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.
Description:
Includes bibliographical references (pages 92-98)
California State University, Northridge. Department of Geology.