Australia: The Land Where Time Began

A biography of the Australian continent 

These ranges, red dome-shaped hills, bornhardts, are eroded from acid volcanic rocks dated to 1540 Ma. This is a massif developed on silicic volcanic rocks (Gawler Range Volcanics) of Mesoproterozoic age. The rocks are thought to be ignimbrite, cooled ash flow deposits laid down by nuee ardente-type eruptions. Some of the hills are isolated, though arranged in ordered rows, their relief varying from 150 - 180 m. Orthogonal fractures define the blocks each bornhardt is developed on. Convex-upward sheet fractures are the basis of the dome shape of the hills. Vertical partings define columns that are prominent in detail.

The crestal flats of the many bevelled domes combine to form an ancient land surface at the summits. It has been suggested that glaciers must have overridden the present area of the Gawler Ranges during the Permian, the glaciers probably removing any regolith that had formed prior to that time, making the resulting planation of Mesozoic age, as it must be younger than the time when the ice withdrew from the area.

In the formation of the Gawler Ranges of  the present, by the Jurassic the Beck Surface, a planation surface, had developed, beneath which the volcanics were weathered differentially, a thicker regolith developing along fracture zones. The present landscape was shaped by the subsurface weathering, the valleys of the present being initiated by weathering that was fracture-controlled. The intervening massive blocks were rounded by either weathering that was guided by sheeting fractures, as the result of corners and edges of blocks being preferentially weathered, or by both mechanisms (Twidale & Campbell, 2005).

It has been suggested that there had been an extensive mantle covering the Nott Surface at an earlier time that has since been eroded and removed, as 2 sites on valley-side slopes have preserved a regolith. It has been found that Mt Anna Sandstone, that is exposed to the north of the upland in the southwest part of the Eromanga Basin, is the deposition site for regolith transported from the Gawler Ranges to the south. Lenses in the Mt Anna Sandstone have been found to contain boulders and cobbles from the Gawler Ranges Volcanics, the degree of weathering of the included fragments decreases down the profile, as would be expected of the stripping of a weathered profile, with the top portion being most heavily weathered because it has been subject to weathering for the longest period of time, and the least alteration occurring near the bottom, making this consistent with the stripping of a regolith.

The Nott Surface is considered to be best described as as an etch surface, a former weathering front, not much regolith being present, the age of which is derived from the presence of fossiliferous strata beneath and above it of Neocomian-Aptian, the earliest Cretaceous of 130-120 Ma. The conclusion is that the present landscape was exposed during the later Mesozoic, not changing significantly during the Cretaceous, as there is little volcanic debris in the adjacent basins. There is a skin that is iron-rich that is believed to be the old weathering front base that has been preserved in patches on the crests and slopes of many hills. Within the valley floors of the Gawler Ranges silcrete formed in the Early Tertiary, as well as in the piedmont zone that was marginal to the upland. In the upland the valleys are virtually intact, but dissection has taken place in the marginal plains.

Sources & Further reading

1. Veevers, J.J. (ed.), 2000, Billion-year earth history of Australia and neighbours in Gondwanaland, GEMOC Press Sydney
2. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd

Ranges