Iron bacterial and fungal mats, Bajocian stratotype (Mid-Jurassic, northern Normandy, France)

The Oolithe ferrugineuse de Bayeux Formation is located at the historical B ajocian stratotype of Sainte-Honorine-des-Pertes, north of Bayeux, Normandy . The condensed formation ranges from the base of the Humphriesianum Zone t o the Parkinsoni Zone and is divided into four beds of decimetric scale. Three main microfacies are present. (1) oncoid rudstones, (2) ooid bioclast ic packstones and (3) silty burrowed wackestones/packstones. Sedimentation took place in a very quiet environment, below the photic zone and below or near the storm wave base. The general setting is a distal carbonate ramp, i ts lower part characterized by hemipelagic sedimentation indicated by the p resence of planktonic foraminifers. The inferred depth is around 100 m. Fre e oxygen concentration was low. Dysaerobic conditions are indicated by a sc arcity of benthic macrofauna. Ferruginous structures are numerous in the first two microfacies, and absen t in the last. Hematite staining is not uniform and follows many sedimentar y patterns. Among the more widespread Fe structures are perforation infilli ngs with endolithic microorganisms, microstromatolites, oncoids, ooids, bli sters, coatings and hardgrounds. These structures can be associated and non e are mutually exclusive. Hematite-coated filaments of different sizes and shapes are observed in the micrite matrix: the walls of various organisms; the calcite crystals associated with the Fe cortical laminations; the perfo rations and burrow; the hard-grounds; and microstromatolites. Petrographica l and SEM examinations suggest that the laminated crusts (oncoids and hard- grounds) are formed by microbial iron mars dominated by filamentous bacteri a and fungi. Seven types of microbes are recognized: filaments (five morpho types), spheroidal bodies and stalked bodies. Filamentous microfossils of t ype 1 to 4 resemble the present-day filamentous bacteria (Beggiatoales and Cytophagaceae). Because of their large diameter and their branching nature, filaments of type 5 are possibly filamentous fungi. Another argument in fa vor of fungi is the presence of stalked and spheroidal bodies that resemble zoosporangia and oogonia of some Oomycota. In deep, calm and dysaerobic waters, many interfaces (e.g. between aerobic and dysaerobic waters) are present in the sediments. The stability of the s oluble reduced state of iron (Fe2+) is higher at such interfaces, and many ferric iron-encrusted microbial fossils are observed. iron could thus serve as an electron donor for microbial iron-oxidation processes. Other microbi al iron deposition pathways are also possible. It appears that, regardless of geological age (Paleozoic, Mesozoic) and geo graphical location, the same microbiological mechanisms an probably respons ible for the red color in calcareous stratified or unstratified bodies. The presence of fossilized iron-encrusted bacteria and fungi at interfaces may therefore serve as an indicator of anoxic to dysaerobic conditions in vari ous paleo(micro)environments. (C) 2000 Elsevier Science B.V. All rights res erved.