Robert Matheson

B. Sc. Honours Thesis

Sedimentation of the Barrow Group, Exmouth Plateau, Northwestern Australia

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The Exmouth Plateau is a major promontory of the north western Australian continental shelf and ranges from longitude 111^oE to 117^oE throughout southern latitudes 16^o to 21^o. This plateau has an aerial extent of approximately 150,000 square kilometres at the plateau margins.

Past research in the area indicates that the Exmouth Plateau is an isolated block of continental crust which was created by rifting during the breakup of India and Australia. (Von Rad and Exon, 1983). The plateau is bounded to the north, west and south by margins which were created as a result of rifting and shearing associated with this major tectonic event.

Sedimentation on the Exmouth Plateau has varied substantially over time, reflecting the different environments which were encountered throughout the rifting process. Early Triassic sediments are fluvial-deltaic in origin but are unconformably overlain by deposits which show a steadily increasing marine influence. This generalized stratigraphic sequence reflects the fact that the plateau has undergone progressive subsidence since it was first exposed to the sea by the breakup of Gondwanaland.

The continental margin deposits of the Barrow Group are examined in detail. This major unit is Tithonian-Hauterivian in age and represents the progradation of an entire continental shelf complex into a shallow marine basin which formed as a result of crustal attenuation, (Boote and Kirk, 1989). The geology of the Barrow Group is unique in that the progradation of the continental shelf takes place over the subsided continental crust of the Exmouth Plateau rather than onto oceanic crust as is usually the case in coastal settings.

Because the plateau is composed of continental crust, it sits at a much higher level than would a similar block of oceanic crust. For this reason, the seas which covered the plateau during Barrow sedimentation were extremely shallow and the entire coastal margin was compressed into a vertical section of several hundred metres rather than the usual three to four kilometers. Despite its vertical limitations, the prograding continental margin of the Barrow Group contains all of the typical coastal facies, ranging from alluvial plain to deep sea fan.

The objective of this study is to examine the depositional history of the Barrow Group, using seismic and well log data, in order to draw conclusions regarding the source area, the environments of sedimentation, and the extent of Barrow Group deposition. In particular, this study will focus on the issue of the Barrow Group. This problem has been addressed in the past by Veevers and Powell (1979), who proposed that Barrow Group sedimentation was caused by the formation of a rim basin along the south western margin of the Exmouth Plateau as a result of its interactions with the greater Indian Plate. According to these authors, the uplifted edge of the plateau provided a source for sediments which were carried to the north east, forming the progradational wedges of the Barrow Group.

Boote and Kirk (1988), dispute this theory, proposing instead that Barrow Group sediments were supplied from the Australian mainland to the south where uplift associated with the rifting event initiated erosion.

In this thesis, the direction of Barrow Group progradation will be assessed from seismic data, thereby providing evidence which can be used to determine conclusively which of these two theories is valid.

Keywords:
Pages: 51
Supervisor:  Ron Boyd