Orthoclase distribution and authigenesis in the Franciscan Formation of a portion of western Marin County, California

Abstract In part of western Marin County, Calif., the Franciscan 'Formation' may be divided into four parallel zones based on lithologic assemblageand graywacke composition; a central spilitic zone and clastic zones. Discrete grains of K-feldspar generally occur only in the western zone; average K-feldspar content is 8.2 percent; smaller subzones which do not contain K-feldspar, are associated with faults related to the San Andreas system. Obvious authigenic orthoclase is present in 46 percent of the western zone samples, and in 19 and 12 percent of samples from the other two clastic zones. Field interpretation of the structure indicates that the western zone is older than the other clastic zones. It is suggested that secondary movements may have an important effect in K-feldspar distribution in the Franciscan Formation.