The Maastrichtian breakup of the Otway Basin margin ? a model developed by integrating seismic interpretation, sequence statigraphy and thermochronological studies

Abstract

The generally accepted model of a 95 Ma breakup event in the Otway Basin has several unresolved inconsistencies, including evidence of significant post-95 Ma aged crustal extension. An alternative model is proposed: oceanic breakup took place at the end of the Late Cretaceous. A phase of extension during the pre-Aptian was followed by thermal subsidence before renewal of rifting (Cenomanian). Rifting of the already attenuated crust allowed the region to subside below sea level, resulting in marginal to nearshore marine conditions throughout most of the Late Cretaceous. Continental breakup in the Late Maastrichtian (approximately 67 Ma) was marked by regional uplift along the northern basin margin, and towards the incipient continent?ocean boundary. This uplift is parallel to the present-day shelf. The age of this event is derived from palynological and seismic interpretations, and is consistent with previous thermochronological work. Broad anticlinal features on the basin margin may be associated with ridge-push associated with early oceanic crust development. The first post-rift depositional cycle was during the latest Maastrichtian. A significant transgression flooded a pene-plain (67 Ma unconformity surface). A maximum flooding surface was developed near the Cretaceous/ Tertiary boundary, with subsequent highstand deposition developed through most of the basin. A second unconformity marks a depositional hiatus at the end of the deposition of this sequence. Later cycles of post-rift sedimentation began in the Late Paleocene (55 Ma) with deposition of the 'Wangerrip Megasequence'. Several eustatic sequences were deposited in a deltaic environment during the subsequent 8 million years.