Characteristics and formation of bedrock mega-grooves (BMGs) in glaciated terrain: 1 - morphometric analyses

Abstract

Bedrock mega-grooves (BMGs) are subglacial landforms of erosion that occur in glaciated terrain in various geological and (palaeo)glaciological settings. Despite a significant literature on BMGs, no systematic morphometric analysis of these landforms has been undertaken. This is a necessary step towards exploring BMG formation and has been successfully applied to other subglacial landforms of similar magnitude (e.g. mega-scale glacial lineations (MSGLs) and drumlins). In this study, BMGs from ten locations across the world are systematically mapped, sampled and measured. Based on the 10th–90th percentile of the aggregated global population (n = 1242), BMGs have lengths of 224–2269 m, widths of 21–210 m, depths of 5–15 m, elongation ratios of 5:1–41:1, and the spacing between adjacent grooves is 35–315 m. Frequency distributions for all metrics are unimodal, strongly suggesting that the sampled BMGs form a single landform population. This establishes the BMG as a geomorphic entity, distinctive from other subglacial landforms. The variability of the metrics and their correlations between and within sites most likely reflect site-specific geological characteristics. At sites which have been associated with fast-ice flow, BMGs display the largest dimensions (especially in terms of length, depth and width) but lowest elongation ratios, whereas BMGs formed under a primary geological control occupy smaller size ranges and have higher elongation ratios. Morphometrically, BMGs and MSGLs plot as different populations, with BMGs being on average 4 × shorter, 3.5 × narrower, 3.5 × more closely spaced and about 2 × deeper. It is suggested that future research focuses on numerical modelling experiments to test rates of erosion in different bedrock lithologies under varying glaciological conditions, and on adding to the body of existing field-derived empirical observations. The latter remains key to validating geological controls over BMG formation and assessing the efficiency of erosion mechanisms.