TIDES AND DELTAIC MORPHODYNAMICS (Invited Presentation)

Tide-dominated and tide-influenced deltas are not widely recognized in the ancient record, despite the numerous modern and Holocene examples, including eight of the twelve modern largest deltas in the world, like the Ganges–Brahmaputra, Amazon, Chang Jiang, and Irrawadi.

Tide-dominated or -influenced deltas are assumed to be more common in inner-shelf or embayment settings, because wave energy is expected to be higher and tidal energy lower in outer shelf and shelf-edge. However, there are ancient examples of tide-influenced shelf-edge deltas, indicating that the controls on tidal morphodynamics in deltas are not yet well understood.

This paper asks the following questions: (1) How do tides influence delta deposition, beyond creating tidal facies? (2) Does tidal reworking create specific geometries in delta clinoforms? (3) Does tidal reworking change progradation rates of deltas? (4) Is significant tidal reworking restricted to inner-shelf deltas only? (5) What are the conditions at which deltas may be tide-influenced or -dominated at the shelf edge? (6) What are the main morphodynamic controls on the degree of tidal reworking?

The paper utilizes a dataset of multiple ancient and modern deltas, situated both on the shelf and shelf edge. We show that beyond the commonly recognized shore-perpendicular morphology and the recognizable tidal facies, there are changes in delta clinoform morphology, morphodynamics of delta lobe switching, delta front progradation rates, and the nature of the delta plain. Strong tidal influence is here documented to promote subaqueous, rapid progradation of deltas, by efficiently removing sediment from river mouth and thus reducing mouth bar aggradation and fluvial delta plain construction rates. The delta front clinoforms become gentler and longer. Tide-dominated deltas tend to maintain a funnel shape and show low lobe switching rates. The funnel and thus river mouth position is further stabilized by fine-grained sediment accumulation on marginal tidal flats due to the flood current sediment transport. All these effects weaken as the deltas prograde to the shelf edge, due to the loss of vertical (and lateral) restriction and tidal amplification. Here significant tidal reworking is restricted to topographic irregularities.