MID-CONTINENT PRAIRIE MOUNDS ARE NEBKHAS (COPPICE DUNES)

Found on flat lying geomorphic surfaces older than late Holocene, usually fluvial terraces, approximately 40,000,000 mounds can be found extending from west of the Mississippi all the way to eastern Texas and from southern Missouri to the Gulf Coast. The question as to the origin of these mounds has sparked nearly two centuries of literature debate, with hypotheses such as erosion, gophers, seismicity, wind accumulation, and many others attempting to explain mound formation. Mounds of the mid-continent region are normally between 0.5-1.5m in height and between 10 and 30m in diameter and found abundantly in groups. Through detailed grain size analysis of 1 mound at each of 4 sites in differing regions of Arkansas (the Ozarks, the Arkansas River Valley, and the Gulf Coastal Plain) we obtained adequate textural data to conclude that the mounds in this region owe their origin to eolian deposition, much like the Nebkhas forming in New Mexico.

Each mound was cored at 4 points around its periphery in line with cardinal compass directions at a distance of 20% of the total mound diameter from the edge of the mound. A core was also taken at the apex of the mound and one in the intermound area. Core depth was determined in reference to the intermound. The cores were described, horizons defined and correlated, and a detailed grain size analysis was performed on each horizon of each core (totaling 144).

Our data indicates the coarse sediment fraction is concentrated on the north and west flanks of the mounds, suggesting that eolian deposition is the primary mode of origin for these mounds. The Nebkhas of New Mexico, known to form through accumulation of wind blown sediment trapped by vegetation, have shown to have coarse fractions on the upwind side suggesting a possible correlation between the two phenomena. If substantiated, significant paleo-wind data is available in mid-continent prairie mounds. Moreover, recognition of an eolian depositional regime extending from the Great Plains to the Mississippi River in mid-Holocene is crucial to our understanding of late Quaternary climate change.