Abstract

It is suggested that the cherts of the Franciscan Formation are biogenic pelagic deposits, formed above rugged volcanic terrains that were possibly close to an oceanic ridge. They may be formed, together with pelagic limestones, as a result of topographic shielding from density currents, and not from direct precipitation of silica from volcanic emanations. The cherts are thought to represent some of the oldest strata on volcanic oceanic basement, and their thin development to indicate a young age for the oceanic plate that was to form the material within the Franciscan mélanges.

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