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Fluvially Generated Heterolithic Stratification as a Tool for Determining Process Dominance and Location in the Fluvial-Marine Transition


At the mouths of rivers, and for a considerable distance inland in low-gradient settings, fluvial and tidal processes interact to create a unique suite of deposits. Most rivers display variations in discharge, and their hydrographs can be divided simplistically into two parts: the river flood when most of the sediment discharge occurs, and the longer-duration interflood period when sediment discharge is small. Tidal currents, by contrast, act throughout the year, although their strength varies on various time scales (e.g., neap-spring cycles). Most importantly, the increase in the strength of fluvial currents during river floods decreases the tidal influence by pushing the limit of tidal intrusion (and also salt-water intrusion) seaward. In the more proximal parts of the fluvial-marine transition, depositional conditions alternate between being river-dominated during river floods and tide-dominated during interflood/low-flow periods. This is commonly reflected in an alternation between coarser sandy deposits with a unidirectional seaward paleocurrent and minimal bioturbation (river-flood deposits), and finer-grained, commonly muddy, deposits in which tidal lamination and higher levels of bioturbation are present (interflood deposits). These deposits are indicative of a fluvially dominated, tidally influenced environment. In more distal areas where tidal currents are stronger, tidal lamination and/or reversed paleocurrents begin to occur in the waning-flow portion of the flood deposits, until eventually the tidal currents become strong enough to overprint the entire river-flood bed. In such settings, river-flood deposits can be cryptic, but can be marked by the coarsest sand and thickest fluid-mud beds. Bioturbation is typically more intense, but restricted, in the interflood deposits. This pattern indicates a tidally dominated, fluvially influenced environment. Considerable local variability is expected, but general trends in the character of river-flood and interflood beds are a powerful tool for determining location in the fluvial-marine transition and the nature of coastal environments at a larger scale.