Formation of a Push Moraine at the Margin of Höfdabrekkujökull, South Iceland

The NE margin of the glacier Höfdabrekkujökull - a piedmont lobe extending from the SE part of the ice cap Myrdalsjökull—is currently advancing, and along the glacier terminus a dislocated push moraine as high as 5–10 m is being formed. In the period 1979–1982 the average frontal advance amounted to 10 m per year. During the following period 1982–1984 the average glacier advance increased to 32 m per year. During the advance, the glacier forefield is intersected by thrust planes dipping 18–25° upglacier. At the same time supraglacial debris roll and fall downslope the steep glacier front, and flow till is deposited in tongues at the foot of the push moraine. These features, which are formed and destroyed concurrently with the glacier advance, contribute to a very heterogeneous texture of the push moraine. Digging in the distal part of the push moraine showed glacio-tectonic disturbances by folding and thrusting. Floes were pushed as unfrozen slabs, 50–100 cm in thickness, forward in front of the advancing glacier to form an imbricate structure.It is concluded that the push moraine is formed by recurrent pro-glacial thrusting, folding, and stacking. During further advance the glacier and the upper part of the push moraine will override the lower part. Once the ice overrides the formed ridge, subglacial deformation by basal shearing may create a glacier bed of heavily disturbed deposits, which far back from the advancing ice margin may be overlain by basal till. If a glacier recession sets in, the push moraine under discussion will be transformed to a ridge about 3–5 m high. In accordance with this, the majority of older push-moraine ridges found in the forefield of Höfdabrekkujökull does not exceed 5 m in height.