ASPECTS OF THE LATEST PLEISTOCENE GLACIAL GEOLOGIC RECORD OF THE JAMES LOBE OF EASTERN SOUTH DAKOTA

The late Pleistocene Laurentide ice sheet advanced generally southward across eastern South Dakota to within 6 km of its maximum Wisconsin extent along the modern Missouri River valley. This significant glacial advance occurred during latest Pleistocene time of the latter part of the Bølling/Allerød interstadial warm period. Maximum ages on the southward advance of at least 160 km are on ¹⁴C dates (12,880 – 12,050 14C yrs BP; ~14.5 ka cal) on wood that was deeply buried (up to 58 m) by late Wisconsin glacial deposits (mainly till). These dates are from 12 sites in 10 counties, and are consistent with paleoecologic studies that indicate that spruce parkland occupied the region that was partially overridden by the glacial advance. Close minimum ages on the glacial advance are provided by ²³⁰Th/U dates ranging from 13.02 +/- 0.23 ka to 10.48 +/- 0.57 ka on travertine laminae of groundwater discharge deposits that overlie the glacial deposits; as well as age constraints on younger Laurentide moraines of readvances to positions 300 km to the northeast and that preceded Lake Agassiz. Consistent with geochronology, loess cover is largely absent to very thin on till surfaces in the James River lowland except for areas near postglacial sources such as the Missouri River valley. The sparcity of loess contrasts with thick loess mantles in adjacent glaciated areas of southeastmost SD, eastern Nebraska, and northwest Iowa. The ¹⁴C dates span the Bølling time interval that ice core records show was markedly warmer than preceding glacial conditions. Thus, the late Wisconsin readvance of the James lobe, and similarly timing of readvances of the Des Moines and Lake Michigan lobes, may be complex dynamic responses of the Laurentide ice sheet to climatic warming, and increased meltwater production.