Mode de formation de la roche silicifiée à bactéries fossiles du Crétacé supérieur du Bassin de Mons (microbialite de Saint-Denis)

The Saint-Denis microbialite is an unusual rock consisting of silicified, biologically-derived structures (bacteria, fungi, etc.) preserved in davities within the siliceous beds of Saint-Denis, Belgium (Saint-Denis Silicite - Upper Cretaceous). Morphological analysis of the fossil microorganisms leads to the distinction of single structures and bioconstructions that are associated in various organic microfacies. The paleoenvironmental interpretation of the microbialite supports and extends the understanding of the still disputed genesis of the Saint-Denis Silicite, especially in terms of biosedimentary evolution. Textural analysis of the quartz fabrics (megaquartz, microquartz, length-slow - and length-fast chalcedony) and other minerals such as siderite shows that, on the one hand, fossil microrganisms served as mineral seeds and, on the other hand, their influence on crystal growth most probably consisted in supersaturation variations due to biologically-driven inhibition and complexation processes (see proposed model of biogeochemical interaction). Reports on cases strictly or partly analogous to the Saint-Denis microbialite exist but are scarce. This stresses the need for the recognition of a new, specific microbialite type of great interest in diverse research fields such as subsurface paleogeography and astrobiology.