32,000-34,000 OSL AGES FOR LARGE, SINUOUS LATE PLEISTOCENE CHANNELS IN THE LACHLAN VALLEY, SOUTHEASTERN AUSTRALIA

OSL techniques have been used to obtain the first dates on large, sinuous, Late Pleistocene channels in the Lachlan Valley, southeastern Australia. The Lachlan is a 700 km long tributary of the Murray Basin. Its palaeochannels are associated with two major alluvial terraces, which grade into extensive alluvial plains downstream from Forbes. Late Pleistocene channels of the Ulgutherie Fluvial System are characterised by sinuous, regular and scrolled meanders, with wider and shallower channels than the present-day, irregularly meandering, suspended-load river. Reconstructed channel cross-sections are 6 times larger than the average for the present Lachlan River, and bankfull discharges based on channel geometry formulations were estimated to be 4 to 7 times larger. Two OSL dates from channel sand and overlying source-bordering dune sand indicate that these channels were fully established by 34,000 yr BP and geomorphic evidence suggests the system may have declined soon afterwards. This conclusion is consistent with regional lake-level and geomorphic evidence of cool, pluvial conditions, followed by an arid glacial maximum.