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Bedforms and Bars
Bed-Form Distribution and Internal Stratification of Coarse-Grained Point Bars, Upper Congaree River, S.C.
Ground and aerial studies demonstrate that point bars along the upper Congaree River show considerable variation in size, topography, morphologic features, facies and grain size patterns. A generalized depositional model for these coarse grained meander deposits consists of bar-apex, midbar, distal-bar, and the scour-pool and crossover facies. Point-bar facies are distinguished by their surficial morphologic form, internal stratification, and grain-size characteristics.
Well-developed coarse-grained point bars contain sand and gravel near the bar-apex. The mid-bar facies is covered by sandy megaripples that commonly migrate over the stoss side of large transverse bars attached to the point bar margin closest to the channel thalweg. Distal portions of the bar may develop both chute channels and chute bars resulting from high-discharge events. The scour-pool and crossover facies are characterized by transverse bars, sandwaves, and megaripples in between consecutive meander bends and deep scour troughs within the meander bend.
Observations of sediments at the surface and in shallow trenches indicate that the apex gravel material represents a lag deposit. Large-scale tabular cross beds of transverse bars and small trough and tabular bedding of megaripples characterize the mid-bar facies. Chute bars contain complex bedding that indicates their deposition during multiple high-stage flow events.
Grain-size characteristics, stratification and physiographic features that occur in these coarse-grained meander deposits establish criteria for the recognition of point bar sequences in ancient rocks.
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