CHRONOLOGY OF LAURENTIDE ICE SHEET (EAST WHITE SUBLOBE) ADVANCE TO ITS MAXIMUM WISCONSINAN LIMIT, SOUTHEASTERN INDIANA, USA

The chronology of Laurentide Ice Sheet (LIS) advance to its maximum position in Indiana (USA) during Oxygen Isotope Stage 2 is not well constrained. The advance chronology of the Miami sublobe of the Huron-Erie Lobe (Ohio) and Decatur sublobe of the Lake Michigan Lobe (Illinois), to the east and west of the East White sublobe (Indiana), respectively, has been studied in detail. Limited published chronologic data exist for the East White sublobe, and the primary objective of this project is to develop a chronology of LIS fluctuation in Indiana to compare with well-dated records to the east (Ohio) and west (Illinois). We report here on the discovery of wood below till along a 225-m-long exposure of Quaternary sediments in a limestone quarry near Flat Rock, Indiana. The exposure reveals ca. 5 m of outwash, till, and lacustrine sediment above Devonian Jeffersonville Limestone. Near the west end of the exposure, a representative stratigraphic section contains 2.5 m of sand and gravel (outwash) overlying 0.75 m of diamiction (lodgement till) that overlies 2 m of interbedded silt and sand (lacustrine sediment) that rests on bedrock. Wood and organic fragments are concentrated in the upper 50 cm of the lacustrine sediment with small amounts of wood reworked into the till. At this section, two radiocarbon ages from the outer five rings of an unrooted, upright log sheared off and overlain by till and three ages from wood fragments taken in the upper 50 cm of lacustrine sediment range between 18,250 and 17,800 ¹⁴C yr BP (22,100 and 21,540 cal yr BP). Two similar ages, 17,950 ¹⁴C yr BP (21,740 cal yr BP) and 18,050 ¹⁴C yr BP (21,890 cal yr BP), were acquired from the same stratigraphic position ca. 50 m to the east. Taken together, the group of seven ages pass a chi-squared test at the 95 percent confidence level and are thus statistically the same with a pooled mean radiocarbon age of 18,040 ¹⁴C yr BP (21,860 cal yr BP). This establishes an estimate of the maximum age for advance of the East White sublobe, roughly 20 km north of the maximum Wisconsinan limit in southeastern Indiana. This age is between 1 and 2 ka younger than maximum ages from the adjacent Miami and Decatur sublobes. Additional chronologic control from similar stratigraphic settings in Indiana will help resolve questions of synchroneity between sublobes.