Why did some levels of chert-forming sediment have more silica than others?
There are two theories for the original differences in silica levels that led to the formation of chert beds. The first theory is that silica levels were controlled by changes in the oceanographic process called upwelling, which brings nutrient-rich water to the ocean surface and allows the Radiolaria to thrive. Some evidence indicates that upwelling in the world’s oceans is cyclic and strongly controlled by the Earth’s orbital cycles. The second theory is that the silica-rich beds were deposited by small underwater landslides that sorted the fine clays out from the heavier radiolarian shells. There is some microscopic evidence of graded bedding that supports this possibility. Both processes, cyclical upwelling and underwater landslides, could have played a role in the formation of the Franciscan ribbon chert.
