Description
Four sedimentary formations were mapped on the west side of the Temblor Range. The oldest, Panoche-Lodo Formation, is a cartographic unit combining the Upper Cretaceous Panoche Formation and the Paleocene-Lower and Middle Eocene Lodo Formation. It consists of 8,000 feet or more of an olive-drab succession of "dirty" thin-bedded and massive siltstones and wackes deposited under mostly bathyal marine conditions. The Point of Rocks Sandstone is Upper Eocene. It consists of tan to light-gray fine- to coarse-grained quartz-rich calcareous arenite; cannonball concretions and cavernous-weathering characterize generally bold exposures. At least 2,300 feet of sandstone and minor gray shale were deposited under tropical to subtropical marine neritic to bathyal conditions. During Miocene time the Monterey Formation was deposited under cool, shallow marine conditions. More than 4,000 feet of siliceous shale, porcellanite and opaline chert were accumulated largely from siliceous tests of marine micro-organisms during the Zemorrian through Mohnian Stages. Abundance of fine clastic shales and sandstones increased during the lower Mohnian. Sandstone lenses are found distributed in irregular fashion throughout the Monterey Formation. More than 7,500 feet of nonmarine Paso Robles Formation accumulated during Plio-Pleistocene time. Terrestrial gray to brown clay through boulder-sized debris accumulated rapidly in an inland basin. At least three periods of major deformation can be recognized in the area. At the end of the Panoche-Lodo deposition the strata were folded and eroded before deposition of the Point of Rocks Sandstone. An angular unconformity of +30 degrees exists between the two formations. At the end of Monterey deposition, deformation folded and eroded the Miocene and Upper Eocene beds. The Paso Robles Formation was deposited with angular unconformity on the eroded Monterey Formation, and it is believed to have filled in synclinal valleys formed by post-Monterey deformation. Intense deformation at the end of Paso Robles deposition further folded the previously initiated folds and caused overturning of some of them. Contemporary with, and probably succeeding folding, the crest of the Temblor Range was uplifted 3,000 to 5,000 feet along the Salt Creek reverse fault. Continued uplift and erosion have shaped the Temblor physiography to its present form. Oil and gas exploration in the past has been without success. Several dry holes were drilled on both sides of the San Andreas Fault. Structural traps such as the Cottonwood Spring anticline and the footwall of the Salt Creek reverse fault merit further investigation. Gypsum deposits are subeconomic on the west side of the Temblor Range.
