Abstract: Factors Influencing Sediment Distributions and Patterns on Shallow, High-Energy Insular Platform, Northern Channel Islands, California

Peter C. Day

A shallow platform within the southern California borderland exhibits irregular surficial sediment distributions. Examination of high-resolution profiler records and vertical sediment sections from more than 300 box cores delineates thicknesses and distributions of these sediments and suggests environmental conditions controlling their deposition.

South of the four northern Channel Islands, the platform margin is characterized by thin, patchy sediments overlying a wave-cut surface. Sediments are biogenic, residual, or relict-palimpsest with only a minor island-derived component. On the lower energy, depositional northern platform margin, modern detrital sediments are beginning to prograde over shallower terrace levels, but in volume remain subordinate to those of relict or palimpsest nature. Surface layers of tens of meters of terrace cover reflect reworking by organisms and intermittent storm activity. Scoured passages between the islands connect the two exceedingly different margin environments. Current-meter data show that bottom currents within these passages are strongly influenced by tides and commonly attain high velocit es.

In the absence of additional bottom-current measurements, sediment-derived trends suggest influences on sediment compositions and distributions present on the northern Channel Islands platform. When parameters of mean-grain size, sorting, skewness, kurtosis, calcium carbonate content, and composition are mapped, coincident trends within the data emerge. Relations between the observed trends and dynamic processes are highly interpretive but provide a first approximation that also suggests tidal currents and storm waves as the major influencing agents.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC