LATEST GLACIAL AND EARLY HOLOCENE MEGAFLOODS IN THE UPPER MISSISSIPPI RIVER VALLEY

We report on geomorphological and geochronological evidence for latest glacial and early Holocene megafloods in the Upper Mississippi River valley between Keokuk, Iowa and Hannibal Missouri. We have identified a series of fluvially cut benches and slackwater sediments that record minimum heights of megafloods that affected this part of the valley. Radiocarbon dates and crosscutting relationships with regional stratigraphic units document two distinct periods of flooding between 11,600 and 9,200 ¹⁴C yr B.P. The older flooding episode is of greater magnitude and is represented by a series of low benches cut into older Quaternary sediments and Mississippian bedrock along the western valley margin. Lag deposits consisting of erratic cobbles and boulders rest on these cut surfaces, and are overlain by loess-free alluvium, colluvium or alluvial fan deposits. The younger flooding episode, dated to 9,600-9,200 ¹⁴C yr B.P., is represented by distinctive reddish brown silty clay laminae and beds that occur at elevations lower than that of the modern floodplain in slackwater settings such as distal floodplains and abandoned channels. Both megaflood episodes were produced by drainage of glacial lakes. The oldest episode's timing suggests that it may be related to drainage of Glacial Lake Agassiz through the southern outlet during the Lockhart Phase. The occurrence of Superior-source reddish brown silty clay sediment indicates that the younger megaflood episode's source was drainage from Glacial Lake Superior via the St. Croix Valley.