The INNAMINCKA 1:250 000 map area is located in the north-east of the state, adjacent to the SA-Queensland state border. It lies between latitudes 27°00 and 28°00 and longitudes 139°30' and 141°00. It is one of the best explored areas of the state in the subsurface and, together with STRZELECKI, contains in the subsurface the whole of South Australia's known petroleum resources. The surface geology is dominated by Quaternary to Recent Holocene dune sands and clays which rim the incised and eroded Tertiary Innamincka Dome, exposing a Cretaceous core. A smaller dome located to the south, called the Tickerna Dome, has no Cretaceous exposed. The Middle Tertiary silicification on the Eyre Formation provided a silcrete cap (Tsi) which now visually defines the domal structure. Erosion of the dome provided clastic sediments (Tmo - Doonbara Formation) which post-date Tsi; they formed a Tertiary sandstone which was lateritised at the top. All exposed rocks were subsequently affected by a secondary silicification. The last, and main diagenetic process to have occurred was the gypsite process, which weathered, bleached, and partly ferruginised underlying strata, as previous processes did, but this last process has masked to some extent the effects of earlier processes.