Authigenic zeolites present in the generally tuffaceous Miocene Briones Sandstone and related formations of the central Coast Ranges of California indicate three stages of diagenetic history: (1) Initial alteration of pyroclastic materials to clinoptilolite (and montmorillonite) that is widely distributed in small amounts throughout the region. (2) Subsequent crystallization of heulandite followed by stilbite in fractures at a few places. (3) Widespread development of laumontite in only the southern part of the region, where the sandstone appears to have been downfolded and faulted to greater depths than elsewhere. Laumontite occurs both as pervasive cement of sandstone and as filling of fractures, and was produced through the reaction of interstitial solutions with other zeolites and with such major constituents of the sandstone as plagioclase, montmorillonite, and calcite at temperatures of 100° C or higher. Mordenite was found at only one locality, closely associated with clinoptilolite and opal. Analcite occurs in diverse settings, and its relation to the other zeolites is obscure. Sparry calcite and coexisting stilbite, laumontite, or analcite in veins seem to make up nonequilibrium assemblages.
