Mt Babbage, Northern Flinders Ranges, South Australia

Australia: The Land Where Time Began

Mt Babbage, Northern Flinders Ranges, South Australia

A prominent summit surface is preserved throughout the Flinders Ranges, though the ridge and valley topography predominates. The strata preserved at Mt Babbage, a small mesa that is prominent on the high plain cut across various sedimentary metamorphic and igneous rocks, in the northernmost part of the upland, indicate the age and character of the surface (Woodward, 1955).

The sandstone strata that date to the Early Cretaceous, about 130 Ma, were deposited in marginal areas of the seas which inundated much of Australia at that time as well as at several times throughout the Cretaceous (Frakes, 1897).

Crucial to the dating of large parts of the Australian landscape are marine transgressions that occurred in the Cretaceous as many surfaces and forms, that were later exhumed, had been buried and preserved by associated sediments. There are also remnants of the strata from the Cretaceous in valleys and basins, therefore a minimum age for the adjacent topography can be obtained, and many old plains were arguably eroded by rivers flowing and graded to a shoreline from of Cretaceous age.

A remnant of the Bopeechee Regolith underlies unconformably the beds of Cretaceous age at Mt Babbage (Sheard, 2001), and following exposure they were silicified. The preservation of a regolith is unusual as it was covered by marine deposits. Mt Babbage is a outlier of younger rocks and is the only one that has been preserved within this upland, though beneath adjacent plains there are equivalent beds. It is demonstrated by the Mt Babbage remnant that the surface of the upland predates the Cretaceous, and that at the base of Mt Babbage the unconformity separating the Precambrian strata from the Cretaceous strata is level with the surrounding plain. Erosion of the strata deposited during the Cretaceous that at a previous time had covered the entire surface of the northernmost Flinders Ranges has re-exposed the high plain. It is therefore an exhumed surface from which a regolith of pre-Cretaceous age has been stripped by wave action. The surface originated more than 130 Ma and predates the Earl Cretaceous.

Riverine planation surface

Plant fossils from a littoral or shoreline environment have been found in strata that were preserved in Mt Babbage (Alley & Lemon, 1988), which suggests Mt Babbage of the present was near the southern limit of the Cretaceous sea and the summit high plain that lies to the south is of different age and character. Rivers flowing to the shoreline in the Early Cretaceous have been suggested by the author¹ to be the most likely to have eroded it, which would date it to about 130-120 Ma. The crests of quartzite or sandstone ridges are the places where it mostly preserved, and some sites it clearly cuts across strata that are tilted. It is developed on shale and mudstone in the central Flinders Ranges, rocks that are easily erodible, with the result that there are several levels at different, though closely related elevations, that occur in the same area, as they vary in resistance, as occurs around the Stokes Hill Lookout, in the central part of the upland.

It is suggested by the author¹ that another effect of deep erosion may be the conservation of this surface in argillites. The central section of the Flinders Ranges is essentially a regional anticline, the crest of the structure having being in tension, and it has been weathered and eroded. It is suggested by consideration of stratigraphic sections that several kilometres of rock have been eroded away, the depth of the erosion possibly being so great that the lower compressional zone has been exposed. The buttressing effect of quartzite and sandstone ridges that have been exposed to both the east and west of the core area protected the central region. Together such protection and strata has gone some way to explain the preservation of a high plain between Wilpena Pound and Blinman, in spite of it being developed in weak rocks.

Remnants of earlier landscapes

In the south at Boolcunda and at Springfield, and to the north around Leigh Creek and Copeley, sediments that include brown coal, have been preserved in basins of Triassic age. The fine-grained sediments of the Springfield Basin, becoming finer-grained with height in the sequence, i.e. they display upward-fining, which suggests they were derived from a terrain where the relief is diminishing. It has been suggested that 250-210 Ma they were derived from a surface of low relief, though there are no remnants of a such an age from the Triassic that have been found in the Flinders Ranges. According to the author¹ this does not rule out the possibility that within the ridge and valley topography some of the bevelled ridge crests are remnants of such a landscape, and if so, they could be correlated with the lateritic surface of the Gulfs Area (Twidale, 2000c). The Webb Surface, named after Bruce P. Webb (1926-2000), has been suggested as a name for this implied Triassic landscape, Webb having investigated and mapped many parts of the Flinders Ranges while working for the South Australian Geological Survey.

Sources & Further reading