PALAEOZOIC HISTORY OF THE ST. VINCENT GULF REGION, SOUTH AUSTRALIA

Abstract

The St. Vincent Gulf Region comprises portions of the Gawler Craton and Adelaide Geosyncline. It extends from Yorke Peninsula and a portion of Investigator Strait (within the craton) to the Mt. Lofty — Kangaroo Island fold belt (inclusive). An area of potential hydrocarbon accumulation exists in a marginal area of the geosyncline adjacent to the craton.

After latest Precambrian deformation of Proterozoic strata, Lower Cambrian deposition commenced with transgressive arkoses, overlain by carbonates. The upper portion of the carbonates near or on the craton grade eastwards into mudstones in the geosyncline proper. A major retreat of the sea from marginal areas of the craton and geosyncline occurred during late Lower Cambrian time. The exposed land mass constituted a source for clastics deposited in the remainder of the geosyncline, which was undergoing strong downwarping movements. Intercalated carbonates and clastics of latest Lower Cambrian and Middle Cambrian age document return of the sea to the craton.

Deposition in the geosyncline probably terminated during Middle Cambrian time, with pronounced earth movements and regional metamorphism culminating during Ordovician time. The Palaeozoic and Proterozoic rocks are unconformably overlain by Lower Permian and Cainozoic sediments.

Three major fault patterns can be recognized in the region. Within most of the western portion, the fault pattern is similar to that of the Gawler Craton. Two later patterns of faulting in the remainder of the region evolved during Upper Proterozoic-Cambrian and Cambro-Ordovician times respectively, as the consequence of pronounced folding. Episodic movements of blocks bounded by faults in the western and northwestern portions of the region were the result of strong compressional forces directed towards the craton. Weaker earth movements also occurred during Lower Permian-Eocene time rejuvenating some of the old structures.