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 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 Inspection of published and unpublished data from a number of outcrops of Recent and Tertiary deposits of the Rhine reveals that also in a sedimentary sense a transitional zone can be recognized.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.

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Large bank failures, comprising up to 10 6 m 3 of sediment, are common features along steep channel banks in estuaries and large rivers that consist of clean, ®ne sands, and are mostly assumed to be generated by sudden liquefaction of large masses of very loosely packed sand. Another less commonly recognized type of failure is manifested by the gradual retrogression of a very steep wall, steeper than the angle-of-repose. Instead of the voluminous surging plastic sediment-water¯ow, or hyperconcentrated density¯ow (sensu Mulder & Alexander, 2001) generated by liquefaction, this type of failure, known as breaching by dredging companies and hydraulic engineers, produces a sustained quasi-steady, turbidity current. To date, sedimentologists have not recognized the process of breaching as such. In this paper, it is suggested that breaching may be the origin of many thick, massive sand layers known from ancient deposits from various environments, notably in some turbidite successions. Possible differences in the sedimentary structure of the deposits produced by breach failures vs. liquefaction slope failures ( liquefaction¯ow slides) can be deduced from a knowledge of the sediment transport processes initiated by the failure. A ®eld study is presented on some poorly structured beds in the Eocene Vlierzele Sands in Belgium, which are supposed to have originated from liquefaction failures, but are reinterpreted to be the products of breaching. It is postulated that the local steep slope disturbance required to initiate an active breach can be produced by a small liquefaction slope failure ( liquefaction¯ow slide failure) or local erosion by river or tidal channel¯ow at the initial stage of the failure event.