ANCESTRAL MISSISSIPPI RIVER: DISCHARGE EVIDENCE FROM THE UPLAND COMPLEX

Paleochannels interpreted from satellite images of northwest Mississippi provide a basis for an assessment of the timing and nature of ancestral Mississippi River discharge. Three modern arc-shaped valleys in northwest Mississippi are 15.8 to 19.7 km diameter, have a relief of 35 to 40 m, and a width of 0.8 to 1.6 km in Cenozoic uplands (Chickasaw Bluffs) near the Mississippi River where both loess and underlying Upland Complex gravel are thick. Similar size and close proximity of these arcs suggests a common cause. We sampled and measured the Upland Complex at five quarries on the Chickasaw Bluffs in northwest Mississippi (three on or near the arcs) and five on Crowley’s Ridge, Arkansas. Arc interiors contain large (1 to 3 m) cross-beds commonly orientated up-valley, and we conclude that draping of loess over very large meanders in Upland Complex gravels is the most likely cause of the arcs. This being the case, the large size of the arcs, the 0.27 m/km southerly slope of the base of the Upland Complex, abundant 10 cm cobbles, presence of meter-scale boulders (possibly ice rafted), and the up-valley cross bed dips suggest deposition by a meandering Mississippi River ancestor, herein called Ring River. Based on empirical formulas, we estimate that Ring River had a velocity of 18 km/hr (11 mph) and a discharge of 25 to 100 million liters per second (1 to 4 million cfs), twice the velocity and four to ten times the volume of the modern Mississippi River. We anticipate that additional data will enable us to choose among alternative hypotheses about the origin of the Upland Complex that include: 1) distal glacial discharge, 2) a very large pre-glacial drainage basin, or 3) a wetter pre-glacial climate.