The structure and genesis of Weichselian to early hologene aeolian sand sheets in western Europe

  • Schwan J
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Abstract

Weischselian to Early Holocene aeolian sands are widespread in the lowlands of western Europe. To a large extent these deposits occur in the form of sheet-like coversands with slipfaced dunes being much rarer. Whereas the latter type of landform corresponds to a single facies with dune-foreset cross-bedding (= aeolian facies 1), two structurally different facies are distinguishable in the sand sheets. This paper concentrates on the two sand-sheet facies which are referred to as aeolian facies 2 and aeolian facies 3. Data on these two types are based on (1) a survey of existing literature, and (2) a detailed analysis of lacquer peels and larger-scale exposures in England, The Netherlands and the Federal Republic of Germany. Aeolian facies 2 is defined by the spatial position of its beds, which may be either horizontal, inclined or oppositely dipping in the case of structures formed by fluctuating winds. Horizontal bedding is by far the most common type. Inclined bedding is related either to small, isolated dome dunes or to scoop-shaped deflation surfaces. The structures resulting from fluctuating winds are rare and do not represent an inherently large measure of directional variability of the wind regime. The internal structure of the beds is dominated by aeolian planebed lamination with or without concordantly infilled wind-scours. Degradation of this stratification type because of interference by coarse particles is discussed. Aeolian facies 3 is uniquely typified by the alternation of coarser- and finer-grained horizontal thin beds that are either wavy or even in shape. Depositional models of facies 3 are given at both the local and regional scales. In both cases the coarser-grained layers result from tractional deposition of saltating and creeping grains whilst the finer-grained strata were laid down by settling from suspension. Periodic changes in surface wind speed, the presence of a damp depositional surface and the availability of both sand and silt in the source area are necessary conditions for the working of the models. The large-scale model is associated with specific environmental conditions of the Weichselian Upper Pleniglacial. The local-scale model accounts for the fact that facies 3 is also found in units of Weichselian Late Glacial or Early Holocene age. The regional-scale model involves stepwise tractional transport over long distances so that grains of distant provenance were mixed with material from sources nearer to the receiving site. This process was an important control on the mineralogical composition of the resultant deposits. The prevalence of aeolian planebed lamination in both facies 2 and the coarser-grained layers of facies 3 is attributed to the interaction of three factors, viz. the rarity of topographic barriers, the sparseness of vegetation cover and a high ratio between wind energy and sand availability during the processes of transport and deposition. In the lowlands of Europe, sand sheets are gradually replaced in an easterly direction by coeval wind dunes. The possible causes of this phenomenon are considered. © 1988.

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Authors

  • J. Schwan

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