SEDIMENT DISTRIBUTION ALONG CONFINED MEANDER BENDS IN FLUVIAL AND TIDAL-FLUVIAL CHANNELS

Meander bends that are laterally confined are prohibited from expansion, which often leads to down-dip point bar translation. In highly sinuous bends, translation commonly results in concave bank deposition and preservation, and formation of a counter-point bar deposit. Grain size trends associated with the point bar to counter-point bar transition have been recently documented in fluvial meandering rivers; however, tidal-fluvial examples have been under-reported. Counter-point bars in fluvial meandering rivers are located immediately downstream from and adjacent to convex point bars, and their deposits consist of finer-grained sediment distributed along the concave bank.

In this study, six point bar to counter-point bar transitions are examined along meander bends from rivers with varying channel scale, discharge and tidal influence. The data indicate that observed trends of grain size fining from point bar to counter-point bar are consistent regardless of these parameters. We document high net sand to gross thickness (>0.7) point bars and low net-to-gross counter-point bars (<0.3). The average decrease in net-to-gross across the transition is 57%; the transition length scales to channel size, and is approximately three times channel width. Tidally-influenced counter-point bar deposits are recognized despite the absence of concave scroll patterns in floodplain/tidal flat areas. The lack of scroll bar topography contributes to the challenges in identifying counter-point bar deposits in tidally-influenced settings. Comparisons between fluvial and tidal fluvial examples studied show that increased tidal influence results in overall lower net-to-gross point bar deposits, however, counter-point bar deposits have equally low net-to-gross in both fluvial and tidal-fluvial examples. The results indicate that the overall proportion of fine-grained sediment in meander-belt deposits is not an indication of tidal influence, and purely fluvial examples are capable of preserving significant accumulations of inclined heterolithic stratification (IHS). Heterogeneity related to the point bar to counter-point bar transition in ancient fluvial and tidal-fluvial deposits is considered, with specific implications for fluid flow in analogous hydrocarbon bearing deposits.