Roxana silt of the Upper Mississippi Valley: Lithology, source, and paleoenvironment

Abstract

Significant lithological differences exist between the middle Wisconsinan Roxana Silt (loess) of the Upper Mississippi Valley (the valley north of 41.5°) and that of the type area near St. Louis, Missouri. Primary differences are that the Upper Mississippi Valley Roxana Silt is much thinner, finer textured, and more weathered than that of the type area. Extrapolation of radiocarbon dates brackets sedimentation of Upper Mississippi Valley Roxana Silt between 55 and 27 ka, which is in close agreement with dates from the type area. Radiocarbon dates and paleobotanical data indicate that the Upper Mississippi Valley Roxana Silt accumulated at a maximum long-term-average sedimentation rate of about 7 cm/1,000 yr within a boreal forest environment, whereas the type area Roxana Silt accumulated at a maximum long-term-average sedimentation rate of about 60 cm/1,000 yr within a prairie-like environment. Multiple regression analysis shows that more than 75% of the variance in thickness and texture between the Upper Mississippi Valley Roxana Silt and that of the type area is explained by parameters of source-valley width, source-valley azimuth, and distance of the depositional site from the source-valley. This indicates that although the lithologic characteristics and sedimentation rates between the two regions are quite different, they are consistent with a unified loess sedimentation sys- tem linked to the ancient Mississippi River valley. A proglacial source for the Roxana Silt and a middle Wisconsinan advance of the Lauren-tide Ice Sheet is a topic of scientific debate because recent findings show a lack of Roxana-correlative till deposits in the Midwest. Alternative hypotheses include a glaciolacustrine source, a regional slope erosion source, and a cold-climate desert dust source. Tests of these hypotheses support a proglacial origin for the Roxana Silt. Multiple regression analysis of loess thickness and texture, along with paleobotanical evidence, indicates that the Roxana Silt was blown from the ancient Mississippi Valley, rather than from a cold-climate desert source or from flood plains linked to a glaciolacustrine source. Geochemical and mineralogical composition of the Upper Mississippi Valley Roxana Silt, compared to out-wash-derived Peoria Loess, indicates that detritus from regional hillslope erosion is not a unique component of the Roxana Silt. Combined data indicate that the midcontinental Roxana Silt was derived as loess blown from flood plains of middle Wisconsinan proglacial rivers between 55 and 27 ka.