Kamal Kolo

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-m-thick 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.

Fossil and Subrecent Fungal Communities in Three Calcrete Series from the Devonian of the Canadian Rocky Mountains, Carboniferous of Northern France and Cretaceous of Central Italy

Paleosols and other types of subaerial exposures are present in ancient shallow water limestone sequences and continental facies (Wright, et al., 1988). They provide important information on sea-level changes, sedimentation rates and early diagenetic processes. Calcrete petrography is now well established, mainly from the soils on Recent and Pleistocene limestones (Freytet & Plaziat, 1982, Esteban & Klappa, 1983, Wright & Tucker, 1991). Although microbial organisms are frequently associated with calcretes, their role is still debated and poorly understood. Especially interesting is the association of surficial layers with microbial communities referred to as biofilms. In Recent subaerial environments such biofilms are ubiquitous and are dominated by microscopic fungi (micromycetes). Fungi are also dominant in modern soils and are therefore to be expected to occur in all buried environments. However, it is not clear, how subaerial biofilms and associated paleosols together with inhabiting organisms, are fossilized and preserved. These communities are particularly dominated by microscopical fungi, which were present in the Earth history since Precambrian (e.g. Tyler & Barghorn, 1954). Fungi are numerous in fossil record, however, their identification is difficult and prepares a lot of problems for mycologists and paleontologists (Sherwood-Pike & Gray, 1985). The aim of this paper is to describe fungal communities found in three distinct geological calcretes and to discuss the extent to which they participated in the calcrete carbonate factory.

Probable Fungal Colonization and Carbonate Diagenesis of Neoproterozoic Stromatolites from South Gabon, Western Congo Basin

A petrographic and SEM study of fresh Neoproterozoic carbonate stromatolites from the old Mouila quarry in South Gabon reveals abundant micrometer-sized solution cavities infilled with 0.1–3.0 μm-thick filamentous microbe-like structures, rhombohedral dolomite and quadratic-shaped dolomite crystals that may originally have been oxalates. The filaments appear entangled within an amorphous, organic matrix reminiscent of microbial extracellular polysaccharides (EPS), to which a number of small (0.5 μm) spherical grains of dolomite are attached. A number of characteristic fungal features are evident, including per-mineralized (now dolomitic) sporangia attached to their sporangiophores, dichotomous hyphae, columella, sporangial wall ornamentation, probable spore masses, and ghost traces of fungal remains. In one instance a mineralized zygopsporangium with suspensors appears totally embedded in the matrix. Detailed observation of the solution cavities suggests three distinct stages of formation: (1) incipient fungal colonization, physical penetration of primary carbonate grains boundaries by penetrating fungal stolons, (2) EPS production, organic acid dissolution and neomineral formation on crystal boundaries, and (3) an advanced weathering stage where well-developed cavities form with flanking dolomite ‘collars’. Based on the observations that the cavities and fungi are stratigraphically confined to the same depths; there is no compaction, no grain interpenetration, or collapse of the former sulfate or microenterolithes (i.e., very rapid dolomitization); thin, delicate laminae in the stromatolites are well preserved; and oxygen isotope data which indicate penecontempraneous dolomite with no post-depositional meteoric fluid interaction, all imply that the weathering features occurred during, or immediately after, dolomite formation. Similar diagenetic morphostructures were reproduced in vitro through fungal interaction with the fine-grained dolomite of Terwagne Formation (Visean, France) used as a substrate. The results strongly suggest that Precambrian fungi played a role in post-depositional alteration of stromatolites, and importantly, may provide early physical evidence for fungi in the rock record.