Abstract

Extensive wave-cut exposures at Ekuk Bluffs in the Nushagak lowland of southwestern Alaska provide a 7-km longitudinal section through deposits related to advance of Pleistocene glaciers from the Ahklun Mountains onto the adjoining piedmont. Facies analysis of the proglacial sediments reveals the following successive depositional environments: (1) lower sandy tidal flats, (2) upper muddy tidal flats, (3) upper foreshore/beach face, and (4) braided sandur. Horizontal compression at the toe of the advancing glacier induced large-scale imbricate thrusting and folding of the proglacial sediments, producing a distally accreting wedge of deformed sediment beyond the ice margin. The presence of this topographic wedge subsequently dictated the over-all pattern of sediment dispersal, forward during ice advance and rearward during retreat, and led to extensive gravity-driven resedimentation. Previous studies of deposition in glacial-tectonic settings generally have not emphasized these consequences of glacial-tectonically produced relief. The major stratigraphic implication is that sediment flows related to a single ice advance can generate multiple till-like diamicton units, some of which are separated by angular unconformities.

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