RECONSTRUCTING FILLING OF A RED RIVER PALEOCHANNEL USING GEOPHYSICAL, SEDIMENTOLOGIC, AND GEOMORPHIC APPROACHES

The Red River, located in Southwest Arkansas, is a highly active meandering fluvial system with prolific abandoned channels, most of which are neck cutoffs. Because of the rapid migration, active channels commonly erode into incompletely filled older abandoned channels. The fill of these channels is not likely to be the conventional oxbow lake fill that accumulates in a quiet-water lacustrine environment throughout the fill cycle. Instead, it is hypothesized that where an active channel cuts into an incompletely filled abandoned channel, floodwaters will crevasse across the relatively low bank of the abandoned channel intersection and deliver relatively coarse sediment (sand and silt) into the abandoned channel. These coarse crevasse deposits will interfinger with fine-grained lacustrine sediment that accumulates between flooding events.

Three cores were taken along a transect across an abandoned channel and its associated cutbank. The transect is less than 90 meters from a younger paleochannel that cross cuts the channel being studied and could have potentially crevassed into the older abandoned channel being studied. Descriptions and grain size analysis show that the fill is interbedded very fine sand, silt, sandy silt and a few beds of clayey sediment. Near the bank, the amount of sand relatively decreases and the silt and clay content relatively increases compared to the center of the channel. These core descriptions do not match the cutoff bar sand proximal to the neck cutoff, the fine-grained lacustrine fill at a distance from the neck cutoff, or a chute channel cutoff and tentatively support the crevasse hypothesis.