The late quaternary deposits of blares opening and the middle huon valley, Tasmania

Abstract

A 10-12 m sequence of late Quaternary deposits from a large river bank section is described. Morphologic, stratigraphic, sedimentologic, palynologic and radiometric analyses demonstrate the presence of both pre-Last Glacial age glacigenic deposits and Last Glacial age fluviatile terrace and alluvial fan deposits. The pre-Last Glacial age till and outwash gravels are associated with 10-30 m high morainic ridges at Blakes Opening and occur throughout the middle Huon and lower Picton valleys. These deposits are strongly chemically weathered and are believed to belong to the Penultimate or to an earlier glacial age. At Blakes Opening the glacigenic deposits are overlain by a stratified sequence of organic-rich fluviatile sands and silts deposited by the Huon river, and by alluvial fan deposits derived by erosion from the adjacent moraines. Most of the fluviatile sands and silts are of early to middle Last Glacial age (70000-30000 B.P.) and the alluvial fan deposits are mainly of late Last Glacial age. The surface soil, an iron podzol, is of Holocene age. Pollen analysis of the organic sands and silts revealed that the vegetation had altered from Wet SclerophyllEucalyptusforest to Temperate Rain Forest which reverted to Wet SclerophyllEucalyptusforest. Although the influences of possible habitat changes and frequency of firing on the vegetation are discussed, it is concluded that the changes were primarily climatically induced. The vegetation changes probably represent a sequence of climatic changes from drier and colder conditions to moister and warmer conditions with a return to drier and colder conditions. Radiocarbon dating suggests that the moister warmer phase may represent an interstadial of mid Last Glacial age. Observations of glacial and periglacial landforms and deposits in the wider area suggest that during the early glaciation thick valley glaciers occupied the middle Huon and lower Picton valleys, whereas, during the Last Glacial Stage only local cirque and short valley head glaciers were developed at high elevations on Mount Picton and on the South Picton Range.