Abstract

Mapping a strip of 15′ quadrangles across the northern Coast Ranges of California by student colleagues and myself has resulted in the development of a model for the origin and emplacement of the Franciscan complex and its relation to the coeval Great Valley sequence that differs from earlier models.

Rocks of both Franciscan and Great Valley are predominantly graywacke, silty mudstone, and local conglomerate. Most of the Great Valley rocks are graded, and bottom markings are common; these features are present but not characteristic of the Franciscan rocks. Franciscan graywacke and silty mudstone tend to be broken and sheared, or chaotically deformed. The chaotic zones typically contain exotic blocks, mostly of ophiolitic rocks, but also blue-schist, mica schist, metagraywacke, and blocks similar to the enclosing sheared matrix. These chaotic units are best characterized as mélanges.

Within the Franciscan complex, mappable units have been defined, based on lithology of matrix and content of exotic blocks. The units dip and face easterly and are oldest to the east (Tithonian or older for the South Fork Mountain Schist). Progressively younger rocks occur down section to the west; the youngest are the Coastal Facies largely of Eocene age. The probable origin involves deposition of Franciscan sediments in a trench, followed by thrusting of the sediments against and under the landward side of the trench, while the axis of the trench migrated progressively westward. The process began in Tithonian time, slowed during Late Cretaceous, and ceased in the Eocene. Models are suggested for the formation of mélanges during sedimentation and underthrusting.

Each mélange unit contains a unique assemblage of exotic blocks. It is suggested that these assemblages in part reflect the lithology of the basement rocks on which the sediments were deposited. If true, the composition of the basement varied from dominantly ophiolite-suite rocks to dominantly blueschist. A mélange of ophiolite blocks also formed, the basement beneath a part of the Great Valley sediments.

Samples of radiolarian chert from the ophiolite suite throughout the Franciscan were determined to be of Tithonian age by Emile Pessagno. If the ophiolite represents ocean floor, formed at a spreading ridge, the ridge must have been located adjacent to the trench in Tithonian time; after the Tithonian, no oceanic rocks were added to the Franciscan pile. A diapiric origin for the Tithonian ophiolite is suggested.

Northwest-trending isolated slabs of Great Valley—type rocks and of shallow-water to continental coal-bearing clastic rocks of Cretaceous to Miocene age are scattered within the Franciscan complex. Evidence is presented that these were deposited in separate, probably fault-controlled basins, developed on the surface of the tectonically active Franciscan complex during and following underthrusting. They do not appear to represent a klippen of a Coast Range thrust fault, as has been suggested by others.

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