Cretaceous Fault Growth and Linkage in the Ceduna Shelf, Bight Basin, Southern Australia

Abstract

The Ceduna Shelf is part of the Bight Basin which formed during several phases of extension and thermal subsidence in the Middle Jurassic to Late Cretaceous. The Ceduna Shelf comprises two large prograding delta systems, the Cenomanian White Pointer and the Campanian-Maastrichtian Hammerhead supersequences. Detailed investigation of Cretaceous fault evolution provides critical insights into fluid migration and trap formation. Fault styles within the two delta systems are markedly different, although the location and growth history of faults in the Hammerhead supersequence is strongly influenced by faults in the underlying White Pointer supersequence. Faulting in the White Pointer supersequence is attributed to gravitational extension associated with shale mobilisation in the Cenomanian. Faults are curved in plan view and highly listric, detaching on the Albian Blue Whale Shale. Syn-extensional wedges and rollover anticlines are developed in hanging walls of listric faults, with significant offset within the Cenomanian White Pointer sequence, decreasing upwards into the Turonian Tiger sequence. Fault styles within the Late Cretaceous Hammerhead supersequence are markedly different despite similar strike directions, suggesting a different style of growth and partial detachment in the underlying Tiger shale sequence. Faults are steep and planar, often partially linked to the underlying White Pointer supersequence. Detailed displacement analysis of regional (south-dipping) faults show a displacement maxima in the mid-Hammerhead sequence and a decrease in displacement within the lower Hammerhead sequence, suggesting that most fault growth occurred in the late Campanian or Maastrichtian and that fault linkage with the White Pointer supersequence occurred by downward propagation. Most regional faults show a degree of vertical linkage with faults in the White Pointer supersequence, with the largest displacement and greatest degree of linkage in E-W trending segments. Horizontal segment linkage is visible as a series of hangingwall synclines 2–3 km across within the lower Hammerhead sequence. In contrast, the counter-regional faults behave as isolated faults with regular tip lines and displacement maxima towards the middle of the faults. They show a later displacement maxima within the Maastrichtian sequence which decreases to zero towards the base of the Hammerhead sequence, suggesting they developed only as the regional faults reached maturity.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015