Ch. 2— Resource Assessments and Expectations • 57 



ment (deep) rocks include metasediments, schist, 

 andesites, and dacites. Thick sequences of Tertiary 

 sediments were deposited in deep marine basins 

 throughout the region. The shelf is fairly narrow 

 (3 to 12 miles) and is transected by several subma- 

 rine canyons extending to the edge of the shelf. 

 From Point Conception north along the moun- 

 tainous coast to Monterey Bay, the shelf is quite 

 narrow in places, but north of San Francisco to 

 Cape Mendocino it widens again to 6 to 25 miles. 

 The coast in this area is generally rugged with a 

 few lowland areas along river valleys. Wave energy 

 is high along the entire coast and uplifted wave- 

 cut terraces indicating former higher stands of sea 

 level are common. 



Northward along the coast of Oregon, the con- 

 tinental shelf is as narrow as 6 miles and averages 

 less than 18 miles in width. Off Washington, the 

 shelf gradually widens to over 30 miles and is un- 

 derlain by a varied terrain of sedimentary rocks, 

 mafic and ultramafic intrusives, and granite rocks. 

 The Washington coast also has been influenced by 

 glaciation, and glacial till and alluvium extend out 

 onto the shelf. The Columbia River is a major 

 source of sediment in the southern Washington and 

 northern Oregon region. Beyond the shelf, but 

 within the U.S. EEZ, the seafloor spreading centers 

 of the Gorda and Juan de Fuca ridges and related 

 subduction zones at the base of the continental slope 

 contribute to the tectonic activity of the region. 



Sand and Gravel 



The narrow continental shelf and high wave 

 energy along the Pacific coast limit the prospects 

 for recovering a great abundance of sand and gravel 

 from surficial deposits. In southern California, de- 

 posits of sand and gravel at water depths shallow 

 enough to be economic are present on the San 

 Pedro, San Diego, and Santa Monica shelves. Most 

 coarse material suitable for construction aggregate 

 is found in relict blanket, deltaic, and channel de- 

 posits off the mouth of major rivers. One deposit 

 of coarse sand and gravel within 10 miles of San 

 Diego Bay in less than 65 feet of water has been 

 surveyed and estimated to contain 26 million cu- 

 bic yards of aggregate. Total resource estimates for 

 the southern California region indicate about 40 

 billion cubic yards of sand and gravel. ^^ However, 



"Williams, "Sand and Gravel Deposits Within the U.S. Exclusive 

 Economic Zone," p. 382. 



excessive amounts of overlying fine sand or mud, 

 high wave energy, and unfavorable water depth 

 may all reduce the economically recoverable ma- 

 terial by as much as an order of magnitude. Indi- 

 vidual deposits would need to be studied for their 

 size, quality, and accessibility. 



Sand and gravel resource estimates for northern 

 California are based primarily on surface informa- 

 tion with little or no data on depth and variability 

 of the deposits. As is typical elsewhere, the sand 

 and gravel deposits are both relict and recent. Much 

 of the relict material appears to be too coarse to 

 have been deposited by transport mechanisms oper- 

 ative at the present depth of the outer continental 

 shelf. ^° These relict sands are thought to be near- 

 shore bars and beach deposits formed during lower 

 stands of sea level in the Pleistocene. Recent coarse 

 material is nearer the coast and generally depos- 

 ited parallel to the coastline by longshore currents. 

 Sand and gravel estimates for the northern Cali- 

 fornia shelf, assuming an average thickness of about 

 1 yard, are 84 million cubic yards of gravel, 542 

 million cubic yards of coarse sand, and 2.6 billion 

 cubic yards of medium sand.^' Most of this mate- 

 rial would lie in State waters. 



Off the coast of Oregon and Washington, sea 

 level fluctuations and glaciation controlled the loca- 

 tion of coarse sand and gravel deposits. Most of 

 the gravel lies to the north off Washington, where 

 it was deposited in broad outwash fans by glacial 

 meltwater streams when the sea level was about 650 

 feet lower than present. Promising gravel resource 

 areas convenient to both Portland and Seattle are 

 off Gray's Harbor, Washington, and the southern 

 Olympic Mountains. Smaller gravel deposits off 

 Oregon lie in swales between submarine banks in 

 relict reworked beach deposits. Little data on the 

 thickness of individual deposits are available, but 

 general information on the thickness of outwash and 

 beach sediments in the area suggest that estimates 

 of 3 to 15 feet average thickness are reasonable. ^^ 



'"S.G. Martindale and H.D. Hess, "Resource Assessment: Sand, 

 Gravel, and Shell Deposits on the Continental Shelf of Northern and 

 Central California," Program Feasibility Document — OCS Hard 

 Minerals Leasing, app. 9, U.S. Department of the Interior, 1979, p. 5. 



"Ibid., p. 7. 



^^G.W. Moore and M.D. Luken, "Offshore Sand and Gravel Re- 

 sources of the Pacific Northwest," Program Feasibility Document — 

 OCS Hard Minerals Leasing, app. 7, U.S. Department of the In- 

 terior, 1979, p. 8. 



72-672 0-87—3 



