HOLOCENE BIGNELL LOESS CHRONOLOGY, STRATIGRAPHY AND PALEOENVIRONMENTAL RECONSTRUCTIONS FROM WITHIN A LOESS TABLE, SOUTHWESTERN NEBRASKA

Plateau-like features, excavated in the Wisconsinan loess deposits during the Holocene, occur in proximity to the Platte River valley in central and southwestern Nebraska. The Bignell Table, one such table along the south valley wall of the Platte River in southwestern Nebraska, contains a rainwater basin (A.K.A. playa, buffalo wallow) that is undergoing rapid dissection by a gully that breached the lip of the table. The resulting 70+m high gully face, located on the Sargent Ranch, has exposed the Late Illinoian Sangamon Soil, Middle Wisconsinan Gilman Canyon Formation, Late Wisconsinan Peoria Loess, Brady Soil, Holocene Bignell loess, pre-agricultural soil, and overlying post-agricultural fill. An array of radiocarbon ages and stable isotope, phytolith, and rock magnetic data were obtained from gully-exposure profiles and a series of cores extracted from within the rainwater basin and along a transect to the upper table surface. On the basis of patterns in these data sets, four zones were defined above the Late Wisconsinan/Early Holocene Brady Soil (Younger Dryas Chronozone) within the gully exposure: (1) 11.6-9.6ka—transition from Brady Soil pedogenesis to Bignell loess accumulation, characterized by a vastly increased rate of loess fall and rapid climate amelioration; (2) 9.6-4.4ka—further increase in the rate of loess fall and heightened aridity, an expression of the Altithermal; (3) 4.4-0.1ka—decreased rate of loess fall and development of multiple Entisols, with a slight increase in warm-season precipitation; and (4) historical—accumulation of post-agricultural laminated fill (sheet-wash contributions from the adjacent cultivated table surface).