Lake Tyrrell is the largest (208.6 km²) of many inland Quaternary salinas (salt lakes and pans) in the Murray Basin of northwestern Victoria. The Murray Basin is a shallow geological basin that covers about 300 000 km2, across the States of Victoria, South Australia and New South Wales.
Salt lakes are common in Australia, occurring in arid or semi-arid regions. What is extraordinary is that many of these lakes have been in existence for a very long time, resulting from past dramatic changes in climatic conditions.
Where did the salt come from?
Lake Tyrrell is a long way from the sea – so why is it so salty? Well, the salt is actually oceanic in origin! In past geological time large areas of continental Australia were subjected to marine inundation (flooding). Major depositional (literally sediments being deposited) phases are linked with successive cycles of marine transgression (higher sea level) and regression (lower sea level) by shallow epicontinental seas (inside the continent).
This was true of the Murray Basin in the late Miocene to Early Pliocene (Figure 3) where the sea transgressed from the present coast between the Fleurieu Peninsula and Mount Gambier in South Australia, forming a large gulf that extended into NSW (including Lake Tyrrell). The sea level was approximately 65m above present levels!
Following this transgression there was a slow regression, with the shoreline moving back to the southwest to its current position.
Lake Tyrrell is a groundwater-fed lake (discharge zone) with minor surface-water inflow from Tyrrell Creek (Figure 4). This means that the floor of the lake intercepts the regional water table.
Minerals of the lake
Lake Tyrrell is rich in sodium-chloride (Halite), however, as you wonder around the lake you would be forgiven for thinking it looked a lot like a mine tailings dam. This is because, in addition to Halite and Gypsum, there is also Alunite and Jarosite as well as other Iron-Oxides forming around the lake (reference).
The sediments at the surface of the lake are comprised of sands and clays which have been transported via aeolian (wind) processes (source).
Interestingly, many of these minerals, which are found in hyper-saline lakes all over Australia, are also found in environments on Mars (source). This is a cool example of how knowledge about an environment on Earth can help us understand the types of environments which may have existed on Mars or other planets in our Solar System!
Living things at the lake
With a pH less than 4, it might seem impossible that anything could live at the lake. There are, however, many microbial communities which have been identified, cycling iron and sulfur species (the reddish-brown organic film seen in Figure 7).
When crystalline salt is visible on the surface as at Lake Tyrrell, vegetation is restricted to higher land – in this case, the islands (see Figure 4). Salt bush and samphire dominate these thin strips of land which support numerous species of bird life.
Salt – Economically Important
Salt lakes are economically important, producing gypsum, anhydrite, salt (sodium-chloride) and potash (potassium, used for fertiliser). At lake Tyrrell, as in all areas of salt production, annual evaporation exceeds rainfall (340mm/yr). Each year, this process leaves a salt crust which is commercially extracted by Cheetham Salt Works at the northern end of the lake (see Figure 4). Watch the video below to learn how salt is processed.
Studies at the lake (referenced below) have concluded that there was Aboriginal occupation at Lake Tyrrell prior to 30,000 years ago. At this time the water level was higher and much less salty. Bone fragments, flaked stone artefacts, a quartz tool, charcoal and numerous other small artefacts have been found at this site.
To the Aboriginal people the constellations and movements of the night sky holds great significance, particularly to the Boorong people who once occupied the country around Lake Tyrrell. In fact, Tyrrell is a Boorong word that means ‘sky’.
Figure 9 shows an image of Bunya, who was chased up into a tree by the evil emu Tchingal. He waited so long in the tree he turned into a Possum! For more information about the stories of the Boorong people, head to this website.