Australia: The Land Where Time Began

A biography of the Australian continent 

Palaeochannels in the Channel Country

It has been found that in past times river meanders in the Channel Country were much longer than at the present, and along the channels of the Cooper Creek and Diamantina River there are are large deposits of alluvial sand that underlie the mud of the present, 2-3 m down. To carry so much sand for such long distances the rivers must have been flowing much more strongly than they do today.

These sand deposits have been shown to be of 2 ages. The oldest sand layer was deposited between 260 000 and 220 000 years ago, which puts it approximately at the time of the penultimate interglacial period. This indicates that the climate was much wetter than present. This was followed by a drier phase, when the rivers slowed greatly. Next there is another major pluvial period when sediments dated to about 110 000 years ago was deposited.

The present regime of river flow was begun at the end of this pluvial. The large meandering sand-filled channels were replaced by anastomosing, braided streams of much lower energy, transporting mud across the extensive floodplains. Dating of the mud deposits place them between 85 000 years ago and the present. This fits with the increasing aridity that reached a peak about 30 000-10 000 years ago during the last glacial.

The period between the last 2 interglacials is believed to have been mostly arid in Australia. An age of 274 000 +/- 22 000 years has been measured for an upper Diamantina dune. Most dunes would be moved about during each dry period, so not many older dunes have been found.

Gypcrete, that form in conditions of extreme aridity, has been used to date the beginnings of aridity in the Lake Eyre Basin. Major changes in the Murray-Darling system hydrologic regime have been instigated by climate changes in about the last 40 000 years, according to research in the inland plains of south-eastern Australia. The chronology being elucidated for the Channel Country shows that the last glacial and Holocene climate changes had little effect on the river systems. It seems that major changes in the  regime predates them by a long period of time. In the Lake Eyre Basin the present interglacial is less humid than the preceding interglacial.

Coolibah trees (Eucalyptus microtheca) line most of the channels of the Cooper floodplain. Away from the belts of active channels, few distinct sinuous channels and networks of small channels, commonly anastomosing, are found on the almost flat floodplains, that can be up to 10 km wide.

These floodplains have extremely low gradients and mounds resulting from gilgai formation in the soils may have influenced the formation of drainage networks. They have been affected by human activity. It is believed Aboriginal burning practices may have altered the vegetation and cattle grazing has definitely increased erosion.

Sources & Further reading

Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000

 

Rivers
Home
Journey Back Through Time
Geology
Biology
     Fauna
     Flora
Climate
Hydrology
Environment
Experience Australia
Aboriginal Australia
National Parks
Photo Galleries
Site Map
                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading