123"00 



12230' 



Width of image is 4 millimeters. 



Width of image is 2.25 millimeters. 



Length of bar is 0.5 millimeters. 



38 ; 00'N 



Photographs taken through microscopes of samples of 

 bottom sediments collected from the Gulf of the Faral- 

 lones (1 millimeter equals 0.04 inch). A, Sample consisting 

 mostly of medium-grained shell material. B, Sample con- 

 sisting of "light" (less than 2.96 times as heavy as water) 

 mineral grains; most light-colored grains are of the miner- 

 als quartz or feldspar; dark grains are glauconite, a clay 

 mineral. C, Grains of two "heavy" minerals nypersthene 

 (left) and hornblende (right) mounted on a glass slide for 

 examination under a microscope in polarized light. D, The 

 same grains photographed in cross-polarized light. The 

 characteristics of the grains under each type of illumina- 

 tion were used to identify the minerals. 



8ATHYMET HIC IDEPTHI CONTOUR INTERVAL 10 METERS OOWf, 



TO 100-METER DEPTH 



li METER = 3.281 FEET; 



Map of the Gulf of the Farallones. showing sites where the U.S. Geological Survey collected bottom-sediment samples for mineralogical analysis. Grains of fine and 

 very fine sand (about 0.002 to 0.01 inch in diameter) were separated and then divided into "light" minerals (less than 2.96 times as heavy as water) and "heavy" 

 minerals (more than 2.96 times as heavy as water). Numbers on the map show the abundance of heavy minerals as a percent of sample weight, and the sizes of the 

 shaded boxes are proportional to this abundance. The thick blue line marks the approximate boundary between areas covered by different heavy-mineral assemblages. 

 The mineralogy and geochemistry of sediments from the Gulf of the Farallones define two main depositional provinces one composed of granitic debris and one that 

 indicates a source in the Franciscan terranes of northern California The granitic sediment comes from two sources. The dominant source of granitic sediment is the 

 Sierra Nevada. The sediment from this source was transported down the Sacramento-San Joaquin drainage and through San Francisco Bay and the Golden Gate. 

 The lesser source of granitic sediment is the basement rocks of the Salinian terranes west of the San Andreas Fault system, including the Farallon Islands. Minerals 

 derived from the Franciscan terranes characterize surface sediment on the Continental Shelf in the northern Gulf of the Farallones. These minerals represent reworked 

 Continental Shelf deposits, erosion of dune deposits accumulated during the last lowstand of sea level, and coastal erosion of Franciscan bedrock. 



Heavy-Mineral Provinces on the Continental Shelf 29 



