Australia: The Land Where Time Began
Lake George See
the Big Dry,
A lake on the Southern Tablelands, about 100 km inland. It is a fluctuating closed lake that is situated between the south-east mountain catchments and the arid inland. It has a catchment area of about 932 km2. Lake George occupies about 16 % of the area of the catchment basin.
The lake is in a large internal drainage basin, about 9302 km in area, a half graben that was formed tectonically and is bounded in the west by the Lake George Fault, that is north-south trending, which forms a sharp escarpment that reaches about 250 m above the floor of the lake, the floor having an overall altitude of 673 m. The water level of the lake varies over a wide range, falling rapidly during major droughts, being completely dry in recent times (Burton & Wilson, 1973; Jacobson & Shuett, 1979). Studies have been carried out on ancient shorelines that were about 37 m above the present floor of the lake (Coventry, 1976). If the water reached heights above 37 m the lake would have overflowed through Geary's Gap that is 500 m wide. Reviews have been carried out in recent time of the fluctuations of the water level in the lake over the last 160 years (Burton & Wilson, 1973; Jacobson & Shuett, 1979). The geomorphology of the surficial sediments and soils of the Lake George Basin (Coventry & Walker, 1977), in which they paid particular attention to shore lines that had been abandoned. The fire and vegetation history in the area around Lake George, as recorded in core LG2 (8.6 m) for the last 350,000 years have been studied (Singh et al., 1981a). The late Cainozoic history has also been described from B.M.R. Scout Hole No. 4 (72 m) and Core LG4 (36 m) from the northwestern corner of the lake (Singh et al., 1981b). Singh et al. (1981b) had taken the upper 2.94 m of the LG4 core for their palaeomagnetic study and it is this same length of the core that was the basis for the study by the authors6 of the fluctuations of the level of the lake over approximately the last 60,000 years and comparisons with the results of palynological studies and the analyses of facies (Singh et al., 1981a, 1981b), and shorelines that had been abandoned (Coventry, 1976; Coventry & Walker, 1977).
See Source 6
Ages determined from cores
A core drilled into the sediments of Lake George found pollen and charcoal that were dated to more than 100,000 years ago. In the top 8.6 m, covering 350,000 years - are the longest continuous record of fire and vegetation history in Australia. The base of the core, which is 72 m long, is estimated to be between 4.2 and 7 million years ago. It is estimated to extend down to about 134 m. This would put the origin of Lake George at about 20 Ma. The sediments show alternating layers that indicate 8 times when the lake was full and 8 when it was dry, indicating 8 glacial and 8 interglacial periods in the past 750,000 years.
See Lake George Project
The groundwater below the catchment is mainly low salinity, but under the actual lake bed, it is of higher salinity, and is confined by a layer of clay. As the lake dries by evaporation the salt it carries is concentrated in the remaining water. There are complicated interactions between the lake water and the pressurised high saline aquifer beneath the lake bed. The result is that salt has been stored in the aquifer at 10-12 m below the sediment surface, at a level of hydrostatic balance. This has been occurring over the last 1-2 million years.
Faulting formed the lake as a normal fault-angle basin in or before the Mid Miocene. The sediment deposits in the lake show the history of the area since it formed. Below the lake bed there is a series of river and lake sediments that reaches 150 m thick. 3 sediment layers have been identified.
The Gearys Gap Formation is the oldest, made up of deeply weathered sand and gravels. Palaeozoic bedrock underlies these Early Tertiary river sediments. The north-west flowing rivers were incised into the bedrock, so are associated with the previous drainage system. The palaeochannels can be traced below the lake bed. Between this sediment layer and the following 2 layers there is a break.
The next layer is comprised of river sands and lake clays, the Ondyong Point Formation. There is a laterally persistent layer of sand and silt that separated the upper 2 layers. There is no break in sedimentation.
Lake clays and silts of the Bungendore Formation make up the uppermost layer. The sediments from the top 2 layers represent deposition in a closed drainage basin. The differing sediment types result from climate changes.
The low watershed of the Great Divide in the east and the Lake George Range in the west form the margins of the basin. The northern and southern ends of the lake have subdued topography with low saddles and a complex of natural and artificial drainage lines. There are also swampy and lagoonal areas.
In the upper reaches of the streams in the basin occupy wide, open valleys. These streams converge towards the lake, from the north, Collector Creek, from the east, Alliianoyonyiga Creek and Butaroo Creek and Taylor Creek, from the south, Turallo Creek. The extreme variations of the lake during the Quaternary are evidenced by the ancient strand lines up to 37 m above the lake bed. Past this level the lake overflowed via Gearys Gap into the Yass River drainage system. This occurred between 27,000 and 21,000 years ago.
The faults that formed Lake George cut across pre-existing drainage lines. Prior to faulting, Taylors Creek used to flow across the area from east to west. It has been dammed to form a lake in the fault angle depression. In the process of creating the basin the fault disrupted the drainage system of the area, beheading headwater streams of the Yass. A number of creeks in the north that used to flow north-west were diverted by the fault, after which they flowed along the fault scarp base into Lake George. The fault crosses the Molongolo River to the south. This river has a much bigger catchment than the northern rivers, so was capable of continuing in is course across the rising fault block, cutting through the rock to form the Molonglo Gorge.
At the time of the arrival of Europeans in the area around Lake George it was mostly eucalypt woodland. The sediment contains pollen that shows that fire increased from about 130,000 BP, associated with an increase in eucalypts in the surrounding area. Prior to this eucalypt proliferation the vegetation was less sclerophyllous, with vine thickets and casuarinas.
The present landscape around the lake, with increased erosion, with widespread gullies, is the result of European clearing and agriculture and grazing. The land has been degraded even further by sand mining of the strand lines and aeolian deposits on the southern and northern margins of the lake. Land degradation has been increased greatly by roadworks in the area.
Lake George has been subject to cyclical wet and dry times. Salt is carried to the lake from the catchment in wet times. In dry times the lake acts as a playa, with capillary zone evaporation. It has been called a natural analogue of a waste water disposal basin.
The pollen record
Pollen cores contained pollen indicating the presence of cool temperate rainforests, especially Nothofagus, in wetter times during interglacial phases (Colhoun & Van der Geer, 1988). During the glacials the forests were replaced by open sclerophyll woodland and treeless herb fields and grasslands (Singh & Geissler, 1985). The woodlands were replaced by treeless vegetation during the coldest parts of each glacial. This process reached a peak in the second last glacial, the area being covered by almost unbroken grassland. In the LGM the vegetation became more diverse, having some grasses but also a variety of shrubs. Casuarina originally dominated the sclerophyll forests, by the start of the last glacial they were replaced by eucalypts (Johnson, 2006).
Water levels in Lake George between 1819-1990 Approximate depth in metres, estimated from a graph on p. 111 of Mary White's book, Running Down.
According to the authors1 the results of their study of the ostracods lead to conclusions that were similar to those of Coventry (1976) and Coventry & Walker, (1977) from dated stranded beach lines around the lake and those from studies of facies changes and plant microfossils (Singh et al., 1981b). Broad water quality changes, fresh to saline, can be indicated by ostracods, as well as the type of water body, ephemeral of permanent. High species diversity of ostracods is associated with times of water level of the lake is high, as there is an increased variety of niches that are available for ostracods. At times of low water level there is reworking of sediments and of ostracods, with sorting and breaking of ostracod shells. Ostracod valves are not present in some samples, and this is a problem that hadn't been solved by the time the authors1 wrote this paper. The questions the authors asked were, were they absent from some samples because the lake was dry or because they had been present in the area of the samples when there was water present then been dissolved by pedogenic processes some time later.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|