--> Structural Architecture of the Ceduna Sub-Basin: The Effects of Older Fault Orientations, and More Recent Exploitation of Weaknesses

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Structural Architecture of the Ceduna Sub-Basin: The Effects of Older Fault Orientations, and More Recent Exploitation of Weaknesses

Abstract

The Ceduna Sub-basin is a large, underexplored sub-basin on the continental margin of southern Australia in the Great Australian Bight. Recent acreage releases in the basin have resulted in renewed exploration interest. Murphy and Santos were jointly awarded the EPP43 acreage on the western margin of the basin in 2013 and PGS have subsequently acquired broadband 3D seismic across EPP43. A large delta system formed in the Ceduna Sub-basin during the Early–Late Cretaceous, depositing over 10km thickness of sediments. Detailed seismic stratigraphy has illustrated a series of major transgressions and regressions across the delta. Consequent variation in the amount of sediment input has effected the local sediment loading patterns, hence effecting the fault patterns. Distinct sets of listric faults exist vertically, and show varying character from proximal to distal. Structurally, the Ceduna Sub-basin is unusual in Australia, with faulting being sediment load driven (listric), rather than tectonically derived. Syn-sedimentary listric faulting is pervasive throughout the Cenomanian and Turonian deltaic section. The faults rotate with depth and often detach into a mobile stratigraphy, usually ductile shales. Down dip the listric faults feed mobile thrust belts at the toe of the delta system. Few external forces affect the orientation of faults, and the delta system forms using gravity sliding and gravity spreading in an arcuate form. Underlying the decollement surface is a Jurassic rift system formed by the separation of Australia from Antarctica commencing in the Middle Jurassic. These rift-related faults show very different orientations (mostly east-west) in comparison to the listric faults. The interaction of the Jurassic faults with the overlying Cretaceous delta system appear limited, suggesting that the initial rift has seen little tectonic activity since its initiation. Eocene volcanics have been documented in the east of the Ceduna Sub-basin. The PGS Springboard 3D seismic data has shown that similar volcanics are also present in the west of the sub-basin. Cinder cones are often located above the tip of listric faults showing that the faults are clearly acting as weaknesses that have been exploited by the volcanics rising through the thick sediment package.