Australia: The Land Where Time Began

A biography of the Australian continent 

Lake Bungunnia

The Pinnaroo Block, in the south-western part of the Murray Basin, was uplifted during the Pliocene. About 3.2 Ma the uplifting block began to dam the Murray, and other ancestral rivers of the system, the Darling, the Lachlan and the Murrumbidgee. It is believed that at that time rivers such as the Murray River carried at least double the amount of water carried by the same rivers at the present. The lake that formed inundated an area of 68000 km² at its peak, covering the Mallee region. It extended along the Murray Valley, covering large areas of south-eastern South Australia, south-western New South Wales and north-western Victoria. In the Late Miocene-Early Pliocene there was a marine intrusion in the Murray Basin. The sediment deposited formed the Parilla Sands formation. This deeply weathered and laterised Karoonda Surface was now overlain by the deposits resulting from Lake Bungunnia.

Alluvial plains extended to the north and east of the lake. To the south there was a shallow marine shelf on the continental edge between the Pinnaroo Block and the present-day coastline.

The lake sediments have been dated by magnetic pole reversals that occurred during the time of the deposit formation. These deposits, the Blanchetown Clay Formation, formed from 3.2 Ma to 700 000 years ago. The fluctuating water level of the lake, as recorded by the deposits, indicate the climatic conditions over the period of their formation.

• From 3.2 - 3 Ma there were 4 low water events.
• Between 3.0 and 2.8 Ma, the water was moderately deep - a humid climate.
• 2.8-2.6 Ma there was a relatively dry phase, with sand deposits and a wetter fluctuation at 2.53 Ma
• 2.5-2.4 Ma the lake dried up completely. This coincided with the formation of the northern ice cap, with dry conditions over much of the globe.
• 2.4-1.9 Ma the lake was filling.
• 1.9-1.7 Ma there were minor fluctuations

From then on it remained huge until 700 000 years ago when it began draining. A breach in the raised southern margin started the drainage. The draining of Lake Bungunnia resulted in the formation of the southern end of the Murray River. Lakes Menindee and Tandou, as well as other minor lakes along the Darling and Murray Rivers. Lake Tyrell is the largest of the smaller basins the Lake left as it fragmented.

see Salt

Sources & Further reading

1. Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
2. Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994