Mode of subglacial transport deduced from till properties, Mýrdalsjökull, Iceland

Abstract

Abstract A multi-methological study of recent deposited till yields new insights into the mode of subglacial transport. At Slettjokull, the northern margin of the Mýrdalsjokull ice-cap, the ice-flow direction, transport distance, transport path, and glacier regime are known for a well defined point source. This knowledge permits an investigation of the lateral development of till properties and the reconstruction of a sedimentological model comprising some of the parameters affecting the modification of sediment in subglacial transport. One of the most remarkable results from this investigation is the very short transport distance that is required for the different till properties to reach a mature state. Within the first 250 m of transport distance significant changes occur in the grain-size distribution, clast-morphology, geochemical and mineral magnetic properties. However, the variation beyond 250 m suggests that till properties are likely to be a function of parameters other than transport distance. It is inferred on the basis of a renewed depositional model that the physical processes and conditions in a deformable bed beneath Slettjokull are highly responsive to any change in type of substratum. Furthermore, it is proposed that the ability of comminution processes to act on the sediment in transport is controlled by the prevailing subglacial drainage conditions and its influence on the rheological behaviour of a deformable bed. As a consequence, it may be demonstrated that the sedimentary properties of the deforming bed till might change significantly over very short distances without direct correspondence to transport distance.