Intricated Cross‐stratification Due to Interaction of a Mega Ripple With Its Lee‐side System of Backflow Ripples (Upper‐pointbar Deposits, Lower Rhine)

Abstract

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.

Sorting in grain flows at the lee side of dunes

“…Of course the counterflow must be strong enough to form the ripples, although not so strong as to induce much resuspension. Boersma et al (1968) found thick sandy bottomsets below cross-stratified deposits of straight crested dunes in gravelly sand. Boersma et al were able to identify climbing counterflow ripples and wavy lamination from the counterflow, irregular foresets from the reattachment zone, and coflow ripples from downstream of the reattachment zone.…”

“…The laboratory conditions of Ribberink (1987), Kleinhans (2002) and Blom et al (2000) probably fit in cell 4, whereas the Termes (1986) data fits in either cells 4 and 8. The case described by Boersma et al (1968) is an extreme case of cell 8. The relict gravel dunes described by Carling (1996) and by Shaw and Gorrell (1991) should probably be classified in cell 6.…”

“…These fine deposits can be interpreted as toesets and bottomsets. It is not clear from the data whether the foreset itself was enriched with the sand, or whether the lowest sampled layer included both an unenriched lowest foreset and the underlying sandy bottomset and Allen (1963Allen ( , 1970 sand small flume Blom et al (2000) trimodal sand -gravel large flume Dillingh (1990) sand field active dunes Kleinhans (2002) sand -gravel large flume Love et al (1987) sandy -gravel field relict dunes after flood Ribberink (1987) sand -gravel large flume Termes (1986) sand -gravel small flume, Gilbert-type delta Hunter and Kocurek (1986) sand small flume, Gilbert-type delta Allen (1984) sand small flume Bagnold (1954) sand laboratory, air Bak et al (1988) sand laboratory, air Boersma et al (1968) gravelly sand field deposits Carling (1996) gravel field relict dunes after flood Carling and Glaister (1987) gravel flume Hunter (1985a,b) sand small flume, Gilbert-type delta Jopling (1965) sand small flume, Gilbert-type delta Klaassen (1987Klaassen ( , 1991 gravelly sand large flume Nemec (1990) gravelly sand field, Gilbert-type delta Shaw and Gorrell (1991) Makse and Herrmann (1998) stratification and segregation microscopic grain interactions thus was finer than the layer of foreset material just above the lowest one. Ribberink (1987) sampled the vertical sorting in dunes of about 0.04 m high after experiments in a large flume with a bimodal sediment with modes at 0.78 and 1.19 mm (see Fig.…”

Dune‐Phase Fluvial Transport and Deposition Model of Gravelly Sand

“…The result is an upward fining deposit. Allen (1963Allen ( , 1970, Boersma et al (1968) and many others found a distinct upward fining within subaqueous dunes in the laboratory and in the field, indicating that sorting in the grain flow plays an important role. Although this sorting principle related to grain flows is well known, a mathematical description of the process is not available.…”

“…Two types of deposits are distinguished genetically, according to Boersma et al (1968), Allen (1970), Ribberink (1987) and Kleinhans (2001): the cross-bedded deposit with an upward fining, and the lag deposit. Cross-bedded deposits are formed by the propagation of dunes by discontinuous grain flows of bedload sediment at the lee side of a dune.…”