The Gason 1:250 000 sheet area is centred about 850 km north of Adelaide, in the South Australian portion of the Great Artesian Basin. The area mapped includes most of the Gason Dome, a gentle upwarp in a relatively flat-lying sequence of Mesozoic and Tertiary rocks. The oldest outcropping geological unit is the Upper Albian-Cenomanian Winton Formation (up to 720 m thick in Delhi-Santos' Kalladeina 1) which is a sequence of lithic sandstones, claystones and siltstones. Oil exploration and water bores have also penetrated older Mesozoic sedimentary (maximum known thickness 1100 m) and Palaeozoic sedimentary rocks (maximum known thickness 1705 m) that can extend down to a depth of 3762 m. Cainozoic units exposed on the Dome include sandstones of the Tertiary Eyre and Doonbara Formations and equivalents of the Etadunna Formation and Cadelga Limestone. Pleistocene fluviatile sediments underlie the sand dunes of the Simpson and Strzelecki Deserts. Most of the surface geological units have been subject to alteration including silicification, kaolinization and gypsification. Structures on the Gason sheet area show a dominant northerly trend which is consistent with surrounding areas of the Great Artesian Basin.