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Stratigraphy, Structure and Oil Possibilities in Monterey and Salinas Quadrangles, California
The 7,600-foot thick Cenozoic section in the Monterey and Salinas 15-minute quadrangles, northern Santa Lucia Mountains, lies on a basement of Mesozoic granodiorite and Paleozoic(?) Sur Series schist. A Paleocene age for the 800-foot thick Carmelo Formation is confirmed by new micro- and megafossil material. Eocene, Oligocene and Lower Miocene rocks are missing. The 850-foot thick Middle Miocene Chamisal Formation consists of an upper marine sandstone, the Los Tularcitos Member, and a lower non-marine red-bed sequence, the Robinson Canyon Member. Conformably above the Chamisal Formation is the Monterey Formation consisting of 3 mappable members: a lowermost Luisian sandstone up to 200 feet thick, the Los Laureless Sandstone; a Luisian through Mohnian siliceous shale 2,000 feet thick, the Aguajito Shale; and an uppermost Delmontian diatomite unit up to 800 feet thick, the Canyon del Rey Diatomite. A 60-foot thick olivine basalt flow lies between the Monterey and Chamisal Formations, dating this volcanism at Middle Miocene. Conformably overlying the Monterey Formation and in part interfingered with it is the Santa Margarita Formation, up to 1,600 feet thick.
Choice of the Monterey vicinity as the type locality of the Monterey Formation was unfortunate as the section there is not typical of the Monterey Formation in well known localities elsewhere in California in either age, thickness or completeness. A few miles east of the type section the shale and diatomite members begin to interfinger with the Santa Margarita Formation, and the entire Miocene section becomes sandstone toward the Sierra de Salinas.
Overlapping the Miocene and basement units is the Plio-Pleistocene Paso Robles Formation, up to 500 feet thick, the 200-foot thick Aromas Formation, and several thinner later Quaternary units.
The structural pattern is essentially a series of northwest-trending open folds interspersed among fault blocks involving relatively rigid crystalline basement rocks. In each of these blocks the Miocene section is warped over the ends, and the inference is that these began as simple, compressional, anticlinal fold features developed above rising fault blocks. A long, narrow trough exists along the west side of the Salinas Valley where 8,000 feet of beds lie against the King City fault. The major Tularcitos fault, the importance of which is partially obscured by alluvial fill of Carmel Valley, and the related Chupines fault have planes which stand near vertical and show features of both normal and strike slip type. Thrusting does not appear to be of major importance in the area mapped with the possible exception of the King City fault.
The best chances for oil accumulation are possible fault-stratigraphic traps along the west side of the Salinas Valley northeast of the projected King City fault. The Guidotti anticline northeast of Laguna Seca has surface closure but the prism of sediments there is 2,000 feet deep or less and the entire section may be sandstone.
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