Stromatolites and Cyanobacterial Mats in Peritidal Evaporative Environments in the Neoproterozoic of Bas-Congo (Democratic Republic of Congo) and South Gabon

The 'Schisto-Calcaire Subgroup' is a muddy predominantly subtidal shelf succession that crops in the West Congolian Belt. The approximately 300-mthick CI (Bas-Congo) and approximately 20-m-thick SCIII (Gabon) series were deposited in a very shallow marine evaporitic environment. The evidence for this interpretation includes sedimentology of dolomite and sulfate minerals and oxygen isotopes. Cyanobacteria (probable Oscillatorians) formed mats on the inland marshes fringing ponds of channeled belts. In Gabon, they are associated with abundant domal stromatolites deposited in shallow to lower intertidal settings. While diagenetic overprints (dolomicrosparitization, sulfate growth, silica replacement) may be significant, several microbial laminar mudstones retained their original fabric. SEM analysis revealed well-preserved three-dimensional (3D) cyanobacterial communities associated with the stromatolites. During progressive lithification in the upper part of shallowing-upward evaporitic sequences, the stromatolites constituted a favorable substrate which has been invaded and colonized by probable fungal hyphae. These produced characteristic features that have been reproduced in vitro in experiments.