Australia: The Land Where Time Began

A biography of the Australian continent 

Changes to drainage systems that resulted from rifting

Unlike any other continent, the drainage of the continental interior and the middle portion of the southern margin flow to the centre of the continent, not to the sea. The rifting of Australia from Antarctica to the south and India to the west wasn't a single event at a specific time, it began in the northwest of the continent, probably about 150 Ma, over millions of years, progressing down the west coast, along the south coast and up the east coast. Before rifting began it would have shared rivers with all the surrounding landmasses, and the flow of these rivers would have been changed as the they were broken or beheaded as rift valleys formed around the Australian coastline.

In Western Australia, rivers would have flowed between India and Australia, but evidence of the palaeodrainage channels has been obliterated by later geological activity. In the southwestern province of Western Australia there are a number of south-north trending valleys, the diameter of which indicates they were cut by major rivers, that converge on the Swan River. They presently contain no rivers, just series of salt lakes. It is believed they may be amongst the oldest, if not the oldest, surviving drainage channels in the world. The valleys were formed prior to the start of rifting, as early as about 150 Ma in this part of the continent, by rivers with headwaters in Antarctica. This would put the formation of the valleys an unknown number of millions of years earlier than 150 Ma. They would have been the first rivers from Antarctica to be beheaded.

When the margin of the continent was uplifted following rifting, a new watershed formed, the Ravensthorpe Axis. The upwarp of this axis across the drainage lines resulted in rejuvenation to the south of the axis.

The palaeodrainage systems of the Bunda Plateau (the Nullarbor Plain) was partially obliterated by the deposits laid down by marine incursions of the Eocene and Miocene, but some, such as deep channels cut by the rivers running from the Musgrave Block, the Stuart Range and the Gawler Ranges, that were cut during the Cretaceous, can be detected by satellite. These large channels were flooded by the epicontinental sea during the Eocene, becoming filled in by the marine deposits of the Pidinga Formation from the sea that penetrated a long way up the rivers. The edges of the continent in the east and west are believed to have already bee upwarped by the Eocene, but evidence suggests that in the south, uplift had still to take place. With no drainage divide provided by uplift of the southern margin the sea was allowed full access to these ancient river channels.

In the Late Miocene, the limestone of seafloor off South Australia was raised to form the Nullarbor Plain (the Bunda Plateau). To the north of the Nullarbor, the desert palaeorivers are chains of salt lakes for most of the time, but when they do flow the water disappears at the edge of the Nullarbor, flowing into the underground water of karst desert in which there is no surface water, though there is a lot of subterranean water.

Block faulting occurred in South Australia to the east of the Nullarbor in the Tertiary, forming the internal drainage of the Lake Eyre Sub-basin, and the Mt Lofty Ranges and the Flinders Ranges, and also the formation of the St Vincent Gulf and the Spencer Gulf. There is disagreement about when the mountain ranges were uplifted. Some believe there may have been the equivalent of the Flinders Ranges in much earlier time that has left no trace.

It has been suggested that the Murray River turns sharply to the south at Morgan because the Flinders Ranges were uplifted, blocking its path to the west, though no evidence has been found that the Murray had once flowed west to the Great Australian Bight.

Prior to the subsidence of the Lake Eyre Sub-basin the rivers flowed to the south into the Bight. After the subsidence they flowed backwards into Lake Eyre, when they flow at all, which isn't very often.

Palaeodrainage of Southeastern Australia

The New Zealand Subcontinent formed the eastern margin of Australia during the Jurassic and the first half of the Cretaceous. The rivers of the Tasman Divide that formed its highlands, flowed into the Eromanga-Surat Basin, the approximate area covered by the Great Artesian Basin. The Victoria Divide, an east-west palaeodivide that separated the Eromanga Basin to the north from the Otway Basin, Torquay Basin and the Gippsland Basin to the south. During the Cretaceous, there was a high proportion of the volcanic material derived from the New Zealand subcontinent in the sediments accumulating in the basins.

The drainage from the Victoria Divide, approximately coincident with that of the present, flowed in the Eromanga Basin to the north and the Otway-Gippsland Basins to the south.

Sources & Further reading

  • Mary E. White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
  • J. J Veevers (ed.) Billion-year earth history of Australia and neighbours in Gondwanaland, GEMOC Press Sydney, 2000
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 21/10/2016

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading