ABSTRACT

Pliocene sandstones with a striking bluish hue are prominent along the western base of the Sierra Nevada Range in the Mehrten Formation, and in the Coastal ranges of central California in the Tehama, Neroly, Cierbo, Purisima, San Joaquin, and Etchegoin formations. The color has previously been shown to be due to a thin authigenic montmorillonoid coating on the dark-colored sand grains. These coated sandstones have in common a preponderance of andesitic detritus and a high degree of permeability. Interbedded coated and uncoated sandstones occur as a result of differing degrees of permeability, or, alternate derivation from andesitic and non-andesitic source rocks. Compositional and cross-stratification studies indicate that andesitic sandstones of the Mehrten, Etchegoin, and San Joaqui formations were derived from the Sierra Nevada; those of the Cierbo and Neroly formations, in addition to a major source in the Sierra Nevada, probably had some contribution from the Coast Ranges; the Tehama sediments were derived from the Coast Ranges; and the Purisima blue sandstones may have been derived from either Coast Range of Sierran effusive rocks to the east.