Formation of afterflow silt loam deposits and structural modification due to drying under warm conditions: an experimental and micromorphological approach

In earlier papers the authors discussed several micromorphological and textural characteristics of loamy afterflow deposits derived from loess material and formed during overland flow without splash. This discussion is completed by considering the critical load concentrations ccr for the start of sedimentation, taking into account the hydraulic properties of laminar flow. Furthermore, a remarkable combination of clayey and medium coarse, sandy laminae was observed in the deposits produced under laboratory conditions. Mechanisms are proposed to explain the affinity of these fractions for simultaneous deposition. When laminated silt loam deposits are found in the Pleniglacial Middle loess deposits in the southern part of The Netherlands or in the Belgian Hesbayan loess, showing structural modifications, for example cracks formed after sedimentation, it is not always clear whether these modifications were the results of cold or relatively warm conditions. This paper describes micromorphological and laboratory investigations aimed at identifying criteria representative of warm conditions. The experiments examined the effect of drying on the structure of laminated silt loam deposits produced by afterflow and enabled the characterization of the modifications formed. Drying could be done at room temperature, with circulating warm air, or at temperatures below 0°C. Elsewhere (see Coutard and Mücher, 1985), the influence of drying at temperatures below 0°C on the structures of an afterflow silt loam deposit will be discussed. In comparison with the deformation in laminated silt loam deposits due to freeze‐drying, these new formations are small and have only locally influenced the sedimentary structures. These structure modifications are: formations of cracks dominantly in clay layers and less (about 25 per cent of the cracks) in very fine silt layers, consisting of grains smaller than 10 μm in diameter, with only very locally deformations of the laminated structure; only incidentally the cracks are partly refilled by a subsequent overland flow deposit, mostly only followed by sealing of cracks; only incidentally the cracks are partly refilled by a subsequent overland flow deposit, mostly only followed by sealing of cracks; curling or argillaceous laminae, maximum 5 mm in upward direction; only very locally, formations of the tiny waved laminae, with a vertical amplitude between 100 and 400 μm, in combination with the occurrence of vesicles.