A Sediment Wedge and an Instantaneous End-Moraine: a Twofold Ice-marginal Product of the 1890 Glacier Surge of Bruarjokull, Iceland

Contemporary understanding of the behaviour of surging glaciers and ice streams is hampered by the lack of data on landsystem evolution and sedimentary environments. This study concerns the ice-marginal environment of the surge-type Brúarjökull in Iceland. The sediment distribution in the glacier forefield as well as the morphology, sedimentology and tectonic architecture of the 1890 end moraine is investigated for highlighting the interaction between very dynamic ice and sediment/landform associations. As a result of substrate/bedrock decoupling during the 1890 surge, subglacial sediment was dislocated across the bedrock surface and deformed compressively, leading to gradual substrate thickening and the formation of a sediment wedge in the marginal zone. A drop in subglacial porewater pressure at the very end of the surge led to substrate/bedrock coupling and a stress transfer up into the sediment sequence causing brittle deformation of the substrate. Simultaneously, the glacier toe ploughed into the topmost part of the marginal sediment wedge initiating the moraine-ridge construction. Fine-grained and incompetent sediment deformed in ductile manner, resulting in a narrow rooted-fold-dominated moraine while coarse-grained and competent sediment deformed in brittle fashion, resulting in wide imbricated moraine. A new sequential model of subglacial and ice marginal processes operating during a glacier surge is proposed, illustrating the stepwise formation of a surging-glacier marginal sediment wedge and an instantaneous end moraine - a twofold, inseparable marginal end-product of the 1890 surge. As a result of high ice-flow velocities (100-120 m/day) the sediment wedge is thought to have formed in approximately five days and the end moraine in about one day.