90 



California Division of Mines and Geology 



Bull. 197 



per foot of depth for the main body and 100,000 tons 

 per foot of depth for the small southern mass. Until 

 average thicknesses are determined for each body, 

 however, total reserves cannot be accurately estimat- 

 ed. Even so, total reserves at the main body are cer- 

 tainly large and may well be 50 million tons or more. 

 The smaller body to the south, if structurally uncom- 

 plicated, would have an average thickness of about 50 

 feet and total reserves of 5 million tons. It is thought 

 that the limestone from the entire deposit might aver- 

 age 90% or more CaCOa and thus may be a future 

 source of cement raw material. Specific reserves of 

 sugar rock are much smaller than total limestone re- 

 serves but cannot be determined without careful drill- 

 ing and sampling. In 1962, the owner reported having 

 a 7- to 10-year supply of limestone blocked out as a 

 result of shallow drilling in the quarry area. 



Other reference: Franke, 1935, p. 425. 



Lopez Canyon deposits. SE corner sec. 36, T. 30 

 S., R. 13 E., M. D., and vicinity, 8 mileseast of San Luis 

 Obispo; San Luis Obispo 1 5-minute quadrangle. Own- 

 ership: Not determined; probably U.S. Forest Service 

 land (1965). 



Limestone and dolomite have been mentioned by 

 several investigators to occur along the course of 

 Lopez Canyon on both the southwest and northeast 

 walls. These carbonate rocks occur as beds, lenses, and 

 concretions commonly associated with siliceous and 

 clay shales of the Monterey Formation of Miocene 

 age. The beds are confined to the lower part of that 

 formation and apparently comprise a stratigraphic in- 

 terval having a ma.ximum thickness of 300 feet (Fair- 

 banks, 1904, p. 4). According to Laizure (1925, p. 523), 

 the main limestone sequence, which is about 100 feet 

 thick and dips 45° SW into the ridge, is prominently 

 exposed in Fern Canyon where it joins Lopez Canyon. 

 Here, the strata consist of dolomitic limestone and 

 limy dolomite which are thinly interbedded with cal- 

 careous, dolomitic, and siliceous shales (Oliver E. 

 Bowen, 1962, oral communication). The carbonate 

 rocks generally are brown to dark gray, bituminous, 

 fine grained, dense, hard, laminated, siliceous, and ar- 

 gillaceous. Weathered surfaces bleach to a light yel- 

 lowish tan or gray. Four samples of typical carbonate 

 rock were collected from the Fern Canyon section 

 (SE cor. sec. 36) by Oliver E. Bowen and analyzed by 

 Pittsburgh Testing Laboratories in 1959. The analy- 

 ses, given below in percent by weight, show the 

 material to be impure dolomite of variable composi- 

 tion. 



Limestone from Lopez Canyon reportedly was used 

 as a source of lime in the 1880.S (Irelan, 1888, p. 532), 



but there is no record of production. In 1924, Peter 

 Rodriguez and S. J. Rhyne located three claims near 

 the mouth of Fern Canyon and erected a small kiln, 

 but only a small amount of lime was burned (Laizure, 

 1^25, p. 522). Because the carbonate deposits of Lopez 

 Canyon are impure and of variable chemistry, they do 

 not appear to be of future interest. 



Other references: \\Aiurf, 1906, p. 80; Franke, 1935, p. 425; Logon, 1947, 

 p. 305. 



Morgan deposit. Location: Sec. 36 T. 32 S., R. 14 

 E., M.D., 5 miles northeast of Nipomo and 8 or 9 miles 

 east of Arroyo Grande; Nipomo 1 5-minute quadran- 

 gle. Ownership: Not determined. 



Dark-gray limestone is exposed in isolated patches 

 30 to 40 feet wide over a considerable distance along 

 the tops of the ridges near Loma Pelona (Aubury, 

 1906, p. 80). The limestone probably is part of the 

 Monterey Formation which underlies the southwest 

 half of sec. 36. Apparently, the deposit was worked to 

 a minor extent around 1900 as a source of lime rock. 

 A kiln is situated in adjacent sec. 35. 



Navajo deposit. S'/j sec. 28 and N'/ sec. 33, T. 29 

 S., R. 16 E., M. D., 20 miles east of Santa Margarita and 

 7 miles northwest of Pozo; Pozo 1 5-minute quadran- 

 gle. Ownership: Trinidad Mining Company (c/o Karl 

 and Feree Pierce, Morro Bay) owns eight unpatented 

 claims (Navajo 1 to 8) (1963). 



The Navajo deposit consists of several westward 

 trending lenses of Sur Series (?) limestone set as pend- 

 ants in granitic rock. Although exploratory and assess- 

 ment work has been fairly extensive, there has been no 

 commercial production. The largest mass is half a mile 

 long by a maximum of 600 feet wide. Its east end lies 

 at the summit of hill 2805 near the SE cor. sec. 28. To 

 the southwest, in N'/j NE'X sec. 33, there is a smaller 

 body about a quarter of a mile long and a maximum 

 of 300 feet wide. A much smaller lens lies due east at 

 the crest of hill 2707, and a few minor patches of 

 limestone are exposed nearby to the west. Internal 

 structures (bedding?) in the larger bodies generally 

 strike N 80° E and dip about 30° N, but there are some 

 variations in attitude. Dikes and sills of decomposed 

 granite pervade the limestone and may amount to 20% 

 of the lenses. 



The limestone is white to blue gray, medium to 

 coarse crystalline, locally siliceous and sometimes 

 blotchy or banded. Chemical analyses of 1 1 samples 

 (table 19) showed an average of 51.82% CaO, 1.73% 

 MgO, and 2.42% Si02. Dolomite rock was not recog- 

 nized in the field but may be present locally. 



Development work, done mainly about 1959, con- 

 sists of extensive cuts, a few test pits, an assessment 

 tunnel, and some shallow stripping. Some shallow 

 drilling is reported to have been done in the early 

 1960s, but there was no activity in May 1963 when the 

 deposit was visited by this writer. Limestone reserves, 

 excluding 20% for granitic dikes, are estimated to be 

 60,000 tons per foot of depth at the main mass and 



