The Willochra Lake, Flinders Ranges, South Australia

Australia: The Land Where Time Began

The Willochra Lake, Flinders Ranges, South Australia

A period of planation occurred that resulted in the area being reduced to low relief, the only rises remaining being outcrops of quartzite and other resistant rocks, followed by a period of deep dissection during which outcrops of weaker rocks, such as shale and mudstone, were worn down to leave quartzite and sandstone ridges and ranges. The seas that covered the area during the Cretaceous withdrew and the baselevel was lowered which was part of the cause of stream incision, the other part being a period of renewed faulting and uplift. The ridge and valley landscape of the present was produced by the rivers eroding deep valleys in weaker strata in later Cretaceous and the earliest Tertiary leaving formations of sandstone and quartzite as the ridges. The dissection occurred about 60 Ma, before the Middle Eocene, in the southern section of the Flinders Ranges as can be seen at the northern margin of the Willochra Plain.

In the Early Tertiary the rivers that drained this plain were blocked, which is believed to have resulted from the upfaulting of the western margin of the upland (e.g., Williams, 1973; Quigley et al., 2006). In the northern part of the Willochra Plain a lake formed as a resulted of this blockage of the rivers draining this plain, with sediments being deposited in the Basin tongue up adjacent valleys. The presence of plant fossils has allowed the dating of these sediments to the ?Middle Eocene (Harris, 1970), indicating that at least at this part of the Range the ridge and valley topography had been developed by this time. Remnants of the old valley floor that had been silicified were capped by coarse introduced debris, and graded to the margin of the lake are common in the valleys of the Kanyaka and Mt Arden.

In the Middle Eocene arms of the Willochra Lake extended up the valleys eroded by Kanyaka, Wirreanda and Mt Arden creeks, which occasionally flow at the present. They are very old streams, as is the River Torrens which drains the central part of the Mt Lofty Ranges (Twidale, 1997). In Mt Arden Creek valley, remnants of a surface cut across strata from the Cambrian, and on which lake beds were later deposited, have been uncovered close to the junction between that creek and the Willochra.

According to the author¹ it is uncertain whether the ridge and valley topography age adjacent to the northern Willochra Basin and Plain can be considered to be characteristic of the entire fold mountain belt. Stratigraphic evidence has not been found, though the bevelled ridge crests of putative Cretaceous age are well preserved, occurring throughout the upland, as is the case with the old valley floor remnants. Abandoned pediments, and mesas in locations where the old piedmont has been preserved by coarse debris that has fallen from nearby bluffs, indicate erosion low in the landscape. The old valley floors are 5-10 m higher than the valley plains of the present and can be correlated in many instances with fragments of platforms that are perched on adjacent scarps. Kaolinised regoliths that have been stripped in some places to produce local etch plains have been formed beneath the former valley floors. Also, widespread accelerated soil erosion, that is most obvious in the form of gullying, has resulted from the arrival of humans, especially Europeans (Twidale & Bourne, 1996).

The modern Flinders landscapes are basically those that were present in the Cretaceous and earliest Tertiary, though these recent changes are minor in the overall view they are significant in terms of landscape development models. The planation surfaces are from the Early Cretaceous 130-120 Ma, and in the central southern region the high plains are from the Late Cretaceous 100-70 Ma, and the ridge and valley assemblages are about from about 60 Ma.

Sources & Further reading