THE TIRRAWARRA SANDSTONE AND MERRIMELIA FORMATION OF THE SOUTHERN COOPER BASIN, SOUTH AUSTRALIA — THE SEDIMENTATION AND EVOLUTION OF A GLACIOFLUVIAL SYSTEM

Abstract

An extensive sedimentological study of cores from the Permian Gidgealpa Group of the southern Cooper Basin has produced a new understanding of the relationship between the Tirrawarra Sandstone and the Merrimelia Formation. These two sequences were previously considered to be everywhere separated by a regional unconformity. This study has shown that the facies states, transitions and associations of both formations interdigitate so that typical 'Tirrawarra- type' fluvial deposits are present within glacigenic 'Merrimelia-type' sediments.The Merrimelia Formation was examined regionally in 29 cored wells. Typical facies occurring within the Formation include glaciofluvial outwash, extensive developments of subaerial and subaqueous diamictites, glaciolacustrine ripple-laminated sandstones and siltstones, and thick, monotonous mudrock sequences with clay-dominant rhythmite horizons.The Tirrawarra Sandstone was also examined regionally in 32 cored wells. Four major facies associations were identified within the Sandstone and indicate the temporal and spatial evolution of a fluvioglacial to predominantly fluvial depositional system. Locally the interrelationship between the Tirrawarra Sandstone and the Merrimelia Formation is clearly evident where basal outwash fan deposits of the Tirrawarra Sandstone are interbedded with typical Merrimelia lithotypes. The dominant, low-sinuosity fluvial channel sand facies of the Tirrawarra Sandstone is also an integral part of typical Merrimelia sediment patterns.The Merrimelia Formation and Tirrawarra Sandstone sediments are intimately related and illustrate the evolution of a predominantly glacigenic system into one in which fluvial processes dominate both in time and space. It is evident that no unconformity separates the two sequences and the inclusion of the Merrimelia Formation within the Gidgealpa Group is proposed.A distinct sedlmentological and stratigraphic identity has been assumed in evaluations of the hydrocarbon potential of the Tirrawarra Sandstone. The Merrimelia Formation has been considered generally non-prospective. This study indicates that potential exists for additional reservoirs of 'Tirrawarra type' to be locally developed within the Merrimelia Formation.