18 



California Division of Mines and Geology 



Bull. 197 



carbonate material was selectively mined, crushed, 

 screened, and sold as terrazzo under the names 

 "Golden Travertino" (travertine) and "California 

 Onyx". Reserves of the two materials are not known 

 although the travertine appears to be more abundant 

 than the onyx. 



Other references: "HaUi, 1890, p. 668; Waring, 1915, p. 162-163, 165; 

 Laiiure, 1927, p. 210; Logon, 1947, p. 332. 



Tomales Bay Shell deposit. Location: Approx. 

 sec. 5 (proj.), T. 3 N., R. 9 W., M.D., 2 miles north- 

 northwest of Inverness; Inverness IS-minute quadran- 

 gle. Ownership: Not determined. 



This undeveloped shell deposit is described here to 

 demonstrate that sizable accumulations of shells exist 

 in coastal bays other than San Francisco Bay (see un- 

 der San Francisco Bay district) and that different ex- 

 ploration methods can be used in combination to 

 delineate submerged shell deposits. 



According to C. C. Daetwyler (unpublished report, 

 196.'i, p. .^6, 184), a shoal area on the northeast side of 

 Tomales Bay is covered with Holocene shells and as- 

 sociated mud at least 7.8 feet thick. The shells are 

 shown to cover an irregular area of more than 100 

 acres with maximum dimensions of about 4000-by- 

 2000 feet. By combining an acoustic-reflection survey 

 (Sonoprobe) with shallow cores and test borings, Da- 

 etwyler (p. 48, 56, 67, 75, 184) was able to determine 

 the surface distribution of the shell accumulation. A 

 description of test boring No. 13 (Daetwyler, p. 184), 

 made in 13 feet of water east of Pebble Beach, shows 

 shells (predominantly Ostrea lurida Carpenter) in a 

 silt and clay matrix, with scattered pebbles and angu- 

 lar quartz grains to a depth of 7.8 feet below the 

 present bay bottom. Although 5 to 30% shells are in- 

 dicated to be present on the bay bottom (Daetwyler, 

 p. 56), the average thickness and percentage of shells 

 present for the entire deposit is unknown. Based on 

 this limited information, roughly 100,000 to 500,000 

 tons of shell are believed to be present in this shallow 

 deposit. 



Using similar exploration methods, Daetwyler (p. 

 112, 180) also found other shell deposits 1 to 2 miles 

 to the northwest at the base of an older bay mud se- 

 quence. Because these older deposits are small and 

 covered by 34 feet of mud, they are not of commercial 

 interest. However, the younger shell deposit may be 

 of economic value because of its shallow depth and the 

 low cost of dredging and washing. On the other hand, 

 various other economic and political factors may tend 

 to conflict with dredging activities. 



Other reference; Eckel, 1933, p. 359. 



of San Joaquin and Santa Clara Counties. There is a 

 wide variety of limestone and shell deposits in the 

 district, but no dolomite is known. Because the district 

 encompasses a highly populated and industrialized 

 area, virtually all carbonate occurrences have been 

 examined; and many, including some very small or 

 impure ones, have been worked as sources of lime- 

 stone. 



As many as 14 or 15 deposits have been worked for 

 limestone and shells beginning about 1851. Since then, 

 large amounts of limestone and shells have been pro- 

 duced for cement (San Francisco Bay Shell deposits. 

 Lime Ridge deposits) and for crushed rock uses 

 (Rockaway deposits. Skyline deposits). Limestone 

 and shells have also been used for a variety of other 

 purposes, including lime manufacture, livestock feed, 

 soil conditioning, and decorative uses. In 1968, the San 

 Francisco Bay Shell deposits and the Rockaway depos- 

 its were still being used extensively as sources for 

 cement, crushed rock, livestock feed, and soil condi- 

 tioning. 



The carbonate materials are widely distributed in 

 the San Francisco Bay district in a variety of forms. 

 Most important, in terms of both past and future re- 

 sources, are the shell deposits of southern San Fran- 

 cisco Bay and the thin-bedded chert-associated 

 Cretaceous limestone of the Franciscan Formation in 

 San Mateo County. 



The shell deposits, which are largely associated 

 with soft bay mud, reflect population explosions of the 

 native oyster, Ostrea lurida Carpenter, during Holo- 

 cene time. Although impure, these deposits contain 

 extensive reserves at a shallow depth and are easy to 

 dredge. Other deposits may exist outside of the south- 

 ern arm of the bay, but none of commercial value is 

 known. 



The thin-bedded limestone of the Franciscan For- 

 mation — commonly referred to as Calera limestone — 

 is also important. Deposits of this relatively impure 

 limestone (usually 20 to 50% chert) are distributed 

 discontinuously along a narrow belt for 11 miles 

 southeast of Rockaway in Pacifica (Walker, 1950b; 

 Darrow, 1963, plate 1). Of at least nine quarries, Rock- 

 away was the only active deposit in 1968. Unless the 

 deposits can be beneficiated, future development will 

 depend on construction and other crushed rock needs. 



Quaternary deposits of travertine, calcareous tufa, 

 and other surficial materials have been important in 

 the past (Lime Ridge deposits) but only relatively 

 small reserves remain. Other limestone deposits of 

 pre-Cretaceous to ([Quaternary age are either too small 

 or too impure to be of future commercial use, except 

 as local sources of crushed rock. 



SAN FRANCISCO BAY DISTRICT (B-3) 



This district encompasses the deposits in and about 

 San Francisco Bay in Alameda, Contra (>)Sta, San 

 Francisco, and San Mateo Counties and adjacent parts 



Cahill Ridge deposits. Location: Sees. 2 and 3, T. 

 5 S., R. 5 W., and sees. 33 and 34, T. 4 S., R. 5 W., M.D., 

 4 miles southwest of San Mateo; Montara Mountain 

 7'/2-niinute quadrangle. Ownership: C^ity and C^ounty 

 of San Francisco Water Department (1962). 



