ABSTRACT

The braided-to-meandering transition zone of the Red River in Oklahoma and Texas consists of four principal facies and two subfacies (channel bar, point bar, flood flat, flood plain, with the point-bar facies divided into sheet flow and channel flow point bar subfacies). Each facies and subfacies contains an assemblage of sedimentary structures which occur in response to the varying discharge regime of this sand bed stream. The channel-bar facies is characterized by fine-grained sand and a predominance of Pi- and Gamma-cross-stratification, plane-bed stratification, and scouring surfaces. Medium-grained sand with Epsilon-like- and Pi-cross-stratification, plane-bed stratification, and sand-filled mud cracks characterize the sheet-flow point-bar subfacies, while Pi-, Beta-, and Nu-cross-stratification and soft sediment faulting in fine grained sand characterize the channel flow point bar subfacies. Medium-grained sand with Piand Gamma-cross-stratification, plane-bed stratification, and scouring surfaces characterize the flood-flat facies, with very-fine-grained sand, Pi-, Nu-, and Kappa-cross-stratification, mud cracks, desiccated mud beds, thick mud slabs, contorted bedding, and burrows characterizing the flood plain facies.

Grain-size distributions and sedimentary textural parameters of these facies and subfacies are distinctive, and can be used to differentiate them. Flood-plain facies sediments are fine-grained, fine-skewed, and poorly sorted, with channel flow point bar subfacies sediments being well sorted. Channel-bar facies sediments are fine-skewed, with sheet flow point bar subfacies and floor-flat facies sediments tending to be coarse-skewed. Mean grain-size, standard deviation, and skewness are the most powerful discriminators. Scatter diagrams plotting these parameters display separations between the two point bar subfacies, the flood-flat facies, and the flood-plain facies. Channel bar facies sediments were scattered in all plots. Combining structural data with grain-size data is effective in separating transition zone facies, and may be useful in analyzing ancient sand bodies.