Diagnostic sedimentary structures of the fluvial-tidal transition zone – Evidence from deposits of the Rhine and Meuse

Abstract

n mesotidal settings the transition of a coastal plain estuary to the river is marked by the change of a multiple ebb and flood channel configurationto a single channel system. At high river discharge fluvial processes operate, whereas in periods of low discharge the flow is complicated by a tidalcomponent and a landward intrusion of the salt wedge. These hydraulic and morphological characteristics make the transitional zone differentfrom the ‘pure’ fluvial and estuarine environment. Inspection of published and unpublished data from a number of outcrops of Recent andTertiary deposits of the Rhine reveals that also in a sedimentary sense a transitional zone can be recognized. In order to separate this zone fromthe upstream fluvial and downstream estuarine environment a sedimentological definition of the fluvial-tidal zone is proposed being the part ofriver that lies between the landward limit of observable effects of tidal-induced flow deceleration on fluvial cross-bedding at low river dischargeand the most seaward occurrence of a textural or structural fluvial signature related to the high river stage.Many structures of the fluvial-tidal zone suggest conditions of rapid deposition. This may reflect the high mobility of channels that characterisethe fluvial-tidal zone, or deposition in a bay-head delta setting. The diagnostic features of the fluvial-tidal transition zone, as manifested in thestudied outcrops, are described and illustrated in detail. They primarily refer to large-scale dune cross bedding and decimetre- scale successions(cycles) in vertical sense:Dune cross bedding: Foresets tend to be more or less regularly spaced or bundled. These bundles are often delineated by slightly erosive discontinuities,arranged in a daily variance mode due to remote tidal effects. The discontinuities are draped by silt and peat detritus rather than byclays, as is the case in the estuarine environment. In the pebble size grade, peat also predominates over clay. The dune foresets often passdownward into thick bottomsets, produced by suspension load on the gently inclined toe of the dune, leading to a knicked (‘shovel’) appearanceof this assemblage. In contrast with the fluvial and tidal counterparts, bottomsets in the transitional zone show a disorder in direction of crosslaminations.Foresets are frequently used by ripples moving up high on them (‘setclimbers’). These ripples usually are not of the backflow typeas found in fluvial deposits. Directions of ripple movement parallel to foreset-strike are also found. Incidentally, setclimbing is developed to sucha degree that entire crossbedded sets are made up of multi directional small scale cross lamination.Decimetre-scale successions: In the case of tidal deposits decimeter-scale rhythmic successions are generated either by seasonal changes or springneaptidal cyclicity. Fluvial cyclic successions can be found in fills of abandoned channels and represent high and low discharge alternations. Incontrast to this, diagnostic decimeter-scale successions of the transitional environment consist of alternating units reflecting fluvial dominanceat high river discharge versus dominance of tidal currents at low discharge.Although none of the sedimentary features of the fluvial-tidal zone is unique for this environment, they allow a rather safe signature of thisenvironment when several are found together.