Lake Amadeus, Central Australia : modern processes and evolution

Abstract

Lake Amadeus, a large saline piaya 120 km long and up to 12 km wide, forms a major feature of arid Central Australia. Surrounded by stabilized dunefields, it has been selected here as the site for a range of historic and process oriented studies. The Amadeus piaya is today a groundwater controlled system of a type largely unrepresented by detailed studies elsewhere in Australia. Moreover, it lies in an area from which Late Cenozoic environmental data remain sparse. With its associated variety of stabilized gypsum and quartz dunes associated with saline sedimentary facies, it provides sensitive indicators of past arid depositional events. Analyses of the stratigraphic and chronologic record are supplemented by detailed studies of modern hydrologic, chemical and sedimentary processes. These provide the framework controls within which interpretations of the past record are reconstructed. Field work was carried out during winter and spring seasons in 1984, 1985 and 1986. Four 15m cores and more than 20 short cores up to 1.7m long were taken. Across the piaya and its marginal land, groundwater, shallow stratigraphy and sediments were studied by piezometers, trenches and auger holes. Evaporation was measured with a method of sediment blocks. Palaeomagnetism and thermoluminescence dating methods were used to establish the piaya chronology. Thin section, chemical, mineral and texture analyses help in evaluation of the sedimentary facies assemblage. The Cenozoic sediments are divided into two major units: Uluru Clay in lower part and the overlying Winmatti Beds. The Uluru Clay sequence, at least 60m thick, overlies Proterozoic dolomitic limestone. Of uniform lithology, it consists of clay horizons with minor intercalated gypsum. The Clay was deposited in a shallow lacustrine and fluvial enviroument with periodical saline and frequently dry conditions. The basal Uluru Clay is estimated to be over 5 Ma old. The transition from Tertiary to Quaternary, coincident with the Gauss/Matuyama palaeomagnetic boundary, occrred within the uniform Uluru Clay sequence. The Winmatti Beds comprise the top several metres of basin sediments. The beginning of Winmatti Beds coincides probably with Jaramillo subchrone (0.91 Ma). The appearance of gypsum-clay laminae, thick gypsum sands and aeolian quartz, characteristic of the Winmatti Beds, marks the onset of a new sedimentary and climatic environment. In this the dominance of saline groundwater marks the first development of a groundwater discharge playa system. The association with aeolian deposits signals the dominance for the first time of major aridity. On the landward margin, two rings of gypseous dunes and associated quartz dunes represent facies equivalent of arid units in the playa. The older gypseous dune possibly formed soon after the Uluru Clay. The younger gypseous dune is correlated with a gypseous clayey sand layer within the Winmatti Beds. The gypseous dunes were deposited by deflation of near-shore gypsum accumulating in the groundwater seepage zone during a period of high watertable. The hydrologic and climatic history since the younger gypseous dune formation is correlated broadly with events identified in Southern Australia. The younger gypseous dune formed around 45 to 60 Ka B.P. (TL dates), when a high regional watertable was associated with a wetter climate. A period of regional dune activation followed the younger gypseous dune formation resulting in an aeolian sand deposit in the playa and the thick quartz sand mantle on the gypseous dunes. This represents a drier and windier period which may correlate with the low water level period of 25 to 16 Ka in Southern Australia. The deposition of shallow water gypsum layer, which comprise marginal terraces and low terrace islands, represents a relatively high water level period. This may correlate with the relatively high water levels of Holocence time in Southern Australia. The chronology and stratigraphy predating the younger gypseous dune remain unclear. They are complicated by major breaks in the depositional record. Groundwater bevelling, deflation and soil formation help explain the hiatuses and low rates of deposition. A new surface feature is identified which has both morphologic and stratigraphic expression. Termed GYPSUM GROUND it comprises a large area of the playa surface. A brown undulating salt encrusted surface developed over a nearly pure layer of sand-sized gypsum lies some 40 cm above the local watertable and above the level of periodic annual flooding. Thin section and detailed sedimentologic studies establish this as a degradational remnant of a previously more extensive gypsum sand associated with a high watertable environment equivelant to the deposition of gypsum marginal terraces and low terrace islands. The gypsum ground, now largely independent of groundwater evaporative processes, is one of three morphologic and sedimentary units recognized as characterising the modern playa surface. The other two at lower surface altitude, salt flat and sulphide lowland, are controlled by a combination of groundwater and surface interactive processes. Evaporation pattern for the playa surface are divided into two types. One represents a very low rate from the encrusted surface (El phase); the other is a much higher evaporation phase after the crust is dissolved by rain (E2 phase). Evaporation of the El phase is estimated to be of the order 70mm/y. Since all rain water is not totally evaporated during the E2 phase, this value (El) can only be used as an upper limit for net evaporation, the difference between the total evaporation and the rain water on the surface. The quantity of rainfall not evaporated during E2 phase (therefore a recharge component) seems to be significant compared to the annual El evaporation. Therefore, the net evaporation and discharge rate may be very low, consistent with a very low salt concentration rate in the system. This evaporative regime provides new insights into the question of evaporite formation in a context where the absence of salts seems anomalous when considered in the light of present processes. The playa lacks any substantial salt deposits (other than gypsum and glauberite), either on the surface or in the sediment column, eventhough it has been experienced saline conditions. The surficial salt crusts are commonly 1 cm thick and never exceed 5 cm although the watertable remains in the capillary fringe and the groundwater is highly saline (250g/l). The total quantity of dissolved salts in the groundwater pool are less than expected from present processes considering the long existence of the saline phase. The thin salt crusts on the surface today are ephemeral being subject to periodic dissolution and reformation. Crusts cannot develop to a significant thickness because of combined low net evaporation, leaching by.rainfall, possible downward ionic diffusion associated with groundwater body unsaturated with respect to sodium chloride. Groundwater salinity has probably never exceeded chloride saturation due to a combination of processes including slow chemical concentration rates, processes of groundwater body expansion, past salt loss through deflation and salt leakage by deep groundwater outflow during early Quaternary or even Tertiary time. The priciple of simplistic uniformitarian interpretations is once more called into question by these studies. In its Quaternary hydrologic history, the groundwater discharge playa, today delicately balanced between discharge and recharge regime, has sometimes existed as a prolonged groundwater recharge zone as evidenced by fossil soils with vegetative biotubule remnants. Thus the present hydrologic processes are not representative of past regimes emphasizing the dangers of using modern processes as analogues for past regimes. The data confirm that Lake Amadeus has rarely operated as a true surface water lake since Tertiary time. The groundwater processes and history demonstrated here provide a new basis for understanding playa systems both here and in comparable arid to semi-arid regions of low relief elsewhere in the world.