Evolution of an ancient cratonic upland, the Gawler Ranges of inland South Australia

The Gawler Ranges is a massif comprising bornhardts developed on Mesoproterozoic dacite, rhyodacite and granite. The country rocks are subdivided by orthogonal fractures into massive compartments within which are developed sets of concentric arcuate partings or sheet fractures. Similar arcuate structures were superimposed on the western sector of the upland by a meteorite impact during the earliest Palaeozoic. Following the Permian glaciation, the area was reduced to low relief. Fracture controlled differential weathering beneath the planation surface resulted in an irregular weathering front with prominent domical rises and intervening linear depressions. Uplift near the southern limit of the Mesoproterozoic outcrops caused north flowing rivers to strip the regolith and deposit the detritus in the ocean that then occupied the Eromanga Basin. There it was intercalated with marine strata that demonstrated a Neocomian-Aptian age for the alluvial deposit, and hence also for the etch or two stage bornhardt assemblage that was revealed by the evacuation of the weathered mantle. It has persisted virtually unchanged because of resistance of the country rocks. The veracity of the great age attributed to the Gawler Ranges topography is discussed, as are the possible reasons for its survival, the origin of the sheet fractures, and the reasons for the widespread development of etch or two stage features, though it is suggested that in view of the influence of shearing and resultant fractures on landscape development, it is more accurate to regard the upland as of multistage origin.