Abstract: Flow Patterns and Bar Morphology in Braided-to-Meandering Transition Zone--Red River, Texas and Oklahoma

Daniel E. Schwartz

Among fluvial sedimentary systems, the transition between the braided and meandering pattern has had limited coverage in the literature. This reorganization of major patterns involves a complex association between regional (climate, discharge, vegetation, etc.) and local (sediment size, flow velocity, permeability, etc.) parameters. The Red River in Texas and Oklahoma changes from a braided to a meandering pattern within a fluctuating transition zone. A 100-km segment of this zone is the subject of this study.

Medium sand- through pebble-sized sediments are abundant in the upper and middle parts of the transition zone, with fine-sand through coarse-grained sandsized sediments dominating the lower reach. Clasts, ranging from 2 to 13.5 cm, are common in the upper and middle reaches, less so in the lower.

Large scale channel bed forms are predominantly linguoid and transverse bars. Lateral, braid, and longitudinal bars are present in channels with straight, large-radius meander bend, or point-bar crossover segments. Channel-bar slip faces rarely exceed 1 m in height and chord length ranges between 10 and 200 m. Point bars are present throughout the zone, with height increasing and width decreasing downstream. Point-bar formation results from three distinct processes: (1) bar-toward-channel deposition, in which sand waves and linguoid, transverse, and chute bars migrate over the point-bar surface toward the channel; (2) apparent upstream bar migration resulting from the erosion of the downstream end with deposition on the upstream end of the next successive point bar; and (3) channel-toward-bar (normal "helical") deposition. Four types of small-scale ripples are present in close lateral proximity on many bar surfaces. These are: linguoid, sinuous, straight, and lunate. Straight and sinuous small-scale ripples of eolian origin cover subaerially exposed bars.

Stratification in the transition zone is variable, with no major trends in size or type of bedding feature. The most readily preserved structures result from flood deposition that represents less than 2% of the average annual discharge. Channel deposits are characterized by basal lag, graded large- and small-scale trough and tabular cross-stratification. Point bars contain large-scale trough, tabular, and small-scale trough cross-stratification as well as parallel lamination. Overbank deposits of the river flood plain are characterized by small-scale trough cross- and climbing-ripple stratification.

The transition zone is a preservable fluvial pattern. The structural and formational associations on the Red River differ greatly from previous descriptions of braided and meandering and braided-meandering rivers. Because of these differences, further examination of other "transfluvial" rivers is needed to understand the complexity and variability of modern and ancient fluvial systems.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC