The Geology of the Franciscan Complex in the Ward Creek-Cazadero Area, Sonoma County, California

In 1963, Coleman and Lee published their classic study of the glaucophane-bearing metamorphic rocks in a small exposure in the channel of Ward Creek, about 1 mile (2 km) west of Cazadero, Sonoma County, California. These rocks are glaucophane, lawsonite, aragonite, and sodic (jadeitic) pyroxene-bearing metabasalts, with associated stilpnomelane metachert. The presence of sodic pyroxene makes it clear that these are unusually high-pressure metamorphic rocks (Newton & Smith, 1967). Coleman and Lee's 1963 paper on Ward Creek was the foundation for many others in the 1960's and 1970's. In 1988, Maruyama and Liou published an exhaustive study of mineral composition and stability relationships in metabasalts and metacherts, which was in turn the foundation for many other papers. At least 32 articles on petrology, mineralogy, oxygen isotopes, and geochronology, including three field guide, have been published on this exceptional sequence (Erickson, 1992a). Possibly the most intensely studied body of metamorphic rocks in the world, Ward Creek's small outcrop is the American standard for glaucophane-bearing metamorphic rocks. In contrast, the geology of the area surrounding the Ward Creek rocks is poorly known. To learn the larger-scale setting of the Ward Creek rocks, I mapped 14 square miles (35 km2) of the Franciscan Complex centered on the Ward Creek locale. In this article, I combine my studies of the geology surrounding Ward Creek and those of many others on the classic exposure itself into an overview of the areal geology. Wakabayashi (1992) provides a good modern overview of the whole Franciscan Complex. Any recent physical geology text should provide a good discussion of the basic plate-tectonic model, including the trench - accretionary wedge - volcanic arc structure of subduction zones.

Original publisher: Department of Conservation Division of Mines and Geology. This version includes the author's correction to Photo 3.