Australia: The Land Where Time Began

A biography of the Australian continent 

Arcoona Plateau and the Tent Hills

Desert conditions prevail to the north and west of the upland spine, that is based in the Adelaide Geosyncline, over the Stuart Shelf and the Gawler Craton and various basins, as well as older structural remnants of South Australia. Neoproterozoic strata comprise the Stuart Shelf region that has essentially remained undisturbed, deposited on, and underlain by, the crystalline basement of the craton (Johns, 1968). The sediments of the Shelf are expressed in the Arcoona Plateau and the Tent Hills region to the west and north of Port Augusta (Twidale et al., 1970; Twidale, 1994).

The landscape's age

Interbedded quartzites and siltstones dipping gently down to the north comprise the sedimentary sequences. When these sedimentary sequences are dissected the result is plateaux and mesas, as the dip is so gentle. A change in elevation of 17-18 m/km is implied by a dip of 1", and over a distance of several scores of kilometres successive quartzite layers are exposed, each of which forms a capping on plateaux and high plains, though clearly bevelled, and the summit surface cuts across bedding. The thin remnants of weaker sediments survive above the caprock, leading to many plateaux being domed. Movement along the Torrens Fault delineated the Arcoona Plateau and its composite summit surface on the east. In the downfaulted basin on the east, that is presently mainly occupied by Lake Torrens, a sequence of freshwater sediments was deposited, the basal members of which are from the Eocene. Therefore the faulting is at least from the Eocene, and the dislocated high plain surface of the Arcoona Plateau is also of a similar age, at least (Johns, 1968).

Other palaeogeographic evidence indicates that the age is actually much greater than is suggested by the age of the faulting, as is the case with the Mt Lofty Ranges. The valley floors cut below the Plateau in the Woomera area and elsewhere are underlain by marine strata from the Early Cretaceous (Johns et al., 1981). The valleys in which these beds were deposited must have existed prior to the marine transgression of the Early Cretaceous, and the surface they are incised into is even older, which is suggested by the author1 to probably be of Jurassic, or possibly Triassic, age.

This is another case in which the use of geological method and common sense allows the sequence of development of the landscape to be established. The landscape - plateaux and valleys - has been shown to predate the Early cretaceous. As the summit surface or plateau had developed before the rivers incised the valleys and the sea had advanced into the area during the Cretaceous, it is even older (Twidale, 1994). To the west of the Plateau Early Tertiary silcrete occurs in the floors of valleys (e.g. Hou et al., 2003). It has also been preserved in old valley floors, and in piedmont zones, as skins on blocks and boulders, to the southeast of the Plateau (Twidale et al., 1970; Hutton et al., 1972). Given the Eocene age of the faulting that delineates the Plateau on its eastern side the age of the silicification appears to be of younger rather than older Tertiary age, though its age is difficult to determine.

Adjacent plateaus and high plains

To the west and southwest of Port Augusta there are remnants that are probable of this old surface in the Tent Hills and Baxter Range, and probably also on the Baxter Hills, that is an outcrop of a folded Proterozoic conglomerate that is located further to the west (Bourne & Twidale, 2003). Silcrete is also present in the Plateau piedmonts.

 

Sources & Further reading

  1. Twidale, C.R., 2007, Ancient Australian Landscapes, Rosenberg Publishing Pty. Ltd. , NSW
Author: M. H. Monroe
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Last Updated 14/10/2013
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading