REGIONAL LITHOFACIES AND PALAEOGEOGRAPHY OF THE GIDGEALPA GROUP

Abstract

The Permian Gidgealpa Group contains major reserves of gas and minor amounts of oil in fluvial to marginal marine sandstone reservoirs within coal measures in the Southern Cooper Basin. Lithofacies analysis of a sequential set of palynologically defined time-rock units through the Gidgealpa Group has clarified depositional trends, identified regions of maximum channel development, located positions of shore-lines and shown directions of marine transgressions and regressions.Braided streams deposited the oldest unit, the Tirrawarra Sandstone, on a possibly glacially scoured land surface. As topographic gradients declined, rivers began to meander, and sandstone, shale and coal deposits of the Patchawarra Formation encroached over Tirrawarra Sandstone. Northwards flowing rivers gradually filled pre-Permian valleys with detrital sediments.During uppermost Patchawarra Formation deposition, the Cooper Basin was invaded from the east by a restricted sea, which then deposited the Murteree Shale. As this sea retreated, shoreline sediments of the Epsilon Formation prograded towards the east. A second transgression inundated the land, and the Roseneath Shale was deposited. Finally, delta deposits forming the Daralingie Beds prograded eastwards in the wake of the retreating sea.A period of erosion followed, during which time the present structural grain was imposed on the basin. Subsequently, eastward flowing meandering rivers deposited sandstone, shale and coal of the Toolachee Formation on an essentially flat topography, with only minor hills. The Toolachee Formation grades upwards into non -coal bearing beds, which perhaps register a climate change.