Abstract

Seismic images along Australia's southern continental margin reveal the internal geometry and depositional history of the Hammerhead Delta, a Late Cretaceous shelf-margin delta complex in the Ceduna Subbasin of the Bight Basin. The Hammerhead Delta comprises the late Santonian to Maastrichtian Hammerhead supersequence, which is divisible into three, third-order sequence sets and their component sequences. Sequence set 1 (late Santonian to early Campanian) comprises three progradational sequences deposited after a major fall in sea level under a high sediment supply regime. Sequence set 2 (early Campanian to early Maastrichtian) comprises two, sandy progradational sequences. Basinward shifts in facies caused by forced regression are apparent between each progradational sequence. Sequence set 3 (early to late Maastrichtian) is a thick, aggradational succession deposited when the rates of creation of accommodation space and sediment supply were balanced. A transgressive episode at the top of sequence set 3 marks a rapid decrease in sediment supply and the end of deltaic sedimentation.

A long-lived sand-rich sediment supply, most likely derived from erosion of the eastern Australian highlands to the northeast, was the major influence on delta formation. Rapid progradation and the formation of thick shelf-margin clinoforms resulted in slope instability, growth-faulting, and load-induced collapse of the shelf margin during the Campanian. The Hammerhead Delta is characterized by sandy, progradational clinoforms and lacks the thick coeval prodeltaic shales and shale tectonism that are common to many other large deltas. The results of this study, which included seismic facies mapping, well correlations, and comparisons to other large shale-poor deltas, suggest that the Hammerhead Delta has excellent reservoir potential.

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