This item is only available electronically.An area to the north of Lake Torrens, encompassing the newly proclaimed Stuart Creek Precious Stones Field, was studied, with particular reference to the opal- bearing Cretaceous sediments.
The pre-Mesozoic basement consists of Adelaidean rocks of the Wilpena Group in the east, separated from Lower Cambrian rocks by the Arthur Fault
The relatively flat-lying Cretaceous sequence consists of two mappable units. The lower unit, called the "Stuart Creek Beds" in this thesis, is probably a transitional unit marking the transgression of the Lower Cretaceous sea. The early Aptian Marree Formation comprises bioturbated brown montmorillonitic muds with sandy and conglomeratic beds more prominent near the base. Scattered "erratics" occur. The original sediment, deposited in a marginal marine environment was black, pyritic and carbonaceous. It has been affected by a number of weathering events during the Tertiary. The uppermost portion has been very heavily weathered and bleached. Only a thin remnant exists now, due to erosion prior to Tertiary deposition
The Tertiary sediments are mostly sands with some clays, silts and limestones. Milky quartz pebbles are a common characteristic. The sediments are non-marine, dominantly fluviatile. Silicification is common but variable. The sequence is correlated with the Mount Sarah Sandstone of Miocene age.
Opal, occurring throughout the area, is believed to have formed associated with silicification of the Tertiary. The precious opal is notable in that it occurs within relatively fresh sediments, rather than highly weathered rocks as elsewhere in South Australia. Precious opal is only found on the Field, but all the Cretaceous must be considered potentially opal-bearing until further work is done.Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 197
