The origin of horizontal alternating bedding in weichselian aeolian sands in Northwestern Europe

Abstract

Periglacial acolian sands of Weichselian age are widespread in the lowlands of western and central Europe. One facies of this sediment type, corresponding to the coversand sensu stricto, is characterised by a wavy parallel bedding and a generally horizontal position of the strata. Since this layering results from an alternation of finer- and coarser-grained beds, “horizontal alternating bedding” is the appropriate designation for it. By a detailed analysis of representative lacquer peels eight characteristic stratification types could be distinguished. These units are the smallest fundamental building blocks constituting the sediment under consideration. An idealised annual cycle of sedimentation is proposed. In summer, light and variable winds carry silt in suspension to the site of deposition whilst the sandy saltation load is left behind in a throughput zone farther upwind. Upon settling, the fines adhere to the surface which is wet or damp due to the thawing of the seasonally frozen ground. In winter, wind strength is generally greater so that deposition of sand prevails in the area whereas the fine-grained suspension load settles downwind. Deposition by snow-free winds results in the accretion of patches of well-sorted sand. Snow flurries, on the other hand, produce niveo-aeolian strata of generally more poorly sorted sand. In the cycle it is implied that: (1) The aeolian sedimentation basin must have been large and uniform enough to permit areal separation of sand and loess-like material; and (2) a seasonal shift in the centres of sand and silt deposition was inherent to the atmospheric pressure system of the formative period. The exposed part of the northern and eastern North Sea floor as it existed around the Middle Weichselian glacial climax assumedly was the principal source area for the deposits. The relationships between the periglacial aeolian sands with horizontal alternating bedding, loess and their integrades as described from the Belgian lowlands and Niedersachsen can be interpreted in the context of the proposed depositional cycle.