A high resolution geophysical investigation of spatial sedimentary processes in a paraglacial turbid outwash fjord: Simpson Bay, Prince William Sound, Alaska

Simpson Bay is a turbid, outwash fjord located in northeastern Prince William Sound, Alaska. A high ratio of watershead:basin surface area combined with high precipitation and an easily erodable catchment create high sediment inputs. Fresh water from heavy precipitation and meltwater from high alpine glaciers enter Simpson Bay through bay head rivers and small shoreline creeks that drain the catchment. Side scan sonar, seismic profiling, and high resolution bathymetry were used to investigate the record of modern sedimentary processes. Four bottom types and two seismic faces were described to delineate the distribution of sediment types and sedimentary processes in Simpson Bay. Sonar images showed areas of high backscatter (coarse grain sediment, bedrock outcrops and shorelines) in shallow areas and areas of low backscatter (estuarine mud) in deeper areas. Seismic profiles showed that high backscatter areas reflected emergent glacial surfaces while low backscatter areas indicated modern estuarine mud deposition. The data show terminal morainal bank systems and grounding line deposits at the mouth of the bay and rocky promontories, relict medial moraines, that extend as terrestrial features through the subtidal and into deeper waters. Tidal currents and mass wasting are the major influences on sediment distribution. Hydrographic data showed high spatial variability in surface and bottom currents throughout the bay. Bottom currents are tide dominated, and are generally weak (5-20 cm s-1) in the open water portions of the bay while faster currents are found associated with shorelines, outcrops, and restrictive sills. Tidal currents alone are not enough to cause the lack of estuarine mud deposition in shallow areas. Bathymetric data showed steep slopes throughout the bay suggesting sediment gravity flows. Central Alaska is a seismically active area, and earthquakes are most likely the triggering mechanism of the gravity flows.