Exposure of Late Pleistocene Mississippi River Meander-Belt Facies at Mt. Pleasant, Louisiana

Whitney J. Autin, A. Todd Davison, B. J. Miller, W. J. Day, Brian A. Schumacher

Exposure of a sedimentary sequence along a Mississippi River bluff at Mt. Pleasant, Louisiana, provides insight into the construction of the Prairie Terraces. This site serves as a type section for a late Pleistocene meander belt of the Mississippi River, and stratigraphic features have been traced beneath the Prairie Terraces in southeastern Louisiana.

A 23.35-m measured section reveals upper units of Peoria loess and mixed loess. Loessial sediments overlie proximal flood-plain deposits of a meandering river system. The upper flood-plain deposits are stratified natural levee facies consisting of coarse silt to fine sand with small-scale trough cross-stratifications, horizontal stratifications, climbing ripples, scour surfaces, and various soft sediment deformation structures. Within this deposit, a silt loam paleosol was identified with moderately developed blocky structure, clay films, and oxide stains concentrated along root traces. The flood-plain sequence grades downward into abandoned channel-fill facies, with five distinct cycles recognized by textural gradations. Each cycle fines upward from basal loamy or sandy textures to s lt loam or silty clay textures. Oxide stains and concretions are common in the upper fine-grained parts of each cycle. The third channel-fill cycle is characterized by a mottled paleosol with blocky structure, clay films, and oxide stains in its upper part. This paleosol represents an unconformable surface upon which the overlying fluvial sequence was deposited. About 400 m downstream from the measured section is a point bar sequence consisting of fine sand to granule gravel with large- and small-scale trough cross-stratifications, horizontal stratifications, scour surfaces, and clay drapes. The vertical and lateral succession of strata at Mt. Pleasant Bluff indicate deposition by a large-scale mixed-load meandering fluvial system. The modern Mississippi River has facies characteristics nalogous to those described in the upper Pleistocene sequence.

Correlation of individual strata away from the Mt. Pleasant Bluff measured section is difficult. However, the upper contact between mixed loess and stratified natural levee facies can be traced along the Mississippi River bluffs for considerable distances along with the underlying upper paleosol. Identification of the lower paleosol away from Mt. Pleasant Bluff is more difficult due to its limited exposure and possible lateral variability.

The described meander-belt facies are of a probable Wisconsin age and are here named the Mt. Pleasant Bluff alloformation. This age designation is based on position in the stratigraphic section, degree of preservation of sedimentary facies, character and degree of development of the upper paleosol, preservation of constructional topography beneath the loess, and correlation of this sequence to nearby sites with Wisconsin-age radiocarbon dates.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.