Australia: The Land Where Time Began
Lake Buchanan - An Atypical Salt Lake
At 23 x 7 km, this is a large lake. It has a surface area of 117 km2 and a catchment of 3000 km2, along the 450 m high north-south trending ridges to the east and west. It is almost 300 m above sea level. It is situated in a depression among the hills of the Great Divide in central Queensland. The annual rainfall of 60 mm, falls mostly in the summer.
As a result of the mainly east to west wind in the area of the lake, a series of ridges have built up along its western margin. These ridges represent high lake stands. Some of the depressions between the ridges occasionally retain water for a while. The largest of these inter-ridge lakes is Lake Constant.
The normal condition of Lake Buchanan is mostly dry, with a pool 4 x 2 km near the south-eastern end, the lowest point in the depression. The bed of the lake has a slight slope, rising to the north-western end. Every few years the lake fills, but occasionally the entire lake dries up. When the lake driest up the area of the pool in the southeast has a thin coating of halite.
The salt-tolerant water-plant Ruppia grows when there is water present and large numbers of ostracods and other crustaceans are present in the zooplankton.
The brines of this lake differ chemically from that of other, more typical, Australian playas. The water of typical Australian playas have the concentrations of their dissolved minerals in proportions similar to sea water, but in this lake the sulphate content is lower than sea water, and gypsum is absent. The calcium-magnesium is different and there are very high levels of strontium and fluoride. The different concentrations of minerals in the water result from the different source rocks. In the typical Australian playa the runoff contains dissolved minerals derived from different source rocks, in this case mostly marine sediments, from the inward drainages, with the addition of concentrations of salt aerosols.
The lake has remained at its high location in the ranges throughout the Tertiary Period because it is situated at a place where the palaeodivide, the Nebine Arc, coincides at this point with the current Great Divide.
The lack of chemical evolution over such a long time span, throughout the Tertiary, is explained by the altitude of the lake. Unlike the playas of the central arid regions, its source rocks remained constant, and the minerals necessary for the formation of gypsum were not available. It also explains the slow rate of sedimentation compared with the drainage systems of the interior, where the infrequent floods bring very large amounts of eroded material from a very large area, whereas the only available source for the deposit of sediments derived from the hard rocks of the divide.
Research has shown that there have been 4 major wet phases over the last 730,000 years, during which the lake was considerably deeper than 2 m. The first 2 were separated by a short dry phase. The resulting sediments display layers of alternating wet and dry phases. The record of these sediments is incomplete because at times when the lake dries up the surface of the lake bed blows away (are deflated).
A delta has formed in the north of the lake where streams enter, and there are beach ridges on along the shore. The change to arid-type deposition at about 2.4 million years ago coincides with the dry phase of Lake Buchanan. The same dry phase has been detected in sediments of this age in many places throughout the continent.
Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
|Author: M. H. Monroe Email: firstname.lastname@example.org Sources & Further reading|