Characterization of the loamy surficial sediments in a type area of the Iowan Erosion Surface

Wysocki, Donald
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An area of approximately 20,000 Km('2) in northeastern Iowa, known as the Iowan Erosion Surface is blanketed by a mantle of loamy surficial sediments approximately 40-50cm thick. The origin and nature of these sediments are poorly understood. These sediments were studied in a 360ha watershed that contained catenary sequences of soils and hillslope forms typical to the Iowan Erosion Surface. Properties of the loamy surficial sediments in this area were investigated for the purpose of characterization of the sediments and were tested for agreement with a model of hillslope evolution;Six hillslope transects, ranging in length from 240-510m, initiating on hillslope summits and terminating on alluvial toeslopes, were established to characterize the loamy sediments. The sediments were sampled by soil coring at 30m intervals along the transects. Seventy-nine cores were obtained, with all cores penetrating the loamy sediments into the underlying material. The pedologic characteristics of the cores were described and the loamy sediments within the cores were divided into 5cm depth increments;Particle size characteristics, thickness of the sediments, and clay mineralogy were investigated as functions of hillslope position. Clay content, fine/coarse particle ratios, thickness, and organic carbon content of the sediments progressively increased along the six transects in the downslope direction. Sand content, available phosphorus, bulk densities, and geometric means of the particle size progressively decreased downslope. Short, simple hillslopes expressed trends which were systematic and could be expressed mathematically through regression analysis with a high degree of correlation. Sediments along long, compound hillslopes expressed trends which were not as systematic and often variable. Analysis of the clay mineral fraction revealed that interstratified clay minerals were the most abundant, with lesser amounts of other clay minerals;Evaluation of the properties of the sediments indicates that they have resulted primarily from the process of fluvial erosion on hillslopes of glacial till, subsequent to the cessation of loess accumulation;Comparisons of observed properties with those predicted by Ruhes's model of hillslope evolution show that the hillslopes within the study area could have evolved as described by the model.