Quaternary evolution of glaciated gneiss terrains: pre-glacial weathering vs. glacial erosion

Abstract

Vast areas previously covered by Pleistocene ice sheets consist of rugged bedrock-dominated terrain of innumerable knolls and lake-filled rock basins -the 'cnoc-and-lochan' landscape or 'landscape of areal scour'. These landscapes typically form on gneissose or granitic lithologies and are interpreted (1) either to be the result of strong and widespread glacial erosion over numerous glacial cycles; or (2) formed by stripping of a saprolitic weathering mantle from an older, deeply weathered landscape.We analyse bedrock structure, erosional landforms and weathering remnants and within the 'cnoc-andlochan' gneiss terrain of a rough peneplain in NW Scotland and compare this with a geomorphologically similar gneiss terrain in a non-glacial, arid setting (Namaqualand, South Africa). We find that the topography of the gneiss landscapes in NW Scotland and Namaqualand closely follows the old bedrock-saprolite contact (weathering front). The roughness of the weathering front is caused by deep fracture zones providing a highly irregular surface area for weathering to proceed. The weathering front represents a significant change in bedrock physical properties. Glacial erosion (and aeolian erosion in Namaqualand) is an efficient way of stripping saprolite, but is far less effective in eroding hard, unweathered bedrock. Significant glacial erosion of hard gneiss probably only occurs beneath palaeo-ice streams.We conclude that the rough topography of glaciated 'cnoc-and-lochan' gneiss terrains is formed by a multistage process: 1) Long-term, pre-glacial chemical weathering, forming deep saprolite with an irregular weathering front;2) Stripping of weak saprolite by glacial erosion during the first glaciation(s), resulting in a rough land surface, broadly conforming to the pre-existing weathering front ('etch surface');3) Further modification of exposed hard bedrock by glacial erosion. In most areas, glacial erosion is limited, but can be significant beneath palaeo-ice stream. The roughness of glaciated gneiss terrains is crucial for modelling of the glacial dynamics of present-day ice sheets. This roughness is shown here to depend on the intensity of pre-glacial weathering as well as glacial erosion during successive glaciations.

Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers

“…Joint analysis uses the principle of the circle inventory method (Davis and Reynolds, 1996, p. 720) to prevent any directional bias, but was speeded up by using the georeferenced outcrop photos. Circles of known area were drawn in the GIS and all joint traces were digitised within that particular circle (see also Krabbendam and Bradwell, 2014). Joint orientations can then be easily extracted from the data set and presented in rose diagrams, using the same method as the striae.…”

Sliding of temperate basal ice on a rough, hard bed: creep mechanisms, pressure melting, and implications for ice streaming

“…and Bradwell, 2014). The sudden, short jump is probably due to true sliding as basal ice is pushed higher onto (but not over) sloping obstacles due to an increase of P w .…”

“…The bedrock geology of Greenland is dominated by Precambrian gneisses, and such rocks almost certainly underlie much of the inland track of the NEGIS. Deglaciated areas of similar Precambrian gneisses in Canada, Scotland, Scandinavia and West Greenland generally show a lack of till, extensively exposed bedrock and a rough landscape of rock knolls and rock basins (Roberts and Long, 2005;Krabbendam and Bradwell, 2014). On these grounds, widespread deformable till beneath the NEGIS is unlikely.…”

“…Hard, rough beds are widespread on the beds of the former Pleistocene ice sheets and also likely beneath large parts of the present-day Greenland and Antarctic ice sheets (e.g. Kleman et al, 2008;Eyles, 2012;Rippin, 2013;Krabbendam and Bradwell, 2014;Krabbendam et al, 2016). Evidence for palaeo-ice streaming on hard beds has been reported from the former Pleistocene Laurentide and British ice sheets and deglaciated parts of West Greenland…”

“…Drilling in Greenland Ice Sheet adjacent to the Jakobshavn Isbrae has recorded a basal layer of temperate ice below cold ice of some 30 m thickness (Lüthi et al, 2002), equal to or greater in height than typical bedrock obstacles (roches moutonnées, whalebacks) in most crystalline gneiss terrains (Krabbendam and Bradwell, 2014). Temperate ice has also been modelled to occur beneath other parts of the Greenland Ice Sheet (e.g.…”