Coral-microbialite reefs in pure carbonate versus mixed carbonate-siliciclastic depositional environ-ments: the example of the Pagny-sur-Meuse section (Upper Jurassic, northeastern France)

Middle to Upper Oxfordian reefs of a shallow marine carbonate platform located in northeastern France show important facies changes in conjunction with terri-geneous contents. The Pagny-sur-Meuse section shows coral-microbialite reefs that developed both in pure car-bonate limestones and in mixed carbonate-siliciclastic deposits. Phototrophic coral associations dominated in pure carbonate environments, whereas a mixed photo-trophic/heterotrophic coral fauna occurred in more sili-ciclastic settings. Microbialites occur in pure carbonate facies but are more abundant in mixed carbonate-silici-clastic settings. Reefs seem to have lived through periods favourable for intense coral growth that was contempo-raneous with a first microbialitic layer and periods more favourable for large microbialitic development (second microbialitic layer). The first microbialitic crust probably developed within the reef body and thus appears to be controlled by autogenic factors. The second generation of microbialites tended to develop over the entire reef surface and was probably mainly controlled by allogenic factors. Variations in terrigeneous input and nutrient content, rather related to climatic conditions than to water depth and accumulation rate, were major factors controlling development of reefs and their taxonomic composition .