Re-evaluation of the Mentelle Basin, a polyphase rifted margin basin, offshore south-west 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 re-processed and integrated with recent high quality 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 with a greater degree of confidence. Basin stratigraphy can be subdivided into several seismically defined megasequences, separated by major unconformities related to both the Valanginian breakup between India-Madagascar and Australia-Antarctica, and tectonically-driven switches in deposition through the Albian. Resting on the Valanginian 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 Barremian-Aptian silty-sandy mudstones were deposited. As subsidence continued, thick Albian ferruginous black clays were deposited. Internally, black clay megasequences 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, followed by rapid seafloor spreading from the middle Eocene to the present.