Abstracts No.1: 1st AGC Proterozoic Geology, 1975, Adelaide by GSAustralia

TH1 QKOLOCMCAL SOCKTV OF AUSTRALIA INCORPORATED

ABSTRACTS

of :• FIRST AUSTRALIAN QIOLOQICAL CONVENTION

PROTEROZOIC GEOLOGY

ADELAIDE 12-16 MAX 1975 ^BBmummmsKm

FIRST

AUSTRALIAN

GEOLOGICAL

12-16

MAY

CONVENTION

1975

PROGRAMME

PAGE NO. Monday

12th

9.30

11c 30

11.45

a.m„

1 2 . 00

May a.m.

1 2045

pom

R E G I S T R A T I ON WELCOMING

ADDRESS

RETIRING

PRESIDENT'S

The

Hill)

ADDRESS

Society:

(Professor

past, present

and

2.00 - 5 . 30 p.m o Chairman: R . Wc R0 Rutland

KEYNOTE

2 o 00

2.45

pomo

Transvaal and Mt B r u c e S u p e r g r o u p s parallels and some c o n t r a s t s in basin d e v e l o p m e n t and m i n e r a l deposits ( Dr o Ao Button)

2 c45

3 o 30

p c. m ,

Palaeotectonics and s e d i m e n t a t i o n in Precambrian Belt Basin, northwestern United States. (Dr0 J0E0 Harrison)

SPEAKERS

3 o 30

4 o 00

pcm<

AFTERNOON

4 o 00

4 . 45

p o m(

Proterozoic geology A. Tuga r inov 0)

4 o 45

5 o 30

p ra, .

Proterozoic styles (D r o M o J o F r a r e y )

5 o 30

p.m.

Tuesday

13th

SOCIAL

1 0 0 00

10o00

1 0 . 45

TEA in

U0S.S0R0

the

11.15

12.00

(Professor

Canadian

Shield

FUNCTION

a cmo

a om o

THE

ADELAIDE

Tectonics (R.W.R.

GEOSYNCLINE

the

Rutland

Adelaide & B.

A r e d e f i n i t i o n of Group in the l a t e Supergroup, South

a o m(

MORNING

AND

ITS

Fold

BASEMENT

Belt.

Murrell).

the base of Proterozoic Australia.

Adelaidean sedimentation Ranges. (B0 M u r r e l l ) 10.45

the

May

9c 1 5 - 1 2o45 pomo Cha i r m a n : K c R . Yates

9o15

future

the Burra Adelaidean (RoB0 Kitch).

the

Willouran

TEA

N o t e s on g l a c i a l and p r e - g l a c i a l sequences in the A d e l a i d e G e o s y n c l i n e (B . G. Forbes)_

10.

- 2 -

PAGE 'M. 1 2 o 00

2 000

12.45

5 o 30

pomo

p.m o

Isotopic datings of the basement-cover boundaries within the A d e l a i d e "Geosycl (j.A. Cooper), Deposition Hill (K.D.

of the A d e l a i d e a n Tuckwell)

System

CONCURRENT

SPECIALIST

MEETINGS

(1)

ine". ]_2. Broken 13.

GROUP

A.A.P.

2 o 00

2o40

pomo

Diversification of e a r l y Precambrian microb ictas up t o t h e e n d o f t h e t i m e of f o r m a t i o n of the major banded iron ores, (M.J. Muir)

2C40

3 c 05

po mo

Microfossils from cambrian stromatol Western A u s t r a l i a

3.05

3o30

p0m0

Microfossils Bas i n o (C.J.

3o30

4 0 00

4 o 00

4C45

4.45

5 o 30

2.45

2 o45

17 .

AFTERNOON

Pom.

relationA review of the Pa 1 a e o b i o 1 o g i c a 1 ships of s t r o m a t o l i t e s to the o r i g i n of higher organisms ( W. V . Pre i s s )

18.

3 o 30

p om o

pDmf

Roper

Group,

15.

pomo

p o m.

from the Peat)

Prein

McArthur _

TEA

The d i v e r s i f i c a t i o n s (RoJoFo Jenkins) (2)

2 o 00

a newly d i s c o v e r e d i t i c i r o n f o r m a t i on (M . R . Walter)

A.SoGo

Ancient

early

metazoans __ 1 9 .

Sediments

An e n v i r o n m e n t a 1 s t u d y of the r o c k s containing the E d i a c a r a assemblage in the F l i n d e r s Ranges (R.JoFo Jenkins) _

21.

Proterozoic sedmentary rocks the s t r o m a t o l i t e controversy Donaldson)

23.

of Canada (J.A.

and

3 c 30

4 o 00

p o mo

AFTERNOON

4 .00

4 . 30

p om.

Dewatering structures in l a t e Precambrian Marino Group sediments, Hallett Cove South A u s t r a l i a (V.A. Gostin & J0A. Donaldson)

24.

The d e p o s i t i o n a l environments of the Tindelpina S h a l e Member of the T a p l e y Hill Formation (late Proterozoic), Adelaide Geosyncline - a geochemical study (J. Sumartojo)

25.

Faulting contemporaneous with Umberatana Group sedimentation (late Precambrian) southern Flinders Ranges, South Australia (P.S. PIumme r )

26.

4.30

5.00

5 . 00

5 . 30

p.m o

p o m0

(3)

TEA

S.G.T.S.G. mechanisms

: Deformation styles and in the A u s t r a l i a n Proterozoic

- 3 -

PAG£ NO. 1.00

2.30

p0m,

The tectonic history and s i g n i f i c a n c e retrograde s h e a r z o n e s a t B r o k e n hi i l l (M .A . Etheridge)

3 o00

p0m.

S t r u c t u r a l g e o l o g y of s i l l i m a n i t e the N o r t h e r n Leases, Broken Hill, (W. Laing)

,.00

3o30

p t. m „

The

;.30

4.00

p om o

AFTERNOON

r.00

4,30

p0m.

Basement - cover relationships in t h e Boolcoomata area, Olary Province of South Australia, (R,G. Wiltshire)

structure

Complex,

-.30

>.00

5-00

Franks

Wales

28.

are a ,

(R . A . Glen )

30.

Old 32.

R e c o g n i t i o n of t h r e e s u p e r p o s e d tectonic phases in t h e n o r t h e r n p a r t o f t h e Flinders Ranges (J,P„ Richert)

33.

314#

80 00

pc m .

ANNUAL

14th

GENERAL

MEETING

May

p Cm O

THE

M U S G R A V E

AND

A R U N T A

dLOCKS

RCW0Nesbitt

10 c 2 5

acm,

T o w a r d s a s t r a t i g r a p h y of the (R0D0 Shaw & A.J. Stewart)

Arunta

P r e s e n t s t a t u s of g e o c h r o n o 1 o g i c a 1 in t h e A r u n t a B l o c k o (L.P0 Black) -

Willyama

TEA

Block —

A n i n t e r p r e t a t i o n of t h e t e c t o n i c s of A u s t r a l i a , (R0 D • S h a w & W 0 A n t i l o f f )

0.25

Comments and speculations concerning the p o l y g e n e t i c b r e c c i a s of the S o u t h Australian diapirsc, (W.D. Smith)

12,45

South

M t

schists N,S.W.

pcm0

ha i r m a n : 15

the

27.

5c30

ednesday .15

pom,

New

10 o 45

a cm o

Metamorphism in N oT o (S0 Dobos)

the

northeast

36.

research 37.

Arunta

Complex. 38.

0.45

11 o 1 5

a om o

M O R N !NG

1o 15

11o40

a j ,

B a s e m e n t and c o v e r r e l a t i o n s on the northern m a r g i n of the A m a d e u s B a s i n , central A u s t r a l i a0 (R0W. Marjoribanks)

U40

12 .45

p0 m o

35,

central

TEA

review

the

metamorphic

geology

the

Tomkinson,

Rangeso

(R0L0

Oliver)

and Mann

39.

igneous and

Musgrave

—

An interpretation of t h e e v o l u t i o n of the Musgravian Orogenic Cycle, eastern Musgrave Ranges, central Australia. (C0H0 Conor) The the

Proterozoic sediments and volcanics Musgrave Block (R.B* M a j o r )

4 3 ,

of .

CONCURRENT

SPECIALIST

GROUP

MEETINGS PAGE N O .

(1)

A0A0P0

Early

Archaeocyatha.

Earliest molluscs. A F T E R N O O N TEA

Rozanov)

(B. Runnegar

S P.Jell)

46.

T r o p h i c a n a l y s i s a p p l i e d to s o m e e a r l y Devonian communities from southeastern Australia. (R. Mawson)

48.

Devonian b r a c h i o p o d and tetracoral assemblages from New South W a l e s . ( A . J . Wright),

49.

(2)

S.G.IoG.O.D. deposits.

: Proterozoic

base

metal

R e l a t i o n s h i p s b e t w e e n o r e , b a n d e d iron f o r m a t i o n a n d P o t o s i g n e i s s at B r o k e n H i l l o (D.H. MacKenzie s N0 Gow)

51.

C o p p e r m i n e r a l i z a t i o n in m i o g e o s y n c 1 ina 1 c l a s t i c s of t h e B e l t S u p e r g r o u p , n o r t h w e s t e r n U n i t e d S t a t e s . (J0 H a r r i s o n )

52.

AFTERNOON 7

(Yu A .

TEA

R e v i e w of t h e g e o l o g y of t h e S u l l i v a n British Columbia (H. M o r r i s ) C o m m e n t s on m i n e r a l i s a t i o n in the British Columbia (F.R. Edmonds)

Mine.. j

Belt,

(^J

The Sullivan ore b o d y , British Columbia: p r e l i m i n a r y s t u d i e s in s u l p h i d e p e t r o l o g y Q (F.Ao C a m p b e l l , U . G . E t h i e r S R 0 A 0 B o t h ) CONCURRENT (1)

SPECIALIST

GROUP

53.

MEETINGS

S o G . T . S . G . : D e f o r m a t i o n styles and m e c h a n i s m s in t h e A u s t r a l i a n P r o t e r o z o i c 0

T h e s t r u c t u r e of H a r t s R a n g e s , N o r t h e r n Territory0 (M.J. Rickard) — Geometric analysis across a ductile mylonite: the W o o d r o f f e T h r u s t , M u s g r a v e R a n g e s , (T.H. Bell)

55.

B a s i n a n d d o m e d e f o r m a t i o n in t h e M o u n t G e o s y n c l i n e ( B . A 0 D u f f S I'.J*. W i l s o n )

56.

Isa ——

PAGE WO 7c 00 -

7.25

7o25

7.50

7 0 50 -

8.15

8 *15

8 c 40

8040 -

9o05

Thursday

PcfTio

p.m.

pomr

p.m o

15th

10.00

a oirio

1 0 o 0 0

10.45

1 1 o 15

11,15

1 2 o 00

p.rru

a o m c.

noon

12.45

G e o c h e m i s t r y of the Gawler Range VolcanicSo (C.D. Branch) Stratigraphic units in the Gawler Range V o l c a n i cs^ S o u t h A u s t r a l i a (A.M. B ' i s s e t t printed abstract only)

A s p e c t s of the g e o c h e m i s t r y and p e t r o l o g y of the m a f i c g r a n u l i t e s of p o r t i o n s of the Arunta, H u s g r a v e and F r a s e r B l o c k s . (A0 F . Wilson)

6 2.

G e o c h r o n o 1 o g i c a 1 s t u d i e s o f some Proterozoic rocks in A u s t r a l i a 0 (P „ Ao Arriens)

6 3.

C o r r e l a t i o n of u r a n i u m . d i s t r i b u t i o n and mineralogy i n f r a c t i o n a t i o n o f t h e Mt D a v i e s gabbro intrusion,, ( J . D . Kleeman)

64.

THE

NORTHERN

AUSTRALIAN

5,30

E v o l u t i o n and p r o b l e m s N o T 0 (K.Ao Plumb)

the M c A r t h u r —

MORN I NG

p.m o

2O45

Group Hill area

The s o - c a l l e d P i n e Creek G e o s y n c l i n e n o t h i n g t o do w i t h a g e o s y n c l i n e 0 ( E 0

CONCURRENT

SPECIALIST

A0S.G0

T h e

ringwood barred-basin

zoic

66.

TEA

The s t r a t i g r a p h y of the Mount I s a equivalents in the Mount I s a - L a w n ( R * Cavaney) — — p0m o

Basin,

E v o l u t i o n of P r o t e r o z o i c t o p o g r a p h y and the f o r m a t i o n of m i n e r a l i s e d b a s i n s in northwest Queens1 and0 (G„M. D e r r i c k & I Wilson)

(1) 2o00

59.

PROTEROZOIC

has Rod)

G r a n i t e d i a p i r i s m i n t h e Rum J u n g l e area, Northern Territory (0. S t e p h a n s s o n & K. J oh n s o n ) — — — — 2.00

57.

May

9.15 - 12.45 p.m. C h a i r m a n : W. D 0 S m i t h 9 o 1 5

A c h r o m a t o g r a p h i c m o d e l o f t h e s e d i men t o s p h e r e or the d i f f e r e n c e in c o m p o s i t o n of Precambrian and y o u n g e r s e d i m e n t s & ( T . C . Van M o o r t )

the

: Ancient

GROUP

MEETINGS

sediments

evaporite deposit marine evaporite in Amadeus

Basin.

(A.J0

a classical the ProteroStewart)

- 6 -

PAGh1 NO. !045

3o30

p .m0

Devonian conglomerate Basin. (B.G0 Jones)

1.30

4.00

p o m#

AFTERNOON

\c00

4.30

p.m.

D e v o n i a n and the s o u t h e r n (W. M a y e r )

fo30

ieOO

5 .00

5.30

Chairman: >.00

p .m o

pom.

R.A.

2 o ^5

Both

pomo

I ok5 -

3.30

pomo

5 o 30 -

4 o 00

p.m o

nOO

4C45

* .45

5 o 30

pom.

p .m o

'.25

• 50 -

o15 .45

.10

2o 25

2o50

3.15

3.45 40 1 0

4.35

pom<

p0me

p 0m o

71.

Carboniferous sedimentation in part of the Tamworth T r o u g h 0 72. s t r a t i g r a p h y and N.T0 (D0 C l a r k e )

structure 73.

S e d i m e n t o 1 o g y of the Corunna Conglomerate near Iron Knob, South A u s t r a l i a ^ (N0 Lemon & V. G o s t i n ) (2)

SoG.IoG.0oD, depos i t s.

The g e o l o g y River, NCT.

and (J.

Proterozoic

base

74.

metal

m i n e r a l i s a t i o n at M c A r t h u r Binnecamp & Logan)

75.

G e o c h e m i c a l and m i n e r a 1 o g i c a 1 investigations McArthur area, N0T0 (K.M0 Scott & I . B . Lambert)_76. AFTERNOON

TEA

Economic i m p l i c a t i o n s Syncline, Ashburton0

of the Duck (G. D o u s t )

Creek 77,

The P a r a b a r a n a Copper P r o s p e c t , a p o s s i b l e volcanogenic sedimentary deposit in the C a r p e n t a r i a n Mount P a i n t e r B l o c k , S 0 A . (K.Eo Every) : Petrogenesis Proterozoic rocks 0

S . G o G o M .

of -

Amadeus

TEA

Heavitree Quartzite near A l i c e S p r i n g s ,

(3)

!o 00

deposition,

The M o r d o r ultrabasic affinities Langworthy

and

metamorphism

Complex: A P o t a s s i c intermediate intrusion with kimberlitic central Australia0 (A 0 P • £ L.P. Black)

The T o l l u V o l c a n i c s , a bimodal igneous ( D . F. B l i g h t , R.W. N e s b i t t & p r o v i n ce o Bowden) P e t r o l o g y of A r c h a e a n p e r i d o t i t e s , R o c k s , Wo A . ( I . A . N i c h o l l s )

78.

80.

P.R,

81.

Corsair 82.

p.m.

AFTERNOON

p.m.

P e t r o g e n e s i s of metamorphosed ultramaficm a f i c c o m p l e x e s in the G r e e n v a l e a r e a , North Queens 1and 0 ( M 0 J C R u b e n a c h & G o 0 o A r n o l d )

83.

B u r i a l metamorphism of basic volcanics. (RoE.

84.

p.mo

TEA

the F o r t e s c u e Smith)

Group

- 7 -

PAGE NO. .35

.00

5.00

5.30

riday

p.m.

Porphyroidal G e o s y n c l i ne .

0.00

—

p0m.

A metamorphosed r e g o l i t h from the Arunta Block, Central Australia,, (R G. Warren)

7.30

p.m.

CONVENTION

16 t h

May

.15- 12.45 p c m 0 THE hairman: R . Halligan 1015

rocks in the Warramunga (D0McP. Duncan)

10,00

a c m.

10.45

a.m.

AND

Stratigraphy, the Bangemall P.Co M u h l i n g )

BANGEMALL

11o15

a.m.

MORNING

!1C15

12.00

a.m.

Synthesis Bangemall

sedimentation and B a s i n , W, A . ( A . T 0

structure Brakel &

!.00

12.45

4,30

Chairman ',00

-.30

;. 0 0

p<m#

B.P.

2.30

3 . 00

3 . 30

p.m.

Webb

p.m.

p om0

p.m.

f c 00

4 o 00

4 o 3 0

p0mo

p „m.

87.

88.

TEA for the Groups.

d e p o s i t i o n of the Wyloo and (R.E. Smith & R.C. Horwitz)

CONCURRENT

91.

92.

SESSIONS

(1)

General

Precambrian

The (I.

geological Genuts)

evolution

the

East

—

Kimberleys, — 94.

A c o m p a r i s o n of P r o t e r o z o i c g e o l o g y of the Kimberley B l o c k and T a n z a n i a * (Ro H a l l i g a n ) The P r o t e r o z o i c of central Australia,

the and

Granites-Tanami , regional correlations

Blake)

The P r e c a m b r i a n (P.A. Arriens) Precambrian Boul ter)

90.

and

Regional geology, stromatolite biostratigraphy and i s o t o p i c d a t a b e a r i n g on t h e age o f a Precambrian sequence near lake Carnegie, Western A u s t r a l i a , (W0V. P r e i s s , M.J. Jackson, R 0 W 0 P a g e & W. C o m p s t o n ) —

( Do H . 30

in - —

The s o u t h e r n b o u n d a r i e s of the H a m e r s l e y Bangemall b a s i n s of sedimentation0 (R.CoHorwitz) -

86.

BASINS

The N a b b e r u B a s i n : a newly discovered Proterozoic basin in W e s t e r n , A u s t r a l ia ( W . D,M„ Mall & A.T„ Goode)

2.00

DINNER

HAMERSLEY

0.45

85.

geochronology

Tasmania J N.J.

of —

95.

96,

Antarctica

Turner

97. & C.A0

99.

-8-

:ha i r m a n :

!.00

>.30

3.00

3.30-

4.00

MCJ. Rickard

2c30

3.00

3.30

4.00

4.30

pofTio

p.m.

p.me

(2)

General PAG£

NO.

T h e a p p l i c a t i o n of r e f l e c t a n c e determinations on c o a l i f i e d and g r a p h i t i s e d plant fragments to m e t a m o r p h i c s t u d i e s . ( R . O f f l e r & C . F . KQ Diessel)

101.

S t r a t i g r a p h y a n d e n g i n e e r i n g g e o l o g y of the A d e l a i d e C i t y area.. ( J C M . L i n d s a y & Ju S e l b y )

102.

T h e s t r u c t u r a l s e t t i n g of southeastern New England. Beat tie)

103.

Structural patternso

serpentinites in (E .C „ Leitch & R .

envelopes in A u s t r a l i a n (E «S „ T • 0 ' D r i s c o l l )

tectonic

S t r u c t u r e a n d s e d i m e n to 1 o g y of Archaean metasediments near Lawlers, Yilgarn Block, Western Australia, (J.A0 Donaldson & J .P o Piatt)

0 O 0

104.

106.

THE T R A N S V A A L A N D MOUNT BRUCE SUPERGROUPS - P A R A L L E L S A N D SOME CONTRASTS IN BASIN D E V E L O P M E N T A N D M I N E R A L DEPOSITS

A. Button Economic Geology Research Unit University of the Witwatersrand The Transvaal and Mount Bruce Supergroups are preserved on stable Archaean blocks- known respectively as the Kaapvaal and the Pilbara. The sediments within the basins are little deformed or metamorphosed, except near craton-marginal tectonic belts. The basins are largely contemporaneous, having been deposited between 2350 and 1850 million years ago. The transvaal and Mount Bruce can be subdividec1 into three major stratigraphic units. A basal volcanic and clastic unit is gradationally and conformably overlain by a chemical sedimentary unit, the latter being unconformably overlain by an upper clastic unit. The basal volcanic and clastic unit comprises the Wolkberg Group of the Transvaal Basin and the Fortescue Group of the Hamersley Basin. In both, deposition was strongly influenced by palaeotopography and by Archaean tectonic elements. The successions commence with fluvialcoarse clastics and sub-aerial volcanics, and grade up to marginalmarine finer-clastics,volcanics and stromatolitic carbonates. The Fortescue comprises a large proportion of volcanics, while the Wolkberg contains greater thicknesses of marginal-marine arenites. Sub-economic Au-U occurrences are found in conglomerates of both units, as are epigenetic vein deposits of Au, Pb, Zn, Ag and fluorite.. Cupriferous shales are known from the Fortescue Group, but have not proved to be economic as yet. The chemical sedimentary unit comprises the Chuniespoort and CampbellGriquatown Groups of the Transvaal Basin and the Hamersley Group in Australia. Both consist of chemical sediments and some fine-classics. The units in the Transvaal are dominated by carbonates, while those in the Hamersley are dominated by iron formation, and include some acid volcanics. Iron formation and carbonates are facies-equivalents, and can grade into one another both vertically and laterally. Ore deposits common to both units are those of iron and crocidolite. The Transvaal Basin has, in addition, important deposits of limestone, dolomite, amosite, fluorite, gold (both epigenetic ana detrital) and some chrysotile. The upper clastic unit consists of marine and marginal-marine sediments, mainly shale with arenites, volcanics and carbonates. In the Transvaal Basin, the unit is represented by the Pretoria and Postmasburg Groups; in the Hamersley, by the Wyloo and possibly the Manganese Groups. Upward coarsening, sharp-topped cycles, indicative of progradation into a marine basin, are prominent in both. Oolitic ironstone, haematized and manganized sediments and various types of vein deposits are common to the successions. In particular, manganese deposits associated with chert breccias along the basal unconformity of the group (where they rest on dolomites) are remarkably similar in the two basins; as are occurrences of haematized conglomerate above the unconformity where it truncates iron formation. The Transvaal and Hamersley Basins bear far greater similarities to one another than to any other basins of comparable age. They were probablyformed contemporaneously on the same super-continent, the parallels reflecting widespread evolution in biologic, atmospheric and crustal conditions. The basins could conceivably have been joined. Comparisons between them spotlight areas for1 research in both basins, and indicate in addition, target areas for mineral exploration. oOo

2. PAL A E O T E C T O N ICS AND SEDIMENTATION IN THE PRECAMBRI AN BELT BASIN IN T H E NORTH WESTERN UNITED S T A T E S

Jack E. Harrison U.S. Geological Survey, Federal Centre, Denver, Colo. 80225 U.S.A. The Belt Basin represents a slowly sinking reentrant on the North American craton that began to form about 1,500 m.y. ago and persisted for more than 600 m.y. This sinking block resembles an aulacogen in some respects, but the basin is not a true grabenlike trough extending into the^craton at a plate separation. The sinking block was at times shaped like a right triangle: its southern E-W base was bounded at times by an island or cratonic prong; almost at right angles on the west was the Cordilleran geosyncline trending generally north all along the North American craton; and the hypotenuse was a hinge line trending SE back into the Canadian Shield. Within this basin, perched on the craton, tectonics were largely confined to downwarps along the cratonic hinge lines and to gentle, warps in the rrain basin, although a segment of the southern edge in the southeastern part of the basin ^was bounded by a high-angle fault that was active during the first third of B elt time. Filling of the basin ended during the East Kootenay orogeny about 850 m.y. ago, and the greenschist or higher grade metasedimentary rocks within the basin have subsequently acted as pseudocraton. Sedimentation within the basin was in four major cycles of marine or marginal marine clastic and carbonate sediments that accumulated without apparent widespread interruptions to a thickness of at least 20 km. Early deposits, particularly along the join of the perched basin and the geosynclinal trough to the west, contain abundant turbidites; the first cycle is terminated by a black shale. Subsequent deposits are all red-bed sequences or carbonate. The second cycle is represented by a deltaic sequence prograded from the south that interfingers with fan-delta systems built out from the northeast and east. The third cycle is a carbonate sequence that contains shelf carbonate (stromatolitic) in the east and northeast, grading westward into slope breccias and large slumps, and then black shale still farther west. Clastic debris from a southern source interfingers with the carbonate sediments from the east. A dome in the western part of the basin began forming by differential subsidence late in the carbonate cycle and affects western facies. The fourth cycle consists of clastic red-bed sequences prograded into the basin from both the south and northeast. The lower part of this cycle contains the Purcell Lava (about 1,130 m.y. old), which forms the only time line known within the Belt Supergroup. The middle part of the fourth cycle appears to represent braided stream deposits prograded across a pediplain. Two events of Precambrian intrusion are recorded. The oldor event at about 1,400 m.y. consists of gabbroic to dioritic sills and a few granodiorite stocks intruded mainly into the oldest formation (Prichard). The younger event at about 800 m.y. accompanied the East Kootenay orogeny and also consists of gabbroic to dioritic sill intruded into most formations. All intrusives tend to be most abundant along the old basin hinge lines or intrabasin troughs.

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A REVIEW OF THE GEOLOGY OF THE SULLIVAN MINE H.C. Morris Cominco, Vancouver

ABSTRACT

The Sullivan orebody is a large, conformable massive sulphide lens of Proterozoic age occurring in rocks of the Purcell supergroup in southeastern B.C. To date, roughly 110 mtons of ore have been mined. Mineralization occurs in conformable layers ranging from 0.5 m to 50 m thick, inter-bedded with fine-grained argillaceous sediments. The mine assemblage totals 50-100 m and contrasts strongly with the typical medium to fine-grained alternating quartzite-shale sequences of the host Aldridge Formation, which probably represent turbiaite deposition. The orebody covers approximately 3 sq km and is concentrically zoned. The central portions of the mineralized area are characterized by a footwall breccia zone, by intensive tourmaline alteration in the footwall- — * — and albitization of Hanging Wall rocks. Roughly half of the orebody displays extreme regularity and delicate lithologic fabrics. The other part is complex, both structurally and petrologically. Various genetic models have been offered in the past, hydrothermal-replacement and submarine-exhalative being the most favoured. It is presently considered that a submarine exhalative origin, complicated by later metamorphic and tectonic events, is the most acceptable concept.

GEOLOGICAL SOCIETY OF AUSTRALIA INC. FIRST AUSTRALIAN GEOLOGICAL CONVENTION

Proterozoic Highlights of the Canadian Shield M•J• Frarey Geological Survey of Canada Structural divisions and time-classification in the Canadian Shield are reviewed• The Stockwell timeclassification is based on major tectono-thermal events in Shield history; such a scheme is preferable to those based on arbitrary units. Proterozoic time began at the end of the Kenoran Orogeny. Proterozoic rocks of all sedimentary, volcanic, and intrusive types occur in the Canadian Shield. Supracrustal sequences are most commonly of shallow marine or continental origin and high-hevel intrusions predominate. "Classical" geosynclinal cycles have been restricted to the Aphebian Era. High-grade metamorphic rocks of Aphebian age in mid-Churchill Province mark the peak of Hudsonian tectonism. Subsequently, the Shield became relatively stable except for the Grenville Province, and deposition and deformation resulted from epeirogenesis, block-faulting and rifting, mainly in the western Shield. Proterozoic sedimentation and volcanism diminished markedly in Helician time and terminated in early Hadrynian time, circa 800 m.y. ago. The history of the Grenville Orogen remains obscure. The continuity of major Arch:-an to Paleohelikian rock units into the Grenville from adjacent provinces constrains proposals for a marginal plate boundary and convincing evidence for Proterozoic plate boundaries within the Grenville Province is lacking, paleomagnetic evidence notwithstanding.

3. TECTONICS OF THE ADELAIDE FOLD BELT

R.W.R. Rutland & B. Murrell Department of Geology University of Adelaide The morphological expression of the Adelaide fold belt is largely due to Tertiary tectonics. Tertiary faulting was of particular importance in the Torrens Hinge Zone which also separates the Adelaidean geosynclinal area of the Adelaide Fold Belt from the Gawler Platform area. The fold belt, developed from the geosynclinal region, is essentially intracratonic but the fold trend in the Mt. Lofty-Olary arc is parallel to the cratonic margin at the end of the Cambrian. No stable platform corresponding to the Gawler Platform occurred to the north or east of the fold belt where the present boundaries had only ephemeral strato-tectonic significance during the Adelaidean deposition. A Cambrian basin extended under the Frome Embayment and a CircumDenison arc of early Palaeozoic deposition also extended from the Officer Basin through the Warburton Basin to the Bancannia trough. Possibly the Permian Cooper Basin is renewing the trend of the Adelaide "geosyncline" in the same way that the Pedirka basin continues the Amadeus Basin trend. The general area of the Torrens Hinge zone can be attributed some role in separating contrasting depositional environments throughout the history of the belt but it is best defined for the Marinoan and Cambrian. The platform area was a major source of sediments for deposition of clastic wedges especially during Torrensian and Marinoan times. However, in Marinoan and Cambrian times the platform was itself a depositional area, the Stuart Shelf, (albeit with a major break in sedimentation between the presumed ABC Range Quartzite equivalent and the Cambrian). The older Pernatty Grit and Pandurra Formation on the platform are most probably equivalent to the Upper Callanna Beds and Lower Burra Group of the Geosyncline. If so the platform carries no record of deposition corresponding to most of the Burra and Umberatana groups in the geosyncline, other than the Woocalla Dolomite which may be a composite formation. The Woocalla Dolomite may therefore correspond to one or more of the transgressions within the geosyncline represented by the Skillogalee Dolomite, the Tapley Hill Formation and Brighton Lineston equivalents, and the Nuccaleena Dolomite. The geosynclinal area shows several phases of development with substantial changes in the nature and locus of deposition. A plot of maximum preserved sediment thickness against time indicates a much greater net rate of sedimentation after the beginning of the Sturtian than formerly. The Lower Callana Beds mark an early extensional phase with local (evaporite) basin development and basic volcanism. The upper Callanna Beds and Burra Group mark a first cycle of basin filling with clastic wedges derived from the Gawler Platform. In the Willouran ranges the basin was clearly fault controlled and was also filled from the northeast. During the Sturtian, deposition including the main glacial deposits, was controlled by contemporaneous faulting and extended through the central Flinders Ranges, the Olary Province and the Broken Hill area, where the preceding Burra Group is absent. Much of the

glacigene material was apparently derived from fault blocks within the geosyncline. During the deposition of the Wilpena Group clastic wedges were again derived from the west and a broad basin extended to north and east beyond the present limits of the fold belt. As the locus of deposition shifted east in the Lower Cambrian the principal source ofclastic material was from the south east. Renewed tectonic activity led to deposition of the Lake Frome Group and to the thick clastic deposits of the Kanmantoo trough. A consideration of the stratotectonic history of the belt, summarized above, suggests that the aulacogene concept has little relevance, especially in the currently popular sense where it is related to plume generated triple junctions. Faulting was however of great importance in controlling the different patterns of deposition observed in the Lower Callanna Beds, the Burra Group, and the Umberatana Group. Faulting was less important during deposition of the later units which extended north and east beyond the present limits of the fold belt. The possibility of a major orogenic episode prior to Burra Group deposition in the south east of the belt is indicated by recent isotopic work on the Houghton Inlier. Within the main part of the fold belt local open folding and erosion produced several unconformities within the sequence, especially beneath the Burra Group and Umberatana Group. In part, these were related to mobility of the Lower Callanna beds, which produced piercement structures. Subsequent piercement structures are generally fault controlled and largely confined to the Flinders tectonic zones. The main folding of the belt is clearly post-Cambrian but there is evidence of earlier folding in the north-west of the fold belt and later in the north-east. The folding can be related in part to basement structures (and especially to retrograde shear zones within the basement) and it also exhibits control by faulting and be depositional trends developed during the Adelaidean and especially during Torrensian and Sturtian Times. The main distinction between the Flinders tectonic zones, on the one hand, and the Mt. Lofty-Olary fold belt on the other, reflects their different stratotectonic histories. In general, although a decollement zone exists, the shortening in the Adelaidean cover reflects a shortening in the basement. The evidence also opposes application of the orocline concept, both for the Nackara and Fleurieu arcs. Metamorphism and strain tend to increase across the belt from Port Augusta to the Kanmantoo metamorphic belt in the south east; and the broad metamorphic zones are parallel to the structural trends. The highest grade metamorphics and associated granites, however, appear to cut across the structural trends and may antedate the main deformation. The evidence available gives little support to concepts of large scale horizontal displacements on the Darling or Torrens lineaments or on the postulated Gambier Fracture Zone. The possibility of displacements

of the cratonic margin by large fault zones prior to Adelaidean deposition is less easily discounted. There is no reason to suppose however that the local cratonic margins at various times, corresponded to the limits of continental crust.

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6. A R E D E F I N I T I O N OF THE BASE OF THE BURRA GROUP IN THE L A T E PROTEROZOIC A D E L A I D E A N SUPERGROUP, SOUTH A U S T R A L I A

R.B. Kitch, Utah Development Company An unconformity separating two contrasting styles of sedimentation and tectonism in the basal portion of the Adelaidean Supergroup is described. I regard this unconformity as fundamental to the location of chronostratigraphic boundaries. This unconformity is in the previously defined River Wakefield Group in the Far North of South Australia (Orrorool:250,0G0 geological sheet). The stratigraphic break was recognised using sedimentological and structural methods when interpreting surface and subsurface mineral exploration data. The unconformity is exposed in four regional anticlines in the Orroroo sheet area. In the Carrieton and Yednalue Anticlines the unconformity passes laterally into a disconformity, which can be traced around these folds. In the Johnburgh and 'Yatina area Cainozoic and intermontane deposits largely cover Proterozoic structures. However, on the exposed western limbs in thse latter areas an angular unconformity is recognised at the base of the redefined Burra Group. In tie' Carrieton Anticline the name Eurelia Beds is tentatively proposed for the sediments underlying the unconformity. I correlate these beds with the Lower Callanna Beds. For the sediments overlying the unconformity, the name Carrieton Beds is proposed. I correlate these beds with the River Wakefield Group exposed on the Burra 1:250,000 sheet and the Upper Callanna Beds. I suggest all sediments overlying the unconformity being described, be placed in the basal Burra Group. There is a contrast in the sedimentological and structural style between the beds above and below the unconformity. The lower prism of sediments were produced in tectonically controlled depressions in a marginal marine environment. The rocks overlying the unconformity are the product of a prograding deltaic environment. The deformation style below the unconformity is conical (en echelon domes and basins), and above, cylindrical. The Eurelia Beds comprise greater than 400 m of sediments deposited in intracratonic half grabens in which hypersaline, shallow water, marine deposition occurred in a restricted circulatory environment. Lithologically these beds are characterized by allogenic chert nodules, pyroclastics and carbonaceous dolosiltstones containing congruent pyrite and chalcopyrite minerals. Pyroclastics, with flow textures, probably related to a lineament trending northwesterly through Carrieton Township intertongue with the sequence. Much of this material was re-worked to form volcaniclastic units. Portions of the sequence have slumped down the depositional slope and have piled up against a sumbarine scarp situated on this lineament. This area is referred to as the Carrieton Fold Zone and was formerly interpreted as a diapiric structure. The Carrieton Beds consist of greater than 300 m of sediment, which on the Orroroo Sheet area were deposited in a deltaic-marine system which gave way westwards (Emeroo Range) to fluvial conditions and

southwestwards (ranges east of Nelshaby), to fluvio-deltaic conditions. The Carrieton Beds are placed below the major arenite of the Burra Group (formerly the Rhynie Sandstone equivalent, but more probably a Yednalue Quartzite correlative). These beds commence with a lenticular, heavy mineral laminated, arenite infilling depressions in the unconformity surface. Thick stromatolitic limestones, minor magnesite horizons, intensely reworked pyroclastics and minor sulphide mineralization characterize the beds. 0O0

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8. ADELAIDEAN SEDIMENTATION IN THE WILLOURAN RANGES

Burton Murrell Department of Geology University of Adelaide The sediments of tip Willouran Ranges are of Upper Proterozoic age and form a 3000 km north-western extension of the Flinders Ranges. In South Australia these rocks belong to the Adelaide System which is divided into the Willouran Series (oldest) Torrensian, Sturtian and Marinoan Series (uppermost Precambrian). The pattern of tectonism shows a number of distinct events with a return to stability between. (a) Willouran: crustal tension; widespread development of sunklands, basic volcanism, block faulting. (b) Torrensian: warping; development of elongate restricted marine basins. (c) Upper Sturtian-Marinoan: epeirogeny plus warping; shallow epicontinental sea with open circulation. (d) Cambro-Ordovician (Delamerian Orogeny): crustal compression; fold zones with cleavage imposed over earlier structures. The Willouran Series consists of an evaporite sequence several thousand metres !:hick. Contacts on the basement inliers show a completely nonviolent transition from an erosional to a depositional regime at the onset of Adelaidean sedimentation. Basic volcanics occur in the lower part of the sequence which is always disrupted but has a constant lithologic association. The proportion of this disrupted sequence made up by the volcanics and basal carbonate (each of measurable thickness) suggests that it represents a small thickness of beds which acted as a zone of detachment between basement and cover. Local facies changes and piercements are common. Initial folding and piercement of Willouran rocks predates the Torrensian, which is represented by a transgressive sequence initiated as a NNW trending trough developed between the Norwest Fault and the eastern side of the ranges. Marginal facies deltaic sandstones were separated by a shallow water mid-basin facies of carbonaceous siltstones. With transgression onto the forelands, the western margin was overstepped by a magnesite bearing dolomite/shale sequence and the eastern by the mid-basin facies. Slumping in siltstones near the top of the sequence and pre-Sturtian truncation of folds show late syndepositional tectonism. East of the ranges and south of Mt. Norwest on the western side, the Sturtian glacial sequence consists of a bedded arkose sandwiched between lenses of tillite. North of Mt. Norwest this is progressively replaces by a sequence of boulder-beds, tillites and siltstones which thickens and coarsens northwards. In addition to recognisable Willouran and Torrensian sources, the boulder-beds show derivation from a complex igneous and metamorphic basement source, and so lend substance to a hypothetical basement high to the north, the "Mooloorinna RidgeTT. Basal beds containing rounded granite and porphyry cobbles in a calcareous matrix and associated ironstones probably represent a regolith belonging to a pre-Sturtian land surface. When marine sedimentation resumed following erosion of the Torrensian rocks and the Sturtian glacial period the basin of deposition became for the first time the Adelaide Geosyncline. The Torrens Hinge Zone, associated with strong facies changes throughout the Marinoan, was near the western margin and sedimentary source, and as far east as the Norwest Fault, Marinoan sediments lie on the magnetic basement.

Continued growth of pre-Sturtian structures caused cyclical slumping within the area, which only ceased prior to the Upper Glacial. The Yerelinna Tillites near Marree contrapose a westward spread of fining clastics denying the temptation to correlate their red feldspathic granules with the Gawler Ranges source of porphyries. Following the Nuccaleena Formation's (?terrestrial) dolomites and redbeds clastic sedimentation from westerly sources resumed. Red sediments and thick wedges of sandstone along the Torrens Hinge Zone intertongue with the green and grey siltstones of the eastern facies. While an almost complete Marinoan sequence is preserved 20 km to the east, along the Torrens Hinge Zone itself the upper part is missing and Cambrian sediments lie on the truncated surface. Cambrian sediments show restricted basin faunas passing upwards into a red-bed evaporite sequence which closes the sedimentary record.

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10. NOTES ON G L A C I A L A N D P R E G L A C I A L S E Q U E N C E S OF T H E A D E L A I D E " G E O S Y N C L I N E " SOUTH A U S T R A L I A

B.G. Forbes Geological Survey of South Australia These notes summarise some Geological Survey contributions by R.P. Coats, W.V. Preiss and the writer. It seems to be generally agreed that the Adelaide "Geosyncline" is not a geosyncline in the strict sense, although the sedimentary accumulation resembles that of a miogeosyncline. The tern basin or trough might^be appropriate. It is thcaght that "Adelaide aulacogene" is an over-simplification for wnat appears to have been a variably subsiding ba^in complex. The middle part of the Adelaidean sequence in South Australia (Umberatana Group) near its base and top is characterised by tillites or coarse poorly sorted sediment of gldcial derivation. The term tillite is extended here to include what may be glacio-mai'dne deposits. Glaciation is indicated by the presence of dropstones, facetted and striated clasts and the distant provenance of some boulders. There are two main glacial sequences, of Sturtian and Marinoan age, which will be referred to here respectively by the informal terms lower and upper glacial sequences. In the COPLEY region tney are the Yudnamutana and Yerelina Sub-groups of the Umberatana Group. The earlier glaciation provided the thickest deposits and can be tentatively subdivided into three units. It is best exposed in the Umberatana and Manunda regions. The upper unit is characterised by the presence of reddish quartzite and porphyry boulders prcbably derived from the Gawler Craton to the west and lies unconformably on older glacial or preglacial rocks. Correlated with this upper unit is the tillite of Mount Jacob (Wooltana region), the Wiiyerpa Formation (Bibliando and Olary regions), the Appila Tillite and the Sturt Tillite. A middle unit, sometimes containg hematitic beds (e.g. the Braemar Iron Formation), possibly represents an interglacial period. The lowest unit contains thick tillites and includes the Bolla Bollana Formation and tillite of the Pualco Range (Manunda). The lower glacial sequence reaches over 5000m in thickness in the Umberatana and Manunda regions but is elsewhere generally only a few hundred metres. The environment appears to have been mainly marine. The upper glacial sequence is separated from the lower by a thick section of mainly siltstone and local carbonate and contains a smaller proportion of coarse detritus. Thickness reaches about 1300m in the Umberatana region, where it has been best described. In the Parachilna region it is represented by the Elatine Formation (up to 250m) and to the south and southeast by the Pepuarta Tillite and associated beds. The sequences in the Umberatana and Olary regions contain similar partly calcitic sandstones and siltstones with sparse pebbles or dropstone. Clasts in the upper and lower glacial sequences were derived from the Gawler Craton, the Mount Painter and Willyama Inliers and other crystalline basement inliers and exposed piercement structures, as well as more local older Adelaidean rocks. The upper glacial sequence probably had a mainly easterly provenance.

Within the preglacial beds of the Adelaidean there has been some revision of stratigraphic order, mainly resulting from the work of W.V. Preiss on the Spalding Inlier. From this work and mapping southward to Rhynie it appears that the River Wakefield "Group" is underlain by the Rhynie Sandstone and overlain by a quartzitic sequence correlated with the Yednalue Quartzite and is thus part of the lower arenaceous beds of the Burra Group, such as the Aldgate Sandstone and Emeroo Quartzite. Black titaniferous hematite detritus appears to by typical of the basal Burra Group and is present at the base of the Aldgate Sandstone, Erneroo Quartzite and throughout the Rhynie Sandstone. Thickness of the Aldgate Sandstone and equivalent beds is quite variable, but reaches over 300m in the Clare and Port Pirie regions. Shallow marine^ to tidal flat and possibly fluviatile environments are indicated.

12. ISOTOPIC DATINGS OF THE BASEMENT-COVER BOUNDARIES WITHIN THE A D E L A I D E " G E O S Y N C L I N E "

J.A. Cooper Department of Geology University of Adelaide Three rubidium-strontium total-rock dating studies have been made in attempts to place reasonable limitations on the age of the stratotype Torrens Group (Torrensian) and the presumed older Wooltana Volcanics of the Mt. Painter region. 1)

Further measurements on the pre-Torrensian, upper amphibolite metamorphics at Inglewood show some samples directly supporting an earlier 870 m.y. date, whilst others indicate further variations of the initial 87Sr/86Sr ratio in the Houghton metamorphics (Houghton "Diorite") studies,

Many measurements on selected Wooltana volcanics from the Mount Painter region produce considerable deviation from isochron alignment. Arguments are presented which indicate that initial 87Sr/86Sr are not older than about 800 m.y. This is the same preferred age recorded by Compston et al. (1966) using less data. There is no support for the hypothetical 1350 m.y. calculation made by these authors using the same data.

The alkali rich Pepegoona Porphyry of the pre-Callana Beds, Radium Hill metamorphics, Mt. Painter Complex, produced two isochrons from a suite of indistinguishable samples. This+relationship is not clearly understood. The older result 1309- 129 m.y. (I.R. = '739- '£73) is probably ^n emplacement age where as the younger 1035 - 62 (I.R. = *933- "038) may be related to an updating by later amphibolite intrusions located nearby. Both are younger than earlier published calculations using single total rock (1650 m.y.) and microcline (1900 m.y.) measurements respectively. These three investigations collectively indicate that both the Torrens Group, and the Callana Beds as mapped in the Mt. Painter region, are less than 900 (possibly 800) m.y. old, and that the Radium Hill metamorphics are less than the older igneous assemblages of the same area.

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13.

DEPOSITION <3F THE ADELAIDEAN SYSTEM IN BROKEN HILL Kevin D. Tuckwell University of New South Wales W.S. 6 L.B. Robinson College Broken Hill A new subdivision of the Upper Precambrian in the Broken Hill area was proposed by Cooper and Tuckwell (1971), and will be briefly discussed. Sound correlations are established between the Broken Hill section in N.S.W. and the Mt. Painter section in South Australia. Deposition took place largely under shallow marine conditions, in an environment controlled by the relative subsidence of major fault bounded basement blocks. Activity along these faults coincided with deposition of the Yancowinna Sub-Group (Yudnamutana Sub-Group equivalent) and the Teamsters Creek Sub-Groop (Yerelina Sub-Group equivalent) probably in response to the advance/retreat of continental glaciation. Originally glacially derived sediments have been extensively reworked by submarine mud flows (in the Yancowinna Sub-Group) and in places by intermittent fluviatile action (in the Teamsters Creek Sub-Group). Continued uplift of the Euriowie Block during sedimentation occasioned the entire Torrowangee Grovp, to be developed in an on-lap situation against the Block. As a consequence of this, sediment deposited against these blocks has been subject to erosion and redeposition within the be in. Interstadial sequences are characterised by extensive carbonates, and post glacial deposits were laid down in a stable shelf environment.

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DIVERSIFICATION OF EARLY PRECAMBRIAN MICROBIOTAS UP TO THE END OF THE TIME OF FORMATION OF THE MAJOR BANDED IRON ORE

M.D. Muir Royal School of Mines, London Evidence for life in the Early Precambrian can be found in (a) structurally preserved microfossils, (b) flat 'algal1 laminations, both on a microband and mesoband scale, and (c) stromatolites. In Early Precambrian times, structurally preserved microfossils are rare, probably because of metamorphism, or else because depositional conditions did not favour the preservation of microfossils. Living forms may not have been present in all areas in Archaean times, although the still slender evidence of 6 13c ratios appears to confirm the presence of photosynthetic organisms in many greenstone belts. Stromatolites are also infrequent, but this may reflect, in part, the absence of carbonate sediments. Flat falgal11 banding is common in greenstone belts from many of the Archaean shields, and can be recognised even where the rocks are highly metamorphosed. At the present time, there is no' way of determining conclusively that the laminations are algal in origin, and it is possible that they may represent bacterial laminations similar to those found in modern geyserite. The greatest development of banded iron formations occurs in the Lower Proterozoic, but they have many features in common with the Archaean occurrences, both in their lithologies, and in their possession of similar fossils.

13. MICROFOSSILS FROM A NEWLY DISCOVERED PRECAMBRIAN STROMATOLIT1C IRON FORMATION IN WESTERN A U S T R A L I A

M.R. Walter Bureau of Mineral Resources5' Geologists from BHP recently discovered in W.A. an iron formation with a granular texture, unlike the typical well laminated iron formations of the Hamersley Ranges. The presence of detrital granules indicated a similarity with the Gunflint and Biwabik Iron Formations of the Lake Superior region, which contain stromatolites and microfossils, and led to a search for such fossils here. Unattached stromatolites (oncolites) and microfossils have been discovered in the W.A. iron formation. The microfossil assemblage is indistinguishable from that in the Gunflint and Biwabik Iron Formations, and includes Gunflintia spp., Eoastrion sp., Kakabekia sp. and Huroniospora spp. These all occur within oncolites. The filamentous forms are frequently arranged with their filaments parallel to the laminae of the oncolites, and are intertwined, and unicellular and rosett-forming forms occur as clusters within masses of filaments. These distributions indicate that most or all of these micro-organisms lived within the oncolite-forming mats, and thus were benthonic. The biological affinities of these microfossils are not always clear. Some examples of Gunflintia minuta from the Gunflint Iron Formation have been shown bv Gerald Licari and Preston Cloud to possess enlarged cells (heterocysts) which now occur only in cyanophytes (!tblue-green algae"). No such enlarged cells have been found in the Western Australian examples, and they are rare in the Gunflint. Many G. minuta may'have been filamentous bacteria. Huroniospora is probably a unicellular cyanophyte. Eoastrion is probably a bacterium, perhaps an iron- and manganese-oxidising form such as Metallogenium. The affinities of Kakabekia are unknown, although a famous extant homeomorph occurs in the soil near the walls of Harlech Castle. The microfossil assemblage is broadly comparable with present day examples of stromatolite-forming cyanophytic and bacterial mats. The preservation of all the microfossils is as iron oxide replacements and moulds and is not as good as in the Gunflint. Because of this, identification is difficult, but there appear to be at least four species in common between the W.A. iron formation and the Gunflint (Gunflintia minuta, Huroniospora psilata, Eoastrion simplex, Kakabekia umbellata). In contrast, the only possible microfossils known from the iron formations of the Hamersley Ranges are crude spheres of organic matter, some of which may be internally partitioned (these have been described by Gene LaBerge). Thus the microfossil assemblages vary with sedimentary facies. The biostratigraphic significance of the similarity of the North American and Western Australian assemblages is not clear, as very little is known about the time ranges of Precambrian microfossils. However, all of these iron-formations seem" to be of about the same age. The W.A. iron formation is older than 1100 m.y. and ovelies the Archaean greenstones and granites; consideration of the regional geology leads to the conclusion that the iron formation is similar in age to those of the Hamersley Ranges, i.e. about 2000 m.y. old. The Gunflint and Biwabik iron formations are generally considered to be 1900 m.*y. old. The fact that nearly identical microfossils assemblages assemblages occur in widely separated rock units of similar ages

encourages further investigation of the biostratigraphic usefulness of Precambrian microfossils. 0O0

* All B.M.R. contributions in this volume are presented viith the permission of the Director.

17.

MICROFOSSILS FROM THE ROPER GROUP

C.J, Peat Royal School of Mines, London A highly diverse assemblage of microfossils has been found in black shale horizons of the 1.3 x 10^ year old McMinn Formation of the Roper Group, McArthur Basin, Northern Territory, Australia. The structures found range from sijiiples spheres and filaments through microcolonies and multicellular structures to large (10 x 120um) 'fronds', presumably of algal origin. These fossils show a higher level of organisation than other microfloras of comparable age and are therefore of great evolutionary interest. A representative selection of these forms will be shown, together with a summary of results and work in progress.

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18. A REVIEW OF THE PAPAEOBIOLOGICAL RELATIONSHIP OF STROMATOLITES T O THE ORIGIN OF HIGHER ORGANISMS

W.V. Preiss S.A. Department of Mines A line of reasoning on the relationship of stromatolites to the origin and diversification of metazoa and metaphytes is reviewed. The thane was introduced by Fischer who argued front the point of view of atmospheric evolution and the early requirement of shielding from ultraviolet radiation. Early heterotrophs were thought to have evolved around "oxygen cases" in the vicinity of blue-green algae. Garrett, and later Awramik, argued that Recent stromatolites are areally restricted to unfavorable environments by grazing and burrowing animals. Awramik then correlated the apparent decline in stromatolite diversity in the Vendian with the origin of predatory metazoa. Schopf and others reviewed the major evolutionary* grades attained during the late Precambrian and leading up to the evolution of multi-cellular organisms. These events were dated as closely as possible from the fossil record. The development of exoskeletons was related to body size, mobility, feeding and symmetry. Stanley's theory is based on the ecological principle that increased diversity at a given trophic level leads to increased diversity at the next lower level. The autotrophic metaphytes divesified as soon as the heterotrophic metazoa had appeared. This was thought to explain the 200 - 300 m.y. time lag between primitive eucaryotic . floras (Bitter Springs Formation) and the multicellular organisms fo the latest Precambrian and early Cambrian. If stromatolite taxa are in any sense related to algal species, then Stanley's theory should have predicted major diversification of stromatolites after the origin of the metazoa, if these grazed on algal mats. Monty disputes the Garrett-Awramik hypothesis by pointing out (1) that in the Recent, hard resistant stromatolites ("heads") are the most restricted, (2) that many freshwater mats can cope with grazing and (3) the toxicity of cyanophytes to animals and to other algae. Their predominance in the Precambrian nay have actually prevented the evolution of higher organisms (cf. Fischer). The cause of the decline would have to be sought elsewhere. Finally, caution is urged in using data on stromatolite distributions since the majority of stromatolite taxa have been defined from the Riphean of the USSR. Further work on other continents and in beds of different age may show that some taxa are more long ranging than previously thought. oOo

iy. THE D I V E R S I F I C A T I O N OF E A R L Y METAZOANS

Richard J.F. Jenkins Department of Geology University of Adelaide The "Ediacaran" radiation of large, soft bodied metazoans is now relatively well documented from a dozen or so occurrences around the world. The event postdates late Precambrian occurrences of tillites. Fossils apparently representing an early phase of the radiation and found in England and particularly the Russian platform and Siberia occur in sequences dated at about 680 to 635 m.y. It is to this episode that the Ediacara assemblage of the Flinders Ranges probably belongs. The Cnidaria are the most diverse and abundant elements of the Ediacara assemblage (AznAu 6t/u,cto), possible supportive evidence for classical zoological ideas as to their lowly position in the animal kingdom. Initial radiations of Protostomia (?Mollusca, Annelida, Arthropoda) and Deuterostomia (?Echinodermata) had already occurred, but with the exception of the annelids, had seemingly gained little momentum. A section of the Cnidaria, the perinatulaceans, reached large size and moderate diversity; they are of interest because of their world distribution. Specimens are sometimes found with their anchoring discs still buried as in life; it is probable that different forms inhabited separate micro-environments. There seems little hard evidence for tne occurrence of trace fossils significantly earlier than the appearance of large body fossils and it is considered that these two events were essentially synchronous. Examination of the supposed pre-Ediacaran trace fossil BunyoAsLcknuA datga/inoi Glaessner suggests that it is a composite of markings occurring on two separate bedding planes; a metazoan origin thus seems improbable and it is considered to be a unique accidental set of markings made by a tethered implement moved by the current. The crawling and burrowing animals which appeared as part of the Ediacaran radiation left abundant bioturbation in sediments of appropriate facies (e.g. silty intertidal deposits). Organisms excavated horizontal penetrative (post depositional) feeding burrows, but vertical burrows were evidently absent. The petalate form PteA^diyilum Gurich which occurs in the Ediacara assemblage s.s., is found in association with other complex frond and sac-like organisms (respectively Rang da Gurich and the !T Erniettomorphan Pflug) at an apparently later stage. The interpretation of these is problematic. They occur most numerously in the Nama System of South West Africa. An occurrence in Siberia of Fte/LccLcnlum together with sac-like organisms is dated at about 610 m.y. Complex, many fronded organisms forming a major component of a late Precambrian assemblage from Newfoundland show some similarities to Rangna. Also occurring in all the carbonate phases of the Nama System are calcareous tubes (Cribricyathida) with a distinctive wall structure; similar forms occur in the Lower Cambrian of Siberia. Calcareous worm tubes occurring in the late Vendian of Siberia (e.g. AnabaniXeA sp., dating at about 580 m.y.) are also represented in the Cambrian. Chitinous SaboJULLcLLtzA sp., tubes occur in the latest Vendian and Lower Cambrian of the U.S.S.R. Early megaphytes, ribbon-like brown algae (IImdota&vua sp.) are first found in Russia in rocks dated at 595-610 m.y.

20.

On a. world-wide basis vertical dwelling burrows (e.g.SkotitkoA Haldeman) seem to have appeared at about the same time as did other trace fossils typical of the early Cambrian (e.g. RuAopkycuA Hall, PlcLgwgrnuA Roedel). Mineralised skeletons became numerous at about the same time. It is considered that the idea that these events were related to the advent of large predators requires further attention. The sudden appearance in the late Precambrian of large metazoans, together with trace fossils, provides a biological marker for the start of the Phanerozoic. A problem with this approach is that there seems to be no major geological event at the same time. Diversification of early metazoans seems to have occurred over a period of at least 70-100 m.y. before the Cambrian. Tube inhabiting organisms appeared in the latest Precambrian. Vertical burrows together with other distinctive trace fossils and mineralised shells and skeletons mark the base of the Cambrian. 0O0

AN ENVIRONMENTAL STUDY OF THE ROCKS CONTAINING THE EDIACARA ASSEMBLAGE IN THE FLINDERS RANGES R.J.F. Jenkins Department of Geology University of Adelaide

The Ediacara fossil assemblage occurs widely in the Flinders Ranges in a relatively thin unit composed of several distinctive and easily recognizable lithologies, and located near the base of the Rawnsley Quartzite of the Pound Sub-Group (proposed new nomenclature)5 Wilpena Group. The start of deposition of the Rawnsley Quartzite corresponds to a marine transgression. In the western flank of the Flinders Ranges bedding sets containing cycles of trough-cross stratified sandstones, ripple bedded sandstones with sand volcanoes, and flat laminated sandstones, apparently represent tidal deposits which faced open water, and indicate a tidal range of the order of 1.8 m. Sandstones deposited offshore occur elsewhere. The succeeding fossiliferous unit is readily mappable and is designated the Ediacara Member of the Rawnsley Quartzite. The Member contains five major lithofacies; facies A, green or maroon siltstone and fine-grained sandstone; facies B, purple or mauve, laminated siltstone and interbedded sandstone flags; facies C, light coloured, flaggy wavy bedded sandstone with silty partings; facies D, light coloured flaggy to thick-bedded sandstone with silty partings; and facies E, light coloured, mainly flat stratified thin-to-massive bedded quartzite lacking well developed parting. The Member was evidently deposited in variable tidally influenced, near-shore and shoreline environments. Features such as rhythmical and flaser bedding, interference and flat-topped ripples, winnowed coarse sand residues, abundant mudstone clasts, and rare desiccation cracks suggest that the intercalated siltstone- sandstone beds and sandstones with silty partings (facies B, C and D) were deposited mainly in sheltered intertidal flat environments. The laminated siltstones or fine sandstones of facies A are possibly of lagoonal origin. The mainly flat-bedded sands of facies E are interpreted as possibly representing parts of an offshore bar. Coarsening -upwards cycles involving the facies sequence A, C and D or the sequence B, C and D, overlain by either facies E or other unfossiliferous sandstones, are commonly developed. These cycles tend to be repetitive at any one locality. The individual cycles show distinctive minor lithological features (e.g. colour, minor grainsize variations) and can be correlated widely between localities in the western Flinders Ranges, suggesting that in this area at least, deposition occurred within a continuous (single) water body. The rich body fossil assemblage occurs chiefly in the deposits of probably intertidal origin (facies B, D and D) and for the most part appears to be composed of organisms stranded by the tide away from their normal habitat. Some organisms, such as Cowm&diiixLtte Glaessner and Wade and pennatulaceans evidently inhabited pools or channels; individuals and groups of pennatulaceans bent over by currents from their anchored life position are found occasionally. The trace fossil assemblage of the Member is in general accordance with the tidal environment suggested, but is unusual, in that vertical dwelling burrows are apparently not present.

22.

Outcrop of the Ediacara Member in the western flank of the Flinders Ranges probably represents the infill of an elongate, tidally influenced lagoon sheltered behind an arcuate offshore bar perhaps 200 km long. Although not yet investigated in detail, a similar belt of outcrop of the Ediacara Member extending between Mount Scott Range and Reaphook Hill may have also been deposited in a lagoon and bar complex, which need not have been precisely contemporaneous with the western belt. 0O0

23. PROTEROZOIC SEDIMENTARY ROCKS OF CANADA AND THE STROMATOLITE CONTROVERSY

J.A. Donaldson, • Carleton University, Ontario Studies of algal stromatolites in present-day environments have shown that gross morphology and internal lamination are significantly influenced by environmental factors, substantiating earlier conclusions based on studies of stromatolites in rocks of Proterozoic age. An opposing school of thought, primarily sustained by several groups within the U.S.S.R., maintains that at least some Proterozoic stromatolites have restricted geologic ranges as a reflection of evolutionary changes, and thus can be used for time-stratigraphic correlation. The "environmentalists" have largely ignored the biostratigraphic approach, whereas most "evolutionists" have failed to provide information useful for environmental interpretations, such as the types and relative abundance of sedimentary structures associated with stromatolites. Assessment of the environmental and biostratigraphic significance of stromatolites requires extensive collection of data sufficient to test both interpretations, from radiometrically dated stromatolitic sequences at numerous levels in the Proterozoic rock record. Until recently, biostratigraphic studies have been concerned mostly with stromatolites of Riphean age ( 1650-560 m.y.). Because most distinctive stromatolite "groups" and "forms" are reported to have ranges entirely within the Riphean, the validity of presently designated ranges for Riphean stromatolites can be tested by comparing them with stromatolites from significantly older artd younger strata. In Canada, several sedimentary sequences of Aphebian age ( A/ 25001800 m.y.) contain stromatolites that closely resemble some of the major Riphean groups; examples from the Belcher Group of Hudson Bay resemble Kussiella, Omachtenia, Boxonia, Gymnosolen, Anabaria, Inzeria, Baicalia, Tungussia,~inella and Jacutophyton. These comparisons can be validly criticised on the basis that these "similar" stromatolites have not been reconstructed according to the rigorous method of serial sectioning. However, most Aphebian stromatolites in Canada, particularly those of the Belcher Group, can be readily observed in numerous mutually perpendicular longitudinal sections and in transverse sections that are abundantly displayed in the field along joints and on glaciated surfaces. Furthermore, critical evaluation of the serial-sectioning method suggests that significant operator error can be introduced during preparation of the templates that constitute the framework for reconstructions. Moreover, biostratigraphic descriptions of stromatolites rarely provide information sufficient for reliable evaluation of stylolitic solution and other diagenetic effects that can extensively modify gross morphology, laminations, margins and textures. Intercontinental exchange and study of type material with a view toward standardizing methodology is regarded as an urgent priority for the advancement of stromatolite studies. The use of stromatolites alone for assignment of geologic age is presently unwarranted. oOo

24. D E W A T E R I N G S T R U C T U R E S IN L A T E PRECAMBRIAN MARINO GROUP SEDIMENTS, H A L L E T T COVE, SOUTH A U S T R A L I A

V.A. Gostin, Dept. of Geology, University of Adelaide, S. Aust. J.A. Donaldson, Dept. of Geology, Carleton University, Canada. Thin tabular sub-vertical silt-free zones within poorly sorted sandstones represent zones of water excape during and following very repid deposition. These may bifurcate upwards or downwards and may be associated with sand mounds (similar to munroes) on the bed surface. Structure similar to these subvertical escape channels have been misinterpreted as organic burrows in some ancient sediments.

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25. THE DEPOSITIONAL ENVIRONMENTS OF THE TINDELPINA SHALE MEMBER OF THE TAPLEY HILL FORMATION (LATE PROTEROZOIC), ADELAIDE GEOSYNCLINE - A GEOCHEMICAL STUDY

J. Sumartojo Department of Geology and Mineralogy, University of Adelaide One of the best stratigra!phic markers of the Adelaidean (Late Proterozoic) rocks in the Adelaide Geosyncline is the Tapley Hill Formation. The basal member of this formation which is called the Tindelpina Shale Member, is the object of a geochemical study. The mineralogy of the Tindelpina Shale is very simple. X-ray diffraction and microsopic examination indicates the presence of quartz, muscovite, and chlorite. Feldspars, calcite, iron oxides, carbonaceous material, and pyrite occur in minor amounts. The quartz-muscovite-chlorite assemblage shows the metamorphic character of the low greenschist-facies of this stratigraphic unit. The concentrations of fourteen trace-elements (Ga, Rb, Sr, As, Ba, B, Cr, Cu, Li, Ni, Pb,V, Zn, and Zr) were analysed in about 120 samples giving the following results for their average abundances in parts per million (standard deviations are in parenthesis): Ga 19 (4), Rb: 146 (39), Sr: 160 (178), As: 14 (10), Ba: 858 (192), B: 184 (71), Cr: 83 (37), Cu: 30 (18), Li: 22 (9), Ni: 40 (17), Pb: 21 (12), V: 191 (55), Zn: 62 (28), Zr: 225 (33). Based on the contents of B and Ga-B-Rb ratios it is suggested that the Tindelpina Shale is of shallow marine origin. This suggestion is substantiated by the Fe/Mn ratios of those two rock units. Silicate analyses of about 150 samples of the Tindelpina Shale result in the following percentages (standard deviation is between brackets): Si0 2 : 59.77 (7.88), A1 2 0 : 13.65 (2.49), Fe 2 0 3 : 5.45 (2.85), MgO: 3.75 (1.72), CaO: 4.01 (5.44), Na 2 0: 1.17 (0.65), K 2 0: 3.35 (0.85), Ti0 2 : 0.95 (0.19), P 2 0 5 : 0.18 (0.07), MnO: 0.07 (0.10), Loss on ignition 7.31 (4.67), Free quartz: 40.40 (8.64). Their composition suggests the granitic nature of the parent rocks. Trend surface analysis based on the Niggli parameters (si, al, fm, alk, and k) K 2 0/Na 2 0, Vogt-index, quartz, zircon, adjusted boron, and copper suggest the source material for the Tindelpina Shale lay to the west of the Adelaide Geosyncline, in the Gawler Block. \ minor source contribution came from the northeast, the Willyama Block. The general trends for the variables used in trend surface analysis conform with the main north-south elongate shape of the Adelaide Geosyncline. A correlation matrix of 25 variables (Si, Al, Fe, Mg, Ca, Na, K, Ti, P , Mn, Ga, Rb, Sr, As, Ba, B, Cr, Cu, Li, Ni, Pb, V, Zn, Zr, and quartz) indicates the associations of most of the trace elements with the main phyllosilicate fraction (muscovite). Factor analysis of the 25 variables result in six factors which account for most of the data variance. Those factors are: provenance/sorting, Eh-pH, biogenic activity, salinity, diagenesis, and proximity to source rock. oOo

26. F A U L T I N G CONTEMPORANEOUS WITH UMBERATANA GROUP SEDIMENTATION (LATE PRECAMBRIAN), SOUTHERN F L I N D E R S RANGES, SOUTH A U S T R A L I A

P. S. Plummer Department of Geology University of Adelaide Adelaidean interglacial sedimentation within the Umberatana Group was greatly influenced by contemporaneous activity along Spring Creek Mine Fault. A study of the stratigraphy on either side of the fault and of the fault zone has revealed a period of contemporaneous faulting, and a subsequent phase of downwarping. The interrelationship of these movements with a regressive-transgress ive-r egress ive marine cycle resulted in palaeoenvironments ranging from shallow marine, through intertidal and supratidal, to possibly floodplain.

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27. THE TECTONIC HISTORY AND SIGNIFICANCE OF RETROGRADE SHEAR ZONES AT BROKEN HILL

M.A. Etheridge Dept. of Earth Sciences Monash University Some new Rb/Sr isotope measurements on a retrograde shear zone (R.S.Z.) at Broken Hill are presented, together with a summary of other published and unpublished information relevant to their role in the tectonic history of the Willyama Complex and the overlying Proterozoic rocks. It is concluded that the localized retrograde metamorphism was initiated during the second "highgrade" deformation, and that most deformation since that time has been concentrated within the R.S.Z.Ts. The general role of mylonite (shear) zones in the tectonic history of high metamorphic grade areas is discussed, and the problem of determining their ages is commented upon. It is concluded that palaeotectonic interpretation of such areas must include assessment of the age and role of such zones.

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28. S T R U C T U R A L GEOLOGY OF S I L L I M A N I T E SCHISTS OF THE N O R T H E R N LEASES, BROKEN H I L L , N.S.W.

W.P. Laing Department of Geology arid Mineralogy University of Adelaide Structural analysis of the leases held by North Broken Hill limited, north-east along stride from the Broken Hill line of lode, has added to the understanding of regional structures in the Willyama Complex, The area is bisected by a major structural discontinuity, the north-east trending Globe Vauxhall shear zone. Three generations of folding (apart form folds related to retrograde shear deformation) are recognised at present. The first two are each associated with thr formation of high grade, sillimnite-bearing schistosities, repectively Sj^ and S 2 . A third generation of folds deforms both S]_ and S2. Fi folds occur in limited areas of the Willyama Complex, and are nob observed in the Northern Leases. Here S l 5 which is the more general expression of Fj_ deformation, is parallel to bedding. F2 folds account for most of the mapped regional structure, and most mesoscopic folds appear to be of this generation. They fold Si, and may or may not possess an axial plane schistosity S2. In the northern part of the Northern Leases F2 folds generally plunge steeply north-east, while in the southern and western part mosr F2 folds plunge gently to moderately south-west. F3 folds are open structures, generally lacking an axial plane schistosity, and fold Si and S2. They plunge consistently to the south-west. F3 folds have been identified only on a mesoscopic scale, and it is not known whether they occur on a more regional scale. Their similarity to many of the south-west plunging F2 folds makes distinction difficult between F3 and F2. Three problems in the structural geology of the Northern Leases are of particular interest: 1) Attention has been drawn previously to variations in plunge of F2 folds on a regional scale. In the Northern Leases F2 fold plunges vary systematically on all scales. The mesoscopic data, particularly the relationship between S || Sn and S2, indicates that the plunge variation may result from the superposition of F 2 folds on a non-planar S || S-j_ form surface. This implies a ph^se of warping between Fj_ and F^- Limited evidence suggests a systematic, though cross-cutting, relationship between warp axes and S2 orientation, indicating that the warping may be an early phase of F2, prior to the development of S2. 2) There is a marked regional change in attitude of the F2 axial surface across the Globe Vauxhall shear zone, from upright on the north-western side, to inclined on the south-eastern side. This may be a result of differences in progressive deformation of F2 on each side of the Globe Vauxhall zone. In the south eastern block F 2 may have "overlapped" into F 3 , producing similar orientations of axes and axial surfaces, whereas in the northwestern block F2 and F3 may have been separated temporally by a

distinct change in orientation of the axial surface. The Broken Hill orebody lies within the south-eastern block, in the Tlinclined S2 11 regime, 3) A strong sillimanite lineation generally plunges gently south-west, irrespective of the plunge of F2 folds. Rare outcrops in the north-east show instead, a steep north-easterly plunging sillinanite lineation, parallel to F2 axes, in an area characterised otherwise by high angle relations between the two elements. The gentle south-west plunging lineation is L»2, formed by rotation of a north-east plunging Li lineation, lying in the S II Si form surface. These observations ar§ explained in terms of the non-planar S || Si form surface suggested above. The greater directional stability exhibited by L2 compared with F2, suggests that the mineral lineation rather than the fold axes, reflects the maximujii extension direction.

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30. THE STRUCTURE OF THE MT. FRANKS AREA, WILLYAMA COMPLEX, NEW SOUTH WALES.

R.A. Glen, Department of Geology and Mineralogy University of Adelaide. Early workers in the Broken Hill area suggested that the regional structure consisted of a series of open south plunging synforms separated by tight but non-traceable antiforms. The Mt. Franks area in the western part of the Willyama Complex covers the area between two such synforms - the south-west plunging Mt. Robe synform to the west and the Parnell synform to the east. This central area consists of andalusite schists in contrast to the Parnell synform and parts of the Mt. Robe synform which consist of sillimanite schists. Three north to north-east trending domains have now been defined in this area: 1. a western domain containing beds dipping,moderately west. Andalusite schists in this domain are underlain by sillimanite schists (the Robe schists) which are exposed in a later second generation, D2, anticline. The mapped isograd is parallel to the bedding trace and is folded around the D2 fold. 2. a central domain in which beds dip steeply east. In this domain, andalusite schists are separated from quartzites with interbedded andalusite schists by the Mt. Franks retrograde zone. 3. an eastern domain containing beds dipping gently east. Sillmanite schists (the Parnell schists) dominate this domain and occur in the western limb of the Parnell synform. The boundary between the western and central domains is anticlinal in nature with both bedding and facing changing from westerly to easterly dips. The absence of a hinge is attributed to a slide. This boundary is located wholly within andalusite schists. The boundary between the central and eastern domains is defined by the Apollyon retrograde zone. In their present structural position, the Parnell schists overlie the andalusite schists, but it is considered that they are probably equivalent to the Robe schists and that there has therefore been a component of vertical movement on the Apollyon fault. Recent structural work in the sillimanite schists of the Parnell synform and around Broken Hill has identified two sillimanite schistosities with a third locally developed in addition to retrograde sohistosities restricted to narrow zones. S^ is a tectonic fabric often parallel to bedding (Sq)• From a comparison with the Parnell schists, the schistosity parallel to Sg in the Robe schists is also regarded as Si. The andalusite. schists overlying the Robe schists in the western domain also have a schistosity parallel to bedding; this is also regarded as Si,. In the central domain, however, bedding has changed orientation so that Si is at a high angle to So. Several Fi fold pairs with Si as an axial plane fabric occur in this domain. In the eastern domain, Si is again layer-parallel. The change is Si/So relationship is regarded as a large Fi fold with a high interldmb angle.

31.

In contrast to the two periods of syntectonic high grade metamorphism in the sillimanite schists, the andalusite schists show the following events: H^

high grade metamorphism (andalusite and biotite) M2

folding accompanied by the formation of a schistosity (muscovite and elongate quartz) and the partial retrogression of andalusite to sericite and quartz•

D2 M3

folding and crenulation of S^ with the local development of a new schistocity (muscovite and quartz)

local folding and kinking of S2.

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32. B A S E M E N T - C O V E R RELATIONSHIPS IN T H E O L D B O O L C O O M A T A A R E A , O L A R Y PROVINCE OF S O U T H A U S T R A L I A

R.G. Wiltshire School of Applied Geology South Australian Institute of Technology In the Olary Province^, Rrecambrian schists and gneisses of the Willyama Complex crop out in three blocks whose eastern margins are unconformably overlain by and whose western margins are in faulted contact with Adelaidean metasedijnents. At Old Boolcoomata three periods of folding are evident in the Willyama Complex schists. First and second generation folds are tight with schistosity parallel to their axial planes. Third generation folds are close with a crenulation foliation parallel to their axial planes. The macroscopic structure in the Willyama Complex is an easterly plunging third generation synform, imposed on first and second generation folds giving a modified type 2 interference outcrop pattern. The Adelaidean cover rocKS of this area belong to the Umberatana Group and consist of interbedded siltstones and quartzites with a proininent tillite horizon and minor dolomite lenses. This sequence has been folded into a close easterly plunging syncline , with slaty cleavage in the siltstones parallel to the axial plane of the syncline. The northeastern limb of this fold is faulted out by the MacDonald Fault, bringing the Willyama Complex rocks into contact with the Adelaidean sequence. Adjacent to the McDonald Fault there is local development of crenulations and some mesoscopic second generation folds in the Adelaidean siltstones . The style and orientation of the major syncline in the Adelaidean rocks suggests that folding of the cover took place at the same time as the third generation of folding in the Willyama Complex.

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RECOGNITION OF THREE SUPERPOSED TECTONIC PHASES IN THE NORTHERN PART OF THE FLINDERS RANGES

J.P. Richert Aquitaine Australia Minerals

Three superposed tectonic phases were individualized during a one month survey of the Proterozoic of the Mount Painter block (Northern Flinders Range) in 1974. The first phase is characterized by isoclinal folds, a general metamorphic foliation, flat lying structures, and three thrustsheets including material of the basement. The second phase has formed E-W concentric upright folds which refold the first phase fold axis and lineations giving superposed tectonic patterns. The third phase, mainly of brittle deformations, is related to large strike slip faults. A metamorphism of the green schist facies is contemporaneous with the first phase of isoclinal folds. It is due to late stage tectonic effects and repeated metamorphism of the metamorphic rocks of the basement can be observed. These observations are important for the general tectonic interpretation of the Proterozoic formations overlying the basement of the Mount Painter block.

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3b. COMMENTS AND SPECULATIONS CONCERNING THE POLYGENETIC BRECCIAS OF THE SOUTH AUSTRALIAN DIAPIRS

Walter D. Smith Carpentaria Exploration Co. Ltd.

The present characteristics of the diapirs are attributed to a composite four phase process as summarised below. The first three of the phases are primary in character and the fourth is secondary. FIRST ^ PHASE Fault block uplift occurred locally in a discrete and differential .manner during Adelaidean sedimentation as a result of isostatic crustal. adjustment due to upper mantle movements in the floor of the depositional area. Movement in the diapirs during this phase was upward. SECOND PHASE Differential gravitational sliding of incompletely lithified sediment overlying the uplifted fault blocks occurred during Adelaidean sedimentation as a response to palaeoslopes developed, siesmic agitation and (perhaps) over-pressured pcre water. A variety of preconsolidation pull-apart breccias and slump features developed, extending beyond the edges of the causal fault block into the surrounding sediments. Movement in the diapirs during this stage was essentially lateral. The first and second phases were not single episodes but repetitive processes of the types described, acting over long periods of time, THIRD PHASE Preferential deformation of the already brecciated zones, due principally to the Delamerian Orogeny, caused mobilisation which was responsible for injection, upwarp features, and retexturing of breccias. Movement in the diapirs during this phase was essentially upwards. FOURTH PHASE Surficial brecciation and alteration due to hydrological and weathering agencies near old and present landsurfaces (mostly Tertiary and Quaternary) obscured the surface aspect, limiting availability of clear evidence relative to the primary origin of the diapirs. Movement in the diapirs during this phase was downwards. Each of the four phases may be developed to different extents in any individual diapir, thus accounting in large part for their extreme variability.

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35.

TOWARDS A STRATIGRAPHY OF THE ARUNTA BLOCK

R.D. Shaw and A.J. Stewart v Bureau of Mineral Resources, Canberra The Arunta Block is the roughly triangular body of basement rock that crops out between the Amadeus and Georgina Basins, in the southern part of the Northern Territory; it extends for about 1000 km east-west and 400 km north-south. The Block consists of Early Proterozoic (or older) sedimentary and volcanic rocks which were complexly deformed, metamorphosed, and copiously intruded by granite in early Middle Proterozoic time. Later events include dolerite dyke intrusion, migmatizaticn in the southern area during the Late Proterozoic, and widespread faulting and nappe formation with associated retrogressive metamorphism and pegmatite emplacement in the Late Palaeozoic. Rock-types in the Arunta Block are many and varied, but for simplicity of presentation the metamorphic rocks can be grouped into three units, while the granites constitute a fourth. Representatives of the three metamorphic units are present in most parts of the Arunta Block, but it is not yet known whether they are chronostratigraphic as well as lithologic correlates. The first unit consists chiefly of granulites of pelitic, felsic, and mafic compositions, with smaller quantities of compositionally similar gneisses and calc-silicate rock of the amphibolite facies. It is everywhere in discordant contact (fault or possible unconformity) with the second unit, which consists of greenschist facies slate, schist and calc-silicate rock grading in places into higher-grade felsic, pelitic, and calcareous gneisses, with associated mafic flows or silxs. In some places, such as the Harts Range, the second unit consists of the higher-grade rocks alone. As a whole, the second unit contains a much higher proportion of mature sediments (now metamorphosed) than does the first unity and conversely, a lower proportion of meta-igneous rock. The second unit is unconformably overlain by the third unit, which is characterized by even greater sedimentary maturity, and comprises quartzite, shale, lenses of limestone and dolomite, and sills of porphyry, including a lopolith 20 km across. In the Alcoota and Alice Springs areas, the second and third units are metamorphosed to the amphibolite facies, whereas in the Reynolds Range the same two units pass along strike into gneiss and granofels of the granulite facies. The fourth unit comprises large batholiths of granite, including even-grained or porphyritic, and massive or foliated varieties. Smaller plutons unrelated to the granites include diorite sills and bosses, and ultrabasic complex with kimberlitic affinities, and a carbonatite. The field relations suggest that the first unit may be the oldest in the area, but the evidence is not conclusive. The second and third units are tentatively correlated with the Warramunga and Hatches Creek Groups (respectively) of the Tennant Creek Block.

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AN INTERPRETATION OF THE TECTONICS OF C E N T R A L AUSTRALIA R.D. Shaw and W. Antiloff Bureau of Mineral Resources Canberra The southern Arunta and Musgrave granulite belts extend as easterlytrending blocks for at least 550 k., and are flanked by Upper Proterozoic to Palaeozoic sedimentary basins, namely the Ngalia, north and south Amadeus, and Officer Basins, There is good correlation between granulites in zones of upfaulting and major positive gravity anomalies, and between the sedimentary basins and negative anomalies. The sedimentary basins cannot, by themselves, account for the negative anomalies. Consequently large troughs containing older metasediments and a high proportion of granite are postulated to underlie the basins. The granulite belts are truncated by two northerly-trending gravity lineaments interpreted to be major zones of crustal dislocation. Similar granulite belts occur elsewhere in Australia, notably at Fraser Range, Western Australia, where they are flanked by granitic rocks. The first recognizable episode in the tectonic history of central Australia -was the formation of major troughs. The base of the troughs subsided to a level where partial melting occurredcausing granitegranulite separation. In mid-Proterozoic time, the granulites were upfaulted through a layer of granitic rocks, and established zones of weakness in the crust which provided loci for later faulting. The cycle of subsidence and uplift was repeated at least once more. Subsidence of both granites and granulites took place over much of central Australia, resulting in the base of the granitic layer reaching a depth ^ near the Conrad Discontinuity. Subsidence was accompanied by deposition of up to 10 km of sediment. Finally, the granulites were upfaulted at least 5 km at 600 m.y. to form the Musgrave Block, and at 400-300 m.y. to form the southern and northern Arunta Blocks. From analysis of the gravity anomalies in both central Australia and at Fraser Range there appears to be a compositional discontinuity at a depth of about 20 km below which major density irregularities do not persist. The following tectonic model is suggested to explain this situation: During each subsidence a compositional discontinuity formed in the crust as a result of granite-granulite separation. Following each uplift, the granulite blocks became stabilized in the upper crust whereas the lower crust was sufficiently hot and weak to cause the inhomogeneties produced by the upfaulting to be largely annulled by chemical diffusion, plastic flow, phase changes, and gradual sinking of dense material. The physical discontinuity between a relatively homogeneous lower crust and an inhomogeneous upper crust would have been reinforced by contined granite intrusion because granites would tend to accumulate under the more solid upper crust. This compositional and physical discontinuity is considered to represent the Conrad Discontinuity. We postulate that in central Australia the Conrad Discontinuity represents a horizon below which major -irregularities in bulk rock density are not maintained. —oOo

37. PRESENT STATUS OF G E O C H R O N O L O G I C A L R E S E A R C H IN THE A R U N T A BLOCK, N.T.

L.P. Black Bureau of Mineral Resources, Canberra Current Rb-Sr isotopic data indicate a complex geochronological history for the Arunta Block, The earliest clearly documented event is a widespread granulite facies metamorphism at about 1800 m.y. A regional metamorphic event of amphibolite grade affected the area around 1700 m.y.: this apparently caused retrogression and consequent resetting of total-rock ages in some granulite localities. Data from an ultrabasic complex in the Georgina Range indicate an intrusive age of about 1200 m.y. A previously reported thermal event, which produced migmatisation in the Ormiston Gorge area, has been further documented at approximately 1100 m.y. Granite emplacement in the Arunta Block has occurred frequenlty from 1800 to YDOO m.y. The Palaeozoic Alice Springs Orogeny has severely limited the usefulness of K-Ar and Rb-Sr mineral studies in the area since it has often masked the ages of earlier geochronological events. It appears that this orogeny was sufficiently intense to produce local melting in the Huckitta Dome area of the Harts Range.

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38. METAMORPHISM IN THE NORTHEAST ARUNTA BLOCK, N.T.

S.K. Dobos School of Earth Sciences Macquarie University The Jervois Range area lies 260 km. ENE of Alice Springs N.T., and contains the major northeasternmost exposures of the Arunta Block. The rocks within this area exhibit a wide range of compositions, the major types being pelitic, felspathic, ferroan and ferro-pelitic schists and gneisses. Orthoamphibolites of both extrusive and intrusive origins are also common, and the sequence includes lesser amounts of carbonate and calc-silicate rocks. The above rocks were folded and metamorphosed during regional tectonism which was closely followed by the intrusion of granitic rocks dated at 1800 m.y. Mineralogical, chemical and isotopic data suggest that these granites were formed by partial melting of an igneous body or bodies of andesitic composition rather than by the anatexis of metasedimentary rocks. Apart from minor metasomatism, not necessarily associated with the intrusion of the granitic rocks, there is no evidence of contact metamorphism in the schists, gneisses and amphibolites. The marbles and calc-silicate rocks however, did suffer recrystallization and metasomatism in proximity to some intrusions, and were hosts to widespread but largely uneconomical tungsten mineralization. The pelitic rocks, originally calcium-poor shales and mudstones of probable marine origin, have been metamorphosed to assemblages of quartz + muscovite + magnetite + biotite + andalusite cordierite +_ chlorite; in the highest grade areas, sillimanite (fibrolite) is the stable aluminosilicate; the chlorite in these assemblages is not always retrograde. As the iron content increases, these rocks become ferro-pelites with assemblages consisting of quartz + magnetite + almandine + chlorite ± staurolite ± andalusite ± biotite. Metamorphosed banded iron formations are represented by the assemblage quartz + magnetite + spessartine + chlorite ± staurolite. The amphibolites are represented by hornblende + plagioclase ± epidote ± quartz assemblages and though metamorphism has obscured much of their finer chemical characteristics, the parent basalts are thought to have had oceanic tholeiitic affinities. Regional metamorphism of the carbonate - calc-silicate group has produced common hornblende and garnet-diopside marbles and quartzgarnet-diopside-epidote parageneses. Wollastonite is restricted to two localized shear zones in which the mobility of metasomatic components was relatively high. The migration of these fluids through the rock effectively removed CO 2 and diluted its molefraction in the vapour phase, allowing the formation of wollastonite. Consideration of mineral compositions and their parageneses in the Jervois Range area indicates a peak of metamorphism in the amphibolite facies at approximately 560°C 3 kb. This in effect calls for a high geothermal gradients, and partial melting of appropriate metamorphic rocks is to be expected in regions of higher temperatures.^ The presence of granitic rocks differing isotopically and chemically from those already mentioned may be indicative of such anatexis. The metamorphism at and to the west of Jervois Range is of the low pressure, andalusite-sillimanite type and contrasts sharply with the higher pressure kyanite-sillimanite type found to the south and south-west in the Valley Bore - Harts Range areas. 0O0

39. BASEMENT AND COVER RELATIONS ON THE NORTHERN MARGIN OF THE AMADEUS BASIN, CENTRAL AUSTRALIA

R.W. Marjoribanks Department of Geology University of Adelaide A major, linear, west-trending deformed zone (the Redbank zone), 350 km long and at least 20 km wide can be identified within the Arunta Complex immediately north of the Amadeus Basin. The marked linearity of this zone and of the coincident gravity anomaly probably result from thrustfault movement during the Palaeozoic Alice Springs Orogeny. However, in the Ormiston area there is evidence that the zone originated at least 1800 m.y. ago and has acted as a major crustal feature controlling orogenic events dated at circa 1600 and 1100 m.y., as well as the circa 4-00 m.y. Alice Springs Orogeny. In this area the latter event affected the Arunta Complex Basement and the overlying Proterozoic and Lower Palaeozoic sediments. Uplift to the north took place along re-activated, steep north-dipping thrusts within the Redbank Zone. These faults penetrated the Heavitree Quartzite - the basal unit of the cover sequence to drive wedges of basement, with attached veneers' of Heavitree Quartzite for up to 20 km into the overlying Bitter Springs Formation. These nappes did not reach the surface or penetrate formations above the Bitter Springs. Accompanying nappe emplacement the Basin to the south rapidly deepened to receive a 3000 m thick wedge of syn-orogenic mollasse sediments. Gravity, sedimentary and structural features combine to suggest that the major Alice Springs Orogeny movements reached their maximum in the Ormiston area - the central part of the northern Margin of the Amadeus Basin. Other nappe complexes, developed on the northern margin of the Basin (particularly the Arltunga Nappe Complex), may have formed prior to the Alice Springs Orogeny. It is suggested that the Alice Springs Orogeny served to obscure the effects of this earlier tectonism. oOo

40. A REVIEW OF THE METAMORPHIC AND IGNEOUS GEOLOGY OF THE TOMKINSON, MANN AND MUSGRAVE RANGES

R.L. Oliver Department of Geology University of Adelaide Predominant throughout the Tomkinson, Mann and Musgrave Ranges are felsic and mafic layered granulites *x and cross cutting mafic and felsic igneous rocks of charnockitic aspect. The layered felsic granulites are thought to represent pelitic and quartzofeldspathic geosynclinal sediments, but the intercalated mafic varieties are probably meta intrusives. Felsic compositions are by far the most abundant throughout the whole region. It has been suggested that ortho-granulites in the Michael Hills area of the west Tomkinson Ranges constitute a pre-layered granulite basement but, elsewhere such a basement is not evident. The cross cutting mafic igneous intrusions comprise the Giles Complex and consist of a number of sheet-like bodies in which stratification due to crystal settling is characteristic. "Giles" mafics and ultramafics in the east Tomkinson Range were originally emplaced into granulite terrain at pressures corresponding to the lower crust but, to the west, evidence of deep emplacement is lacking and presumed shallow intrusions may indeed by genetically related to basic volcanic extrusives of the Tollu Group. Eastward, in the Mann and Musgrave Ranges, mafics of Giles Complex affinity outcrop less abundantly and are virtually unstudied. There is little doubt that the Giles magma is derived from the mantle, despite the relatively high Srs7/Sr86 ratio of .706-.708. Anorthosite, which likewise intrudes the granulites, is also of probable mantle origin. A contrasting (to the above) view of Tollu-Giles relationships holds that the volcanics are cut by, and thus predate, the Giles intrusions; and yet a third opinion emplaces the Giles mafics 100 million years before the extrusion of the thus unrelated Tollu Group. In addition to the voluminous mafic igneous rocks, as indicated, there are large intrusive granitic and granodioritic bodies (e.g. almost the entire Mann Range) and, in the west Tomkinson Range, abundant representatives of such compositions in the extrusive Tollu Group. Most likely, the granitic and granodioritic bodies originated by anatectic fusion (and subsequent differentiation?) of the layered granulites. It seems possible that the felsic components of the Tollu Group (if not the mafics as well) were similarly derived, though it has been suggested that these, also, are

* The word "granulite" is here used to mean a rock crystallised in the granulite facies. x

The word "charnockite" is used to include hypersthene "granite" and norite and intermeciate rock types.

derivations of the Giles magma. High potassium values, common to both layered felsic garanulites on the one hand, and granites and extrusives on. the other, high initial Sr ratios of .708-. 718 of the granites and extrusives, and migmatitic structure in many of the granites and granodiorites, seem to support the anatectic derivation of the felsic intrusives and extrusives. Many of the granitic bodies are charnockitic, inferring crystallization from a dry magma, thus adding support to the concept of these rocks crystallising from fused granulite metasediments. Throughout the region, layering in the granulites, accompanied in appropriate rocks by a parallel schistosity, is axial plane to mesoscopic Fj_ folds which developed prior to emplacement of the Giles magma. Fj_ folding was presumably associated with the crystallisation of the granulites which detailed analysis shows to have been under pressures of 8-12 Kbrs and temperatures of 800-1000°C in the eastern Tomkinson and Musgrave Ranges and probably the Mann Range. Water pressure was undoubtedly less than load pressure. The date of this metamorphism is variously held to be between 1650 and 1250 million years ago. Emplacement of the "Giles" magma is thought to have taken place between 1250 and 1150 M.Y. , and there is abundant evidence that the formation and crystallisation of the granitic and granodioritic melts (from anatexis) occurred in the vicinity of 1100 M.Y. High temperature metamorphism of some of the Giles mafic bodies, probably under PT conditions similar to those above, resulted in the formation of localised gabbro-gneiss zones within these bodies, and possibly also in the development of a weak foliation in some of the acid charnockitic masses of deep-seated origin. Subsequent meso- and macrofolding, in the Tomkinson Range area, deformed both the layered granulites and a number of the "Giles" intrusive bodies and is responsible for the present areal distribution of these rocks. Fold axes trend E-W with variable plunge. Intensification of this folding appears to have led to fracturing along E-W trending faults and shears. In the Tomkinson Range such faults appear to be near vertical but eastward, in the Mann and Musgrave Ranges, the faults, or thrusts, are gently inclined. In the east Tomkinson Range the Hinkley fault cuts, and thus post dates, mafic rocks of the Giles Complex; whereas in the Musgrave Ranges, similar though fewer, mafic intrusives are roughly aligned along, and thus appear to postdate, the Woodroffe Thrust, although post mafic deformation in the plane of the thrust also occurs. Throughout the region, movement along faults and shears was accompanied by localised retrograde metamorphism. Characteristic features are the rimming of nearly all ferromagnesian minerals and opaques by granular concentrations of garnet. In the Musgrave Ranges, movement along the Woodroffe Thrust and the Davenport Shear have resulted in the juxtaposition of slabs of terrain metamorphosed under different (granulite, amphibolite and

42.

transitional facies) PT conditions. Amphibolite Facies terrain to the north of the Woodroffe Thrust is thought to represent basement to Granulite Facies meta-cover rocks; but, in the Mann Range, rocks, lithologically similar to those north of the Woodroffe Thrust in the Musgrave Ranges, appear to intrude charnockitic granite and also mylonitic rocks resembling those of the Woodroffe Thrust itself. The period of faulting, which presumably accompanied uplift, facilitated the emplacement, over a prolonged period, of numerous basic dykes and minor intrusions, criss-crossing, in some cases schistose and re-crystallised and/or folded and, in other cases, virtually unmodified. The dykes represent the last phase of igneous activity in the region, though in the west Tomkinson Ranges, volcanics of the Tollu Group continued to be extruded "post Giles" and may be, in part, contemporaneous . 0O0

43. A N I N T E R P R E T A T I O N OF THE E V O L U T I O N OF THE M U S G R A V I A N OROGENIC CYCLE, EASTERN M U S G R A V E RANGES C E N T R A L A U S T R A L I A

C.H.H. Conor South Australian Department of Mines Adelaide Australian geology has been controlled by .deep primal trans-continental structures. Two dominant set trend WNW and NE. The former largely controls the shape of the Archaean Blocks in Western Australia. The latter is more important in its influence over Lower Proterozoic basement of eastern Australia. The Musgrave Block occupies a central position at which the dominance of the structures changes from WNW in the west to NE in the east. The material forming the Musgrave Ranges was deposited in a Lower Proterozoic trough. The sediments were mainly arkosic but with frequent intercalations of aluminous and mafic clays, pelitic limestones and ferruginous cherts. At depth the sediments were isoclinally folded, dehydreated and converted into high grade metamorphics. This event was roughly contemporaneous with the Kimban Orogeny of,Eyre Peninsula (1,800 m.y.b.p.). A second tectonic event, the Musgravian Orogenic Cycle, is recognised from detailed mapping of the 2,700 sq km comprising the EATERINGINHA 1:100,000 sheet area. The Musgrave Block was uplifted, flanked to the north and south by the Tproto-TAmadeus and Officer Basins. Deformation within the Block was initially expressed by folding, but, as plasticity decreased the underlying primal trans-continental structures were transmitted by faulting into the metasedimentary mass. THE MUSGRAVIAN OROGENIC CYCLE The gneisses forming the original (Kimban Fl) Isoclines were refolded during the early part of the Musgravian Orogenic Cycle. On EATERINGINNA the F2 folding in the east takes the form of a broad arcuate structure convex easterly (F2a). Parasitic geniculate folds (F2g) occur centrally withing the main structure, and vertical warping of the Fl axes is evident (F2W). Although the gneisses may be generally considered to occupy a niche transitional to Amphibolite and Granulite Facies, their metamorphic state is not homogeneous and provides evidence of progressive metamorphism. Intrafolial movements have resulted in local recrystallisation of the T Kimban1 metasediments so that augen gneisses or similar drawn-out textural forms occur associated with F2a trends. TSchist zonesT are developed associated with some F2g folds. Textural and mineralogical differences occur across the Marryat Fault Zone, so that there is a tendency for garnet to substitute for cordierite south of the^fault zone. Migmatites contain a hypersthene-garnet mineralogy similar to the parent gneisses and commonly are the loci for sulphide concentration. Granitoids have developed by anatexis. They are the Kulgeran granitoids rediometrically dated at 1,100 m.y. Since some occur as WNW en echelon dykes that show little sign of folding, anatexis occurred near the end of F2 folding when the Block was becoming brittle. In the Eastern Musgraves the main granitoid masses are spatially related to the Ferdinand Lineament. It is suggested that heat, introduced up fractures by volatiles or mafic magma , raised the temperature of the countryrock adjacent to faults so causing local fusion. The Kulgeran granitoids vary from granite to granodiorite and from syenite to mangerite. Hypersthene and clinopyroxene may co-exist with the more

44.

usual hydrated mafics. Mafic intrusives are equated with the Giles Ultrabasic Complex and are mainly located along the primal WNW trend. The earliest pulse preceded anatexis and is represented by folded peridotitic dykes whose reaction with the host metasediments suggest crystallization at high temperature. Post anatectic dykes show little obvious sign of folding or metamorphism, although, the earlier representatives are dislocated along structures axial to the F2g folds. Intrusion ceased following a stage of intense shattering and mylonitization Final uplift is marked by reworking of pre-existing faults initially characterized by an epidote-quartz-potash feldspar mineralogy, and finally by simple brecciation. The presence of sediments unconformably overlying the crystalline basement of the eastern Musgrave Block indicates that uplift liad ceased and the Block had become stable by mid-Adelaidean times. 0O0

T H E PROTEROZOIC SEDIMENTS AND V O L C A N I C S OF THE MUSGRAVE BLOCK

R.B.Major South Australian Department of Mines Adelaide, Relatively unmetainorphosed Proterzoic sediments and volcanic rocks occur on and around the crystalline basement of the Musgrave Block in Central Australia. The oldest of these rocks are in Western Australia and the Northern Territory. They are the Dixon Range Beds, Bloods Range Beds and the Mount Harris Basalt. The latter two units are, in part, metamorphosed and granitised and are then seen to be gradational with the Olia Gneiss of the northern Musgrave Block. «

Along the northwestern part of the Block these older rocks are overlain by the Dean Quartzite which is conformable to the west and unconformable to the east. The Dean Quartzite is the basal unit of the Proterozoic succession in the southwest part of the Amadeus Basin which lies to the north of the Block. There is some disagreement in literature as to the stratigraphic position of the Dean Quartzite e.g. it may be part of the Olia Gneiss. The Dean Quartzite, the overlying Pinyinna Beds and the underlying tl'iree sedimentary and volcanic units have all been metarmophosed (at least in part) by faulting (thrusting) during the Peterman Ranges Orogeny which has been dated at 600 million years. On the southwestern part of the Musgrave Block in Western Australia, is a sequence of unmetajnorphosed volcanics (acid and basic) and interbedded clastic sediments, together with acid volcanics in areas of couldron subsidence. This sequence (the Bentley Supergroup) is younger than the crystalline basement and older than Upper Proterozoic (Sturtian) glacial sediments. The youngest unit of the Supergroup is the Townsend Quartzite, which, with its lateral equivalent in South Australia (the unmetamorphosed Pindyin Beds), are tentatively equated to the Dean Quartzite. Along the southern and eastern margins of the Musgrave Block are clastic sediments of Upper Proterozoic age with representatives of the Burra, Umberatana and Wilpena Groups of the Adelaide System. These rock types include tillites, basic volcanics, clastic sediments, carbonates and oolitic chert. The chert also contains stromatolites. Representatives of the Adelaidean Groups are found in the Amadeus Basin. Pre-Sturtian conglomerates were deposited in east-west oriented grabens in the Musgrave Block south of the Musgrave Ranges. The Peterman Ranges Orogeny had no metamorphic effect along the southern part of the Musgrave Block but block faulting of the crystalline basement with resultant folding and faulting of the Proterozoic sediments, could be attributed to this Orogeny. There are no known economic occurrences of minerals in these rocks. Minor amounts of copper, lead, silver and gold are found in the Mount Harris Basalt and Bloods Range Beds and these, together with zinc, are known from the Bentley Supergroup. Copper has been found in a quartz vein, intruding sediments of the Umberatana Group, off the southeastern part of the Block. 0O0

THE EARLIEST MOLLUSCS

Bruce Runnegar, Dept. of Geology, University of New England, Peter A. Jell, Dept. of Earth and Space Sciences, State University of New York The oldest fossils known or alleged to be of molluscan origin are: 1, tiny shells from Tommotian (earliest Cambrian) deposits in Siberia (Rozanov at. 1969); 2, large trails of latest Precambrian or earliest Cambrian age that might have been formed by naked molluscs (VlagiogmuA, VidymauJtickvuM and '&inyo/UchmJiA-9

Glaessner 197?); and 3, an apparatus or association of 1denticles1 from the earliest Cambrian of California that has been interpreted as the radula of a metre-long squid (Firby £ Durham 1974). Additional knowledge of primitive molluscs comes from younger Cambrian forms (Runnegar £ Pojeta 1974). These fossils and new collections etched from limestones of the Middle Cambrian of New South Wales and Queensland allow us to use the dual approach of stratigraphic position and comparative skeletal anatomy to reconstruct the early history of the shelled Mollusca. In this reconstruction operculate conical shells known as Hyolitha are referred to a separate but related phylum (Runnegar eX at. 1975). This phylum diverged from the Mollusca in the late Proterozoic.

There is excellent direct or circumstantial evidence that the primary radiation of the shelled Mollusca occurred before the end of the Early Cambrian. Important thresholds were: 1, the origin ana diversification of the helcionellacean Monoplacophora in the late Proterozoic; 3, the development of primitive Gastropoda (Atdane££a,PeXxzg^eXZa) from the helcionellacean Monoplacophora before the Tommotian; 2, the origin of the Pelecypoda (FoKduittoi) ^ via the pseudobivalved Rostroconchia (Myona, 1 HeACLuZtla"), by the Atdabanian; and 4, a differentiation of the Mono-placophora into crytoconic (helcionellacean), limpet-shaped (tryblidioid), ai'id planispiral (bellerophont) lineages before the end of the Early Cambrian. Other developments occurred later. In the Middle Cambrian some tube-bearing Helcionellacea began to curve exogastrically, providing an evolutionary pathway for the formation of primitive cephalopods in the Late Cambrian. The Scaphopoda probably arose from the Rostroconchia about the same time, and the fossil record of the Polyplacophora begins in the Late Cambrian. Most Early and Middle Cambrian shelled molluscs are less than 5 mm in size. Consequently, most of the early evolutionary experiments probably took place in organisms of microfossil body size. This tendency may explain existing stratigraphic and morphologic gaps in the fossil record. Fortunately the latter are unusually rare. In fact the gradations in form between the primitive molluscs of the Early and Middle Cambrian are sufficiently complete to make the differentiation of higher taxa (subphyla, classes etc.) a matter of hindsight rather than Early Cambrian reality. If the late Precambrian trails (Vlag-LogmuA etc.) were really formed by naked molluscs, we can conclude that they attained a large body size much sooner than their shelled relatives. If the size increase follows CopeTs Rule (Stanley 1973), it is likely that naked molluscs had a long evolutionary history m the l^roterozoic, perhaps comparable to that of the shelled molluscs in the Cambrian.

Firby, J.B. S Durham, J.W. 1974. Mulluscan radula from earliest Cambrian. J. Pal&ont. 48, 1109-1119. Glaessner, M.F. 1972. Precambrian palaeozoology. StAatlgfiapkic. problems ofi tht latojt PtidcambhAjm and EanJLy Cambrian, University Adelaide, 43-52. Rozanov, A. Yu. et al. 1969. The Tommotian Stage and the problem of the lower limit of the Cambrian. Thudy gzol. lni>t. Ahad. Na.uk. S.S.S.R. 206, 1-308. Runnegar, B. S Pojeta, J. 1974. Molluscan phylogeny: the paleontological viewpoint. Science. 186 311-317. Runnegar, B. eX at. 1975. Biology of the Hyolitha. LeXkcUa 8, 183-194. Stanley, S.M. 1973. An explanation for Copers Rule. Evolution 27, 1-26.

48. TROPHIC ANALYSIS APPLIED TO SOME E A R L Y DEVONIAN COMMUNITIES F R O M SOUTH-EASTERN A U S T R A L I A

Rath Mawson School of Earth Sciences, Macquarie University K.R. Walker (1972) has pointed out that fossil communities may be analysed in terms of trophic relationships, using an adaptation of E.R. Turpaevajs (1948, 1949, 1957) classification of Recent marine benthic communities into five trophic groups (high and low level suspension feeders, sediment surface feeders, infaunal deposit feeders and awaiters) the Relationship between these being defined by a series of four empirical 'rules'. Using species abundance data from previously described fossil communities, Walker concluded that the Turpaeva 'rules' could be applied meaningfully in the fossil context. To do so, certain assumptions have to be made, perhaps the most dubious of which is that fossil 'biovolume1 can be roughly equated with original biomass (conversion factors unspecified). On general grounds, one would be reluctant to accept such a relationship, even within rough limits, and there are problems of definition here (e.g. what dpes Walker mean by f biovolumef?). Nevertheless if some consistent parameter related to the growth of the organism can be reasonably measured, an attempt can be made, on Walker's terms, to estimate trophic relationships, and thereby develop some sort of framework for making generalized comparisons from community to community; this is what I have done. Clearly, procedures are simplified if the dominant/prominent constituents of a community are shells: brachiopods and/or mollusks - the case with many Palaeozoic communities. In applying trophic analysis to fossil communities we are confronted by the nettlesome problems of loss of soft-parts and perhaps total loss of information regarding soft-bodied organisms (sometimes represented by tracks and trails). Even with the serendipity of silicified faunas, problems arise from the differential silicification of certain shell fabrics, e.g. preferential silicification of brachiopods relative to certain mollusks, and incomplete silicification of interiors of rugose and tabulate corals, stromatoporoids and large trepostome ectoproct zoaria. Additionally there is the problem of forms moulting shells, specifically the arthropods. Faunas weathering out completely from matrix can be similarly analysed, but either curly corrective factors have to be applied for matrix enclosed within brachiopods, molluscs and so on or, and this is undesirable, destructive procedures have to be applied. Conceivably, faunas preserved as moulds could be studied in this way, but laborious laboratory procedures would be necessary if a reasonable impression of 'biovolume' for each constituent species were to be obtained. This style of trophic analysis has been applied to a number of silicified Ordovician, Silurian and Devonian faunas from New South Wales and eastern Victoria. What I present here is a sample; some results of investigations under way on silicified Early Devonian faunas from the Windellama Limestone (N.S.W.) and the Buchan Group (eastern Vic.). In the examples, silicified shell mass has been used warily as a reflection of original biomass. Autochthonous fossil communities analysed in this way appear to conform well with Turpaeva's frules'.

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D E V O N I A N BRACHIOPOD A N D T E T R A C O R A L ASSEMBLAGES F R O M NEW S O U T H WALES

A.J. Wright Wollongong University College Study of fossiliferous Devonian strata in the Lachlan "Geosyncline" suggests that additional refinement in biostratigraphy can be provided largely through study of common macrofossils. The limited lateral extent and stratigraphic thickness of most exposures compels the adoption of a "composite" biostratigraphy from numerous sections. Sorting of faunas into order of relative depth of deposition is not simple^although it is clearly essential to distinguish between faunas differing because of evolution from those representing different "communities". One approach is to attempt this after inferring a broad age for a fauna, so that some ordering can be achieved within a zone. Although this, may then permit some circular reasoning, it seems the best approach until a highly refined^biostratigraphy is available. (Alternatively, as in Savage (1971)steady lithological and faunal changes in a sequence may be informative.) For example, Gedinnian shelly faunas have been recorded from Victoria (Boola Beds); Manildra, New South Wales (Maradana Shale and Mandagery Park Formation); Mudgee, New South Wales (Mullamuddy Formation); near Bathurst, New South Wales (Merrions Tuff); and Bowning, New South Wales (Elinside Formation). These units yield a variety of faunas, from calcareous and non-calcareous beds, which will be discussed on a quantitative basis. With reference to diversity, it is concluded that this need not be related to depth alone. Examples will be discussed. Using a dynamic model, this may be related to substrate, sediment supply and other environmental attributes. Some attempts can be made to determine absolute (as well as relative) bathymetry. The relevance of KLovan's (1974) work is discussed. In determination of depths of deposition, the existence of possible hermatypic corals is significant in limestone sequences. In general, faunas containing true life assemblages of a significant variety of both brachiopods and corals are rare. Examples of these will be discussed, along with the infrastructure of these assemblages. Collections studied to date have led to the following provisional faunal units based on appearances of brachiopods and corals. The 1 relative positions of certain taxa are doubtful. (oldest)

Notoconchidium - Molongia - Notoleptaena

Spirigerina - Tyersella.

Megakozlowskiella - Muriferella - Fascicostella.

- Pleurodictyum - Notanoplia - Schizophoria - Lyrielasma

Nadiastrophia - Calceola.

Malurostrophia - Spinella - Buchanathyris - Taemostrophia - Dendrostella - Xystriphyllum - Zelolasma - Reomeripora.

5a. Howittia - Carinatina - Phragmophora - Phillipsastrea

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Macgeea - Trapezophyllum - Eridophyllmn - Bernhardt Ina. (youngest) 6.

"Zdmir" - Megastrophia - "hadjia" - Tabulophy].luin - Stringophyllum - Sociophyllum - Fromeophyllum.

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51. RELATIONSHIPS BETWEEN ORE, BANDED IRON FORMATION AND POTOSI GNEISS AT BROKEN HILL

D.H. MacKenzie 8 N. Gow C.R.A. Exploration P/L Two distinctive rock types are closely associated with the siliceous lode horizon which contains the Proterozoic massive lead-zinc-silver orebody at Broken Hill. They are Potosi gneiss, a quartz-feldspargarnet-biotite gneiss, and banded iron formation (B.I.F.) a quartzgarnet-magnetite-apatite- rock. In the mines area one or1 other of the several bands of Potosi gneiss is spatially close to ore whereas the B.I.F. bands are more distant but have close chemical affinities to the ore. The horizontal widths and lateral extents or ore, lode horizon, Potosi gneiss and B.I.F. as indicated by deep drilling at the Zinc Corporation and New Broken Hill Consolidated mines have been plotted on a series of longitudinal projections. This reveals a gross three dimensional pattern which is dominated by thick ribs and lenticular swellings of Potosi gneiss around the margins of which occur the orebodies and B.I.F. horizons. However, ore and B.I.F. which lie in the same plane are antipathetic in that they lie on opposite sides of the thick bodies of Potosi gneiss. The larger orebodies are associated with thickenings in the Potosi gneiss. The relationship holds for three of the bands of Potosi gneiss and is thought therefore, to reflect a primary disposition of the rocks and ore with respect to each other. B.I.F. is interpreted as an oxide rich lateral equivalent of the sulphide ore and the Potosi gneiss is interpreted as a dacitic volcanic rock which foimed positive areas between areas of oxide and sulphide deposition. The chemical role of the Potosi gneiss in ore genesis still requires resolution.

52. COPPER M I N E R A L I Z A T I O N IN M I O G E O S Y N C L I N A L CLASTICS OF THE BELT SUPERGROUP, NORTHWESTERN U N I T E D STATES

J.E. Harrison U.S. Geological Survey, Federal Centre Denver, Colo. 80225 U.S.A. The belt Supergroup is a stack of fine-grained clastic and minor carbonate rocks at least 20 km thick, deposited in an epicontinental basin on the western margin of the Canadian shield in the period about 1,450 to 850 m.y. (million years) ago. Most of the stack is of shallow-water marine origin, though the lowest exposed rocks include extensive turbidites and the uppermost m y contain some fluvial sandstones and conglomerates. The lower third of the stack consists of graywackes and graphitic shales, and the upper two-thirds of red-bed sequences, black shales, and carbonates. The entire stack has been metamorphosed to greenschist or higher facies. Anomalous copper occurs in almost every Belt formation except the lowest one (Prichard) and in localities scattered throughout the 130,000 km2 of exposed Belt. The copper is principally in chalcocite, bornite, and chalcopyrite that have replaced groundmass and clasts of the host rocks. Highly anomalous copper is found only in green and white beds within the red-bed sequences; no anomalous copper has been found in purple or red (oxidized) beds. Though copper concentrations tend to follow more permeable (coarser) layers, they also cut across the bedding, indicating that the copper has been mobilized and reconcentrated. Copper of ore grade and quantity has as yet been found only in what are now impermeable white quartzites and siltites of the Revett Formation, in a zone about 30 km wide and 100 km long in northwestern Montana. The structural position of -the zone on a post-Revett dome suggests that copper was reconcentrated epigenetically in permeable strata of a structural-stratigraphic trap prior to or during regional rnetamorphism of the formation. 0O0

JL oL^tt ^

53. T H E S U L L I V A N ORE B O D Y , B R I T I S H C O L U M B I A : P R E L I M I N A R Y S T U D I E S IN S U L P H I D E P E T R O L O G Y

F.A. Campbell1 V.G. Ethier1 and R.A. Both1 The Sullivan mine is the largest of several stratiform lead-zincsilver deposits located near Kimberley in south eastern British Columbia. The ore is enclosed beds of the Aldridge Formation (Purcell Supergroup), a sequence of metasedlments of Middle Proterozoic age. The metamorphic grade of the Aldridge Formation in the vicinity of the Sullivan mine is greenschist facies. Recent experimental data on the systems Fe-Zn-S and Fe-As-S provide the basis for estimation of the P-T conditions of metamorphism of the ore. The iron content of sphalerite coexisting with pyrite and hexagonal pyrrhotite decreases with increasing pressure (Scott 8 Barnes, 1971; Scott, 1973). The composition of sphalerite coexisting with pyrrhotite and pyrite in the Sullivan ore body was determined by electron microprobe analysis of 24 samples representing all major ore-bearing stratigraphic horizons. The mean weight percent FeS in the sphalerite is 12.9 (14.2 mole percent), with a standard deviation of 0.6. On the basis of ScottTs (1973) experimental calibration, this indicates a pressure of approximately 5 kb, a figure well in excess of that which can be accounted for by lithostatic load during the East Kootenay Orogeny. Analyses of sphalerite from sphalerite-pyrrhotite assemblages show the composition to be very similar to that in sphalerite-pyrrnotite-pytie assemblages. The composition of arsenopyrite is a function of (a) the assemblage present and (b) temperature (Kretschmar, 1974). Arsenopyrite in samples of Sullivan ore were also analysed using electron microprobe methods. Arsenopyrite-pyrrhotite assemblages were observed within the ore body but arsenopyrite-pyrite-pyrrhotite assemblages were observed only in mineralization in the hangingwall and footwall. Arsenopyrite coexisting with pyrrhotite and pyrite in the footwall contained 32.3 atomic percent As, and that in the hangingwall contained 33.0 atomic percent. According to experimental studies by Kretschmar, these compositions indicate temperatures in the range 410 to 490°C. Arsenopyrite coexisting with pyrrhotite in the ore has a mean of 32.0 atomic percent As, and can be interpreted as indicating temperatures of approximately 400°C. Preliminary sulphur isotope studies support these temperature estimates. oOo 1

Department of Geology, University of Calgary

Department of Economic Geology, University of Adelaide.

References: Kretschmar, U., 1974: Phase relations involving arsenopyrite in the system Fe-As-S and their application. Unpubl. Ph.D. thesis, University of Toronto. Scott, S.D., 1973: Experimental calibration of the sphalerite geobarometer. Econ. Geol. , 613, 466-474. Scott, S.D., and Barnes, H.L., 1971: Sphalerite geothermometry and geobarometry. Econ. Geol., 66^, 653-669.

THE STRUCTURE OF THE HARTS RANGES N.T.

M.J. Rickard Geology Department, Australian National University

The main Harts Range is made of strongly folded foliated amphibolites. These are flanked by gneisses with subordinate quartzite, clacsilicate and marble layers. The complex outcrop pattern is controlled by tliree of four phases of folding. The stratigraphic sequence is uncertain because of discordant folding, attenuation and sliding. In addition there was tectonic interlayering and disruption associated with the calcareous rocks and largescale boudinage of amphibolites. The Irindina gneiss^ is variable but typically rich in garnet and biotite. Layering was generally passive so that fold hinges are represented by a stripe lineation on the gneissosity. The Riddock amphibolite occurs in the T upper1 part of the Irindina formation and calc-silicate and marble members divide it in two. The Bruna gneiss ^ contains remarkably big feldspars and the Oonagalarbie formation consists of very fine grained quartzo-feldspathic rock, probably a recrystallised mylonite. The Cadney gneiss is characterised by abundant calc-silicates and its junction with the other units is everywhere discordant. The first folds, and in places also the second, are tight isoclines in the gneissic layering that only occasionally affect the outcrop pattern. They were probably N-S recumbent folds, now tilted into reclined positions. Hinge-plane schistosity is uncommon and gneissic layering passes around the hinges of even the earliest folds. Second folds are overturned southwards and have an E-W trend. They vary from tight to open structures and from reclined to horizontal with only a rarely developed hinge-plane schistosity. Strain was strongly inhomogenous within the axial plane. Upright third folds accompanied by a rough fracture cleavage trending northeastwards or eastwards warp the recumbent folds. The lack of characteristic schistosities and the considerable variation in plunge and fold style impedes analysis. Poles to bedding and gneissosity lie in a broad girdle about a moderate NE-axis; a second partial girdle about an E-W axis probably represents the second folds. F1 and F2 fold hinges, where distinguishable, cluster about the poles to these girdles but generally all/fold hinges lie in a broad girle that approximates the F2 hinge plane. Mineral streaking (extension) lineations are common but their distribution is the same as the fold hinges and it is difficult to assign them to folds. Metamorphism reached upper amphibolite facies during the second deformation. Retrograde effects associated with the pegmatite dykes of the TMica Mines1 are much younger, probably accompanying the Alice Springs deformation. There is a remarkable similarity between the deformational styles observed in these ancient Arunta rocks (1600 My) and those in the deformed cover of the Arltunga Nappes. 0O0

GEOMETRIC ANALYSIS ACROSS A DUCTILE MYLONITE: THE WOODROFFE THRUST, MUSGRAVE RANGES

T.H. Bell, The James Cook University of North Queensland The mylonites of the Woodroffe Thrust in the Amata Area, form axial plane to folds in country rock lying to either side of the mylonite zone. These folds are quite open below the sole of the thrust. However, as the axial plane mylonitic schistosity intensifies towards the mylonite zone, the folds progressively tighten until they are isoclinal. The formation of the mylonitic schistosity is extremely heterogeneous in space and time. The heterogeneity is such that mylonitic schistosity appears to form and deform concurrently in adjacent localities. The extremely strong lineation on the mylonitic schistosity surface is a combined intersection and mineral lineation. The intersection lineation is due to the intersection of the country rock and mylonitic schistosities. The mineral lineation is an alignment of elongate aggregates and grains of quartz and feldspar. This lineation commonly bends through 180° in the plane of the mylonitic schistosity (which remains unfolded) on a mesoscopic and macroscopic scale (the fold axes behave in the same manner). Associated with this bending of the mylonitic lineation there is a ductile rotation of two large ovoidal blocks of country rock relative to one another about the pole to the mylonitic schistosity. These blocks of country rock are separated by a zone of intense mylonitization and their relative rotation is about 90°.

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56. BASIN AND DOME DEFORMATION IN THE MOUNT ISA GEOSYNCLINE

B.A. Duff, Department of Earth Science, University of Leeds I.H. Wilson, Geological Survey of Queensland Basin-and-dorne Jrype of double folding is recorded in the Eastern Fold Belt, the Leichhardt (basement) Block and the Western Fold Belt of the Mount Isa Geosyncline. Generally, the folds are ellipsoidal *_n shape with a marked elongation parallel to the regional trend of the Geosyncline. Axes plunge shallowly north and south. It is envisaged that the ellipsoidal basins and domes resulted from a protracted period of progressive deformation during which early concentric buckling of rectilinear north-trending axes was succeeded by inhomogeneous flattening producing the singletrending axial-plane foliation and B lineation. The Corella Breccia originated as a "chocolate-tablet" type of boudinage in the axial plane. In the field, different stages in the deformation are reflected in the appression of basins and domes which is accompanied by steepening of plunges; greater intensity of plate-type boudinage, foliation and lineation; and the partial transposition of quartzofeldspathic veins injected near-normal to the axial plane after initial buckling. The intensity of deformation was controlled by the homogeneity and rheology of the undeformed rocks, the presence of layering and the proximity to large homogeneous blocks such as granite plutons. Boundary effects on sediments adjacent to such masses, and subsequent faulting have locally perturbed the otherwise consistent north-south structural fabric. Developing fold limbs were also deformed by north-trending thrust faulting. Subsequent east-trending normal faulting of the folds (north block up) attests a stage of adjustment to dilation. The final response to continued east-west appression was conjugate northeast and southeast trending strike-slip faulting of both basement and cover. The marked uniformity in size, orientation, and distribution of the basins and domes and related minor structures and the orthorhornbic symmetry of the gross fabric suggests a spatially uniform regional strain. This uniformity constrains the selection of a tectonic model. It precludes single leading-edge deformation (e.g. subduction) but is consistent with a "vice-like" model in which the Geosyncline is appressed from both sides as in the ensialic mobile belts of Africa.

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57. A C H R O M A T O G R A P H I C MODEL OF T H E SEDIMENTOSPHERE: OR T H E D I F F E R E N C E IN COMPOSITION OF PRECAMBRIAN A N D Y O U N G E R SEDIMENTS

J. C. van Moort University of Tasmania Since the beginning of the century variations in the composition of sediments as a function of age have been recognised. Already in the beginning of the century it was noted that the post-Silurian carbonate rocks in America contain increasingly higher proportion of calcite and that the older carbonate rocks are largely dolomitic. A similar evolution can be demonstrated for carbonate rocks from the Russian platform and the Australian platform. These mineralogical trends correspond to higher CaO values in the younger carbonate rocks and gradually higher MgO values in the older rocks. Carbonate cement in shales shows the same trend. Pre-Devonian shales from the platforms of the northern hemisphere show an increased illite and chlorite content. Pre-Ordovician shales from the southern hemisphere show a similarly increased illite and chlorite content. This mineralogical evolution corresponds to an increased potassium and also magnesium content, even when corrected for presence of carbonate admixtures. Clay and silt admixtures in carbonate rocks show a similar evolution. The observed chemical compositions of the shales and carbonate rocks in each period agree reasonably well. Information on the mineralogical and in particular on the chemical evolution of sandstone is limited. Mineralogically sandstones become depleted in heavy minerals when old but the chemical consequences of this are unclear. Chemical variations in Phanerozoic sandstones are erratic; Proterozoic sandstones are high in K2O and MgO. From the data from Ronov et al. it can be calculated that the cement in the younger sandstones is relatively rich in calcium sarbonate. The younger sediments contain in general more sodium than the older ones. Younger sediments are moreover very much enriched in strontium as was shown by Vinogradov et al (1952) for carbonate rocks and for shales by Reimer (1972a). As the relative proportions of the preserved sediments are reasonably well known, it is possible to discuss the chemical evolution of the entire sedimentosphere. From known distributions of the sediment types and their accepted average composition the chemical evolution of the entire non-metamorphic sedimentosphere is calculated quantitatively for Na, Sr, Ca, Mg, Rb and K during the last 950 million years. Concentrations of Na and Sr increase gradually in the younger sedimentosphere, concentrations of Ca and Mg are highest in the early Palaeozoic sedimentosphere and concentrations of Rb and K are highest in the Proterozoic rocks. The chemical composition of the collective sediments in given periods seem not to be compatible with any known simple source. Consequences of the hypotheses explaining the chemical trends as primary or as a consequence of later leaching are investigated. An hypothesis of alternative slow chromatographic evolution of the sediments is given. Chromatography has been defined as the summation of separation processes used for the separation of ionic or molecular mixtures. Typical of most clrromatographic processes is the support which consists of a

58.

column of solid material, which is largely insoluble in the solvent(s) introduced into the system. Secondly, a suitable solvent must be available to form a solution of the solutes to be separated. The final ionic or molecular distribution in the column will depend on the differential rate of migration and the time evolved. The importance of chromatographic processes ira "geology were first recognised by Day (1897) and have been stressed by Ritchie (e.g. 1964, 1966)*in multiple papers. The concept of chromatography goes back to Schttnbein (1661) who coined the term geochemistry. The preserved proportions of the non-metamorphic sediments and the distribution of Na, Sr, Ca, Mg, Rb and K in the sediments is a function of time. Both distributions can be explained by chromatographic processes during which the elements in the order of being mentioned will show an increased tendency to get fixed. The different composition and proportions of the ancient shales, carbonate rocks and sandstones is largely of secondary nature. oOo

59. THE PETROLOGY AND GEOCHEMISTRY OF THE GAWLER RANGE VOLCANICS IN THE KOKATHA AREA, AND SOME ECONOMIC IMPLICATIONS

C.D. Branch, School of Applied Geology South Australian Institute of Technology

The Gawler Range Volcanics in the Kokatha area are dominantly terrestrial and rest unconformably on Lower Proterozoic Glenloth Granite and amphibolite. The volcanics, dated about 1535 m.y. old, are intruded by the comagnatic Kokatha Granite, about 1500 m.y. old. The basal volcanic unit of mainly viscous pink to brown rhyolitic flows, tuffs, thin ignimbrites in simple cooling units, and spectacular amygdaloidal flows contains some interbedded basaltic andesite flows. An overlying compound cooling unit of red-brown rhyodacite ignimbrite grades from quartz-rich near the base to quartz-poor at the top. This is overlain by a succession of generally grey rhyolitic lavas, tuffs, and thin ignimbrites in simple cooling units: near the top an avalanche breccia derived from a collapsed volcanic dome forms a widespread marker bed. The uppermost unit is a compound cooling unit of uniform dacite ignimbrite which ranges from black where densely welded to red-brown where moderately welded (Chandabooka Dacite). The preserved volcanic sequence is about one kilometre thick, and is intruded by flat-roofed stocks of Kokatha Granite. In the Kokatha region the volcanic units form flat-lying sheets which generally are unmetamorphosed and undeformed: thus they provide a good opportunity to study the geochemistry of a Proterozoic acid volcano-plutonic province. So far, 33 full silicate analyses (28 volcanics, three intrusives, 2 greisens) have been completed, together with trace element analyses for Rb, Sr, Ba, Pb, Mo, Cu, Zn, Ni, V. In 23 volcanics Si02 ranges from 77% to 66% and these rocks are classified as rhyolite, dellenite, rhyodacite and dacite, although in most handspecimens phenocrysts of quartz are absent. In five volcanics from early in the succession Si02 ranges between 51% and 54%, and these rocks are similar to mugearite and doreite. When plotted on a FMA diagram the analyses form a trend a little on the alkali side of the caic-alkali trend, and the silica gap between 66% and 54% suggests the suite is bimodal. In 21 acid volcanics K20>Na20 (average K2O 5.07%, Na20 3.13%), but in two volcanics from the sequence between the major ignimbrite sheets Na20>K20 (average Na20 5.62%, K2O 2.95%) and one contains a possible phenocryst of Na-rich amphibole. The three granites analysed contain 72 to 77% of Si02 and are chemically similar to volcanics with the same silica content. Greisenized granite and volcanic have been analysed and the greisenized volcanic contains 9200 ppm Pb and 740 ppm Zn. All but four of the acid volcanics with K> Na, and all the granites are corundum normative, and for all Na20 + K20 + CaO < 1.1 (generally o.7) AI2O3 suggesting a metasedimentary origin for the magma. However, the few Sr°VSr86 ratios determined for volcanics and granites elsewhere in the Gawler Range province range from 0.7045 to 0.706, suggesting an igneous source. Additional data are being obtained in an attempt to rationalise this conflict. 0O0

60. S T R A T I G R A P H I C U N I T S IN T H E G A W L E R R A N G E V O L C A N I C S , S O U T H AUSTRALIA

A.H. Blisset Geological Survey of South Australia The thick sedimentary sequence in the Adelaide Geosyncline accumulated in a subsiding trough to the east of the Gawler Block in Eyre Peninsula, a part of the Australian Precambrian shield and a source of much of the detritus. The basic Roopena Volcanics of eastern Eyre Peninsula are generally correlated with the Willouran Wooltana Volcanics in the lower part of the Adelaidean in the northern Flinders Ranges. During later periodic transgressions in Adelaidean times, the Pandurra Formation (Torrensian), Woocalla Dolomite (Sturtian) and the Tent Hill Formation (marinoan) were deposited on the Sturt Shelf on the eastern margins of the Gawler Block. The Stuart Shelf responding to subsequent earth movements by epeirogenic block faulting and gentle warping of the sedimentary cover in contrast to the intense folding in the Adelaide Geosyncline. The last stage in the consolidation of the Precambrian crystalline basement upon which the Adelaidean was deposited was the largescale eruption of the Carpentarian Gawler Range Volcanics (c. 1535 Ma) over the Cleve Metamorphics and "older granites", followed by the intrusion of the Hiltaba Cranite (c. 1500 Ma). An unknown volume of volcanics was stripped off before the deposition of the Torrensian Pandurra Formation, and again before the Marinoan when the Tent Hill Formation overlapped on to the volcanics east of Lake Gairdner. The Gawler Range Volcanics are a vast calc-alkaline province of lavas and welded ash flows, agglomerates and minor air fall tuffs, consisting predominantly of a dacite-rhyodacite-rhyolite assemblage, with subordinate andesites and basalts. The main mass of volcanics occupies an area of more than 25 000 km in the Gawler Ranges and northwards to within a few kilometres of Kingoonya. That the volcanics once covered a much larger area is indicated by scattered remnant masses of similar rocks in the Tarcoola region, up to 200 km to the northwest; and at Mt. Cooper, 80 km to the southwest. Eastwards the volcanics are concealed by Adelaidean formations and it is not known how far they extend in this direction. The volcanics in the western portion of the province have been subdivided into major rock units. The Yardea Dacite is the uppermost formation in the Gawler Ranges proper, cropping out over an area of several thousand square kilometres south and west of Lake Gairdner, and on islands in the lake. The dacite overlies rhyolite near Lake Acraman, and is overlain unconformably by the Marinoan Tent Hill Formation east of Lake Gairdner. The Chandabooka Dacite in the Kokatha district may be equivalent to the Yardea Dacite, though erupted from localised vents. East of Kokatha homestead, the dacite rests unconformably upon a variable succession of rhyolites, rhyodacites, andesite and basalts, which has been named the Chitanilga Volcanic Complex. Basalt in the lower part of the complex rests unconformably upon Cleve, Metamorphics near Kokatha. About 18 km to the east, the Chandabooka Dacite overlies the Lake Gairdner Rhyolite which m y be equivalent to rhyolite within the Chitanilga Volcanic Complex. North of Kokatha and in the Glenloth district, the Lake Gairdner Rhyolite apparently is unconformable upon Cleve Metamorphics and the "older granites".

61.

Volcanics in the Glyde Hill Volcanic Complex south of Lake Everard probably were erupted from another system of vents. Dacite and rhyodacite overlie the Wheepool Rhyloite, at the base of which is a band 5 m thick of gently dipping coarse rhyolitic agglomerate. Basalt, andesite and fine-grained rhyolite occur lower in the succession. The base of the Gawler Range Volcanics is not exposed in this region. Contacts between the highly porphyritic Yandoolka Rhyolite and the surrounding volcanics are obscured by Quaternary sand. It may represent the infilling of vents from which the agglomerate in the lower part of the Wheepool Rhyolite was ejected. The Gawler Range volcanics were intruded by scattered porphyritic rhyolite dykes whose age is unknown, though possibly genetically related to the Hiltabe Granite.

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62. ASPECTS OF THE GEOCHEMISTRY AND P E T R O L O G Y OF THE MAFIC G R A N U L I T E S OF THE ARUNTA; MUSGRAVE AND FRASER BLOCKS

Allan F. Wilson University of Queensland

Pyroxene granulites are major components of the Proterozoic metamorphic belts in the Fraser Range (W.A.), Musgrave Range (S.A.) and Strangways Range (N.T.). Field, petrographic and chemical data show that basic volcanic flows, basic intrusions of several forms, and basic sediments (tuffs and otherwise) are well represented in these belts. Many of the pyroxene granulites from the northeast portion of the Strangways Range appear to be the metamorphosed equivalents of olivinepoor tholeiitic basaltic rocks with rare interlayers of olivine-rich basalts. These are more magnesian and chemically more akin to primitive oceanic tholeiites than some from the western portions of the Strangways Range which show affinities with continental tholeiites. Many of the pyroxene granulites of the eastern Musgrave Range are tholeiitic, but are more aluminous and less magnesian than those of the strangways Range. Sr is high (commonly 200 ppm) whereas Sr is unusually low ( 80-100 ppm) Sr is high (commonly 200 pprn) whereas Sr is unusually Strangways and the eastern Musgrave Ranges, mafic granulites comprise only about 10-15% of the rocks, whereas in the Fraser Range mafic granulites are dominant and make up 70-80% of most of the region. Moreover, in the Fraser Range most of the mafic granulites are dominant and make up 70-80% of most of the region. Moreover, in the Fraser Range most of the mafic granulites are more common than bipyroxene granulites in both the Strangways and eastern Musgrave Ranges. Relict anygdaloidal and major element chemistry of almost all mafic granulites is compatible with slightly silicasaturated tholeiites. The rare earth and trace element data suggest that these granulites were derived from at least two basaltic magma types. Detailed sampling across a well established metamorphic gradient shows that there has been no noticeable expulsion of metallic elements during metamorphism of the "whole rocksf!, even though the metamorphic pyroxenes of the rocks themselves show distinct and predictable compositional changes. Similar lack of widespread element migration has been observed in some of the larger masses or thicker layers of mafic granulites of the Strangways and Musgrave Ranges. However, the composition of some narrow mafic pyroxene granulite layers have been affected by a large volume of surrounding felsic layers. It would appear that hydrous mafic rocks, or mafic rocks intruded into hydrous sediments, are more susceptible to whole rock chemical modification and textural reconstitution than large mafic intrusions or thick compact basaltic flows. Notwithstanding the marked tendency for large bodies of mafic granulite to remain closed systems with respect to most chemical elements, preliminary stable isotopic and general chemical data of these ana associated felsic granulites suggest C02 and F have been active fluids in these rocks, and the unusually low ratios of -Le0/-LD0 found m many of these rocks, may imply contact with a mantle-derived reservoir of oxygen. —oOo

63. GEOCHRONOLOGICAL STUDIES OF SOME PROTEROZOIC ROCKS IN AUSTRALIA P.A, Arriens Eucumbene Drive, Duffy, A.C.T. This ^ study contributes new Rb-Sr data, and/of re-assesses previously published ages from: 1-

Pilbara Region, Western Australia

Amata district, Central Australia

Eyre Peninsula, South Australia

The often quoted 2000 m.y. age for the Woongarra Volcanics of the Hamersley Group in the Mt. Bruce Super group is based on 46 Rb-Sr analyses. These data can be resolved into at least three populations with parallel isochrons of 2000 m.y. age and separate initial ratios in the range from .70 to .78. Field observation by the Geological Survey of Western Australia now favours an intrusive, rather than extrusive origin for the rocks, and the 2000 m.y. age accordingly becomes a minimum estimate for the age of the sedimentary rocks intruded by the acid sills. At Whim Creek, the published 3000 m.y. age of acid lavas might well be revised to a younger age, in the light of new data from Whim Creek, and a re-assessment of the possible range of initial 87sr/86sr in older Proterozoic and Archaean rocks. At Amata, Rb-Sr studies of amphibotite and granulite facies gneisses of great structural complexity give no definite ages. The best fitted isochrons are for mylonites in the Woodroffe Thrust and Davenport Shear, where "ages" between 2600 and 2000 m.y. are indicated The anomaly is that the rocks on either side of the' fault zones are younger. Leuco gneisses forming a small band within granulite facies gneisses were selectively sampled at Amata, for such rocks often have high values for Rb/Sr. An isochron of approximately 1350 m.y. with in 07 no - initial °'Sr/ Sr of not less than 1.2 was obtained from these leuco gneisses, which have 87Rb/86sr in the range from 1 to 193, and measured 87sr/86sr from 1.2 to 5.3 The Rb/Sr field of the leuco gneisses lies quite outside the range for all other rocks analysed from Amata, and a selective mechanism of partial melting is the most probable hypothesis for the origin of these rocks. j J

In Eyre Peninsula, 24 total-rock sampels of gneisses were analysed to obtain data additional to 5 total-rock samples which give the 1780 i 120 m.y. age reported by Compston and Arriens (1968). ^ The new data indicate a much longer sustained and more complex history for Eyre Peninsula than suggested by the earlier data. Although no satisfactory isochron was obtained, it is likely that at least some of the rocks are over 2000 m.y. old, and others may be as young as 1600 m.y.

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64. C O R R E L A T I O N OF U R A N I U M D I S T R I B U T I O N A N D M I N E R A L O G Y IN F R A C T I O N A T I O N OF T H E MT. D A V I E S GABBRO I N T R U S I O N

J.D. Kleeman Department of Geology, University of New England It has been shown previously that uranium exhibits a definite partition relationship between clinopyroxene and basic liquid phases, the partition ratio being 100-250 times in favour of the liquid. Consequently, during fractionation of a gabbro, uranium will concentrate in the liquid, and the uranium concentration of precipitating clinopyroxene should reflect the uranium concentration of the liquid from which it formed, due to the partition ratio. In a large fractionating gabbro body, the degree of fractionation may be correlated with stratigraphic position, and therefore in the simplest case, uranium concentration of clinopyroxene should be correlated with position in a fractioned sequence. The picture would, however, be complicated by any later pulses of liquid during fractionation, and be upset by reaction with liquid trapped in the pore space between accumulating phases. There are a number of basic and ultrabasic intrusives in the Giles Complex, Central Australia. Many of these have fractioated hi t>ttu from initially tholeiitic magmas, and vary in both stratigraphic height and overall degree of fractionation (Nesbitt oJt aZ., 1970). An initial study of uranium distribution and concentration in mineral phases showed that most of the intrusions do not give a consistent correlation between fractionation and changing uranium concentration in clinopyrozene. The south side of Mt. Davies does show a reasonably sonsistent trend of increasing uranium concentration with stratigraphic height and degree of fractionation. The south side of Mt. Davies is apparently unrelated to the north side. Over thirty samples have been studies, from top to bottom of the south side. Uranium concentrations have been determined in all phases using the fission trach method.Clinopyroxene has from 0.001 to 0.17 ppm U, orthopyroxene has 0.0006 - 0.03 ppm and piagioclase and olivine have less than 0.0002 ppm U. These uranium concentrations are correlated with a mineral variation of clinopyroxene Ca[+8Mg46Fe6 to Ca47Mg39Fem and orthopyroxene Ca]Mg§4Fei5 to CaiMggLfFeis and piagioclase An87 to An64* Mineral compositions were determined with an electron microprobe The inferred concentration of uranium in early, relatively unfractionated liquid is consistent with a tholeiitie, and has been concentrated by up to two orders of magnitude during fractionation.

Nesbitt, R.W., Goode, A.D.T., Moore, A.C. and Hopwood, T.P., 1970: The Giles Complex, central Australia: a stratified sequence of basic and ultrabasic intrusions. Spec. PuhU Qdol. Soc. S. Afit.,7, pp. 547-564.

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65. E V O L U T I O N AND PROBLEMS OF THE MC.ARTHUR BASIN, N.T.

K.A. Plumb Bureau of Mineral Resources, Canberra The McArthur Basin is a relatively undeformed structure within which up to 12km of mainly shallow-water sediments, of the Carpentarian Tawallah and McArthur Groups and the Adelaidean of Carpentarian Roper Group, and their :stratigraphic equivalents, were deposited. It is exposed over an area of about 170,000 km , and is bounded by, and unconformably overlies, the early Proterozoic inliers of the Pine Creek Geosyncline, Murphy Tectonic Ridge, and northeast Arnhem land. Sedimentation commenced about 1700 m.y. ago, and ceased before the emplacement of 1280 m.y.-old dolerite sills. The basin contains the major McArthur River (or H.Y.C.) syngenetic lead-zinc deposit. The Tawallah Group and equivalents comprise quartz-rich arenites and subordinate basic volcanics, carbonates, and lutites up to 6 km thick. They are overlain by a dominantly carbonate sequence-the McArthur Group and equivalents- up to 5.5 km thick. These are overlain, with regional unconformity, by alternating quartz arenites and micaceous lutites up to 5 km thick-the Roper and Malay Road Groups. Most formations are laterally uniform over large areas. Detailed sedimentological studies are limited, but transitional to shallow marine facies seem to predominate. The Carpentarian succession is very much thicker in the 50-60 km wide northerly trending fault-bounded Batten Trough than on the adjoining Arnhem, Caledon, Bauhinia, and Wearyan Shelves. At least some of the bounding faults were active during sedimentation. An exceptionally thin succession was deposited on the westerly trending Urapunga Tectonic Ridge, between the Arnhem and Bauhinia Shelves. The Batten Trough may be bisected by an eastward estension of the ridge. The Batten Trough first developed in Arnhem Land, to the north, where it controlled accumulation of both the Tawallah and McArthur Group equivalents. In the McArthur River area, to the south, the trough achieved full significance only during McArthur Group^time. During Roper Group time the zone of maximum sedimentation shifted westward onto the Bauhinia Shelf. Contrary to expectations, the quantity of flood basalts falls off markedly in areas of major syndepositional faulting. Subsequent deformation was in response to complex block faulting along pre-existing basement faults, which reflect the dominant north-northeast and north-west trends of northern Australia. It is most intense on northerly trending faults along the Batten Trough, where vertical displacements of up to 7.5 km uplifted the trough into an anticlinorium, or horst, during the Adelaidean. The Urapunga Tectonic Ridge is a zone of concentration of west-trending high-angle reverse faults. Conjugate strike-slip fault systems, with small displacements, are observed on the shelves, and a right-lateral strike-slip fault system existed beneath the Batton Trough before the development of the McArthur Basin. The late uplift of the Batten Trough is an unusual platform phenomena. oOo

66.

E V O L U T I O N OF PROTEROZOIC TOPOGRAPHY AND T H E FORMATION OF MINERALISED BASINS IN NORTHWEST QUEENSLAND

G.M. Derrick, Bureau of Mineral Reasources I.H. Wilson, Geological Survey of Queensland In the Proterozoic Northwest Queensland Province a median ridge of volcanic and plutonic rocks separates younger sediments and volcanics of the Western and Eastern troughs. Between about 1700 m.y. and 1650 m.y. ago, deposition in the Western trough was influenced by an active, fault-controlled eastern margin from which basal conglomerate wedges, quartzite and overlying fissure basalts emanated. The western margin is an ill-defined foreland of plutonic, volcanic, and metasedimentary rocks which are possible source rocks of thick blanket sands overlying the basalts. Some east-trending lineaments partly control thickness variations in the trough. In the Eastern trough basalt and sandstone were deposited, but major lineaments of similar age appear to be less prominent. From 1650 m.y. to about 1550m.y. granite intrusion and deep crustal movements reactivated many old fault lines, and produced numerous narrow meridional shelf, trough, and basinal areas. The subsequent deposits, from about 1550 m.y. onwards, were the base metal-bearing Mount Isa Group, Surprise Creek Beds, Gunpowder Greek Formation, and Paradise Creek Formation in the Western Trough, and the Mary Kathleen and Mount Albert Groups in the Eastern trough. Basal conglomeratic arenites in many lines trending both north and east. The areas rapidly stabilized, provenance matured, and sediments graded upwards into sand silt, shale and dolomite. Shales of Gunpowder Creek Formation grade upwards into stromatolitic dolomite and chert of the Paradise Creek Formation. The Surprise Creek Beds contain a stromatolitic shelf facies and a flysch-like trough facies in which copper traces are abundant; the adjoining Mount las trough subsided for a longer period, and lead-zinc deposits formed in euxinic zones in the trough. In the east, near Deighton Pass, continued local depression of the Mary Kathleen Group between basement rises led to deposition of zincbearing pyritic and pyrrhotitic black shales in the deeper-water facies, and traces of copper in shallower-water silty arenite facies. Continued subsidence localised deposition of younger fluviatile sands. Further east near Cloncurry, more stable basement rises are flanked by proximal jaspilite, which may be transitional into zinc and copper-bearing shales in distal troughs and basins. oOo

67. THE STRATIGRAPHY OF THE MOUNT ISA GROUP EQUIVALENTS IN THE MOUNT ISA-LAWN HILL AREA

R.J. Cavaney Carpentaria Exploration Company P/L The post-Haslingden Group Carpentarian rocks west of the Mount Gordon Fault Zone can be sub-divided into ten distinct formations, nine of which fall logically into a new group named the McNamara Group. The new group is justified because it is geographically, and sufficiently lithologically distinct, from its equivalent, the Mount Isa Group. In addition, several formations in the new group are unrepresented in the Mount Isa Group. The post-Myally history begins with a tectonic event of regional' magnitude. Associated with this tectonism, probably in its early phase, is a rock unit characterised by acid volcanicity, although volcanogenic material is not always the dominant rock type. This unit has unconformities above and below it, and acid igneous intrusions associated with it. Following this tectonism, widespread floodings and minor regressions of the sea resulted in the deposition of a clastic unit which filled and covered topographic irregularities such as up faulted quartzite horst blocks left by the preceding tectonic event. Gradual deepening allowed deposition of sericitic siltstones, sandstones and shales. At the top of this unit, dolomitic siltstones becomes prominant and pass upward into dolomites. The base of this dolomitic formation is taken to be a very widespread banded chert bed. This also marks the initiation of acid volcanicity as shown by the increase in potassium feldspar content of the sediments and by tuff marker beds. Widespread shallowing is suggested by the development of massive stromatolitic cherts overlying the dolomitic unit. The chert unit exhibits a lateral facies change to quartzites. Overlying the predominantly stromatolitic formation is a series ofdolomitic siltstones and shales and quartzite units. The uppermost Carpentarian unit is the Lawn Hill Formation. Stratigraphic correlations can be made between the Carpentarian sediments west of the Mount Gordon Fault Zone, and sequences to the north at Hedleyfs Creek, to the east in the Surprise Creek Beds, and to the south in the Mount Isa Group.

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68. T H E S O - C A L L E D PINE C R E E K G E O S Y N C L I N E H A S N O T H I N G TO DO W I T H A GEOSYNCLINE

Emile Rod 9 Minns Road, Gordon N.S.W. 2072 Since 1930, the area in the Barwin-Katherine region where Lower Proterozoic rocks are exposed, has been known under the name of "Pine Creek Geosyncline". This term for a structural feature is widely accepted and also figures prominently on many tectonic maps, as for example on the 1971 Tectonic Map of Australia and New Guinea. T h e Pine Creek Geosyncline is described as the approximately triangular area bounded on the west and southwest by the Litchfield Block and the Adelaidean and Palaeozoic sediments of the Daly River Basin, and on the east and southeast b y the contact with the overlying Carpentarian beds of the McArthur Basin. How far the Lower Proterozoic rocks of the Pine Creek Geosyncline extend towards north is not known. They certainly continue beyond the islands in the Tinior Sea north and northeast of Darwin. It should also be quite obvious that under the cover of the Carpentarian and Adelaidean sediments the Lower Proterozoic rocks, which constitute the Pine Creek Geosyncline, might continue for a great distance towards south and especially towards east. Most probably the Lower Proterozoic Pine Creek Geosyncline might still occur under the Western Gulf of Carpentaria. A s the term Pine Creek Geosyncline was established to describe the depositional basin of the Lower Proterozoic sediments of the Darwin- ^ Katherine a r e a , it is proposed to analyse here at first the configuration of the basin from its inception, at the close of the Archaean. ^Afterwards it should be possible to decide if the term"geosyncline" is appropriate for such a structural feature or not. The palaeogeography at the beginning of the Lower Proterozoic sedimentation in this region can be envisaged as follows: Huge dome-like Archaean granitic and metamorphic complexes, 30 to 20U Km in diameter, are scattered in a very shallow sea. These domes of low relief stick out like huge bold heads in a landscape of low lands, wide mudflats crossed by meandering rivers, and arcuate sea arms. In the waterchannels and along the shore, there is a lush growth of blue-green algae. A s the relief is extremely gentle the rivers do not carry any coarse clastics so that siltstones are the dominant deposits formed. ^ Much organic material is produced by the blue-green algae. This organic matter is especially accumulated in oxbows, quiet water pools and m silled basins of the sea. It is exactly in those localities, where organic matter is abundant, that uranium oxides, leached out from the Archaean granites,- are being preferentially precipitated .and enriched. Thus the dome-like Archaean granitic and metamorphic complexes form an integral part of the sinuous basins of deposition. Such a structural feature, combining a widely scattered group of dome-like landforms with the tortuous shallow seas surrounding them, can according to modern terminology certainly not be called a geosyncline. It is advocated that a special name be given to those Lower Proterozoic basins of sedimentation. oOo

69. G R A N I T E DIAPIRISM IN THE RUM JUNGLE AREA NORTHERN T E R R I T O R Y

0. Stephansson University of Lulea, Sweden K. Johnson Bureau of Mineral Resources, Canberra Early studies in the Rum Jungle area suggested an intrusive relationship between the Rum Jungle and Waterhouse "Granites", and the overlying sediments. It was later shown that the granitic "intrusions" at least in part, were Archaean basement complexes on which Lower Proterozoic sediments had been deposited. Fold superposition involving three and possibly four deformation phases was postulated as being responsible for doming of the basement and cover rocks. The purpose of this talk is to show that the domed structures in the Rum Jumgle area, and the emplacement of some Middle Proterozoic granites in the Pine Creek Geosyncline were related, and caused by diapiric intrusion of granites into the basement complexes and cover rocks. Structural and met amorphic evidence in support of diapiric intrusion in the Rum Jumgle area includes: pebble deformation within steeply dipping beds of quartz conglomerate, disappearance of polyphase fold structures away from the basement complexes, bending of folded country rock strata into concordance with the complex-sedijiient contact', and metamorphic and metasomatic alteration of sediments in contact with the basement complexes. Gravity data show mass deficiencies in the Archaean complexes which possibly coincide with young granite diapirs. ' aapir i:n granites in the Pine Greek Geosyncline may have been a source of metals and non-metals, and could also have mobilized and concentrated any pre-existing syngenetic mineralization.

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70. T H E RINGWOOD EVAPORITE DEPOSIT-A CLASSICAL BARRED-BASIN MARINE E V A P O R I T E IN T H E PROTEROZOIC OF T H E AMADEUS BASIN

A.J, Stewart Bureau of Mineral Resources, Canberra The Ringwood evaporite is a deposit of gypsum, anhydrite, and dolomite in the Late Proterozoic Bitter Springs Formation of the Amadeus Basin. The Bitter Springs is a shallow-marine deposit consisting of stromatolitic dolomite and limestone, red, green, or white shale, clean quartzite, and evaporites. The evaporite deposit at Ringwood comprises two parts. The lower part (127 m) consists of four interbedded rock-types: (1) a breccia of grey gypsiferous dolomite clasts cemented by colourless gypsum with anhydrite inclusions, (2) dark grey fine-grained bituminous pyritic dolomite, (3) soft grey dolutite, (4) laminae of blue-grey anhydrite. The rock-types in the upper part (133m) are similar to those in the lower, except that pyritic dolomite is absent. The evaporites are overlain by limestone breccia and massive algal limestone. The normative gypsum: normative anhydrite ratio calculated from chemical analyses increases exponentially up the deposit, consistent with an origin of the gypsum by hydration of anhydrite by meteoric water. Gypsum in the upper part of the deposit contains abundant celestite, indicating that the pre-existing anhydrite contained considerable strontium, and hence constituted the primary sulphate. The main hypotheses of ancient evaporite origin are evaporation of sea-water in a barred basin, epeiric sea, sabkha , or•dessicating deep ocean basin. The known occurrences of evaporites in the Bitter Springs Formation do not show the concentric distribution of the dessicating deep basin model, and the associated clastic rocks are shallow-marine or continental, not deep-water. The diagnostic features of a sabkha, such as red-beds, algal-mat stromatolites, dessication cracks and flat-pebble conglomerate, nodular anhydrite, laminae of aeolian silt, and association of celestite with dolomite, are all absent from the Ringwood rocks. Deposition in the shallow-water oxidizing environment at the margin of or around shoals in an epeiric sea is contra-indicated by the abundance of pyrite, organic matter, and grey or black colours of the Ringwood deposit. The deposit meets all the criteria for an origin according to the barred-basin model of Ochsenius, beginning with a basal pyritic bituminous dolomite, interbedded with and ultimately overlain by gypsum and gypsiferous dolomite (originally anhydrite and anhydritic dolomite), and capped by limestone breccia and massive stromato1itic limestone, indicative of an algal reef. The reef allowed a stagnant pool or lagoon characterized by reducing conditions to form in the otherwise oxidizing environment of the Bitter Springs sea, and evaporation of the lagoon water resulted in the rocks of the Ringwood deposit.

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71. D E V O N I A N C O N G L O M E R A T E DEPOSITION, AMADEUS BASIN .CENTRAL A U S T R A L I A

B.J. Jones, Department of Geology, University of Wollongong The Polly Conglomerate at the base of the Finke Group and the Brewer Conglomerate at the top of the Pertnjara Group are both probably Upper Devonian in age and they have many characteristics in common. However they are related to separate tectonic events and their interrelationship through a series of lateral and vertical facies changes is coincidental. The Polly Conglomerate crops out in the southern part of the Amadeus Basin between Horseshoe Bend and Umbeara homestead. It is a wedge shaped unit which thickens and becomes coarser grained southwestwards towards the Musgrave-Mann basement block from which it was derived. The conglomerate overlies an irregular erosion surface with some topographic highs not being covered. In the Umbeara homestead area the unit consists of 200=m of massive to flat bedded boulder, cobble and pebble conglomerate. To the north near Horseshoe Bend the unit is only about 40 m thick and consists of flat bedded cobble to pebble conglomerate with some thin sandstone interbeds. Eastwards it also becomes finer grained. The Polly Conglomerate grades upwards into the sandstones of the Langra Formation which is in turn overlain by the Horseshoe Bend Shale. These three units represent a typical laterally and vertically fining sequence derived from the continued erosion of an uplifted block on the southern margin of the basin. In contrast the Pernjara Group in the northern and western portion of the Amadeus Basin exhibits a coarsening upwards sequence with the basal Parke Siltstone overlain by the Hermannsburg Sandstone and the Brewer Conglomerate forming the top of the succession. The Brewer Conglomerate £s mainly confined to a 20 to 50 km wide belt along the northern margin of the basin. It is also a wedge shaped unit which is thickest in the north ( 3000 m) and it rapidly becomes thinner southwards away from the source area. The contact with the Hermannsburg Sandstone is gradational in some areas but disconformable in others. Maximum clast sizes in the Brewer Conglomerate range from about 1 m in the north to 10 or 15 cm near the southern margin of the conglomerate. Clast lithologies can be matched reasonably well with the older exposed rocks to the north and successively older lithologies occur at progressively higher levels within the Brewer^ Conglomerate i.e. a classical uncovering sequence. When the distribution of the first occurrence of a distinctive clast lithology is mapped the arcuate pattern tends to suggest that deposition of the conglomerate occurred as a series of broad piedmont alluvial^fans. This would also account for the local disconformities recognized within the conglomerate. To the south the conglomerate grades laterally into pebbly sandstones which show a progressive decrease in the proportion of unstable clast lithologies downstream from the source area. The Pertnjara Group is interpreted to have been deposited during the progressive uplift of a source area along the northern margin of the Amadeus Basin with the Brewer Conglomerate being deposited during the nain phase of the Alice Springs Orogeny.

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72. D E V O N I A N A N D C A R B O N I F E R O U S S E D I M E N T A T I O N IN THE S O U T H E R N P A R T OF T H E T A M W O R T H T R O U G H

W. Mayer Geology Dept., Canberra College of Advanced Education This paper discusses the sedimentation of Devonian and Carboniferous rocks in a small area east of Gloucester in the southern part of the Tamworth Trough. The Tamworth Trough, a tectonic segment of the New England Fold Belt, is an ^ elongate, slightly arcuate northwest to north trending basin in which sediments and volcanic rocks of Devonian to Permian age accumulated. Its boundaries are defined today by the Mesozoic sedimentary rocks of the Sydney Basin to the west and southwest and by the Peel Fault System to the east and northeast. The basin was marginal to a volcanic mountain chain which lined its western shore and which supplied it with much of its detritus. The eastern boundary of the basin, now largely obscured as a result of fault movements, is generally believed to have been one of transition to a deeper oceanic environment in which the Mid-Palaeozoic rocks now outcropping east of the Peel Fault were deposited. The area of the Tamworth Trough discussed here lies hard against the southern extension of the Peel Fault System where the fault zone swings from a northwesterly trend to assume an east-west alignment over a short distance. The rocks are Lower Devonian to Lower Carboniferous in age and include volcanic breccias and conglomerates, lithic and feldspathic sandstones, coralline limestones and limestone breccias, radiolaria-bearing siltstones and acid to intermediate flows and tuffs. They are steeply dipping, they have been broken into a number of fault segments and they are intruded by a variety of igneous rocks which are associated with the fault system. The detrital sedimentary rocks were derived predominantly from a volcanic source which changed in composition from mainly intermediate in the Devonian to more acidic in the Carboniferous. The sediments were deposited in a basin probably close to the eastern margin of the Tamworth Trough. A series of volcanic islands, some of them fringed by coral reefs, probably existed within this part of the basin. Sediment reached the area of deposition from sources in the southwest and in the north and northeast. Much of the sedimentary material, in particular that of coarsest grain size, reached the deeper parts of the basin by a process of mass-movement. Slumping and sliding of large masses of debris off the volcanic areas resulted in extensive local disturbance and erosion of the seafloor and the incorporation of large rafts of bedded sediment into the deposits. The basin had sufficient depth to allow some of the dislodged sediment to be taken into suspension to produce turbidite deposits. Current structures in turbidites indicate that the depositing currents flowed at times longitudinally down the axis of the trough. The available geological data suggests that a considerable amount of sediment, particularly during Lower and Mid-Devonian times, was derived from a land area to the northeast as well as from some nearby volcanic islands within the basin. Whether the northeastern source Represented a substantial land mass or consisted of a series of ... volcanic islands could not be determined. 0O0

73. H E A V I T R E E Q U A R T Z I T E S T R A T I G R A P H Y A N D S T R U C T U R E NEAR A L I C E SPRINGS, N.T.

David Clarke Northern Territory Geological Survey Measured sections of the Adelaidean Heavitree Quartzite, basal unit of the central Australian platform cover, are presented from Tommy'rs Gap, Mangeraka Gorge, Heavitree Gap and Ormiston Gorge localities m tne correctable units are attempted and it is proposed that the four main subdivisions be formalized as members of the formation. At their type^section locality, Heavitree Gap, the basal 'Undoolya Siltstone* which nonconformably overlies the metamorphic and igneous rocks of the Arunta Complex, is thought to have been deposited in a lacustrine environment. However, fluvial interbeds and equivalents are known elsewhere. The overlying 1Temple Bar SandstoneT includes a fluvial lower unit, which is conglomeratic and feldspathic at T Tommyfs Gap in the east, and fines to silt and very fine sand in a tnicker section at Mangeraka Gorge to the west; and an upper, probably open marine facies sand, possibly littoral at the base and fluvial near the top in the east, which thins to,the west. The stratigraphical ly highest member, the TBlatherskite QuartziteT, is thought to comprise a lower open marine unit (absent at TommyTs Gap) overlain by littoral sands, These grade upward into the pelites and stromatolitic carbonate rocks of the sitter Springs Formation. Both palaeo-current directions and gross sandstone geometry suggest a provenance to the east or northeast, and a measured section of the stratigraphically equivalent Vaughan Springs Quartzite at Central Mount Wedge is illustrated for comparison. A section of the quartzite outcropping at Simpson Gap west of Alice Springs is also presented and correlation is proposed. This correlation provides new evidence for interpretation of the Baltherskite Nappe and StewartTs (1967) conclusions are thought to be substantially confirmed. Further considerations suggest that recumbent folding was not an important mechanism, and that thrusting occured in a northeast over southwest direction, with the relative displacement reaching 14 km. near Simpson Gap.

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n. S E D I M E N T O L O G Y OF THE C O R U N N A C O N G L O M E R A T E NEAR IRON KNOB, SOUTH A U S T R A L I A

Nicholas Lemon B.H.P., Whyalla Victor Gostin, Dept. of Geology, University of Adelaide University of Adelaide The Corunna Conglomerate is a sequence of shales, sandstones, and conglomerates which outcrops over an area of 200 sq. kms. some 64 kms. west of Port Augusta and.5 kms. north of Iron Knob in SouthAustralia. This sequence is of Carpentarian age and directly overlies the Gawler Platform basement in the area. The Carpentarian age of 1535 m.y. was determined using a shale from within the unit. This age ties in well with an age of 1610 m.y. given to the underlying Burkitt Granite, an age of 1535 m.y. for the contemporaneous Gawler Range Volcanics and 1480 m.y. for the dykes which cut the sequence. The basal member of the sequence is a coarse, poorly sorted conglomerate which unconformably overlies the Burkitt Granite and a dolomite and chert unit. Immediately overlying this is an undetermined thickness of green marine sandstones. These sandstones intertongue and grade ^ into the lower part of a red conglomerate member which was deposited under sub-aerial conditions. Evidence of alluvial fan and flood plain environments can be found'in this unit. Deposition of this unit stopped and an erosional surface with up to 100 metres relief was etched into these sediments. Deposition commenced again with a white conglomerate unit filling the old valleys. This unit fines up the sequence and the top of it is marked by a thin continuous horizon of a well-rounded granule conglomerate which was derived by the reworking of the underlying unit by waves in a marine environment. Above this marker horizon is a sequence of well sorted, cross bedded white marine sandstones. These contain some green shales bands at the base and the shale becomes more prominent up the sequence, eventually constituting up to 60% of the unit. There is no record of further sedimentation in the Corunna Conglomerate above this point. The sediments were then faulted, mildly folded and intruded by dykes during the Wartakan phase of tectonic and igneous activity. In summary, the Corunna sequence commenced with rnirone deposits, then, as the land rose in relation to sea level, the sedimentation changed to paralic and terrestrial. A period of erosion followed. Then as sea level rose again, sedimentation recommenced with terrestrial sediments at first grading back into marine as the sea transgressed.

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75. GEOLOGY A N D M I N E R A L I S A T I O N IN THE NORTHERN TERRITORY

M C . A R T H U R DISTRICT

J.G. Binnekamp and R. Logan Carpentaria Exploration co. P/L

The McArthur Group is a sequence of predominantly carbonate sediments of Middle Proterozoic (Carpentarian) age. These sediments were deposited on the floor of a wide shallow trough, the McArthur Basin. Tuffaceous horizons indicate volcanic activity. A depression, the Bulburra Depression, developed in this trough. A short period of inc increased volcanic activity coincides with its initial development. Bituminous pyritic shales carrying above-average lead-zinc content were deposited in this depression. Deposition of significant amounts of lead-zinc sulphides took place contemporaneously with the deposition of shales in several local sub-basins within the Bulburra Depression. The H.Y.C. deposit is by far the largest deposit of this type. It comprises a 50 m sequence of fine grained thinly bedded lead-zinc bearing bearing shales,^ interrupted by several wedge-like turbidite breccias and and slumps, derived from surrounding areas of carbonate sedimentation. East of the H.Y.C. deposit a facies change from shale to a reef and lagoonal environment takes place. Cu-Pb-Zn sulphides were deposited contemporaneously with algal dolomites. Remobilisation and recrystallisation during diagenesis and later brecciation resulted in the formation of several coarse-grained sulphide deposits. Coarse grained base-metal mineralization occurs in one of the dolomitic units of the McArthur Group. Megascopical relations of the various deposits and their possible genesis are discussed. The potential of further base-metal deposits in the area is discussed.

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76. G E O C H E M I C A L A N D M1NERALOG1CAL I N V E S T I G A T I O N S , M C . A R T H U R A R E A NORTHERN TERRITORY

K.M. Scott and I.B. Lambert Division of Mineralogy, CSIRO

The unmetamorphosed Proterozoic McArthur deposit occurs witiiin the H.Y.C. Pyritic Shale Member which consists essentially of pyritench, tuffaceous, dolomitic shales and siltstones. Throughout the McArthur region, these sediments are characterized by anomalously high contents of Fe, K, Zn, Pb, As and Hg and normal contents of most other elements, including Au, Ba, Bi, Cu, Co, Ni and Sn. Their organic carbon contents are low to moderate. The contents of certain other elements can be correlated with various mineralogical components of these rocks. For instance, Ti and P are largely associated with the detrital fraction, Na with albite (generally low), B and Li with clay minerals, Rb with potash feldspar and Sr with carbonates (generally low). These mineralogical and chemical relationships permit certain conclusions to^be drawn concerning the physico-chemical conditions in the depositional environment around the time of ore formation. Ferroan dolomite is the predominant mineral in the carbonate-rich members ovelying, underlying and beside the H.Y.C. Pyritic Shale Member. The Fe contents decrease gradually to normal values over a distance of 15 km or so from the deposit. Mn is also often high in these dolomites, but they rarely hav anomalous contents of the metals enriched in the pyritic shales. Pyroclastic debris is relatively abundant in the mineralized shales and it is considered that the igneous acticity associated with tuff eruptions was responsible for generating the ore fluid. This fluid probably formed at moderately high temperatures, and may origianally have contained significant amounts of Cu which precipitated out as it cooled during ascent. The abundance of thin-bedded sediments within the ore indicates that mineralization occurred over a protracted period which has been estimated to be 0.5 - 1.5 x 10 years on the basis of extrapolations from sedimentation rates in modern sedimentary environments. Etching with nitric acid reveals a significant proportion of readily etched pyrite within the ore, often in the form of rims around small crystals of relatively unreactive pyrite. The reactive pyrite is rare in the country rock shales, suggesting that it was introduced with the sphalerite and galena, whereas the bulk of the pyrite formed from a different Fe solution. This interpretation is in accord with available isotopic data. A thick dolomite member occurring between the ore and a nearby major fault zone contains minor discordant mineralization, which possible precipitated from the McArthur ore solution. It is envisaged that the bulk of this ore solution ascended in the fault zone and migrated through permeable zones in the carbonate reef or bank before spilling onto the basin floor. oOo

77. ECONOMIC IMPLICATIONS OF THE GEOLOGY OF THE DUCK CREEK SYNCLINE ASHBURTON REGION, WESTERN AUSTRALIA

G. Doust Anaconda Australia Inc. The projection of an Archaean basement ridge into an otherwise regular, curvinear Proterozoic sedimentary basin (the Ashburton Trough) forms the structure known as the Duck Creek Syncline. As well as forming this superficial structure, the basement ridge also effects the partition of the 'Stuart Basin' from the main part of the sedimentary basin. Sediments of the Wyloo Group up to the top of the Ashburton Formation were deposited in the entire basin, at which time the culmination of the Opthalmian Fold Episode occurred. Acid to intermediate pyroclastic vulcanicity erupted into the Stuart Basin at this time, in places narking the upper limit of the Ashburton Formation. The unmetajnorphosed sediments which overlie the Ashburton Foirration have been designated the 'Stuart Formation'. These overlie the lower rocks with apparent structural unconformity on a shallowly undulose decollement surface which may, in places, represent an erosional unconformity as well. The'entire Wyloo Group contains minor copper and lead mineralization in quartz veins and shears, this mineralization having maximum intensity in the Ashburton Formation. The overlying S'cuart Formation is completely barren, as far as is known, so that the time of maximum mobility of base metals in this area appears to occur at or about the volcanic" phase of sedimentation. This coincidence of mineralizing and volcanic episodes at a time when a tectonic basement structure acts to partition a subbasin of a major sedimentary domain, encourages the implication of possible base-metal accumulations being formed at this time in this area.

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78. T H E P A R A B A R A N A COPPER PROSPECT A POSSIBLE V O L C A N O G E N I C S E D I M E N T A R Y DEPOSIT IN T H E LOWER P R O T E R O Z O I C M O U N T P A I N T E R BLOCK, S O U T H A U S T R A L I A

K. Every South Australian Department of Mines

The Parabarana Prospect lies in the ME corner of the Mount Painter Block in the northern Flinders Ranges, 55 kin north of Adelaide. North Flinders Mines Ltd. holds title to the area. The Mount ^ Painter Block is a complex crystalline basement inlier to the Adelaide System. It comprises a thick sequence of metasediments (Radium Creek Metamorphics) of probable Lower Proterozoic age, and is intruded by Carpentarian alkali granites (Older Granite Suite). At the Parabarana Prospect metasediments and acid volcanic rocks, which are tentatively correlated with the Brindana Schist of the Radium Creek Metamorphics, are host to the copper mineralisation. These metasediments are interpreted as stratigraphically overlying an an older augen gneiss besement in an overturned sequence. The Parabarana copper deposit is interpreted as being primarily of volcanogenic-seddmentary origin. Metals, which were supplied to a closed muddy basin from a nearby volcanic source, combined with biogenic sulphur to deposit sulphides. The general sequence of rocks in the Parabarana area, in stratigraphic order from north to south is as follows 1)

A granitic (to syenitic) paragneiss consists of K-feldspar-rich augen m a foliated matrix of chlorite with or without quartz. A strong gneissic foliation dips steeply to the NW. The augen gneiss unit also contains subparallel bands of semi-pelitic schists and amphibolites. It has previously been interpreted as the stressed equivalent of the intrusive Carpentarian Terrapinna Granite but is now thought to have been mostly an arkosic sedimentary pile".

The Brindana Schist forms a northerly dipping sequence, the middle portion of which is host to the mineralisation. It is interpreted to be of volcano-sedimentary origin and may be divided into the following subunits:i)

The hanging wall rocks include andalusite, graphite and retrograded chlorite schists, minor amphibolites, mylonites and breccias.

ii) The mineralised zone comprises banded hornfels, acid igneous rocks and sericite-chlorite rock. The banded hornfels consist of alternating pink and dark greenish-grey layers up to 1.5 cm thick. The pink bands are interpreted^as being tuffaceous arenites; the dark bands were calcic pelites and contain the bulk of the mineralisation. The acid igneous rocks, now represented by pink massive microadamellite with •relic micro-banding, are considered to have been flow rhyolites. The sericite-chlorite rock is strongly sheared and is a metasomatised metasediment. iii) The footwall calc-silicate hornfels consists of pink feldspathic and green amphibolltic and epidotic bands and are thought to have been tuffaceous silts tones and calcareous shales.

79.

^ Mount Neil Granite Porphyry is a slightly stressed granite wmch has intruded the Radium Creek Metamorphics.

The mineralisation comprises chalcopyrite, pyrite and arsenopyrite with traces of marcasite, molybdenite, sphalerite, uranium minerals and gold; it^occurs mostly in microfractures and shears with only minor disseminations, and is stratabound predominantly within the ban banded hornfels. The mineralised zone occurs as a surface truncated lens, which dips about 550NNW? ^ i s 10 _ 30 m t h i c k 5 5 Q 0 m i n ^ ^ ^ strike width and 900 m in proven down-dip length, Cu assays average 1.1% over the thickest part of the mineralised zone. Assays within the banded hornfels unit vary from 0.5% to 12% Cu. Up to 7% Cu and 2% Mo ^occur in the narrow sheared contact zone in,the sericitechlorite rock immediately below the acid volcanics. Except for minor mineralised shear zones all other units are barren. Discontinuous mineralisation occurs in racks of similar lithologies up to 15 km from Parabarana. The complicated metamorphic and structural history of the area can be related to a prijnal crustal break now represented by the Paralana fault system. This was responsible for at least five recognisable tectonic episodes, which are as follows:1)

Conversion of sediments to augen gneiss of lower amphibolite grade by dynamic regional metamorphism.

Sedimentatation, volcanism and mineralisation.

Dextral strike slip movements causing brittle deformation, ^ crushing and retrogressive metamorphism.

Intrusion of Mount Neil Granite Porphyry causing contact metamorphism, metasomatism and MhydrothermalM mobilisation of sulphides into fractures, and shears.

Post Carpentarian thrusting of the augen gneiss block and development of a closely-spaced imbricate faulthing of the Brindana schist, which resulted in an apparent upward rotation of the steeply dipping mineralised zone.

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80. THE MORDOR COMPLEX: A POTASSIC INTERMEDIATE TO ULTRABASIC INTRUSION WITH K I M B E R L I T I C AFFINITIES, C E N T R A L AUSTRALIA

A.P. Langworthy and L.P. Black Bureau of Mineral Resources, Canberra The Mordor Complex is a roughly equant intrusion of potassic intermediate to ultrabasic rocks 50 km east-northeast of Alice Springs in Central Australia. The complex is 6 km in diameter, and crops out inside Mordor Pound, which is an area, 12 km across, made up mainly of leucocratic gneiss and amphibolite of the Arunta Block, almost completely enclosed by a roughly horseshoe-shaped wall of Heavitree Quartzite. The authors mapped the Mordor Complex in 1972-3 during regional mapping of the Arunta Block at 1:100 000 scale, and tnis paper summarises the preliminary laboratory investigations. The Complex is made up of a suite of highly fractionated rocks ranging from phl'ogopite peridotite through pyroxenite, shonkinite, melamonzonite, and monzonite, to syenite. The syenite is a coarsegrained. homogeneous intrusion occuyping the western half of the complex. This is intruded by basic differentiates that occupy the eastern half, and are themselves intruded by numerous ultrabasic plug-like bodies, up to 200 rn across, and by pegmatite dykes. The ultrabasic plugs consist essentially of olivine (F075), diospidic augite, bronzite, phlogopite, calcite, apatite, and iron oxides. The more fractionated rocks contain interstitial microcline, and with decreasing basicity, sodic plagioclase. The Complex as a whole is low in silica and soda but enriched in aluminium, calcium, magnesium, potassium (up to 7.2%K), barium (up to 1.5%), rubidium, and strontium (up to 0.6%). It has a high K/Na ratio a low K/Rb ratio (110-280), and a low Mg/Fe ratio. Thirteen wholerock samples lie on a Eb-Sr isochron which gives a date of 1230±100 m.y., and has a high initial 87s r /86 Sr r a t i o (0.711*0.001). The unusual chemical features of the Mordor Complex indicate a similarity to the high-K rocks of the Leucite Hills in Wyoming, the volcanics of the Bufumbira province of Uganda and the Roman province of Italy, the kimberlites of Basutoland, and the leucitites of W.A. Q-mode factor analysis of these and comparable provinces confirms the chemical similarity between the ultrabasic rocks of the Mordor Complex and kimberlites. However, the ultrabasic rocks in the Mordor Complex have crystallised slowly under lower P-T conditions than kiinberlites. They do not contain pyrope, jadeitic-diopside, chrome-spinel, or high-magnesium olivine, nor are they porphyritic, brecciated, or tuffaceous, and they do not contain ultrabasic autoliths. The parent magrra was intruded to a fairly high level in the crust, in much the same way as envisaged for kimberlites; however, instead of them being explosively extruded, it was probably held at a high temperature in a zone of hot metamorphic rocks sufficiently long for differentation to syemtic end-products to take place. 0O0

THE T O L L U VOLCANICS: A BjMODAL IGNEOUS PROVINCE

81.

D.F. Blight, R.W. Nesbitt S P.R. Bowden Department of Geology and Mineralogy, University of Adelaide. The volcanics of the Tollu Group outcrop at the western end of the Musgrave Block, Central Australia. In the Tollu area a sequence of acid'"and basic terrestrial volcanic rocks, granophyres, granites and gabbros, rest upon and intrude a basement of foliated quartz feldspar-biotite gneisses. The area of volcanism studied is surrounded on three sides by mafic stratiform intrusions. (Giles complex). Presumed Ordovician.sediments of the Officer Basin unconformably overlap the volcanics on the fourth and southern side.' This paper describes the geology and geochemistry of the area and attempts to elucidate the relationships between the volcanics, the granophyres and the Giles Complex#Gabbros. We suggest that the major igneous events are interrelated and that the bimodal nature of the rocks results from a major melting episode of sialic crust about 1100 m.y. ago.

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82.

PETROGENESIS OF METAMORPHOSED U L T R A M A F I C - M A F I C COMPLEXES IN T H E G R E E N V A L E AREA, N O R T H - Q U E E N S L A N D

M.J. Rubenach and G.O. Arnold James Cook University The Gray Creek, Boiler Gully and Sandlwood Complexes occur along the boundary of the Proterozoic Georgetown Province('Block1 )and the P Sf° Z °i C B r °S e n R i v e r E v i n c e CEmbayment'). These metamorphosed ultramafic-mafic complexes were fomed during a series of intrusive metamorpnic and deformational events ranging over a period of at least 700 m.y. in the Proterozoic and Lower Paleozoic. Following emplacement ^ of layered ultramf ic-gabbro bodies into pelitic sediments and (?) basic volcamcs, the first deformation (D-,) and associated metamorphism resulted in isoclinally folded amphifiolites and schists wrapped around relict lenses of ultramafics and gabbros. Di probably occurred around 1,100 m.y. Following Di, the Gray Creek Complex was intruded by: a) gabbros, tonalites and trondjhemites, b) dyke swarros comprising a variety of porphyritic calcalkaline rocks, and c) tholeiitic dykes. Non-conformable deposition of quartz-rich flysch and tholeiitic basalts over the Gray Creek Complex preceded a Mid-Ordovician deformation, D 2 . D2 produced isoclinal folding in the three Complexes and was associated with amphibolite facies metamorphism. Following D2 the Georgetown Province was affected by granitic intrusions and provided xhe source for voluminous rhyolitic/dacitic volcanic detritus in the SiluroDevonian Flysch sequences of the Broken River Province. Abundant andesitic volcanics and dykes of the Upper Ordovician Everett Creek Volcanics, faulted against the eastern margin of the Gray Creek Complex, are believed to have derived during this regime. Further folding of both basement and cover rocks occurred in the Devonian and again in the Carboniferous,

The complex history of the metamorphosed ultramafic-mafic complexes is beleived to result largely from their early emplacement along the edge of a Proterozoic sialic area which continued as a continental margin through the Lower Paleozoic. Three major peroids of igneous activity represented in rocks within or adjacent to the Gray Creek Complex (the initial ultramafic-mafic complexes; the post - D]_, pre - D2 gabbro, trondjhemite and tonalite intrusives, and calcalkaline and tholeiitic dykes; and the andesites of the Everett Creek Volcanics) are beleived to have formed during three distinct tectonic regimes affecting this continental margin, over a large interval of time. Rocks from the various suites comprising the Complexes have been analysed for major elements and Cr, Ni, Zr, Y and Nb. Comparisons have been made with rocks from various ophiolite complexes and from various Cenozoic oceanic environments. Details of the geological history place severe restrictions on the application of simple plate tectonic models to the area. This is well illustrated by the Gray Creek Complex which contains most of the constituents on an ophiolite complex. The interpretation of this Complex as an obducted slice of oceanic crust underlying the SiluroDevonian Flysch would provide an elegant plate tectonic model. However such an interpretation neglects abundant evidence for the origin of the constituents of the 'ophiolite* during three different tectonic regimes.

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83. B U R I A L M E T A M O R P H I S M OF THE P R O T E R O Z O I C F O R T E S C U E GROUP, WESTERN AUSTRALIA - A RECONNAISSANCE STUDY

Raymond E. Smith C.S.I.R.O, Division of Mineralogy Examination of authigenic mineral growth in basic volcanic rocks of the Fortescue Group shows that metamorphic adjustment has taken place under hydrous conditions within a moderate to low pressure and moderate temperature regime. Two broad zones of metamorphic adjustment can be recognized at this stage. The lower grade zone belongs to the prehnite-pumpellyite f&cies and the higher is.the greenschist facies which is marked by the first appearance in basic rocks of a fibrous amphibole in the tremolite-actinolite group. Two associations can be seen within the prehnite-pumpellyite facies: some combination of quartz-albite-epidote-prehnite-pumpellyitechlorite-sphene; OR quart z-albite-epidote-chlorite-sphene-sericitecarbonate. Within the greenschist facies the typical association is quartz-albite-epidote-chlorite-actinolite-sphene with or V/ithout sericite and carbonate but carbonate is not abundant. Samples from the south side of the Hamersley Basin show general greenschist facies associations while samples from the north side of the basin and from those areas lapping across the Archaean Pilbara Block show lower-grade associations belonging to the prehnite-pumpellyite facies. An exception to this general picture is in the far west of the north side of the Hamersley Basin where higher-grade associations belonging to the greenschist facies are developed.

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84. PETROLOGY OF A R C H A E A N PERIDOTITES, CORSAIR ROCKS, W.A.

I.A. Nicholls Research School of Earth Sciences Australian National University Canberra, A.C.T. Archaean peridotites from Corsair Rocks, near Kalgoorlie, W.A. consist of 1% chromian spinel, 4-5% serpentine pseudomorphs after equant to skeletal olivine, 20% skeletal clinopyroxene and 35% finely crystalline to glassy matrix. Clinopyroxene and matrix material probably represent the products of rapid cooling of liquid during extrusion or shallow intrusion of a partially crystallized peridotitic magma. The estdjnated composition of this interstitial liquid is itself ultramafic, indicating that magma generation and subsequent crystallization took place at temperatures probably exceeding 1500°C. Bulk rock trace element abundances suggest that the source of peridotitic magma had geochemical characteristics broadly similar to those of the present-day mantle.

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Present address: Department of Earth Sciences Monash University Clayton, Victoria

85. P O R P H Y R O I D A L ROCKS I N T H E W A R R A M U N G A G E O S Y N C L I N E

D. McP. Duncan Western Mining Corporation

The Lower Proterozoic succession of the Warramunga Group, Northern Territory, Australia, is composed of shales, greywackes and conglomerates with minor haematite shales and cherts having a total thickness of 8,000 metres. At Tennant Creek, the lower part of the succession contains conformable horizons, discontinuous lenses, dykes and other isolated bodies of porphyroidal rocks. Although showing varying degrees of recrystallization and deformation due to greenschist facies -regional metamorphism, the porphyroidal groundmasses contain relict textural patterns typical of volcanic pyroclastics. The megacrysts bear a remarkable number of embayrnents and cavities whose presence is attributed to magnetic corrosion, rather than irregular, skeletal growth. The highly-ordered structural condition and the compositions of the potassium feldspar and plagioclase, are consistent with recrystallization at temperatures typical of greenschist facies metamorphic, rather than magmatic, conditions. The major element compositions of the porphyroids show similarities to those of calc-alkaline rhyolites. The petrographic, mineralogical and chemical similarity of the porphyoids to high-level granites and enclaves of the neighbouring Tennant Creek Complex, suggests a genetic association in which the granites have intruded and disrupted their own volcanic and hypabyssal framework within the Warramunga Group sediments. The widespread record of porphyroidal rocks in the region indicates that acid volcanism, at least in part of calc-alkaline nature, formed an integral part of the development of the Warramunga Geosyncline.

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86. A METAMORPHOSED REGOLITH FROM THE ARUNTA BLOCK CENTRAL AUSTRALIA

R.G. Warren Bureau of Mineral Resources, Canberra Rocks collectrd from three localities in the Arunta Block, one on Delmore Downs station and two in the Reynolds Range, contain mineral assemblages derived from chemical composition low in SiC>2 ^ C a 0 b u t high xn^MgO and A1203- Metamorphic assemblages, depending on regional grade, include chlorite-anthophyllite-cordierite and sapphirinespinel-enstatite. All the sites are close beneath a major unconformity there is no association with basic or ultrabasic rocks; and detrail zircon is present in the rocks. Hence the parent material is considered to be supracrustal. Modern ^ materials that approach the required composition include superficial magnesite deposits, magnesium-rich solonized soils, parna deposits and alkaline lake or lagoon fills. Material of the requisite composition could have also been produced by the diagenic addition of magnesite to a kaolinitic weathering profile by grtound water. The stratigraphic age of the parent material is considered to by younger than that of the sapphirine-bearing rocks of the Harts and Stmngways Ranges, and the latter are also probably different genetically Unlike the sapphirine -bearing • and related rocks present at Oonagalabi (and several other similar metalliferous prospects in the Ausnta Block) these are not associated with sulphide minerals.

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87. STRATIGRAPHY, SEDIMENTATION AND STRUCTURE IN THE BANGEMALL BASIN, WESTERN AUSTRALIA

A.T. Brakel and P.C. Muhling Geological Survey of Western Aust. The Bangemall Basin is a folded, Middle Proterozoic sedimentary basin with an arcuate east-weat elongation, discordantly overlying the older tectonic units of the Western Australian Shield, A regional facies change in the Bangemall Group occurs in the central, part of the basin near the 119°E meridian and is the result of an easterly lensing out of dolomite, chert and sandstone units. It partly corresponds to a major northeasterly trending zone of faulting and basement arches. Correlation of much of the eastern succession with the formations to the west and north is uncertain. In the western portion of the basin most of the lower units were deposited in a shallow marine environment, but local, discrete lenses of coarsegrained, terrestrial, alluvial fan deposits occur on the basal unconformity. These lower units of dolomite, lutite arid sandstone are characterized by lensing, interfingering and lateral gradations. Near the top of these units is the Discovery Chert, representing a period of stable conditions over most of the western region. The rest of the sequence was laid down in deeper water and consists of laterally extensive lutites with interbedded sandstone sheets, two of which persist for distances of over 175 km. In the eastern part of the basin the succession is simpler, with laterally persistent lutites and a prominent sandstone. The lower lutite • rests on the basal unconformity or is separated from it by locally developed sandstones and conglomerates. Along the northern margin of the basin there is a thick dolomite-bearing sequence, equivalent to all the shallow water and some of the deeper water formations of the western succession. Basic sills and dykes arc common. The only confirmed volcanic rocks are two minor felsic lavas near the zone of the regional facies change. Folding has occurred on two arcuate trends, one concentric with the preBangemall plateform to the north the other concentric with the present limits of Archaean outcrop to the south. Northeasterly trending zones of basement arches, facies changes and faulting are prominent in the westtern portion of the basin. These zones were active during and after sedimentation. Other northeasterly lineaments within*,- the eastern succession are frequently the sites of copper mineralization. Several major faults, two of which are known to be thrusts, occur in:the centre of the basin where the arcuate trends converge. 0O0

T H E N A B B E R U BASIN: A N E W L Y D I S C O V E R E D LOWER BASIN IN W E S T E R N A U S T R A L I A

PROTEROZOIC

W.D.M. Hall 8 A.D.T. Coode B.H.P., Melbourne

The Nabberu Basin lies along the northern margin of the Yilgarn Block north of Wiluna and Meekatharra in central Western Australia. It extends at least 600 km in an ESE-WNW direction and is about 120 km wide in the vicinity of Lake Carnegie. The sediments lie unconformably on an Archaean granite/greenstone basement, and are unconformably overlain by the Middle-Upper Proterozoic Bangeirall Group to the north. In the east, the Nabberu sediments are intruded by dolerites and are partly concealed beneath sediments of the Officer Basin. The Nabberu Basin is thought to be Lower Proterozoic in age on regional grounds and on limited absolute age dating evidence. Along the southern margin of the basin the Lower Proterozoic rocks dip very gently northwards. Deformation increases north across the basin until along the northern margin large asymmetric folds are slightly overturned southwards, and accompanied by north dipping thrusts. A strong slaty cleavage becomes progressively more conspicuous northwards. The fold belt trends WNW across the eastern portion of the basin before swinging to the SW. To the north and west the Archaean basement becomes increasingly involved in the deformation, and becomes progressively more gneissic as the Lower Proterozoic rocks become more strongly schistose. Refolding of early structures becomes pronounced in the west. The Nabberu sediments are essentially unmetamorphosed in the southeastern part of the basin but become progressively more metamorphosed towards the fold belt, reaching a maximum grade of granulite facies west of the Robinson Ranges. In the least metamorphosed southeastern area, the basin is occupied by about 6000 metres of shallow water sediments. Thin quartzose to arkosic clastics (Yelma Formation) at the base rest unconformably on Archaean rocks, and are overlain by chert, shale, banded iron formation and minor carbonate (Frere Formation), and thinly bedded carbonate, shale and sandstone (Windidda Formation). The overlying Wandiwarra Formation (sandstone and shale) is locally transgressive and disconformable, and steps over onto the Frere Formation. The Wandiwarra Formation is conformably overlain by clean supermature sandstones and siltstones (Princess Ranges Quartzite), siltstones and fine sandstones (Wongawol Sandstone), thinly bedded carbonate, shale and fine sandstone (Sholl Creek Formation) and the Kulele Creek Limestone. Distinctive assemblages of stromatolites occur in carbonate units throughout the sequence. The lower group of formations comprises an initial transgressive sequence followed by a regressive phase leading to the subaerial exposure of the Windidda Formation. The upper group represents a second transgressive^ sequence. Glauconite is found at the base of both transgressive sequences. The banded iron formations of the Frere Formation are shallow water (reworked or granular) in origin and are similar to those of the Lake Superior and Labrador provinces in North America. Pelletal (intraclastic) ferruginous cherts are the most common rock type, although oolitic iron formations are found locally. Benthonic microfossils identical to those in the Lake Superior iron formations are found in oncolites (unattached stromatolites) in one locality.

89.

West towards the Peak Hill - Robinson Ranges, area the basal clastics become Considerably thicker and more varied (generally finer grained), and are commonly interbedded with basic volcanics and greywackes. The Peak Hill Beds of MacLeod (1970) are lateral equivalents of the Yelrna Formation, while the overlying Horseshoe Range Beds, Labouchere Beds and Robinson Range Beds are lateral equivalents of the Frere Formation. 0O0

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S T R U C T U R A L SYNTHESIS FOR T H E DEPOSITION OF T H E WYLOO AND B A N G E M A L L GROUPS

R.E. Smith and R.C. Horwitz C.S.I.R.O. Division of Mineralogy The Proterozoic Wyloo and Bangemall Groups lie in a complex position between the Archaean Pilbara and Yilgarn Blocks. Coinciding with the distribution of the Wyloo and Bresnahan Groups is a major positive Bouguer anomaly which does not appear to be explained by the outcropping Proterozoic lithologies. This, together with the shape of the anomaly has led to our proposal that the positive gravity anomaly represents a crustal separation feature where the density contrast is provided by a slab of basaltic material coinciding with the position of separation. Interpretation of gravity pattern together with examination of the distribution of dolerite dykes and sills has led to tentative mapping of the boundaries of the two silalic plates that correspond to the Archaean Yilgarn and Pilbara Blocks. A major lens-shaped negative Bouguer anomaly located within the intermediate zone at least in part coincides tS/ith a Proterozoic granitic intrusive. Initial separation would be pre-Wyloo in age. Perhaps the basic volcanic episode that caused the eruptives of the Fortescue Group is a result of this separation. Erupted basic material is postulated as marking the position of separation. Later subsidence of the dense slab could have provided the locus of a trough into which Wyloo and Bresnaham sedimentation took place. The abundance of dolerite sills in parts of the Bangemall Basin too, bears evidence that there was access to large quantities of basic magma. This also m y be related to the situation of the basin at the junction or separation of the two sialic plates. 0O0

91. T H E S O U T H E R N B O U N D A R I E S OF THE H A M E R S L E Y A N D B A N G E M A L L BASINS OF S E D I M E N T A T I O N

R.C. Horwitz C.S.I.R.O. Division of Mineralogy

The southern boundaries of the Hamersley and Bangemall basins, against the Archaean Yilgarn Block are re-defined. This results from field work, initiated by the Bureau of Mineral Resources near Lake Carnegie (Jackson, this Symposium) and extended west to about longitude 118°• However, a comparison to the Preliminary Bouguer Anomalies map shows that the boundaries may be extended, with fair confidence, further to the west (Smith and Horwitz, this Symposium). Between Lake Carnegie and Peak Hill, the Yilgarn Block is overlapped by three sequences which each transgress onto the Archaean. The oldest, which contains oolitic cherts and banded iron formations, has more in common with the Lake Superior Gunflint and Biwabik iron formations than with those of the Hamersley Ranges. Facies distributions suggests a derivation from the north. The youngest sequence, in this complex grouping, with an unconformity at its base has been dated on glauconites as Carpentarian by the Australian Mineral Development Laboratories, initially in the east (Jackson, this Symposium) then further west. This complex grouping is overlain, with angular unconformity, by sediments of the Bangemall Group. A new palaeogeographic interpretation, based largely on facies distribution, is proposed for the two following basins of sedimentation: the Lower Proterozoic Hamersley and the younger Bangemall. Reference is made to tectonic evolution and structural patterns developed by Smith and Horwitz (this Symposium).

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92. R E G I O N A L GEOLOGY, S T R O M A T O L I T E BIOSTRATIGRAPHY A N D ISOTOPIC DATE BEARING O N THE AGE OF A PRECAMBRIAN SEQUENCE NEAR LAKE CARNEGIE, WESTERN A U S T R A L I A

W.V. Preiss M.J. Jackson R.W. Page £ W. Compston^

At the northeastern margin of the Yilgarn Block, southeast of Lakes Carnegie and Wells, the Archaean basement is unconformably overlain by a thick, almost undeformed sequence of Proterozoic clastic and carbonate rocks containing mafic intrusions. Recent reconnaissance mapping and seismic work in the Officer Basin Area to the east indicates that this thick Proterozoic sequence can be divided into an older unit up to 12,00Qm thick (seismic interpretations) unconformably overlain by a younger unit up to 7,000m thick. Isotopic dating and studies of stromatolites provide the first indications of the age of these two sequences. Based on seismic interpretations and regional geological comparisons, the younger sequence is equated with the sedimentary rocks (Townsend Quartzite and overlying rocks) that flank the southwest margin of the Musgrave Block. The Townsend Quartzite undonformably overlies the .. Musgrave Block which contains acid volcanics in the Tollu Group dated at about 1100 m.y. Consequently, this implies a late Proterozoic age for the younger of the two sequences. This is supported by the presence of gcUcaLici cf. B.BaAAa from this sequence at two widely separated localities within the Officer Basin area; BalcaLia buAAa is a widespread stromatolite in the Burra Group of South Australia. Glauconites from a bed 5 m above the base of the older sequence were dated isotopically and stromatolites from beds 15 m and 1500 m above the basement were studied palaeontologically. If the previously known time ranges of stromatolite groups were accepted, then stromatolite biostratigraphy and geochronology would provide conflicting evidence as to the age of the older Proterozoic sequence. Ages of glauconite from six sandstone samples are closely grouped between 1670 and 1710 m.y. by K-Ar, and lie between 1590 and 1710 m.y. by Rb-Sr, so that the original age of the glauconites must be at least 1700 m.y. Provided the glauconites are not reworked, this also represents the minimum age of the sediments. A dolerite sill that intrudes the sequence about 1500 m above the base has been dated by the Rb-Sr technique, and a sill of similar dolerite (assumed coeval) has been mapped intruding the sequence only 20 m above the level of the stromatolites and glauconite. The Rb-Sr mineral isochron gives an age of approximately 1050 m.y. This is interpreted as a ndnimum estimate for the original emplacement or secondary alteration of the rock. The stromatolites examined include specimens from the same locality and the same stratigraphic level as the glauconites (5 - 15 m above the basement). Stromatolites from about 1500 m higher were also examined. New forms#of the stromatolite groups MlnjcvUcL, TunguAtla. and Tcuiiou^^Ua were identified. Previously these groups were only known from Late Proterozoic beds. TaAAsOU^eMcL and llinjcuiiahad been described only from the Late Riphean*(950 - 680 m.y.) and TunguAAla from the Middle Riphean to Vendian (1350 - 570 m.y.). A fourth stromatolite, ? cf. Kalpcuila is also present; elsewhere in Australia, KulpcuUa occurs in Late Riphean or Vendian equivalents. It is uncertain whether these occurrences represent disjunct time ranges (and possible convergent evolution) of groups, or simply downward extensions of continuous time ranges. Based solely on

93.

group level identifications of stromatolites a late Proterozioc age might have been assigned to these rocks. Caution is not only needed in assigning ages to unknown Precambrian sediments on the basis of stromatolites alone, especially if identified only at group level, but also in interpreting isotopic results from isolated samples, especially of glauconite. Initially, it was hoped that the three-fold approach to dating the sequence would provide an unambiguous age; this, however, was not achieved.

2 3

Dept. of Mines, Adelaide Bureau of M m . Res. Canberra Australian National University, Canberra

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9**. G E O L O G Y OF T H E EAST K I M B E R L E Y REGION, WESTERN A U S T R A L I A

I. Gemuts Anaconda Australia Inc. The Kimberley region occurs in the rugged northern extremity of W.A. and is composed mainly of Precambrian rocks. The oldest rocks are the Halls Creek Group which were probably deposited over the entire East Kimberley region in Archaean time. These were then tightly folded, slightly metamorphosed and intruded by extensive mafic bodies, some of which produced ultrairafic differentiates. A narrow zone of intense folding and high grade metamorphism was superimposed on this terrain, along the Halls Creek Mobile Zone. This produced the Tickalara Metamorphics, which have been subdivided into 3 zones ranging from green schist to granulite facies. The Mabel Downs Granodiorite m y have been generated during this episode, and posttectonic batholiths and stocks of the Lamboo Complex completed the igneous/metamorphic cycle. In early Carpentarian times, the area became stabilized into competent blocks which deformed almost entirely by dislocation along major faults which parallel the Mobile Zone, Sedimentation upon these platforms began with vast eruptions of acid volcanics along the fault zones, followed by deposition of extensive monotonous arenaceous and argillaceous facies across them. A glacial epoch comprising at least two separate episodes in Adelaidean time left a remarkable record of extensive ice caps, and a thick cover of glacial sediments. Outpourings of basalt in the Lower Cambrian covered almost the entive area, and presently obscure extensive areas of Precambrian rocks in this region.

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95. A COMPARISON OF THE PROTEROZOIC GEOLOGY OF THE K I M B E R L E Y BLOCK A N D T A N Z A N I A

R . Halligan North Broken Hill Ltd. There are striking similarities between the broad geological patterns of the Kimberley Block and the Pre-Cambrian of Tanzania. Both consist of a broad central shield which is flanked by elongate mobile zones with high-grade metamorphic rocks, and overlain by unmetamorphosed clastic sediments. In the Kimberleys, the central shield or eraton is not exposed, being covered by the thick sequence of Lower and Middle Proterozoic quartzrich sediments of the Speewah and Kimberley Groups, and the Adelaidean glacial successions of the Kimberley Plateau. The Halls Creek and King Leopold Mobile Zones include probable Lower Proterozoic clastic metasediments, limestones, acid and basic volcanics of the Halls Creek Group, and their metamorphic equivalents of the Lamboo Complex and Tickalara Metamorphics. The mobile zones form a V-shape about the central plateau, or a reversed Y-shape if we include the rocks of the Granites - Tanami area. Large intrusive masses of granitic and gabbroic rocks are a feature of the mobile zones. In Tanzania, the typical Archean granite shield (the Dodoman is flanked by the Middle Proterozoic Ubendian mobile belt, which includes amphibolite to granulite facies rocks, and the Usagaran mobile belt of Mozambiquian a g e , (possible Middle Proterozoic). Large granitic and basic intrusives are well developed in the mobile belts. As in the Kimberleys, the mobile belts have a V-or Y-shaped disposition about the central shield area. The overlying sediments in both areas include greywacke, arkose, orthoquartzite, shale, limestone, "red beds" and flood basalt lithologies. Several systems, groups and unconformities occur within the sedimentary sequence, but the trend is for the older units to include greywacke and arkose, while the younger units are mainly shale and orthoquartzite. In both areas, limestones and "red beds" are closely associated. In the Kimberleys, the youngest rocks are the tillites, limestones and clastic sediments of the Adelaidean glacial sequences, but these lithologies are not known in Tanzania. The overall geological similarities suggest a similar geological history for the two areas. The development of a stable Archean craton, flanked by structurally controlled depositional troughs in which Lower Proterozoic lavas and sediments were accumulated. This was followed by a period of intense tectonism, metamorphism and intrusion. The overlying sediments represent a period of rapid erosion and deposition of shelf-facies sediments, which become progressively more mature and well sorted. The geochronology of the two areas is complex, because of the intense metamorphism and tectonism and is not well understood, but it seems clear that most of the Proterozoic rocks were deposited in Lower and Middle Proterozoic time. It is unfortunate that Proterozoic glacial rocks are unknown in Tanzania, for this would have been an invaluable geological datum, between Australia and Tanzania,^and would also have allowed correlations with the important Proterozoic sequences of Zambia and Zaire. Rocks of possible glacial origin occur in the lower part of the Bunyoro Series of Uganda, but the relationship of these rocks to Proterozoic sediments of Tanzania is not clear. 0O0

96. T H E P R O T E R O Z O I C OF T H E G R A N I T E S - T A N A M I , C E N T R A L A U S T R A L I A , AND REGIONAL CORRELATIONS

D.H. Blake Bureau of Mineral Resources Canberra The oldest rocks exposed in The Granites-Tanami region are tightly folded,low grade metasediments and metavolcanics of the Tanami complex: these are correlated with the Archaean or Lower Proterozoic Halls Creek Group of the Kimberley region to the northwest, and with part of the Arunta Complex of the Arunta region to the south. The Tanami complex of the Arunta region to the south. The Tanami complex is overlain unconformably by moderately folded, mainly clastic, Lower Proterozoic units, one of which, the Mount Winnecke Formation, includes acid volcanics dated at about 1800 m.y. These volcanics are inferred to be comagmatic with the Winnecke Granophyre, also dated at about 1800 m.y., and are probably younger than the Whitewater Volcanics of the Kimberley region. Other granitic bodies which intrude the Tanami complex have been dated at 1700 to 1800 m.y. The granitic intrusions and older rocks in the region together make up The Granites-Tanami Block, the basement on which Carpentarian and Adelaidean, mainly clastic, sedimentary rocks of the Birrindudu Basin were deposited. Most of the Carpentarian sedimentary rocks are mapped as the Birrindudu Group, which comprises the Gardiner Sandstone at the base, dated at between 1400 and 1550 m.y., the overlying Talbot Well Formation, which includes stromatolitic chert, and, at the top, the Coomarie Sandstone. These formations are moderately to gently folded. The Birrindudu Group is correlated with the Mount Parker Sandstone and Bungle Bungle Dolomite of the Kimberley region and with the Limbunya Group of the Victoria River region. It is overlain unconformably by the more gently folded Adelaidean Redcliff Pound Group, which is correlated with the Heavitree Quartzite and Bitter Springs Formation of the Amadeus Basin to the south and with the Vaughan Springs Quart zite of the Ngalia Basin to the southeast. The rocks of The Granites-Tanami Block and Birrindudu Basin are overlain unconformably by Cambrian Antrim Plateau Volcanics and younger Palaeozoic terrestrial sediments, and they are overlapped to the west and east respectively by Palaeozoic sediments of the Canning Basin and Wiso Basin successions. oOo

T H E P R E C A M B R I A N G E O C H R O N O L O G Y OF A N T A R C T I C A

P.A. Arriens Eucumbene Drive, Duffy A.C.T. Four zones of Antarctica are covered in this regional study. 1.

George V Land, using samples collected by the 1911-14 Australasian Antarctic Expedition.

Wilkes Land, in the Windmill Islands and mainland areas near the stations of Wilkes and Casey,

Princess Elizabeth Land, in the Vestfold Hills near Davis, and also at Landing Bluff near the Amery Ice Shelf and in the Rauer Islands,

MacRobertson Land, from the Mawson Coast to the Southern Prince Charles Mountains.

The geochronology of these regions is based on the interpretation of over 800 new Rb-Sr analyses of rock and mineral samples. At Cape Denison, George V Land, ages for muscovite and biotite from rocks collected in situ range from 1500 to 1700 m.y. Erratic boulders from moraine give ages from 1300 to 1700 m.y. In the Windmill Islands and on mainland areas near the stations of Wilkes and Casey, isochrons of between 1400 and 1100 m.y. are obtained for total-rock suites of gneisses of upper amphibolite to granulite facies. Muscovite and biotite ages of near 1100 m.y. in pegmatites are similar to the younger total-rock ages obtained from gneisses. The usual re-assembly of Gondwanaland, before Australia separated from Antarctica, places Eyre Peninsula in Australia, adjacent to George V Land, and the Albany-Esperance/Fraser Range area of ^ Western Australia opposite to Wilkes Land. The close comparison of the Antarctic age data with the corresponding geochronology of the Australian regions, is entirely consistent with an earlier conjugate geological history for these two continents extending well back into Proterozoic time. In Elizabeth Land, ages slightly over 2500 m.y. are. obtained from gneisses in the Vestfold Hills. K-feldspar ages indicate that pegmatite dykes in the region were emplaced before 2000 m.y. but biotite ages are reset to 500 m.y- A suite of total-rock analyses of dolerite dykes in the Vestfold Hill gives an age of near 1400 m.y. but no meaningful result was obtained from the analysis of separated mineral concentrates. Gneisses farther south in Princess Elizabeth Land are not intruded by dolerite dykes, and m the Rauer Islands give an age of apprcxijietely 1100 m.y. Granite at Landing Bluff near the Amery Ice Shelf is 500 m.y- old. In MacRobertson Land, Archaean rocks have been discovered at Mt. ^ Bayliss and in parts of the Mawson Escarpment. A complex geological history in the Prince Charles Mountains has produced poorly aligned total-rock isochrons for many of the gneisses, but the 2600-2800 m.y. total-rock ages are confirmed by a 2580 m.y. age obtained for muscovite from a pegmatite dyke cutting quartzite on Mt. Stinear.

98.

Younger total-rock ages for gneisses and granites cluster between 1200-1000 m.y. and between 700-500 m.y. One isochron for granite approximately 1100 m.y. old has an intercept for initial 87Sr^6 Sr of about 1.1, illustrating the hazard of quoting "ages" for single samples, based on some assumed value for initial 87Sr/86Sr. The extremely high initial 87Sr/86Sr for the Mt. Rymill granite gives clear evidence for derivation from pre-existing sialic crustal rocks. Pegmatite dykes and veins feature prominently in many rock exposures in the Southern Prince Charles Mountains, and where present in large stockworks may represent as much as 10% of the total volume of the rock. Most ages for muscovite, biotite and K-feldspare from pegmatites in MacRobertson Land range between 700-500 m.y., and similar ages are given by total-rock isochrons for massive granites in various localities. It is very likely that regional thermal heating attended the production and emplacement of these granites and pegmatites, because biotite ages in older rocks throughout MacRobertson Land are constantly reset to near 500 m.y. Similar up-dating of biotite ages to about 500 m.y. has also been observed in Princess Elizabeth Land, and in India.

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93. PRECAMBRIAN OF T A S M A N I A

N.J. Turner Tasmanian Department of Klines C. Boulter University of Tasmania The oldest rocks in Tasmania are predominantly of sedimentary origin and include regionally metamorphosed, greenschist facies assemblages and relatively unmetamorphosed assemblages. The main tract of unmetamorphosed terrain is the Rocky Cape Region which occupies the North Western part of the State. Transecting the Rocky Cape Region is a belt of greenschist facies, regional metamorphics called the Artnur Lineament. The belt is transitional with adjacent unmetamorphosed rocks along its western margin near Wynyard and probably also at Arthur River. A variety of sedimentary sequences occur west of the Arthur Lineament, At Rocky Cape there is a thick ( + 5500 m) sequence of siltstone and orthoquartzite with minor dolomite and subgreywacke (Rocky Cape Group). Analysis of cross-bedding in the orthoquartzites indicates that for much of their depositional period the palaeoslope was to the NW with a foreland to the SE. An ephemeral foreland may have existed to the NW. At Arthur River the metamorphics pass westward into an orthoquartzitepelite sequence thence into a subgreywacke-pelite sequence. Overlying the Rocky Cape Group with angular unconformity at Black River is a sequence of conglomerate, orthoquartzite, chert and dolomite, Orthoquartzite with minor conglomerate and overlain by dolomite also occurs at Arthur River. Stromatolites nave been obtained from the latter locality and a type belonging to the group Baicalia and closely resembling Baicalia Burra (71300 m.y. - 700 m.y.) has been found in clasts in a conglomerate unconformably overlying the dolomite. East of the Arthur Lineament along the north coast is a sequence of quartzwacke-pelite called the Burnie Formation. Similar sequences occur at Badger Head, Mt. Bischoff and in the Pieman River-Zeehan area. A small part of the sequence near Zeehan has been regionally metamorphosed. Deformation and metamorphism in the Rocky Cape Region occurred during the Penguin Orogeny which included two main phases of deformation. K-Ar radiometric age determinations of syntectonic dolerites from Cooee near Burnie indicate the first phase occurred at 700-720 m.y.B.P. Rb-Sr radiometric age determinations of muscovites from syn-orogenic granites on King Island give values of 742-753 m.y. B.P. Possibly the deformation on King Island is equivalant to the Penguin Orogeny. No Precambrian granites are known in mainland Tasmania, but, granite boulders occur in basal Cambrian beds in the Zeehan area. Metamorphism within the Tyennan nucleus, which is the major tract of metamorphic terrain in the state, and in the small Forth nucleus attained a maximum grade of upper greenschist facies in the Barrovian metamorphic series. The rocks were mainly derived from orthoquartzite, pelite, and subordinate carbonate. One small occurrence of banded iron formation is known. It occurs within an inter layered amphibolite-quartzite sequence. Minor amphibolites derived from basic rocks occur and minor eclogite is present. Probably the concensus of opinion is that the metamorphic terrains of the Tyennan and Forth nucleii are older than the relatively unmetairorphosed terrains but this has not been proven. The only contact between the two so far observed is at Goat Island and it has been interpreted as

100.

a thrust surface bringing unmetamorphosed rocks over metamorphosed rocks. The Frenchman Orogeny has been postulated as the cause of metamorphism and polyphase deformation in the Tyennan and Forth nucleii. Radiometric data from the Frenchmans Cap area may support the contention that an orogeny preceding the Penguin orogeny occurred. Two events have been detected; one at 780 m.y. G.P. (?Penguin Orogeny) and another at 960 m.y. B.P. (?Frenchman Orogeny). These results are preliminary and are yet to be substantiated.

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T H E A P P L I C A T I O N OF R E F L E C T A N C E D E T E R M I N A T I O N S ON C O A L I F I E D A N D G R A P H I T I S E D P L A N T F R A G M E N T TO METAMORPHIC S T U D I E S

R. Offler and C.F.K. Deissel Geology Department The University of Newcastle Plant fragments (phytoclasts) are ubiquitous in terrestial and marine sediments. When metamorphosed, they undergo pregressive coalification and then graphitisation with increasing grade. Concomitant with the coalification and graphitisation is an increase in the mean maximum reflectance of the phytoclasts. This change in reflectance may be used in metamorphic studies to: (1)

show the variation in grade in a metamorphic terrain

(2)

to determine the grade of metamorphism in areas where index minerals are not developed

(3)

show that coalified and graphitised phytoclasts are more sensitive indicators of thermal variations than index minerals

(4) show that local fluctuations in PH2Q 'nave occurred during the formation of mineral(s) used for metamorphic zonation.

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102. S T R A T I G R A P H Y AND E N G I N E E R I N G G E O L O G Y OF THE A D E L A I D E C I T Y AREA

J.M. Lindsay and J. Selby Geological Survey of South Australia The city is near the S.E. edge of the Adelaide Plains Sub-Basin (St. Vincent Basin), and on the W. edge of the arcuate Para Fault Block. This Block has a Proterozoic - Cainozoic succession intermediate between that of the Adelaide Plains (up to 600 m subsurface Cainozoic) and the exposed Proterozoic Mt. Lofty Ranges nearby. The Block is bounded on the W. edge of the City by the Para Fault system, and only 5 km S.E. of the City by the active Eden-Burnside Fault system. The Eden Fault scarp 150 m high, forms the southern backdrop to the Adelaide Plains. Movement on this fault in 1954 gave the city an earthquake of Richter magnitude 6, resulting in $6 million damage. The Para Fault scarp, just W. of the City, is only 20-30 m. high, and this system is aseismic to date. Under the City, Cainozoic strata have a thickness of only 35m at Montefiore Hill, North Adelaide, but about 150 m near the S.E. corner. Montefiore Hill itself overlies a bedrock high of deeply and highly weathered siltstones. The Tertiary succession beneath the City, dipping gently S.E. to S. , is broadly similar to the coastal sections S. of Adelaide but there are significant differences. North Maslin Sands may be represented by thin and patchy aquifer sands overlying weathered bedrock in a few City bores. Widespread non-marine more or less carbonaceous clays silts and impure lignites, resting on such sands or on bedrock, and 5-35 m thick, may be termed Clinton Formation (middle-late Eocene). As in the classic Maslin Bay sections, marine influence appears in the overlying South Maslin Sands, and the transgression peaked in lower Blanche Point Marls. The Late Eocene planktonic foram. zone of Hantkenina psUnvctiva, locally well-developed and 1-2 m thick, is a useful biostratigraphic marker near basal Blanche Point Marls. . It seems to represent a warmer, more open-marine episode. Blanche Point Banded Marl, with tough siliceous bands and abundant sponge spicules5is a potential deep bearing member beneath the City, particularly North Adelaide. Overlying equivalents of Blanche Point Soft Marl, including an aquifer sand, are more marginal-marine and regressive. Late Eocene marine sand and clay/silt equate with the earliest part of the Port Willumga Beds and are the youngest midTertiary strata known with certainty below the City. Hallett Cove Sandstone (shallow-marine, Late Pliocene) truncates the Eocene succession with low-angle unconformity as at Maslin and Aldinga Bays. Subsequently the Sandstone was karst-weathered and eroded to a mesa form in the central City, with remnants in North Adelaide. A Quaternary alluvial sequence, deposited on and against erosional surfaces, includes Plio-Pleistocene Carisbrooke Sand, early Pleistocene Rindmarsh Clay and Keswick Clay, and clastic fill of the ancestral Torrens valley. Silty to gravelly carbonate and calcrete are capped by surficial red-brown clay and silt soils. Until recently, most large multistorey buildings in the City were supported on piles or piers into the Hallett Cove Sandstone. However this formation is weathered and cavernous, and contains clay-filled sinkholes and bands of loose sand. These factors combine to make it an unreliable bearing horizon, and large buildings now tend to be founded on reinforced raf?s within the very stiff Hindmarsh Clay Design of shallow foundations is complicated by the shrink-swell characteristics of the upper fissured zone of the Clay. Excavation in the City is hampered by shallow groundwater seepages associated with fissuing and g^gai development. Shoring is usually necessary in tne Clay, where gi-L&cuL. n^11c,pq nroeressive deterioration of the walls. Groundoccurs at^Sious levels within Hallett Cove Sandstone.

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103.

THE STRUCTURAL SETTING OF SERPENTINITES IN SOUTHEASTERN NEW ENGLAND

E.C. Leitch and R. Beattie University of Sydney

Numerous serpentinite bodies occur in the southeastern part of the New England Fold Belt. We are concerned here with those at Mount George and Yarras. At Mount George an elongate, steeply dipping mass of serpentinite outcrops over an area of 11 km. It has been emplaced along a major fault separating Middle Palaeozoic chert, jasper, siltstone, sandstone and basaltic rock from several thousand metres of Early Permian diamictite, sandstone and siltstone. Cold intrusion of the Mount George body, which contains blocks of jasper, amphibolite, albite, dolerite and schist, occurred at an advanced stage of Late Permian orogeny. Near Yarras numerous small pods, elongate lenses and irregular masses of serpentinite occur within a structurally disjointed mass of slightly metamorphosed sedimentary and basic igneous rock, Major lithologics in the ?Middle Palaeozoic host include chert, jasper, siltstone, basalt, dolerite and basaltic breccia. Schistosity in the serpentinites is frequently strongly folded, mainly about steeply plunging axes, approximately parallel to fold axes in the surrounding rocks. Inclusions of lithologies other than those of the host unit are absent from the serpentinite bodies. Differences between Mount George and Yarras serpentinites are believed to result from differing mechanisms and times of emplacement. The Yarras bodies occur within a probable melange formed during Middle-Late Palaeozoic subduction, and were detached from the mantle by thrusting in the downgoing slab. In contrast, the Mount George mass was only detached after cessation of subduction; it probably arose diapirically along a deep fracture that allowed egress of water into the uppermost mantly. Recognition that ultramafic rocks have frequently been derived as solid or largely soldd bodies from the mantle has led to them being accorded special significance in tectonic interpretation. However, our studies indicate that clear distinctions must be made between the ages of generation, alteration, detachment and emplacement of mantle-derived ultrabasic rocks. These rocks are not uniquely related to plate tectonic processes and they should not be indiscriminately grouped together and identified as ancient suture lines. 0O0

104. S T R U C T U R A L E N V E L O P E S IN A U S T R A L I A N T E C T O N I C P A T T E R N S

E.S.T. 0TDriscoll Western Mining Corporation

The tectonic pattern of the Australian continent is seen to contain major structural elements that are repeated on minor scales within local regional patterns. In morphotectonic studies of Australia, Sherbon Hills recognised four principal regional lineament directions, namely ENE, NNE, NNW and WNW. Of these, it appears that the dominant NNW trend ^ corresponds to the major axes of many structural and material distribution envelopes as well as to many prevailing directions of regional fold axes. It is therefore postulated that the liNW trend represents a fundamental orientation of the long axes of bulk deformation ellipsoids, repeatedly aligned in that direction from region to region. The four principal ingredients of the continental pattern are recognised in Australian mining districts by specific structural and mineral distribution alignments. In the Broken Hill district, for example, it is found that the total group of mineral occurrences lies at the intersection of two regional linear belts aligned respectively ENE and WNW. Although at a local scale the main "line of lode" is aligned along the ENE trend, the all-mineral distribution for the district as a whole is nevertheless related to a principal envelope of maximum incidence in the form of an ellipse trending NNW. In terms of systematic geometry, the NNW trend is seen as representing the direction of the maximum diametric plane - the AB plane - of the regional deformation ellipsoid. These particular trends in similar structural settings, with similar NNW elliptic distribution envelopes, are also identifiable in other mining districts such as Mount Isa, Cobar and Kalgoorlie. The principal lineaments are again reflected in other associated phen- . omena, including topographic distributions, drainage patterns and, on a grander scale, in the transcontinental gravity lineaments revealed by BMR geophysical surveys. The angular relationships between the principal structural lineaments and the alignments of their associated distribution envelopes provide a clue to identifying differential movements3 in contrast to derivative lineaments that are the resultants of these primary movements. In this respect, primary status must be regarded as relative. A primary generator of one set of derivative trends may itself be derived from yet a larger system of primary generators. Field observations show that conspicuous trends in both the northeasterly and northwesterly quadrants appear to exchange status at different scales and from place to place. In many ways, the tectonic framework of eastern Australia is a mirror image of that of western Australia. However, considerations of regional structural asymmetry and of the nature of observed local structural displacements suggest that one of the earliest continental deformation systems was developed by means of oscillatory left-hand (anticlockwise) displacements along faults and shear zones parallel to Sherbon Hills' WNW. lineaments. Such a conclusion is consistent with the widespread dominance of NNW distribution ellipses, including fold trends, throughout the continent. Where field deformation patterns are reproduced in minature kinematic models, they indicate that Sherbon Hills' four principal lineaments are predictable and mutually dependant components of the one unified system of related movements. They also show that under oscillatory conditions, the one system will generate folds along each or'any of

105.

the ingredient linear trends. A general deformation pattern, with associated distribution envelopes, is therefore depicted for the Australian continent as a whole, with the same basic structural ingredients appearing at a variety of scales. The pattern m y untimately be seen to account for the long sub-linear segments of the Australian coast-line as noted by Vening Meinesz. Terrestrial rotation and equatorial drag nay be invoked as fundamental sources of movement.

0O0

106. S T R U C T U R E A N D SEDIMENTOLOGY OF A R C H A E A N METASEDIMENTS NEAR LAWLERS, Y I L G A R N BLOCK, WESTERN A U S T R A L I A

J.A. Donaldson* and J.P. Piatt University of Adelaide. Uppergreenschist-facies metasedlments occupy a second generation (D2) isoclinal syncline (the Scotty Creek syncline) between the Lawlers mafic and ultramafic sequence on the east, and amphibolite-facies gneisses and granite on the west, Ultramafic conglomerate overlies pendotite on^the east side, and granite-boulder conglomerate overlies the unconformity that cuts acress the boundary between a greenstone belt and granitic terrain. The eastern contact and the ultramafic conglomerate are complicated by^first generation (Dl) isoclinal folds and by D1 and D2 tectonic slides. Weak deformational fabrics associated with Dl are largely obscured by later fabrics. The Scotty Creek syncline appears to be related to the major NNW-trending folds and strike-faults characteristic of much of the Eastern Scotty Creek syncline. Sedimentary textures and structures are well preserved in the eastern limb; schistosity is oblique to bedding and intersection lineations plunge north. In the western limb, sedimentary structures have been destroyed; schistosity is layer-parallel, and rodding lineations are subhorizontal. Several subsequent minor deformations have locally produced chevron folds and crenulation cleavages. In the well-exposed weakly-deformed eastern limb of the D2 syncline the component lithologies (siltstone, sandstone, pebbly sandstone, and conglomerate) display an upward coarsening sequence, with only minor reversals in the overall trend. Three environmental facies are recognised on the basis of textures and primary sedimentary structures. The lowermost facies consists mainly of interbedded siltstones and pebblefree sandstones that display abundant ripple marks, small-scale crossbeds, and structures formed by soft-sediment deformation; these sediments appear to have been deposited in a deltaic environment. Intercalated sandstone and pebbly sandstones in the central part of the section are characterized by alterations of parallel-laminated and crossbedded units, both of which display numerous channels and scours. This facies probably accumulated in flood plains traversed by braided rivers, or in alluvial fans subjected to episodic sheet floods. The conglomeratedominated uppermost facies, containing distinctly bedded units of confan deposition. These three facies constitute a progradational sequence; paleocurrent determinations based on crossbedding suggest a southwestern source. The source rocks were mainly granitoid, although mafic and ultramafic rocks did provide significant local contributions of detritis. Derivation of the coarse conglomeratic detritus was probably related to uplift of the source area along faults that were active during sedimentation. Similar sedimentary rocks occur in the Jones Creek area, 60km north of Lawlers. If rocks of the two areas are correlative, coarser and more angular granitoid detritus in conglomerates of the Jones Creek area may represent talus accumulations that are but sparsely represented in the Lawlers area.

*ARGC Senior Research Fellow on leave from Carleton University, Canada.

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