Early-middle Frasnian (early Late Devonian) sedimentology and magnetic susceptibility of the Ardennes area (Belgium): Identification of severe and rapid sea-level fluctuations

This paper focuses on the early and middle Frasnian stratigraphic interval from the Ardennes area in Belgium, with a multidisciplinary and high-resolution study, including detailed sedimentology and magnetic susceptibility study, to identify the main paleoenvironmental changes affecting this interval. Three mud mound levels are observed, in stratigraphic order, the Arche, La Boverie and Lion mounds, surrounded and separated by shales. The three mounds are characterized by the same facies and relatively similar facies evolution through time. The base of the mounds shows deep mound facies (mud, stromatactis, crinoids) and the upper part of the mounds shallow mound facies (lagoonal facies, laminites). The mound succession is interpreted as related to the following events: the base corresponds to transgressive and high stand system tracts; overlain by a sharp transition with shallow facies that correspond to a main regression; followed by the next transgressive system tracts, which corresponds to the upper part of this mound and the lower part of the next mound. Then, again a sea-level drop occurs below the top of this lithostratigraphic unit and is followed by the next transgressive stage etc. A similar transgressive-regressive history is interpreted for all three mounds. As the La Boverie mound is only 35 to 45 m-thick, compared to the 100 m-thick Arche and Lion mounds, the two sea-level fluctuations occurring at the base and top of the La Boverie mound are considered as very severe and rapid, occurring within 1 My. An important transgression is interpreted as occurring during the global negative carbon excursion, the punctata Event, recorded worldwide (synthesis in Racki et al., 2008). During this interval, strong and sharp variations are also recorded in the magnetic susceptibility curve.