Fluvial conglomerates in continental red beds of the Buntsandstein (Lower Triassic) in the Eifel (F.R.G.) and their palaeoenvironmental, palaeogeographical and palaeotectonic significance

Abstract

Fluvial conglomerates occur in a variety of facies within the continental red bed sequence of the Buntsandstein (Lower Triassic) in the Eifel at the western margin of the Mid-European Triassic Basin. The alluvial rudites are classified into channel-floor lag, channel-bar, channel-top lag, crevasse-splay and floodplain-lag conglomerates. The conglomerates were predominantly deposited in Scott-type, and in subordinate amounts also in Donjek-type and South Sasketchewan-type braided rivers.The vertical distribution of clast size and frequency within the megacycles reflects the palaeotectonic sequence of uplift events in the source area, resulting in supply of large amounts of coarse detritus which cause recurrences in fluvial style. Gradually upwards diminishing size and abundance of gravel indicate continuous lowering of source area relief during weakening endogenic activity or even tectonic quiescence after final peneplanation. The palaeoenvironmental interpretation of the individual members and the palaeotectonic analysis of the conglomerate successions are combined with a reconstruction of the depositional history of the Eifel Buntsandstein which comprises an evolution of fluvial deposition. The terminal stage of the palaeoenvironmental evolution is highlighted by a climax of regional diversification of conglomerate sedimentary milieu. The horizontal distribution of clast size and abundance delineates the palaeogeographical evolution of the elongated Eifel Basin, giving evidence of a progressive sidewards extension of the depositional area with time. Lateral supply from parts of the western margin and mixing of proximally derived coarse clasts with distantly delivered fine detritus result in highly inhomogeneous gravel distributions within the sedimentary region often not being characterized by gradually decreasing size and abundance of gravel in downstream direction.The stepwise southwestwards shift of the marginal centre of supply of coarse gravel with time during the course of the lateral extension of the basin results in ascension of the fluvial conglomerate wedges into successively younger formations comprising consecutively Middle Buntsandstein, Upper Buntsandstein, Lower Muschelkalk, Middle Muschelkalk and finally Middle Keuper. The extraformational conglomerates mainly delineate the palaeogeographical framework, whereas the intraclastic rudites primarily give evidence of processes operating within the depositional area, reflecting erosion and reworking of overbank sequences with floodplain fines, palaeosols and aeolian dune sands during lateral channel shift. The Bröckelbank intraformational carbonate breccias to conglomerates originating from reworking of calcrete palaeosols are a peculiar facies element with particularly high palaeoenvironmental significance. They document the frequency, spatial extension and temporal persistence of pedogenesis within the alluvial plain. The size of the extraformational clasts gives evidence of the range of current velocities during aggradation of the channels. The distribution of conglomerate type and clast characteristics within cyclothems indicates the presence of stage fluctuations and variations in discharge in parts of the sequence. The preservation of complete cyclothems or the multistoreying of channel bar conglomerates reflects the differential importance of the two main mechanisms controlling accumulation of sediments in the alluvial plain: primary-depositional restriction to suppression of formation and secondary-erosional removal to destruction of finer-grained channel sediments, floodplain deposits and aeolian dune sands or palaeosols. Condensation of a series of depositional events within monotonous stacked conglomerate sequences obliterates the full range of sedimentary processes and mimics periods of long-term stability of conglomerate formation. Waning-flow mud drapes are particularly significant instruments to split up the individual genetical units of formation of fluvial conglomerates, often comprising discrete flood phases separated by stage fluctuations of different magnitude. The homogeneous or microcyclic composition of the coarse member reflects uniphase or multiphase aggradation of watercourses with gradually declining or abruptly waning individual flood periods. The fluvial rudites thus are of high significance in the reconstruction of the depositional milieu in the Eifel Buntsandstein and provide a key to detailed interpretation of palaeoenvironmental processes, palaeogeographical constellations and palaeotectonic events which could not have been identified to such an extent in exclusively sandy sequences.