Boring algae, micrite envelopes and lithification of molluscan biosparites

The preservation of molluscan shells as calcite casts is commonly a result of their centripetal replacement, while on the sea floor, by a stable micrite envelope. This grows in three stages: (1) algae bore into the shell wall, (2) the algal filaments die and decay, (3) micritic aragonite fills the tubes. Later, if the skeletal sand is exposed to fresh water as a result of uplift or a fall in sea level, or to a suitable brine deep under the sea floor, the residual molluscan shell is dissolved but the envelope remains intact. It forms an empty mould that is subsequently filled with drusy calcite. Complete replacement of skeletal grains by micritic aragonite yields cryptocrystalline grains like Ming's “grains of aragonite matrix” and the grains in Beales's “bahamite”. Through a study of compaction fabrics it may be possible to decide whether an envelope‐bearing limestone was cemented above sea level or below the sea floor