Abstract

A radiocarbon-dated sediment core from Powers Fort Swale, located in the Western Lowlands of southeastern Missouri, provides sedimentological and palynological evidence for dynamic changes in fluvial regime and vegetation in the Central Mississippi Alluvial Valley during the late Pleistocene and Holocene intervals. During the late Wisconsinan glacial maximum, glacial meltwater carried by the Mississippi River maintained active braided streams. Sand grains in full-glacial and late-glacial sediments of Powers Fort Swale show evidence of reworking by eolian activity, with late Pleistocene formation of sand dunes continuing until 11,600 yr B.P. within the Western Lowlands. The sedimentological sequence indicates a progressive, stepwise loss of glacial meltwater and coarse sediment supply during the late-glacial interval, resulting in abandonment of braided-stream terraces. The initial, partial diversion of Mississippi River flow occurred in the late-glacial interval, possibly as early as 16,300 yr B.P., as the Mississippi River shifted from the Western Lowlands through the Bell City-Oran Gap in Crowleys Ridge and into the Eastern Lowlands. By 14,500 yr B.P., the complete diversion of meltwater through this gap coincided with a change in the Powers Fort sediment core from a spruce-dominated to an oak-dominated pollen assemblage. At 11,500 yr B.P., sedimentological changes reflected a changeover to a meandering river regime. The transition to pollen assemblages characteristic of Holocene temperate swamp forest occurred at 9500 yr B.P.

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