Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

Glacial Ripping in Sedimentary Rocks: Loch Eriboll, NW Scotland

Version 1 : Received: 20 April 2021 / Approved: 21 April 2021 / Online: 21 April 2021 (12:20:51 CEST)

How to cite: Hall, A.M.; Mathers, H.; Krabbendam, M. Glacial Ripping in Sedimentary Rocks: Loch Eriboll, NW Scotland. Preprints 2021, 2021040578 (doi: 10.20944/preprints202104.0578.v1). Hall, A.M.; Mathers, H.; Krabbendam, M. Glacial Ripping in Sedimentary Rocks: Loch Eriboll, NW Scotland. Preprints 2021, 2021040578 (doi: 10.20944/preprints202104.0578.v1).

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.

Subject Areas

glacial ripping; groundwater overpressure; breccia; rubble till; Cambrian quartz-arenite

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