The role of endolithic cyanobacteria in the formation of lithified laminaein Bahamian stromatolites

The microboring activity of endolithic cyanobacteria plays a major role in the formation of the dominant lithified laminae in modern marine stromatoli tes in the Exuma Gays, Bahamas. These stromatolites are composed primarily of fine-grained carbonate sand that is trapped and bound by the filamentous cyanobacteria Schizothrix sp.. Periodic introduction of coccoid endolithic cyanobacteria Solentia sp. during hiatuses in stromatolite growth associat ed with very low rates of sedimentation results in the formation of lithifi ed horizons, 200-1000 mum thick. These layers consist of micritized grains that are welded together at point contacts. The micritization is caused by extensive microboring and carbonate precipitation within boreholes concurre nt with endolithic activity. Grain welding occurs when boreholes cross from one grain to another at point contacts. Thus, microboring destroys origina l grain textures but, at the same time, plays a constructional role in stro matolite growth by forming lithified layers of welded grains; These lateral bands of fused carbonate grains help to stabilize and preserve the stromat olite deposits.