High-resolution facies analysis of a Lower Devonian deltaic marine-terrestrial transition (Nellenköpfchen Formation, Rheinisches Schiefergebirge, Germany): implications for small-scale fluctuations of coastal environments

The homogeneous succession of high-energy intertidal, predominantly sandy deposits in the Lower Devonian Nellenkopfchen Formation of Alken (Mosel area, Germany) is interrupted by two finer-grained fossiliferous units, LAFU (Lower Alken Fossiliferous Unit) and UAFU (Upper Alken Fossiliferous Unit) which have been subject to high-resolution facies analysis. The LAFU is composed of five subunits that are characterized by continuous trends in grain size, sedimentary structures, and the degree and kind of bioturbation. They are separated by distinct boundaries. Except for a basal twofold silting-up cycle rich in autochthonous to parautochthonous plant material, no superimposed trend is obvious in the LAFU. In contrast, the UAFU is composed of five distinct subunits, in general showing a shallowing-upward trend starting with rhythmic high-energy pulses at the base. Towards the top, some organic(plant)-rich layers reflect repeated flooding each time followed by silting-up and terminating in colonization of the surface by plants. In contrast to previous single-event interpretations for the two fossiliferous horizons, a number of distinctive sedimentary units prove a more complex origin. They reflect the combination and interaction of small-scale sea-level change, lateral shift of depositional facies, synsedimentary tectonics, and meteorological events in a sensitive land-sea transition. The present high-resolution approach shows that both fossiliferous units are the result of sedimentary processes in a distal deltaic setting that comprised a number of different depositional sub-environments including interdistributary areas of the lower delta plain.