Cooper Basin ridges — are they structural?

Rodney Boucher (Linex Pty Ltd)

Introduction their origin. In the following decades, doming of sediments on pre-existing Hydrocarbon-bearing ridges of the unsubstantiated maps and sections ridges in the Cooper Basin is Cooper Basin are traditionally assumed showing extensional faults were often commonplace (e.g. Stuart, 1976). Thus, to be structural. For three decades, printed in the literature. Heath (1989) isopachs will display thickness changes, published maps and sections have pronounced it was widely accepted that regardless of any later structural portrayed various unsubstantiated Cooper Basin ridges were dominantly modification. The compactional doming structures on the assumption that extensional. Yet throughout this time process will result in closure on depth topographic expression must have there had been very little published structure maps imprinted above existing evolved from structural movements. scientific evidence to support these basement ridges which might also be There has been no consensus and claims. misinterpreted as tilting. Care needs to structures have been variously Earlier, Sprigg (1958) and Wopfner be taken when relating onlap and considered to be extensional, compress- (1960) suggested that surface topo- compactional doming to structural ional and strike-slip faults or folds. graphic features represented ancient, movements. Though structural modification of many reactivated transcurrent faults within the An additional factor that has resulted ridges is common, new evidence and basement. Winsor (1984) indicated that in the over-interpretation of structural interpretations indicate that geomorphic compressional tectonics occurred in the features arises from the misuse of processes played a significant role in Cooper Basin area, and Kuang (1985) vertical exaggeration. There has been a shaping the basin. proposed that extensional, compress- tendency to exaggerate the vertical scale ional and strike-slip faults evolved from on geological cross-sections or to Features of Cooper regional simple shearing. When authors compress the horizontal scale on seismic Basin ridges began looking for structures beneath the until they suitably show notable relief. Basement to the Cooper Basin is Z horizon, elaboration on these themes Gross vertical exaggeration, commonly considered to be all pre-Permian evolved. For example, Carroll (1990), up to 50:1, contributes to the mis- sediments, metasediments and igneous Apak (1994) and Sun (1996) show conception that significant relief is rocks. The interface is mapped various compressional, extensional and commonplace. It is therefore easy to be seismically as the Z horizon. Ridges are strike-slip interpretations. misled into believing these features must basement features that are replicated Additional structural alternatives be structural. through the overlying Cooper and have been proposed. Mancktelow (1979) Eromanga Basins. Faults are rarely suggested that cross-folding produced An alternative imaged to penetrate through the the domes and basins within the Cooper geomorphic model Z horizon and into the Cooper Basin on Basin. Battersby (1976) and Stuart The discovery of an altered zone at the seismic. The most obvious fault extend- (1976) described the main ridges as a top of basement (Boucher, 1996) ing into the basin is the Big Lake Fault, series of anticlines. removed the requirement for an which displaces <10% of the total unconformity in the Moomba Field. post-Carboniferous stratigraphic section. Evidence supporting structural movements Previously, the altered zone was inter- Due to the economic significance of preted as Tirrawarra Sandstone (and the Cooper Basin and the difficulties There is good evidence to show that conglomerate) that was uplifted and imaging intra-basement reflectors on structural movements do occur. eroded prior to deposition of the seismic, very little work has been done Merrimelia Formation sediments at Patchawarra Formation. A revised interpreting the structures beneath the Gidgealpa, Merrimelia and Packsaddle interpretation is that Patchawarra Z horizon. Instead, basement structures are uplifted relative to younger Formation sediments onlap the granitic have traditionally been interpolated from Patchawarra Formation sediments Moomba palaeohigh. Previously, Permian and later depositional patterns adjacent to the ridges. The Big Lake Boucher (1994) proposed the complex and structures. This analysis has given Fault has demonstrably offset the Early ridge at Kobari might actually be a rise to multiple and possibly misleading Permian sediments, and Permian Cambrian volcanic palaeohigh rather interpretations. sediments have been eroded from ridges than a structural ridge. The basal glacial at Gidgealpa and Murteree. However, and fluvioglacial sediments of the Previous work care must be taken elsewhere when Cooper Basin would indicate that the Kapel (1972) published a schematic assuming erosional truncation is landscape was modified by glacial sketch portraying subsidence in the evidence for uplift because this is not activity. Glacial processes would northern Cooper Basin along extensional necessarily always the case. provide dramatic topographic relief at faults. These are not described in the text It is unanimously agreed in the what would later become the top of and no evidence was provided to explain literature that onlap and compactional basement unconformity.

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Onlap and compactional doming occurs at all ridges. The difficult task is Mount Buffalo area to separate structurally evolving ridges from palaeohighs. It is interesting to compare the size of the ridges in the Cooper Basin with palaeohighs from elsewhere to assist to determine which ridges could realistically be palaeohighs. Comparisons with Mount Woodroffe and Mount Buffalo Mount Woodroffe, the highest mountain in SA at 1440 m, rises 750 m above the surrounding landscape. Less than 10 ridges in the Cooper Basin Z horizon have a larger amplitude. In comparison, Mount Buffalo in eastern Victoria is larger in amplitude and area than most of the Cooper Basin ridges (Table 1). Only the Innamincka, Pondrinie–Packsaddle, Murteree and Gidgealpa ridges are larger in these respects. Of these, Gidgealpa is similar in geometry to Mount Buffalo (Fig. 1). Mount Buffalo is a residual palaeohigh resulting from differential 0 10 erosion of the resistant granite (O’Sullivan et al., 2000). The similar kilometres geometry of Mount Buffalo indicates that a ridge of this size does not require a structural origin. Though at Gidgealpa, Gidgealpa Ridge area thrust-repeated lithologies in the basement (Daily, 1964; Carroll, 1990; Sun 1996) and uplifted and erosionally truncated Early Permian sediments indicate that the Gidgealpa dome has been subjected to structural activity. Elsewhere in the Cooper Basin area the Z

Table 1 Relative sizes of the largest Cooper Basin ridges compared to Mount Buffalo. Amplitude Area (m) (km2) Della 1800 96 Innamincka 1500 600 Gidgealpa 1400 162 Nappacoongee 1350 40 Murteree 1300 490 Pondrinie–Packsaddle 1300 280 Merrimelia 1200 90 Big Lake 750 34 Coobowie 750 180 Moomba 700 418 Warra 700 126 Toolachee 450 198 Daralingie 400 72 Coonatie 350 24 Strzelecki 350 30 0 10 Dullingari 250 48 kilometres Munkarie 200 15 200670-005 Tirrawarra 150 56 Mt Buffalo (Victoria) 1200 154 Fig. 1 Z horizon depth structure map of Gidgealpa and DTM of Mount Buffalo, Victoria.

MESA Journal 21 April 2001 31 Petroleum

horizon landscape was produced in the References Kapel, A.J., 1972. The geology of the Late Carboniferous and Early Permian Patchawarra area, Cooper Basin. APEA by glacial activity. It is not unreasonable Apak, S.N., 1994. Structural development Journal, 6:71-75. to expect that many of the apparent and control on stratigraphy and sedimentation in the Cooper Basin, Kuang, K.S., 1985. History and style of structures in the Cooper Basin are . University of . Cooper– structures. palaeohighs followed by onlap and National Centre for Petroleum Geology Exploration Geophysics, 16:245-248. compactional doming of Cooper and and Geophysics. Ph.D. thesis Mancktelow, N.S., 1979. Structure and Eromanga Basin sediments. (unpublished). tectonics of the Cooper Basin. Delhi Petroleum report (unpublished). Conclusion Battersby, D.G., 1976. Cooper Basin gas and oil fields. In: Leslie, R.B., Evans, H.J. O’Sullivan, P.B., Belton, D.X. and Orr, M., Despite decades of research, the origin of and Knight, C.L. (Eds), Economic 2000. Post-orogenic thermotectonic the Cooper Basin ridges remains geology of Australia and Papua New history of the Mount Buffalo region, uncertain and new ideas continue to Guinea, 3, Petroleum. Australasian Lachlan Fold Belt, Australia: evidence emerge. As a result of this uncertainty, Institute of Mining and Metallurgy. for Mesozoic to Cenozoic wrench-fault numerous models have been evoked, Monograph Series, 7:321-368. reactivation. Tectonophysics, 317:1-26. describing the structures variously as Sprigg, R.C., 1958. Petroleum prospects of extensional, compressional or strike-slip Boucher, R.K., 1994. Igneous associations in the eastern Warburton Basin. In: 12th the western parts of the Great Australian faults or from folding. However, much Artesian Basin. AAPG Bulletin, of the basement topography is glacial Australian Geological Convention, Perth, 1994. Geological Society of Australia. 42(10):2465-2491. rather than structural. It is geometrically Abstracts, 37:40. plausible for all Cooper Basin ridges to Stuart, W.J., 1976. The genesis of Permian and Lower Triassic reservoir sandstones be deposited on palaeohighs. Mount Boucher, R.K., 1996. Big Lake Suite not during phases of southern Cooper Basin Buffalo is a residual granite in Victoria Tirrawarra Sandstone in the Moomba development. APEA Journal, Field, Cooper Basin, SA. South and similar in size to Gidgealpa. Though 16(1):37-48. Gidgealpa and other ridges display Australia. Department of Mines and evidence for episodic structural Energy. Report Book, 96/31. Sun, X., 1996. Sequence stratigraphy, movements on basement faults, sedimentology, biostratigraphy and Carroll, P.G., 1990. Pre-Permian structure palaeontology of the eastern Warburton geomorphic processes significantly and prospectivity at Gidgealpa, South shaped these and other apparent Basin (Palaeozoic). . Australia. University of Adelaide. University of Adelaide. National Centre structures. National Centre for Petroleum Geology for Petroleum Geology and Geophysics. and Geophysics. M.Sc. thesis Ph.D. thesis (unpublished). Acknowledgements (unpublished). Winsor, C.N., 1984. Structural geology and The author thanks PIRSA Petroleum and Daily, B., 1964. Appendix 3 part B and genesis within the Nappacoongee– Publishing Services staff for access and Addendum to Appendix 3 part B. In: Murteree Block of the Cooper Basin, provision of data, and Natural Resources Harrison, J. and Higginbotham, G.T., South Australia — evidence of hydro- and Environment, Victoria, for Delhi-Santos Gidgealpa No. 1 South carbon potential. Delhi Petroleum report permission to publish the Mount Buffalo Australia, well completion report. South (unpublished). DTM image. Australia. Department of Primary Wopfner, H., 1960. On some structural For further information contact Industries and Resources. Open file Envelope, 363 (unpublished). developments in the central parts of the Rodney Boucher (ph. 03 5443 3216, Great Artesian Basin. Royal Society of email [email protected], 2 Heath, R., 1989. Exploration in the Cooper South Australia. Transactions, McGowan St, Bendigo Vic, 3550). Basin. APEA Journal, 29(1):366-398. 83:179-193.

Selected unpublished Departmental reports

2000/22 Watson, S. Resources production statistics for the six-monthly period ended 31 December 1999. 2000/23 Hough, J.K. and Flintoff, M.W. Appraisal of the Lady Edith and Lady Alice gold mines. 2000/28 Hore, S.B. Gravity base station project — progress report to July 1999. 2000/32 Lindsay, J. Source rock potential and algal-matter abundance, Cooper Basin, SA. 2000/33 Crooks, A.F. Mineral potential of the proposed Gawler Ranges National Park. 2000/39 Cockshell, C.D. Statement of environmental objectives for seismic operations in the Otway Basin, SA. 2000/41 Langley, K.R. Example environmental procedures for seismic operations in the Cooper and Eromanga basins, SA.

32 MESA Journal 21 April 2001