Origin of Quaternary deposits west of Marianna Gap, Mississippi Alluvial Valley, Eastern Arkansas

A 13 km-wide gap in Crowley's Ridge is present near the town of Marianna in eastern Arkansas. Formation and timing of the gap were examined using geomorphic relationships of 5 surfaces surrounding the gap. These surfaces were identified in topographic maps, aerial photographs, and digital elevation models of the area. Eleven cores from this study and 10 cores from a previous study (Blum et al., 2000) sampled litho- and pedo-stratigraphic units 2 these surfaces. Samples for grain size, stable isotopes, and elemental composition aided in identifying the origin, mode and timing of deposition, and paleovegetation present during deposition of the sediment associated with the Dudley surface. Within and east of the gap, Pleistocene surfaces that might have provided information on gap formation have have been removed by Holocene meandering of the Mississippi River and subsurface data from these areas was not collected for this study.;The Marianna study area west of Crowley's Ridge is comprised of two terraces of the ancient Mississippi River which formed from 83--46 ka in response to fluctuations in sediment input and discharge driven by glacial cycles. The oldest and highest fluvial surface, the Rondo terrace is separated from the younger and generally lower Dudley fluvial surface by Lick Creek and Big Cypress channels but both terraces have a similar physical stratigraphy. After the braided rivers that formed these surfaces ceased to actively deposit sand (Rondo at 83.3 +/- 6.7 ka and Dudley at 46 ka), overbank clay aggraded across the area. Silt lamina within clayey overbank sediment of the Dudley braid belt indicates that initial loess deposition in the region was sporadic across an aggrading floodplain. Soil formation has obliterated the silt laminae in the upper part of the overbank deposit as both fluvial and eolian sedimentation rates decreased ca. 30--25 ka. Loess deposition rates rapidly increased between ca. 25--10 ka based on regional studies and the resulting Peoria Loess buries both terraces but is thicker adjacent to Crowley's Ridge and generally thins to the west irrespective of the surface it covers. Other than colluvium accumulating at the base of slopes and alluvium accumulating in the modern channels, deposition is minimal in the study area and soil development has prevailed during the Holocene.;The low elemental concentrations of fine sand from the Dudley braid belt are elementally similar and may be sourced largely from the Arkansas River drainage basin which contributed sediment to the Pleistocene Mississippi River in the study area.;Stable carbon isotopes indicate that vegetation in the area has been dominated by C3 plants throughout the past 80 ka, with a slight increase in C4 vegetation in the modern soil. This suggests that forested or cool climate grasses have persisted throughout most of this time. Warm season grasses have increased in abundance, but are not dominant during the late Holocene.;Attraction toward, but deflection southward of braid channels at Marianna Gap during active Dudley braid belt sedimentation, as well as thinner loess deposits West of the gap support the conclusion that Marianna may have existed in an ancestral form < 46 ka, but was probably fully breached shortly before 10 ka. The gap has been modified and enlarged after 10 ka by a meander bend of the Mississippi River meander belt 2.