Article révisé par les pairs
Résumé : More than 5,500 carapaces, valves and fragments of ostracods, were extracted from 48 samples collected in the stratotype of the Terres d'Haurs Formation (= Fm) and in the very base of the stratotype of the Mont d'Haurs Fm, at the Mont d'Haurs, close to Givet. Fiftytwo species belonging to several assemblages of the Eifelian MegaAssemblage have been identified. They are generally indicative of neritic marine environments below fair-weather wave base, some even below storm wave base. Ostracods indicative of semi-restricted environments are also present but the sedimentological analysis displays that these ostracods have been mainly transported from these shallow settings. Close to the boundary of the Terres d'Haurs Fm and the Mont d'Haurs Fm, thick-shelled ostracods indicate that the energy of the environment increased. The richness in ostracods and their great diversity in the two studied sections prove that the living conditions were particularly favourable for these crustaceans. Ten microfacies are recognized, the succession of which (from 1 to 10) constitutes a standard shallowing upward sequence, with environments ranging from open marine, near storm wave base, to coastal, close to subaerial exposure. The microfacies analysis points to a carbonate ramp system with oolitic shoals and algal shoals separating semi-restricted and coastal areas from the open marine environment. Storm events and those related to the wave activity redistributed many organisms, which formed diversified communities with abundant echinoderms, bryozoans, molluscs and brachiopods in the peri-shoal environments. The lithological curve reflects a progessive and transgressive evolution in two phases marked by two parasequence sets: the first set records the destabilization of the overlain carbonate platform (Trois-Fontaines Fm) leading to the establishment of a shallow "open lagoon" in the inner ramp, the second set corresponds to the development of several shoals at the inner-mid ramp zones. No important reefal episode is present and shoals are of modest relief. The salinity and energy were the key parameters controlling the zonation of the organisms and the distribution of the environments. Low-field magnetic susceptibility (XLF) values are weak and eight magnetic susceptibility evolutions are reported along the lithological column. The magnetic susceptibility and microfacies curves are more or less mimetic in the lower half of the section and opposite in the upper part of the Terres d'Haurs Fm. A significant decreasing trend of the low-field magnetic susceptibility values across the boundary between the Terres d'Haurs Fm and the Mont d'Haurs Fm is presented. Two linear regression models show a moderately positive correlation between XLF values and microfacies in the lower half of the section and a moderately strong negative correlation between these two parameters in the upper part of the Terres d'Haurs Fm. The water agitation is highest in the mid and inner ramp, associated to the oolite and algal shoal environments and corresponds to the lowest mean XLF values presented in the models. The lower half of the section corresponds more to a carbonate platform profile in opposition to the second model, which confirm the carbonate ramp morphology. The average XLF values in the upper part of the Terres d'Haurs Fm are more homogeneous compared to those reported in the lower half of the Terres d'Haurs Fm. High-resolution stratigraphie correlation for the base of the Mont d'Haurs Fm in Belgium and France is proposed due to similar XLFdata established in the "Les Monts de Baileux" section 40 km distant from the stratotype area.