Quaternary fluviatile deposits from the Pieman Dam site, western Tasmania

Abstract

At the Pieman Dam site in western Tasmania, a sequence of 25-30 m of fluviatile gravels, sands, silts and clays overlies a strath terrace. The sediments represent a graded series in which the 2-11 m of gravel and sand deposits at the base represent either outwash or fluviatile deposits derived from outwash. The succeeding 5.6 m of organic-rich, fine sands, silts and clays were deposited in a river backswamp. These deposits contain abundant pollen and plant remains. Pollen analysis indicated that a riparian scrub backed by temperate rainforest dominated by Nothofagus cunninghamii and Eucryphia lucida , very similar in composition to the present temperate rainforest, developed on the site and that it was replaced by subalpine and alpine shrub and herbaceous vegetation. These deposits are sealed by over 10 m of organically sterile clays and silts, which are overlain by 4 m of slope deposits. The latter formerly mantled all the fluviatile deposits. A buried rock channel occurs on the bed of the Pieman River and deepens rapidly downstream, from -3.8 m upstream of the dam site to -22 m downstream of the dam site. The channel is infilled with gravels to form the bed of the present river at -5 to -7 m. The age of the buried channel is not known, but it is older than the basal gravel and sands of the fluviatile sequence which, at the youngest, may be of Penultimate glacial age. A 14 C assay of more than 54000 B. P. from the surface of the organic-rich fine sands, silts and clays indicates that the interglacial rainforest vegetation is probably of Last Interglacial age, but it may be older. The upper part of the clay and silt sequence is interpreted as being of early Last Glacial age and the surface slope deposits are interpreted as having been formed under non-forested conditions during the later part of the Last Glacial Stage.