Glacial sedimentary evidence supporting stick-slip basal ice flow

Abstract

Last Glacial (Late Devensian) subglacial sediments in the southern Sperrin Mountains, north central Ireland, comprise tabular bedrock rafts which are interbedded with diamicton (till) and brecciated bedrock and separated by glaciotectonic shears. Bedrock rafts and diamicton beds alternate laterally and vertically in the profile, suggesting the ice-bed interface was chequered with both high-strength (rock rafts) and low-strength (diamicton) patches during accumulation of the sediment pile. The overall facies arrangement is interpreted qualitatively as reflecting stick-slip basal ice flow and is related to the redistribution of free subglacial meltwater at the ice-bed interface. It is inferred that stick phases occur over bedrock rafts as meltwater migrates to lower-pressure regions, infilling shallow cavities on the surrounding diamicton surface. Slip occurs when porewater pressure increases and the meltwater layer backfills across bedrock raft surfaces, causing ice-bed uncoupling. The presented field evidence suggests a stick slip rather than pervasive subglacial deformation model may be a better approximation for the flow of parts of the Late Devensian mid-latitude ice sheets.