Australia: The Land Where Time Began
Palaeodrainage - Ancient Rivers
A palaeodrainage is one that was cut by a stream or river but isn't now used by that river. To be classified as a palaeodrainage a watercourse needs to be old and be long enough to be thought of as a river or stream. An oxbow lake is an abandoned part of a watercourse, but because it is not as long as a stream it is not considered to be a palaeodrainage.
There are 7 major palaeodrainage systems that have been found in Western Australia:
Valleys that drained into the Indian Ocean via the Great Sandy Desert.
Some palaeodrainages have been found in the Great Sandy Desert, part of the Canning Basin. They have been named the Mandora palaeodrainage line, Wallal palaeodrainage line and Percival palaeodrainage line. They were found, in part, because of the vegetation associated with a particular soil type, and the presence of underflow, when a river continues to flow, but through the alluvium deposited by an earlier stage in its history, though surface water is no longer visible. Lake Percival and Lake Gregory, both salt lakes in a chain, are places where the evidence of the drainage system can be seen on the surface.
Valleys that are believed to have formed the only true internal drainage system in Western Australia, in the northeastern Gibson Desert and the southeastern Great Sandy Desert.
An elaborate system of abandoned river channels dating to the Eocene have been identified in the Yilgarn of Western Australia that were dismembered by the onset of arid conditions. Their courses can be recognised by a complex series of elongate, aligned salinas. Some of the southern salinas are arcuate in shape, possibly reflecting ring structures in the crust.
Valleys draining into Bonapart Gulf across the Northern Territory.
Valleys that drained to the east across the Northern territory to the Finke River, then on to Lake Eyre. The Central Groundwater Discharge Zone is now marked by a chain of playas that includes Lake Hopkins, Lake Neale and Lake Amadeus, that traces the route of the palaeodrainage system.
Where the Finke River crosses the James Range, the river occupies a sinuous course through the uplands, but at a higher elevation is an older, abandoned river channel that intertwines with the present river channel. It is uncertain what caused this channel to be abandoned. The 2 river channels are of similar type, the mystery is why the original channel would be abandoned for another just like the first.
In the southwest of Western Australia there are valleys that trend from south to north in which the palaeorivers flowed into the Indian Ocean, such as the Avon River, Swan River and Canning River, as well as the Beaufort Palaeochannel and the Darkan Palaeochannel that are believed to have entered the Perth Basin across the Darling Fault.
Valleys draining into the ancestral Great Australian Bight prior to the uplift to form the Nullarbor Plain of the present that isolated the Cowan Drainage Basin. The edge of the Nullarbor forms an abrupt end of the channels that once flowed into the Bight before the floor was uplifted. Playas such as Plumridge Lakes and Lake Boonderoo.
Since the Nullarbor emerged from the sea the climate of the area has been arid, with limited surface water. On the plain there are only a few palaeodrainages that are isolated and discontinuous.
The Murray and Murrumbidgee Rivers have nearby many channels that were occupied by these rivers in the past. The Leichardt River and Flinders River in north-west Queensland, both the meandering type of river, have former courses near these river channels.
Some rivers regarded as palaeodrainage channels are still flowing in courses that were formed in the distant past. The River Torrens, draining a large part of the central Mt Lofty Ranges, then flows through Adelaide to the sea in the Gulf St Vincent.
The ranges are defined by faults, that are still active, the source of the minor earthquakes and tremors, dislocation still occurring occasionally. Strata of marine origin were deposited in the Eocene in the downfaulted Gulf St Vincent, indicating that in the Late Cretaceous and early Tertiary the faults were active. This was the time when the Ranges were formed. In the Eocene, at least according to 1 interpretation, the Torrens River debouched from the upland, flowing into a lake in the fault-angle depression that existed between the back slope of the Para Block and the scarp of the Eden Fault. The sediment load of the river was deposited in the lake as its velocity decreased. Coarse boulders were deposited first, that are now near the present Athelstone, followed by cobbles, near Paradise, and further to the west, gravel was deposited near Beefacres. The fluvial deposit remnants, interfingering laterally with lake beds that are fossiliferous, are still visible. This dates the Torrens River back at least 60 million years to the Eocene.
On the crests of ridges and ranges of the Flinders Ranges, between valleys deeply eroded during the Late Cretaceous and Palaeocene, 100-60 Ma, there are remnants of a high summit plain in the southern section of the ranges.
See Australian Landscape - Cretaceous to Present
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|