Australia: The Land Where Time Began

A biography of the Australian continent 

Flinders Ranges - Deep Erosion and River Patterns

Included in the Flinders Ranges of the present are elements of several ages, and erosion of at least 6 km of rock has been involved, though it is not suggested that the ancient ranges reached a great height at any time as the marine sediments were folded and broke the surface of the then sea they were subjected to weathering and erosion and so were being worn down to some extent as they rose above the level of the water.

The rivers and streams were shaped by the ridge and valley topography, the resulting patterns largely reflect natural selection. A stream that rises in a weak, non-resistant layer such as shale develops more rapidly than a stream flowing on a less resistant rock such as sandstone, eventually becoming the dominant or master stream. It becomes more prominent as it cuts lower, as the lower it cuts the more runoff it attracts. This results in stream patterns that are usually adjusted to the occurrence of weak and resistant rocks or to a pattern of fractures that may be planes of weakness. Therefore, in the Flinders Ranges, as in other fold mountain belts, trellis and annular patterns are dominant. Strike streams are long streams that develop in weak beds, so called because they follow the strike or bedding of the strata, ands shorter dip or antidip streams that extend back into and through ridges by headward (or regressive) erosion. Seepage and slumping achieves such erosion, most commonly along fractures.

In some cases river patterns do not adjust to structure, instead cutting across hard strata and folds: they are termed transverse or anomalous. In the Flinders Ranges there are several notable examples of this type of river form. There are a number of explanations for anomalous stream sectors, such as diversion by faulting, glaciation, volcanism, antecedence, superimposition and inheritance (see Twidale 1976a, 2004). The explanation of many can be in terms of the deep erosion that has occurred in the Flinders Ranges (Twidale, 1966a, 1972, 2004; Twidale & Bourne, 1996). The distribution and geometry of folds vary in depth as well as plan. The early river patters that determined the distribution of weaker strata differ from that of the land surface of the present. A stream that had adjusted to a weak layer that was exposed in a higher surface could have been let down on to a more resistant formation, in places where there has been deep erosion. With sufficient energy it could have carved a gorge, which is anomalous in terms of the structural pattern of the present.

Odd drainage patterns, such as those present on the Mern Merna Dome, where a gorge has been cut in quartzite by a stream, in spite of being only a few hundred metres from a plain that has been eroded in weak rocks, may be explained by erosion deep into rocks of contrasting character and structure. There is a small stream flowing directly across the domal structure that cuts through 2 quartzite ridges. The development of the 2 gorges cut by Kanyaka Creek and Wirreanda Creek at a point where they pass through a quartzite ridge near Partacoona Homestead in the southern Flinders Ranges is said by the author1 to be even more curious. After both cut gorges they converge and then flow onto the Lake Torrens Plain. As well as these there are many other anomalous patterns that can be explained in terms of the local strata disposition, the changing strata distribution with depth and deep erosion.

Sources & Further reading

  1. Twidale, C.R., 2007, Ancient Australian Landscapes, Rosenberg Publishing Pty. Ltd. , NSW
 
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 30/09/2013 
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading