Ashburton Range

Australia: The Land Where Time Began

Ashburton Range

Prominent summit bevels are preserved on ridges of folded quartzite from the Neoproterozoic in the Ashburton Block and the Tennant Block. A bit to the north and northeast is the Cretaceous shoreline and the summit surface, called the Ashburton Surface, that was constructed as part of the landscape in the Cretaceous (Hays, 1967). An alternative suggestion is that it and remnants of terraces that are preserved on valley fills from the Cambrian have been interpreted (Stewart et al., 1986) as possibly of Cambrian age, though according to the author¹ the evidence is equivocal, and he suggests that any epigene form from the Cambrian would surely have been destroyed or modified by the action of frost during the Permian, a time when the uplands of central Australia were glaciated (BMR Palaeogeographic Group 1992). Also, there is no known evidence of the terrace deposits marking the upper limit of deposition. As the beds have been described as being ferruginised and silicified, which suggests periods of weathering that result in duricrusts that are similar to those that have been recognised from the Mesozoic and Cenozoic.

It is suggested by regional considerations that the surface is more likely to have either been exhumed from beneath a Cretaceous cover, or be a remnant of a complex epigene surface graded into shorelines of seas that extended over adjacent basins just to the east during the Cretaceous (Frakes, 1987). No evidence has been found in the local area that indicates which of the 2 possibilities may be correct. In the adjacent regions laterite and silcrete from the Miocene are known (e.g., Daly River, MacDonnell Ranges) and a siliceous caprock with columnar structure is responsible for the preservation of Castle Rock, a mesa, located near Renner Springs.

The Cambrian age of the terraces that were previously described (Stewart et al., 1986) has since been analysed by apatite fission track thermochronology (AFTT) and cosmogenic nuclide analysis (Belton et al., 2004), the conclusion being that they are much younger than had previously been suggested. They also dispute the postulated Cretaceous age of the Ashburton Surface (Twidale, 1994).

Very low rates of erosion were announced (Stewart et al., 1986), but he didn't consider the possibility of a cover of sediment or regolith having been stripped from the surface. When considering the age of the beveled ranges AFTT analysis indicates deep erosion, but not when it occurred. Whether or not there was a substantial Cretaceous cover, the morphostratigraphic argument is suggestive but not compelling. The author¹ suggests that it is unwarranted to extrapolate their conclusion to all high surfaces in central Australia in light of the field evidence, though he suggests the paper by Belton et al. is welcome as a contribution to the debate between the 2 schools, physical and morphostratigraphic, though when considering the situation of the Ashburton terraces the question is still open.

According to the author¹ AFTT dates are not uniformly reliable. In some places morphostratigraphic interpretation is corroborated by AFTT analysis, though in other parts of southeastern Australia and South Africa its findings are at odds with the geological evidence (e.g., Partridge & Maud, 1987; Twidale, 1990; Birkenhauer, 1991; Fleming et al., 1999; Bishop & Goldrick, 2000), as is the case with cosmogenic nuclide dates.

Sources & Further reading