This is the text portion of U. S. Geological Survey Open-file Report
91-375-A. The appendices are not included.
Station Locations and Grain-Size Analysis of Surficial Sediment
Samples collected on the Continental Shelf, Gulf of the Farallones during
Cruise F2-89-NC, January 1989
N. M. Maher, H. A. Karl, J. L. Chin and W. C. Schwab
INTRODUCTION
The U.S. Geological Survey began a major geologic and oceanographic study
of the Gulf of the Farallones in 1989. This investigation, the first of
several planned adjacent to major population centers, was designed to establish
a good scientific data base on a segment of continental shelf adjacent to
the San Francisco Bay area so as to evaluate and monitor human impact on
the marine environment. Two cruises were conducted in January 1989 on the
continental shelf between Cordell Bank and Half Moon Bay east of the Farallon
Islands (Chin et al., 1989). Reconnaissance side-scan sonar and high-resolution
seismic-reflection surveys were conducted along a rectilinear grid of tracks
spaced nominally 4 km apart. This report describes the collection and analysis
of 268 surficial sediment samples collected during the second cruise, F2-89-NC.
DATA COLLECTION
A regional grid of 97 samples of surficial sediment was collected with a
Soutar Van Veen sampler at the intersections of the reconnaissance tracklines
and, at other sites of interest, at intervals of about 4 km (Figure
1). A denser grid of 171 samples spaced nominally 1 km apart was collected
just east of the Farallon Islands (Figure
1 and Figure
2).
Three systems were used to navigate the ship during the sampling: (1) Global
Positioning System (GPS), (2) LORAN-C, and (3) a shore-based transponder
net. The primary system used for real-time positioning was chosen either
manually by the navigator or automatically by the computer. Positional accuracy
varied from about 100 m using GPS and LORAN-C to a few meters using the
shore-based transponder net.
LABORATORY ANALYSIS
Grain -Size Analysis
The following procedures used to determine grain-size distribution of the
sediment samples were standardized by the U.S. Geological Survey Marine
Geology laboratory, Menlo Park. Similar methods are described in Carver
(1971) and Theide and others (1976).
Oxidation of of organic matter
Five to ten gram portions of sample were placed in a beaker with 200 ml
of distilled water and 5 ml of 30% hydrogen peroxide. The samples were stirred
to fully disaggregate the sample and heated overnight at 70-80° C to
drive off excess hydrogen peroxide following oxidation of organic matter.
Removal of soluble salts
Following oxidation, the samples were washed into plastic 250 ml bottles
and centrifuged at about 1700 rpm for 45 minutes. The water and soluble
salts were decanted, replaced with distilled water, centrifuged a second
time, and again decanted.
Size segregation
After removal of soluble salts, the samples were wet sieved with distilled
water into three size fractions: Gravel (>2 mm), sand (63 
- 2 mm), and mud (silt + clay) (<63 ). The gravel and sand fractions were transferred to pre-weighed
evaporating dishes, dried, and reweighed. The weight of the sample was then
calculated by subtracting the weight of the dish. The silt + clay fractions
were transferred into graduated cylinders and filled to the 1 liter mark
with distilled water. Five ml of 10% sodium hexametaphosphate solution was
added to disperse clay and clay size particles and prevent flocculation.
Determining the weight and size distribution of the silt + clay (<63
)
fraction
A representative 20 ml aliquot was pipetted from each 1 liter graduated
cylinder, transferred to a preweighed aluminum weighing boat, oven dried
and reweighed. The total weight of the <63 fraction was calculated by
multiplying that weight by 50 and subtracting out the sodium hexametaphosphate
weight. Weight percents for half-phi intervals from 4.0 to 9.0 
(phi
sizes are converted to millimeters as explained below; see also Table
1) were determined using a hydrophotometer (Jordan et al., 1977).
Determining the size distribution of the sand fraction
A Rapid Sediment Analyzer (RSA), commonly known as a settling tube, was
used to measure weight percents for half-phi intervals from -1.0 to 4.5
phi. In the RSA, the sand grains are released at the top of a 2 m water
column and settle to a weighing platform . The cumulative weight vs time
is recorded on a chart recorder from which the size distribution can be
measured using a calibrated overlay. Theory, equipment, and techniques employed
are described in detail by Theide and others (1976).
Calculation of grain-size statistics and parameters
Statistics of the grain-size distribution were computer calculated with
a U.S. Geological Survey Marine Geology grain-size program (McHendrie 1988).
Grain-size statistical parameters and graphic representations are given
in phi units. The phi unit () is a logarithmic transformation of millimeters into whole
integers, according to the formula:
where d = grain diameter in millimeters.
The parameters calculated for these analyses include:
Method of Moments
All of the above statistical parameters can be calculated using the method
of moments. This method gives a more rigorous treatment of the sediment
characteristics. The computer program used for the sample analyses in this
study performed the necessary calculations for parameter determination.
The first moment measure corresponds to the mean, the second to the standard
deviation, the third to the skewness, and the fourth to the kurtosis.
All data derived from these analysis are shown in Appendix I and II. Grain-size
parameters calculated using the method of moments and graphically derived
values are both reported.
PRELIMINARY RESULTS AND INTERPRETATION
The distribution of surficial sediment textures based on the regional grid
of 97 stations suggests that depositional processes in the Gulf of the Farallones
are complex. A 20-km wide corridor of sand extends westerly from the Golden
Gate to the Farallon Islands (Figure
3). Sediment textures in fig. 3 are grouped according to the classification
of Shepard (1954). 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
(Figure 3).
More detailed analysis of the sediment texture reveals a slightly more complex
regional distribution as shown, for example, by a plot of mean grain size
(Figure 4).
The increased complexity is well illustrated by examining the cross-shelf
corridor of sand defined in Figure 3. Plotting the mean grain size at 1-phi
intervals shows that patches of medium and coarse sand exist within a field
of fine sand and that sediment texture becomes coarser closer to the Farallon
Islands (Figure
4). Increased sampling density reveals an even more complex pattern
of sediment texture. Note the area of dense stations (171 spaced 1 km apart)
on Figure
1 and Figure
2. Sediment in this area, based solely on analysis of samples from the
regional grid of stations, is uniformly fine sand (Figure
4). However, data from the 171 sample grid of stations (also plotted
at a 1-phi interval) shows that the area actually consists of a complex
pattern of mean grain sizes that range from fine to very coarse sand (Figure
5). This level of sampling density provides data important to interpretation
of the modern day depositional and oceanographic processes operating in
the Gulf of the Farallones.
REFERENCES
Carver, R. E., 1971, Procedures in Sedimentary Petrography: New York, John
Wiley and Sons, 653 p.
Chin, J.L, Rubin, D.M., Karl, H.A., Schwab, W.C., and Twichell, D.C., 1989,
Cruise report for the Gulf of the Farallones cruise, F1-89-NC, F2-89-NC
off the San Francisco Bay Area, January 6 through 28, 1989: U.S. Geological
Survey Open-file Report 89-317, 4. p.
Folk, R. L., 1968, Petrology of Sedimentary Rocks: Austin, University of
Texas Publication, 170 p.
Inman, D. L., 1952, Measures for describing the size distribution of sediments:
Jour.Sedimentary Petrology, v. 22, #3, p. 125 - 145.
Jordan, F.J., Jr., Fryer, G.E., and Elze, H.H., 1971, Size analysis of silt
and clay by hydrophotometer: Jour. Sedimentary Petrology, v. 41, p. 489-496
McHendrie, G., 1988, sdsz - A Program for Sediment Size Analysis: U.S. Geological
Survey, Branch of Pacific Marine Geology, Menlo Park, CA.
Shepard, F. P., 1954, Nomenclature based on sand-silt-clay ratios: Jour.
Sedimentary Petrology, v. 24, p. 151-158.
Thiede, J., Chriss, T., Clauson, M., and Swift, S.A., 1976, Settling tubes
for size analysis of fine and coarse fractions of oceanic sediments: School
of Oceanography, Oregon State University, Reference 76-8, 87 p.
Trask, P. D., 1950, Dynamics of sedimentation, in Trask, P. D., ed., Applied
Sedimentation: New York, John Wiley and Sons, p. 3-40.
http://walrus.wr.usgs.gov/reports/ofr91-375/
maintained by Molly Gowen Groome
was last modified May 2, 2000.
Department of the Interior, U.S. Geological Survey, Western Region Coastal and
Marine Geology
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