Sedimentology of a Modern Point Bar at Raven Camp on the Red River, Central Louisiana
LOCK, BRIAN E., and CATHERINE E. BISHOP, University of Southwestern Louisiana, Lafayette, LA
The Red River waterway is a Congressionally funded project designed to establish a navigation channel from the Mississippi River to Shreveport. Channel straightening in the Raven Camp area, at river mile 136, has artificially cut off a meander loop and lowered local base level so that the point bar is almost completely exposed at low water stage in the late summer and fall while winter and spring high waters still completely submerge the bar and reactive sediment transport. Air photographs taken over the past twenty years record a surprising rate of channel migration and shifting sites of sand accumulation; these processes continue today, despite artificial constraints by the U.S. Army Corps of Engineers.
As the spring floodwaters recede, they expose a variety of sedimentary structures, dominated by sand waves over most of the bar surface. At the downstream end of the bar, where floodwaters rejoin the navigation channel, the sand waves have heights of up to 2 ft, although over most of the bar they average 6 in.; crest-to-crest interval is 10-15 ft. Climbing linguoid ripples cover the uneroded surfaces of the sand waves and grade up to mud drapes which mark the last stages of individual floods. Large-scale foreset bedding characterizes the bar margin adjacent to the low water channel, and well-developed reactivation surfaces are common.
During the stage of receding waters, symmetrical wave ripples having a wavelength (crest to crest) of slightly more than one inch are formed by wind-generated waves in very shallow standing water close to the water's edge.
Dropping water stages are recorded by convolute lamination and by internally chaotic beds of fluidized sand. These are structures that relate to dewatering of the sediment. Desiccation cracks and curled-up mud flakes are common. Many mud clasts are incorporated in subsequent flood-stage sands.
After exposure, point bar sands are further modified by raindrop impacts and associated erosion and by the formation of rills and deep gullies during stormwater runoff. During dry weather periods, eolian transport begins and texturally distinctive ripples form. Adhesion structures develop on damp sands close to the water's edge during eolian episodes.
Biological traces are surprisingly common and include the effects of rooted plants (particularly from the fast-growing willow seedlings), vertebrate and insect traces, and algal mats, as well as associated biogenic gas escape structures.
Deposits of one winter/spring season are commonly scoured and removed the following year by fresh deposition over a different part of the bar. Scour surfaces are abundant internal features of the point bar.
Study of the Raven Camp point bar over a period of several years provides important lessons for the explorationist seeking hydrocarbons in fluvial sands. Many of the sedimentary structures that might be seen in conventional cores are not ones traditionally regarded as indicative of point bar sands, and the reservoir heterogeneity and facies architecture are far more complex than much of the literature would suggest.
AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)