par Kolo, Kamal;Konhauser, Kurt;Prian, Jean-Pierre;Préat, Alain
Référence Geology and Resource Potential of the Congo Basin, Springer Berlin Heidelberg, page (77-96)
Publication Publié, 2015-01
Partie d'ouvrage collectif
Résumé : 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 (Viséan, 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.