A Chronology of Glacial Retreat and its Influences on Sedimentary Strata in a Turbid Outwash Fjord: Simpson Bay, Prince William Sound Alaska

Simpson Bay is a small, macrotidal turbid outwash fjord located in southcentral coastal Alaska. This Y- shaped fjord is compartmentalized in to three morphologically distinct basins. Fresh water from heavy precipitation and the melting of high alpine glaciers enters Simpson Bay through bay head rivers and small shoreline creeks that drain the catchment. A high watershed:basin surface area ratio combined with high precipitation and an easily erodable catchment creates high sediment inputs. Seismic profiles were collected to determine the temporal source to sink relationships of terrestrial sediment input. The profiles show punctuated changes in sediment flux and type over millennia as the fjord changed from a glaciated basin, to an estuary under the influence of a tidewater glacier, to an estuary with a glacier grounded in the drainage basin, to it's present form as a fjord with very few high alpine glaciers and whose sediment input is driven by precipitation. The specific strata indicative of these types of glaciofluvial influence can be used to determine a chronology of glacial retreat and sedimentary environments in the fjord. As a glaciated basin, bedrock was scoured from the drainage basin and deposited beneath and in front of the glacier as a glacial diamicton. Directly above this basal surface is a unit of coarse laminated sediment indicative of glacial retreat and the establishment of a tidewater glacier. This subglacial/proglacial strata has thick lamina of parallel to sub- parallel continuous and discontinuous reflectors. Transparent and chaotic units within this layer indicate debris flows in the proglacial environment. Sedimentation rates during this stage were rapid due to large input of sediment from the glacier. Deposits of fine grain, finely laminated sediment found in depressions atop this layer indicate glacial retreat from the marine environment and into the drainage basin. Coarse grain material is entrained in the drainage basin, rivers, and fjord head deltas and sedimentation rates decreased significantly during glacial retreat.