions, and bounded by faults parallel to metamorphic layering and foliation. The structures are similar to the metamorphic "core complexes" of western North America. Lenses of ultramafic rock lie on the bounding faults, and the same faults have served as locii for Quaternary andesitic volcanic activity. Metamorphic grade in the northern islands (Fergusson and Goodenough) is amphibolite facies, with pockets of granulite (Goodenough) and eclogite (Fergusson), and is greenschist in the southern island (Normanby). In all three islands there is a tectonostratigraphic sequence from leucocratic felsic metamorphites at the base, or internally, through more mafic, to ultramafic at the top, or externally. Eclogites s.s. and albite-omphacite-garnet granofels carry minor rutile, quartz, biotite, and/or phengetic muscovite. Partial retrogression, with early poikiloblastic and late invasive crosscuting amphiboles, spanning a compositional range from pargasite to actinolitic hornblende and actinolite, is universal. Textural relations record complex parageneses. Symplectitic intergrowths of albite and omphacite, albite and biotite, and biotite rims on muscovite indicate increasing temperature and/or decreasing pressure late in the metamorphic evolution. However, in other specimens, atoll garnets may indicate falling temperatures. Available PT indicators give maximum pressures of 15-17 kbar at 600-800 C for the eclogites. Mafic granulites from Goodenough Island contain garnet (to pyrope >0.5), orthopyroxene and augite, oligoclase and rare scapolite. Pressures for various specimens has been estimated at 11-13 kbar and 1517 kbar, with temperature circa 800^C. The presence in the felsic rocks of mosaics of albite with biotite, of phengitic muscovite, and of garnets of unusual composition is taken to indicate that the felsic rocks followed a PT path in common with the mafic rocks, and that the water for the abundant migmatites may have been provided by the breakdown of hydrous phases, particularly pargonite. Calcareous rocks contain garnet, scapolite and clinozoisite; rare pelites consist of zoned garnet, kyanite, quartz, staurolite, pargonite and phengetic muscovite. The hightemperature regime is reflected in the apparent absence of minerals such as lawsonite and chloritoid, the comparative rarity of chlorite, the composition of the retrogressive amphiboles, and the absence of true blueschists. The metamorphic rocks are believed to have Cretaceous protoliths, which were downwarped in a north-dipping Paleogene subduction system. The ultramafic rocks are probable outliers of the PUB on the mainland. The metamorphic rocks had been exhumed to upper crustal levels by the Early Miocene, the domal stuctures are much younger features, related to continuing extension in the region.

408

8.30 DISTRIBUTION OF METAMORPHIC FACIES IN THE ARUNTA BLOCK, CENTRAL AUSTRALIA R.G. Warren^ and B.J. Hensen^

^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Department of Applied Geology, University of New South Wales

Prograde metamorphism in the Arunta Block, central Australia, which varies from greenshist-transitional amphibolite to granulite in exposed rocks, was of the high T, low P type. Metamorphism equally affected supracrustals, granites, and the relatively undeformed late mafic dykes which crosscut the granites. Throughout much of the northern zone, the grade is low (2micas + chlorite). In the Reynolds Range region, where continuous section from low grade metamorphites to granulite is exposed, the locus of prograde PT conditions crossed from the andalusite to sillimanite fields within the Crd-Kfs-Qtz-Bt-AS stability field, so that nearly all dehydration reactions occurred at comparatively low pressures, leaving only amphiboles, biotite and cordierite persisting as hydrous phases to higher grade. Similar conditions appear to have prevailed farther east in the Tower Rock-Deep Bore area, though the section is not as well exposed; but in the Jervois district fibrolite co-exists with muscovite, possibly pointing to higher P/T in the east of the Arunta Block. The grade in the central zone is generally granulite, falling to lower grade near the Plenty River structure in the extreme east. Assessment of the prograde metamorphism in the southern zone is rendered difficult by the absence of diagnostic assemblages, but appears to be amphibolite grade everywhere, with pressure decreasing southwards from the bounding faults. Granulite-facies assemblages are believed to have formed through dehydration melting. Segregation of water-undersaturated melts varied with rock type, but the melt-fraction was essentially retained within the rocks in which it formed. Thus, the high-grade metamorphism was near-isochemical, reflected, for example, in high K2O, high Rb, and low K/Rb being maintained in felsic rocks at granulite grade. Moreover fluid composition was locally buffered and fluid flow restricted; as shown by Cal-Wo-Scp-Grs in areas with Opx-Cpx and Opx-Kfs. Therefore we have concluded that high heat-flow did not result from fluid advection. Early cooling was marked by the formation of retrograde biotite, absorbing water from the crystallizing partial melts. The cooling has been described as near-isobaric, but there is evidence for slight warping: Cordierite from the central zone and part of the northern zone has OpxSil along grain boundaries, and there are rare examples of garnetovergrowths in such rocks; elsewhere cordierite overgrows garnet and fine-grained symplectic Opx-Crd replaces garnet. Geo-barometers and thermometers based on Grt-Opx-Pl-Qtz and Cpx-Pl-Qtz have been used for PT estimates where these are available. Elsewhere, Fe-Mg distribution in Grt-Crd has been used to estimate pressure by extrapolation from the values in those localities with other geobarometers as benchmarks.

409

The Arunta Block is laced by faults and shear zones. In the high grade areas these may be narrow mylonites or belts up to several kilometres wide of hydrated schistose rocks; in the low grade areas equivalent fractures are marked by topographically prominent quartz veins. Textures and mineral assemblages in the rocks in such shear zones indicate a prolonged and repeated history of deformation and retrogression. In the granulite regions; the earliest assemblages in the shear zones formed in the kyanite stability field, in the Reynolds Range the retrogression is zoned into And, Sil or Ky bearing assemblages which show the same distribution from low to high pressure as the zoning in the prograde assemblages. Overprinting relationships in the shear zones provide information on the unroofing history of the Arunta Block, differential movement along the shear zones has produced the present distribution of prograde metamorphism in the ARunta Block.

410

8.2

GEOCHEMICAL STUDIES IN THE ARUNTA BLOCK, CENTRAL AUSTRALIA

R.G. Warren^, L.A.I. Wyborn^ and M.T. McCulloch^ ^Bureau of Mineral Resources, Geology and Geophysics, Canberra 2Research School of Earth Sciences, Australian National University, Canberra The geochemical data available for the Arunta Block has been supplemented by a collection, mainly of granites s.l., but including a limited number of samples of other rock types. Assessment of the results shows that the geochemical characteristics of the Arunta Block are those common to the Early Proterozoic throughout northern Australia. The granites of the northern Arunta Block can be subdivided on field characteristics into [a.] strongly gneissic, sheet-like, pre-metamorphic [b.] poorly foliated, post-folding, batholithic, mostly, probably all, premetamorphic and [d.] Non-foliated, leucogranites, possibly in part post-metamorphic. Type [a] and some type [b] granites are chemically very similar, with high K2O and Kb (for the Si02 content), K/Rb in the range 150-220, and other characteristics of the Kalkadoon-Ewen-Leichhardt association, as for example, high Y, low Nb and high REE. Some of the type [b] granites have still higher KpO, Rb (for the Si02 content) and lower K/Rb, <150 (enriched [b] granites). They also have high U, commonly >9ppm, higher Th and higher REE. In outcrop many have euhedral megacysts of K feldspar, and contain allanite and/or fluorite. Some carry anomalous Sn, and those in a narrow belt extending westwards from Barrow Creek also carry high Li. Type [c] granites occur in an arcurate belt from northwest of Barrow Creek to near Jervois Mine. They appear to be related to the pegmatites enplaced circa I68O Ma. The leucocratic, apparently aluminous granites east of the Tarlton Fault may also belong to this suite. Chlorine-bearing hastingsite is common in granites in the northeastern Arunta, but rare elsewhere. The granites in the central zone are small bodies: field observations and available analyses indicate that these too are K-rich, notable exceptions being the Huckitta Granodiorite and part of the Wuluma granitoid. Granites with the characteristics of type [a] also occur in the southern zone. The Burt Bluff Gneiss and the unnamed deformed megacrystic gneiss west of Jay Creek; and the Atneequa Granite and the unnamed granite in the Casey Inlier belong to this association. Some of the granites in the southern zone display chemical characteristics, such as high Na/K, atypical of the bulk of the Arunta granites; preliminary Sm/Nd analyses indicate these granites were derived from a younger source than that of the K-rich granites. Quartzofeldspathic gneisses from the central zone mainly are K-rich, similar to Type [a] granites. However, two units, the Erontonga Granulite and the Entia Gneiss, in part have high Na/K. Because these are considered to lie towards the base of the sequence in the Arunta Block, it seems unlikely that these were derived from the same source as the sodic granite suites in the southern Arunta Block. The regional geochemistry indicates a potential for deposits of U, REE and Y, particularly in the northern Arunta. Other deposits of W-Mo, similar to Molyhil may be present where enriched [b] granites intruded calc-silicate rocks; but the high Fe202/FeO, universal in the granites of the Arunta Block, suggests the region is not prospective for Sn, despite high Sn in some units. 411

12.8 THE PERMIAN-TRIASSIC CONTACT IN THE HIMALAYAS OF NEPAL, AND ITS RELEVANCE TO EPISODIC LIFE CASTASTROPHE J.B. Waterhouse^ and P. Flood^ ^Department of Geology and Mineralogy, University of Queensland ^Department of Geology, University of New England

A newly discovered Permian-Triassic contact is described from the Manang district of northern Nepal, north of the Annapurna Range. The Permian, in the traditional sense, is well represented, with the basal Permian Chulu Formation, containing diamictitite, followed by the Shokang Formation, containing the first Glossopteris to be discovered in Nepal. The Late Permian Senja Formation commences with the Popa Member, followed by the Pija Member with fauna correlative with those of the upper Zewan Formation (members C,D) in the Vale of Kashmir, and by further members with a new Megalosia chuluensis faunal assemblage, younger than the Zewan faunas, and probably of late Permian age, equivalent to high Changhsing and high Dorashamian in China and Armenia. A very late Permian Marsyangdi Formation contains diverse Permian brachiopods (especially Retimarginifera and Spirigerella), with rugose corals and a few bivalves. It contains carbonate bands that are extremely close in appearance to carbonates of the overlying traditionally Early Triassic Panjang Formation, an ochre-coated carbonate with successive zones of Otoceras woodwardi, Ophiceras tibeticum, and then three zones with so-called ''Meekoceras*' faunas of the Himalayas, dominated by Gyronitin ammonoids. Five further zones follow, of late Scythian (Smithian, Spathian) age, in the overlying Nar, Pisang, and Gungdung formations, succeeded by Anisian ammonoid faunules. The Permian-Triassic contact, to follow Diener, is placed at the base of the Panjang Formation, which is discriminated from the Marsyangdi Formation by a slight change in colour, and a thin uneven clay, 1-2 mm thick, or less. The carbonates do not otherwise change perceptibly. (Our chemical analyses are still awaiting completion). In faunal terms, the principal change is in the shift from predominant brachiopods (Permian) to predominant ammonoids (Triassic). Permian-type brachiopods persist into Panjang Formation, especially in the lower part, some probably derived, others apparently not. Basal Panjang ammonoids are little different from general Permian suites, but differ in their abundance. Bivalves are much the same as late Permian bivalves. There is a very marked change in ammonoid faunal composition at the start of the influx of Gyronitin ammonoids (start of the Dienerian in terms of the Canadian Arctic), but again, no marked lithological change. In earlier studies, Waterhouse had suggested that Dienerian could be regarded as a possible Permian Triassic contact, but this must be revised, because it was based on analyses of Triassic ammonoid ranges set out by Tozer (1971), which have had to be substantially changed (i.e. Tozer, 1978, 1981). From the crude time scales now available, it appears that the Permian-Triassic faunal catastrophe falls a little out of phase with an apparent episodicity of substantial but not catastrophic life changes, perhaps lending support to the thesis by Waterhouse since 1963 and 1973 that the event was an unusual one, due to substantial but short-lived increase in heat from the sun. The event definitely post-dated the drastic reduction in marine shelf-area that occurred during the Middle Permian, after the Kazanian Stage, but did either coincide or shortly follow a great marine invasion at the start of the Otoceras-Qphiceras

412

faunas. Thus some possibility of widespread poisoning must be allowed, at least within the marine sphere, related at least indirectly to renewed volcanicity and sea-floor spreading, or reworking of long emerged shelf sediments with undesirable mineral concentrations. However such postulations require much more geochemical analyses, and these must be based on carefully measured and sections with detailed faunal control. Of these, the Manang sequence promises to be one of the most complete, and therefore amendable to close scrutiny.

413

19.3

TOWARDS AN ORDOVICIAN BIOSTRATIGRAPHY OF AUSTRALIA A PROGRESS REPORT

B.D. Webby^, R.A, Cooper^, A.H.M. VandenBerg^ r.s. Nicoll'^, I. Stewart^, J.H. Shergold^^, C.F. Burrett^, B. Stait^ and B.J. Cooper^ ^Department of Geology and Geophysics, University of Sydney ^New Zealand Geological Survey, Lower Hutt ^Geological Survey of Victoria "^Bureau of Mineral Resources, Geology & Geophysics, Canberra ^Department of Zoology, Monash University ^Department of Geology, University of Tasmania "^Department of the Library, Parliament House, Canberra ®South Australian Department of Mines and Energy The only comprehensive Australian Ordovician stage and zonal scheme in general use is that established by T.S. Hall, Keble, Harris and Thomas between 1899 and 1938 based on the Victorian graptolite succession. It was summarized by Harris & Thomas (1938) and Thomas (1960), and subsequently updated by VandenBerg (i^ Webby & others, 1981). Not only is it the basis for correlation of Ordovician basinal deposits in Eastern Australia and New Zealand, but it is the most workable scheme for correlating graptolitic successions throughout the entire 'Pacific province', that is, through some three-quarters of the known Ordovician world. Indeed, it is now recognized as the most detailed and well established global Ordovician biostratigraphic scheme. The chief disadvantages of the scheme are that the key Victorian sections exhibit a comparatively sparse and poorly documented record of associated shelly faunas and this limits the number of tie points to other biostratigraphic schemes. Secondly the appropriate stratotypes for the Victorian stages have yet to be defined. Much basic field work remains to be done before type sections can be designated, and the ranges of species can be revised. In terms of the defined limits of the System, the Cambrian/Ordovician boundary cannot be located because it lies below the lowest records of planktic graptolites, as currently used to recognize the earliest Victorian stage, the Lancefieldian. Harris & Thomas (1938) introduced a tripartite, subsystemic subdivision of the Ordovician, into Lower, Middle and Upper, and this has been adopted by most later workers. However it does not equate with tripartite subdivisionsused in other parts of the world. Accordingly it now seems preferable to follow recommendations of R.A. Cooper and VandenBerg and substitute a twofold division with the Lower/Upper Ordovician boundary being taken at the base of the world-wide zonal fossil Nemagraptus gracilis (that is, at the base of the Gisbornian). The Ordovician/Silurian boundary is recognized in an apparently continuous succession near Darraweit Guim, north of Melbourne, with representatives of key graptolites of the persculptus Zone below, and the acuminatus Zone above the boundary. VandenBerg has revised the latest Ordovician stage, the Bolindian, to include the persculptus Zone. Recent discoveries of conodonts by Stewart in the graptolitic succession of Victoria are providing some biostratigraphically useful links. The conodonts are of 'North Atlantic' provincial aspect and allow, for example, the early Bendigonian (Be 1) Zone of Tetragraptus approximatus and ^ fruticosus (4-branched) to be tied to the conodont Zone of Prioniodus elegans, the late Bendigonian - Chewtonian (Be 3 - Ch 2) interval to the Gepikodus evae Zone, and the latest part of the Darriwilian (Da 4) Glyptograptus teretiusculus Zone - early Gisbornian Nemagraptus gracilis Zone to the Pygodus anserinus Zone. Separate latest Cambrian-Early Ordovician local stages, the Datsonian and Warendian, have been established by Jones and others (1971) in the platformal sequences of the Georgina Basin, with accompanying conodont zones of mainly North American Midcontinental aspect. When the Cambrian/Ordovician boundary 414

is finally ratified internationally it is likely to be located precisely within the Datsonian zonal succession. Nicoll has recorded conodont associations but not a continuous record through the 'Arenig-early Llanvirn' equivalents of the Canning, Amadeus and Georgina Basins, to the youngest which belongs to the 'North Atlantic' Zone of Eoplacognathus suecicus (that is, it is about mid-Darriwilian in age) . These assemblages include elements of North Atlantic, North American Midcontinent and Chinese provincial aspect. There is no faunal record of Upper Ordovician deposits in these mainland platformal areas of Australia. These platform sequences have varied shelly faunas (especially nautiloids and trilobites) and palynomorphs which provide useful supplementary biostratigraphic data. Shergold emphasizes certain faunal events which are widely correlatable in these platform areas: (1) the incoming of Cordylodus proavus assemblages initiating the Datsonian; (2) the incoming of the scolopodids and chosonodinids defining the start of the Warendian; (3) though not yet adequately documented, the first appearances of Histiodella, ambonychiid bivalves and the trilobite Carolinites; and (4) the trend to gigantism in bivalves, nautiloids and trilobites near the 'Arenig-Llanvirn' boundary. The Canning Basin has a mixed graptolite and conodont facies but provincial differences in these faunas limit the closeness of correlations with Victoria. Some workers consider that local stages should be introduced for the entire Lower Ordovician (Tremadoc-Llanvirn) interval in mainland platformal areas. The Ordovician succession of Tasmania is far more complete but there are parts of the sequence with endemic faunas, and a considerable proportion of supratidal to intertidal strata at some levels. Brachiopods and nautiloids are w e l l represented but data especially on the brachiopods remain largely unpublished, as does the conodont and trilobite data. Graptolites are useful at a few horizons, and so are some occurrences of corals, stromatoporoids and gastropods. Burrett recommends that the generalized 'OT' assemblages of Banks & Burrett (1980) now be ignored but more refined zonal. schemes have yet to be published. He proposes to erect local stages based on the Florentine Valley sequence, for the interval from about 'middle Tremadoc - Llandovery* equivalents, and probably independently of those suggested for platform areas in mainland Australia. Significantly external correlations of Tasmanian successions are mainly with North American platform sequences, and North American Stage and Series names such as Chazyan and Whiterockian can be comparatively easily applied. L o c a l usage of subdivisions lower, middle and upper are employed, and approximate to the tripartite divisions of the Ordovician used in North America. There are no recognizable major faunal events in the Tasmanian sequence which can be used for correlation purposes. It presently remains difficult to correlate directly between the Victorian graptolite and Tasmanian shelly faunal schemes, but mixed graptolite and shelly facies like those represented in the Upper Ordovician of central New South Wales m a y , when more adequately documented, provide tie points between the two schemes. Already it is possible to establish broad correlation between the Upper Ordovician carbonate sequences of Tasmania and central N.S.W. using corals and stromatoporoids (WdDby and others, 1981), and conodonts may be expected to provide more precise zonal links. References Banks M . R . & Burrett C.F., 1980, J . geol, Soc. Aust., 26, 363-376. Harris W . J . & Thomas D.E., 1938, Vict. Min. geol. J . , 1, (3), 62-72. Jones, P.J., Shergold J . H . & Druce E.G., 1971, J . geol. Soc. Aust., 18, 1-32Thomas, D.E., 1960, J . Proc. R . Soc. N.S.W., 9 4 , 1-58. Webby, B . D . & others, 1 9 8 1 , Int. Union. Geol. Sci., Publ. 6 , 1-64.

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17.3

INTRUSIONS BENEATH LARGE INTRAPLATE VOLCANOES P. Wellman

Bureau of Mineral Resources, Geology and Geophysics, Canberra

Large, intra-plate volcanoes, when dissected by erosion, are found to overly a oogenetic intrusive complex. The intrusive complex causes a positive gravity anomaly, a magnetic anomaly with a positive about 10 km across, and a net uplift of the country rock. Modelling of the gravity anomaly alone gives a lower bound for the density of the intrusive complex of 3.0-3.05 t.m."^, and depth to the centre of mass of less than 10 km. For volcanoes in stable intra-plate regions the space for the intrusive complex is likely to be created solely by uplift of the country rock above it and to the side. Hence the uplift volume gives the volume of the intrusive complex. The residual, gravity anomaly together with the mean density of the country rock gives the total mass, so the density of the intrusion can be determined directly from its mass and volume. For some Australian and New Zealand volcanoes the inferred density of the intrusive complexes is about 3.0 t.m."'^, and the volume of the intrusive complex is about one third the volume of volcanics. Hence the average composition of the intrusive complex must be gabbro, so it is more mafic than the average outcropping intrusion. The intrusive complex is inferred to be roughly equidimensional in shape, so it is best modelled as a sphere, with a centre at 6 to 8 km depth.

416

2.30 OROCLINAL BENDING OF THE NEW ENGLAND OROGEN A REFINEMENT USING MAGNETIC AND GRAVITY ANOMALIES P. Wellman and R. Korsch Bureau of Mineral Resources, Geology and Geophysics, Canberra

The New England Orogen consists of a fore-arc basin succession, the Tamworth Belt, originally in the west, and accretionary wedge sequences, the Woolomin, Sandon and Coffs Harbour tectonostratigraphic associations of mainly greywacke, originally in the east. These associations formed as relatively straight bands, that were bent into three oroclines, the Manning, Texas and Coffs Harbour Oroclines in the Early Permian. Each orocline consists of an almost 180° change in direction. The pre-Permian rocks forming the oroclines are now partly obscured by Permian and Triassic granitic intrusions and volcanics in the New England Tableland region, and Triassic to Cretaceous sediments in the Clarence-Moreton and Surat Basins. Gravity and magnetic anomalies have been used to refine this geological model, by mapping the extent, and internal units, of the bands. The Tamworth and Woolomin associations are found to extend from the northernmost outcrops almost due north to at least 27°30'S in Queensland, with almost no change in character. The originally-eastern, youngest third of the Sandon association is denser and more magnetic, allowing the mapping of the Sandon and Coffs Harbour associations around the Texas and Coffs Harbour Oroclines. Internal units parallel most association boundaries. However in the Woolomin and adjacent Sandon, the units are at an angle of about 20° to the association boundaries, suggesting that here time horizons are truncated by association boundaries. The Demon Fault can be traced for about 300 km, and twelve geophysical features are consistent with a dextral displacent of 22 km.

417

4.9 THE ORIGIN OF MID-PROTEROZOIC UNCONFORMITYRELATED URANUM-GOLD-P.G.E. MINERALIZATION, ALLIGATOR RIVERS, NORTHERN TERRITORY A.R. Wilde^, R. Maas^, M.T. McCulloch^ ^Department of Earth Sciences, Monash University, Melbourne ^Research School of Earth Sciences, Australian National University, Canberra

Genetic models for the origin of Mid Proterozoic unconformity-related uranium deposits range from those which call for deposition close to the surface in karstic collapse caverns to those which advocate a more deep-seated origin (over 1 km) and involvement of meteoric or metamorphic fluids. In some models, deposition is thought to result from breakdown of uranyl carbonate complexes although no unequivocal evidence has been advanced to substantiate this hypothesis. In this paper we present a study of fluid inclusions, alteration mineralogy and Nd-Sr isotopic systematics in the Nabarlek, Jabiluka and Koongarra deposits, which imposes significant constraints on genetic models. Fluid inclusion studies demonstrate that the temperature of the mineralizing fluids was between 100 and 250'C at pressures of up to 500 bars. Salinity of the inclusion fluid ranges from 0 to 45wt% eq NaCl, with two distinct salinity groupings apparent: a hypersaline group (>20wt%) and a low salinity (<15wt%) group. The fluids were oxidized (in equilibrium with hematite and anhydrite) and had high Cl/S. A pH range of 3-4 was computed using the calculated logK values for chlorite/mica equilibria and Mg-K analytical data for fluid inclusion leachates. Thus chloride complexing of the metals is suggested. Massive desilification in the ore zone indicates that the fluid was entering the deposits from above, and that large volumes of water (W/R 500-1000) were focussed through the mineralized breccia zones at the unconformity. Calculations have shown that ore-forming concentrations of uranium, gold and palladium are mobilized (as chloride complexes) only in such oxidized and acidic brines. Platinum becomes mobile under high fo2» and fluids with an oxidation state comparable to that of seawater are required for its transport. Deposition of the ore metals may have been in response to mixing of reduced CH^-bearing fluid with oxidized uraniferous fluid and/or change in pH as a result of fluid-wallrock reactions. Highly oxidized fluid is thought to be either a function of evaporite dissolution in the overlying sediments or dissolution of the abundant hematite in the overlying Kombolgie Formation sandstones. Sm-Nd dating of primary ores at Nabarlek and Jabiluka II indicates primary high grade mineralization between 1610-1620 Ma, in close agreement with Sr model ages on hydrothermally altered schists associated with these ores. Sm-Nd model ages for Koongarra 418

yield a similar age. This suggests mineralization occurred after deposition of at least part of the Kombolgie Formation and burial of the unconformity which is dated at 1650 Ma, consistent with elevated temperatures and pressures derived from the fluid inclusion studies. Later partial remobilization of the ores occurred between about 1100 and 900 Ma and is reflected in reset Sm-Nd and Rb-Sr ages as well as published U-Pb ages. Initial Nd and Sr isotopic compositions of uraninites are generally within the range found for host Cahill Formation rocks, Kombolgie Formation sandstones, and felsic to mafic volcanics present in the Kombolgie Formation, but they show consistently lower ^'^Sr/^^Sr (0,710-0.720) and higher E^^ (-3 to -9) than are found in underlying Archean basement and a U-rich post-tectonic high-level granite spatially associated with the Nabarlek mineralization. This is most easily explained by extensive fluid-rock interactions at the site of ore deposition which may have partly modified the original fluid Nd and Sr isotopic signatures. However, mass balance calculations suggest that at least some of the Nd and HREE, and most of the U were derived from other sources. While deep-seated sources cannot be completely ruled out, the isotopic data can be explained more easily by a derivation of the ore fluids from (and isotopic equilibration with) the sedimentary and volcanic rocks of the Kombolgie Formation above the unconformity. This is consistent with the fluid inclusion and mineralogical evidence for highly oxidizd and acidic ore fluids.

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15,2

STRUCTURAL DEVELOPMENT OF THE WESTERN LORD HOWE RISE B, Willcox and P- Symonds

Bureau of Mineral Resources, Geology and Geophysics,

Canberra

Reconstructions of SE Australia and the Lord Howe Rise (LHR), juxtapose the sharply defined SE Australian margin against a 200 km w i d e rift-zone on w e s t e r n LHR (Weissel & Hayes, 1977; Shaw, 1979; Fig. 1). The rift-zone, comprising eroded (?) Cretaceous tilt-blocks and half-graben, abuts a highstanding planated basement platform w h i c h forms eastern LHR (Jongsma & Mutter, 1978; Willcox & others, 1980; Fig. 2). Both structural provinces, and the flanking Middleton and Lord Howe Basins, are progressively offset by NE-trending (?)transfer faults. Recently, the LHR has been interpreted as a 'lower plate margin' (Lister, Etheridge & Symonds, in p r e s s ) w h i c h possibly formed, together with the Dampier Ridge, between a series of westerly dipping branching detachments. Modern seismic data (Roeser & others, 1985; W h i t w o r t h & Willcox, 1985, Fig. 1) indicate that the•grabens on western LHR (Fig. 2) are up to 50 km wide, and extend over several tens of kilometres on a NNW-trend. Their abrupt termination at transfer-like faults and the oblique angle that these faults make w i t h the normal extensional faults, suggest formation w i t h i n a 'transtensional zone'. These features are best developed on the w e s t e r n LHR north of Lord Howe Island, w h e r e their sediment fill is up to 4500 m thick in places. Extensional basins to the south of Lord Howe Island appear to be less complex. Some fault-blocks in the southern area may contain dipping sedimentary strata of Mesozoic age, rather than solely Tasman Fold Belt rocks as previously believed (Fig. 3). If so, sediments may have been deposited in this area at the same time as the older sediments were accumulating in the Gippsland (i.e., Strzelecki) Basin. The nature of the synrift sediment fill w i t h i n the LHR basins (riftzone) is a matter for conjecture. It is generally assumed to be late Mesozoic, but correlation w i t h older eastern Australia basins (such as the Permo-Triassic Sydney Basin and Esk Trough, and the Triassic-Jurassic Clarence-Moreton Basin) cannot be completely discounted. If however, the LHR basins formed according to classical models for passive continental margins, much of the earliest sediment fill would have been fluvial-lacustrine. During recent surveys, diapiric structures have been identified w h i c h suggest the movement of shale, or possibly salt, within the basins (Fig. 4). In the overlying section, there is evidence of (?) Maastrichtian w a v e - b a s e erosion having caused planation of several of the tilt-blocks as the LHR subsided. (Burns, Andrews & others, 1973). Abbreviated

references

Burns, R.E., Andrews, J.E., & others, 1973 - Initial Reports DSDP, 21. Lister, G.S., Etheridge, M.A., & Symonds, P.A., in press - Detachment models. Tectonics. Roeser, H.A., & others, 1985 - BGR Cruise Report, S0-36(2). Shaw, R.D., 1978 - Seafloor spreading in Tasman Sea.... Aust. Soc. Geophys. Bull., 9, 75-81. Weissel, J.K., & Hayes, D.E., 1977 - Evolution of Tasman Sea reappraised. Earth Plan. Sci. Lett., 36, 77-84. Willcox, J.B., Symonds, P.A., Hinz, K., & Bennett, D., 1980 - Lord Howe Rise. BMR J., 5, 225-236. Whitworth, R., & Willcox, J.B., 1985 - Lord Howe Rise Research Cruise. BMR Report, 266.

420

Figure l.Tasman Basin/LHR region showing rift-zone on western LHR and planated basement on eastern LHR. Boxed areas show BGR •Sonne' survey (north) and BMR 'Rig Seismic' Survey (south), MB = Middleton Basin; LHB = Lord Howe Basin;LHI = Lord Howe Island. Modified from Lister & others, ( in .press ).

Figure 2.

LHR cross-section

(Willcox & others,

1980).

LINE s o 36A-13 SW

Figure 3.Dipping strata within infra-rift sequence (IR) on southern LHR. CB = crystalline basement; R = rift-fill; P-BA = post-breakup aggradation.

421

Figure 4.Apparent diapir on seismic monitor from northern LHR. Y, 0, etc are major unconformities from Roeser & others (1985).

9.8

EARTHQUAKE INDUCED LIQUEFACTION POTENTIAL IN Q U E E N S L A N D D.J, Williams

Department of Civil Engineering, University of Queensland

The hazard associated with earthquake activity is not a function only of the earthquake magnitude expected for a particular recurrence interval, at a particular location. It is a function also of geological and surficial soil conditions, of the level of development at the location and of the degree to which potential earthquake activity was allowed for in the design and construction of man-made structures. In Queensland, historical earthquake magnitudes are moderate by world standards, with a maximum recorded earthquake Richter local magnitude of 6.3. However, most of the development in Queensland has taken place along the east coast, where about eighty percent of the known Queensland earthquakes have occurred. Further, the coastal development is centred on rivers, with increasing construction on deep estuarine deposits comprising interbedded soft clays and loose sands. The estuarine areas are low-lying and the soils are therefore largely saturated. Under these conditions, the estuarine areas may amplify low amplitude, low frequency earthquake motions and amplification may be sufficient to cause loose saturated sand layers to liquefy. The potential for earthquake induced liquefaction in the developing estuarine areas of the east coast of Queensland has been investigated. The basis for the investigation was a study of local soil conditions and earthquakes that are known to have occurred in Queensland, together with American, Japanese and New Zealand experience of earthquake induced liquefaction. The investigation was extended to consider possible future earthquakes which are predicted by analysis of the historical earthquake data. The results of the investigation indicate that for known earthquakes some estuarine areas would have been on the borderline of liquefaction and that future slightly larger earthquakes may cause major liquefaction induced damage to the coastal development that has largely taken place over the last two decades. During that time there have been few earthquakes of any significance in Queensland. The significance of these findings is that, with few exceptions, the possibility of damaging earthquake induced ground motions has not been considered in the design and construction of coastal structures in Queensland. Action to minimise the potential for earthquake induced liquefaction of the supporting ground would also improve the load bearing and settlement performance of the ground. The most effective way of reducing the potential for a loose saturated sand layer to liquefy is to densify the deposit. This can be achieved by vibrocompaction or by the installation of stone columns. Alternatively, a susceptible layer may be neutralised by piling through it to firmer strata or hammer-base piles may be founded within the susceptible layer and be used to densify it. This would at the same time provide the required bearing capacity. If designers are aware of the possibility of earthquake induced liquefaction, they may take steps to guard against that possibility, which will also improve the bearing capacity and settlement performance of the ground.

422

BIBLIOGRAPHY Fairless, G.J. and Berrill, J.G. (1984). Liquefaction During Historic Earthquakes in New Zealand. Bulletin of New Zealand Nat.Soc. for Earthquake Engineering, Vol. 17, No. 4, pp.280-291. Ishihara, K. (1985). Stability of Natural Deposits During Earthquakes. Proc. 11th Int. Conf. on Soil Mechanics and Foundation Eng., San Francisco, Vol. 1, pp.321-376. Japanese Soc. of Civil Engineers.(1973). Earthquake Resistant Design for Civil Engineering Structures, Earth Structures and Foundations in Japan. Kuribayashi, E. and Tatsuoka, F. (1975). Brief Review of Liquefaction During Earthquakes in Japan. Soils and Foundations, Vol. 15, No. 4, pp.81-92. Lee, K.L. and Fitton, J.A. (1969). Factors Affecting the Cyclic Loading Strength of Soil. Vibration Effects of Earthquakes on Soils and Foundations, ASTM, STP 450, pp.71-95. Rynn, J.M.W. (1984). Earthquake Seismology. Introduction to Earthquake Engineering Workshop Notes. Brisbane, 12-13 Nov. 1984, 157 pages. Depts of Civil Eng . and Geology and Mineralogy, Univ. of Queensland. Rynn, J.M.W. (1985). The Earthquake Hazard in Relation to Queensland's Coastal Environment. Proc. Short Course on Geological and Environmental Aspects of Coastal Management, Brisbane, Feb. 1985, pp.119-144, Queensland Institute of Technology. Rynn, J.M.W. (1986). Quantitative Assessment of Seismic Risk in Queensland and Northern New South Wales (Australia). Final Report for Queensland Seismic Risk Study to Queensland State Government, 191 pages. Dept of Geology and Mineralogy, Univ. of Queensland. Rynn, J.M.W. (1987). The Assessment of Seismic Risk in North-Eastern Australia. Submitted for Publication in lEAust, Civil Eng. Trans. Seed, H.B. (1976). Evaluation of Soil Liquefaction Effects on Level Ground During Earthquakes. State-of-the-Art Paper. ASCE Nat.Conv.on Liquefaction Problems in Geotechnical Engineering,Philadelphia,27 Sept.-l Oct, 1976, pp.1-104. Seed, H.B. and Idriss, I.M. (1982). Ground Motions and Soil Liquefaction During Earthquakes. EERI Monograph. U.S.A. National Research Council Committee on Earthquake Engineering (1985). Liquefaction of Soils During Earthquakes.Pub.Nat.Academy Press, Washington, D.C. Youd, T.L. and Hoose, S.N. (1977). Liquefaction Susceptibility and Geologic Setting. Proc. 6th World Congress on Earthquake Engineering, New Delhi, Vol.3, pp.2189-2194. Williams, D.J. (1987). Some Engineering Risks in the Earthquake Hazard to the East Coast of Queensland. Submitted for publication in lEAust, Civil Eng. Trans.

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8.10

DATING THE SOURCES OF BEGA BATHOLITH GRANITES BY ION MICROPROBE

I.S. Williams^, Y. Chen^, B.W. Chappell^ and W. Compstonl ^Research School of Earth Sciences, Australian National University, Canberra ^Geology Department, LaTrobe University, Bundoora ^Geology Department, Australian National University, Canberra

All the granites of the Bega Batholith, southeastern N.S.W., are I-type in the sense of Chappell and White (1974) and can be inferred, from their chemical compositions, to have been derived from sources that in large part have not been fractionated chemically by the processes of weathering and erosion. The granites can be grouped into seven supersuites of compositionally related plutons that form narrow strips extending up to 300 km parallel to the meridional elongation of the batholith. The most useful elements for discriminating between the supersuites are Na and Sr, which decrease significantly from east to west across the batholith. There is also a systematic change in the isotopic cormosition of the granites (McCulloch et aL, 1982), with a progressive westward rise in initial ^Srr^Sr (Ecj.) and fall in initial ^^^Nd/^^Nd (Emj), implying a westwardly increase in the amount and/or age of an evolved component in tne granites and therefore in their source regions. The Bega BathoUth granites contain small amounts of inherited zircon which are a directly datable sample of their source material. By using the SHRIMP ion microprobe it has been possible to analyse the magmatic zircons and their inherited cores separately and to determine the ages of magmatism and of each of the inherited components without the problems of averaging inherent in even single crystal conventional analyses. The inherited cores are very difficult to distinguish opticallyfromthe magmatic zircon which overgrows them and not every visible core contains inheritance. Obtaining ages for a sufficient number of inherited zircons for a useful assessment of the ages and abundances of the different inherited components is therefore difficult. Nevertheless, analyses of about 40 zircons from each of several plutons and dioritic inclusions have led to the identification of four principal zircon components in the batholith: 1. Magmatic zircons about 390 Ma old. 2. Toung' inherited zircons between 420 and 650 Ma old. 3. Intermediate age' inherited zircons about 1000 Ma old. 4. 'Old' inherited zircons between 1150 and 3350 Ma old. With few exceptions, each component is present in each granite, but in different proportions. The granite with the highest E^^, lowest £5^ (the Moruya Tonalite on the NE side of the batholith) contains the least and youngest inheritance; the granite with the lowest highest (the Blue Gum Tonalite on the SW side of the batholith) contains the oldest inheritance. Tlie dioritic inclusions all contain significantly less inheritance than their granite hosts. The'young'inherited zircons are the most widespread and abundant inherited component in both the granites and inclusions. They probably originate from an igneous component in the source formed at an intermediate stage in magma genesis. If that was underplated material, then there was widespread underplating of at least the eastern part of the Lachlan Fold Belt in the late Precambrian and early Palaeozoic. The 'old' inherited zircons, with their wide range in ages, probably are from a sedimentary component in the source, not country rock assimilated during granite emplacement. A westwardly increase in the amount of that sedimentary component would explain the Na, Sr, £5^, S^^ and zircon age trends observed. The 'intermediate age' inherited zircons may be a dominant age group in the sedimentary component or may represent an earlier phase of underplating approximately contemporaneous with the Grenville activity in North America. ChappeU, B.W. & White, AJ.R. (1974): Two contrasting granite types. Pacific GeoL, 8, 173-174. McCulloch, M.T., Chappell, B.W. & Hensel, H.D. (1982): Nd and Sr isotope relationships in granitic rocks of the Tasman Fold Belt eastern Australia. Abstracts ICOG V, 246-247, Nikko.

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2.4

TWO-STAGE LOWER CRETACEOUS RIFTING IN THE OTWAY BASIN P.E. Williamson, M.G. Swift and G.W. O'Brien Bureau of Mineral Resources, Geology and Geophysics, Canberra

The rifting history between the southeastern Australian continental margin and Antarctica is poorly understood, and a number of hypotheses have invoked either an initial wrench component or a uniform rifting history which reflects the current rift-drift trends. Examination of existing well and seismic data, as well as regional multichannel seismic data collected over the offshore Otway basin by the Australian Bureau of Mineral Resources vessel "Rig Seismic", has led us to the postulation of a two-phase Early Cretaceous rifting history for the region. The first phase, from 140 MaB.P. (corresponding to the oldest known sediments in the Otway Basin) to 120 MaB.P., had E to ENE trending rift faults. These rift treuds are particularly well-preserved on the Crayfish Platform, where, in the western part of the Otway Basin, the early rift is imaged by seismic reflection data and has been penetrated by petroleum exploration wells. This early rifting episode coincided with deposition of the Pretty Hill Sandstone. The post-Pretty Hill sedimentary section is particularly thin on the Crayfish Platform; and this, along with the relative absence of second phase faulting, may facilitate the recognition of the early rift trends in that area. The first rift phase trends are approximately perpendicular to, and are probably associated with, approximate SSE accommodation and transform directions exhibited, for instance, by the Sorrel fault on the west Tasmanian margin. The first-phase trends are also in evidence within the onshore Otway basin and as basement-involved features in the Bass and Gippsland basins. The E to ENE orientation of the first-phase trends are similar to the general rift trends for the southwestern Australian margin. Thus, the orientation of the rifting from 140-120 MaB.P. may have been largely constant right across the Australian Southern Margin, and through Bass Strait. The termination of the first Early Cretaceous rift phase „ in the" Otway Basin probably corresponds to the initiation of seafloor spreading between Australia and Antarctica along the southwestern Australian margin at approximately 120 Ma B.P. The resulting major unconformity at 120 MaB.P. is penetrated in wells on the Crayfish Platform and corresponds to a change in sediment type from quartz-rich sands (Pretty Hill Sandstone) to volcanogenic sandstones, mudstones and shales (Eumeralla Formation). The second phase of Early Cretaceous rifting in the Otway Basin took place from approximately 120-95 MaB.P. Termination of this rifting event at 95 MaB.P. approximately corresponds to the change from Otway Group to Sherbrook Group sedimentation. This second phase has rift fault trends between ESE and SE and is responsible for the final rift and drift trends which dominate the observed trends in the region. The first-phase faults are, however, reactivated in some areas with a relatively thick post-Pretty Hill section and are preserved in the Otway Basin as minority fault trends at the Top Otway Group and Top Sherbrook Group (Late Cretaceous) unconformities, and even at shallower levels.

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2.22

FOLDING, THRUSTING AND STRIKE SLIP FAULTING IN THE SOUTHWEST PORTION OF THE LACHLAN FOLD BELT C.J.L. Wilson^ and R.B. Watchorn^

^Department of Geology, University of Melbourne ^Western Mining Corporation Ltd, Stawell Joint Venture, Victoria

Two sections through the SW margin of the Lachlan Fold Belt (LFB) provide an insight into the deformation of the Cambrian to Early Devonian sequences in the Ararat-Stawell region. The structural evolution of this portion of the LFB is interpreted in terms of a thin-skinned tectonic model superimposed on which is a strike slip regime. Deformation events D 1 - D 3 predate the emplacement of the granites and were synchronous w l & regional fold forming events and movements along discrete decollement surfaces to produce thrusts, and involve an eastwards translation of tectonic units. These are associated with emplacement of slivers of Cambrian volcanics at higher stratigraphic levels within the younger Cambro-Ordovician turbidite cover. Deformation event D4 is characterised by west dipping shears that also juxtapose crustal segments of different metamorphic grade and history and may coincide with the emplacement of granites and porphyries. All these deformation features are characterised by linear, roughly NW-SE trending folds and thrusts. They are followed by deformation D5 which is a strike slip faulting event, predominantly in a sinistral (left lateral) sense. We interpret the gradual change from folding, thrusting to a strike slip faulting regime to reflect a smooth change in the average stress state in this portion of the LFB. This suggests that the complex intracontinental deformation of the LFB during the early to middle Devonian can be described as a continuum of deformation rather than relating it to discrete events occurring in a small number of discrete blocks. The sedimentary sequence in the Grampians is deformed by north-south and northwest trending high angle faults, en-echelon folds, systematically arranged arrays of reverse and normal faults and descrete Joint sets which are typical of major wrench fault zones. These structural characteristics have probably been induced by transcurrent movements during the Tabberabberan Orongeny on underlying basement faults and are a result of a transpressional environment {strike slip plus compression)

426

Tertiary and Quaternary deposits

143°

Devonian granite Sg

Silurian Grampians Group Cambro - Ordovician quartz - rich turbidites

Cambrian (?) greenstone

\.fj

.... 'v

€o

J.; ••:. St. .. Arnaudv^-..

—Anticline Syncline Fault

50Km

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MAGDALA ANTICLINE Concongella Hill

STAWELL' ARARAT FAULT ZONE

/ greenstone

A' f/i

LANDSBOROUGH FAULT

AVOCA FAULT

greenstone 5km

B-

Grampian Group

JALLUKAR FAULT

MOYSTON FAULT ZONE /

STAWELL-ARARAT FAULTZONE

5km

427

2.25

TECTONIC HISTORY OF THE PALAEOZOIC BROKEN RIVER PROVINCE, NORTH QUEENSLAND I.W. Withnall and S,C. Lang Queensland Department of Mines

The Broken River Province occurs in the northern part of the Tasman Orogen. The first systematic study was the 1:250 000 mapping program by the BMR and GSQ {White, 1965). This and subsequent studies were reviewed by Arnold & Fawckner (1980) and Wyatt & Jell (1980). Remapping by the GSQ (now RGIP) since 1982, and concurrent university studies, have provided much new data (Withnall & others, 1987). The Broken River Province is bounded to the west by major faults, which include the Burdekin, Halls Reward, and Teddy .Mount Faults, and which range from ductile mylonite zones, interpreted as thrusts, to brittle normal faults. They separate the Broken River Province from the largely PreCambrian Georgetown Province to the west. The latter also includes SiluroDevonian granitoids and probable Early Palaeozoic metasediments and metavolcanics. The Clarke River Fault, which was a ductile mylonite zone with transcurrent movement early in its history, and later a brittle normal fault, separates the Broken River Province from the Lolworth-Ravenswood Province. The latter consists of probable Precambrian rocks, CambroOrdovician sediments and volcanics, and Ordovician to Early Devonian granitoids. The nature of the eastern margin of the Broken River Province is uncertain, as it is obscured by Late Palaeozoic igneous rocks.

The Broken River Province is divided into two subprovinces, the Camel Creek Subprovince (CCSP) to the east, and the Graveyard Creek Subprovince (GCSP) to the west. Each has had a different tectonic history.

The CCSP consists predominantly of multiply deformed alternating belts of quartz-rich and quartz-intermediate flysch of Ordovician to Early Devonian age. A predominance of westward younging suggests that the belts represent imbricately-stacked thrust sheets. Contacts between most of the belts are interpreted as tectonic, although in a few places the two types of flysch are interbedded, indicating that they were deposited contemporaneously, presumably from different sources. The quartz-intermediate flysch was derived from a plutonic/metamorphic terrane (like the Georgetown Province) and reworked sediments (including limestone) from within the basin. Felsic to mafic volcanic detritus is a persistent, although volumetrically minor, component of the rocks. Some calc-alkaline andesitic to basaltic lavas and volcaniclastics associated with Late Ordovician limestone occur along the western margin, but the volcanics present elsewhere in the CCSP are tholeiitic basalt lavas. The CCSP was deformed by two folding events (the earliest being associated with the thrusting) before deposition of the overlying Clarke River Group commenced in the latest Devonian.

The GCSP to the west contains Ordovician quartz-rich flysch and tholeiitic volcanics, separated by an angular unconformity from Silurian quartzintermediate flysch and shallow water sediments including limestone. These are overlain disconformably by Early to Middle Devonian shallow marine sediments, including abundant limestone. They are separated by a slight angular unconformity of a few degrees from a Late Devonian to Early Carbon428

iferous fluviatile to shallow marine sequence, which includes redbeds. The main deformation was moderately tight folding in the Early to Middle Carboniferous. W e interpret the Broken River Province as an e x t e n s i o n a l intracratonic rift rather than the forearc basin and accretionary prism of previous models (Arnold & Fawckner, 1980; H e n d e r s o n , 1987). This new interpretation is consistent w i t h models being developed by other RGIP workers for the Hodgkinson Province further n o r t h . The Hodgkinson Province and CCSP were probably continuous originally. The GCSP may be an aulacogen or failed arm to the H o d g k i n s o n - C a m e l Creek intracratonic rift system, or a pull-apart basin related to strike-slip movement on the Clarke River F a u l t . The two subprovinces are separated by the Gray Creek Fault, w h i c h is interpreted as the southern continuation of the Burdekin Fault, along w h i c h the Georgetown Province was thrust over the CCSP in the Early D e v o n i a n . The GCSP is interpreted as having ridden 'piggy-back' on the Georgetown Province and escaped the thrust imbrication.

REFERENCES A r n o l d , G . O . & Fawckner,J.F., 1980: The Broken River and Hodgkinson River Provinces. ^ S t e p h e n s o n , P . J . & H e n d e r s o n , R . A . (editors). The Geology and Geophysics of Northeastern Australia G e o l o g i c a l Society of A u s t r a l i a , Queensland D i v i s i o n , pp 175-189. Henderson,R.A., 1987: An oblique subduction and transform faulting m o d e l for the evolution of the Broken River Province, northern Tasman Orogenic Z o n e . Australian J o u r n a l of Earth Sciences, 34, 2 3 7 - 2 4 9 . W h i t e , D . A . , 1965: The geology of the Georgetown/Clarke River a r e a , Q u e e n s l a n d . Bureau of M i n e r a l Resources, A u s t r a l i a , Bulletin 7 1 . W i t h n a l l , I . W . , L a n g , S . C . & Jell,J.S., (editors) 1987: Stratigraphy, sedimentology, biostratigraphy, and tectonics of the Ordovician to Carboniferous Broken River Province, north Q u e e n s l a n d . Australasian Sedimentologists G r o u p , Field Guide Series N o . 5 G e o l o g i c a l Society of Australia Inc. W y a t t , D . H . & Jell,J.S., 1980: Devonian and Carboniferous Stratigraphy of the northern Tasman Orogenic Zone in the Townsville h i n t e r l a n d . In S t e p h e n s o n , P . J . & H e n d e r s o n , R . A . (editors). The Geology and Geophysics of Northeastern Australia G e o l o g i c a l Society of A u s t r a l i a , Queensland Division, pp 201-228.

429

8.11 THE PERALKALINE GRANITES NEAR TEMORA, SOUTHERN NEW SOUTH WALES - PETROLOGICAL, GEOCHEMICAL AND TECTONIC IMPLICATIONS R.J. W o r m a l d Department of Geology, LaTrobe University,

Bundoora

The Devonian peralkaline granites which outcrop north of Temora in southern New South Wales occur in the Temora Rift on the western margin of the Began Gate Synclinorium (Fig. 1). The Temora Rift is a tectonic feature formed by Late Silurian divergent left lateral strike-slip movement between the Wagga-Omeo Terrane, to the west of the Gilmore Fault, and the Kosciusko Terrane, to the east of the inferred fault line A-B (Fig. 1). The peralkaline granites are closely associated in space and time with metaluminous granites, syenites, diorites and pyroxene-plagioclase cumulate gabbros (Fig. 2). Aeromagnetic data indicate that basic rocks are more abundant at depth in the intrusive complex. Petrographically the peralkaline granites are hypersolvus in that they contain only one feldspar. This is generally strongly perthitic, rimmed by albite and replaced by "chessboard" albite indicating the superposition of a sodium dominated metasomatic event on textures already developed during subsolidus unmixing of intermediate alkali feldspar. The main mafic phases are arfvedsonite and aegirine which occur interstially. Accessory phases include fayalite, magnetite, zircon and apatite. Although albitization appears to be associated with late stage postmagmatic metasomatism, primary crystallization texures for the aegirine and some of the arfvedsonite indicate that peralkalinity was attained in the melts prior to the seperation of a sodium-rich fluid. The late crystallization of arfvedsonite and its high F/OH indicate that the melts had very low water activities. Chemically the granites have high Si02, Fe/Mg, total alkalis, Zr, Nb, Ga, Y and rare earh element abundances and relatively low AI2O3, MgO and CaO. The rare earth element patterns are distinctive with pronounced negative europium anomalies (Eu/Eu* = 0.45-0.08). The geochemical features are similar to those exhibited by the A-Type granites of the Bega Batholith. The abundances of Sr, V, Cr and Sc are exceptionally low, as is Ba abundance in the more strongly peralkaline granites. The textural features of the peralkaline granites, their close association with plagioclase-pyroxene cumulates and the general features of the chemistry and chemical variation between the granites (curved plots on Marker variation diagrams) indicate an origin best explained by crystal fractionation from an intermediate parental magma with limited post-magmatic modification by a sodium and fluorine rich fluid phase. Indications of plagioclase fractionation (depletion in Sr and Eu/Eu*) and pyroxene fractionation (depletion in Sc) as peralkalinity is approached indicate that removal of these phases was important in obtaining this condition in the magmas. Alkali feldspar fractionation (depletion in Ba and enrichment in Rb) is only important once peralkalinity is attained. The dry nature of these granites indicates that they must have been fractionated from a relatively anhydrous parental magma. This may have been derived from a dry residual lower crustal source which had already had a granitic magma extracted from it. If this is the case then the major element characteristics of these granites, particularly the high Fe/Mg and low CaO and AI2O3, common to all A-types, cannot be explained unless these granites have been fractionated. The A-Type granites belong to an association of intrusive rocks including cumulate gabbros, syenites and high temperature l-type granites commonly found in regions of high heat flow such as rift zones. They are genetically related by crystal fractionation in which water

430

activity plays a critical role in the evolutionary sequence. At low values of water activity there is a wide temperature gap between the plagioclase saturation boundary and the solidus in these granite types. Plagioclase fractionation would under these conditions be operative over a large temperature range eventually inducing peralkalinity in the residual melts. With higher water activity the plagioclase saturation boundary is closer to the solidus and plagioclase therefore crystallizes over a narrow temperature range. Plagioclase crystal fractionation will therefore be less efficient and peralkalinity may not be attained. Under conditions of high water activity early crystallizing fayalite and clinopyroxene will react with hydrous derivative melts to form hornblende and biotite before peralkalinity is induced.

14730 QUATERNARY |V|covr SILURO-DEVONIAN

SILURIAN Sedlmentt RHBasaltIc i

| H [ Sediments FT3 B i o 111 e. h ornblende L I D granite Aeglrlne,

Trouoh margin • 40

Sample l o c a l i t y

2Km

BTonallte H H Gabbro

Fig. 2. Geology of the Temora region.

Fig. 1. Regional geology.

431

4.1 METALLOGENIC OVERVIEW OF THE PROTEROZOIC OF AUSTRALIA WITH SPECIAL REFERENCE TO GOLD, PLATINUM AND RARE EARTHS L.A.I. Wyborn, R.G. Warren, R.W. Page, and M.A. Etheridge Bureau of Mineral Resources, Geology and Geophysics, Canberra

The tectonic evolution of the Proterozoic can be subdivided into three successive periods of basin formation at 2OOO-I87O Ma, I8OO-I6OO Ma, and 1100-600 Ma, separated by two major periods of deformation at around i860 Ma and l600-l400 Ma. Igneous compositions and intrusive styles appear to be time-controlled, with specific magmatic events occurring continent wide within restricted time intervals; deformation and metamorphic events also appear to have similar time-dependent styles across the continent. Most Proterozoic deposits are either related to the basin forming events, or to deformation episodes; very few deposits are directly associated with igneous processes. In the Proterozoic there are very few magmas of andesitic type, as are commonly found in subduction zones, and most Proterozoic igneous provinces are bimodal. Not only are felsic igneous rocks of Proterozoic age compositionally dissimilar to those found in modern subduction zones, they also differ in crystallisation style. With the exception of some granites emplaced between l840-l820 Ma and I6OO-I5OO Ma, few Proterozoic granites are capable of fractionating to produce magmatic concentrations of oreproducing elements; this could explain the lack of granite-related Au, Mo, Sn, or Cu deposits in the Proterozoic (e.g. Meyer, I98I). New geochemical and petrological data highlight two distinctive features of the Proterozoic environment that are important to metallogenesis. 1). Felsic igneous rocks have been shown to have above average concentrations of U, Pb, F, rare earths and Y; these concentrations, particularly for U, are higher than those observed for the late Archaean. Above average contents of Au, Pt, and Cu are also commonly observed in mafic igneous rocks, especially those of early Proterozoic age, whilst Au and S are high in some felsic igneous rocks. 2). Geochemical and petrological studies of metamorphic rocks, shear zones, altered igneous rocks, and regolith profiles have shown that these rocks have equilibrated with oxidised, low temperature fluids that had the capacity to move not only U, Cu, Pt and Au, but also elements believed to be immobile such as Zr, Y, Nb, and rare earths. This alteration is most commonly observed near oxidised, F-rich, felsic igneous rocks and sedimentary evaporites, both of which are widely distributed in Proterozoic domains. The combination of these enriched source rocks and fluid compositions have given rise to a range of polymetallic deposits which appear to be specific to the Proterozoic e.g. Olympic Dam, Mount Isa, Coronation Hill, Tennant Creek. Our data highlight aspects of significance for exploration in the Proterozoic of metals which are currently prime targets:Gold: Despite the presence of at least five significant gold-producing areas; Olympic Dam, Telfer, Tennant Creek, Pine Creek, and the Granites-

432

Tanami, and records of gold occurrences from Mount Isa, Halls Creek, Gascoyne, and Arunta, exploration interest has not been as active for Au in the Proterozoic, as it has been in the Archaean or Phanerozoic. Gold concentrating processes are likely to be distinct from those of the Archaean and Phanerozoic and exploration models should involve the unusual fluid compositions, and the observed association of Au with U, Pt, Th, and rare earths. Platinum: Most of the Proterozoic layered mafic igneous complexes, occurring in either the early Proterozoic sequences (pre-l870 Ma) or else 1100 Ma (the Giles Complex), are too small to concentrate magmatically derived platinum as in the Merensky Reef. A second type of platinum occurrence is exemplified by the Coronation Hill prospect. At least two possible ore-forming processes might be considered as having generated such deposits. It is possible that this deposit formed by epithermal processes related to the host felsic volcanics. If this is so, then there is a high prospectivity of finding similar deposits, as the host volcanics are part of a felsic volcano-plutonic suite covering at least 40 000 sq km in Northern Australia at around i860 Ma. However, units of this suite, although containing above average Au contents, do not show evidence of fractionation, and are unlikely to have high Pt values. Another probability is that the deposit formed by high-level circulation of oxidised fluids. Pt has been found in anomalous concentrations in low temperature altered rocks at Mount Isa and Tennant Creek, and there is a possibility that Pt will be found in the Halls Creek, Gascoyne, and Paterson domains as these have similar geological characteristics to the Pine Creek domain. Rare Earths and Yttrium: These elements, which are important in the new generation of high temperature superconductors, are in high concentrations in Proterozoic igneous rocks, including carbonatites. However, the lowtemperature, oxidising fluids, characteristic of the Proterozoic, can also produce economic concentrations and the mobility of Y and rare earths in deformed and altered rocks has been documented at Mount Isa (Wyborn, 198?). Uranium deposits in Pine Creek and the western Mount Isa Inlier carry anomalous Y and heavy rare earths, whilst Olympic Dam and Mary Kathleen are noteworthy for their high contents of light rare earths. Other concentrations of Y and rare earths are in the Brockman deposit (Halls Creek) and xenotime-rich sandstones such as in the Granites-Tanami Block. Our observations show that these targets, Au, Pt, Y, and rare earths, are all mobilised by similar low-temperature oxidised fluids. Such fluids can also move U, Cu, Th, and Zr, thus producing useful pathfinders for the target metals. Metal ratios in the deposits are controlled primarily by the source rock composition and subtle variations in the fluid compositions. References Meyer, C., I98I. Ore-forming processes in geologic History. Economic Geology, 75th Anniversary Volume, 6-4l. Wyborn, L.A.I., 198?. The petrology and geochemistry of alteration assemblages in the Eastern Creek Volcanics as a guide to copper and uranium mobility associated with regional metamorphism and deformation. Mount Isa, Queensland. In: Pharaoh, T.C., Beckinsale, R.D., & Rickard, D.T., (editors). Geochemistry and Mineralisation of Proterozoic Volcanic Suites. Geological Society Special Publication, kZ^-k^k.

433

8.4

GEOLOGICAL EVOLUTION OF GRANITE COMPOSITIONS WITH TIME IN THE AUSTRALIAN CONTINENT - IMPLICATIONS FOR TECTONIC AND MANTLE PROCESSES

L.A.I. Wybornl, Wyborn^, B.W. Chappell^, J. Sheraton^, J.F. Tarney^, W.J. Collins"^ and B.J. Drummond-^ ^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Geology Department, Australian National University, Canberra ^Department of Geology, University of Leicester, UK ^Geology Department, University of Newcastle

A review of the chemical compositions of major I-type granite batholiths of Australia and New Guinea shows regular, systematic changes in compositions with time. There are some significant and consistent differences in the major and trace element compositions of crustal components which have important implications for crustal growth processes. Granites may be divided into two broad types. Either they are Sr undepleted and Y depleted or they are Sr depleted and Y undepleted. The Y depleted type infers derivation from a source which has residual garnet, but not plagioclase, whereas the Sr depletion infers sources with residual plagioclase but not garnet. Most of the granites have at least a two stage origin and hence, these chemical features reflect P-T conditions at either the time of melting or during the formation of the source from the mantle. In time there are essentially four main groups of I-type granite : (1) early Archaean tonalites and trondjhemites, (2) late Archaean to middle Proterozoic K-rich tonalites and granodiorites, (3) early Palaeozoic granodiorites, and (4) late Palaeozoic, Mesozoic, and Cenozoic tonalites. A characteristic of groups (1) and (4) is that their model source ages are nearly coincident with their emplacement ages and that group (4) is thought to be associated with subduetion. However, with groups (2) and (3) there is usually a significant crustal residence time which for some members of group (3) can be up to 1000 Ma. These changes can be related to changes in seismic refraction profiles which suggest that lower crustal underplating of mantle derived material is important in the Proterozoic and lower Palaeozoic, but not so in the Archaean. Profiles also suggest that Archaean granites are unlikely to be derived by small degrees of partial melting of mafic volcanics. The change to more K-rich granites in the late Archaean and particularly the early Proterozoic has been observed for some time, and was taken to indicate a change from essentially primary mafic-source derived granites to those that were derived by remobilisation of Archaeein felsic crust. This leads to the assertion that most of the felsic crust was produced by the late Archaean, and that granites are then remelted from this Archaean crust. However, it is difficult to derive the relatively K-rich

4 34

Proterozoic and early Palaeozoic granites from pre-existing Archaean crust, as most of this is depleted in K, Rb, Th, U. Y. and the HREE, whereas the vast majority of the Proterozoic and early Palaeozoic granites are enriched in these elements. In addition, their Rb-Sr and Sm-Nd model ages are mostly Proterozoic or younger, thus substantiating that they are not derived from an Archaean proto-crust. Most Proterozoic and Palaeozoic granites have trace element signatures similar to continental tholeiites in adjacent areas, implying that the same subcontinental mantle was being tapped at different times for both the granite source rocks and the tholeiites. These significant temporal changes in granite composition need to be assessed with respect to tectonic models. It seems that a particular type of granite apparently dominates over a certain time period, although it is not necessarily exclusive to it. Post 400 Ma granites have some similarities with, but are subtly different from Archaean granites, and reflect generation in subduction zones, with their sources in the mantle wedge. K-rich Proterozoic and Palaeozoic granites are not easily reconciled with subduction processes, and probably reflect melting at <45 kms of mafic continental crustal underplate. One anomaly to be reconciled is that these K-, Rb-, Th-, and U-enriched late Archaean to Palaeozoic granites compliment the depleted early Archaean tonalites. The possibility exists that these elements were stored in the sub-cratonic lithosphere during Archaean crustal generating processes, and they were not tapped until a change to a dominance of a different type of crustal process that began in the late Archaean and dominated through to the early Palaeozoic. Alternatively, different mantle types were being tapped at different times, with possible implications for the extent and the time scale of mantle convection.

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12.3

FRASNIAN-FAMENNIAN EXTINCTION EVENT - THE EVIDENCE FROM DEVONIAN VERTEBRATES G.C. Young

Bureau of Mineral Resources, Geology and Geophysics, Canberra

The fossil record of Devonian vertebrates exemplifies the importance of systematics in assessing fossil range data from which periodicity of extinction is inferred, and the problems of using a non-global data base to deduce a global phenomenon. Bias introduced by including Recent taxa with a short (and sometimes ancient) known fossil record has been called 'The Pull of the Recent' (Raup 1987), but inferred cladistic relationships may also indicate that apparent rahges based on known fossil occurrence grossly underestimate the time between evolutionary events defining the actual range of a taxon (speciation, and extinction). This 'Pull of the Sister-Group' may be an all-pervading bias in compilations such as those of Sepkoski (I982, 1986) based on standard taxonomic treatments of groups which have not been analysed cladistically. The statistical treatment and other analytical aspects of the periodicity hypothesis have been criticised (e.g. Hoffman 1985) f but the quality of the taxonomic data base also requires attention. Patterson L Smith {I987) sorted the fish and echinoderm families used by Sepkoski (1982) into clades showing valid extinction on the one hand, and non-monophyletic groups showing pseudoextinction or other invalid results on the other; only the latter category exhibits extinction peaks (Late PermianEocene) in the 26 Myr cycle of Sepkoski & Raup (I986). Analysis of genera in the same groups (Patterson & Smith in press) showed a decrease in the proportion of monophyletic groups showing valid extinction, in contrast to Raup & Sepkoski's (1986) claim that extinction peaks are more clearly defined by genera than families. Similar analyses have not been carried out for Devonian vertebrates, but McGhee (1982) used the compilations of placoderm and acanthodian fishes by Denison (1978, 1979) to conclude major extinctions at the Frasnian Famennian boundary in both groups, with marine species suffering higher losses than fresh-water species. The placoderms were the most successful and diverse group of Devonian vertebrates, with a relatively good fossil record, a cosmopolitan distribution, and a known range of Late Silurian to latest Devonian. Some 250 valid genera are now recognised. This is a increase on the compilation of Denison (I978), with most new taxa occurring in poorly studied regions outside Europe and North America (South America, South Africa, Antarctica, Australia, Middle East, China, Soviet Arctic, Arctic Canada). The taxonomic sample has been biased towards the well studied regions which formed the Devonian 'Old Red Continent' (Young 1987a), but there is no clear evidence of major extinction at the Frasnian -Famennian boundary when other palaeogeographic areas are considered. Some of these formed the major Palaeozoic continent of Gondwana, with East Gondwana being one of five faunal provinces based on vertebrates recognised for the Early Devonian (Young I98I). In the Euramerican Province a major feature of Late Devonian vertebrate successions is the complete replacement of psammosteid heterostracans at the end of the Frasnian by adaptively similar phyllolepid placoderms. However the latter group has an earlier history in Australia and Antarctica (Young 1984), and this and other biostratigraphic discrepancies can be explained by a biotic dispersal model involving shallow marine invertebrates and vertebrates in the early Middle Devonian, and exchange of continental faunas and floras at or near the Frasnian-Famennian boundary as Gondwana and Euramerica came together (Young 1987b). For Devonian

436

vertebrates, and probably for other groups, major faunal change in particular sections may be due to biogeographic factors, the resulting extinction events are only of local significance, and the cause of faunal replacement is significant change in global palaeogeography. In such cases extra-terrestrial influences have at most a very indirect cause and effect relationship to biotic extinction. It is possible that apparent periodic extinction may reflect periodicity in fossilisation, as do diversity peaks in the fossil record. REFERENCES Denison, R.H., 1978. Placodevmi Handbook of Paleoichthyology, volume 2, P.Schultze (Editor), Gustav Fisher Verlag, Stuttgart, 128 pp. Denison, R.H., 1979-

Acanthodii.

Handbook of Paleoichthyology,

H.-

Volumes.

H-P. Schultze (Editor) Gustav Fischer Verlag. Stuttgart, New York.

Hoffman, A. I985. Patterns of family extinction depend on definition and geological timescale. feature 315, 659-662. McGhee, G.R. 1982. The Frasnian-Famennian extinction event: a preliminary analysis of Appalachian marine ecosystems. Geological Society of America, Special Paper 190, 491-500. Patterson, C. L Smith, A.B. I987. Periodicity of extinction: a taxonomic artefact? Nature, in press. Patterson, C. & Smith, A.B. in press. Periodicity in extinction: the role of systematics. Ecology. Raup, D.M. 1987. Major features of the fossil record and their implications for evolutionary rate studies. Pp. l-l4. In Campbell, K.S.W. & Day, M.F. (Eds) Bates of Evolution. Allen & Unwin, London. Raup, D.M. & Sepkoski, J.J. 1986. Periodic extinction of families and genera. Science, 231, 833-836. Sepkoski, J.J. I982. A compendium of fossil marine families. Milwaukee Public Museum Contributions in Biology and Geology, 51, 1-125. Sepkoski, J.J. I986. Phanerozoic overview of mass extinction, pp. 277-295, In Raup, D.M. & Jablonski, D. (Eds.) Patterns and Processes in the History of Life. Dahlem Konferenzen I986. Springer-Verlag, Berlin. Sepkoski, J.J. L Raup, D.M. I986. Periodicity in marine extinction events. Pp. 3-36, In Elliott, D.K. (Ed.) Dynamics of Extinction. Wiley, Somerset, New Jersey. Young, G.C. 1981. Biogeography of Devonian vertebrates. Alcheringa, 5, 225243.

Young, G.C., 1984. Comments on the phylogeny and biogeography of antiarchs (Devonian placoderm fishes), and the use of fossils in biogeography. Proceedings of the Linnean Society of N.S.m. 107, 443-473Young, G.C. 1987a. Devonian vertebrates of Gondwana. pp. 41-50, in McKenzie, G.D. (editor) Gondwana Six. Stratigraphy, Sedimentology, and Paleontology. AGU Geophysical Monograph 4l. Young, G.C. 1987b. Devonian palaeontological data and the Armorica problem. Palaeogeography, Palaeoclimatology, Palaeoecology, 60, 283-304.

437

A B S T R A C T S OF S C I E N T I F I C P O S T E R S

438

COMPOSITIONAL VARIATION OF ZONED TOURMALINES ASSOCIATED WITH Sn-W MINERALISATION W.H. Barth Geology Department, University of Tasmania

Tourmaline i s the most common borosilicate mineral and is widely found in igneous, metamorphic and sedimentary rocks. Its abundance in rocks associated with Sn-W ores is well known. Tourmalines from the Cleveland tin deposit, the adjacent Meredith g r a n i t e , F o l e y ' s q u a r t z - p o r p h y r y dike and related veins have been examined. The p u r p o s e of t h i s i n v e s t i g a t i o n i s t o o u t l i n e t h e chemical c h a r a c t e r i s t i c s of tourmalines from the west Tasmanian tin province and assess their usefulness in characterizing Sn-W mineralization. Tourmaline o c c u r s as p o i k i l o b l a s t i c sieve-like aggregates in the porphyry dike, is as sub- to anhedral clusters in the granite but i s w i d e l y distributed as euhedral crystals in veins and in the tin lode where often i t is present as massive l a y e r s or ' t o u r m a l i n i t e s ' . Euhedral c r y s t a l s have blue to blue-green cores and are rimmed by alternating yellow-brown to green-brown bands. Detailed geochemical s t u d i e s of about 1500 tourmalines indicate a variation of Fe/Fe+Mg from 0.980 in the porphyry, 0.750 to 0.950 in the granite, 0.60 0 t o 0.8 0 0 in the t i n l o d e and 0.500 to 0.750 in the tourmaline veins. All tourmalines are close to endmember ' s c h o r l ' . Mn c o n t e n t s are g e n e r a l l y low, Na/Ca ratios are high and F/Cl ratios are very high with overall high F contents. Na is 0.1 to 0.2 cations/formula unit higher in the dike than elsewhere. However, the characterization of tourmaline is biased by a large compositional v a r i a b i l i t y w i t h i n a given grain. Within individual crystals, the cores show a prominent Na deficiency, low F contents but high A13+ with excess A1 o b v i o u s l y e n t e r i n g the Y site in tourmaline. In some pegmatitic tourmalines the X s i t e is only f i l l e d with 0.2 ions Na (31 oxygens/formula) with Fe2+ p r o b a b l y compensating in the X s i t e . The core/rim boundaries are marked by a decline in A13+ and a smooth increase in total Fe and Mg . Na f i l l s the X position"to some 0.850 ions and may approach rarely 1.000. F is also very high at the edges emphasizing o v e r a l l F-A13+ avoidance. The transition from core to rim is characterized by a compositional hiatus. Uncorrected formula calculations show a r a p i d i n c r e a s e in Si (Si < 6.200 ions/31 oxygens) coupled with a cation excess of up to 0.4 units beyond the ideal 16.000. Formula r e c a l c u l a t i o n s on the basis of a 6.000 Si approximation and a sum of 16.000 cations yield Fe3+ estimates that show a sharp i n c r e a s e in Fe3+ commencing at the core/rim boundary. At this boundary very high Fe3+ contents are found in bands with i n t e n s e green c o l o u r s p r o b a b l y as a result of Fe2+ -Fe3+ charge transfer processes. The gradually increasing oxidation state in the rim of t o u r m a l i n e i s p o s i t i v e l y correlated with reduced vacancies- in the X site (Fig.lA). With the replacement of A13+ by Fe3+ an o v e r a l l t r e n d away from a l k a l i - f r e e •(Fe^^Al'^)Al^^ (BO. ).Si.O-,.(OH) . and proton-deficient schorls N a ( F e ' ^ A l ' ' - ( D ' h ) 3 ^(Fig. towards a h y p o t h e t i c a l endmember Na ^ ^(Fe^^Fe^ AP^^ ^^^^^ place.

439

The u n i q u e c h e m i c a l characteristics and the similar compositional trends from tourmalines occurring in ores and associated rocks of the Cleveland deposit can be related to crystallization from hydrothermal fluids of the main Sn-W m i n e r a l i z i n g e v e n t . The g e o c h e m i c a l c h a r a c t e r i z a t i o n of tourmalines from less known terrains thus has potential as a prospective tool for Sn-W mineralization.

6.0

Fig.1

440

6.5

7.0

MAGNETOSTRATIGRAPHY OF SEDIMENTS FROM THE LORD HOWE RISE

C.E. Barton Bureau of J^ineral Resources, Geology and Geophysics, Canberra The main objective of Leg 90 of the Deep Sea Drilling Project was to correlate Neogene biostratigraphic zonations in oceanographic environments ranging from tropical to sub-Antarctic. A sequence of holes was drilled along a north-south transect starting from the Ontong-Java Plateau northwest of Bougainville, along the Lord Howe Rise, and ending on the southern margin of the Chatham Rise 220 km east of South Island, New Zealand. Equatorial Site 589 was drilled during Leg 89. Drill sites were located above the carbonate compensation level in water depths ranging from 1000 to 2200 m. Very long continuous sequences of sediments were recovered comprising very pure pelagic carbonates, typically foraminiferal-bearing nannofossil ooze (or chalk) and nannofossil ooze (or chalk). A total of 3.7 km of core was recovered from eight sites (589 to 594), giving records back into the Eocene at Sites 588, 592 and 593. Palaeomagnetic work was undertaken to provide an independent chronological framework to calibrate the different biostratigraphies. The bulk of the sediments are extremely weakly magnetized with NRM intensities seldom greater than 0.05 |IG. The quality of the palaeomagnetic records deteriorates with increasing depth caused by the combined effects of removal of primary magnetic oxides by sulphate reduction processes and the disalignment of magnetite grains during compaction. Sediments are characterized by extremely high calcium carbonate contents (>95%) with almost no biosilicious component. Blebs and streaks of pyrite are common, and the presence of iron sulphides with poor magnetic stability properties is suspected. Viscous components of magnetization are common, sometimes to the extent of dominating the primary remanence. There is evidence to suggest that a component of magnetic remanence is imparted during core recovery. Siliceous carbonate oozes are better palaeomagnetic recorders than pure carbonate oozes. Magnetic reversal sequences are generally recognizable back to the Gilbert chron (3.4 to 5.35 Ma), except at equatorial Site 586 where only the Brunhes/Matuyama boundary (0.73 Ma) can be identified. Longer reversal records were obtained at Site 588 (to Chron 13, about 13 Ma) and Site 594 (to the base of Chron 5, about 5.9 Ma). Site 594 is unique in having a sequence of hemipelagic sediments resulting from the input of terrigenous material after about 6 Ma following the Kaikoura orogeny in New Zealand. A diatom-radiolarian biostratigraphy has been obtained for Site 594, but this disagrees with the calcareous nannofossil scheme. The palaeomagnetic data favour the latter. There is a remarkably long Brunhes record (100m) at this site that includes one brief interval of reversed polarity at about 0.3 Ma, and several isolated data points with positive (reverse) inclinations v/hose interpretation is uncertain. 441

GEOPHYSICAL, SEABEAM AND SEAFLOOR CAMERA INVESTIGATIONS OF TASMANTID SEAMOUNTS S.J. Carty, R. Coleman, T.C. Hubble, C.J. Jenkins, J.B. Keene, T.R. Pritchard and P.M. Schneider Ocean Sciences Institute, University of Sydney

A variety of marine geophysical techniques have been applied to the seamounts in recent years through cooperation between the R.A.N, and Sydney University. This display illustrates some of the results. The immediate aim has been to characterize seafloor shape and physical properties in the vicinity of these large structures, with broader geoscience objectives also being met. Seafloor photography, SeaBeam multibeam echosounding, reflection seismic profiling, magnetic surveys and direct sampling by dredging and coring have provided much new information at dimensional scales ranging from grainsizes (mm) to the entire seamount (km). Research cruises have covered aspects of the Gascoyne, Kimbla, Taupo, Derwent-Hunter and Queensland Seamounts and also the south Tasman hotspot/earthquake site near 4oOs 1560E. Additional work has been carried out over the Heemskirk and Zeehan Seamounts of the southern Lord Howe Chain and at a set of older (?01igocene-Eocene) seamounts in the SE Tasman Sea. Features of large scale include: (i) parasitic eruptive cones and lobes low on the flanks, (ii) post-volcanic sediment cover on the summit platforms, (iii) aprons of volcanic detritus spread up to 70km from the seamount and buried by later sedimentation, (i) large slumped sediment masses low on the flanks, (v) acoustic reflections which are apparently related to underlying intrusive bodies and (vi) local crustal depression and outer-rise uplift due the load of the seamount. Associated special deposits include (vii) manganese nodule pavements and blockfields of remnant volcanic materials in erosional moats alongside the peaks and (viii) on the upper slope, basaltic epiclastic breccias and conglomerates cemented with zeolites, biogenic carbonates and manganese oxides. The data contributes to ideas on processes of initial seamount construction, volcaniclastic sediment dispersal, erosion of the summits which have attained sealevel and the tectonic subsidence due to loading. Each seamount has a prolongued effect on the region's deep-sea sedimentation through interaction with bottom-water flows and the shedding of sediments to the surrounding seafloor.

442

STUDIES OF GROUND MOTION ATTENUATION B.A.

IN W E S T E R N

AUSTRALIA

Gaull

B u r e a u of M i n e r a l R e s o u r c e s , G e o l o g y & G e o p h y s i c s , Mundaring Geophysical Observatory

Recordings from 7 accelerographs in southwest Western Australia (SWWA) generated during an ML 4.5 earthquake were used to plot peak horizontal acceleration (A, in m . s " 2 ) and velocity (V, in mm.s"^) as a function of hypocentral distance (R, in km). A further 24 points from the SWWA data base were added to the (R, A) plot by normalising amplitudes to ML 4.5 by multiplying them by (a) where log (a) = 0.72(4.5-ML) and ML represents the magnitude of the event whose amplitude was being normalised. This expression was obtained from the assumption that the ground motion at peak horizontal acceleration (PHA) could be approximated to SHM and a (ML, T) function derived from 137 local data points and is given by log T = 0.14 ML 1.68, where T is the ground period at PHA on an "average site" during an earthquake of magnitude ML whose range is 0 . 4 i M L i 6.2. The resultant curves are defined by log A = 0.72 ML - 1.53 log R - 0.0033R - 1.6 (Figure 1) and log V = 0.45 - 1.23 log R - 0.0017R (Figure 2; for ML 4.5 only). To give control up to ML 6.9 the Meckering event of 1968 was also incorporated into the data using the estimates of acceleration in Everingham and Gregson (1970). The mean difference in the derived curve and observed values was 33%. This (ML, R, A) relation compared well with McCue's (1986) equivalent relation derived in eastern Australia out to distances of about 50 km. The strong ground motion network in WA is also being used to provide the necessary control in the near field for a local magnitude study. The preliminary result obtained was that the attenuation function for mean peak Wood Anderson amplitude (A3, in mm) for an earthquake of Richter magnitude, ML 3.0, is given by: log A3 = 2.22 - 1.07 log R - 0.00075R, where R is the hypocentral distance in km and l i R $ 2200, (Figure 3). REFERENCES Everingham, I.E., and Gregson, P.J., (1970) - Meckering earthquake intensities and notes on earthquake risk for Western Australia. BMR Record 1970/97 (unpublished). McCue, K., (1986) - Strong motion attenuation in Eastern Australia. Earthquake Engineering Symposium, Institute of Engineers, Australia.

443

PROBABILISTIC B.A. Gaull^,

Geology

E A R T H Q U A K E R I S K M A P S OF

M.O. Michael-Leiba^

and

AUSTRALIA

J.M.W.

Rynn^

^Bureau of M i n e r a l R e s o u r c e s , and G e o p h y s i c s , M u n d a r i n g G e o p h y s i c a l O b s e r v a t o r y ^ B u r e a u of M i n e r a l R e s o u r c e s , G e o l o g y and G e o p h y s i c s , C a n b e r r a ^ D e p a r t m e n t of G e o l o g y and M i n e r a l o g y , U n i v e r s i t y of Q u e e n s l a n d

New earthquake risk maps of the Australian continent and its continental margins have been prepared. The risk is depicted as contours of peak ground velocity, acceleration and intensity with a 10% probability of being exceeded in 50 years. These maps are based on the Cornell-McGuire probabilistic method. As the earthquake process is assumed to follow a Poisson distribution, foreshocks and aftershocks were eliminated from the analysis. The maximum magnitude in each earthquake source zone was defined to be half a magnitude unit greater than the largest recorded Richter magnitude for that zone. For areas outside the defined source zones, normalised background seismicity levels were used in the risk estimates. Separate relationships for the attenuation of ground intensity with hypocentral distance using a scaling law of the Kanai form were obtained for western, southeastern and northeastern Australia and for Indonesian earthquakes felt in northern Australia. These were based on the determination of mean isoseismal radii from isoseismal maps of Australian earthquakes. Conversions of such ground motion intensity the Modified Mercalli scale) to peak ground motion acceleration (A, in m.s ) and velocity (v, in mm.s" ) were carried out using the empirical relations log A = 1/3.1 2.3 and = 7v/5, respectively. Recordings of peak ground motion accelerations, where available, were used to check the attenuation relations. Land regions subject to the greatest earthquake risk, considered herein as the 10% probability that in a 50 year perjod peak ground motion accelerations of 1.0 m.s (or velocities of 100 mm.s or greater will be exceeded are: Simpson Desert,_ 2.4 m.s (or 2|0 mm.s ); Yilgarn Block to the east of Perth, 1.6 m.s Lor 160 mm.s ) D a l t o n - G u n n i n g area to the north of Canberra, 1.2 m.s" (or 120 mm.s ); Canning Basin - Fitzroy Trough in Western Australia, 1.2 m.s (or 1^0 mm.s ); Wi^e Bay-Burnett region of central-eastern Queensland, 1.0 m.s (or 100 mm.s ); an^ easternmost ^^ Flinders and Cape Barren Islands in Bass Strait, 1.0 m.s (or 100 mm.s ). It is stressed that the results of this study pertain to average site conditions and no account is taken of the long-period ground motions generated by Indonesian earthquakes that have been known to affect structures in Darwin, Perth and Adelaide.

444

CHLOROFLUOROCARBON GENERATION - GEOLOGICAL MODELS BASED ON DEVONIAN ENVIRONMENTS DURING PLANETARY CHANGE E.J. Heidecker Department of Geology and Mineralogy, University of Queensland

Thinning of the earth's protective ozone layer is attributable to chlorofluorocarbon (CFC) emissions. Chlorine atoms liberated from CFCs in the upper atmosphere are known to be capable of destroying vastly greater numbers of ozone molecules. Special industrial geological processes are unlikely sources, so little is to be learnt from the geological record. In view of the present environmental crisis, even remote possibilities deserve consideration in case they cast light on effects and controls. A geological conceptual model for CFC generation needs coincidence of: (i) (ii) (iii)

to account

for

hydrocarbon source, fluorine and chlorine sources, energy source.

Upper Devonian environments preserved in the Burdekin Basin of northeastern Queensland provide these requirements with 'acid' volcanic fall-out rich in fluorine, deposited above hydrocarbon-rich sediments. Upper Devonian environments may indicate changes likely in modern times if destruction of the ozone layer is allowed to continue. The Devonian record provides indications of algal blooms, shallow fauna loss, and ultra-violet oxidation effects which deserve study for modern guidelines.

445

M A R R Y A T CREEK EARTHQUAKE,

30 M A R C H

1986

K. McCue^, B.C. Barlow^, D. Denham^, T. Jones^ G. Gibson^ and M.O. Michael-Leiba^ ^Bureau of Mineral Resources, Geology and Geophysics, Canberra 2phillip Institute of Technology, Bundoora

Large intraplate earthquakes have an almost universal property of occurring in unexpected places, and on March 30, 1986, the Marryat Creek earthquake confirmed this behavioural pattern. The earthquake took place in central Australia {26.2'S, 132.8°E) in a region that was cratonized in the Early to Middle Proterozoic and that has no record of significant previous seismic activity. The main earthquake had a magnitude of and was associated with a surface fault scarp that is in the shape of a boomerang (convex to the northeast)13 km long, with a maxmum displacement of 0.8 m. ^ preliminary fault plane solution for the earthquake indicates that the mechanism was a conbination of thrust and strike-slip faulting, with the pressure axis striking at N 220'E.

446

PROBABILISTIC EARTHQUAKE RISK M A P S OF TASMANIA M.O. Michael-Leiba^

and B.A.

Gaull^

^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Bureau of Mineral Resources, Geology and Geophysics, Mundaring Geophysical Observatory

New earthquake risk maps of Tasmania have been prepared depicting risk by contours of peak ground velocity, acceleration and intensity with a 10 percent probability of being exceeded in a 50 year period. The CornellMcGuire method was used. The maps are based on seismicity up to 1984 and take into consideration the events of the 1883-1892 earthquake swarm east of Flinders Island and other historical data. The earthquake process was assumed to be aftershocks were eliminated before the relations were determined for the zones.

Poissonian, so foreshocks and magnitude-frequency recurrence

For this earthquake risk assessment, average eastern Australian seismicity and attenuation for average site conditions were used.

background

The earthquake source zones most affecting the risk in the Tasmanian region are the West Tasman Sea Zone and the Western Tasmanian Zone. The West Tasman Sea Zone extends from east of Flinders and Cape Barren Islands into the Tasman Sea to 150.3'E. The I883-I892 swarm appears to have occurred in this Zone, with at least one intensity-deduced Richter magnitude 6.0 - 7.0 event in each of the years 1884, I885 and 1892. Consequently, the highest risk land areas are Plunders an^ Cape Barren^Jslands ^hich lie predominantly between the 60mm.s /0.6m.s and 120mm.s /1.2m.s contours with the risk increasing to the east. The Western Tasmanian zone includes western and northwestern Tasmania and seismicity off the west coast and in the vicinity of King Island. The largest event recorded in the zone was in I88O. It had an intensity-deduced Richter magnitude of 5.5. It occurred in southwest Tasmania and was felt with a maximum intensity of MMVI in southern Tasmania^ The northern part of western Tasmania (enclosed by the 59 mm.s /0.55 m.s contour) is the second highest risk region. The chief contributions to uncertainty in the estimates of earthquake risk are uncertainties in early earthquake locations and magnitudes, and in strong ground motion attenuation, for which accelerograph recordings and more Tasmanian isoseismal data are needed.

447

GEOLOGICAL EXPLORATION BY J.D. DANA AND THE REVEREND W.B. CLARKE IN THE ILLAWARRA, 1840 R.G. Middletonl, p.p. Carr^ and E.G. Jones^ ILOS Angeles County M u s e u m ,

USA

2Department of Geology, University of Wollongong

The Reverend WilUam Branwhite Clarke, who studied geology under Adam SedgS It Cambridge, arrived in Sydney in 1839 and was the first t S S field geologist to make his permanent home in Australia. In the simryear the eminent American scientist James Dwight Dana also arrived in Australi as geologist for the United States Exploring Expedition ( I S s t - l s S r u n L r ^ t h e ^ c o m m a n d of Lieutenant Charles Wilkes Clarke accoLaitied Dana during his travels through the lUawarra in 1840 when ? S r ^ e T e d and described the geology of the region by traversing from S n g o r r through Kangaroo Valley to 'Coolangatta' and returned to WoUonlonl v S Gerringon| and Kiama. Dana visited many places dumng Ws four fears with the Wilkes Expedition but he was very ^^P^^f f ^^^^ Sie Illawarra and wrote "the Illawarra District is a perfect gem of a place S r GZO^ as well as for landscape beauty; it is one of the lovehest spots on the globe". Throuffh their diaries, sketches, letters and various pubUcations, these t w o S e e r i n g scientists, whose influence is still very much in evidence hive S t b S d a rich and colourful account of their brief but s i ^ i c a n t ?ivels toge^^^^ through the Illawarra. In particular the volume on Geology produced by Dana as part of the report on the Wilkes Expedition S n S S an extensive chapter on the geology of New South Wales whereas Clarke's diary not only describes the g e o l o ^ and g e o ^ P h y of the Illawarra but also provides an insight into the hfestyle at that time. On the basis of his observations during his visit to New South Wales, D ^ a proposed a tripartite subdivision of the sedimentary strata of the i S w a r r a region with a conformable sequence of argillaceous sandstone and W t T t th? Sase, a coal-bearing formation in the middle and an upper u S t of sandstone with subordinate shale. These umts are equivalent to the present Shoalhaven Group, the Illawarra Coal Measures and the Narrabeen Group plus Hawkesbury Sandstone respectively. The fnterStion h e t L L Dana and Clarke was not without conflict of opinion butTed to proposals for the age and environment of d^posityn of these units together with a discussion of the igneous rocks and landscape evolution. Considering the short duration of Dana and Clarke's visU and the paucity of information on the geology of Australia at the time, the correspondence ? e t w e r S geologicfl assessment of the Illawarra and the inte^retation accepted today bears testimony to the scientific insight of the men.

448

PALAEOZOIC

FOREARC V O L C A N I S M IN THE SOUTHERN NEW FOLD BELT, NEW SOUTH W A L E S R. Offler^,

J. G a m b l e ^ and

ENGLAND

Fardy^

^Geology Department, The U n i v e r s i t y of Newcastle ^Geology Department, V i c t o r i a U n i v e r s i t y of Wellington, ^CSIRO Division of Energy C h e m i s t r y , Lucas Heights

Within the fault slices of the Peel Fault System, Glenrock Station, are forearc basin sequences containing distinctive volcanic and subvolcanic suites. Those in the western fault slice (Group I) are associated with sedirnents of the tevonian Tamworth Group. Ihey consist of basalts, dacitic tuffs and less canmon dacites. MORB-like characteristics for the basalts are apparent in Hf-Ih-Ta, Ti-Zr, TiA00-3Y-Zr, Ti-V and Cr-Y discriminant diagrams. This feature is also recognised in REE patterns vîich are flat and slightly fractionated (LaAb)p^ = 0.96-1.37). Hov^ver, Rock/>K)RB patterns show a selective enrichne^ of mobile inconpatible elements and Th, a feature normally associated with lAT rather than MORE basalts. Further, Ce and P are slightly enriched which^ suggests that these basaltic magmas have a transitional tholeiitic — calc-alkaline affinity. Magmas with this type of chemical signature have been noted in marginal and fore-arc basins. It is suggested that these lavas have been erupted onto the seafloor as a r e ^ l t of rifting of the forearc basin. Although most basalts have this distinctive chemistry, others lower in the sequence have more affinities with the basalts in forearc sequences elsevîere in the area. Ihey have LIL and HFS element contents characteristic of tholeiitic and calc-alkaline magmas erupted in island arc settings. Ihe felsic volcanic rocks associated with the Group I basalts show a flat Ti02-Fe0/Mg0 trend and noderate, LREE enriched patterns ((LaAb)^^ = 3.39-5.42) typical of calc-alkaline, island arc lavas. In the forearc basin sequences of the overlying fault slice (Group IV), basalts and acid volcanics are intimately associated with distal, turbidite, volcanogenic sediments. Ihe basalts are strongly tholeiitic, and ccmironly ccntain low Zr and Y, and high Cr and Ni contents shewing chemical affinity to boninitic n^^lts. In addition, they have lower total REE contents and show MORB-like LREE depleted to flat patterns ((La/Yb)_ = 0.48-2.34). However, Ti/100-3Y-Zr, Ti-Zr, Ti-V and Cr-Y plots in<?icate that they are of ARC origin. Rock/MORB patterns substantiate this interpretation. Ihe acid volcanics show a similar LREE enrichrrent to those in the Group I sequence ((La/Yb)^ = 2.54-4.71) but total REE and Zr are higher. Ihe tectonic setting ?or these volcanics and the basalts is believed to have been the outer reaches of the forearc basin close to the trench. Intruding the forearc basin sequences are dolerites showing ARC or MORE affinities according to immobile element contents. Some have flat REE patterns, others enriched patterns ((LaAb)^^ = 1.26-3.29). When normalised against N-MORB, three patterns emerge, the first exhibiting positive Ce-P and Ih anorialies similar to the Group I basalts, the second Zr-Hf depletion and Ce-P enrichment like the Group IV basalts and the third, an enrichment in HFS elements, as well as Ce, P and Ih. Thus all dolerites show a "subduction component" and in one suite a "wirnin olate ojrnpo^.ent". It is likely that the basaltic and -âle-alkaline magmas have been derived from the mantle wedge overlying Liiu pj-av-o, liiis mantle source would need have been variably depleted to explain the significant variation in chemistry shown by the different magmatic suites.

449

THE PATUKI OPHIOLITE - IMPLICATIONS OF SEQUENTIAL PRE TO POST-SPREADING MAGMATIC EPISODES WITHIN A FAST-SLIPPING FRACTURE ZONE W. Sivell^ and J.B. Waterhouse^ ^School of Science and Technology, Nepean College of Advanced Education ^Department of Geology and Mineralogy, University of Queensland

Three stages of geochemically distinct, predominantly mafic (sub-)seafloor magraatism occurred during the evolution of the ophiolitic Early Permian Patuki Volcanics, East Nelson, New Zealand. They can be related to distinct mantle sources and melting regimes in the vicinity of a fastslipping transform fault. The Patuki Volcanics comprise a steeply-dipping, NNE-trending belt of mainly mafic-ultramafic rocks, some 150 km in strike length, in Nelson Province, South Island. Consisting chiefly of spilitic lavas interbedded with volcanogenic sediments, red and green tuffs and serpentinised ultramafic rocks, the Patuki lies in faulted contact with the southeast margin of the Dun Mountain Ultramafics of the Dun Mountain ophiolite belt with which it shows some similarities, but also a number of important differences. At north D'Urville Island, a 400 m thick volcanosedimentary sequence within the Patuki Volcanics is comprised of two lithologically distinct ophiolitic units. These are (1) a lower unit of spilitic pillow lavas, volcanic breccias, minor basic intrusions and graded volcanic arenites; and (2) an upper unit of massive and brecciated mafic lava flows, abundant argillite and tuff. The volcanic sequence is interrupted by continuous, sub-parallel bands of highly sheared serpentinite that crop out over many kilometres along strike. At south D'Urville Island, gabbro and plagiogranite (metamorphosed to amphibolite and/or greenschist facies during early ocean-ridge metamorphism) intrude the volcanics. Metabasalt, gabbro and amphibolite comprise a sequence up to 600 m thick, the metabasalt overlying gabbro with an unfaulted contact. The earliest-emplaced basic volcanics of the Patuki sequence are olivinephyric - titaniferous salite bearing lavas and intrusives of alkaline affinity. They show geochemical features (e.g. strong LREE-enrichment and low Zr/Nb ratios) closely resembling basalts from fracture zones and within-plate settings (WPB). Relative to other Patuki suites, these rocks possess high Cr, Ni, P, Zr and Nb contents, in addition to high LREE levels. The primitive magmas of this suite originated from a plume-related mantle source similar to that proposed for many ocean island basalts (GIB). Geochemically similar volcanics have not been recognised among Permian ophiolitic assemblages elsewhere in South Island, including the Dun Mountain ophiolite belt. The second suite of later-formed Patuki lava flows and volcanic breccias is comprised of plagioclase and clinopyroxene-phyric basalts which exhibit pronounced LREE-depletion and high Zr/Nb ratios similar to N-type mid-ocean ridge basalts (MORB). These evolved MORB-like lavas were erupted from periodically replenished, shallow-level, steady-state magma chambers. They display overall low Nb, P and Ni levels and enhanced rates of incompatible element enrichment (e.g. high Zr/ZrQ) due to magma mixing. Parental magmas for this suite were derived by extensive shallow-level melting of depleted upper mantle.

450

The compositions of irregularly cross-cutting dykes, sills and small intrusions that invade the Patuki volcanic succession range from primary mantle-derived basalts, identical in some respects to primitive MORB (M^^ 74; Al203/Ti02=20.9), to highly differentiated, silicic plagiogranite The intrusives display some arc-like geochemical signatures, including pronounced decoupling between REE and high field-strength elements (i.e. high La/Nb), and slightly LREE-enriched to LREE-depleted REE distributions. In contrast to earlier-generated Patuki magmatic suites, these rocks show evidence of extensive polybaric, closed-system fractionation dominated by removal of ferromagnesian phases (chiefly clinopyroxene and olivine). Ca0/Al203 ratios, as well as Ni and Cr contents in the mafic rocks decrease rapidly to very low levels during early-stage crystal fractionation, the data adhering closely to geochemical trends of subduction-related magmas. Some evolved Patuki plagiogranites contain Tipoor, magnesian pyroxene xenocrysts (Mg^ = 87) clearly in disequilibrium with the chemistry of their host rocks. These pyroxenes crystallised from liquids with Mg^ = 72, similar to the most primitive Patuki dykes. Furthermore, the intermediate-silicic rocks lie on geochemical trends that are continuous with the basic Patuki dykes. Consistent with data for the more mafic rocks, the plagiogranites show geochemical signatures (e.g. low Y + Nb, Rb etc) of granitoids in primitive, immature arc settings. Primary magmas for the intrusive suite were generated by high degrees (>20%) of hydrous partial melting of a depleted upper mantle source similar to the source for the Patuki upper lavas, but selectively reenriched in large ion lithophile (LIL) elements due to subduction-related processes. The intrusives were emplaced prior to termination of early high - T/P, amphibolite-forming, spreading-ridge metamorphism of the Patuki sequence, but following abandonment of the shallow-level, steady-state (ridge-crest) magma chambers. Seafloor spreading may take place on fast—slipping fracture zones e.g. in the North Fiji Basin. It is considered here that the geochemically anomalous Patuki ocean crust magmatic assemblage reflects sequential episodes of "leaky transform" magmatism, seafloor spreading within a fracture zone environment, and closely ensuing subduction-influenced plutonism within the cooling oceanic lithosphere. Within the early-formed (alkaline) metavolcanic suite, pillow lavas locally pass into brecciated flows, implying a rugged bathymetry. The overall strike-slip influenced tectonic regime led to accumulation of unusually large volumes of ophiolite— derived sediment, due to the combined effects of rugged bathymetry and localised mass-flow processes. The sediments include distinctive trace metal enriched umbers, monomict-polymict breccias, chert, graded volcanic arenites and abundant argillite. Such large amounts of sediment are not observed in association with Permian ophiolite successions elsewhere in New Zealand. Distinctive higher-than-average (for oceanic crust) P/T metamorphic assemblages characterise wallrocks adjacent to faults and fractures within the volcanic sequence. Subsequent to magmatism, the intensely brecciated and highly fractured Patuki oceanic crust sequence was tectonically intruded by parallel, thin but continuous (and conformable) bands of partially serpentinised, mantle-derived (harzburgitic) ultramafic rocks. Prior to serpentinisation, however, the ultramafic protolith had been intruded by Patuki magmas.

451

AUTHOR INDEX KEY

8.27

Session number, scientific paper

K e y n o t e paper

Scientific poster

270

Page number

452

A D A M , J.D. A D A M S O N , D.A. AGOSTINl, A. A H M A D , R. A N D R E W , A.S. A R A K E L , A.V. A R C H B Q L D , N.W. A R N E , D.C. B A L E , R. B A M B E R R Y , W.J. B A R L O W , B.C. B A R T H , W.H. B A R T O N , C.E. B E A R D S M O R E , T.J. B E L P E R I O , A.P. B I C K F O R D , J.L.C. B O G A C Z , W. B O N E , Y. B O U R K E , D.J. B O Y C E , W.H. B O Y D , R. B R O W N , C.M. B R O W N , M.C. B R O W N L O W , J.W. B U I C K , l.S. B U L T I T U D E , R.J. B U R R E T T , C.F. B U R T O N , N. C A N N , J.H. C A R R , P.F. C A R T E R , R.M. C A R T Y , S.J. C A T T , P. C H A P P E L L , B.W. C H E N , Y.D. C H E N H A L L , B.E. C H I V A S , A.R. C H O P R A , P. C L A R K , D.A. C L A R K E , I. C O E , R.S. C O F F I N , M.F. C O L E M A N , R. C O L L I N S , L.B. C O L L I N S , W.J. C O L W E L L , J.B. C O M P S T O N , W.

8.24 398, 8.28 33 9.2 152 11.4 35 7.1 40, 14.1 37, 14.3 39 6.1 42, 6.6 183 7.3 44, 7.4 45 14.4 46, 19.7 117, 19.8 48 6.8 50 8.36 301 13.1 51 9.13 259, P 446 P 439 3.3 53, P 441 4.4 57, 6.2 55 5.3 290, 7.17 59 9.10 6 1 13.5 63 2.7 65 2.20 66, 7.13 207, 7.20 358 9.9 68 20.2 131 7.6 373 17.14 70 2.28 72, 2.31 74 8.6 141 7.2 162 9.3 414 10.1 76 14.6 78 7.23 206, 8.34 86, P 448 5.8 80, 18.6 82 P 442 16.4 180 8.4 434, 8.8 377, 8.9 379, 8.10 424, 17.5 130, 17.10 120 7.4 45, 8.10 424, 8.29 84 8.34 86 7.11 148, 10.2 186, 14.2 88 9.5 89 3.4 90 17.14 70 3.20 401 15.5 101, 20.1 99 15.13 93, P 442 6.17 95 8.4 434, 8.5 97 15.5 101, 20.1 99 2.19 272, 8.10 424

C O O P E R , B.J. C O O P E R , R.A. C R O O K , K.A.W. D A L E , L.S. DAVIES, G.F. DAVIES, H.L. DAVIES, P.J. DE D E C K K E R , P. D E N H A M , D. D I C K I N S , J.M. D O M A G A L A , J. D R U M M O N D , B.J. D U D D Y , I.R. D U G G A N , M.B. D U N C A N , R.A. D Y E , J.E. EGGINS, S.M. E M B L E T O N , B.J.J. E R I K S S O N , K.A. E T H E R I D G E , M.A.

5.1 150, 5.4 102, 19.3 414 19.3 414 2.24 247, 8.20 104 14.3 39 2.1 106 1.1 107, 3.17 109, 3.22 221, 8.23 408, 20.1 99 20.3 110 7.5 113, 14.2 88 9.1 114, 9.6 164, P 446 12.5 115, 19.7 117 7.2 162 2.17 118, 8.4 434, 9.5 89 6.8 50 17.10 120 15.7 262, 17.2 121 7.12 122

E W A R T , A. E X O N , N.F.

15.9 123 3.8 124, 3.14 249 K N 24, 4.8 174 2.3 126, 3.5 128, 4.1 432, 15.1 252 17.5 130 8.20 104, 20.2 131

F A L K N E R , A. F A L V E Y , D. F A R D Y , J.J. F E A R Y , D.A. F E L T O N , E.A. F I E L D I N G , C.R. F I N L A Y S O N , D.M. F L O O D , P. F L O O D , R.H. F O D E N , J. F R A K E S , L.A. F R A N C I S , J.E. F R A S E R , S.J. F R E Y , F.A. F R O S T , K.M. F R O S T , M.T.

13.4 133 3.22 221 14.3 39, P 449 20.3 110 13.7 383 7.19 137, 13.6 135 2.27 406, 2.33 139 12.8 412 8.5 97 8.6 141, 8.21 142 7.22 144 7.22 144 16.2 146 17.8 330 8.27 270 8.9 379

G A G A N , M.K. G A M B L E , J.A. G A T E H O U S E , C.G. G A U L L , B.A.

7.11 148 17.7 372, P 449 5.1 150 9.2 152, P 443, P 444, P 447 11.1 153 P 446 8.33 154 4.8 174

G E R D E S , L. GIBSON, G. GIBSON, G.M. GIBSON, R.G.

453

GIODINGS, J.W. GILLIGAN, L.B. G L E A D G W , A.J.W. G O D F R E Y , N.H.H. G G L D I N G , S.D. G G R T E R , J.D. GOSTIN, V . A . G R A Y , C.M. GREEN, A.A. G R E E N , M.D. GREEN, P.F. GREEN, P.M. G R E E N H A L G H , S.A. G R I F F I N , W.L. GRIMES, K.G. GRIMSTGNE, L.R. G R O V E S , D.I. HABERMEHL, M.A. HAINES, P.W. HAMILTON, L.H. H A M I L T O N , N.T.M. HAMMOND, R.L. H A N C O C K , S.L. H A N D , M. H A N N A N , K.W. H A R R I S , C.W. H A R R I S , P.T. H A Y W I C K , D.W. HEGARTY, K.A. HEGARTY, R.A. HEIDECKER, E.J. HEINRICH, C.A. HENDERSON, R.A. H E N S E N , B.J. HERBERT, H.K. HERCZEG, A.L. HILL, C.M. HILL, K . C . HOLLIS, J.D. H O S T E T L E R , P.B. HOUSEMAN, G.A. HUBBLE, T.C. HUNTINGTON, J.F. HUTTON, A.C. HUTTON, L.J.

3.6 156, 3.15 219, 3.21 158, 3.22 221 11.4 35 6.8 50 13.2 p O 4.5 172, 6.3 295 7.8 160 7.17 59, 19.1 203 17.8 330 16.2 146 2.19 272 6.8 50 7.2 162 9.6 164 17.4 303 11.3 236 2.21 165 8.27 270

JENKINS, C.J.

10.2 186 19.1 203 8.18 310 6.17 95 2.6 168, 8.18 310, 13.10 170 4.3 309 8.36 301 4.5 172 4.8 174 16.1 175 7.16 177 2.5 178, 3.18 192 16.4 180 12.2 182, P 445 6.1 42, 6.6 183 14.7 185 8.30 409 4.5 172 10.2 186 7.14 188, 7.15 190 3.18 192 17.11 389, 17.13 387 7.1 40 2.5 178 P 442 16.2 146 13.1 51, 13.9 194 A7

K E E N E , J.B. K E N T , D.V. K E P E R T , D.A. KIMBER, R.W.L. K I N G S T O N , D. KIVIOR, I. KLOOTWIJK, C. KNUTSON, J. KORSCH, R.J.

I D N U R M , M.

3.6 156, 3.7 197, 9 195 J"Smy J^

J A C O B S O N , G. JAMES, P.M. J E F F R E Y , S. J E L L , J.S.

7.3 44, 7.4 45 12.6 198 2.32 245 7.7 234

JENKINS, R.J.F. JENNER, G.A. J O H N S O N , D.P. J O H N S O N , R.W. J O H N S T O N , D. J O H N S T O N E , D.W. J O N E S , B.G. JONES, L.E.A. JONES, P.J. JONES, R.M. J O N E S , T. J O Y C E , E.B. J U S T , G.D.

KROUSE, H.R. KUDRASS, H.R. L A C K I E , M.A. L A I N G , W.P. LAMBECK, K. L A M B E R T , I.B. L A N G , S.C. L A S T , W.M. L A W , S.R. L A W S O N , T. L E A C H , J.H.J. L E N N O X , P.G. LEVEN, J. LI, Z . X . LINDSAY, J.F. L I S T E R , G.S. L L O R C A , S. LOOSVELD, R.J.H. LOVERING, J.F. LOXTON, C. L U C E Y , C.J. LYNAM, C.

454

15.6 200, 15.12 202, 15.13 93, P 442 19.1 203 8.21 142 7.11 148 8.20 104 7.9 205 2.27 406, 2.33 139, 2.34 360 7.13 207, 7.23 206, 8.34 86, 13.1 51, P 448 9.4 209 19.6 211 10.3 213 P 446 1.4 215 9.12 217 P 442 K N 26 2.18 274 5.2 292 9.4 209 13.5 63 3.15 219, 3.22 221 17.10 120 2.14 223, 2.16 251, 2.30 417 4.5 172 15.3 393 3.12 225 4.4 57, 6.4 229, 6.10 227, 16.4 180 2.11 230, 2.12 231, 15.13 93 6.8 50 2.25 428, 7.7 234, 7.10 232, 7.19 137, 11.3 236 7.5 113 8.13 238 10.1 76 7.21 239, 12.1 241, 16.3 243 2.32 245 2.24 247 3.14 249 2.14 223, 2.16 251 15.1 252 6.14 254 4.6 256 6.8 50 3.22 221 11.5 258 9.13 259

MAAS, R. M c C O N A C H I E , B.A. McCUE, K. M c C U L L O C H , M.T.

4.9 418

2.20 66

P 446 4.9 418, 8.2 411, 15.11 260 15.7 262, 17.2 121 M c D O U G A L L , I. 3.2 263 McFADDEN, P.L. 8.14 264 M A C K E N Z I E , D.E. 8.12 266, 8.17 268 M c L E N N A N , T.P.T. 2.18 274, 2.19 272, M c N A U G H T O N , N.J. 8.3 276, 8.27 270 8.15 278 McPHIE, J. 17.14 70 MCQUEEN, K . G . 2.6 168, 8.18 310, M A L L E T T , C.W. 13.2 280, 13.10 170 6.9 282 M A R S H A L L , B. 7.7 234 MAWSON, R . 17.5 130 MENZIES, M. 9.3 284, 9.4 209, M I C H A E L - L E I B A , M.O. P 444, P 446, P 447 15.10 285 MIDDLEMOST, E.A.K. 2.8 287 M I D D L E T O N , M.F. 7.23 206, P 448 MIDDLETON, R.G. 9.14 289 MORRIS, P.H. 8.35 343 MORRISON, G.W. 8.6 141 M O R T I M E R , G.E. 11.3 236 M U R P H Y , P.R. 2.33 139 M U R R A Y , C.G. M U R R A Y - W A L L A C E , C.V, 5.2 292, 5.3 290 3.19 294 M U S G R A V E , R.J. 6.3 295 M Y E R S , I.A. NANSON, G.C; N E W B E R Y , S.P. N I C O L L , R.S.

7.13 207 4.4 57 12.4 296, 19.3 414

O'BRIEN, G.W. O ' R E I L L Y , S.Y. ODLING, N.W.A. OFFLER, R. OLIVER, R . L . OSTWALD, J . O V E R S B Y , B.

2.4 425 17.4 303 6.5 297, 8.26 299 8.36 301, P 449 8.31 302 6.15 305 8.16 307

P A C K H A M , G.H. P A G E , R.W. PARTINGTON, G.A. P A T T E R S O N , D.J. PATTISON, C.I. R E C O V E R , S.R. P I C K A R D , J. PIGRAM, C.J.

15.13 93 4.1 432, 4.3 309 2.18 274 6.1 42, 6.6 183 8.18 310 6.16 312, 17.15 314 9.2 152 2.10 318, 3.16 316, 3.20 401, 3.21 158, 3.22 221, 20.3 110 12.4 296 4.2 320, 5.5 322 3.10 324, 3.14 249 3.20 401

P L A Y F O R D , P.E. PLUMB, K.A. POWELL, C . M C A . P R A S E T Y O , H.

PREISS, W.V. PREMOLI, C. PRICE, R.C. P R I T C H A R D , T.R. PURVIS, A . J .

5.7 326 6.13 328 8.29 84, 17.8 330 15.5 101, P 442 8.31 302

G U I L T Y , P.G.

15.8 331

R I C K A R D , M. RIGBY, J.F. R O B E R T S , J. R O B E R T S O N , A.D.

2.24 247 14.5 332 2.32 245 1.2 333, 1.3 335, 17.11 389, 17.12 336, 17.13 387 6.12 338, 18.5 340 3.22 221 15.4 342 8.3 276 6.4 229, 8.35 343 2.7 65 8.22 345, 10.4 347 9.6 164, 9.7 349, 9.13 259, P 444

R O C K , N.M.S. ROGERSON, R. ROOTS, W.D. ROSMAN, K.J.R. R U B E N A C H , M.J. R U S S E L L , N. R U X T O N , B.P. R Y N N , J.M.W. SAPPAL, K.K. SCHMIDT, P.W. S C H N E I D E R , P.M. SCOTT, K . M . SCOTT, M. SEET, L . H . S E N A P A T I , N. S E X T O N , M.J. S H E L L E Y , E.P. S H E L L E Y , J.M.G. SHEN-SU, S. S H E R A T O N , J. S H E R G O L D , J.H. SILVER, E . A . SIMPSON, C . SIMPSON, C . SIVELL, W. SLADE, J.C. SLITER, W.V. SMITH, I.E.M. SMITH, K . SPENCER, R. STAGG, H.M.J. STAIT, B. STEPHENSON, A.E. S T E P H E N S O N , P.J. STEWART, A.J. S T E W A R T , I. STOLZ, A.J. S T R U C K M E Y E R , H. STRUSZ, D.L.

455

13.3 351 3.11 353, 3.14 249 P442 6.11 355 2.26 356 2.18 274 2.20 66, 7.13 207, 7.20 358 2.27 406, 2.33 139, 2.34 360 11.2 363 14.2 88 17.6 364 8.4 434 19.2 366, 19.3 414 3.20 401 4.8 174 7.18 368 8.1 370, P 450 2.20 66 3.20 401 17.7 372 11.1 153 15.5 101 20.1 99 19.3 414 7.6 373 8.8 377, 8.9 379, 17.9 375 4.7 381 19.3 414 8.21 142, 8.24 398 13.7 383 19.4 385

SrUART-SMITH, P.G. SUNATA, W. SUTHERLAND, F.L. SVENSON, D. SWIFT, M.G, SWITZER, C.K. SYMONDS, P.A.

2.23 386, 2.24 247 3.21 158 17.11 389, 17.13 387 18.4 391 2.4 425 6.4 229 2.10 318, 15.1 252, 15.2 420, 15.3 393, 20.3 110

TALENT, J. TANAKA, H. TARNEY, J.F. TAYLOR, B. TAYLOR, G. TAYLOR, M.J. TAYLOR, R.G. TAYLOR, W.R. THRUPP, G.A. TORGERSEN, T. TRUSWELL, E.M. TURNER, S.

7.7 234 3.9 395 2.17 118, 8.4 434 8.20 104 17.14 70 8.31 302 6.3 295 8.24 398, 8.25 396 3.13 399, 3.20 401 10.2 186 19.10 403 14.8 405

VALLANCE, T.G. VANDENBERG, A.H.M. VARNE, R. VERNON, R.H. VON RAD, V.

KN 28 19.3 414 8.21 142 8.5 97 20.2 131

WAKE-DYSTER, K.D.

2.27 406, 2.33 139, 2.34 360 4.1 432, 8.2 411, 8.23 408, 8.30 409 2.22 426 12.8 412, P 450 17.7 372 19.3 414 2.30 417, 17.3 416 4.9 418 6.1 42, 6.6 183 15.2 420, 15.3 393 9.8 422 2.19 272, 8.10 424 4.7 381 2.4 425, 20.2 131 2.22 426 2.25 428, 7.7 234, 11.3 236 8.11 430 2.17 118, 8.4 434 2.17 118, 3.5 128, 4.1 432, 8.2 411, 8.4 434

WARREN, R.G. WATCHORN, R.B. WATERHOUSE, J.B. WEAVER, S.D. WEBBY, B.D. WELLMAN, P. WILDE, A.R. WILKINS, R.W.T. WILLCOX, J.B. WILLIAMS, D.J. WILLIAMS, I.S. WILLIAMS, P.R. WILLIAMSON, P.E. WILSON, C.J. WITHNALL, I.W. WORMALD, R.J. WYBORN, D. WYBORN, L.A.I.

YOUNG, G.C. YOUNG, R.W.

12.3 436, 19.4 385 7.13 207

456