THE SEDIMENTARY-STRUCTURE RECORD OF DEPOSITIONAL CONDITIONS IN THE FLUVIAL-TIDAL TRANSITION ZONE (Invited Presentation)

Depositional conditions in the fluvial-tidal transition zone (FTTZ) are inherently variable. The most significant variation through most of the FTTZ in most rivers is the river-flood/interflood cycle; only in the downstream reach of rivers with a tide-dominated mouth are tidal currents the dominant process. This river-flood/interflood cycle causes the strength of river currents to change in a reciprocal relationship with the strength of tidal currents: the strongest river currents occur when the tidal currents are weakest and vice versa. Which process dominates is an example of the more general “magnitude—frequency” discussion that intrigued geomorphologists 40 years ago: do short-lived river floods do more work and create more morphological change than the tidal currents that act continuously throughout the year? The answer appears generally to be “yes”. It is likely that the potential reworking of a river-flood deposit by tidal currents is only an issue in areas near the mouths of tide-dominated systems, and in energetic channel-axis locations. In more landward locations and in areas that are strongly depositional, the preservation potential of the river-flood deposit is high. The sedimentary record of river-tide interaction is expressed differently at different locations through the transition and within any channel cross section. In channel-bottom locations, the tidal signal is cryptic and is shown by subtle variations in the nature of the fluvially generated cross stratification, or by the presence of tidal rhythmites in bottomset deposits. Higher on the bars in lower-energy settings that are strongly depositional, faithful records of the alternation between river-dominated/influenced and tide-dominated sedimentation can be preserved within IHS deposits. The salinity variations that accompany the fluvial-tidal dominance cycle influence the ichnology of the deposits and can generate bimodal bioturbation styles. In most cases, the ichnology reflects conditions during river low-flow periods when the system is least dynamic. If this is not taken into consideration, the long-term salinity will be over-estimated. Careful evaluation of the depositional conditions during the river flood and interflood times offers the potential for significant refinement in our understanding of the FTTZ.