Formation of sub-marginal end moraines and its implications of subglacial ice-flow mechanism during the 1963-64 surge of Brúarjökull, Iceland
|Location||International Geological Congress,oslo 2008|
|Author||Benediktsson, حvar ضrn۱; Ingolfsson, Olafur۱; Schomacker, Anders۱; Kjوr, Kurt H۲|
|Holding Date||07 October 2008|
Present understanding of the link between glacier dynamics and end-moraine formation is limited by the lack of data on glaciological parameters, and morphological and structural properties of end moraines. In this study, the morphology and architecture of an end moraine formed by a ~9 km surge of Brúarjökull in 1963-64 is described and linked to the situation at the ice margin during surge termination, and actual descriptions and measurements of the surge.
Accurate mapping based on digital elevation models and interpretation of aerial photographs recorded at surge termination (1964) and after the surge (2003) allows the exact position of the ice margin at the end of the surge to be compared to the position of the end moraine at present. This comparison shows that the end moraine was positioned beneath the ice along most part of the margin at the end of the surge. The mapping moreover shows that meltwater outlets were frequent along those parts of the ice margin where sub-marginal end moraines were observed, indicating efficient subglacial drainage. Ground Penetrating Radar (GPR) and sedimentological data show a 4-5 m thick deformable bed (resting on bedrock) that was composed of a top layer of till, which extends across the entire end moraine ridge, overlying gravel and fine-grained sediments. GPR and structural geological investigations reveal that the architecture of the end moraine is dominated by thrust sheets.
A sequential model explaining the formation of the end moraine in a sub-marginal environment is proposed. The model assumes that the hydraulic conductivity of the bed had a major influence on the subglacial drainage efficiency and associated porewater pressure at the end of the surge. Thereby, the conductivity also affected the rates of subglacial deformation and the velocity distribution. In the uppermost till, the porewater pressure was high because low permeability restrained downwards escape of basal meltwater. The high porewater pressure decreased the yield strength and raised the strain rate of the till. Consequently, the advance of the glacier was facilitated through shear deformation in the uppermost part of the bed. In contrast to the till, the underlying gravel was highly permeable with low porewater pressure, and thus high yield strength and low strain rate. The low porewater pressure in the gravel increased the effective pressure of the till and the glacier, which induced high friction at the till/gravel interface. The friction transferred stress into the gravel, which consequently became subjected to thrusting with the décollement located at the boundary to the fine-grained sediments below. Because of the difference in strain rates of the till and the gravel, the latter deformed at a rate much lower than the rate at which the deformation occurred within the till. As a result, the principal velocity component was located within the till allowing the glacier to override the thrust sheets that constitute the end moraine.