the Eromanga Basin borders onto old basement rocks which are about 1,500 million years old and consist of granite and gneiss. Also on the western margin lie the sediments of the Officer Basin which comprise sandstone, quartzite, siltstone and shale about 500 million years old. These sediments were tilted by crustal forces and form prominent hills such as the Mount Johns Range near Marla. The opal deposits at Mintabie occur in a sandstone unit which was bleached and weathered by the same process that affected the Eromanga Basin sediments as described above.