Drilling and sampling of basement rocks underlying the south-western Murray Basin, newly performed by CSR Ltd and SADME, have revealed that a hitherto unknown igneous domain exists there. The most abundant rocks are altered and soda-enriched basaltic lavas and tuffs of spilitic type. Less abundant volcanic rocks are picritic metabasalts and trachyandesites. Intrusive rocks include metadolerites and unaltered gabbro and granite. Sediments are rare, and mainly consist of recrystallised silica and chlorite, probably derived from in-place corrosion of the basalts, together with very rare distal terrigenous clastics, mainly micas and clays. Tectonic deformation is localised rather than regional. The overall composition of the predominantly igneous domain as now revealed contrasts with that of terrigenous clastics of the Kanmantoo Group on the north-west and that of the Glenelg River Complex on the south-east. The extent of the buried province in other directions is not known. If subsequent drilling indicates a wide area of submarine volcanic rocks, the likely original setting of this province may be a back arc basin, but the volcanic rocks so far examined were intersected on the Padthaway Ridge, which may have been a tensional or transcurrent tectonic feature at the time of eruption. The area is of high ore-forming potential, particularly for pyritic copper-zinc deposits, and it is suggested that further drilling should be targeted on zones of strong alteration and tectonic disruption within the volcanic rocks.