STRATIGRAPHY AND CHRONOLOGY OF LATE WISCONSIN LAURENTIDE ICE SHEET FLUCTUATIONS OF THE EAST WHITE SUBLOBE, CENTRAL INDIANA

The stratigraphy and chronology of Laurentide Ice Sheet (LIS) initial advance and fluctuations near its maximum late Wisconsin extent in central Indiana has received little attention in the past 20 years. Wayne’s 1963 work, Pleistocene Formations in Indiana (Indiana Geological Survey Bulletin 25), developed the stratigraphic nomenclature for Quaternary units in Indiana and provided an overview and context for radiocarbon ages associated with glacial sediments. Ansel Gooding (Earlham College) detailed the stratigraphy and chronology in southeastern Indiana and adjacent southwestern Ohio in the 1960s and 1970s. Radiocarbon ages compiled by Ned Bleuer (Indiana Geological Survey) in the 1970s and 1980s in west-central Indiana and mapping of Marion County in the 1990s provide the latest effort.

Here, we compile published and unpublished radiocarbon ages (n=81) within Indiana to establish a chronologic framework and to identify spatial and temporal gaps in the existing data set to guide future work. Using this radiocarbon data set, we interpret that the LIS reached ca. 40.5 degrees north latitude (80 km north of Indianapolis) by 26.0 k cal yr BP, based on the youngest maximum ages from cutbank exposures along Wildcat Creek (Carroll County) and boreholes in Tipton and Hamilton Counties north of Indianapolis. The largest concentration of ages occur within 30 km of the late Wisconsin maximum position, and the majority of ages suggest advance to the maximum limit ca. 24–23 k cal yr BP, in agreement with advance to the maximum limit in southwestern Ohio and east-central Illinois. However, two newer sites—Flat Rock and Plainfield—suggest a younger advance (22 k cal yr BP) to the maximum limit, as well as a younger readvance ca. 19.6 k cal yr BP. This potential readvance is not well understood at this time and requires further investigation. One obstacle in the interpretation of this data set is that a majority of the radiocarbon ages are over 30 years old (pre-AMS with large errors) and the accurate location and stratigraphic position of collected organic material is not known at all sites. A reinvestigation of these sites along with additional outcrop and borehole data from central Indiana will help resolve stratigraphic and chronologic questions.