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Late Pleistocene glaciation and geomorphic development of the Von-Oreti valley, northern Southland

Kathryn (Humphreys) Coombes
Department of Geography, University of Otago, Dunedin, New Zealand

 
Abstract

The record of environmental change in the Late Quaternary for Otago/Southland, New Zealand, has received scant attention. It has long been recognised that landforms in the Von-Oreti area of northern Southland were moulded by ice advances of a distributary tongue of the Lake Wakatipu Glacier, but the environmental effects of the climate change and glaciation are poorly understood. No glaciers remain in the area, and the Von River now flows north into lake Wakatipu, where ice once flowed south. The Oreti River - sharing the same valley as the Von - retains a southward flow, with the two rivers separated by the smallest of drainage divides, itself a legacy of past glacial environments. In this study, models of landscape change have been developed using lithofacies-based environmental reconstruction and relative dating. This approach evaluates the environmental change caused by interaction of multiple ice advances and fluvial systems, with respect to both the chronology of change and the character of physical landscape development. The results show that several early advances through the valley eroded bedrock ridges but left only high outwash terrace deposits, while the four more recent advances left remnant terminal moraines in the valley. Because of the environmental setting of distributary ice tongues which flowed against the regional drainage, these advances created a complex sedimentological environment distinct form that document from past and present New Zealand glaciers. The main feature of this environment was a prevalence of dynamic proglacial lakes. The model developed shows that some of the lakes were dammed by moraines, but many were dammed between glaciers and the valley relief. Outwash of the advances drained north rather than south as the low drainage divide to the south became an impediment to flow. The northward drainage has facilitated stream capture and the formation of deep gorges and still affects the landscape as the river system attempts to regain equilibrium.

Because of a lack of carbon suitable for radiocarbon dating, little progress was made in providing absolute ages of environmental change in the Von-Oreti valley. Two carbon dates and an analysis of a pollen core have provided new information which will be valuable for regional climate correlations. The two methods of relative dating used, weathering rinds and soil development, were of limited use. Of the two methods, the soil development dating was of more use, and has potential for further dating of Quaternary deposits in Otago/Southland.

 

Reference

Coombes, K.A.H. 1997. Late Pleistocene glaciation and geomorphic development of the Von-Oreti valley, northern Southland. Unpublished MSc thesis, in Geography, at the University of Otago, Dunedin, New Zealand. 138 pp.

© 2009 Department of Geography, University of Otago, Dunedin, New Zealand