Stratigraphy, sedimentology, chronology and palaeohydrology of Quaternary lacustrine deposits at Madigan Gulf, Lake Eyre, south Australia

摘要

Madigan Gulf is a large bay at the southern end of Lake Eyre North, a major ephemerally flooded playa in arid central Australia at the southwestern margin of a vast (1,300,000 km2) internal drainage basin. The stratigraphy and chronology of the Quaternary sequence in the gulf is described from 8 cores and a cliff exposure at the gulf margin. A number of depositional environments are recognised and their distinctive sedimentary components are described. Facies recognised include deep- and shallow-water lacustrine environments, dominated by surface-water processes, and dry or ephemerally flooded playa environments dominated by groundwater-controlled processes. Sedimentary components include terrigenous clastics from river inflow and shoreline erosion, carbonates of detrital, inorganic or biological origin and gypsum and halite evaporites. Carbonates and gypsum evaporites, precipitated within the basin, are frequently reworked as clastic components. The establishment of a preliminary chronology for the sequence, by the application of thermoluminescence, uranium/thorium disequilibrium, amino acid racemization and radiocarbon dating techniques, has allowed a reconstruction of the last 130 ka of Lake Eyre palaeohydrology. The wettest phase occurred during the last interglacial (early in oxygen isotope stage 5) when an enlarged Lake Eyre was up to 25 m deep. Subsequently there has been a number of dry periods separating successively less effective wet phases culminating in the deposition of a substantial halite salt crust around the time of the glacial maximum. The dry interludes are characterised by deflation of salts and sediment from the basin, a process controlled by lowering of the watertable. The record from Madigan Gulf demonstrates the dramatic and repetitive impact of lake deflation on the Quaternary record of Lake Eyre. In the early Holocene a minor, but mostly perennial, lacustrine event was terminated at about 3–4 ka when the modern ephemeral playa regime was established. The major catchment of Lake Eyre is located in the monsoon-watered areas of northern Australia. As demonstrated by large floodings of the modern ephemeral regime, major lacustrine episodes must indicate enhanced monsoon precipitation in northern Australia. In the Holocene the lake has not risen to levels achieved during the early stage 5 lacustral phase, indicating a marked reduction in the effectiveness of the monsoon in the present interglacial by comparison with its predecessor.