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A Comparison Of Structural Styles East And West Of Cape Otway.

Carter, Steve

Honours Degree, 1991

University of Adelaide

Abstract

An area east and west of Cape Otway, in the Otway Basin, was studied using recently acquired seismic data, well data and onshore geological maps. The aims of the thesis were to understand the structural evolution of the area, explain the structural differences between the Torquay Sub-Basin (east of Cape Otway) and the Otway Basin (west of Cape Otway), and to provide a tectonic model for the area.

Rift phase extension in the region during the Lower-Mid Cretaceous was in a NNW-SSE direction, with faults trending NE-SW to E-W. This faulting was large scale and listric to the west of Cape Otway, and large scale, syn-sedimentary in the Torquay Sub-Basin.

As southern Gondwanaland began to break up in the Mid Cretaceous, faulting changed orientation to NW-SE to E-W, and became much smaller scale on both sides of Cape Otway. This change coincided with the beginning of the drift phase and associated slower subsidence. NW-SE to E-W faulting continued until the Base Upper Cretaceous.

A major phase of compression in the Base Upper Cretaceous caused widespread folding in a NE-SW direction, which was most intense near Cape Otway. Subsidence and NW-SE extensional faulting continued after this on the western side of Cape Otway, but sedimentation did not re-commence in the Torquay Sub-Basin until the Late Upper Cretaceous. Folding in the Cape Otway region at the Base Tertiary is reflected in a regional unconformity elsewhere in the basin, but is not discernible in the Torquay Sub- Basin.

NW-SE faulting on both sides of Cape Otway stopped by the Paleocene, and by the Oligocene, NE-SW folding caused minor structural growth. Major folding did not begin until after the Miocene, and was localised around Cape Otway. In the Torquay Sub-Basin, compression resulted in spectacular inversion of Lower-Mid Cretaceous NW-SE to E-W faults.

Breakup of Antarctica and Australia may have occurred during the Base Upper Cretaceous (90 My), not at Top Lower Cretaceous (95My) as interpreted in the past. This is based on the identification of two major angular unconformities in the Mid Cretaceous. The higher unconformity at Base Sherbrook Group, fits closely with the first occurrence of deep marine sediments and is associated with a major episode of folding.

After the breakup of Gondwanaland, the Cape Otway high acted as a stable edge, trending roughly parallel to the recently formed plate margin. It was the focus of compression and extension, and prevented flexing of the crust and subsidence in the Torquay Sub-Basin during the Upper Cretaceous.

Wrench movement cannot adequately account for compression in the Otway Basin, and an alternative explanation is outlined. This involves Mid to Upper Cretaceous compression resulting from easterly movement of Antarctica with respect to Australia, and the folding of strata against the Cape Otway high. Tertiary compression remains unexplained however.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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