STRATIGRAPHY, SEDIMENTOLOGY, CHRONOLOGY AND PALEOHYDROLOGY OF QUATERNARY LACUSTRINE DEPOSITS AT MADIGAN GULF, LAKE EYRE, SOUTH AUSTRALIA

Madigan Gulf is a large bay at the southern end of Lake Lyre North, a major ephemerally flooded playa in arid central Australia at the south western margin of a vast (1,300,000 km(2)) internal drainage basin. Th e stratigraphy and chronology of the Quaternary sequence in the gulf i s described from 8 cores and a cliff exposure at the gulf margin. A nu mber 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 dominate d by groundwater-controlled processes. Sedimentary components include terrigenous clastics from river inflow and shoreline erosion, carbonat es of detrital, inorganic or biological origin and gypsum and halite e vaporites. Carbonates and gypsum evaporites, precipitated within the b asin, are frequently reworked as elastic components. The establishment of a preliminary chronology for the sequence, by the application of t hermoluminescence, uranium/thorium disequilibrium, amino acid racemiza tion and radiocarbon dating techniques, has allowed a reconstruction o f the last 130 ka of Lake Eyre palaeohydrology. The wettest phase occu rred during the last interglacial (early in oxygen isotope stage 5) wh en an enlarged Lake Lyre was up to 25 m deep. Subsequently there has b een a number of dry periods separating successively less effective wet phases culminating in the deposition of a substantial halite salt cru st around the time of the glacial maximum. The dry interludes are char acterised by deflation of salts and sediment from the basin, a process controlled by lowering of the watertable. The record from Madigan Gul f demonstrates the dramatic and repetitive impact of lake deflation on the Quaternary record of Lake Eyre. In the early Holocene a minor, bu t mostly perennial, lacustrine event was terminated at about 3-4 ka wh en the modern ephemeral playa regime was established. The major catchm ent of Lake Eyre is located in the monsoon-watered areas of northern A ustralia. As demonstrated by large floodings of the modern ephemeral r egime, major lacustrine episodes must indicate enhanced monsoon precip itation in northern Australia. In the Holocene the lake has not risen to levels achieved during the early stage 5 lacustral phase, indicatin g: a marked reduction in the effectiveness of the monsoon in the prese nt interglacial by comparison with its predecessor