Origin and paleoenvironment of the Upper Mississippi Valley Roxana Silt

The Roxana Silt (middle Wisconsinan loess) is the source of debate among Quaternary scientists because its genetic linkage to an advance of the Laurentide Ice Sheet is uncertain in light of recent findings that indicate a lack of till deposits of the same age. This dissertation analyzes the Roxana Silt along the Upper Mississippi Valley (UMV) north of Davenport, Iowa. In addition, a lithostratigraphic classification for loess-derived sediments in Wisconsin is informally proposed, which classifies the Roxana Silt as a member of the Kieler Formation.;The UMV Roxana Silt is lithologically quite different from Roxana Silt at its type area in west-central Illinois because it is more weathered, finer textured, and thinner. However, statistical models of loess thickness and texture show that those differences are well explained by local environmental controls on the middle Wisconsinan loess sedimentation system. Radiocarbon ages from the UMV Roxana show that it is the time-stratigraphic equivalent of the Roxana in its type area. Deposition of the UMV Roxana began shortly prior to 50 ka and ended circa 27 ka.;Tests of glacial versus nonglacial provenance hypotheses favor a glacial source and show that nonglacial detritus was not a distinctive component in the UMV Roxana Silt. Specifically: (1) detrital iron-stained silt grains indicate a glacial source in the Lake Superior region, (2) trace metals of lead and zinc in the Roxana show that nonglacial sources from the Upper Mississippi Valley Lead-Zinc District were insignificant, (3) quartz/feldspar ratios indicate that nonglacial sources from the highly weathered pre-Roxana landscape were insignificant, and (4) the lithology of the Roxana is highly correlated with the Mississippi Valley indicating that the valley was the source, rather than regionally distributed nonglacial sources of dust.;This study suggests that the loess of the UMV Roxana Silt accumulated at a relatively slow rate (;The conclusions of this research indicate that the Roxana Silt is glacially derived, but a correlative glacial till remains enigmatic. However, several glacial tills are noted that possibly are correlative.