Depositional mechanisms and facies models of intertonguing aeolian environment and fluvial milieu in the middle buntsandstein of the mid-european triassic basin

Abstract

Intertonguing aeolian sand seas and fluvial braidplains occur abundantly in the upper part of the Middle Buntsandstein in various parts of the Mid-European Triassic Basin and are especially well-developed in the Eifel North-South-zone at the western margin of the depositional area. Based on assessment of the Buntsandstein regions and on comparative evaluation of other examples of interference between aeolian environment and fluvial milieu, the specific mechanisms controlling the significance of numerous distinctive sedimentary processes and the importance and nature of the mutual relationships are discussed. The main depositional mechanisms comprise migration, modification and merging of bedforms which are characterized by both similarities and distinctive features in each environment. The significance of migration, modification and merging in the aeolian milieu is ameliorated or deteriorated by the association of dune fields and river systems intersecting the erg in comparison to an extensive dune belt without coexisting stream networks. Common features in both aeolian environment and fluvial milieu are the cyclic composition of the sequence as well as depositional and erosional multistoreying of sediments by primary restriction of formation and secondary removal of parts of the cyclothems. The general facies zones of aeolian sand seas comprise the marginal or peripheral erg facies, the medial or transitional erg facies and the central or interior erg facies which are characterized by the extension of aeolian subenvironments, distribution of aeolian bedforms and degree of interference with associated fluvial systems as well as with the morphology of the pre-Triassic basement. The medial erg facies zone has the greatest significance for the evaluation of the mutual relationships as a consequence of the close association of nearly equally distributed aeolian and fluvial milieus. Considerable interference of the fluvial river systems with the aeolian belt result in overprinting and further subdivision of the general erg facies zones by regional environmental diversification which is best expressed in the complex of intertonguing aeolian dunes with fluvial channels and floodplains in the Middle Buntsandstein of the Eifel North-South-zone. The main influence in the narrow elongated depositional area is the regional variation of the dynamics of the fluvial systems as result of the sheltering, deflecting und punctuating function of the pre-Triassic basement bounding the basin at both sides, and as a consequence of supply of additional amounts of sediment material of lateral-marginal provenance to the axial-longitudinal transport. The Eifel North-South-zone is divided into five regions with different distribution and type of lithofacies associations in aeolian environment and fluvial milieu, comprising Southern Saar area, Northern Saar area, Southern Eifel, Western Eifel and Northern Eifel. The regional diversification of aeolian depositional environment stresses particularly the significance of the alluvial network and its behaviour in time and space as the dominant control on the distribution of dunes and interdunes. The distribution of small dune fields and large sand seas which are internally governed by division into the general aeolian facies zones and by regional diversification can be summarized in a basinal depositional model that comprises from the margin to the centre the succession of the proximal zone, medial zone I, medial zone II and distal zone. The occurrence of aeolian patches and complexes within these zones is controlled by major longitudinal and lateral changes of the river patterns as a consequence of variation of distance from the source area, palaeoslope gradient, and climatic and topographic influence of the margins. The four basin zones are characterized by variations in fluvial style predominantly in an ordered manner which are to a minor amount regionally punctuated by random changes. The intertonguing of aeolian and fluvial systems reaches its climax in the medial zone I. The major ultimate controls on the distribution and extension of dune fields and interdune flats are the tectonics of the source area in the proximal zone and medial zone I, and the moisture of the alluvial plain as a result of spatial extension and temporal persistence and periodicity of floods in the medial zone II and the distal zone.