Oceanography and Geology of the Gulf of the Farallones 



Heavy-Mineral Provinces on the Continental Shelf 



Florence L. Wong 



Minerals are integral to every aspect of 

 our lives from esthetically pleasing gem 

 stones to more mundane but essential 

 components of concrete. In geology, min- 

 erals are useful in unraveling certain 

 questions, such as where do large bodies 

 of sediment come from (source), how do 

 they move around (transport processes), 

 and where do they settle (distribution). 

 This chapter looks at these geologic 

 issues in the Gulf of the Farallones 

 by examining small (sand-size) mineral 

 grains of high specific gravity ("heavy 

 minerals" in this study, minerals at 

 least 2.96 times as dense as water). Small 

 grains are apt to be transported great dis- 

 tances by natural processes. In contrast, 

 large mineral grains are not transported 

 far before coming to rest in a depositional 

 basin and thus are not as diagnostic as 

 smaller grains. Therefore, small particles 

 of heavy minerals are good clues to sedi- 

 ment source, transport, and distribution. 

 The Gulf of the Farallones extends 

 from Point Reyes on the north to Ano 

 Nuevo on the south and from the Golden 

 Gate westward across the Continental 

 Shelf to the Continental Slope beyond 

 the Farallon Islands-Cordell Bank ridge. 

 Water depths on the shelf are generally 

 less than 330 feet. Most of the gulf lies 



west of the major tectonic feature known 

 as the San Andreas Fault Zone, which 

 separates two different types of basement 

 rock: granitic rocks of the Salinian block 

 on the west, and various sedimentary, vol- 

 canic, and metamorphic rocks of the Fran- 

 ciscan terranes to the east (see chapter on 

 Earthquakes, Faults, and Tectonics). 



The surficial sediment in the Gulf of 

 the Farallones was initially eroded from 

 rocks onshore and then transported and 

 deposited in the present offshore basin. 

 The agents of this process included fault- 

 ing and folding of the Earth's crust and 

 changes in global climate, including gla- 

 ciation. The sediment of the sea floor in 

 the gulf (see chapter on Sediment of the 

 Sea Floor) is composed of sand, mud, 

 some gravel, and biologic debris, such as 

 shells and bone fragments. 



Sediment on the Continental Shelf in 

 the Gulf of the Farallones west of San 

 Francisco Bay was systematically sam- 

 pled by the U.S. Geological Survey in 

 1989 to study its characteristics, distribu- 

 tion, and origin. Various properties of the 

 samples were analyzed, including grain 

 size and mineralogic and chemical com- 

 position (see chapter on Chemical Com- 

 position of Surface Sediments on the Sea 

 Floor). The minerals present in the sam- 



ples reflect two major sources in the 

 central California region: (1) the large 

 variety of rock types of the Franciscan 

 terranes and (2) granitic materials shed 

 from the Sierra Nevada and similar rocks 

 in the central California region. The two 

 mineralogical assemblages are distrib- 

 uted in different areas of the shelf; how 

 this distribution developed is less clear. 

 The mineral and chemical composi- 

 tion of sediments from the Gulf of the 

 Farallones define four depositional prov- 

 inces two that are composed of granitic 

 debris and two that reflect sources in the 

 Franciscan terranes of northern Califor- 

 nia. The granitic sediment comes from 

 two sources. The dominant source is the 

 Sierra Nevada. The sediment from this 

 source was transported down the Sacra- 

 mento-San Joaquin drainage and through 

 San Francisco Bay and the Golden Gate. 

 The lesser source is the granitic base- 

 ment rocks of the Salinian block west of 

 the San Andreas Fault system. Sediment 

 from a Sierran source is spread over the 

 Gulf of the Farallones to a limited extent 

 north of the Golden Gate and to at 

 least the shelf edge south of the Gate. 

 A small contribution of sediment from 

 Salinian rocks of the Farallon Islands- 

 Cordell Bank ridge, along the west edge 



of the shelf, is also found on the shelf 

 near the ridge. 



Minerals derived from the Franciscan 

 terranes characterize surface sediment on 

 the Continental Shelf in the northern 

 Gulf of the Farallones. These minerals 

 represent reworked shelf deposits, ero- 

 sion of dune deposits accumulated 

 during the last low stand of sea level, 

 and coastal erosion of Franciscan bed- 

 rock exposures. 



28 Oceanography and Geology of the Gulf of the Farallones 



