CHRONOLOGICAL EVIDENCE FOR MULTIPLE PERIODS OF LOESS DEPOSITION DURING THE LATE PLEISTOCENE IN THE MISSOURI AND MISSISSIPPI RIVER VALLEY, UNITED-STATES - IMPLICATIONS FOR THE ACTIVITY OF THE LAURENTIDE ICE-SHEET

The loess stratigraphy of the mid-continental U.S. is an important proxy record for the activity of the Laurentide Ice Sheet in North America. One of the most outstanding problems is deciphering the age of loess deposits in this area during the late Pleistocene. Radiocarbon dating of snails and thermoluminescence dating of the fine-silt fraction (4-11-mu-m) from loess at the Loveland Loess type section, Loveland, Iowa and a recent excavation at the Pleasant Grove School section, Madison County, Illinois provide new chronologic control on loess deposition in the Mississippi/Missouri River Valley, U.S. The thermoluminescence chronology indicates that the Loveland Loess is Illinoian in age (135 +/- 20 ka) but is not correlative with the Teneriffe Silt which is dated to 77 +/- 8 ka. Concordant radiocarbon and thermoluminescence age estimates demonstrate that the Roxana Silt and a correlative loess in Iowa. the Pisgah Formation, is probably 40-30 ka old. These age estimates in conjunction with previous results indicate that there were four periods of loess deposition during the last 150 ka at 25-12 ka, 45-30 ka, 85-70 ka and at ca. 135 +/- 20 ka. This chronology of loess deposition supports the presence of both a late Illinoian and early Wisconsinan loess and associated soils. Thus, there may be more than one soil in the loess stratigraphy of the mid-continental U.S. with morphologies similar to the Sangamon Soil. The last three periods of loess deposition may be correlative with periods of elevated dust concentrations recorded in the Dye 3 ice core from southern Greenland. This is particularly significant because both areas possibly had the same source for eolian particles. Reconstructions of atmospheric circulation for glacial periods show a southerly deflected jet stream that could have transported dust from the mid-continental USA to southern Greenland. Lastly, the inferred record of loess deposition is parallel to a chronology for deglaciation of the Laurentide Ice Sheet deciphered from chronologic and stratigraphic studies of raised glacial and marine sediments in the Hudson Bay Lowlands, Canada. These chronologies indicate that the Laurentide Ice Sheet was quite dynamic during the late Pleistocene, advancing and retreating across North America at least four times during the last 150 ka.