Calcification of filaments of boring and cavity-dwelling algae, and the construction of micrite envelopes

Endolithic (boring) algae play a significant role in the breakdown and alteration of carbonate skeletons, other carbonate structural elements, and sediment grains in reef environments. The activities of endolithic algae affect or control particle angularity and size, sediment porosity and permeability, particle micritization, micrite envelope formation, and other aspects of carbonate erosion and diagenesis.

In controlled experiments in the shallow marine environment at Discovery Bay, Jamaica, endolithic algae boring into crystals of Iceland spar calcite begin to grow out of the bores in the crystals into the sea after 25 days; after 65 to 95 days in the sea, dead and exposed filaments are completely calcified by the precipitation of micrite — size crystals of rhombohedral low Mg calcite on the exterior and interior of the thalli.

The coalescence of calcified algal filaments on the surface of grains will produce a “constructive” micrite envelope, which differs from the envelopes produced by the boring — infilling mechanism described by Bathurst, by forming entirely on the grain exterior. The process of constructive envelope formation can be geologically rapid, occurring within a few years of less. Micrite envelopes of this type are found on carbonate grains from the modern in Jamaica and other islands, and in sediments from the Devonian of western Canada, and the Ordovician of eastern Canada.