GreatBight2

NW Australia Offshore

Southern Carnarvon Geographic Basin

The Southern Carnarvon geographic basin is, mainly. a Paleozoic sedimentary basin covering onshore and offshore regions of NW Australia. It lies to the south of the Mesozoic Northern Carnarvon geographic basin and it comprises several geographic sub-basins. The Gascoyne geographic sub-basin lies in shallow water, while the Bernier platform is in a much deeper water. The Paleozoic Southern Carnarvon geographic basin, mainly, located in onshore of the Western Australian, corresponds to a superposition of cratonic sedimentary basins (Ordovician to the Late Permian). Several geographic sub-basins (depocenters) can be recognized: Gascoyne, Merlinleigh and as well as the Bernier Platform. A major tectonic episode (collision between Gondwana and Laurasia), from which the Pangea supercontinent Pangea was formed, is recognized, easily. Similarly, the breakup unconformity of the Pangea is well marked, on the seismic lines, here, with a Valanginian age.

On this tentative geological interpretation the Atlantic-type divergent margin seems to be formed by Cretaceous and Cenozoic rocks. The breakup unconformity is at Lower Cretaceous, probably, at Valanginian, which is a sub-stage of the Neocomian (Berriasian + Valanginian + Hauterivian). The Pre-rifting unconformity was picked at the base of the Jurassic. The pre-rifting strata, particularly the Permian and Carboniferous rocks belong to a cratonic basin (see next autotrace). Such a cratonic basin developed by a thermal subsidence on an older continental crust, i.e., by a mechanism in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation due to thermal contraction : as the mantle material cools and becomes part of the mechanically rigid lithosphere, it becomes denser than the surrounding material (Wikipedia, the free encyclopedia).

The spacial and temporal stacking of the three different sedimentary basins : (i) Permo-Triassic Cratonic basin ; (ii) Jurassic Rift-type basin and (iii) Meso-Cenozoic Atlantic -type divergent margin is easily recognized on this tentative geological interpretation. The breakup unconformity, which separates the rift-type basin from the overlain Atlantic-type margin is, locally, tectonically enhanced (angular unconformity), while the pre-rifting unconformity is rather cryptic. In fact, in other areas of Western Australia offshore, the rifting seems to occur during Permian / Early Cretaceous (see Plate15). Notice the presence of volcanic sills and dykes within the rifting strata.