Formation of submarginal and proglacial end moraines: implications of ice‐flow mechanism during the 1963–64 surge of Brúarjökull, Iceland

Abstract

The morphology, sedimentology and architecture of an end moraine formed by a ∼9 km surge of Brúarjökull in 1963–64 are described and related to ice‐marginal conditions at surge termination. Field observations and accurate mapping using digital elevation models and high‐resolution aerial photographs recorded at surge termination and after the surge show that commonly the surge end moraine was positioned underneath the glacier snout by the termination of the surge. Ground‐penetrating radar profiles and sedimentological data reveal 4–5 m thick deformed sediments consisting of a top layer of till overlying gravel and fine‐grained sediments, and structural geological investigations show that the end moraine is dominated by thrust sheets. A sequential model explaining the formation of submarginal end moraines is proposed. 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 affecting the rates of subglacial deformation. High porewater pressure in the till decreased its shear strength and raised its strain rate, while low porewater pressure in the underlying gravel had the opposite effect, such that the gravel deformed more slowly than the till. The principal velocity component was therefore located within the till, allowing the glacier to override the gravel thrust sheets that constitute the end moraine. The model suggests that the processes responsible for the formation of submarginal end moraines are different from those operating during the formation of proglacial end moraines.