The sensitivity of subglacial bedform size and distribution to substrate lithological control

Abstract

The varied nature of a glacial substrate has often been invoked to account for the varied distribution, arrangement and morphological expression of subglacial bedforms. The ease and rate of sediment production from the bedrock substrate, and the rheological, mechanical and hydraulic properties of the resultant subglacial sediment layer have been argued, in certain samples or settings, to account for the local-to-regional distribution of bedforms and bedform types, and varying morphological expression such as size, shape, elongation, or spatial arrangement. From locally coherent observations and relationships it is tempting to extrapolate such patterns to express a more fundamental lithological control on subglacial bedform properties and formation. Here we use a large, systematically compiled dataset (> 30,000 subglacial bedforms inscribed by the Irish Ice Sheet) to explore potential lithological controls upon subglacial bedform distribution and morphological expression. We expect any lithological control upon form, size or distribution to be expressed through a spatial correlation between bedform properties and those of the bed lithology; we extract bedform morphometric and distribution statistics and underlying lithology data from datasets held in a geographic information system for comparative analyses. At ice sheet scale, i.e. considering our whole bedform population, we find no apparent relationship between the lithology of either bedrock or till substrate and bedform occurrence, density of arrangement, or size. Regional to local examples do, in contrast, exhibit some coincident changes in substrate and bedform expression. These are typically manifest as an abrupt shortening or lengthening of drumlins at a lithological boundary, superimposed upon more gradual, regional trends. However, not all local variability observed in the bedform population can be attributed to a lithological driver and, where it can be inferred, lithological control is often in tandem with, or subordinate to, other drivers such as topographic setting or ice dynamics. We propose a hierarchy of controls upon bedform form and distribution, strongly coupled to spatial scale: lithological differences can modulate local scale form and expression but bedform incidence and properties are primarily governed by ice sheet scale glaciological patterns and drivers. Only under certain (unknown) conditions at a local scale is the bedforming process sensitive to the lithological properties of the bed.