Paleoenvironment of an Early Devonian land-sea transition:: a case study from the southern margin of the Old Red Continent (Mosel Valley, Germany)

An interdisciplinary approach was used to investigate the facies and paleogeography of the Lower Devonian sedimentary sequence of the Alken quarry, Mosel Valley, Germany. This 87-m-thick sequence consists of stratified sandstones and sandy shales of the Nellenkopfchen Formation (uppermost Lower Emsian). Previous interpretations of the depositional environment include terrestrial, deltaic, and shallow-marine settings. Two distinct fossiliferous units contain abundant terrestrial plant remains and a diverse mixed terrestrial to marine fauna. Physical sedimentary structures are common. throughout, whereas bioturbation is restricted mostly to the fossiliferous intervals. Erosional surfaces frequently separate the beds. Aside from ripple cross-stratification and parallel bedding, longitudinal inclined stratification is most common. Channel-fill structures are less frequent. Scour-and-fill structures exhibit marked disconformities of irregular shape on a smaller scale (din). Mud-pebble lags at the base of laterally prograding cross-bedded layers, scour-and-fill structures, and drainage rills characterize the upper part of the section. Desiccation cracks, Wind-induced striation, and water-level marks occur more sporadically in the exposure. The sedimentary structures and the paleontological information indicate a marine to brackish depositional environment that frequently was emergent. The presence of conspicuous channel-related structures reflects intertidal conditions along the coastal region of a presumed Hunsruck Island /Archipelago. Lagoons and estuaries were bordered by extended tidal flats, in which migrating channels frequently occurred. Terrestrial plant remains, however, indicate a position at the land /sea interface, which was characterized by a complex configuration of different environments. The accumulation of concentrated plant material may have been related to distinct meteorological events such as hurricanes.