The final phase of dead-ice moraine development: processes and sediment architecture, Kötlujökull, Iceland

Consecutive phases of de-icing of ice-cored moraines and the formation of dead-ice moraine were monitored over a 4-year period at the terminus of the Kötlujökull glacier, Iceland. Particularly, the transition from partially ice-cored moraine with isolated dead-ice blocks to the ice-free landscape receives attention in this paper in order to link the final melting processes to the architecture of the sedimentary end product. In the current humid sub-polar climate of south Iceland de-icing of partially ice-cored moraines results chiefly from melting along the bottom surface of ice-cores with an annual average rate of 25 cm. The final de-icing is associated with an interrelated group of re-sedimentation processes and surface features. Series of sinkholes evolve at the toe of dead-ice blocks, which initiate retrogressive rotational sliding or backslumping of the ice-cored slopes and the formation of distinct edges and fractures in the adjacent basins. Although backslumping is the dominant process in this phase of re-sedimentation, structures resulting from this process are rarely recognized in the ice-free landscape. As ice-cores gradually diminish the effect of the latest re-sedimentation events will overprint or destroy most existing sedimentary characteristics. Thus, in the ice-free landscape, structures mainly related to the formation of sinkholes and fractures remain imprinted on the sediment succession. Generally, no inversion of the topography occurs during the final phase of de-icing. The overall topography recognized in the late phase of the fully ice-cored terrain is merely lowered and the amplitude of the relief reduced as de-icing progresses. The sediment architecture of the ice-free landscape is characterized by heterogeneous and often slumped diamict sediments with variable thickness and lateral distribution; clast orientation is related to the direction of slopes, and boulders are found in isolated groups or in linear arrangements.