Meltwater controls on ice-marginal sedimentation

Abstract

This thesis explores the influence that meltwater exerts on styles of ice-marginal sedimentation, using past and present examples from Iceland. The study glaciers display marked contrasts in form, size and composition of moraines which are unlikely to reflect differences in rates of subglacial erosion. This is because the study glaciers occupy a similar climate, show similar relief, sit above similar bedrock, and are inferred to flow at similar speeds. The observed variation in moraine properties must reflect some other process which intervenes to modify sediment transport relationships prior to the arrival of debris at the ice edge. I argue that this key factor which controls sediment transport - and, as a result, the potential to form moraines - is the behaviour of subglacial meltwater flows.Studies of the sediment load of its outlet river show that Solheimajokull is a highly erosive glacier, yet the quantity of debris carried by the ice is extremely small. Consequently, presentday moraine formation is extremely limited. This can best be explained as the product of an aggressive subglacial drainage network which captures and evacuates the bulk of debris generated by subglacial erosion. This state of high efficiency subglacial flushing is likely to dominate the sediment budget of many temperate glaciers.Whereas the present-day margin of Solheimajokull is debris-poor, the present-day margins of Gfgjokull and Steinholtsjokull are debris-rich. This debris consists of two major populations: 1) rounded clasts set in a sorted coarse sand and gravel matrix, derived from a series of englacial debris bands, and, 2) sub-angular clasts in a poorly-sorted matrix, derived from unusually thick sequences of basal ice. Overdeepened basins lie beneath the termini of both Gfgjokull and Steinholtsjokull. It seems that changes in water flow in this zone - rising water pressures associated with water flow upslope cause drainage to take up an englacial route - explain both the debris bands and the basal ice. The debris bands form as sediment-laden englacial channels close-up; simultaneously, the paucity of water at the glacier bed, in conjunction with strongly compressive ice flow, favours widespread preservation of basal ice. I extend Hooke's model of the ice-fall/overdeepening as a process system favourable to subglacial erosion to argue that it also creates conditions favourable to debris retention in ice, and so rapid accumulation of ice-marginal moraines.The idea that contrasts in the behaviour of drainage account for contrasts in moraine development adds depth to studies of the glacial geologic record, and its interpretation in termsThis thesis was written with the specific intent of linking process and form in such a way as to provide a meaningful explanation of moraine development. Different moraine forms are the emergent product of the multitude of process interactions which make up a complex causal network dispersed in time and space. The key factor which regulates this is the behaviour of meltwater. Glacier drainage systems provide the crucial contextual element which links the basic level of process (reductionist analysis) to the level of surface appearance (observations of moraine form). Drainage systems carry genuine causal powers which cannot be broken down into smaller process systems without the loss of explanatory power. This ties in with realist traditions of science, and recent ideas associated with complexity theory.