The Coolcalalaya Sub-Basin: A Forgotten Frontier ‘between’the Perth and Canarvon Basins, WA. Arthur J. Mory, Robert P. Iasky and Sergey I. Shevchenko 1 Abstract The Coolcalalaya Sub-basin is bound by the Darling Fault to the east and the Ajana-Yandi and Urella faults to the west. The only petroleum data are gravity and aeromagnetics from the 1960s and 1970s, and one seismic line shot in 1964. More than eighty water and mineral exploration bores, up to 300 m deep, provide the best geological control for the sub-basin; there is little outcrop. The sub-basin deepens eastwards, to a maximum of 8500 m of ?Ordovician to Permian strata next to the Darling Fault. As in the Perth Basin, movement along the Darling Fault dominated the depositional history of the sub-basin but, unlike that basin, this history commenced prior to the Permian. The three most significant episodes of tectonism were lower-mid Palaeozoic rifting, Permo-Carboniferous rifting, and Lower Cretaceous breakup, which possibly produced gentle folds associated with strike- slip movement along northerly trending faults. The ?Ordovician-Lower Carboniferous section is estimated to be up to 4500 m thick, and is the most crucial for the petroleum potential of the area. The overlying Permo-Carboniferous glacial strata have, at best, sealing potential, and although the younger Permian succession has good source rock characteristics, it is confined to shallow synclines in the north of the sub-basin. Spore colouration and vitrinite reflectance data indicate that the near surface rocks over most of the sub-basin have a low level of maturation and, because these rocks are Devonian and older to the southwest, considerable depositional thinning of the Permo-Carboniferous towards the Northampton Complex may be inferred. The highest measured total organic carbon value from the Devonian is 0.82%, but generating potential is low. On the available data, the sub-basin remains a poorly explored frontier area with low potential. Minor investment in shallow drilling, however, should provide more information on its potential for hydrocarbon generation. Introduction The Coolcalalya Sub-basin (Coolcalalaya Basin of Condon, 1956) lies between the Northampton Complex to the west and the Yilgarn Craton to the east and covers an area of approximately 15 000 km 2 (Fig. 1). The sub-basin has generally been placed within the northernmost part of the Perth Basin (Condon, 1965; Playford et al., 1976) although Arrington (1966) and Condon (1968) suggested it was linked to the Byro Sub-basin in the Southern Carnarvon Basin. The southern boundary with the Irwin Terrace was based on apparent southerly thinning of the lower Palaeozoic section and corresponding thickening of the Permian (Hocking, 1994), but now it is uncertain whether or not this takes place. At that time the possibility of the presence of pre-Permian strata in the Irwin Terrace was not considered and it was thought that the Permian thickened substantially towards the Darling Fault (Le Blanc Smith & Mory, 1995). To the north, Hocking (1994) suggested, on the basis of poor gravity data and regional lineaments, that the sub-basin is bound by a right-lateral fault that separates it from the Byro and Merlinleigh sub- basins. This fault, as mentioned by Crostella (1995), is not particularly clear on images made from the presently available gravity and magnetic data, but appears to be a northwesterly trending tranfer zone that separates faults with significantly different orientations. Little petroleum exploration activity has taken place in the sub-basin; the only records are of gravity and magnetic surveys in the 1960–70s and a single 66 km- long seismic line (Hyde Soak Survey) shot in 1964 for Conoco. The area has been virtually ignored by petroleum explorers, probably because it was thought to contain less 1 Geological Survey of Western Australia, Perth. Acknowledgements: We thank Rio Tinto for access to their coal exploration core and for permission to publish their aeromagnetic data between latitudes 27Þ15’S and 29Þ00’S. Dick Cooper and Associates provided the geochemical analyses from the Irwin River outcrops. Roger Hocking and Angelo Crostella are thanked for reviewing the manuscript. This paper is published with the permission of the Director of the Geological Survey of Western Australia.