Detailed study of the distribution, petrography, and microfossils of the Calera and Laytonville Franciscan limestones allows us to describe their most probable areas of deposition. The Calera Limestone Member was deposited in shallow submerged basins situated on a ridge. Coccolith ooze and rarer radio-larian limestone were deposited on basin floors which were generally below the lysocline. Planktonic foraminifera deposited above the lysocline on basin walls were sporadically swept down into the basins as turbidity currents and, because of this rapid deposition, were preserved. The ridge along which the basins were located may have been located behind an island arc of Mesozoic age. The Laytonville limestone was deposited in deep water, possibly in the arc-trench gap, or as abyssal plain deposits that were subsequently subducted or accreted to the continent.

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