Chapter 1 - Deciphering the relative importance of fluvial and tidal processes in the fluvial–marine transition

Abstract

Abstract Sedimentation in the fluvial–marine transition is governed by the interaction of river and tidal currents. Tidal currents act continuously, albeit with small variations in strength as a result of neap–spring cyclicity and modulation by changes in river discharge. River currents, by contrast, commonly change more dramatically because of the presence of river floods. The superposition of river floods on the tides causes depositional conditions to vary temporally from less fluvially influenced/dominated (and more tidally influenced/dominated) during the times between river floods (the interflood periods) to more fluvially influenced/dominated (and less tidally influenced/dominated) during river floods. These temporal variations in the relative importance of river and tidal currents are recorded within individual beds in the point-bar and mouth-bar deposits of the fluvial–marine transition, creating a spectrum of possible deposit types depending on the longer-term ratio of river flood to tidal energy. In areas of fluvial dominance, tidal action is only present in the interflood deposits, if present at all, whereas in areas of tidal dominance, river-flood sedimentation can become cryptic and is indicated by intervals with coarser sand and a greater abundance of fluid-mud deposits. The ichnological character of the deposits preferentially reflects interflood conditions. A detailed analysis of the deposit characteristics allows deductions to be made about the strength of the tidal currents, the intensity of river floods, and the relative position of a given deposit within the fluvial–marine transition. Discrepancies between the proximality indicated by the nature of the river-flood deposits and the ichnology of the interflood intervals can give an indication of the relative magnitude of river-discharge fluctuations.