Post glacial sediment supply and depositional history for the coastal areas of southern Alaska: new insights from seismic reflection data

Abstract

New high resolution sparker seismic data acquired in Prince William Sound (PWS) and the adjacent Gulf of Alaska provide insights into the post-glacial depositional history of the coastal areas of southern Alaska. Our data suggest Holocene strata lie directly upon Tertiary bedrock below most coastal waterways, the result of active tectonics and extensive glaciation that has uplifted and scoured pre- Holocene strata. We identify two regional unconformities within PWS that we presume to represent the onset of post “last glacial maxima” deposition approximately 15 ka and a neoglacial period estimated at 3.5 ka. Reflector stratification and the general decrease in sediment volume away from Hinchinbrook Entrance suggest prodelta sediments derived from the Copper River dominates late Holocene deposition within eastern PWS. This observation is consistent with the Alaska coastal currents carrying sediment into PWS from the eastern Gulf of Alaska through the Hinchinbrook Entrance. The only outlet for modern sediment back to the Gulf of Alaska from within PWS is through Montague Strait. Below the neoglacial unconformity and above Tertiary bedrock, the distribution and presence of poorly stratified sediments suggest Hinchinbrook Entrance did not supply sediment to PWS during early to middle Holocene times and that sediments deposited within PWS were derived from local sources. Bathymetric data point to sea floor bedrock, active faults, and glacial structures. Subbottom reflector truncations suggest subduction related active reverse faults are abundant throughout southern Alaska. The focus of uplift from both the 1964 M9.2 earthquake and prior great earthquakes is along the western margin of PWS and adjacent Gulf of Alaska.