Australia: The Land Where Time Began

A biography of the Australian continent 

Crater Lakes - Victoria

These lakes have been regarded as a rain gauge for the Holocene. Volcanic activity in Victoria has occurred recently, geologically speaking. The result is a number of volcanic domes and some crater lakes (maar lakes) that are entirely rain-fed, which makes them natural rain gauges.

Of the 30 craters of this type found in Victoria most as centred near Colac and Camperdown, west of Geelong, and Tower Hill west of Warrnambool.

Lake Keilambete is a maar lake 4 km north of Terang, 190 km west of Melbourne. At present the water is a circular body 1.8 km in diameter and an average depth of 11 m. Impermeable mud lines the flat lake bed. The salinity is twice that of seawater, and there are wave-cut notches up the crater walls that show the depths reached by the lake at various times. The layers of the bottom sediment can reveal the history of the lake.

The sediment layers

At 440-400 cm is the basal layer, sediments accumulating prior to 15,000  years ago. Above this is the Basal Soil, a layer of soil, dated to 14,000 years ago, that formed during a dry period. This leached layer is overlain by an organic-rich layer deposited during a swampy phase that lasted until about 10,000 years ago. At this point in time it became a shallow lake that became salty as a result of evaporation.

From 400-375 cm is the Basal Saline Sands, 10,000-8,000 years ago, containing calcareous ooids -that form in shallow carbonate-saturated water. This indicates a shallow lake environment.

From 375-200 cm is the Lower Keilambete Mud, 8,000-4,500 years ago. This is a layer of fine-grained dark muds that are weakly calcareous, this is the main deep water facies. The lake became substantially deeper at 8,000 years ago. This deep water regime continued until the first evidence of shoaling appeared at 5,500 years ago, when the water level dropped. It may have overflowed at the time of its highest water levels between 7000-6000 years ago. The wet period was the wettest in the last 10 000 years.

From 200-0 cm, the Upper Keilambete Mud, 4500 years ago to the present. These are fine-grained muds layered with bands of paler calcareous strata and occasional sandy layers. Shallow conditions are indicated at 3100 years ago by the sandy layer.

After that episode the water level rose briefly, but fell again and remained relatively low, levels fluctuating until 2000 years ago. The last 2000 years have shown only minor fluctuations, until the time of European occupation, the largest reductions of water level occurred in the last 100 years.

Sources & Further reading

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