EARLY PERMIAN DEPOSITIONAL ENVIRONMENTS, SOUTHERN SYDNEY BASIN

Abstract

Seven major lithofacies are present in the Lower Permian sediments of the Jervis Bay area (southern Sydney Basin). These facies, which are areally extensive, are members of the Pebbly Beach Formation (facies A), the overlying Snapper Point Formation (facies B-F) and the Wandrawandian Siltstone (facies G).

Facies A to D represent a regressive shoreface succession which passes upwards into lagoonal and marsh sediments. They were deposited during the Snapper Point regression. Currents were predominantly long-shore with rip and onshore and offshore storm current components.

Facies E to F constitute a regressive (i.e. shoaling upwards) spit profile which Accreted by longshore progradation during the early stages of the Wandrawandian transgression. They form the upper part of the Snapper Point Formation.

Facies G comprises sandy muds of the Wandrawandian Siltstone which were deposited in a marine embayment at temperatures below 0°C as evidenced by glendonites (pseudomorphs after thenardite). The muds represent the culmination of the Wandrawandian transgression which followed upon the disruption of sediment supply to the spits and their consequent inundation.

This depositional history resolves earlier conflicting interpretations of this part of the southern Sydney Basin sequence. It provides a better understanding of the origin of the Snapper Point Formation, which has been a common target for onshore petroleum exploration drilling.