Abstract

A study of the Radiolaria in Coast Range cherts establishes that rhythmically bedded cherts associated with ophiolites below the base of the Great Valley Sequence are Late Jurassic (Tithonian) in age, and rhythmically bedded cherts in the Franciscan complex are of essentially the same age. The only younger pelagic sediments in the Franciscan are pelagic limestones (Laytonville and Calera types) and associated cherts of Late Cretaceous (Cenomanian-Turonian) age. In light of most plate tectonic models, the lack of pelagic sediments other than these seems enigmatic.

The relation between the cherts associated with the ophiolites and the overlying Great Valley Sequence indicates a sudden cessation of pelagic sedimentation in early Tithonian time and probably reflects the rise of the Nevadan orogen. It is postulated that flysch-type turbidite sediments accumulated so rapidly during the remainder of Mesozoic time that they filled in an incipient trench system and spread far out onto the abyssal plain, thus inhibiting pelagic sedimentation.

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