Fig. 13

A two-dimensional depositional model illustrating the palaeoenvironmental evolution for development of the various rock types in the Shipu Group. Stage A: Volcanic ash from a far crater deposit forms tuffite. Stage B: Volcanic ash from a far crater diminishes, and grain shoals develop in a high-energy zone near or above fair-weather wave-base. Stage C: At local high points (grain shoals), serpulid-microbial reefs develop near or above the fair-weather wave-base where waves remove fine volcanic ash and coarser ash is deposited in interreef low spaces due to physical differentiation. Stage D: As the sea level rises, reefs become below FWWB. The waves cannot remove the fine volcanic ash, so the fine volcanic ash inhibits reef growth and is deposited as a laminated fine tuffite layer. MSL: Mean sea level; FWWB: Fair-weather wave-base