CHARACTERISTICS AND GEOCHRONOLOGY OF THE SEVERANCE FORMATION: A NEW MID- THROUGH LATE WISCONSINAN LITHOSTRATIGRAPHIC UNIT IN THE EASTERN PLAINS OF NORTH AMERICA

Studies of late Quaternary landscape evolution in river valleys of eastern Kansas and Nebraska have identified thick packages of mid- through late-Wisconsinan alluvium beneath terraces and thin to moderately thick accumulations of late-Wisconsinan colluvium preserved along margins of valley floors. Previously, these alluvial and colluvial deposits were referred to as the valley facies of the Gilman Canyon Formation (Mandel and Bettis, 2001), a unit of late-Wisconsinan loess on the adjacent uplands, but subsequently have been defined as a new lithostratigraphic unit, the Severance Formation. Recent investigations have revealed that the Severance Formation consists of two members: an early member that sometimes occurs immediately beneath the Gilman Canyon Formation, and a late member that is beneath a terrace surface or a thin veneer of Peoria Loess. Preliminary OSL ages suggest that the early member aggraded between ca. 50 and 30 ka. Hence, the early member records Marine Isotope Stage 3 (MIS 3) alluvial activity that predates the oldest Gilman Canyon Formation deposits and has potential to provide a window into a virtually unknown period on the Eastern Plains. OSL and radiocarbon ages suggest that the late member aggraded between ca. 30 and 14 ka (MIS 3 and 2); hence, it records valley activity just prior to and during cool and dry conditions that coincide with the accumulation of the Peoria Loess and related eolian sand on upland and high-terrace landscapes. The shift from the relatively warm moist conditions of MIS 3 to relatively cool, dry MIS 2 was accompanied by incision, perhaps mimicking the response of the drainage system across the MIS 4 (relatively cool/moist)/MIS 3 boundary.

Both members of the Severance Formation are oxidized, and the late member typically has two or more paleosols forming a pedocomplex similar to the one developed in the Gillman Canyon Formation. The paleosols tend to 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 silans; and many to common macropores. Radiocarbon ages determined on total decalcified organic carbon from the paleosols in the Severance comprise two separate clusters: 28-25 ka and 17-15 ka.