GEOLOGY OF THE TIBURON PENINSULA, MARIN COUNTY, CALIFORNIA

Preliminary results of on-going detailed geologic mapping in and around the NW trending Tiburon Peninsula has revealed a complex but coherent terrain composed of variably deformed and metamorphosed Franciscan rocks consisting principally of graywacke, chert, and basalt (greenstone), with lesser conglomerate and shale. This terrain is overlain by ultramafic (um) rocks composed mainly of peridotite and serpentine which were emplaced along a low-angle fault. Extensive erosion has exposed large portions of the underlying sedimentary and volcanic terrain leaving two areas that remain capped by um rocks: a relatively thin, flat-lying sheet capping Ring Mountain in the NW portion of the peninsula, and an elongate, complexly folded sheet capping the central and SE portions of the peninsula.

The faulted base of the overlying um rocks is recognized throughout the peninsula by the occurrence of pervasively sheared serpentinite and/or chlorite schist which is locally replaced by silica-carbonate mineralization along sub-horizontal shear planes. It is also characterized by the presence of dense, high-grade metamorphic blocks of varying size composed of glaucophane- and lawsonite-bearing blueschist, eclogite, and/or amphibolite that have weathered out of the soft, fault-sheared matrix and remain as isolated blocks above the fault contact. These features are well exposed at Ring Mountain where the faulted base of the relatively flat-lying um sheet has been extensively exposed by erosion. Footwall rocks, composed mainly of chert or graywacke, exhibit alteration characteristics that vary with proximity to the overlying fault: chert becomes intensely folded, bleached, brecciated, and re-silicified; graywacke becomes increasingly foliated and commonly contains quartz-filled cross fractures.

The lower sedimentary and volcanic terrain exhibits a variation in metamorphic grade and deformational character throughout the peninsula. In the NW peninsula, these rocks appear relatively unaltered other than at contacts with the overlying um sheet; in the central and SE peninsula they are folded, exhibit a variable schistosity, and locally contain blueschist-facies minerals such as jadeite, glaucophane, and lawsonite.