Textural and compositional sediment characteristics of the Southeastern Bristol Bay Continental Shelf, Alaska

Abstract

Analysis of 201 surface samples from the continental shelf of southeastern Bristol Bay, Alaska, has yielded an interpretation of the processes responsible for the distribution of sediment composition and texture {mean grain size, standard deviation, skewness, and kurtosis) in the region. The area had been previously described by Sharma {1974, 1975, 1979) and Askren (1972) as a classical graded shelf or one where sediment mean grain size becomes smaller with increased water depth. With the increase in number of samples studied and the increased sample density of this report, it is apparent that the previously described classically graded condition does not exist. Generally, the southeastern Bristol Bay shelf exhibits regional trends of a decrease in sediment-mean-grain-size and an increase in standard deviation, skewness, and kurtosis with increasing water depth. This scheme, however, is disturbed by areas of high variation in sediment textural measures. The pattern seems to be a relict of Pleistocene low-sea level graded outwash plain deposition which has subsequently been modified by wind-generated ocean waves and local ocean currents. During Pleistocene low-sea levels, the termini of Alaskan mainland continental glaciers were located near the present-day Bristol Bay shoreline. As sea level rose, the outwash plain deposits were covered by water. Submerged outwash plain deposits were further modified by hydrologic processes. Today, wind-generated waves, through progressive sorting, are entraining and transporting the relict outwash plain deposits and present day input. Some day they may reach a position of equilibrium between the hydraulic forces and the grain size, shape, and density. On a more local scale, ocean currents produced by a combination of high velocity tidal currents, storm-generated wind waves, water mass constriction, helical flow, and the Coriolis effect have produced currents which have scoured the shelf. Scours are a common feature and their effect on the regional sedimentation pattern is one of disruption. The scouring process selectively erodes surficial sediment, concentrating coarse mateial and removing fines, while leaving adjacent areas relatively undisturbed. Presently, the igneous terrain of the Alaska Peninsula and Unimak Island appears to be the only source of sediments seen in the study area. Surficial sediments have relatively high amounts of titaniferrous magnetite, basaltic hornblende, and volcanic rock fragments. The euhedral and angular nature of the sand grains indicates a short distance of transport. The rate of deposition on the shelf is low, due to an absence of any large sediment input into the area. Assuming a low sedimentation rate, sediments obtained from the uppermost 25 em of the shelf are both Holocene and Pleistocene in age.