In a Lower Carboniferous formation south of Cork, Ireland, characterized by sand/mud alternations at varying scales, several lithotypes could be singled out on the basis of relative sand content and primary sedimentary structures. The lithotypes are arranged in several types of sequences: coarsening upwards, coarsening‐fining upwards, random, and in rare instances: fining upwards. With increased gross sandcontent the sand intercalations thicken and show a richer array of structures, reflecting a wider range of energy conditions. The majority of the structures is interpreted as wave generated. Current formed structures play an insignificant role. Tidal effects could not be ascertained. The alternation of low‐ and high‐energy lithotypes, occurring in sandy as well as muddy parts of the sequences is explained as resulting from the interplay of storm and fair weather conditions. The effect of shoaling bathymetry is reflected in sequences by an increasing importance of high‐energy lithotypes. The environment of deposition can be visualized as a muddy shallow marine platform on which longshore directed sandy shoals were formed under the influence of wave action. Depending on the degree of wave agitation and the availability of sand the shoals reached different stages of maturity: incipient, submerged or emergent. The sequential type encountered in vertical sections depends on the location of the line of logging and varies laterally as sandlenses thicken, amalgamate or die out.

In the mesotidal (tidal range 3·5–4·9 m) Westerschelde estuary (The Netherlands) the intertidal part of a sandbank was the subject of systematic observations of: (1) hydrographic properties, (2) the distribution and response pattern of various types of bedforms, and (3) the sedimentary structures produced. Ebb and flood usually differ considerably in strength, giving rise to a clear dominance of one over the other, which may change over the neap‐spring tide period. Parallel, long‐crested sand‐waves and irregular, short‐crested dunes have a different response to the neap‐spring variation in current velocity. Because one tide (usually the flood in our area) predominated over the other, the internal structure largely consists of unidirectional cross‐bedding, separated into a succession of tidal bundles, each formed during one tide. The tidal bundles are arranged in a lateral sequence reflecting neap‐spring tide periods and differing in character with location. Within the tidal bundle, reactivation, full vortex and slackening structures reflect acceleration, full stage and deceleration of flow respectively in the dominant tide. The full vortex structures tend to be well developed around spring tide but disappear towards neap tide. The subordinate tide carves ‘pause planes’ which enclose the tidal bundles. These pause planes are either erosional or depositional (mud).

In mesotidal settings the transition of a coastal plain estuary to the river is marked by the change of a multiple ebb and flood channel configuration to a single channel system. At high river discharge fluvial processes operate, whereas in periods of low discharge the flow is complicated by a tidal component and a landward intrusion of the salt wedge. These hydraulic and morphological characteristics make the transitional zone different from the ‘pure’ fluvial and estuarine environment. Inspection of published and unpublished data from a number of outcrops of Recent and Tertiary deposits of the Rhine reveals that also in a sedimentary sense a transitional zone can be recognized. In order to separate this zone from the upstream fluvial and downstream estuarine environment a sedimentological definition of the fluvial-tidal zone is proposed being the part of river that lies between the landward limit of observable effects of tidal-induced flow deceleration on fluvial cross-bedding at low river discharge and the most seaward occurrence of a textural or structural fluvial signature related to the high river stage.

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.

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