Eolian Records In The North American Midcontinent

12.6.1. Full Glacial and Late Glacial Times: Loess

During the LGM, loess was deposited over much of the North American midcontinent, particularly in the central United States (Fig. 7). Little loess was deposited in Canada during the last glacial period because the

FIGURE 7 Map showing the distribution of mostly late Pleistocene loess and mostly Holocene eolian sand in the midcontinent region of North America and inferred paleowinds during the last glacial maximum (LGM). Loess distribution data from Thorp and Smith (1952), eolian sand distribution data from Muhs and Holliday (1995), and inferred LGM paleowinds data from Muhs and Bettis (2000). NSH, Nebraska Sand Hills localities (Gudmundsen Ranch and Swan Lake, see Fig. 10; Whitman and Snake River, see Fig. 13); FM, Fort Morgan dune field localities (Logan County, see Fig. 10; Hudson and Coors Quarry, see Fig. 11; Friehaufs Hill and Hillrose, see Fig. 13); GB, Great Bend Sand Prairie localities (Cullison Quarry and Reno County, see Fig. 13); C, Clovis, NM (see Fig. 10); M, Midland, TX (see Fig. 10); BI, Beecher Island, CO (see Fig. 8); BH, Bignell Hill, NE (see Fig. 8); L, Loveland, IA (see Fig. 8); CS, Cottonwood School, IL (see Fig. 8); IRT, Illinois River transect (see Fig. 9).

FIGURE 7 Map showing the distribution of mostly late Pleistocene loess and mostly Holocene eolian sand in the midcontinent region of North America and inferred paleowinds during the last glacial maximum (LGM). Loess distribution data from Thorp and Smith (1952), eolian sand distribution data from Muhs and Holliday (1995), and inferred LGM paleowinds data from Muhs and Bettis (2000). NSH, Nebraska Sand Hills localities (Gudmundsen Ranch and Swan Lake, see Fig. 10; Whitman and Snake River, see Fig. 13); FM, Fort Morgan dune field localities (Logan County, see Fig. 10; Hudson and Coors Quarry, see Fig. 11; Friehaufs Hill and Hillrose, see Fig. 13); GB, Great Bend Sand Prairie localities (Cullison Quarry and Reno County, see Fig. 13); C, Clovis, NM (see Fig. 10); M, Midland, TX (see Fig. 10); BI, Beecher Island, CO (see Fig. 8); BH, Bignell Hill, NE (see Fig. 8); L, Loveland, IA (see Fig. 8); CS, Cottonwood School, IL (see Fig. 8); IRT, Illinois River transect (see Fig. 9).

Laurentide ice sheet covered most of the region during the glacial maximum and extensive proglacial lakes covered the region and served as sediment sinks as ice receded. Eolian silt dating to the LGM in the United States is referred to as Peoria Loess. Peoria Loess is as thick as ~50 m in western Nebraska (Maat and Johnson, 1996) and as thick as ~40 m in western Iowa (Muhs and Bettis, 2000). From western Iowa to the east, loess is thickest adjacent to major valleys, such as those of the Missouri, Mississippi, Illinois, and Wabash Rivers.

Numerous studies show that while the bulk of late Quaternary loess in midcontinental North America dates broadly to the LGM, there are important differences in the timing of deposition within the region. Maximum limiting radiocarbon ages of Peoria Loess range from —30,000 to —20,000 years, and minimum limiting ages range from —14,000 to —10,000 years (Fig. 8). Localities near major loess sources record deposition earlier than those farther away. For example, in west ern Tennessee, immediately east of the Mississippi River, maximum limiting ages for Peoria Loess are as old as -29,00014C years B.P., and an age of 21,800 14C years B.P. has been reported in the lower part of the loess (Markwich et al., 1998). Thermoluminescence and 10Be ages of Peoria Loess from this locality and elsewhere in Tennessee agree with these ages (Markewich et al., 1998; Rodbell et al., 1997). In much of Illinois, Peoria Loess deposition probably began sometime after ca. 25,000 14C B.P. (Curry and Follmer, 1992) and was still in progress at ca. 12,000 14C B.P. (Grimley et al., 1998). In central Illinois, however, maximum limiting radiocarbon ages for Peoria Loess decrease from -25,000 to -21,000 14C years B.P. along a 20-km long traverse away from the source (Kleiss and Fehrenbacher, 1973). Similarly, in Iowa, the maximum period of Peoria Loess deposition was from ca. 24,000 to 14,000 14C B.P. (Ruhe, 1983). Ruhe (1983) points out, however, that initiation of Peoria Loess deposition in Iowa, like Illinois, was

BEECHER ISLAND, COLORADO

BIGNELL HILL, NEBRASKA

LOVELAND, IOWA

COTTONWOOD SCHOOL, ILLINOIS

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