INTERCALATING SLACKWATER LAKE AND OUTWASH DEPOSITS AT A BEDROCK VALLEY CONFLUENCE IN THE LOWER WABASH VALLEY

The Grayville 7.5’ Quadrangle, centered on 87.9373°E, 38.3126°N, includes the confluence of the buried Bonpas Bedrock Valley (BBV) with the trunk Wabash Valley (WV). The BBV is a short, ~1.5 km wide, and filled with up to 30 m of Quaternary sediment. The existing Bonpas Creek is underfit to the valley and is incised into the BBV fill. Bedrock-cored ridges and isolated hills protrude 10-30 m above low relief valley fills. The region was overridden by the Illinois Episode and earlier ice sheets but was affected only by proglacial processes during the Wisconsin Episode. The architecture of the valley fills was investigated with 3.5 km of shallow seismic shear wave profiling (SSW), 1 km of earth electrical resistivity profiling, probing, and study of archived well logs. Strong, undular reflections in the SSW profiles delineate a clear thalweg and slightly stepped slopes along the margins of the BBV. The BBV and WV were excavated before the Illinois Episode. A small tributary to the BBV hangs 4m above the BBV thalweg, possibly indicating glacial erosion, bedrock control, or several stages of excavation. Although the high ridges are covered by a veneer of Illinois Episode till, sediment of that or earlier glacial episodes has not yet been recognized in the BBV fill. During the Wisconsin Episode, rapid glacifluvial aggradation in the WV dammed the BBV, causing formation of a slackwater lake. Some of the outwash, dominantly sand with gravel, transgressed up the lower BBV as a delta or fan. Silt loam to clay sediment was deposited in the lake from eolian and overbank sources. The alluvial and lacustrine sedimentation progressively filled the BBV until the end of the Wisconsin Episode. Two to eight prominent subhorizontal reflectors in the SSW profiles indicate episodic erosion and sedimentation. A late-glacial jökhulhaup overtopped the walls of the WV and formed a wide, gently sloping and low relief swath that crosscuts the lower Bonpas Valley. The central portion of the swath was deposited from the main flow, whereas the bordering ridges were levee- or fan-like deposits that prograded in to the existing slackwater lake. Downcutting of the WV fill may also have occurred during this or related events. The modern floodplain of the WV is set ~7 m below the tributary valley fill deposits.