THE GEOLOGICAL EVOLUTION OF THE CONTINENTAL MARGIN OFF NORTHWEST AUSTRALIA

Abstract

The area comprising the Northwest Shelf of Australia is a good example of an 'Atlantic-type' continental margin. It is characterised by a series of major sedimentary basins of Mesozoic age, which generally parallel the present coastline. In each of these depocentres distinct lithotectonic units can be recognised which are related to phases of rifting and subsequent continental breakup. The pre-breakup rift valley and intracratonic basin stages are represented by a very thick Permian to Middle Jurassic series of mainly fluviodeltaic sediments. Breakup took place near the end of the Middle Jurassic and was accompanied by large-scale block faulting with associated uplift and erosion. As a result the ensuing Upper Jurassic to Lower Cretaceous marine transgression took place over a highly irregular palaeotopographic surface. With continuing post-breakup subsidence, open marine conditions became widespread by Upper Cretaceous time. Since the mid-Eocene the deposition of a thick prograding wedge of mainly carbonate sediments has given a general northwesterly regional tilt to the shelf. Such progradation is characteristic of a fully-evolved Atlantic-type continental margin.

Hydrocarbon occurrences on the Northwest Shelf can be related to the tectonic evolution. Major gas/condensate discoveries have been encountered in fluviodeltaic reservoirs within the block-faulted pre-breakup sequence, sealed by post-breakup transgressive marine shales which also provide important source intervals. In addition, some sandstone units of the transgressive series are hydrocarbon-bearing. The prolonged post-breakup subsidence and accompanying thick sedimentation has ensured that source intervals have locally attained the necessary depth of burial for hydrocarbon generation.