PLEISTOCENE STRATIGRAPHY OF THE BOCO PLAIN, WESTERN TASMANIA

Mapping of surficial glacigenic sediments and logging of drill cores f rom the Boco Plain has revealed a more complete sequence of glacial an d interglacial sediments, and a more complex history of middle to late Pleistocene depositional events than anywhere else in Australia. Radi ocarbon dating of the uppermost fluvial and organic-rich lacustrine se diments indicates that a long record of non-glacial deposition followe d the Penultimate or Boco Glaciation. Prior to this, a sequence of sed iments indicating multiple glacial advances is intercalated with lacus trine silts and fluvial sands and gravels. Radiocarbon dates for the u ppermost unit of organic lacustrine sediments show that they extend fa r beyond the range of the radiocarbon technique. In addition, uranium- thorium dated ferricretes from the older sedimentary sequence provide minimum ages for the glacial events, and suggest ice advances into the plain broadly correlative with marine Isotope Stages 6, 8 and greater than or equal to 10. Normal detrital remanent magnetization of the se diments confirms a middle Pleistocene age for all the glacial episodes recorded in the cores. The glacial sediments overlie Cambrian volcani c bedrock weathered to a depth of 9 m which indicates a long time gap between initial erosion of the valley and the commencement of middle P leistocene glacial deposition.