Oceanography and Geology of the Gulf of the Farallones 



Chemical Composition of Surface Sediments on the Sea Floor 



Walter E. Dean and James V. Gardner 



Sediments cover most of the sea floor in 

 the Gulf of the Farallones, with a few 

 areas of exposed bedrock. To help deter- 

 mine the origin and distribution of these 

 sediments, 112 core samples were taken 

 by the U.S. Geological Survey at sites on 

 the sea floor from the shallow shelf down 

 to a water depth of about 10,000 feet. 

 These samples were analyzed for 28 major 

 and trace elements, organic carbon, and 

 calcium carbonate. 



Many factors have affected the history, 

 transport, and distribution of sediments in 

 the Gulf of the Farallones, including the 

 shape of the sea floor, sea-level fluctua- 

 tions, and current patterns. The Continen- 

 tal Shelf in much of the gulf has a low 

 gradient of about 0. 1 degree, and water 

 depth ranges from less than 1 60 feet to 

 about 400 feet. The Continental Slope in 

 much of the gulf has a steeper gradient of 

 about 3 degrees, and water depth reaches 

 about 1 1 ,500 feet at its base. Although the 

 shelf is uncut by channels and canyons, 

 the upper slope is incised with numerous 

 gullies and submarine canyons, including 

 Pioneer Canyon. 



Before 500,000 years ago, the main 

 sources of sediment to the Gulf of 

 the Farallones were the nearby onshore 

 areas with their variety of sedimentary, 



metamorphic, and igneous rocks. About 

 500,000 years ago, drainage from interior 

 California broke through to the Pacific 

 Ocean at the Golden Gate, providing addi- 

 tional sources of sediment from as far 

 away as the Sierra Nevada. The Conti- 

 nental Shelf between the Golden Gate 

 and the Farallon Islands is covered with 

 sandy sediment, which has been repeat- 

 edly reworked by fluctuations in sea level 

 and great (100-year) storms. Major low- 

 erings of sea level during global glacia- 

 tions have exposed the shelf in the gulf as 

 dry land several times, most recently from 

 about 20,000 to 15,000 years ago. In these 

 glacial periods, sea level was lowered by 

 hundreds of feet, because of the large 

 amount of water tied up in ice sheets on 

 land. During these lowstands, a river prob- 

 ably coursed across all of what is now 

 the Continental Shelf in the gulf, although 

 no channel has yet been identified. About 

 15,000 years ago, rising sea level caused 

 by the melting of the ice sheets once again 

 drowned the Continental Shelf. 



Water movement in the Gulf of the Far- 

 allones affects the distribution of sediment 

 on the sea floor and the movement of sedi- 

 ment across the Continental Shelf to the 

 deep sea (see chapter on Current Patterns 

 over the Continental Shelf and Slope). 



The main oceanographic influences on cir- 

 culation in the gulf include the south- 

 ward-flowing California Current during 

 the winter and the northward-flowing Cali- 

 fornia Undercurrent during the summer. 

 Strong offshore currents can also be 

 caused by summer northwesterly winds, 

 leading to coastal upwelling of cold water. 

 Currents generated by tides also help 

 account for the seaward transport of sedi- 

 ment in the gulf. 



The results of chemical analysis of the 

 core samples collected from the sea floor 

 in the Gulf of the Farallones can be used 

 as "fingerprints" to identify sources and 

 transport patterns of sediment (see chapter 

 on Heavy-Mineral Provinces on the Con- 

 tinental Shelf). The sandy sediments on 

 the Continental Shelf between the Golden 

 Gate and the Farallon Islands contain 

 abundant heavy minerals that are rich in 

 iron, magnesium, titanium, phosphorus, 

 and many trace elements. The sediments 

 immediately adjacent to the Farallon 

 Islands contain low concentrations of 

 heavy minerals and the chemical elements 

 associated with them; instead, these sedi- 

 ments are rich in silica (SiO 2 ). Sediments 

 deposited on the Continental Slope have 

 higher contents of organic matter and 

 clay and, consequently, have higher con- 



centrations of elements associated with 

 clay minerals, such as aluminum, lithium, 

 potassium, and sodium. Calcium carbon- 

 ate (CaCO 3 ), mainly from the shells of 

 pelecypods (oysters, clams, and mussels), 

 is a very minor constituent of surface sedi- 

 ments in the gulf. 



26 Oceanography and Geology of the Gulf of the Farallones 



