Processes, morphodynamics, and facies of tide-dominated estuaries

Abstract

As defined in this chapter, an estuary forms during a shoreline transgression and then fills during a progradational phase that is transitional to a delta. The spatial distribution of processes, grain sizes and facies within tide-dominated estuaries is predictable in general terms. Tidal currents dominate sedimentation along the axis, with wave-dominated sedimentation occurring along the flanks of the estuary in its outer part. Tidal energy increases into the estuary but then decreases toward the tidal limit, with a gradual transition to river-dominated sedimentation at its head. The interaction of the tidal wave with the morphology of the estuary, and with river currents, causes the outer estuary to be flood-dominant, with a net landward movement of sand. By contrast, the inner estuary is ebb-dominant, creating a bedload convergence within the estuary. The axial sandy deposits are typically finest at this location. In transgressive-phase estuaries, the main channel shows a low-high-low pattern of sinuosity, with the tightest bends (sinuosity ≥ 2.5) occurring at the bedload convergence. These bends experience neck cutoff in the transition to the progradational phase of estuary filling. The estuary-mouth region is characterized by cross-bedded sands deposited on elongate sand bars, although wave-generated structures can be important in some cases. Estuaries that are down-drift of major rivers have anomalously muddy outer estuarine deposits. Further landward, upper-flow-regime parallel lamination can be widespread. The margins of the inner estuary are flanked by muddy salt-marsh and tidal-flat deposits that can contain well-developed tidal rhythmites and evidence of seasonal variations in river discharge.