Abstract

Amino acid racemization (AAR) values measured in gastropod shells are demonstrated to be an important aid for correlations and chronology of fossiliferous loessal, lacustrine, and alluvial Pleistocene units in Illinois, central USA. Aspartic acid (Asx) and Glutamic acid (Glx) D/L values were analyzed on a total of 167 Succinea, Hendersonia, and Pomatiopsis shells from 9 geologic units, with clear stratigraphic relationships, at a total of 18 localities in central and southern Illinois. AAR data from Hendersonia and Succinea are less variable and more normally distributed than Pomatiopsis   data, but the latter are locally useful for units lacking preferred genera. Based on analysis of variance tests, Asx- and Glx-D/L data can confidently distinguish among Wisconsin Episode (MIS 2–3), Illinois Episode (MIS 6), late pre-Illinois Episode (MIS 8–14), and early pre-Illinois Episode (MIS 20) deposits. Last glacial Peoria Silt (MIS 2) and Roxana Silt (MIS 3), have mean Asx-D/L values of 0.34–0.37 and 0.42–0.43, respectively, considering all genera. The Illinois Episode Petersburg Silt (∼150 ka) has Asx-D/L (View the MathML source: 0.50–0.56) and Glx- D/L (View the MathML source: 0.17–0.22) ratios that are statistically distinctive from other units. Three late pre-Illinois Episode units (Harkness Silt Member, Belgium Member, and Banner silt units) have similar Asx D/L values (View the MathML source: 0.63–0.71) and, along with stratigraphic context, confirm extensive middle Pleistocene glaciations in the region. Using parabolic kinetic age models, depositional ages of ∼550–250 ka (MIS 14–8) are implied for these units, with a favored correlation with MIS 12 (∼450 ka), a time of especially high global ice volume. The Canteen member, a preglacial alluvium–colluvium below the Harkness Silt, is statistically indistinguishable from other pre-Illinois Episode units with AAR data, but was likely deposited during ∼660–480 ka (MIS 16 or 14), based on parabolic age estimates. The paleomagnetically reversed County Line silt (∼780–830 ka: MIS 20), with the highest mean AAR values, is the oldest known gastropod-bearing Pleistocene unit in Illinois.

Citation

Grimley, D.A. and Oches, E.A. (2015): Amino acid geochronology of gastropod-bearing Pleistocene units in Illinois, central USA. Quaternary Geochronology 25: 10-25. DOI: 10.1016/j.quageo.2014.08.003