Boring algae, micrite envelopes and lithification of molluscan biosparites

Abstract

The preservation of molluscan shells as calcite casts is commonly a result of their centripetal replacement, while on the sea floor, by a stable micnte envelope. This grows in three stages: (1) algae bore into the sheU 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 Illing'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.

Carbonate cements in contemporaneous beachrocks, Jaguaribe beach, Itamaracá island, northeastern Brazil: petrographic, geochemical and isotopic aspects

“…The formation of micritic cement has long been attributed to the filling algal borings with minute crystals of secondary calcium carbonate (Bathurst 1966, Alexandersson 1972a.…”

Extreme cellular adaptations and cell differentiation required by a cyanobacterium for carbonate excavation

“…Intracellular calcium (Ca 2+ i ) then moves cell to cell from the boring front toward the mineral surface, where it is actively pumped out into the liquid medium by distal apical cells. Extruded excess Ca 2+ can either diffuse away or, in some cases, reprecipitate into a micrite rind (12,13). Boring continues into the solid substrate until the organism reaches its limit of light intensity for growth.…”

Biostratigraphy and palaeoecology of the marine Pleistocene of Rhodes, Greece: Scleractinia, Serpulidae, Mollusca and Brachiopoda

“…Commonly, the mould is preserved as a micritic envelope that originated from centripetal micritisation by boring endolithic micro-organisms such as algae, cyanaobacteria and fungi (cf. Bathurst 1966Bathurst , 1975. Although moulds were sporadically encountered during the sorting process, the selective dissolution of aragonitic fossils in the limestone facies implies that these species have been greatly underrepresented in this investigation.…”