Stratification in Modern Sands of the Red River, Louisiana

Abstract

Sedimentary structures of two modern point bars on a meandering portion of the Red River near Shreveport, Louisiana, were investigated by trenching. On one of them, the Beene point bar, twelve sets of trenches, 2-8 feet deep, were dug with a tractor-powered shovel. The trenches ranged from 25 to 250 feet long, and were dug in T-shaped and X-shaped patterns. The point bars consist mainly of fine-grained, well-sorted sands, commonly interbedded with and overlain by thin silt layers. Gravelly sands occur in the deeper parts of some of the trenches. The types of stratification observed include trough-shaped and tabular sets of cross-strata, and horizontal lamination. The most abundant type is trough-shaped cross-stratification. The trough-shaped sets are present on two different scales: the large sets have approximate, average thickness: width: length dimensions of 1:5:15 + (feet); and the small sets ("rib and furrow") have dimensions of 2:10:20 + (cm.). The surface expressions of the small sets appear to be cuspate ripples. The axes of large and small trough-shaped sets have a strong preferred orientation parallel to the local stream-flow direction. The resultant vectors obtained by summing observations on individual cross-stratification planes within large sets also point downstream, but have much weaker magnitudes because of the variability associated with the diverse orientations of cross-strata within any individual set. Spoon-shaped depressions on the surface of the bars, oriented with the tips of the "spoons" pointing downstream, may represent scours incompletely filled with large trough-shaped sets of cross-laminae. At the upstream end of one of these scours, cross-laminae in the upper part of the sand that partially fills the scours are "overturned" downstream. The deformation occurred before deposition of any overlying strata and probably during subsidence of river level.