Rock debris transport and deposition by valley glaciers in South Georgia

• Main Author: Birnie, Richard V.
• Published: University of Aberdeen 1978
• Subjects: 551.4
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.449894

The aim of this geomorphological study is to analyse the forms and processes associated with rock debris on valley glaciers of the sub-Antarctic island of South Georgia (lat 54°S., long 36°W.). Two main types of rock debris are visually differentiated in the field and, on the basis of sedimentary characteristics and modes of occurrences, these are shown to be sub aerially-derived (EXTRAGLACIAL) and subglacially-derived (GLACIOGENIC). A simple systems framework based on a flow diagram is used as a basis for the analysis of the forms and processes associated with these debris types. This framework involves a consideration of debris "supply", "transport" and "depositon" for both extraglacial and glacio-genic debris types. The most common forms of extraglacial debris supply around the glacier margins are snow avalanches and debris falls. Debris arriving by these low magnitude processes is incorporated on the glacier 0.5km from the contributing rockwall or gully. However, a high magnitude rock avalanche on the Lyell Glacier in September 1975 carried debris > 4km from its source rockwall and contributed between 60 and 300 times the 'normal' annual supply of debris due to low magnitude processes. Such high magnitude events are rare in South Georgia. Talus is common along the length of the glacier margins around the ablation zone, and its upper limit coincides with the equilibrium line of the glacier. Talus limits can be used as geomorphological evidence of equilibrium line positions in areas of former glaciation. In transport extraglacial debris is common in englacial positions along planes of sedimentary stratification, and along longitudinal foliation in lateral and medial situations. Thick surface debris occurs where steeply-dipping debris-rich englacial bands intersect the ablation surface and this produces glacier surface relief. High primary relief is associated with thick surface debris in the ablation zones and its distribution is related to the debris source and the patterns of deformation of the containing ice. Surface features >15m high are recorded and relate to differential ablation induced by surface debris. Debris 30cm thick reduces ablation by 75%. Secondary relief is linked to the effects of crevassing and meltwater. Deposition of extraglacial debris at the glacier terminus produces small dump ridges 5m high beside large valley glaciers but small glaciers may have ridges >15m high and at least two have produced rock glaciers. Variations in the area of contributing rockwall and the receiving glacier show that rock glaciers, protalus ramparts and ice-cored moraines form part of a continuum of depositional landforms in South Georgia and thus rock glaciers are landforms of current maritime glacial environments. The supply of glaciogenic debris involves 4 processes on the basis of structural relations: 1) Pressure-melting and refreezing producing debris bands with a high ice content; 2) Subglacial freezing relating to "cold patches" producing bands with a high debris content and undisturbed bedding structures; 3) Differential flow around bed obstacles emplacing basal tractive load into englacial position, giving bands with high debris content; 4) Subglacial freezing at the margins due to seasonal cold wedge, giving thick englacial debris sequences in areas of compressive flow. Glacier surface occurrences of glaciogenic debris are rare in South Georgia, and where they occur the debris fails on ice slopes as low as 12 producing flows and slumps. This contrasts with the stability of extraglacial debris on ice surfaces 35. Surface mass-movement and ice-core degradation is rapid beneath a glaciogenic cover. However, due to the restricted nature of surface debris of this type, flow tills do not form a significant part of the depositional sequences of most valley glaciers in South Georgia. Although depositional landforms include both lateral and terminal moraines only minor terminal ridges are considered in detail. These are 2m high and lie. subparallel to the ice edge. They are produced by a combination of "pushing", "squeezing", and "dumping" which relates to seasonal instabilities at the ice margin. Individual ridges are shown to be hybrid types and are produced annually.