CHRONOLOGY OF LATE WISCONSINAN LAURENTIDE ICE SHEET ADVANCE AND RETREAT NEAR ITS MAXIMUM LIMIT, SOUTH-CENTRAL INDIANA

The chronology of Laurentide Ice Sheet advance to its maximum late Wisconsinan position and subsequent fluctuations of the Huron-Erie and Lake Michigan lobes in central Indiana has been a topic of research for more than 50 years. Limitations associated with conventional radiocarbon dating (large errors), unknown/unclear stratigraphic context of ages, and few sites with multiple ages make interpretation and correlation of all existing radiocarbon ages challenging. We revisited several key exposures in south-central Indiana to improve the chronology and associated stratigraphy of the ice sheet advance to its maximum position and subsequent fluctuations. Previous investigations documented two late Wisconsinan tills separated by sorted sediments (sand, gravel, and fossiliferous silt) within a ca. 40 km zone north of the maximum limit in south-central Indiana. The lower till is associated with glacial advance to the maximum extent, and the upper till represents a readvance during the retreat from the maximum limit. To better constrain the age of the upper till, we obtained 22 new radiocarbon ages on plant macrofossils (Picea needles and stems, Dryas leaves; n=14) and terrestrial gastropods (Succineidae, Discus; n=8) from intertill fossiliferous silts at three sites (Clayton, Plainfield, and Ross). Results indicate the maximum age for the upper till is ca. 21.8 k cal yr BP. Two optically stimulated luminescence samples from glaciofluvial sand between the tills at the Clayton section returned ages within error of radiocarbon control despite being partially bleached and having complications due to U-series disequilibrium. The presence of Dryas, Picea, and a cold assemblage of terrestrial gastropods (Vertigo oughtoni, Vertigo modesta, Columella alticola) at the Clayton section indicate a boreal environment near the tundra border just prior to being overridden by ice. Our new dataset indicates that ice remained less than 30 km from the maximum position in south-central Indiana until at least 21.8 k cal yr BP, which constrains rates of retreat to dated moraines in northeast Indiana. This age also corresponds to dated readvances in southeastern Indiana and south-central Ohio, ice-walled lake plain formation and burial of the Jules Geosol in central Illinois, suggesting regional response to cold climate conditions.