Micritization and Carbonate-Grain Binding by Endolithic Algae

Endolithic (boring) algae are the direct or indirect agents of important erosive and early diagenetic processes in carbonate sediments.

Algae boring into carbonate grains produce micrite envelopes by repeated boring and infilling of borings by precipitated micrite, as described by Bathurst. However, the algae also produce micrite envelopes outside grains by the calcification (cementation) of exposed dead endolithic filaments, a process which reduces intergranular porosity. Algal filaments grow through the micrite envelopes into intergranular pores, and live within the pores as chasmolithic algae (living in holes not of their own creation); after death these may become calcified to produce an intertwined mesh of calcified filaments on which later micrite and microspar cements precipitate. The calcified intergranular filaments and associated cements further reduce intergranular porosity (and possibly permeability) and serve to bind the grains.

Micrite envelopes also may be produced beneath algal-mucous coats through a process of etching and dissolution, resulting in a highly microporous residue micrite.

Girvanella and similar Paleozoic and Mesozoic algae may represent calcified algae similar to those described previously, in which cement precipitates on dead algal thalli. Girvanella may be a diagenetic taxon, produced from many algal genera, rather than a discrete biologic form.