Basin Details and Geological Overview
The Mesozoic to Cenozoic Sorell Basin comprises a series of north-northwest trending transtensional depocentres that underlie parts of the continental shelf and slope off western Tasmania. The basin is contiguous with the Otway Basin to the north. The Sorell Basin contains Early–Late Cretaceous mostly non-marine sediments (fluvial and red beds), overlain by younger post-rift paralic and shallow marine siliciclastics and carbonates.
Along with other Mesozoic basins of the Australian southern margin, the Sorell Basin was initiated in the Late Jurassic to Early Cretaceous as part of the Southern Rift System. This major rift system, which extended from Broken Ridge in the west to the South Tasman Rise in the east, led to the eventual break up of Australia and Antarctica. After the initiation of sea-floor spreading of the Bight Basin in the Santonian, break up appears to have propagated eastwards towards the Otway, Sorell and Bass basins. Break-up in the Otway Basin probably commenced late in the Maastrichtian, however well-developed spreading did not commence until the mid-Eocene, when rapid north–south oriented movement between Australia and Antarctica began. Largely northwest–southeast extension during the Late Jurassic and Early Cretaceous resulted in the formation of a series of west-northwest-trending continental rift basins along the southern margin of Australia, and a series of north-northwest trending transtensional basins along the western margin of Tasmania. The initial rifting phase was followed in the Aptian–Albian by a widespread, early post-rift sag phase. A period of regional inversion followed in the Cenomanian. In the eastern Otway and northern Sorell basins, resumption of rifting resulted in deposition of Cenomanian to Maastrichtian shallow marine to fluvial sediments. Moderate uplift of the rift flanks occurred during the initiation of sea-floor spreading off western Tasmania. Progressive collapse of the margin occurred as seafloor spreading propagated southwards, and resulted in the deposition of thick prograding Paleocene sequences in the north, followed by similarly thick Eocene sequences further south. Subsequent north–south oriented plate movement between Australia and Antarctica during the Middle to Late Eocene resulted in the formation of a transform plate margin and the Australo-Antarctic Gulf, a narrow restricted seaway along western Tasmania. Clearance between the Australian and Antarctic plates occurred around 34 Ma (Eocene/Oligocene boundary) resulting in the development of circum-Antarctic ocean currents—an event recorded by a regional unconformity. From the Late Oligocene to Pleistocene, open marine conditions prevailed in the Sorell Basin.
The Sorell Basin is composed of seven depocentres—King Island, Sandy Cape, Strahan, Port Davey, Toogee and two unnamed sub-basins—along the western Tasmanian margin. Geometry of the narrow, relatively deep depocentres was controlled by transtensional fault systems during the Cretaceous to Paleocene and left-lateral strike slip from Early-mid Eocene. Southward propagation of seafloor spreading resulted in collapse of the margin with thick prograding Paleocene sequences to the north (Otway) and prograding Eocene sequences to the south (Sorell Basin and South Tasman Rise). Continued movement on the transform plate margin along western Tasmania, culminated in final separation and clearance of the Australian and Antarctic plates at about 34 Ma.
The Sorell Basin is sparsely drilled, with only three petroleum exploration wells in the basin—Clam 1, Cape Sorell 1 and Jarver 1 . Limited stratigraphic data has prevented the development of a comprehensive stratigraphic nomenclature scheme for the Sorell Basin, so Otway Basin nomenclature is generally adopted.
The transition from continental shelf to abyssal plain across a continental slope is of relatively gentle and uniform grade. Canyon incision and erosion into the Neogene section affect the upper slope. In the mid-slope, the section is 3-4s TWT thick and probably forms a southeastern continuation of the Eastern Voluta Trough of the Otway Basin.
This cross-section through Cape Sorell 1 in the Strahan Sub-basin provides an example of the geometry, thickness and structure of the basin.
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