MEANDER CUTOFFS, FLOODPLAIN LAKES: GEOLOGIC ARCHIVES IN THE LOWER WABASH VALLEY

Striking geomorphic features, including terraces, scroll bars, and floodplain lakes, in the lower Wabash Valley were constructed by Wisconsin Episode slackwater lake and meltwater sedimentation, followed by postglacial fluvial systems with episodic aggradation and degradation. Previous studies described fluvial and slackwater lake terraces deposited in an island braided river system during the last glacial-interglacial transition, between ~14,000 and 10,500 BP (¹⁴C). Several distinct subsequent meandering systems since 10,500 BP left terraced scroll bar tracts and floodplain lakes. We have been mapping fill of the Wabash Valley to refine the existing geologic framework and complement ongoing process studies of the active meandering channel belt. Meander cutoffs that occur as both relict and extant floodplain lakes are targeted as archives of the sedimentation and ecosystem history. The meander fill sequences typically comprise 2-6 m of fine alluvial and lacustrine sediment with zones of abundant gastropod, bivalve, and wood fossils, which overlie 2-3 m of sandy to gravelly point bar and channel bed deposits. Dating of individual quartz grains by OSL in the lowermost coarse deposits is expected to provide a maximum age for these channel cutoffs, while dating of fossils by ¹⁴C methods, or of intercalated very fine sand beds by OSL, in the lacustrine sequence is expected to constrain the sediment accumulation rates. The sedimentology and stratigraphy of the cutoff termini will be interpreted in terms of cutoff processes. We welcome collaborators to study the biota of these deposits.