PRELIMINARY ANALYSIS OF LATE QUATERNARY SAND DUNES IN THE NORTHWESTERN MISSISSIPPI EMBAYMENT

Fields of sand ridges and elliptical hills, typically rising 5 to 6m above Wisconsin-age alluvial terrace surfaces they occupy, are associated with Late Wisconsin ancestral courses of the Mississippi River in the northwestern Mississippi Embayment. These features are recognized as sand dunes deflated from mid-channel bars in a braided glacial outwash channel, and they are interpreted as a course-grained counterpart of loess that blankets embayment uplands (primarily present toward the east).

We investigated the stratigraphy and textural characteristics of a large dune in a commercial sand pit near Diaz, AR, in order to document the depositional setting. We recognized 5 distinct sand stratal units, at least three paleosols, and a modern soil. The upper sand has large-scale cross-beds and is as much as 4.5 m thick at dune crests, accounting for most of the topographic expression of the dunes. Underlying sands display cm-scale cross-beds or horizontal bedding. One sand unit and an underlying paleosol horizon are locally contorted. Grain size analyses revealed that below the youngest paleosol all sand horizons are well-sorted very fine sand and above this paleosol sand horizons are well-sorted fine sand, suggesting an increase in wind strength during later dune deposition. SEM images of samples from all sand strata show grain-surface plates with up-turned edges characteristic of eolian grain frosting. Dune morphology and all cross-bedding indicate paleo-winds from the southwest (~225deg).

If these sand units are coeval with regional loess, then the westerly or northwesterly paleo-wind direction presumed for loess deposition may be in part inaccurate. IRSL age analyses of these sand horizons are underway.