Implications of upper Mesozoic conglomerate for suspect terrane in western California and adjacent areas

Abstract

Several distinctive kinds of conglomerate occur in upper Mesozoic rocks in different regions of western California. Within each region, conglomerate in the accretionary-wedge Franciscan assemblage is similar to conglomerate exposed nearby in the fore-arc-basin Great Valley sequence, suggesting that both rock units were derived from the same source regions. Long-distance tectonic transport is not necessary to account for the detrital sedimentary rocks in the Franciscan. The types of conglomerate in the lower part of the Great Valley sequence relate to their place of deposition in the fore-arc basin. In the northern Coast Ranges, chert-rich conglomerate derived from the Klamath Mountains was transported longitudinally southward by a relatively large distribution system. Correlative conglomerate in the Diablo Range is rich in volcanic and detrital- sedimentary-rock clasts derived from the western edge of the Sierra Nevada and transported westward by relatively small distribution systems. Conglomerate rich in chert and quartzite in suspect terrane of the Golden Gate-Gilroy and Nacimiento blocks of central California may have been deposited by a large north-flowing system in the southern part of the same basin; an exotic origin is not required for the suspect terrane. In mid-Cretaceous time, the Nacimiento block may have been displaced southeastward along the Nacimiento fault and juxtaposed against the Salinian block, a deep and interior part of the magmatic arc in southern California. This displacement may be reflected in the clast composition of mid-Cretaceous conglomerate of the Nacimiento block, but stratigraphic documentation is still incomplete. Younger and approximately equal right-slip on the San Andreas fault system brought both the Nacimiento and Salinian blocks northwest to their present position. A southward increase in quartzite content in conglomerate of post-Nevadan age is broadly parallel to a similar increase in pre-Nevadan conglomerate of adjacent parts of the Klamath Mountains, Sierra Nevada, and Peninsular Ranges, making it unlikely that any of these rocks were tectonically derived from remote regions. Radiolarians from chert pebbles in Franciscan conglomerate are similar in age distribution and faunal composition to those from pebbles in the Great Valley sequence, from pebbles in pre-Nevadan conglomerate, and from bedded chert in presently adjacent source areas.