The Siegenian delta: land-sea transitions at the northern margin of the Rhenohercynian Basin

Abstract

Abstract Analysis, interpretation and reconstruction of different depositional palaeoenvironments in the land–sea transition zone of the huge Lower Devonian siliciclastic delta system at the northern margin of the Rhenohercynian Basin (Central European Variscides, Germany) are the main subjects of this paper. Seven representative columnar sections have been taken at larger outcrops in the Mid-Rhine region. They furnished the main criteria for this analysis. The understanding and interpretation of the different depositional palaeoenvironments have been supported by regional geological lithological and biostratigraphical data. The sedimentary processes of distal meandering fluvial systems that gradually passed into fluvial-dominated deltaic systems of the elongate bird-foot type have been recognized. Lower delta slope to delta-foot plain conditions have been detected in a subtidal environment probably down to 100 m of former water depth. Thus, the Siegenian delta must have had a low inclined but wide slope down to the basin floor. Palaeoclimatic conditions at a latitude of about 10 to 30° south, in the basinal as well as in the source area of the Old Red Continent in the north, controlled the input of siliciclastic detritus as well as the depositional processes in the basin. Moreover, steady subsidence and an intermittent sea level played a prominent role during sedimentation. They held this system in balance for more than seven million years.