The Meadow Bank (38o2’56”N; o5’31”W) is a nearly linear escarpment, 10 km long by 6 m high, at the mouth of a tributary valleyto the lower Wabash Valley, which was a major meltwater outlet for the Huron and Erie lobes of the Laurentide ice sheet. The Meadow Bank may have been sculpted by glacifluvial processes. However, the landform is similar in appearance to neotectonic features, and occurs within a seismically active area. Our research sought to identify either a sedimentologic or neotectonic origin for the Meadow Bank.Field activities focused on a 1.2 km transect across the escarpment, including tributary and trunk valley fill sequences. Earth electrical resistivity profiling results showed different resistivity characteristics on either side of the escarpment, but there was no evidence of vertical offsets. Eight (8) percussion cores to bedrock along the transect revealed a simple fining upwards deposit, comprised of a 1-3 m veneer of loess, eolian dune sand, and meandering stream deposits, over 15-18 m of last-glacial medium sand outwash, over 5-10 m pebbly coarse sand outwash. One core may possibly have penetrated 10 m of pebbly coarse sand outwash from the penultimate glaciation based on interpretation of a truncated paleosol. Although no marker beds were found, the textural changes could be differentiated into depositional sheets with apparently conformable contacts.The 150-250 μm quartz fraction was extracted from core samples and 16 OSL ages were obtained using SAR protocol on small (2 mm) aliquots. Overdispersion of representative populations of equivalent dose, De,ranged from 6.2-28%. Errors were ~10% of the central age value. In addition, fortuitous sampling of distinct fossiliferous beds enveloping one OSL sample confirmed that age as consistent with 14C ages. In general, the dating results showed good reproducibility and appeared satisfactory in that they conformed to expectations of Wisconsin-and early Holocene-age materials. The ages were mostly in stratigraphic order. The suite of ages supports the explanation that the Wabash Valley was completely filled with outwash between 38 ka, when ice was still advancing from the north and had not yet reached Illinois, and 22.5 ka, when ice reached its furthest extent about 160 km north (~39o30’N latitude) of the study site. This was followed by a restricted period of erosion during ice margin retreat, possibly by one or a few closely-spaced outburst floods, and subsequent reaggradation with outwash until about 17 ka. The eolian dunes were not dated, but extensive dune development at about 17.5 ka has been well-documented in western Illinois. With further deglaciation, the Wabash River transitioned to a meandering style, with the oldest terraces dating from ~15 ka.A zone of anomalously old ages (~45-35 ka) occurs in the middle of the section and is as yet unexplained. Possibly they are not anomalous but can be reinterpreted by adjusting ages within the standard deviations. Alternatively, the zone could represent a lens of remobilized Illinois Episode outwash with some residual luminescence. We are currently testing several sample replicates by multi-grain and single grain SARmethods at a separate laboratory.
|Published - 2014