TRIBUTARY VALLEY FILL IN RESPONSE TO PLEISTOCENE MISSISSIPPI RIVER AGGRADATION WITHIN THE WESTERN LOWLANDS OF THE LOWER MISSISSIPPI VALLEY

Tributary valleys in the northern alluvial valley were inundated by the Mississippi River during the late Pleistocene and Early Holocene and filled with slackwater sediment. Sixteen cores from eight tributaries along both margins of the Western Lowlands expose valley fill up to 18.6 m thick. The basal fill is a poorly consolidated lacustrine dark gray silty clay which may include beds of silty sand and brown silty clay. The absence of root pores, oxidation, and unconformities indicates that the valleys were continuously inundated and water was too deep for plant growth. Sand becomes minimal and clayey silt dominates in the upper 4 m of the deposit. Five meters of oxidized clayey silt with abundant root pores overlies the grey lacustrine sediment. This unit accumulated as the lakes infilled and became aggrading fluvial systems.

OSL dating of the valley fill indicates that aggradation in the tributary valleys began nearly 100 kya and lacustrine sediment continued to accumulate until 9 kya, approximately 10 ky after the Mississippi River permanently abandoned the Western Lowlands. Chemical signatures of the sand fraction in the both the sandy and the silty gray lacustrine sediment that accumulated between 40-12 kya is uniform, except for the brown silty clay beds present in several localities. The lower sandy lacustrine sediment accumulated while the Mississippi River occupied the Western Lowlands. The upper silty lacustrine accumulated after the Mississippi River abandoned the Western Lowlands and probably represents a rapid input of eroded loess from the uplands beginning about 14 kya. Floodplain aggradation of the reworked loess continued throughout the Holocene, but at a progressively slower rate and may include some material eroded from the pre-Quaternary rock/sediment. The surficial oxidized shallow lacustrine and fluvial sediment is chemically distinct from both the gray and brown lacustrine deposits, probably due to secondary precipitation of sand-size concretions in the weathering environment, rather than a different source.