RESPONSE OF THE MISSISSIPPI RIVER AND ITS TRIBUTARIES TO MIDDLE AND LATE WISCONSIN GLACIATION AND DEGLACIATION, LOWER MISSISSIPI VALLEY, MISSISSIPPI

Terraces and floodplains of four Mississippi River tributaries and two Mississippi River terraces in the central Lower Mississippi Valley (LMV) were cored, described, and dated. In the tributary terraces, one Late Wisconsin loess overlies middle to late Wisconsin alluvium. Loess thins from 5.5 m thick near the Mississippi River bluffs to 2.5 m, 14 km up valley. The underlying meandering-stream sediment includes thin overbank and thick channel-plug silt and clay and point-bar sand. The boundary between the loess and alluvium may be gradational or abrupt, but no buried soil was identified. Optically stimulated luminescence (OSL) dates of the upper fluvial sediment vary from 35 to 26 kya. A date on the overlying loess is 16 kya. No loess is present on the adjacent floodplain. OSL dates and previously published radiocarbon dates of floodplain sediment are generally < 14 kya. Stratigraphy and dates on alluvium in these Mississippi tributaries are consistent with those of the Mississippi Valley. Alluvium from Macon Ridge, a terrace on the west side of the valley at approximately the same latitude as the tributaries, is OSL dated 30-40 kya and is covered by up to 5 m of late Wisconsin loess and patchy occurrences of a middle Wisconsin loess (0-2 m) over a well-developed paleosol in the underlying fluvial sands. A low alluvial terrace without loess, to which the tributaries are presently graded, is dated 14.7 kya.

In the LMV, both the Mississppi River and its tributaries aggraded during the Middle and Late Wisconsin until deglaciation. Unlike tributaries in the northern LMV, sediment load of the tributary streams in the central LMV was adequate to aggrade at the same rate as the Mississippi River and no slackwater lake sediment has been identified. During deglaciation, high meltwater:sediment ratios caused incision, despite a rising base level further down valley.