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

Abstract

The Oolithe ferrugineuse de Bayeux Formation is located at the historical Bajocian stratotype of Sainte-Honorine-des-Pertes, north of Bayeux, Normandy. The condensed formation ranges from the base of the Humphriesianum Zone to the Parkinsoni Zone and is divided into four beds of decimetric scale. Three main microfacies are present: (1) oncoid rudstones, (2) ooid bioclastic 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, its lower part characterized by hemipelagic sedimentation indicated by the presence of planktonic foraminifers. The inferred depth is around 100 m. Free oxygen concentration was low. Dysaerobic conditions are indicated by a scarcity of benthic macrofauna. Ferruginous structures are numerous in the first two microfacies, and absent in the last. Hematite staining is not uniform and follows many sedimentary patterns. Among the more widespread Fe structures are perforation infillings with endolithic micro-organisms, microstromatolites, oncoids, ooids, blisters, coatings and hardgrounds. These structures can be associated and none 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 perforations and burrow; the hardgrounds; and microstromatolites. Petrographical and SEM examinations suggest that the laminated crusts (oncoids and hardgrounds) are formed by microbial iron mats dominated by filamentous bacteria and fungi. Seven types of microbes are recognized: filaments (five morphotypes), spheroidal bodies and stalked bodies. Filamentous microfossils of type 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 favor 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 soluble 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 microbial iron deposition pathways are also possible. It appears that, regardless of geological age (Paleozoic, Mesozoic) and geographical location, the same microbiological mechanisms are probably responsible 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 various paleo(micro)environments.