The glacial impact on an exhumed sub-Mesozoic etch surface in southwestern Sweden

Abstract

Along the northern part of the western coast of Sweden, the landscape is characterized by joint-aligned valleys with intervening rock hills. Precambrian bedrock is exposed over large areas and the Quaternary cover is mainly restricted to the bottom of narrow valleys. The well-exposed bedrock exhibits many glacial forms, but recently the major landforms have been interpreted as exhumed sub-Mesozoic etch forms. To gain insight into the relative roles of different processes in shaping the present forms, we studied a granite area where the rock hills contain fractures and narrow clefts with weathering forms and saprolite remnants. Marks of glacial erosion are found only in the upper parts of the clefts, where erosion by subglacial meltwater has exploited weathered preglacial sheeting structures to form furrows with concave cross profiles (p forms). On the summit surfaces, glacial erosion has been of greater importance, but the roche moutonnees found here are also structurally controlled. The main impact of glacial erosion is plucking on the order of tens of metres. Thus, landscapes labelled “areally scoured glacial terrain” can, at some locations, also be described as stripped and slightly incised etch surfaces.

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

“…This comparison is consistent with previous work on glaciated granite-gneiss terrains Scandinavia and Greenland (e.g. Bonow, 2005;Lidmar-Bergström, 1989Lindström, 1988;Olvmo et al 1999). …”

“…In contrast, the geomorphology of deglaciated areas can be studied in great detail, especially with increasing availability of high resolution DTM data (e.g. Bradwell 2013;Hubbard et al, 2000;Johansson 1999;Lidmar-Bergström and Näslund 2002;Olvmo et al 1999). Geomorphological analysis of deglaciated gneiss terrains at different scales can potentially be used as analogues for the subglacial topography of gneiss-dominated beds under present-day ice sheets.…”

“…Lidmar-Bergström, 1989Lidmar-Bergström et al 1997;Lindström, 1988). Where Cretaceous cover rocks were removed earlier (possibly in Tertiary times), landforms indicative of deep weathering as a well as remnants of saprolite are locally preserved (Johansson, et al 2001a,b;Olvmo et al, 1999;. Glaciation in this 'Swedish model' is seen as responsible for stripping the pre-glacial weathering mantle, with only a minor accompanying component of glacial erosion of intact hard bedrock (e.g.…”

Using a GIS filtering approach to replicate patterns of glacial erosion

“…Kleman and Stroeven, 1997;Stroeven et al, 2006). Such results lend support to the notion that some glacially scoured landscapes may have been directly inherited from the pre-glacial pattern of deep weathering (Hall and Sugden, 1987;Olvmo et al, 1999;Olvmo and Johansson, 2002). In fact, the formation of large glacial troughs towards a maximum steady-state shape (Sugden, 1976(Sugden, , 1978Braun, 1989) may have been predominantly established through valley widening, as inferred from (i) restricted glacial erosion in the scoured landscape of Riksgränsen (Stroeven et al, 2002a; this study), as well as for some other scoured landscapes in northern Sweden (cf.…”

Glacial erosion and bedrock properties in NW Scotland: Abrasion and plucking, hardness and joint spacing

“…Hardness and joint density are therefore expected to provide strong controls on the mode of erosion (i.e., abrasion vs. plucking). Some studies have also shown that the final products of glacial erosion of bedrock surfaces vary with the degree of preglacial weathering (Lindström, 1988;Olvmo et al, 1999;Johansson et al, 2001a, b;2 Krabbendam & Glasser: Glacial erosion: Abrasion vs Plucking. MS for Geomorphology Olvmo and Johansson, 2002); in effect this means that the bedrock properties in the top few metres can be different than the bulk bedrock below.…”