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.