Applying luminescence dating on fluvioglacial outwash deposited after the LGM near Chicago (USA)

The Quaternary geology of Will County and the Southern Chicago Metropolitan Area is dominated by glacigenic till, sand and gravel outwash, and glaciolacustrine deposits of the Lake Michigan lobe deposited during the last glaciation. Three moraines make up the Valparaiso Morainic System: Westmont, Wheaton, and West Chicago Moraines. Shallow valleys trending northeast crosscut the moraines and were likely formed by subglacial meltwater channels that evolved during downwasting of the ice. Details of the history of glaciation and deglaciation remain unresolved. Others are beginning to be addressed through radiocarbon dating of plant fossils preserved in slackwater and ice-walled lakes, along with a comprehensive characterization of mapped glaciofluvial sediments. A chronology based on optically stimulated luminescence (OSL) was defined and is presented here. Most samples were retrieved from light-shielded vertical cores (mud rotary and probing), along with some good old hammered pipe into riverbank exposures. This presentation will highlight the advantages and disadvantages of each sampling approach. For luminescence dating it is most likely that only a portion of the sediment would have had its latent luminescence signal fully reset before burial, as these sedimentary units were deposited by (turbid?) meltwater likely proximal to the ice margin. To tackle this issue we opted to measure the OSL from quartz, using a very small aliquot size (1 mm). Multiple samples were measured from stratigraphically equivalent sedimentary units. We obtained statistically identical OSL age from paired samples. In addition, the IRSL (infrared stimulated luminescence) age from K-feldspar, measured from some sample, were also statistically identical to their duplicate OSL age from quartz. Finally, these luminescence ages are concordant with radiocarbon ages, derived from correlated sedimentary units. All these observations bolster our confidence in measuring an accurate luminescence age from these deposits, despite the challenges they offered at the onset.