Australia: The Land Where Time Began

A biography of the Australian continent 

This is a long valley connecting Lake Hopkins on the Western Australia side of the Northern Territory border, stretching for about 500 km to the east, ending in  the dunefields along the Finke River at the edge of the Simpson Desert. It is bounded on the north by the George Gill Ranges and the Cleland Hills, and on the south by the Petermann Ranges and Musgrave Ranges. It includes Lake Hopkins, Lake Amadeus and Lake Neale as well as a number of smaller playas. It is what remains of a palaeodrainage system in the Amadeus Basin that formed in wetter times by rivers flowing into the Finke River, and eventually to Lake Eyre. Since the most recent bout of aridity the river system was dismembered by blowing sand that resulted in the Finke disappearing into the sand of the Simpson Desert, the flood outs ending about 200 km from the lake.

There is no surface drainage systems throughout the length of the valley, all playas and associated landforms and chemical sediments result from transmission of groundwater and a large regional flow system that discharges the water. There are large deposits of silicified groundwater silcrete. The mineral contents of the groundwater is concentrated by evaporation of the water, deposits of gypsum and glauberite being found in the playas. On islands and along the margins of the playas there are gypcrete crusts resulting from induration of gypsum dunes.

No surface water reaches the playa chain in the valley, the groundwater being recharged by water collected by the catchment that covers 90,000 sq km where a number of intermittent streams originating on the ranges to the north and the south, flooding into dunefields from where the water infiltrates to the watertable. The groundwater system is composed of 2 layers. There are fractured bedrock aquifers that are overlain by sediments from the Cainozoic up to 100 m thick in the basin centre that are mostly of aeolian and alluvial origin. They are interbedded with calcrete, gypsite and gypcrete that have been precipitated chemically from the groundwater, the main aquifers being calcrete and alluvial sand units. The water is from 1300 BP to the present. At Ayers rock (Uluru) the rocks at the base of the Cainozoic sediments are 60 Ma, from the Middle Palaeocene. It is believed the original surface of the basin was probably Late Cretaceous.

Gosses Bluff meteorite crater in the northern part of the Amadeus Basin is of Cretaceous age. In northwest Queensland, the Flinders Ranges, the Gawler Ranges and the Macdonnell Ranges, land surfaces of a similar age have been found. Uluru (Ayer's Rock) and Kata Tjuta (the Olgas) are bedrock protruding from the Cainozoic sediments that have been eroded from around them. The runoff of protruding rocks like Uluru and Kata Tjuta recharges the groundwater. At Uluru the result is a vegetation zone that differs from that of the surrounding plains. Between Uluru and  Kata Tjuta there is a very ancient drainage system.

Near Curtain Springs, a chain of small playas receive some of their water from eastward flow down the valley, the remainder coming from the bounding ranges. Areas of brown heaved gypsum form  a 'ploughed field" topography (White, 2000) where groundwater discharges and evaporates on the playa beds. In areas that sometimes flood, and where the groundwater reaches the surface, there are flat, white salt-encrusted areas. There are small sand dune islands on playa beds ringed by gypcrete. Surrounding the playas are 30 m-high quartz sand dunes that are vegetated, which stabilises them.

The climate change history of the area is dated by layers of gypsum and calcite present in the sediments of the playas.

• 75,000  - 35,000 BP    A groundwater regime similar to that of the present with low velocity groundwater regime, with a large amount of calcrete precipitation and greater hydraulic head.
• 27,000 - 22,000 BP    Episodic high rainfall intensity is indicated by the nature of the calcrete. Near the end of this period it is believed humans arrived in the area. Evidence of human occupation has been dated to 23,000 BP at Lake Woods, in the Northern Territory, coinciding with a lake-full episode. 2 older dates on calcrete stringers indicate earlier periods of calcretisation in the system.
• 16,00 - 8,000 BP    Increased aridity indicated by gypsum deposits.

Sources & Further reading

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

Lakes