Neritic Sedimentation of the Lower and Middle Devonian in the Rheinische Schiefergebirge East of the River Rhine

Abstract

During the Early and Middle Devonian the general paleogeographic development of the central Phenohercynian belt was controlled in the north by the Caledonian uplands of the Old Fed Continent and in the south by the variable configuration of the Mid?German Crystalline Pise. In the deposition area between, clastic shallow marine sediments accumulated up to 4000 m in thickness by a large delta complex prograding from the north. Relative lithological and sedimentological continuity and extreme thicknesses of the various clastic wedges strongly supports the concept that the high clastic supply and sedimentation rates were balanced with rather rapid subsidence of the sedimentary troughs for long periods of time. Apart from the Early Devonian sedimentation in the south where clastic debris was also supplied from the Mid-German Crystalline Pise, isopach maps and the distribution of litho- and biofacies reflect the rather continuous and principally unilateral input of siliciclastic material from a northern source area. During the late Early Devonian and Middle Devonian a general retreat of the shoreline and an overall northward migration of the accumulation centers coincided with the gradual denudation of the source area in the north. Irregularities in this broad scheme were probably related to differentiated sediment loading and syndepositional fault movements within the tectonically unstable shelf area. In the Siegenian the first and most extensive depocenter, the “Siegen Trough”, apparently developed as a graben-like structure at the time when nearshore and shallow marine clastic deposits dominated throughout the central Rheinische Schiefergebirge. Since the Late Emsian a shelf/basin contrast evolved which became more accentuated by the reef buildups along the SW-NE trending shelf edge in the late Fifelian and Givetian. In this area smaller depocenters formed as halfgraben structures. Typical facies examples are presented which show the rather low topographic gradient and the extensive continuity of extreme shallow marine facies of the Early Devonian shelf. This contrasts to the more differentiated NW-SE trending facies pattern during the Middle Devonian. Typical facies units are represented by the Hobrack Beds (Eifelian) and Honsel Group (Givetian) which include environments ranging from the nearshore inner shelf area to the distal outer shelf and the bathyal realm. The distal to proximal shelf gradient is indicated by decline of faunal diversity, increase in opportunistic species and changes in characteristic benthic assemblages.