Glacial Sedimentary Processes in Iceland

We have recently completed a study employing macro- and micro-sedimentological techniques on sediments exposed on recently deglaciated fore-fields at nine surging and non-surging glaciers in Iceland. Macro-scale evidence includes section logging (e.g. Figure 1), followed by analysis of particle-size distribution and individual clast shape, angularity and fabric of key units (e.g. Figure 2). Micro-scale evidence involves the identification of rotational structures, fold structures and unistrial plasmic fabric (evidence for ductile deformation) and alignment of grains, shear lines and crushed grains (evidence for brittle deformation) in thin sections viewed under a micro-scope (e.g. Figure 3). The techniques have been used to identify the types of sediments and therefore the processes of deposition (deformation tills, lodgement tills, deglacigenic debris flow deposits, glaciofluvial or glaciolacustrine deposits) (e.g. Figure 4). Many sediments show evidence of subglacial deformation (deformation tills or glacitectonites) and some also contain thin layers of water lain sediments within the deformation till units suggesting ice-bed separation and basal sliding. All the evidence can be used to estimate the relative importance of sliding vs. sediment deformation beneath the glaciers, the styles (ductile vs. brittle) and pervasiveness (depth) of deformation, and whether these vary in space and time. Some important conclusions are i) the classic two tier deformation till (upper ductile;lower brittle) is relatively uncommon in Iceland, despite its documentation in the literature at a few glaciers; ii) the relative importance of sliding and deformation varies throughout the glacier surge cycle, presumably in response to variations in subglacial water pressure; iii) the relative importance of ductile and brittle deformation often reflects the particle size characteristics of the sediments with courser material showing more evidence of brittle deformation. This work is being undertaken in collaboration with Anna Nelson (British Antarctic Survey) and Colm Ó Cofaigh (University of Durham).

Papers relating to this project

• Nelson, A.E., Willis, I.C. and Ó Cofaigh, C. 2005. Till genesis and glacier motion inferred from sedimentological evidence associated with the surge-type glacier, Brúarjökull, Iceland. Annals of Glaciology, 42, 14-22.

Figures

Figure 1. Logs and panoramic photograph of a site in front of Hagafellsjökull Eystri. The exposure is ~30 m long. Solid white line corresponds to a lower thrust fault. Upper dashed lines correspond to a shear plane. Solid grey line corresponds to continuous massive clay layer beneath thrust fault. For interpretation, see Figure 4.

Figure 2 Sediment characteristics of Hagafellsjökull Eystri site, Sections 1 and 6.

Figure 3. Photomicrographs of Unit B, Section 3, at the Hagafellsjökull Eystri site (see Figure 1). (a) Masepic plasmic fabric within diamict. (b) Sheared (from right to left) silt cap beneath pebble-gravel basalt grain within diamict. Direction of shear was from right to left, in a downglacier direction. Subsequent to the shearing event, sediment in-filled between the silt cap and the grain.

Figure 4. Conceptual model of the glacitectonic history at the site in front of Hagafellsjökull Eystri. The red dashed line indicates the zone of decollement, the blue line indicates the thrust fault, which occurs along the zone of decollement, and the green line is the lower contact of the deforming layer. Stage 1 shows the pre-existing subsurface stratigraphy prior to glacier advance and glacitectonism. Stage 2 involves tectonic thrusting of Lithofacies 3 and 4 over Lithofacies 1 and 2, the deformation of Lithofacies 3 and 4, and the deposition of Lithofacies 5. Stage 3 involves glacier overriding, subsequent thrust moraine erosion and subglacial sediment deformation of Lithofacies 5.