Facies distribution and coral-microbialite reef development on a low-energy carbonate ramp (Chay Peninsula, Kimmeridgian, western France)

The Chay Peninsula (western France) succession corresponds to a Kimmeridgian shallow and low-energy ramp system that developed in the northeast of the Aquitaine Basin. The sedimentation shows a transition from a carbonate-dominated to a mixed carbonate-siliciclastic depositional environment that takes place at the base of a major, third-order sea-level transgression. Facies partitioning and coral-microbialite reef levels (i.e., reef windows) suggest higher-frequency (fourth-order) sea-level oscillations. Reef-growth phases (i.e., high-frequency reef windows) are interpreted to have been controlled by fifth-order sea-level oscillations. In the mixed siliciclastic-carbonate system, successive prograding units of shoreface sediments pass into thick marl-limestone alternations over a distance of tens of metres. Such remarkable short-distance lateral facies variations suggest a complex sedimentary system made of juxtaposed deposits that rapidly shift through time. Terrigenous fluxes, in modifying the light intensity and the nutrient level in the water column strongly controlled the facies distribution and notably the reef composition and growth. Identified coral genera suggest mixed photo-heterotrophic mode of nutrition and low-mesotrophic conditions in marine waters. For each reef-growth phase, nutrient increase favoured large amounts of microbialites to develop, forming a crust that entirely capped the coral reef surface. Thus, high-frequency fluctuations in nutrient input in tune with fifth- (or higher) order sea-level fluctuations controlled the reef development at a millennial scale. The Chay Peninsula reef distribution and development provide insight into understanding the response of reef communities under climatically-induced environmental changes.