Mélanges of San Francisco Peninsula—Geologic Reinterpretation of Type Franciscan

The “type Franciscan” at San Francisco Peninsula includes graywacke, shale, conglomerate, limestone, volcanic rock, radiolarian chert, serpentinite, and glaucophane schist. The various types of rock are present mainly as tectonic inclusions, ranging from fragments a few inches in diameter to allochthonous slabs several miles long, in a pervasively sheared shaly matrix. This mixture of blocks and matrix constitutes an assemblage of mélanges. The writers believe that the Franciscan rocks were derived from the fragmentation and mixing of two stratigraphic units. The typically eugeosynclinal rocks belong to a pre-Tithonian San Francisco unit (new). They were deformed and partly metamorphosed during a Late Jurassic orogeny before the deposition of the San Mateo unit (new). The latter yields Tithonian to Late Cretaceous fossils and represents the western extension of the Great Valley sequence. Different associations of tectonic inclusions within the Franciscan mélanges have been noted. Three mélange units and an allochthonous slab were differentiated. The mélanges originally were subhorizontal gravity-slide sheets; they were warped into antiforms and synforms by Cenozoic folding.