DEPOSITION OF THE PRECIPICE SANDSTONE AND THE EVOLUTION OF THE SURAT BASIN IN THE EARLY JURASSIC

Abstract

The Precipice Sandstone, the basal formation of the Surat Basin sequence, is an Early Jurassic quartzose sandstone, which extends to the west and east beyond the limits of the Surat Basin.Throughout most of the basin, the formation consists of medium to coarse-grained, porous, quartzose sandstone with well-developed medium-scale planar cross-bedding. In the subsurface on the Roma Shelf, the formation is more variable in its grain size and porosity/permeability but is generally finer grained than elsewhere in the basin.Sedimentary structures and vertical profile characteristics for the Precipice Sandstone, throughout most of the basin, indicate that the sandstone was deposited as transverse bars in a braided stream system flowing on a gradient of approximately 1m/km. The sandstone on the Roma Shelf shows characteristics of a lower gradient meandering stream depositional system.The palaeogradient and palaeoslope directions predicted from sedimentary data, which indicate a consistent west to east current direction across the northern Surat Basin, are incompatible with previously published interpretations based on use of the datum plane/valley floor isopach method. It is suggested that the isopach method has yielded erroneous results, as a result of the effects of differential subsidence in the underlying Permo-Triassic Bowen Basin sequence.The Precipice Sandstone is a diachronous fluviatile facies, which transgressed in a westerly direction, and was replaced by the finer grained fluviolacustrine Evergreen Formation. The sandstone on the Roma Shelf probably represents the last stages of low-energy fluviatile deposition prior to the transition to the Evergreen Formation. The main sediment source for the Precipice Sandstone probably lay to the west and south of the Great Artesian Basin in Precambrian areas such as the Willyama Inlier.