Re-evaluation of the Mentelle Basin, a polyphase rifted margin basin, offshore southwest Australia : new insights from integrated regional seismic datasets.

Vintage 2-D (two-dimensional) seismic reflection surveys from the sparsely explored Mentelle Basin (western Australian margin) have been reprocessed and integrated with a recent high-quality 2-D seismic survey and stratigraphic borehole data. Interpretation of these data sets allows the internal geometry of the Mentelle Basin fill and depositional history to be reanalysed and new insights into its formation revealed. Basin stratigraphy can be subdivided into several seismically defined megasequences separated by major unconformities related to both breakup between India-Madagascar and Australia-Antarctica in the Valanginian-Late Hauterivian and tectonically-driven switches in deposition through the Albian. Resting on the Valanginian-Late Hauterivian breakup unconformity are several kilometre-scale mounded structures that formed during Late Jurassic to Early Cretaceous extension. These have previously been interpreted as volcanic edifices although direct evidence of volcanic feeder systems is lacking. An alternative interpretation is that these features may be carbonate build-ups. The latter interpretation carries significant climatic ramifications since carbonate build-ups would have formed at high palaeolatitude, ~60° S. Soon after breakup, initial subsidence resulted in a shallow marine environment and deposition of Barremian-Aptian silty-sandy mudstones. As subsidence continued, thick successions of Albian ferruginous black clays were deposited. Internally, seismic megasequences composed of successions of black clays show previously unresolved unconformities, onlapping and downlapping packages, which reflect a complex depositional, rifting and subsidence history at odds with their previous interpretation as open marine sediments. Southwestwards migration of the Kerguelen hotspot led to thermal contraction and subsidence to the present day water depth (~3000 m). This was accompanied by Turonian-Santonian deposition of massive chalk beds, which are unconformably overlain by pelagic Palaeocene-Holocene sediments. This substantial unconformity is related to the diachronous breakup and onset of slow spreading between Australia and Antarctica, which may have led to the reactivation and inversion of basement faults and was followed by rapid seafloor spreading from the Middle Eocene to the present