Lacustrine sediments in Porter Cave, Central Indiana, USA and possible relation to Laurentide ice sheet marginal positions in the middle and late Wisconsinan

摘要

Proglacial lakes formed in the late Quaternary with the advance of the Laurentide ice sheet into Mill Creek Valley of central Indiana. These lakes backfilled into the mouth of Porter Cave deposited organic matter and thinly bedded silty sediment that conforms to the underlying cave floor and shelves. Optical and radiocarbon ages indicate that deposition of fine-grained sediments into Porter Cave occurred in two proglacial lake phases during marine oxygen isotope stage (MIS) 3 (ca. 40 to 30. cal. ka) and MIS 2 (ca. 27 to 18. cal. ka), the latter phase is consistent with other proxy evidence for ice sheet extent and timing. We contend that the ice sheet lobe that dammed a proglacial lake during MIS 3 in central Indiana was ~. 7. km behind the Last Glacial Maximum limit based on current topography. This conclusion is in opposition to previous assessments which infer ice free conditions in central Illinois and central Indiana ca. 130 to 25. ka. The paucity of MIS 3 deposits may be due to erosion by the more extensive MIS 2 ice sheet, with glacial-lacustrine sediments preserved in the protective confines of Porter Cave. This outlet lobe in central Indiana may be short lived and reflects a variable response of lobes of the Laurentide ice sheet during MIS 3, with expansion possibly associated with cooling during Heinrich event 4.