ABSTRACT

A large excavation for a navigation lock, dug in a now-abandoned and partly filled meander loop of the Mississippi River along Old River, exposed around its base a 40-foot vertical section of point-bar sands that had been deposited below mean low-water stage. Data from U. S. Army Corps of Engineers borings made it possible to determine limits of this bar sand and its relationship to enclosing alluvial deposits. The exposed bar deposit is a lithic arenite and is composed of well-sorted medium to fine-grained sands incorporating thin layers of pebbles, macerated plant debris, and sparse heavy-mineral concentrations. Armored mudballs are present in the upper portion of the exposed section; wood fragments are common throughout. The entire exposed section is characterized by festoon crossbedding--the only type of bedding present. The festoon crossbeds formed in response to penecontemporaneous scour-and-fill processes accompanying downstream migration of sand waves during bar deposition. History of river development in this area is well known; it was therefore possible to relate measured directional properties of the sand deposits to current directions of the river at the time of deposition; the average dip direction for the crossbeds was S. 40° E., in close agreement with local channel alignment--S. 45° E. Preferred grain orientation was S. 26° E.