A REVISION OF THE STRATIGRAPHY OF THE WOLLOMBI
COAL MEASURES
(Wollombi Coal Measures Working Party of the Coalfield Geology Council of NSW)
Creech M.K. – Convener, Ives M., Stevenson D., Brunton J., Rigby R., Leary S., Graham P., Smith C., Atkins B., Knight C., Salter G.
ABSTRACT
The Wollombi Coal Measures Working Party was established in August 2000 with the aim of resolving several concerns regarding the nomenclature of the Wollombi Coal Measures and its use in the Hunter Valley Coalfield. Recent exploration of the upper seams of the Wollombi Coal Measures at Broke and Denman have highlighted miscorrelations in the type bore as well as related issues that rendered the use of Wollombi Coal Measure nomenclature problematic at best, and thus requiring considerable modification. Detailed correlations of tuffaceous strata and seams of the both the Wollombi and the Newcastle Coal Measures have clearly demonstrated their similarity and established that the strata are indeed the same across the northern Sydney Basin. Since Newcastle Coal Measure nomenclature was better understood, more practical in its application and had historical precedence, replacement of the Wollombi Coal Measures with Newcastle Coal Measure nomenclature was proposed by the Working Party.
As a result the Wollombi Coal Measures Working Party recommended the following stratigraphic revisions to the Coalfield Geology Council. At the November meeting of the Council the following resolutions were subsequently ratified:
• That the entire Wollombi Coal Measure nomenclature be replaced with Newcastle Coal Measure nomenclature in the northern Sydney Basin.
• That Amoco Wybong DDH1 be adopted as the “Reference Bore” for the Newcastle Coal Measures in the Hunter Valley.
• That the base of the Newcastle Coal Measures in the Hunter Valley be raised to the top of the Watts Sandstone so as to conform to the same nomenclature in the Newcastle Coalfield. (The Watts Sandstone is therefore retained in Hunter Valley nomenclature and is recognised as the equivalent of the Waratah Sandstone in the Newcastle Coalfield.)
• That the Singleton Supergroup in the Hunter Valley now consists of the Newcastle Coal Measures, the Watts Sandstone and the Wittingham Coal Measures.
Significant outcomes of this work include:
• Miscorrelations identified by previous workers in the Type Bore for the Wollombi Coal Measures (DMDCK11) have been confirmed. Unfortunately these miscorrelations involve the same seams targeted in recent exploration programs at Broke and Denman. It should be noted that the Greigs Creek seam (the only seam name in use prior to the ratification of the Wollombi Coal Measures in 1975) has also been miscorrelated in the Type Bore.
• High-resolution correlations across the northern Sydney Basin have now been identified throughout the Wollombi and Newcastle Coal Measures. This has been a necessary outcome required to support the recommendations of the Working Party. Highresolution correlations of seams below the Great Northern and Fassifern seam equivalents have not been recognised nor published previously.
• Newcastle Coal Measure nomenclature has been successfully applied across both the southern and western perimeters of the Hunter Valley and has been shown to be considerably more practical than the existing Wollombi Coal Measure nomenclature. This included the successful correlation between Amoco Wybong DDH1 (AWYB1) and the recently drilled DM Ridgelands DDH10, DM Sandy Hollow and DM Cuan Ck borehole located several kilometres both west and north (Brunton & Moore 2004). This success represents the most practical test of this proposed revision.
• AWYB1 has been photographed and all seams reconstituted. All significant tuffaceous units and coal seams have been correlated with Newcastle Coal Measure equivalents. As this bore is the suggested “Reference Bore” for the Newcastle Coal Measures in the Hunter Valley it is planned that this data will be readily available from the Department of. Mineral Resources, including being available electronically on the internet.
• A table of suggested correlations for all bores studied by the Working Party have been included to assist future workers in using Newcastle Coal Measure nomenclature in the Hunter Valley. Due to the sparse spacing of relevant bores it is anticipated that such correlations will be subject to review in the future.
• Glen Gallic Colliery, the only mine that has exploited a seam in the Wollombi Coal Measures (Greigs Creek seam), was located in 2001. Both this abandoned mine and a nearby outcrop of this seam were recorded and located using GPS.
INCLUSIONS
Table 1 Bore Summary
Table 2 Suggested Regional Correlations
Table 3 Current Stratigraphy
Table 4 Proposed Stratigraphy
Figure 1 Bore Location Plan
Figure 2 Regional Cross Section
Figure 3 DMDCK11 – Miscorrelation of Upper Seams
Figure 4 DMDCK11 – Comparison with Adjacent Bores
Figures 5 to 9 Additional High Resolution Correlations
Plate 1 Nalleen Tuff in DMDCK11
Appendix 1 Geophysics of the Newcastle Coal Measures: Amoco Wybong 1
Appendix 2 Photography of the Newcastle Coal Measures: Amoco Wybong 1
SUMMARY
The Wollombi Coal Measures Working Party (WCMWP) was established in August 2000 with the aim of resolving several concerns regarding the nomenclature of the Wollombi Coal Measures (WCM) and its use in the Hunter Valley Coalfield. Recent exploration of the upper seams of the WCM at Broke and Denman identified miscorrelations in the type bore as well as related issues that rendered the use of WCM nomenclature problematic at best, and thus requiring considerable modification. The WCM stratigraphy was defined on correlations of clastic strata and coal, with no priority given to the more laterally persistent tuffaceous units. This runs contrary to the methodology currently in use in the Northern Sydney Basin.
The well documented similarities between the Newcastle Coal Measures (NCM) and WCM suggested replacement with NCM nomenclature could be considered, such a proposition being likely to promote a better understanding of the WCM strata and promote further research and exploration. Any significant review of WCM nomenclature also suggested the need to replace the type bore DM Doyles Creek DDH11 (DMDCK11). This is a result not only of the identified miscorrelations but also that this bore intersected large sections of strata that are atypical in the Hunter Valley (as it has similarly in the underlying Wittingham Coal Measures). This review was considered timely with the potential exploitation of these seams and the possible renewed interest in exploration that may follow.
BACKGROUND
Exploratory drilling in the late 1990’s near Broke by the NSW Department. of Mineral Resources, and north of Denman by Powercoal (now Centennial Coal Ltd), intersected potentially economic coal seams which lie stratigraphically near the top of the Wollombi Coal Measures. Seams of the Wollombi CoalMeasures have long been recognised as equivalents of those from the Newcastle Coal Measures (David 1907, Booker 1954, Britten 1972 & 1975), however detailed correlations have only recently been proposed (Stevenson 1997 & 1999, Beckett et al., 1999 Creech 2000, Weatherall 1999, Kramer 1999, Kramer et al., 2001). The similarities between the Wollombi and Newcastle Coal Measures is reflected in the correlation of the three tuffaceous units previously identified in the Wollombi Coal Measures (Standing Committee on Coalfield Geology of NSW 1975) with the three tuffaceous units that are Formations in the Newcastle Coal Measures (refer Table 3).
The application of Wollombi Coal Measure nomenclature to the seams targeted in both exploration programs however proved problematic due to miscorrelations identified in the type section in DMDCK11. Such miscorrelations may have been manifest in part at least, to the priority given in Wollombi Coal Measure nomenclature to correlating seams and the intervening clastic strata. In recent times the emphasis has shifted to giving priority to correlating the more laterally continuous tuffaceous units in a given stratum. This is reflected in the current Newcastle Coal Measure nomenclature where seams (and intervening clastics) are informal units within Formations, bounded by tuffaceous units that are themselves Formations (Awaba, Warners Bay and Nobbys Tuffs).
The Wollombi Coal Measures Working Party (WCMWP) was established in August 2000 with the aim of resolving several issues concerning the nomenclature of the Wollombi Coal Measures (WCM) and the nominated type bore. The formation of this Working Party was considered timely with the potential exploitation of these seams and the possible renewed interest in exploration that may follow.
The Working Party had the following objectives defined:
1. To resolve issues regarding the identification and application of Wollombi Coal Measure nomenclature with particular reference to those seams targeted at Broke and Denman.
2. Review the relevance of the existing nomenclature and if warranted propose a stratigraphic revision and/or full replacement with Newcastle Coal Measure nomenclature.
3. Nominate a replacement type bore if appropriate.
In order to resolve the above issues practitioners currently working with Wollombi Coal Measure strata were invited to form the Wollombi Coal Measures Working Party. The inaugural meeting was held 25/8/00. Members of the Working Party were: Michael Creech (Convener) and Malcolm Ives (Centennial Coal), Darryl Stevenson, John Brunton and Russell Rigby, (Dept. of Mineral Resources), Gary Salter (BHP-Billiton – Bayswater Colliery), Chris Knight (Muswellbrook Coal), Bryan Atkins (Wambo Colliery), Peter Graham and Chris Smith (Oakbridge – Bulga and Sth Bulga Coal Mines).
Bores drilled in the Hunter Valley Coalfield that intersected significant sections of the Wollombi Coal Measures were selected by the Working Party (refer Figure 1 for locations). These bores (Table 1) were encoded onto an ECS database and major tuffs and selected seams were then correlated (Table 2). Sections were then prepared for consideration including a regional cross section (Figure 2).
Evidence for a significant miscorrelation in the type bore DMDCK11 was also gathered including re-examining and photographing the relevant units in the type bore. More information was also sought concerning the seam profile of the Greigs Creek seam both in the literature and by locating the Glen Gallic Colliery and recording an adjacent outcrop of the seam. This was the only seam name in use prior to the ratification of the Wollombi Coal Measure nomenclature (Standing Committee 1975). The philosophy of using the current nomenclature was also explored by comparing the holes drilled adjacent to DMDCK11 after 1975 (refer Figure 4). This exercise clearly illustrated the difficulty in using the current nomenclature.
Borehole Amoco Wybong 1 was selected as the preferred “Reference Bore” for the Newcastle Coal Measures in the Hunter Valley because all seams were intact (not destroyed for analysis) and downhole geophysics are available (Appendix 1). This bore is stored at the Department of Mineral Resources Londonderry Core Storage Facility. The relevant strata were photographed (Appendix 2) and some units relogged where necessary. All significant tuffaceous units and coal bearing units in the WCM portion of the bore were subsequently correlated to Newcastle Coal Measure stratigraphy. The highresolution correlations recognised (Figures 5 to 9) in this work have not been recognised nor published previously and were required to support the notion that Wollombi Coal Measure nomenclature could be replaced by Newcastle Coal Measure nomenclature.
MISCORRELATION IN THE TYPE BORE DMDCK11 (ref Figure 3)
The possibility of a significant miscorrelation in the upper seams of the type bore DMDCK11 was first recognised by Darryl Stevenson during the DMR Broke drilling program of 1996. This program was designed to investigate coal resources of the Greigs Creek seam in the Broke-Wollombi district identified previously by Moffit (1983) and Baker (1987). Stevenson (1997) realised that he could not correlate seams in this area with those in the type bore. In particular the Greigs Creek seam appeared to have been misidentified as the Hillsdale seam. He also recognised the Hillsdale and Hobden Gully seams as being very similar to the Great Northern and Fassifern seams from the Newcastle Coal Measures (Stevenson 1999).
The Greigs Creek seam was mined shortly after World War 2 at Glen Gallic Colliery south of Denman (Figure 1). The location of this colliery was poorly recorded however it has now been located by GPS (MGA 284315E, 6400705N 220RL) as has an outcrop of the seam approximately 2km to the east (286095E, 6400219N, 220RL). The details of the outcrop location may be less accurate due to dense trees that effected GPS reception.
Two sections of the Greigs Creek seam have been recorded in the literature. The first section recorded was measured by the Joint Coal Board in December 1951 (JCB Seam Section and Analysis, GlenGallie Coal Company, Plan DC 118), and the second by F. W. Booker (1954). Booker indicated that the Greigs Creek seam outcropped for over 12km along the southern escarpment of the Hunter Valley and he stated that it was the uppermost seam of the Newcastle Coal Measures in this area.
It should be noted that historical precedence for the use of Newcastle Coal Measure nomenclature could have been considered by the Working Party. David (1907) may have been the first to suggest the existence of Newcastle Coal Measure strata in the Hunter Valley by referring to them as an annotation in a geological map of the Wingen area on page 109. Britten had also assigned seam names of the upper Newcastle Coal Measures to two bores in the Millfield area (AOG Millfield – hand written annotation in 1966, and AMAD Millfield – typed annotations in 1971). However these annotations and broad
commentary were apparently not considered when the WCM nomenclature was established a few years later.
The Greigs Creek seam has a distinctive stone band (tonstein) towards the roof recorded in both sections taken from Glen Gallic Colliery. This profile is repeated in drillhole Millers Hunter Valley 1 south of Sandy Hollow where the Greigs Creek seam was correlated by R. A. Britten.and P. A. Raymant in 1969 (refer written log). This same profile (though split) has now been recorded at outcrop 2km east of Glen Gallic Colliery (Figure 1). The uppermost seam in the Broke-Millfield area also includes the same stone band and was correlated as the Greigs Creek seam by Moffit (1983) and Baker (1987).
This same stone band is also present in the Great Northern seam on the western side of the Newcastle Coalfield and the extensive development of this seam is confirmed by drilling north of Denman (Figure 3). It is proposed herein that the Great Northern Seam of the Newcastle Coal Measures is equivalent to the Greigs Creek seam in the WCM, however it should be recognised that the original usage at Glen Gallic Colliery likely refers to the upper split of the Great Northern seam only.
In the type bore DMDCK11 the Greigs Creek seam was assigned to a poorly developed seam about 21m above the Great Northern seam equivalent. This stratigraphically higher seam was also intersected at Denman in DMDCK10 and AWYB1 about 30m above the Great Northern seam equivalent. It would appear that the seam called the Greigs Creek seam in DMDCK11 (the type bore) is equivalent to the Wallarah seam of the Newcastle Coal Measures and has therefore been miscorrelated.
This miscorrelation was compounded by the failure to recognise that the Greigs Creek seam (named the Hillsdale seam in DMDCK11) has split into two separate plies and the sandstone split has been miscorrelated as the Nalleen Tuff (Plate 1). This miscorrelation was recognised by Weatherall (1999) who sampled the tuffaceous unit below recognising this to be the Awaba Tuff equivalent. Subsequent results of chemical fingerprinting supported this modified correlation and confirmed the equivalent relationship of this tuff and the Awaba
Tuff of the Newcastle Coal Measures (Kramer et., al 2001). The miscorrelation of this tuff resulted in the split Hobden Gully seam incorporating the lower split of the Greigs Creek seam, the Nalleen Tuff and the lower split of the Fassifern seam (the upper split not being present). As such there is no corresponding seam name for the combined Fassifern seam in the current nomenclature of the Wollombi Coal Measures.
These miscorrelations in the type bore lead to confusion regarding the naming of the seams explored both at Broke, adjacent to the type bore (DM Broke DDH3 and DM Coolamin DDH1), and at Denman (refer DMDCK10 and AWYB1). In addition it clearly demonstrates, at least in the northern Sydney Basin, the inappropriateness of correlating seams and intervening clastic strata in preference to tuffaceous units. The understanding of this miscorrelation is simplified by reference to regional stratigraphy, in particular by reference to NCM nomenclature and the philosophy of prioritising the correlation of tuffaceous units over seams.
The current confusion involved in using Wollombi Coal Measure nomenclature can also be illustrated by referring to two bores subsequently drilled adjacent to DMDCK11 (Figure 4). The Lucernia Coal in DMDCK11 includes the Hartley Hill seam, Warners Bay Tuff and upper Australasian seam equivalents. An adjacent bore Amoco Wollombi Brook DDH1, contains a split Lucernia Coal in which the Australasian seam is reforming below, removing the Strathmore Formation and including part of the underlying Alcheringa Coal. Locating the boundaries and indeed identifying these units is highly problematic, with the potential for further miscorrelations. Similar difficulties can be seen when comparing DM Whybrow DDH2 to the west, including the already identified miscorrelations of the upper seams.
APPLICATION OF NCM NOMENCLATURE IN THE HUNTER VALLEY
A table of suggested correlations for all the bores in this study has been prepared (Table 2) however it should be recognised that some of these correlations are less certain than others and that additional drilling may result in revisions to this work in the future. These correlations are however included to assist workers to correlate Newcastle Coal Measure strata in areas more distant from the proposed “Reference Bore”, AWYB1.
The general philosophy of these correlations reflects that used successfully in the NCM of the Newcastle Coalfield. Priority is given to identifying the three major tuffaceous units, being the Awaba, Warners Bay and Nobbys Tuffs. Other tuffaceous units are also used for regional correlations, being the Mt Hutton, Stockrington and Edgeworth tuffs. In addition some bands within coal seams are used such as the “clayband” and “lowband” in the Dudley (or Young Wallsend) seam. A similar band has been informally recognised as the Fern Valley tuff (Figure 8).
The identification of one of these regionally significant tuffaceous units is confirmed if a coal bearing sequence is recognised immediately above or below the unit (such as the Great Northern seam above, or the Fassifern seam below the Awaba Tuff). If a correlatable tuff lies within clastic strata then its identification can only be by reference to its position between coal bearing strata some distance above and below (i.e. it is in about the right location). In these instances the correlation is not as certain.
Figures 5 to 9 illustrate these correlation principles by identifying high-resolution correlations of both tuffs and seams of the Newcastle Coal Measures in AWYB1. Examples from other Hunter Valley bores are included. A series of bores from the western perimeter of the Newcastle Coalfield (near Awaba Colliery) were selected for comparison.
A brief description of each seam in the Newcastle Coal Measures is provided below to assist future workers who may not be familiar with
them, to correlate these seams in the Hunter Valley and thereby confirm identification of the major tuffaceous units.
• The Vales Pt and Wallarah seams are not widespread but have been identified in bore DMDCK11 and at Anvil Hill north of Denman. They are generally less than one metre in thickness and are characterised by a tuffaceous siltstone to sandstone floor strata.
• The Great Northern seam is characterised by a prominent tuffaceous band near the top of the seam and a deteriorated basal section. It generally directly overlies the Awaba Tuff, and the roof generally defines the lowermost occurrence of conglomerate in this strata in the Hunter Valley. The Great Northern seam at several localities both in the Newcastle and Hunter Valley appears to thicken by accretion of top-coal. In particular this has occurred in the southwest of the Newcastle Coalfield where the target seam is a combined Great Northern – Wallarah seam at the Mandalong Mine and Wyong Coal Exploration areas. This mechanism has been identified at Denman (Anvil Hill and Ridgelands) where extra coal accretes above a tuffaceous band. This extra coal may represent the same combined Great Northern, Wallarah and Vales Point seams that exist in the southern Newcastle Coalfield.
• The Awaba Tuff can be generally recognised by its sandy nature and inclusion of muscovite mica. When less than about 5 metres thick it is composed of a series of fining upward units. Thicker sections will include overlying laminated and reworked units. It is predominantly off-white, cream or light grey in colour.
• The Fassifern seam (together with the Upper Pilot seam generally located near its base) contains a highly recognisable sequence of tuffaceous bands combined with a laterally persistent but vertically variable ash profile (refer Creech 2000). Across most of the Newcastle and Hunter Coalfields this seam is split, however the characteristic profile generally allows for easy identification of the split units. This seam (or at least the upper split) directly underlies the Awaba Tuff.
• The Pilot seam lies above the Mt Hutton Tuff and contains a recognisable sequence of tuffs, coal and tuffaceous bands. Possibly the most recognisable is a set of three bands towards the base of the Upper Pilot B, and the band midway in the last of the coal units in the Lower Pilot directly overlying the Mt Hutton Tuff (Figure 5).
• The Hartley Hill seam contains a very characteristic sequence of coal and tuffaceous bands at the roof and three tuffaceous bands towards the floor. Otherwise the lower two metres of coal is relatively free of tuffaceous bands. This sequence directly overlies the Warners Bay Tuff (Figure 6).
• The Warners Bay Tuff is predominantly made up of laminated and reworked tuffaceous units. It is not easily discrimanted from other adjacent tuffaceous units when thin (less than a metre in thickness) and adjacent coal strata may need to be referred to.
• The Australasian seam lies between the Warners Bay Tuff and Stockrington tuff and comprises a thick sequence of generally carbonaceous shales and brown tuffaceous bands (Figure 7). The cleanest section of coal is near the base. A light grey tuffaceousshale unit commonly overlies a thin coaly unit that in turn directly overlies the Stockrington tuff.
• The Montrose seam is a relatively clean coal unit of about 3m thickness with a characteristic sequence of three tuffaceous bands at both the top and base of the seam. This unit can be identified directly underlying the Stockrington tuff and if split the first three bands are usually evident (Figure 7). This seam is located at the base of the Australasian seam in many bores in the Hunter Valley.
• The Fern Valley and Victoria Tunnel seams directly underly the Edgeworth tuff and display a complex sequence of carbonaceous shales and tuffaceous bands (Figure 8). One tuffaceous band containing a thin carbonaceous stringer has been informally identified as the Fern Valley tuff. This sequence of coal is best developed north of Denman.
• The Nobbys Tuff is recognisable due to its predominantly sandy nature and inclusion of biotite mica. The occurrence of biotite was also specifically noted in the Monkey Place Tuff (Standing Committee 1975).
• The seams equivalent to the composite West Borehole seam (Lambton Subgroup) and its splits are generally poorly developed in the Hunter Valley. The best examples have been found north of Denman, in particular DM Doyles Creek DDH14 (Figure 9). It should be recognised that these seams are also generally restricted to the north of the Newcastle Coalfield. The “clayband” and “lowband” in the Dudley seam can be identified in many bores in the Hunter Valley, generally located in the second coal split below the Nobbys Tuff.
REGIONAL DISTRIBUTION OF NEWCASTLE COAL MEASURES IN THE NORTHERN SYDNEY BASIN
The similarities between the Wollombi and Newcastle Coal Measures can also be seen in more regional terms with the distributions of seams in both the Hunter Valley and Newcastle Coalfields appear similar. The upper seams are the most consistently developed across both coalfields whereas the lower seams (below the Edgeworth tuff) are poorly developed to the south, towards the centre of the Sydney Basin. The known extent of the upper seams is limited in the Hunter Valley by outcrop to the north and east and by a lack of drillhole data to the south and west. The potential for undiscovered resources of these seams is highlighted by their suggested identification in bores at Merriwa (Amoco East Dunlop DDH1 and Amoco Doolans Creek DDH1 – Figure 2) and near Sandy Hollow (Millers Hunter Valley DDH1).
In common with both the Newcastle and Wollombi Coal Measures the seams between the upper and lower seams are generally poorly developed and banded, and suffer from numerous seam splits. The WCM strata is however more uniform in thickness than the Newcastle equivalents averaging approximately 200m between the Nalleen Tuff (Awaba Tuff equivalent) and the top of the Watts Sandstone (Waratah Sandstone equivalent). Commensurate with the more uniform development of the Wollombi Coal Measures is a lack of conglomeratic strata. This is in contrast with the numerous conglomerate units identified in the Newcastle Coal Measures in the Newcastle Coalfield. In this regard the Wollombi Coal Measures of the Hunter Valley are more akin to the southern Newcastle Coalfield where conglomerates are less dominant and the lower seams in the Lambton Formation are poorly developed.
The above observations suggest that the palaeogeography of the Wollombi Coal Measures in the Hunter Valley is akin to the southern Newcastle Coalfield and that by considering the similarities between the two stratum (including the size, direction and migration of palaeochannels) regional exploration strategies can be undertaken. In addition a better understanding of peat formation in the Sydney Basin may be guided by the high-resolution correlations and regional
based philosophies promoted by this work and by supporting the Working Party’s recommended revisions.