THE TECTONIC ELEMENTS OF THE PERTH BASIN

Abstract

Birth and growth of the highly faulted Perth Basin was dominated by the Darling Fault which down-throws an essentially elongate sedimentary trough against a Precambrian Shield to the east. The Precambrian Northampton and Leeuwin Blocks restrict the basin to the north and the south. The Perth Basin embraces four major sub- basins separated by intra-basin uplifts. These sub-basins are the Dandaragan and Sunbury Troughs and the Abrolhos and Vlaming Sub-basins. Major intra-basin uplifts include the Beagle, Turtle Dove and Harvey Ridges, and the Edwards Island Block.

Surface outcrop is representative of only a small part of the total stratigraphic column. Deposition of Ordovician - Silurian sediments in only the extreme north of the basin was followed by a long period of non-deposition. Sedimentation recommenced in the Lower Permian and continued with minor breaks throughout the Mesozoic and into the Tertiary. The Permian had a widespread distribution, as did the Triassic and Jurassic which attained maximum thickness in the Dandaragan Trough. A major intra-Neocomian unconformity developed in the Vlaming Sub-basin where up to 20,000 ft. of pre-unconformity Neocomian and 5,000 ft. of post-unconformity marine Cretaceous sediments were deposited. Tertiary sediments were deposited in both the Vlaming and Abrolhos Sub-basins.

Limited movement on the Darling Fault in Permian and Lower Triassic times led to gentle basin downwarp. Renewed fault activity in the Upper Triassic resulted in rapid basin subsidence, and less violent fault activity continued through the Lower Jurassic. The most severe tectonic activity and basin subsidence, with local uplift, occurred in the Upper Jurassic and Neocomian, at which time graben-collapse of a mid-basin arch offshore from Perth formed the Rottnest Trench. This intra-Neocomian tectonism was probably associated with .sea floor spreading, the westerly drift of India from Australia, and the break up of Gondwanaland.

This structural synthesis of the Perth Basin is largely derived from geophysical surveys and deep drilling carried out over the past twenty-three years on the present leases of West Australian Petroleum Pty. Limited (WAPET).