1978 



Limestone in hie Coast Ranges 



23 



Antioch for testing as a source of portland cement 

 material (Crawford, 1894, p. }H0) . It has been de- 

 scribed as "an amorphous, compact, bluish-gray, fos- 

 siliferous limestone" by Aubury (1906, p. 67). The 

 limestone is probably part of a sequence of Upper 

 Cretaceous rocks in \\ hich thin limestone beds, con- 

 cretions, and shell horizons arc not uncommon (Col- 

 burn, 1961, p. 11 and plate I). 



Orinda deposit. Location: NE'/ sec. l."* and adja- 

 cent parts of sees. 10 and 14, T. 1 S., R. 3 W., M.D., 2 

 miles south-southeast of Orinda; (Concord 15-minute 

 quadrangle. Ownership: Not determined (1962). 



A large, northwest-trending body of impure lime- 

 stone .^00 feet wide by half a mile long is reported 

 between the Siesta and Moraga Formations of Plio- 

 cene age (Lawson, 1914, p. 22, and Concord map). 

 Where examined in NE'/^NEy, sec. \5, the carbonate 

 unit is exposed only as scattered outcrops and proba- 

 bly is interbedded with softer noncarbonate beds. The 

 carbonate rock is pale buff to tan, hard, dense, fine 

 grained, dolomitic, siliceous and locally brecciated, 

 and recemented u ith calcite. It is likely that the lime- 

 stone was deposited as a tuffaceous or marly lake bed. 

 The deposit is undeveloped and appears to be useful 

 only as a local source of crushed rock. 



Lawson (1914) also shows several small lenses of 

 Siesta and Moraga limestone half a mile to the north 

 in sec. 10 and to the northwest in sees. 5 and 6. 



Patterson Pass deposit. Location: Sec. 10, T. 3 S., 

 R. 3 E., M.D., 7'; miles east of Livermore; Altamont 

 7'/2-minute quadrangle. Ownership: Not determined. 



An unimportant travertine prospect of apparent 

 hot spring origin is reported in the center of sec. 3 just 

 north of the Patterson Pass road (Huey, 1948, p. 62). 



Picardo Ranch deposits. Location: S'/j sec. 12, T. 

 4 S., R. 6 W. (proj.), M.D., I'/j to 2 miles southeast of 

 Rockauav Beach, Pacifica; San Mateo 1. ^-minute 

 quadrangle. Ownership: Louis Picardo, Pacifica 

 (1962). 



Several lenses of Franciscan limestone form a group 

 of deposits that extends two-thirds of a mile north- 

 westerly from the North Fork of San Pedro Creek 

 (Darrow, 1963, plate 1). The largest of these, at the 

 southeast end of the group, is shown to be 800 feet 

 long by 250 to 300 feet wide and is exposed through 

 200 feet of relief. This body, which was not examined, 

 may contain 1 to 2 million tons of limestone and as- 

 sociated rock. 



Just over the ridge to the west is a triangular-shaped 

 body 500 feet long with a maximum width of 300 feet 

 and a relief of 200 feet. The limestone is typical of that 

 of the Franciscan Formation, consisting of thinbed- 

 ded, fractured, dark-bluc-gra\' limestone and interbed- 

 ded chert, with lesser amounts of light-dove-gray 

 limestone. Reserves probably are less than half a mil- 

 lion tons. Development of this deposit consists of a 



small hilltop quarry from which fractured rock was 

 obtained around 1960-61 for use as road base and 

 drain rock in the construction of the nearby high 

 school. The amount of material quarried is unknown. 

 The quarrv was idle in May 1962. 



Several other small limestone deposits are found 

 nearby one of which may be a westward extension of 

 the developed deposit. There are no chemical analyses 

 or other information on the Picardo Ranch deposits, 

 and only the one quarry is known. Total reserves may 

 be 3 to 4 million tons. Considering the presence of 

 chert with the limestone, it seems likely these deposits 

 will be of value principally as local sources of crushed 

 rock. 



Pilarcitos Creek deposits. Location: W'/j sec. 11, 

 T. 5 S., R. 5 VV., M.D., 4 miles northeast of Half Moon 

 Bay (town); San Mateo 15-minute quadrangle. Own- 

 ership: Not determined (1962). 



Typical gray limestone with interbedded chert of 

 the Franciscan Formation is exposed in SW/NW'/ 

 sec. 1 1 on the northeast bank of Pilarcitos Creek. The 

 dimensions of the body are concealed by dense vegeta- 

 tion. However, the Pilarcitos fault, whose trace ap- 

 proximately coincides with the southeast-flowing 

 creek, probably limits the deposit to the southwest. 

 The deposit is developed by a small quarry from 

 which crushed rock reportedly was obtained more 

 than 20 vears ago for local road use. 



Less than half a mile to the south, Pilarcitos Creek 

 veers southv^ estu ard and crosses scattered outcrops of 

 impure crystalline limestone similar to the Sur Series. 

 Although Lawson (1914, map) shows the limestone as 

 a single northviest-trending mass half a mile long on 

 the southwest side of the Pilarcitos fault, he (Lawson, 

 p. 4 and 22) indicates the deposit to be impure (mag- 

 nesia, silica) and small ("limited to a few acres"). 

 Heavy vegetation covers much of the deposit area. 

 Small crystalline limestone deposits of similar type 

 are also reported along the ridge top 1 Vi miles to the 

 west-northwest (Elmo Adams, 1962, oral communica- 

 tion). The limestone outcrops southwest of the fault 

 probably represent small pendants in quartz diorite. 

 Thev are probablv too small and impure to be of eco- 

 nomic value. 



Pleasanton (Pleasanton Lime and Cement Com- 

 pany) deposit. Location: N\V'/^ .sec. 4 and NE'/^ sec. 

 5, T. 4 S., R. 1 E. (proj.), M.D., 2'/; miles south of 

 Pleasanton; Livermore 1 5-minute quadrangle. Owner- 

 ship: Herman Koopman, Route 1, Box 198, Pleasanton 

 (1962). 



Small deposits of grayish-tan, well-indurated, sandy 

 coquina lenses interbedded w ith fossiliferous, massive 

 sandstone of the Briones formation (Miocene) are 

 boldly exposed for half a mile southeast of the railroad 

 cut near the north boundary of sec. 5. The resistant 

 sandstone-coquina unit is several hundred feet thick 

 and dips 70° to 80° SW. The coquina occurs in this unit 

 as scattered lenses as much as 40 feet thick. \'ery local- 



