PRE-JURASSIC IN NORTH CENTRAL QUEENSLAND

Sedimentary rock sequences of early, middle and late Palaeozoic to Triassic age are present in north-central Queensland. Their distribution beneath the Jurassic-Cretaceous Great Artesian Basin sequence and degree of deformation are matters of some importance in the search for petroleum. Identification of the pre-Jurassic structural basins, which contain these sequences, is assisted by data from exploration drilling, geophysical surveys and water bores. The pattern which emerges, while obscure for the lower Palaeozoic, suggests that by the middle Palaeozoic the more active sedimentation and tectonism in the Tasman geosynclinal zone was confined to a region near the present coastline. Between this position of the geosyncline and the stabilised Pre-cambrian block to the west, middle and then upper Palaeozoic sequences were deposited in intracratonic basins. The basins overlapped westwards on to a low-grade metamorphic sequence, which may be the age counterpart of the stable shelf Cambrian-Ordovician sequence of the Georgina Basin. The middle Palaeozoic sequence, ranging from Middle Devonian to Lower Carboniferous, is present in the Drummond Basin, as well as in several small structural remnants further north, east of the Georgetown Inlier. The Upper Carboniferous to Triassic sequence is preserved in the Galilee Basin. The sequences in these basins are distinguished by an interval of folding, igneous intrusions and erosion during the Carboniferous. Several exploratory tests have been drilled in the middle and upper Palaeozoic basins and have indicated a prospective association of sandstone and shale in the two sequences. An oil show from the upper Palaeozoic in Lake Galilee 1 and the gas discovery and oil shows at Gilmore, in the Devonian of the Adavale Basin, are promising and encourage further exploration in the Galilee and Drummond Basins.