Glacial Ripping in Sedimentary Rocks: Loch Eriboll, NW Scotland

Abstract

Glacial ripping is a newly recognized process sequence in which subglacial erosion is triggered by groundwater overpressure. Investigations in gneiss terrain in lowland Sweden indicate that ripping involves three stages of (i) hydraulic jacking, (ii) rock disruption under subglacial traction and (iii) glacial transport of rock blocks. Evidence for each stage includes, respectively, dilated fractures with sediment fills, disintegrated roches moutonnées and boulder spreads. Here we ask: can glacial ripping also occur in sedimentary rocks, and, if so, what are its effects? The case study area is in hard, thinly bedded, gently dipping Cambrian quartz-arenites at Loch Eriboll, NW Scotland. Field surveys reveal dilated, sediment filled, bedding-parallel fractures, open joints and brecciated zones, interpreted as markers for pervasive, shallow penetration of the quartz-arenite by water at over-pressure. Other features, including disintegrated rock surfaces, boulder spreads and monomict rubble tills, indicate glacial disruption and short distance subglacial transport. The field results, together with published cosmogenic isotope ages, indicate that glacial ripping operated with high impact close to the former ice margin at Loch Eriboll at 17.6-16.5 ka. Glacial ripping thus can operate effectively in bedded, hard sedimentary rocks and the accompanying brecciation is significant – if not dominant - in till formation. Candidate markers for glacial ripping are identified in other sedimentary terrains in former glaciated areas of the Northern Hemisphere.

Late erosion pulse triggered by rapid melt in the cold‐based interior of the last Fennoscandian Ice Sheet, an example from Rogen

“…The dilation and brecciation of bedrock outcrops, release of rock blocks and sheets and disintegration into angular boulders are key markers for the glacial ripping process sequence (Hall et al, 2020; Krabbendam et al, 2021, 2022). Similar patterns of damage have been identified on low, abraded bedrock hills in Cambrian quartz arenites in northwest Scotland (Hall et al, 2021).…”

“…The distribution of ribbed moraines at low elevations on the Rogen plateau is typical for central Sweden and Norway, where ribbed moraines occupy the floors of straths and basins (Hättestrand, 1997; Shaw, 1979; Sollid & Sørbel, 1994) and indicates a key role for subglacial meltwater in ribbed moraine formation. Where ribbed moraines occur in association with hydraulic jacking, disrupted bedrock, and with angular locally derived and glacially transported boulder fields (also termed boulder spreads, see Hall et al (2021)), the role of groundwater at overpressure must also be considered.…”

“…The dilation of sandstone fractures and generation of boulder spreads represent the first evidence for glacial ripping in a thawing cold‐based ice sheet interior setting, and a further example of glacial ripping in sedimentary bedrock (Hall et al, 2021, and references cited therein). Other erosion mechanisms, such as meltwater erosion at corridors (Rampton, 2000), winnowing of boulders from till, and freeze‐on quarrying (e.g., Kleman & Borgström, 1994; Sarala, 2006) or plucking, may have been active for locally producing boulders and boulder trains at Rogen, but all these fail to explain the extensive distribution of boulder fields containing tabular boulders (see Krabbendam et al, 2022).…”