40 



California Division of Mines and Geology 



Bull. 197 



the deposits has been verified to some extent by drill- 

 ing and trenching in the vicinity of Black Mountain. 

 Little exploration has been done elsewhere. Consider- 

 ing the relatively small size of the deposits and the 

 presence of chert interbeds, it is probable that the 

 limestone will be useful only for crushed rock pur- 

 poses. 



Other reference: Logon, 1947, p. 312, 317. 



Permanente (Black Mountain) deposit. Loca- 

 tion: W'/, sec. ITandE'/jSec. 18,T. 7 S., R. 2 W., M. D., 

 4 miles south of Los Altos; Palo Alto 1 5-minute quad- 

 rangle. Ownership: Kaiser Cement and Gypsum Cor- 

 poration, 300 Lakeside Drive, Oakland (196.^). 



The Permanente deposit is situated mainly on a 

 steep slope on the north side of Permanente Creek, 1 '/, 

 miles east of Black Mountain. It is by far the largest 

 limestone body known in the Franciscan Formation 

 and may contain the largest reserves of any limestone 

 deposit in the central Coast Ranges. Initial develop- 

 ment of this deposit apparently began around 1900, 

 when El Dorado Sugar Company selectively quarried 

 high-grade limestone for use in beet-sugar refining 

 (Aubury, 1906, p. 82). Later, Alameda Sugar Com- 

 pany quarried limestone which was shipped to Alviso, 

 where it was calcined for use in sugar refining 

 (Huguenin and Costello, 1920, p. X'iS) . Santa Clara 

 Holding Company acquired the deposit during the 

 1920s, but records indicate that they produced lime- 

 stone (for lime) only in 1934. Large scale production 

 of limestone began in 1939 when Permanente Cement 

 Company established a plant to supply 6,800,000 bar- 

 rels of cement for the construction of Shasta Dam. 

 Expansion over the years has increased plant capacity 

 from 2'/, million to 8/2 million barrels of cement per 

 year. Production of limestone and associated materials 

 has increased to more than 2 million tons per year. 

 Most of this has gone to make cement, but significant 

 amounts of overburden and low-grade limestone are 

 sold commercially for aggregate and road construc- 

 tion uses. Until 1951, the company also sold high- 

 grade limestone for use in beet-sugar refining. In- 

 creasing percentages of low-grade limestone encoun- 

 tered in recent years stimulated Permanente Cement 

 Company to explore ways to beneficiate the lime- 

 stone. They completed a froth flotation plant in 1962; 

 and, according to Kleiber and Meisel (1964), this 

 plant was operating successfully by 1963. In 1964, the 

 company changed its name to Kaiser Cement and 

 Gypsum Corporation. 



The limestone deposit at Permanente Creek covers 

 an irregular triangular area having a maximum length 

 of a mile and a maximum width of two-thirds of a mile 

 (fig. 3). It is exposed through 800 feet of relief and 

 may be as much as 700 feet thick in places (Kleiber and 

 Meisel, 1964, p. 39). The limestone body, which con- 

 tains early Late Cretaceous microfossils, is associated 

 with altered volcanic rocks (greenstone) and sedi- 

 mentary rocks of the Franciscan P'ormation. Structur- 



ally the limestone body is complicated by faults and 

 folds, but mainly it dips 2.S° to 35° SE. Moderate 

 northeast dips in the south part of the mass indicate 

 the deposit to be an eastward plunging syncline whose 

 axial trace lies just north of Permanente Creek. 



Internally, the deposit consists of thin beds of light 

 and dark limestone which tend to occur in mutually 

 exclusive sequences. Thin layers and lenses of chert 

 are interbedded in varying amounts with both types 

 of limestone. A thick sill or flow of volcanic rock, now 

 altered to greenstone, and a few thin tuff beds also are 

 present in the quarry area. The stratigraphic relations 

 of these different beds and sequences is not clearly 

 understood, as extensive faulting has severely disrupt- 

 ed the deposit. G. W. Walker (1950b) interpreted the 

 stratigraphic sequence to consist of an "upper light" 

 limestone unit, a "blue" limestone unit, and a "lower 

 light" limestone unit. A more recent interpretation, 

 based on extensive drill data and chemical analyses, 

 was given by Donald Towse of Kaiser Cement and 

 Gypsum Corporation in a paper presented at the 

 American Institute of Mining, Metallurgical, and Pe- 

 troleum Engineers, Southwest Mineral Industry Con- 

 ference, Las N'egas, Nevada, April 24, 1961. According 

 to Towse, the deposit consists of only two limestone 

 units — an upper light-colored unit at least 300 feet 

 thick and a lower dark gray unit at least 200 feet thick. 

 The light limestone is split near its middle by a 90-foot 

 thick greenstone sill or flow. These units may be 

 sliced by two or three thrust faults, causing wide- 

 spread repetition and omission of strata. Further com- 

 plexities resulted from later folding and high-angle 

 faulting. 



The light limestone is typically light gray to dove 

 gray, fine grained, dense, foraminiferal, and well bed- 

 ded. Light and dark interlayers of chert, 1 to 3 inches 

 thick, constitute 10 to 50% of the light unit, being 

 more prevalent near the base. The dark limestone is 

 blue gray to dark gray but weathers and bleaches to 

 lighter shades of gray. It is very fine to medium crys- 

 talline, bituminous, and well bedded to platy. Black 

 chert interbeds are common but not as abundant as in 

 the light limestone. Both types of limestone are 

 strongly fractured. 



The chemical grade of the limestone is quite varia- 

 ble, with high and low grades occurring in both the 

 light and dark units. The variations in grade are re- 

 flected to some extent in the bulk analyses presented 

 in table 8. According to Towse, the dark limestone 

 averages 87% CaCOs and the light limestone 67% 

 CaCOa, the upper part of the light limestone being the 

 best quality (also see Bailey, Irwin, and Jones, 1964, p. 

 70). For purposes of quarry development, three quali- 

 ties of limestone are recognized by Kaiser Cement and 

 Gypsum Corporation (Kleiber and Meisel, 1964, p. 

 39): 1) high grade — consists of dark limestone and 

 averages 87 to 88% CaCOi; 2) medium grade — a mix- 

 ture of light and dark limestone running 70 to 82% 

 CaC'Oj; and 3) low grade — mainly extremely cherty, 

 light-colored limestone containing 55 to70%CaCO3. 



