FIGURE 8 Stratigraphy, radiocarbon ages, and source sediments of selected loess sections from Colorado to Illinois in the North American midcontinent (locations of sections given in Fig. 7). Data from Colorado and Nebraska from Muhs et al. (1999), Iowa data from Ruhe (1983) and Forman et al. (1992a), and Illinois data from Grimley et al. (1998).

Soil or paleosol

Loess time transgressive; maximum limiting radiocarbon ages are —24,500 14C years B.P. near the Missouri River source, whereas they are —19,000 14C years B.P. —280 km to the east of the river.

Loess deposition in Tennessee, Illinois, Iowa, and other areas east of the Missouri River was a function of source sediment availability from the Laurentide ice sheet via the Ohio, Wabash, Illinois, Mississippi, and Missouri Rivers, and the timing of loess deposition probably followed the history of movement of the ice sheet. However, in the Great Plains region of Nebraska, Kansas, and Colorado, west of the area that drained outwash from the Laurentide ice sheet, extensive loess sheets were derived from Rocky Mountain glaciers and, more importantly, Tertiary volcaniclastic sediments (Aleinkoff et al., 1998, 1999). Interestingly, in some places, the timing of loess deposition seems to differ little from that of Laurentide outwash-derived loess found farther to the east (May and Holen, 1993; Martin, 1993; Pye et al., 1995; Maat and Johnson, 1996; Muhs et al., 1999). For example, in eastern Colorado, loess deposition began after ca. 20,000 14C B.P., but ended by ca. 11,800 14C B.P. (Fig. 8). Elsewhere, however, loess deposition differs significantly from that found farther east. In western Nebraska, Peoria Loess was deposited sometime after ca. 30,000 14C B.P., but continued until sometime just before ca. 10,500 14C B.P. (Maat and Johnson, 1996; Muhs et al., 1999), indicating a much longer period of eolian sedimentation.

Loess deposition was not continuous in midconti-nental North America during the LGM. In western Illinois, multiple, poorly developed buried soils are found within Peoria Loess, indicating brief periods of landscape stability with little or no loess deposition (Wang et al., 1998). Radiocarbon ages indicate as many as five cycles of loess sedimentation and soil formation in the period from 20,710 to 17,630 14C B.P. Ruhe et al. (1971) also report multiple buried soils in Peoria Loess in western Iowa, and at least one possible buried soil has been identified in Peoria Loess in eastern Colorado (Muhs et al., 1999).

Multiple lines of evidence indicate that loess sources changed during the LGM over midcontinental North America, even within the area draining the Laurentide ice sheet. The most detailed work has been done in Illinois, where Peoria Loess shows distinctive zonations, based on clay mineralogy (Frye et al., 1968), dolomite content (McKay, 1979), and magnetic susceptibility (Grimley et al., 1998). The compositional changes of the loess in this area are clearly linked to different lobes of the Laurentide ice sheet that provided outwash to the Mississippi and Illinois River systems (Grimley et al., 1998). In western Iowa, Muhs and Bettis (2000) found three zones within Peoria Loess: (1) a lower zone characterized by low carbonate, probably related to syn-depositonal leaching; (2) a middle zone, probably derived dominantly from the Missouri River; and (3) an upper zone that could be derived from both the Missouri River and sources in Nebraska farther west. In eastern Colorado, Aleinikoff et al. (2000) used Pb isotopes in K-feldspars to show that Peoria Loess was derived from two competing sources: outwash derived from Front Range glaciers and volcaniclastic Tertiary siltstone. At least two cycles of alternating dominance by these competing sources are evident in the period of Peoria Loess deposition.

Loess distribution, as well as thickness, particle size, carbonate, and geochemical trends, indicates that most loess in North America was deposited by northwesterly winds (Figs. 7 and 9). There were apparently some episodes of easterly winds, because areally limited tracts of loess occur to the west of some source valleys (see Muhs and Bettis, 2000, for a review of these localities). Nevertheless, such areas are of minor extent, and the picture that emerges is one of dominantly northwesterly winds.

12.6.2. Last Glacial Period: Eolian Sand

Eolian sand deposits are extensive in North America, but until recently were much less studied than loess deposits. The most widespread eolian sands in North America are found in the central and southern Great Plains region, from Nebraska south to Texas (Fig. 7). There are few active dunes in this region at present. However, stabilized dunes and sand sheets cover tens of thousands of square kilometers. Recent studies show that the most critical factor in the amount of eolian activity in this region—assuming that adequate winds and sand supplies are available—is the degree of vegetation cover, which in turn is a function of the balance between precipitation and evapotranspiration (Muhs and Maat 1993; Muhs and Holliday 1995; Wolfe 1997; Muhs and Wolfe, 1999).

Limited radiocarbon ages, archaeology, and degree of soil development suggest that eolian sand sheet, lunette, and dune deposition took place during the last glacial period in Nebraska, Colorado, New Mexico, and Texas (Fig. 10). Radiocarbon results from the Nebraska Sand Hills indicate that there was sufficient w 200

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