Research paperCenozoic structural history of the Gippsland Basin: Early Oligocene onset for compressional tectonics in SE Australia
Introduction
The Gippsland Basin has been one of the most prolific hydrocarbon-producing regions in Australian history, containing some of the largest hydrocarbon accumulations in Australia. From the discovery of oil onshore in 1924, to the discovery of giant oil and gas fields offshore in the 1960's, the Gippsland Basin has contributed extensively to the history of hydrocarbon exploration and production in Australia. A great deal of geological research and exploration was carried out in the basin between the 1960's and the 1990's. However, despite this attention, there is relatively little published literature on the basic geology of the basin. The position of the basin on the south-eastern corner of the Australian continent has meant it has been subjected to multiple phases of tectonic movement, including rifting along the southern margin of Australia, rifting on the eastern margin of Australia, and Neogene compressional tectonics (Dickinson et al., 2001). There has been some focus on early basin formation (e.g. Gunn, 1975; Threlfall et al., 1976; Etheridge et al., 1985; Lowry, 1988; Lowry and Longley, 1991), and on young tectonism (e.g. Dickinson et al., 2001, 2002). However, the general Cenozoic tectonic history of the basin has not been well documented.
Hydrocarbon accumulations in the Gippsland Basin are trapped offshore in broad northeast-southwest trending anticlines. Despite the importance of these structures as traps for hydrocarbon accumulations, the timing of the formation of these anticlines is quite poorly constrained. The timing of anticline growth in the Gippsland Basin has been estimated by various authors to a time period anywhere from the Late Cretaceous to the Miocene (e.g. Brown, 1986; Smith, 1988; Maung and Nicholas, 1990; Rahmanian et al., 1990; Duff et al., 1991; Lowry and Longley, 1991; Johnstone et al., 2001; Holford et al., 2011).
Sedimentation concurrent with tectonism provides a method to identify the history of structures, and is often referred to as ‘growth strata’ (Childs et al., 2003; Jackson et al., 2017). Measurements of thickness changes of sediment across a structure (fault or fold), in conjunction with chronostratigraphic data (e.g. paleontological information), can provide accurate information regarding the history of growth on the structures (Childs et al., 2003). To date, no studies have quantified tectonic growth episodes on structures in the Gippsland Basin. In this study, we have quantitatively analysed the growth history of structures to determine the timing and intensity of the various structural episodes in the Gippsland Basin. We document a hitherto unidentified episode of major tectonism during the Oligocene that appears to represent the onset of compressional tectonism in South Eastern Australia.
Section snippets
Geological setting
This Gippsland Basin, located on the southeast corner of the Australian continent, occupies a unique position in Australia's tectonic history. It is part of a series of rift basins which formed along the southern margin of Australia, as Australia and Antarctica rifted apart in the Late Jurassic to Early Cretaceous (Etheridge et al., 1985). The present day Gippsland basin displays a roughly ESE-WNW-oriented main depocentre, the Central Deep, flanked by the Northern and Southern Terraces, and the
Methodology
2D and 3D seismic and well data are available over most of the basin. There are many 3D seismic datasets located offshore, which have been merged into a ‘Megacube’ seismic volume. This merged 3D seismic was used to undertake seismic interpretation of faults and horizons. The volume is SEG reverse polarity (otherwise referred to as ‘European’ polarity), full stack, and approximately zero-phase. Interpretation of 2D seismic was undertaken on the nearshore and onshore areas (where 3D data was
Uncertainties and limitations
Studies of structural growth on anticlines and faults have used either sediment thickness (e.g. Thorsen, 1963) or two-way-time on seismic profiles (e.g. Mansfield and Cartwright, 1996; Jackson et al., 2017), depending on the data available (seismic, outcrop or wells) and the purpose of the study. Several authors have noted that fault growth plots for faults commonly appear similar whether presented in depth or two-way time, especially if sonic velocity varies gradually with depth and there are
Results
Analysis of growth across a range of structures was undertaken in the Gippsland Basin. These structures fall into two clear groups: those that occur within the Latrobe Group and those that occur within Seaspray Group. These structures are clearly evident on seismic data as large anticlines and faults (Fig. 4, Fig. 5).
Discussion
Growth % plots for the Gippsland Basin in the Late Cretaceous and Cenozoic display two major phases of tectonic activity. The Latrobe Group is exclusively dominated by normal growth faulting, indicating an extensional tectonic regime. Growth across these faults is evident from the oldest intervals analysed (Late Cretaceous – T lilliei), to the youngest Latrobe Group intervals analysed (Late Eocene – end Lower N asperus). In contrast, the Seaspray Group is dominated by reverse growth faults and
Conclusions
The Cenozoic history of the Gippsland Basin is characterized by two major tectonic regimes. The Late Cretaceous-Eocene period is characterized by extensional tectonics that produced abundant normal growth faults. Analysis of growth strata on these normal faults indicates a general waning of extensional activity from the Late Cretaceous through to the Eocene. During the latest Eocene to early Oligocene (~34 Ma), the tectonic regime changed to one of compression. This compressional tectonic
Author contributions
E.M.M and M.W.W. conceived the project, E.M.M. compiled the data, interpreted the seismic profiles and analysed the data. E.M.M and M.W.W. interpreted the results and wrote the paper.
Acknowledgements
We would like to thank IHS Markit for their donation of the Kingdom seismic interpretation software. E. Mahon is supported by a Baragwanath Geology Research Scholarship, and the Melbourne Research Scholarship as part of the Australian Government Research Training Programme Scholarship, in addition to the AAPG Foundation Grants-in-Aid Chandler and Laura Wilhelm Named Grant, and the Geological Society of Australia (Victorian Division) Postgraduate Grant. We would like to thank two anonymous
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