Magnetostratigraphy of glacial lake sediments and dating of Pleistocene glacial deposits in Tasmania

Magnetostratigraphic techniques have been applied to Quaternary glacial deposits of western, central western and central northern Tasmania. The aims of this study were to examine the validity of the application of these techniques to glacial lake sediments, to separate glacigenic deposits that were beyond the range of radiocarbon dating and to compare the stratigraphy determined by these methods with the established stratigraphy, on the basis of their magnetic polarity. The extant model of the glacial stratigraphy of Tasmania is based on morphostratigraphic, lithostratigraphic and biostratigraphic mapping, and the analysis of weathering characteristics, particularly weathering rind analysis. The model recognises at least three glaciations, the Margaret or Last Glaciation, the Henty Glaciation and the Linda Glaciation. The magnetostratigraphic framework developed as a result of this study has largely confirmed the established stratigraphic framework, with the exception of some deposits of Henty Glaciation age. The Henty Glaciation has been considered to be the Penultimate Glaciation in Tasmania, and hence of Middle Pleistocene age. Evidence from the Henty Surface has demonstrated that some of the deposits in the Henty Glaciation type area must be > 730 ka in age and hence of Early Pleistocene age. The Linda Glaciation has been considered to be of very considerable age by many earlier workers. This study has demonstrated that the Linda Glaciation and its equivalents are >730 ka in age, and hence of Early Pleistocene or even Late Tertiary age. The Linda Glaciation appears to have been a complex event, consisting of either a number of glacial advances, or two or more separate glaciations. Magnetostratigraphic techniques have been shown to be useful and valid techniques for the determination and clarification of the ages of Middle and Early Pleistocene deposits in stratigraphic sequences. Glacial lake sediments have been shown to be suitable material for the application of such techniques. The suitability of material for palaeomagnetic analysis varies considerably, so some results are considered to be more reliable than others. Accordingly, a reliability index based on a number of factors has been constructed. Glacial lake sediments derived from sources containing dolerite or volcanic detritals are particularly suitable as they have strong magnetic signatures. Correlation of the glacial events of Tasmania with those of New Zealand and South America is not possible at present. Magnetostratigraphy will provide a means of correlating these events when similar frameworks have been established for these areas.