GreatBight2

SW Australia Offshore

Perth Geographic Basin

The Perth geographic basin is located along the Australian southwestern margin. It was formed as Australia separated from Greater India during Permian / Early Cretaceous oblique rifting. It includes a significant onshore component and extends offshore to the edge of continental crust in water depths of up to 4500 m, in which different sedimentary sub-basins can be considereed. The breakup occur during the Valanginian (Early Cretaceous) and was associated with widespread inversion, erosion, strike-slip tectonics and volcanism, which significantly modified the structural architecture of the Perth Basin. Major structural elements associated with breakup tectonism include a Permian inverted graben and a zone of strike-slip faulting (Zeewyck Sub-basin).

On this Canvas autotrace, the breakup unconformity (Valanginian unconformity) is quite obvious. The pre-rifting unconformity was, tentatively, picked at the base of Permian (hypothesis 1). However, as pointed out in Page 16, the Permo-Triassic interval, not far from this offshore (south Carnarvon offshore) is considered as deposited in cratonic basin. In fact, on this tentative interpretation, the thickness variation of the Permo-Triassic interval can suggest a thermal subsidence characteristic of the cratonic basins. So we propose an alternative picking of the pre-rifting unconformity at the base of Jurassic (hypothesis 2). In reality, we have not data enough to decide what hypothesis is most likely, as illustrated on next autotrace.

As in the previous tentative interpretation, the picking of the pre-rifting unconformity is quite difficile if not impossible on this Canvas autotrace. On the contrary, the breakup unconformity (Valanginian unconformity), which separates the rift-type basin from the Atlantic-type divergent margin is a striking seismic surface. The Atlantic-type divergent margin is, mainly, composed by Cretaceous (post-Valanginian ) and Cenozoic strata, while the rift-type basin comprise lowermost Cretaceous rocks (Berriasian/Valanginian) and Jurassic sediments (hypothesis 2 for the picking of the pre-rifting unconformity, see previous plate).