Detrital K-feldspar Research Articles

Orthoclase Distribution and Authigenesis in the Franciscan Formation of a Portion of Western Marin County, California

ABSTRACT In part of western Marin County, California, the Franciscan Formation may be divided into 4 parallel zones based on lithologic assemblage and graywacke composition. Of the 4 zones, three are composed predominantly of clastic sediments. Discrete grains of K-feldspar are generally present in only one of these 3 zones. Within the belt containing K-feldspar (western zone), the average K-feldspar content is 8.2 percent. Smaller subzones, which do not contain K-feldspar, are contained within the K-feldspar-bearing western zone. It is suggested that these minor subzones are associated with faults related to the San Andreas system and along which the detrital K-feldspar has been removed. Obvious authigenic orthoclase is present in 46 percent of the samples from the western zone and from 19 percent and 12 percent of the samples from the 2 zones which do not contain discrete grains of K-feldspar. The authigenic orthoclase occurs as veins and cavity fillings, as a replacement of the matrix of the graywackes, and as a replacement of albite. On the basis of the field interpretation of the structure, the rocks of the K-feldspar-bearing western zone are older than the rocks of the 2 zones without detrital K-feldspar. This is not consistent with the work further north in the Coast Ranges where the K-feldspar bearing Coastal Belt is thought to be generally younger than the non-K-feldspar-bearing Franciscan. The mobility of the K-feldspar as evidenced by the abundance of authigenic orthoclase and the removal of K-feldspar from, at least, small areas of graywackes suggest that secondary movements may have an important affect in the K-feldspar distribution in the Franciscan Formation.