CENTRAL MISSISSIPPI RIVER VALLEY PLEISTOCENE-HOLOCENE TRANSITION

Mississippi and Ohio River Pleistocene braid belts have been mapped and dated by previous investigators in the Eastern and Western Lowlands. In this research we investigate the three-dimensional aspect of the Mississippi River Quaternary alluvium between 35° and 37° N through the interpretation of 3,374 geologic well logs. The braid belts are capped by a thin silt/clay horizon (Wisconsin loess) that overlies gravelly sand, which in turn overlies sandy gravel. The base of alluvium beneath the Ash Hill (27.3-24.6 ka), Melville Ridge (41.6-34.5 ka), and Dudley (63.5-50.1 ka) braid belt terraces of the Western Lowlands slope southerly 0.275 m/km and all have an average basal elevation of 38 m. Near Beedeville, Arkansas (< 50 km from Little Rock), the bases of these braid belt floodplains descend 30 m across an apparent northeast-striking down-to-the-southeast fault that coincides with the western margin of the Cambrian Reelfoot rift.

The maximum depth of flow (lowest elevation of base of Pleistocene alluvium) occurred in the Eastern Lowlands and appears to have originated from the ancestral Ohio River Cache Valley course in southern Illinois. In traversing from west to east in the Eastern Lowlands, the Sikeston braid belt (19.7-17.8 ka) has a basal elevation averaging 7 m, the Kennett braid belt (16.1-14.4 ka) averages 13 m, the Morehouse (12 ka) braid belt averages 24 m, and the Holocene (< 10 ka) Mississippi River floodplain has the highest average basal elevation at 37 m. Thus, along this easterly traverse the base of the Quaternary alluvium rises and it appears that the age of alluvium decreases. We interpret the eastward thinning of the floodplain alluvium in the Eastern Lowlands as being due to decreasing Mississippi River discharge as the river transitioned from the Wisconsin glacial maximum to the Holocene.

In the Eastern Lowlands the base of the Holocene Mississippi River floodplain averages 23 m higher in elevation than the bases of the Pleistocene braid belt floodplains. This high sub-alluvial surface (platform) is bound by the tectonically uplifted Joiner Ridge, Blytheville arch, Charleston uplift, and Bluff Line fault. The spatial relationship and similar histories of the platform and bounding structures suggest that their formation is tectonically related.