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SUMMARY

Sedimentary structures in the Miocene fluviatile deposits of the Lower Rhine Basin, lying immediately beneath the lignite formations have been studied at Tagebau Frechen near Cologne. The structures investigated include slump structures, load casts, clay breccia, cross-bedding, sand fabric and orientation of driftwood.

The slump structures are composed of alternating laminae of clay and silt and have resulted from sub-aquatic sliding of a sedimentary sheet over a surface of erosion or of a foreset bed of 10–20° inclination. The slumping has caused folding or brecciation of the strata involved.

The base of a sand bed is difficult to obtain as the sediment is weakly consolidated. Load casts, therefore, have been studied exclusively in their internal structure. It is considered that the load casts of smaller dimensions have suffered a diminution of relief through settlement after the process of load casting had ceased.

Two types of clay breccia have been recognised, namely those which have undergone erosion and transportation by water and those which have resulted from slumping.

In an exposure, tabular- and trough-type of cross-bedding alternate with each other in geological succession. From these examples sand fabrics have been studied. Both in tabular- and trough-type of cross-bedding the apparent long axes show two maxima. In the former case, these maxima are disposed to each other at an acute angle, in the latter case, however, they are perpendicular to each other. Imbrication, after tilting the Sf-to original position, is down-current in tabular cross-bedding. In trough type, on the contrary, it may be upcurrent or the long axes of the grains dip symmetrically in up- and down-current direction.