Lake Frome South Australia, like Lake Eyre, is another example of an ephemeral stream floodplain – dune field – ephemeral saline lake complex in the major outflow zone of the Great Artesian Basin. Lake Frome is approximately 100 km long and 45 km wide, with an area of 2700 km2 and lies to the south of Lake Eyre and the east of the Flinders Ranges. Sediments come mainly from the west via a series of short, relatively steep gradient streams, originating in the Flinders Ranges some 40 to 50 km away. Overflow waters from Lake Eyre sometimes spill into the north of the lake via Warrawoocara Channel, which also connects lakes Gregory, Blanche and Callabonna. This spillover prevents Lake Eyre water from today rising to a height of more than 12 metres above the Australian height datum (AHD).
The Adnyamathanha people of the Flinders Ranges tell the story of Akurra, the Rainbow Serpent, who travelled to Mudna (Lake Frome) and drank it dry. Today, the Adnyamathanha still hunt on the lake’s shores but never venture onto its surface as it is too dangerous because the rainbow serpent still resides beneath the salt crust. The Aboriginal name for the lake – Mudna — means trap in two ways. On the one hand, it describes the possibility of being caught in the muddy ooze below the salt crust, and secondly, if one studies the shape of the lake from directly above is the same as the shape of the trap-net known as munda, with its draw-string to close the mouth of the trap, represented by the narrow channel at the northern extremity of the lake, .
The ancient Lake Mega-Frome (Lakes Frome, Blanche, Callabonna and Gregory combined) was last connected to Lake Eyre 50-47 thousand years ago. The region in which it is situated has little rainfall and is very sparsely settled, with the closest settlement to it being Arkaroola Village on the edge of the Flinders Ranges, some 40 kilometres (25 mi) north-west of its nearest shore.
The lake contains many islands mainly in the south, where the lake floor is at its lowest, some 2 m below mean sea level (Figure). These islands are the erosional remnants of earlier now mostly deflated Pleistocene sediments and are composed of gypsiferous quartz sands atop a clay surface. Around the edges of the lake, there are quartzose spits and bars, beach deposits, and delta fans, as well as Pleistocene transverse and longitudinal desert dunes (Draper and Jensen, 1976). As in Lake Eyre, sheet delta fans define the entry points of the larger ephemeral streams and are built by the coalescence of flat lenses of ephemeral stream sediment and dominated by low angle layering. Stream channels are shallow, straight to gently curved features with few point bars. Oxidised organic-rich muds define the lowest parts of the channels. Fine-to-coarse sands with minor gravels form the bulk of the sheet delta sediment. Interchannel areas are composed mostly of reworked eolian sands. Marginward evaporitic flats with widespread capillary gypsum form large areas peripheral to, but sometimes, atop the sheet delta sand prisms.
Drainage into and between lakes in the Lake Eyre- Frome Basin of South Australia.
Like Lake Eyre, the lake also displays large active spring mounds along its eastern side. Mound sediments are made up of varying proportions of detrital sediment and authigenic aragonite, calcite, and dolomite cement forming a microbial tufa bindstone. Active mounds can contain standing ponds of water with flourishing algal blooms. An apron of gypsite fringes the active mounds. As in Lake Eyre, the mounds indicate fault-focused outflow fed from aquifers of the Great Artesian Basin and line up parallel to the major north-south normal fault system that characterises the area.
The inset shows the salinity evolution of a brine sheet from April 1989 until complete desiccation 11 months later in March 1990. The rate of salinity change in the brine sheet was climatically controlled. In April 1989 it was a sheet of mesohaline water (<50‰) covering the previously dry lake floor. Much of the salinity in the water sheet soon after flooding came from dissolution of the salt crust that had previously occupied the central depression. The brine sheet remained for six months through the cool months of winter and spring, with little change in its mesohaline salinity. Then, with passage into the hot summer of the Australian interior, brine sheet salinity rose rapidly. In less than a month it moved from its previous salinity plateau into gypsum saturation and then into the halite saturation field before desiccation three months later. By end March-early April 1990 (almost a year after the flood), the lake floor had dried out, and its 30 cm-thick salt crust had reformed. This style of salinity increase, with a long plateau of mesohaline waters, is vital to life in the lake as it allows sufficient time for a halotolerant macro and microfauna to breed and survive into adulthood. Plateaus in salinity evolution can control the intensity of the “feast or famine” response in many saline water bodies worldwide (seemWarren, 2016; Chapter 9, ).
The gypsum-entraining sediments of Lake Frome, like those of Lake Eyre, preserve a high-resolution record of climatic fluctuations over the last 100,000 years (Cohen et al., 2011). Over the past 20,000 years, the climate in the drainage basin has become increasingly arid. The hydrologic feature known as a “mega-lake” stage indicates a former lake water level that was sufficiently high to merge Lakes Frome, Blanche, Callabonna and Gregory. This Lake Mega-Frome was connected for the last time to adjacent Lake Mega-lake Eyre in the high-water period 50-47 ka; this defined the largest remaining interconnected system of palaeolakes on the Australian continent at that time. The final disconnection and a progressive drop in the level of Lake Mega-Frome indicates a major climate shift to aridification (gypsum beds) that coincided with the arrival of humans and the final demise of the megafauna in the Lake Eyre Basin.
The elevated supply of moisture to the Australian continent at various times across the Quaternary has been inferred to be to a response to an enhanced monsoon. The Lake Frome data provides reliable quantitative evidence for periods of enhanced tropical and enhanced Southern Ocean sources of water that supplied and filled these lakes at different times during the last full glacial cycle.
Gypsiferous surface of lake Frome (image courtesy of CSIRO)