LATE AND MIDDLE (~ 35 KA) WISCONSINAN PROGLACIAL LACUSTRINE DEPOSITION IN CENTRAL INDIANA FROM PORTER CAVE

The limits of Wisconsinan and Illinoian glaciations in the Midwest are defined principally by the distribution of tills and associated moraines. In central Indiana proglacial lacustrine deposition and erosion is associated with these limits, providing a new proxy record for deciphering timing and extent of past ice sheets. Specifically, Mill Creek Valley is at a pivotal geomorphic intersection of glacial and karst terrains where hanging karst springs, mid-drainage waterfalls, and deeply incised, underfit stream valleys reflect a complex glacial history. Prior investigations show proglacial lake sediments intercalated with tills, eolian silts, and paleosols that record advance and retreat of late Quaternary ice sheets. These proglacial lakes flooded caves in central Indiana, leaving behind a sedimentary record of ice sheet incursion. A critical discovery is the ubiquitous presence of well-preserved proglacial lake sediments within Porter Cave that contain organic material ideal for radiocarbon dating and sediments amenable for optical dating. We report chronologic control derived from radiocarbon and optical ages for proglacial lakes within the Mill Creek Valley prior to and during the last glacial maximum. Optical and radiocarbon ages indicate proglacial lakes flooded the cave twice, ca. 40-30 ka and 27-18 ka, associated with expansion of the Laurentide ice sheet into central Indiana. The distribution of lake sediments and associated outlets indicates that the ice sheet margin ca. 35 ka was similar in extent to the last glacial maximum. The Porter Cave chronology is consistent with Midwest loess deposition, >70 m sea-level depression, increased meltwater flux into the Gulf of Mexico and insolation minima which provide a new perspective to evaluate climatic thresholds for Wisconsinan glaciations.