Iron microbial communities in Belgian Frasnian carbonate mounds

The Belgian Frasnian carbonate mounds occur in three stratigraphic levels i n an overall backstepping succession. Petit-Mont and Arche Members form the famous red and grey "marble" exploited for ornamental stone since Roman ti mes. The evolution and distribution of the facies in the mounds is thought to be associated with ecologic evolution and relative sea-level fluctuation s. Iron oxides exist in five forms in the Frasnian mounds; four are undoubt edly endobiotic organized structures: (1) microstromatolites and associated forms (blisters, veils...), possibly organized in "endostromatolites''; (2 ) hematitic coccoids and (3) non dichotomic filaments. The filaments resemb le iron bacteria of the Sphaerotilus-Leptothrix "group"; (4) networks of di chotomic filaments ascribable to fungi; (5) a red ferruginous pigment dispe rsed in the calcareous matrix whose distribution is related to the mound fa cies type. The endobiotic forms developed during the edification of the mou nds, before cementation by fibrous calcite. The microbial precipitation of iron took place as long as the developing mounds were bathed by water impov erished in oxygen.