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Pacific Coastal & Marine Science Center

This is an abstract from the 1992 Eastern Pacific Ocean Conference in Mt. Hood, Oregon.

Surficial Grain-size Distributions Across the Continental Margin, Gulf of the Farallones

Herman A. Karl, John L. Chin, and William C. Schwab

Two cruises were conducted in January 1989 on the continental shelf between Cordell Bank and Half Moon Bay east of the Farallon Islands. Reconnaissance side-scan sonar and high-resolution seismic-reflection surveys were conducted along a rectilnear grid of tracks spaced nominally 4 km apart. A regional grid of 97 surficial sediment samples was collected with a Soutar Van Veen sampler at the intersections of the reconnaissance tracklines and at other sites at intervals of nominally 4 km. A denser grid of 171 samples spaced nominally 1 km apart were collected just east of the Farallon Islands. A 20-km wide corridor of sand extends westerly from the Golden Gate to the Farallon Islands. Silty sand and sandy silt bound the corridor to the northwest and southeast and a tongue of silt from the north extends around Pt. Reyes. Increased sampling density reveals an even more complex pattern of sediment texture. Data from the 171 sample grid of stations collected within the tongue of uniformly fine sand based on the regional grid of stations demonstrates that the area actually consists of a complex pattern of mean grain-sizes that range from fine to very coarse sand. This level of sampling density provides data important to interpretation of the depositional processes operating in the Gulf of the Farallones.

During a cruise conducted in summer 1990, thirty-three gravity cores were collected at 23 stations on the continental slope in water depths that ranged from 300 to 3000 m. Recovered cores varied in length from about 1 m to just under 3 m. Most of the surface samples are very sandy which is unusual for sediment on the continental slope; slopes are characterized by silts and clays. A typical stratigraphic sequence consists of three major lithographic units or facies. Unit 1, the uppermost facies, visually appears to be a homogeneous sandy silt. However, radiographs reveal some structure that may be biogenic, but no strong regular laminations. An irregular contact bounds Unit 1 and Unit 2 which is a clayey silt mottled by burrows. The basal facies, Unit 3, is a bioturbated clayey silt. The uppermost stratigraphic facies, Unit 1, represents present-day depositional conditions. This facies occurs at 11 of the 23 core stations and ranges in thickness from 10 to 85 cm. We are uncertain as to how this facies was deposited. We are undecided as to whether this facies represents sediment that accumulated gradually over an unknown period of time or a mass of sediment emplaced during a single rapid event. The mode of emplacement is fundamentally important to the interpretation of the depositional environment that characterizes the upper and middle slope.

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