Identifying palaeo-ice-stream tributaries on hard beds: Mapping glacial bedforms and erosion zones in NW Scotland

摘要

Ice streams are fed by tributaries that can extend deep into the heart of ice sheets. These tributaries are born at onset zones - the abrupt transitions from slow sheet flow to fast streaming flow that often occur at significant topographic steps on hard beds (bedrock-dominated beds). For this reason, tributary onset zones leave only a subtle erosional geomorphic signature in the landscape record that is rarely studied. This paper examines, in detail, the geomorphic signature of ice-sheet flow on a hard bed at the head of a palaeo-ice stream. We use field survey techniques to map glacial bedforms within an ～200-km~2 area of hard crystalline bedrock in a landscape of 'areal scour' around Loch Laxford in NW Scotland. The bedrock bedforms range from plastically moulded (p-forms) and wholly abraded forms, to stoss-lee forms and plucked surfaces all on an outcrop scale (1-100 m). We devise a five-zone classification system to map (in a CIS) the presence, absence, and abundance of glacial erosional forms within 619 (500-m square) grid cells. We go on to use these erosional bedform zones, along with known glaciological relationships to interpret the spatial and altitudinal pattern of palaeo-ice sheet processes and glacier dynamics in this part of NW Scotland. Our interpretation highlights the strong vertical thermal zonation on mountains, and the spatial variations in ice rheology (softness), ice temperature and, by inference, ice velocity in troughs - intimately associated with the onset of ice streaming in tributaries. Consequently, we define the Laxfjord palaeo-ice-stream tributary - a feeder to the Minch palaeo-ice stream in NW Scotland. Finally, we suggest that this new mapping approach could be performed in other deglaciated hard-bed terrain to examine, more widely, the subtle erosional signatures preserved in areas traditionally thought to represent ice sheet 'areal scour'.