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are seen to contain vast numbers of minute fossil shells and fish scales. These fossils are the remains of marine animals, and show that the shales were deposited in the ocean. It appears probable that the organic remains buried in the Monterey shale are in part, at least, the primary source of the oil and bitumen found in Neocene rocks in California.
At the base of the Mount Diablo Range, in Cholame Valley, is found a thick series of sandstones and shales, resting on the Monterey shale. Conspicuous in this series are beds of bluish-gray sandstone and shale much resembling the Monterey shale below. There are many alternations of the sandstone and shale beds, both of which contain certain well-marked fossil forms. In Priest Valley there is a thick series of sandstone and shale very similar in appearance to the series in Cholame Valley, but it contains, in addition to the sandstone and shale beds noted above, two thin beds of impure lignite. Some of the fossils in the Priest Valley region are identical with those in Cholame Valley. The field work was not done in enough detail to determine the stratigraphic relations of these beds. Our present knowledge only warrants us in saying that both belong to the Neocene and are probably a portion of what is known as the Miocene.
Santa Margarita formation.--Above the Monterey shale on the southwest side of the Salinas Valley is found a series of rocks to which Mr. H. W. Fairbanksa has given the name Santa Margarita formation. It has a most characteristic fauna, and is probably the equivalent of the San Pablo formation, described by Dr. J. C. Mer
In the Santa Lucia Range, where it outcrops at frequent intervals northeast of the Monterey shale belt, it consists of soft gray sandstone. On the northeast side of the Salinas Valley the formation is sandstone at the base, grading upward into soft chalk-like shale and clay shale. It outcrops over a large area in the Salinas Valley near Kings City, where the most characteristic fossil is the large barnacle Tamiosomia gregaria Conr. The sandstone beds of this formation, in many localities in California, are saturated with bitumen and asphaltum. The occurrence of such beds in Monterey County is described on page 18. From the incomplete data at hand it seems probable that the Santa Margarita formation is more recent than the sandstones and shales noted in Cholame Valley and in Priest Valley.
Paso Robles formation. In the upper Salinas Valley is found a series of rocks known as the Paso Robles formation, which has been described by Mr. H. W. Fairbanks. The basal beds of this formation are conglomerates, above which lie alternating beds of reddish clay and conglomerates composed of pebbles of shale, jasper, and granitic rocks. The conglomerates seem to be of local origin and contain a large percentage of Monterey shale pebbles. In the Salinas Valley the rocks of this formation are practically undisturbed, but they are sharply folded in both the Mount Diablo and Santa Lucia ranges. The marked unconformity in bedding between the Paso Robles and all the formations below serves as one of the most satisfactory datum planes in the stratigraphy of the Coast Range. The formation was traced from the upper Salinas into Monterey County, where it covers a large area in the vicinity of Bradley. It is greatly developed in the mesa region on the east side of Salinas River and is found between Monterey and Toro Creek, southwest of Salinas.
u Geologic Atlas U. S., folio 101, U. S. Geol. Survey, 1903. b Bull. Dept. Geology, Univ. California, vol. 2, 1898, pp. 113-116. Jour. Geol., vol. 6, 1898, p. 565.
Later sandstones.-East of Monterey Bay is found a soft, quartzose sandstone of considerable thickness, yellow or reddish brown in color, and on weathered surfaces often containing small nodules of sand cemented by iron oxide. It disintegrates into the soft sand beds which forin the sand hills north and west of Salinas. It appears to have once extended up the Salinas Valley as far as San Lucas, probably as a stream deposit, but most of it has been removed by erosion. This formation is more recent than those that have been described, and rests unconformably on the older rocks.
Stream deposits. The stream deposits in the lower Salinas Valley consist of unconsolidated beds of clay, sand, gravel, and shale pebbles. In the vicinity of Salinas these river deposits yield large quantities of ground water. The wells penetrate alluvial sands and clays, reaching the water supply in beds of gravel 150 to 200 feet below the surface. A further description of these underground waters is given on page 21.
OIL AND BITUMINOUS ROCK DEPOSITS.
In Monterey County bituminous sandstone and shale are found at several points. Near San Ardo (Pl. I) the Monterey shale has a total thickness of about 2,200 feet. Both the Monterey shale and the Vaquero sandstone below have been sharply flexed into a secondary anticlinal fold on the northeast limb of the San Antonio Hills anticline. The shale is here very different from the typical Monterey shale in the Santa Lucia Range. Some beds are soft and chalk-like; others contain an unusual percentage of fine sand and mica flakes, interstratified with beds of quartzose sandstone. The whole series is bituminous, but the sand beds are much richer in bitumen than the others. The soft shales weather white, but when broken are seen to be colored black or brown with bitumen. The upper 770 feet of this formation consists of alternating beds of chalk-like shale and sandstone. Between these beds and the basal sandstones of the Santa Margarita formation is from 3 to 5 feet of cherty shale, characteristic of the Monterey. Small tar springs are found in this series at several points, and also in the typical Monterey shale to the south west.
Above this series to the northeast are found beds of sandstone with shale inclusions and fossils characteristic of the Santa Margarita formation.
The amount of liquid bitumen at this locality is exceedingly small. Both the shales and sandstones are close-textured rocks and would yield oil very slowly even if it should be present in large quantities. The strata are steeply inclined, dipping northeastward at an angle of from 65° to 90°. Oil wells drilled here simply pass down into one or two beds of sand instead of piercing several, the chance of reaching any productive oil sand being proportionately reduced. The rocks bituminized are also without an impervious cover, the crest of the anticline having been removed by erosion.
Several prospect wells have been drilled here, but all have thus far proved unproductive. The Tomboy Oil and Improvement Company drilled one well in sec. 19, T. 22 S., R. 10 E., Mount Diablo meridian. The San Antonio Oil Company drilled on their property in secs. 20, 29, and 30, T. 22 S., R. 10 E., M. D. M., about 2 miles west of San Ardo, and the San Antonio Consolidated Oil Company drilled one well on its property in sec. 12, T. 22 S., R. 9 E., and sec. 7, T. 22 S., R. 10 E., M. D. M.
The Mathews bituminous rock quarry is situated in sec. 29, T. 17 S., R. 8 E., M. D. M., in San Benito County, about 7 miles northeast of Metz. (See Pl. I.) The bituminous rock consists of a bed of coarse sand and bowlders, resting directly upon the eroded surface of the granitic axis of the Gabilan Range, and above this are finer sands, the whole having a considerable but variable thickness. Three prospect holes have been opened and a considerable quantity of bituminous sand rock, which seems to be of excellent quality, has been hauled away. The bituminous sand is covered by thick beds of soft, chalky shale, probably a part of the Monterey shale, resembling that at San Ardo. The shale has a low dip, 3° to 5°, toward the east.
The Mylar bituminous rock quarry is situated in secs. 14 and 15, T. 19 S., R. 9 E., M. D. M., 10 miles northeast of Kings City. The quarry is owned by the City Street Improvement Company, of San Francisco. The bituminous rock is here of considerable thickness and consists of coarse granitic gravels which rest directly upon the worn surface of the granitic axis of the Gabilan Range, followed upward by beds of coarse granitic sand, the whole evidently derived from the wear of the adjacent granite rocks. The gravels and sandstones are bituminous throughout their entire thickness. Resting on the bituminous sandstone are beds of soft chalky shale, probably a portion of the Monterey shale series. The sandstone dips westerly at a low angle, 3° to 5o. Considerable rock has been quarried here and shipped to various localities.
No oil wells have been drilled either at this point or at the Mathews quarry, and it is not probable that wells would be successful, as the
bituminous rock which rests on the eroded surface of the granite is of uncertain thickness and small extent at either locality. There are also beds of similar sandstone, not bituminous, however, at other points along the buried granitic axis of the Gabilan Range. The exact relation of these sands to the shales above is not clearly understood, and they may be beds older than the Monterey shale, which have been impregnated with bitumen by downward infiltration.
About 6 miles northeast of the Mylar quarry the Nonpareil Consolidated Oil Company has drilled several prospect wells in sec. 32, T. 18 S., R. 10 E., M. D. M. These wells were drilled in the Monterey shale east of a great fault which has brought that formation up into contact with the Paso Robles formation. The shales here have a dip of 50° to the northeast and are succeeded above by beds of sandstone. There is a small amount of brea and bituminous sand in the Paso Robles formation along the line of the fault. These wells were not successful and have been abandoned. At several points between this locality and the Peachtree ranch there are indications of oil, but none to justify exploitation. Near the eastern corner of the Peach Tree ranch there is another outcrop of bituminous sandstone and conglomerate, apparently a part of the Paso Robles formation. These bituminous beds rest on crumpled strata of the Monterey shales which are also impregnated with bitumen. The deposit is of no value as bituminous rock, being dry and friable, and there is no probability that a productive oil sand will be reached at this place.
In the East Fork of Little Cholame Creek, northwest of Parkfield (Pl. I), there are many seepages of heavy brea or maltha. These are from the Monterey shale, from the sandstone directly above, and at one point from the Paso Robles formation. The seepages are from thin beds of bituminous sandstone and sandy shale. All of the formations examined here, except the Paso Robles, are close-textured rocks; no open porous sandstones were seen. The strata are greatly disturbed and are in close proximity to the granitic rocks below. The amount of bitumen is small and the conditions are not favorable for its storage.
Several oil wells have been drilled in this region, none of which was successful. The Cholame Valley Oil and Development Company drilled in sec. 31, T. 22 S., R. 14 E.; the Parkfield Oil Company in sec. 16, T. 23 S., R. 14 E., and the Waverly Oil Company in sec. 32, T. 22 S., R. 14 E., M. D. M.
At several points in Monterey and San Luis Obispo counties the basal sandstones of the Santa Margarita formation are bituminous. This sandstone is impregnated with bitumen at the mouth of Thompson Canyon, 4 miles west of Kings City (Pl. I), where it outcrops for some distance in a northwest-southeast direction along the northeast side of the Monterey shale area. Both the sandstone and shale are tilted at a high angle, dipping about 50° NE. This sandstone is now
dry and friable on the surface and there are no oil springs in this vicinity.
There are bituminous sandstones on San Antonio River, 8 miles southwest of Bradley, near the Monterey-San Luis Obispo County line (Pl. I). They are the basal beds of the Santa Margarita formation and outcrop at intervals along the northeast side of the Monterey shale area for several miles. The Monterey shale here contains beds of hard chert, typical of the shale in the Santa Lucia Range, and just below the base of the Santa Margarita formation it contains thin beds of bituminous sandstone. The Santa Margarita formation here consists of a considerable thickness of soft quartzose sandstone, containing many pebbles of Monterey shale, granitic and metamorphic rocks. Both the shale and sandstone are sharply tilted on the south side of San Antonio River, where the dip is northeastward at an angle of 40° to 85°. Toward the northeast the dip is less and the bituminous sandstone is covered by the basal conglomerates of the Paso Robles formation.
Two oil wells have been drilled at this locality, both near the southwest corner of sec. 35, T. 24 S., R. 10 E., M. D. M. One was drilled by the White Oaks Oil Company and another by the Great American Oil Company, which drilled entirely through the bituminous rock into the Monterey shale below without reaching productive oil sand. An oil well was drilled by the Monterey Oil Company in sec. 27, T. 24 S., R. 10 E., M. D. M. It was drilled in the Monterey shale, and was a failure.
Erroneous theories are entertained by some oil men and investors regarding the oil-producing formations in California. It is supposed that because the Monterey shales are bituminous they must of necessity contain commercially valuable quantities of oil, and on this supposition hundreds of thousands of dollars have been wasted in useless exploitation of barren territory throughout the State. At a few localities in California the Monterey shale contains interstratified beds of sand. When these beds of sand are thick enough, of considerable horizontal extent, of porous texture, and have been folded into favorable attitudes, they may contain valuable deposits of petroleum, provided, of course, that the adjacent shale is bituminous.
In the Salinas Valley the Monterey shale, although containing a great amount of bitumen, is far less bituminized than it is in other parts of the State where oil is found. It is here practically without interstratified beds of sand which might be oil producing, and when, in prospecting for oil in this region, the drill enters into a considerable thickness of hard, flinty shale, containing a little oil, gas, and sulphurous water, it is very probable that the drill has reached the Monterey shale, and further exploitation will doubtless result in disappointment.