SEARCH

SEARCH BY CITATION

SUMMARY1

Of the several uncommon types of large-scale cross-stratification encountered in Late Holocene upper-pointbar sandy deposits of the Rhine, one is examined in detail. This type is structurally bipartite as it consists of a relatively coarse-grained, upper interval of large-scale foresets, which indentate with the oppositely dipping small-scale foresets in the underlying finer interval. The lower interval is usually of smaller, and incidentally of the same thickness as the upper one. Together they constitute the so called “interwoven set”.

The interwoven set is regarded as formed by a mega ripple (dune) with in front of it, a simultaneously active system of small-scale, oppositely moving, ripples propelled by the backflow branch of the mega ripple's lee-side vortex.

Comparison of dip, strike, and principal bedding-plane (= horizontal) sections brought to light dissimilarity in the crestline orientations of the mega-ripple and backflow-ripple systems. The latter's oblique, or incidentally perpendicular orientation with respect to the mega-ripple front indicates a conspicious component of lateral water movement, which in the present outcrop was consistent from the left to the right, for the observer looking down(main)stream.

It is suggested that this lateral water movement is due to the radial (transverse) flow in the bend of a meandering river. From the sense of the radial flow the river's bend configuration (here: right hand turn) can be inferred. Relative wander velocities of mega- and backflow-ripple systems were deduced from the configuration in a horizontal section made through the interaction zone of the two structural intervals.