Late pleistocene geocryology of the Bogong High Plains

Abstract

The Bogong High Plains comprises fragmented, elevated surfaces (at 1600m to 1986m elevation),of low to moderate relief and contains relic cryogenic features including cryoplanation surfaces, blockstreams, block fields, block glacis and talus. No glacial features have been observed on the Bogong High Plains.

This thesis has examined the distribution of the cryogenic features and their lateral equivalents in the adjoining valleys of the Mitta Mitta and Kiewa Rivers, with a view to defining processes active in the late Pleistocene.

Three are as at Mt Nelse, Basalt Hill and Pretty Valley were targeted for detailed description and analysis of surficial geology. The cryogenic features defined from this study have been compared to those described in other studies of Alpine areas in south-eastern Australia.

Cryoplanation/blockfield terrain occurs on slopes of <20%, (11°), generally above 1600m. These surfaces are developed on exposed summit tops between 1600m and 1986m, and consist of low angle, broad, stepped, bedrock surfaces with shallow accumulations of blocks and gravelly loam fanning outwards.

Blockstreams are represented by block accumulations in drainage lines on slopes of 10° + 5° at altitudes of 1300m up to 1800m and extending for up to 1km. Talus occurs below elevated bedrock scarps at similar altitudes on slopes >30°. Both types of block accumulations require appropriately jointed bedrock and access to moisture. There is a preference of orientation of blockstreams and talus towards the south. Most large basalt blockstreams have a south-eastern aspect and this aspect coincides with maximum snowbank accumulation. Block glacii exist as relatively level, narrow accumulations of blocks marginal to basalt bedrock scarps, generally with a southerly aspect.

There are many slopes with significant block accumulations which appear to be hybrid blockstream/talus deposits on slopes between 15° and 30°.

Open block accumulations only survive as visible features where the blocks have reversed local relief. Most slopes are vegetated by heath land and/or snow gum woodland. Elevated vegetated slopes of >20% gradient have a distinct striped pattern oriented down slope. The vegetation stripes have probably been induced by etching of, or differential development of, gravelly slope deposits and soil which overlie relic sheets of blocks and cobbles of variable thickness. This striped terrain of >20% gradient exists over an elevation range of 1400m to 1700m, with a preferred southerly aspect and is referred to as block slopes.

The relic status of block accumulations is evident from encroaching vegetation, insitu weathering and adjoining stratigraphy. Frost wedging and associated frost creep appear dormant and the shallow depth of seasonal frost prohibits gelifluction within the present climate.

The various relic block accumulations on the Bogong High Plains would have required colder conditions suggesting an approximate 5°C decrease in average temperatures during the late Pleistocene. The lateral equivalents of these deposits in the adjoining valleys are relic alluvial fans which would have required semi-arid conditions to accumulate and thereby suggest that the late Pleistocene climate was at least 50% drier as well as approximately 5°C colder than present.