Australian School of Petroleum The University of Adelaide Australia
LEBARG
Lake Eyre Basin Analogues Research Group

Lake Eyre

With an area of 1,140,000 sqkm, the Lake Eyre Basin is one of the largest areas of internal drainage in the world, and one sixth of the continental landmass of Australia.The Lake Eyre Basin is a vast, intracratonic, internally-draining, low accommodation dryland fluvial-lacustrine basin. The modern basin is a wide shallow Cainozoic structure superimposed upon the even more extensive magnesium-rich pre-existing megalake of the Mesozoic Eromanga Basin and early Tertiary Birdsville Basin which occupied much of central Australia (Callen et al., 1986). The origin of the lake is predominantly due to a combination of subsidence by basement lineaments and lake sedimentation and aeolian ablation.

Location of the Lake Eyre Basin, highlighting its major depositional elements.
False-colour satellite image of Lake Eyre, with water (dark blue) flowing in from the Warburton and Kalaweerina Creeks (24/04/09; courtesy of MODIS Rapid Response Project at NASA/GSFC).

Rainfall within the basin ranges from 500-400 mm/y in the headwaters to 120 mm/y around Lake Eyre with rivers showing extreme flow variability. Major depositional elements in the Lake Eyre Basin are ephemeral streams, floodouts, dune fields and playas. Between major rivers, Cretaceous sandstones and with local Tertiary strata form uplands capped by sheets of wind-abraded gravel (gibber plains) and thick duricrusts.

Within the south western corner of the basin lies Lake Eyre, which is the fifth largest terminal lake in the world and the largest salina-playa in Australia. It is located some 1000 km north of the city of Adelaide, South Australia. The lake lies asymmetrically in the southwestern corner of the closed inland drainage basin in the heart of the Australian continent. The lake contains the lowest land surface on the Australian continent (Bonython, 1989), with an elevation of -15.2 m below sea level in Belt Bay (Bye and Will, 1989).

Lake Eyre comprises two lakes, Lake Eyre North (144 km long and 77 km wide) and Lake Eyre South (64 km long and 24 km in width), connected by the narrow Goyder Channel. Lake Eyre intermittently becomes a true playa lake only when flooded but is usually a dry salina with an evaporite crust covering the southern low-lying areas.

Flood waters enter Lake Eyre from the several river systems that form the catchment. Rivers flow into the lake from the northern, eastern, southern and western margins. The eastern rivers (Diamantina-Warburton-Kalaweerina system and Cooper Creek system) account for most of the inflow into Lake Eyre (three quarters of the run-off) with floodwaters finding their way via an intricate network of channels known as the Channel Country. These rivers flow through the Tirari, Strzelecki and Sturt Stony deserts to the east of Lake Eyre only after significant flood episodes in their upper catchments, although most is lost through evaporation, infiltration or absorption. Very large flows in Cooper Creek and the Diamantina-Warburton systems are generally responsible for the great fillings of the lake.

During the most common minor floodings, and even during the early stages of rare great fill floodings whilst lake level is rising, the terminus of each of these terminal splay complexes lies above the level of lake fill and is neither inundated by nor abutting the lake waters. The cessation of sedimentary deposition at the terminus of each of these terminal splays therefore implies that mechanisms other than the simple interaction of fluvial waters with standing lake waters are involved. A variety of terminal splay complexes can be found along the shoreline of Lake Eyre North. Three of these, the Neales, Umbum and Douglas Terminal Splay Complexes (TSC), were studied for detailed facies distribution, element architecture and reservoir geometry during Phase I and II of the Lake Eyre Consortium.


The Lake Eyre Terminal Splay Complexes


Aerial views of the Neales, Umbum and Douglas Terminal Splay Complexes

The modern Neales terminus covers an area of approximately 25 sqkm and consists of a fluvial-dominated, high constructive triangular lobate terminal splay complex. Three active rectilinear avulsion distributary channels (ADCs) dominate the system. The distributary complexes are generally highly constructive lobate terminal splays, with a broad middle-ground bar complex incised by deeper distributary channels.

The Umbum Terminal Splay Complex (TSC)is a radial distributive system covering about 15 sqkm. It comprises five major erosive ADCs that deposit locally reworked and flood-borne sediment as splays at the end of each channel. Beyond the terminal splay and across the playa floor, suspended fine silts and parallel laminated muds are deposited by a series of braided channels. Following flood events, suspended sediment within ponded water settles out of suspension, forming mud lenses at the base of channels, which are later desiccated.

The approximately 4 sqkm Douglas TSC is characterized by two ADCs which terminate basinwards, causing propagation through sheetfloods. With increasing radial distance from the source, the splay shows a decrease in grain size, lithofacies thickness, syn-depositional incisional surfaces and primary sedimentary structures. The exposed surface of the splay is etched by a network of minor distributary channels that are often draped by a thin layer of clay.

 

Interested parties should contact Kathryn Amos ().