84 



California Division of Mines and Geology 



Bull. 197 



terey Formation and its lower to middle Miocene 

 equivalents. The carbonate sequence is well developed 

 in the southern part of the district, one sequence being 

 300 feet thick in Lopez Canyon. However, the carbon- 

 ate beds are impure, consisting of siliceous, limy dolo- 

 mite and dolomitic limestone interbedded with 

 calcareous (foraminiferal) and dolomitic shale (see 

 Lopez Canyon deposits). Impure calcareous and do- 

 lomitic beds in thick lower to middle Miocene se- 

 quences also have been mapped to the southeast in the 

 Nipomo 15-minute quadrangle (Hall and Corbato, 

 1967) and to the northwest in the San Luis Obispo 

 15-minute quadrangle by this writer (unpublished). 

 E.xcept for the hard, often blocky dolomite, which is 

 of value mainly for crushed rock purposes, these im- 

 pure deposits probably offer little economic potential. 



Younger deposits of marl and impure limestone ap- 

 parently have been used locally as sources of lime but 

 are no longer of commercial consideration. The wide- 

 spread Jurassic and Cretaceous formations are not 

 known to contain important amounts of limestone in 

 this part of the Coast Ranges, although veins of coarse 

 crystalline limestone cut these formations (Tassajara 

 deposit). 



At least eight deposits have been productive, but 

 only the Lime Mountain deposit has been worked in 

 the last 30 years or more. Prior to that, beginning in 

 the 1880s, si.x of the deposits were worked locally as 

 sources of lime rock and one (Kesseler) provided 

 onyx marble for ornamental use. The Lime Mountain 

 deposit is still active and has yielded the only substan- 

 tial production of limestone in the district. Most of the 

 limestone produced there has been used for beet-sugar 

 refining. No other important source of high-grade 

 limestone is known in the district. However, moder- 

 ately large reserves of cement-grade limestone are 

 available at the Navajo, Lime Mountain, and Dubost 

 deposits. Although none of these alone may be large 

 enough to support a cement plant, it mav be possible 

 to utilize the adjacent Lime Mountain and Dubost 

 deposits cooperatively. Considering the great distance 

 from large marketing centers, however, none of the 

 deposits of the southern Santa Lucia Range district is 

 likely to be developed for regional use in the near 

 future. 



The limestone deposits of the district are described 

 alphabetically below. 



Almaden deposit. Location: SW|4 sec. 34, T. 26 S., 

 R. 10 E., M.D., 1 1 miles west of Paso Robles; Adelaida 

 l.S-minute quadrangle. Ownership: Not determined. 



Fossiliferous limestone at an elevation of 1,.S00 feet 

 is reported by Logan (1947, p. 306). Eckel ctal. (1941, 

 p. .S57) report limestone to the east of the Mahoney 

 (Buena Vista) mercury mine in the same vicinity. 

 There is no known development of the Almaden 

 deposit. 



Aumaier and Rodriguez deposit. Location: Proba- 

 bly .sec. 30 or 31, T. 30 S., R. 13 E., M.D., 7 miles 



east-northeast of San Luis Obispo; San Luis Obispo 

 l.S-minute quadrangle. Ownership: Not determined; 

 probably U.S. Forest Service (196.5). 



S. Aumaier and Peter Rodriguez located a claim of 

 dolomite prior to 192 5, reportedly in "sec. 1 3, T. 30 S., 

 R. 13 E., at the upper end of Little Falls Canyon" 

 (Laizure, 1925, p. 513). An analysis of the dolomite, 

 made by Smith, Emery and Co., showed 20.44% MgO, 

 31.70% CaO, 0.40% SiOj, 0.19% AI2O3, 0.05% FeaOj, 

 and 47.36% ignition loss. It should be noted that "Lit- 

 tle Falls Canyon" is now known as Big Falls Canyon, 

 the head of which lies in sees. 30 and 31, T. 30 S., R. 

 14 E., and does not extend as far northwest as sec. 13. 

 The section location given by Laizure probably is 

 inaccurate. The dolomite deposit in question may be 

 the same as the "vein of white crystalline dolomite" 

 described in "Little Falls Canyon" by Fairbanks 

 (1904, p. 14). 



Other references: Aubury(?|, 1906, p. 80; Franke, 1935, p. 420. 



Dubost deposit. Location: NW'X sec. 30, T. 26 S., 

 R. 10 E., and NE'/ sec. 25, T. 26 S., R. 9 E., M.D., 14 

 miles west of Paso Robles, which is reached over 20 

 miles of good roads; Adelaida 15-minute quadrangle. 

 Ownership: Marion F. Davis, Adelaida Route, Paso 

 Robles (1962). 



Limestone from the Dubost deposit was burned for 

 lime many years ago by A. Gould (Aubury, 1906, p. 

 79) . According to W. L. Stanton, Jr. ( 193 1, California 

 Institute of Technology unpublished thesis), two 

 kilns were built here; one in 1890 near the "Dubost 

 home" (Camp Natoma) and the other in 1894 "over 

 the hill and down the creek" (Franklin Creek?). Stan- 

 ton states that the kilns were last operated in 1912. 

 Although records are lacking, production is believed 

 to have been small, as no extensive quarry site is 

 known. Frank and Mary Dubost owned the deposit 

 for many years prior to 1955, when it was acquired by 

 the present owner. 



The Dubost deposit consists of a sequence of mas- 

 sive limestone and minor sandstone beds that are part 

 of or correlative with the V'aqueros Formation of early 

 Miocene age (Loel and Corey, 1932, p. 102-104, 136^ 

 137). The limestone unit rests on sandstone of proba- 

 ble Late Cretaceous age and is overlain by soft shale 

 of the Monterey (?) Formation. These sedimentary 

 rocks have been broken into three or more fault 

 blocks, which are successively downdropped to the 

 northeast. Older Franciscan rocks have been brought 

 up along Las Tablas fault, thus truncating the deposit 

 on the south. The deposit consists of a main body of 

 limestone and two smaller (thinner) sequences of 

 beds to the northeast (figure 7). Obscure bedding fea- 

 tures suggest a general south or southwest dip for the 

 bodies, but local variations exist. 



Most of the limestone is a pale buff, dense material 

 composed almost entirely of sand-sized shell frag- 

 ments cemented with calcite. Quartz, feldspar, and 

 other impurities constitute a small percentage of the 



