AMINOSTRATIGRAPHY OF PLEISTOCENE SEDIMENTS IN THE METRO EAST ST. LOUIS REGION, ILLINOIS

Fossiliferous Pleistocene sediments from the Robbins Core near Collinsville, Illinois, provide an opportunity to expand the aminostratigraphy of Mississippi Valley sediments into the pre-Illinoian glacial record. A four-fold succession of glacial deposits and interglacial soils is described from this new 174-foot core (Grimley et al., this volume), including the Wisconsin episode Peoria and Roxana Silts; Illinois Episode Glasford Formation and Petersburg Silt, pre-Illinoian Banner Formation and Harkness Silt, all of which are underlain by a nonglacial alluvium and colluvium. The Petersburg and Harkness Silts and the basal alluvium yielded numerous gastropod shells for amino acid racemization analysis. Comparisons are made between total acid hydrolysate amino acid D/L values measured in the gastropod genera Succinea, Hendersonia, and Pomatiopsis from this core with samples from fossiliferous Peoria Silt, Roxana Silt, and Petersburg Silt collected from nearby outcrops.

Past aminostratigraphic studies in the region have measured D-alloisoleucine/L-isoleucine (A/I) ratios in fossil mollusks as a stratigraphic correlation tool. In this study we measure D/L values in several amino acids, including aspartic acid, glutamic acid, valine, as well as A/I, in order to optimize temporal resolution. Aspartic acid D/L measurements in Succinea increase from 0.32 ± 0.04 (n=12) in samples from the Peoria Silt at the Keller Farm section, to 0.41 ± 0.07 (n=2) in Roxana Silt samples from the Schoolhouse Branch section, to 0.58 ± 0.04 (n=5) in samples collected from an outcrop of the Petersburg Silt at Powdermill Creek. In the Robbins Core, Succinea aspartic acid D/L values ranged from 0.55 ± 0.03 (n=2) in the Petersburg Silt to 0.68 ± 0.04 (n=7) in Harkness Silt samples. Hendersonia collected from the Petersburg Silt at Powdermill Creek yielded D/L-glutamic acid values of 0.18 ± 0.05 (n=4). In the Robbins Core, D/L-glutamic acid ratios in Hendersonia were 0.26 (n=1) from the Harkness Silt, and 0.49 (n=1) from the basal alluvium. These data demonstrate a systematic increase in D/L ratios in samples of increasing age. The resulting aminostratigraphy will provide a useful tool for correlating and dating glacial sediments of unknown age in the region.