Abstract

A dismembered ophiolite, tectonically overlain by Great Valley strata, rests upon Franciscan rocks southeast of Cedar Mountain in the northern Diablo Range. The klippe of sedimentary rocks is the first reported outlier of the Great Valley sequence in the interior of the range. The dismembered ophiolite is composed of serpentinized ultramafic rocks, hornblende gabbro, diorite, and plagiogranite, and it is partially altered to greenschist facies assemblages containing chlorite + epidote + albite + actinolite.

The Franciscan Complex northeast of the ophiolite is characterized by lithologic heterogeneity, and it includes mélanges comprised of mixed rock fragments of varied metamorphic grade incorporated in a pervasively sheared shale matrix. The coherent Franciscan terrain southwest of the ophiolite may belong to a structural unit of Late Jurassic age which overlies younger Franciscan rocks in other parts of the Diablo Range. All Franciscan samples examined petrographically contain blueschist facies minerals such as lawsonite, pumpellyite, sodic amphibole, and jadeitic pyroxene.

The Great Valley strata consist of interbedded sandstone and shale. Arkoses of the Great Valley klippe are generally calcareous and contain well-preserved detrital potassium feldspar and biotite. Locally, the sandstone contains megafossils of probable Valanginian and Cenomanian age, including Linearia multicostata (Gabb), Pterotrigonia oregana (Packard), Turritella hearni Anderson, Trigonia sp., Panope sp., and small ammonite scraps similar to Thurmanniceras (?) sp. Incipient low-grade metamorphism is indicated by alteration of calcic plagioclase to albite + calcite + white mica and by partial chloritization of biotite. The lithology and the age, structural, and metamorphic relationships indicate that the Great Valley outlier and its underlying dismembered ophiolite are erosional remnants of the Coast Range thrust which once extended across the Diablo Range.

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