CHARACTERISTICS OF THE SEVERANCE FORMATION: A NEW LATE QUATERNARY LITHOSTRATIGRAPHIC UNIT IN THE EASTERN PLAINS OF NORTH AMERICA

Recent studies of late Quaternary landscape evolution in river valleys of eastern Kansas and Nebraska have identified thick packages of late Wisconsinan alluvium and colluvium preserved along margins of valley floors. The upper 3-4 m of these packages are oxidized and have two or more paleosols forming a pedocomplex developed in them. Radiocarbon ages determined on organic carbon from the paleosols range from ca. 25 to 17 ka. The paleosols have thick, well-expressed Bt horizons with strong brown to yellowish brown matrix colors; prismatic to subangular-blocky structure; iron and manganese oxide stains and nodules; discontinuous clay films and silt patches; and common macropores. The 14C ages and soil properties are typical of paleosols composing the pedocomplex of the Gilman Canyon Formation, a unit of late Wisconsinan loess on the adjacent uplands. We have established a new lithostratigraphic unit, the Severance Formation, for the late Wisconsinan valley fill (alluvium and colluvium). Previously, these deposits were referred to as the valley facies of the Gilman Canyon Formation.