EFFECTS OF SEA-LEVEL AND CLIMATE CHANGE ON THE LOWER MISSISSIPPI VALLEY: PRELIMINARY IRSL DATING OF THE UPPER PLEISTOCENE PRAIRIE COMPLEX

The response of the Lower Mississippi Valley (LMV) to changes in sea level and climate is inferred from changes in the longitudinal valley profile reconstructed from well-preserved late Pleistocene fluvial deposits. To characterize the chrono- and lithostratigraphy of the upper Pleistocene Prairie Complex, 428 m of section from 51 sites was collected within 600 km of the LMV. Sedimentary texture, structure, grain-size trend, Munsell color and pedogenic features of the deposit were used to identify fluvial systems and depositional environments. Twenty representative infrared stimulated luminescence dates on the fine-grained polymineral fraction indicate that the Prairie Complex formed from about 200 to 40 ka, equivalent to Oxygen Isotope Stage (OIS) 6 to early 3. Along the fluvial segment from the northern to central LMV, the Prairie Complex consists of ca. 200 to 80 ka meandering Mississippi River deposits overlain by tributary deposits of the Arkansas River and draped in places by different loess units. This succession is also identified by Landsat-TM imagery where two superimposed generations of meander belts correspond to the older Mississippi River and the younger Arkansas River. A previously identified braided-stream surface of the Prairie Complex is also re-interpreted as a meandering channel based on surface point-bar morphology revealed by satellite imagery. In contrast, the fluvio-deltaic segment in the southern LMV consists of two distinct meandering river alloformations. Along the western valley flank, the older fluvial and deltaic surface is dated to about 115 ka. The younger surface, dated to around 55 ka, is about 10 m lower in the northern sector but converges downstream with the older surface. Downcutting during intervening sea-level falls may represent a hiatus that also created the terraces. The persistence of meander belts along the fluvio-deltaic segment around early OIS 3 may explain the absence of Roxana Silt along the lower reach of the LMV. This chronostratigraphy indicate that (1) upstream meandering channels transformed to braided channels as early as 75 ka while meandering continued to exist downstream until around 40 ka; and, (2) from OIS 5 to early 3, the LMV aggraded upstream and incised downstream in response to simultaneous sea-level fall and increase in upstream sediment supply.