CHRONOLOGY OF PEORIA LOESS DEPOSITION RELATED TO HURON-ERIE LOBE ADVANCE AND RETREAT, CENTRAL INDIANA

The chronology of deposition and provenance of thin Peoria Loess (<1.5 m thick) in central Indiana, distal to major meltwater pathways and dust sources of the Wabash, Ohio, and Mississippi River valleys, is difficult to study owing to the impact of postglacial pedogenesis. Therefore, stable upland surfaces commonly used in loess studies are not the ideal geomorphic locations to capture a detailed record of the timing and source areas for loess in central Indiana. Inspection of lidar and historic aerial photographs in the course of geologic mapping in Morgan County, Indiana, led to identification of a 5-ha closed basin on an Illinoian outwash plain situated 1 km southeast of the late Wisconsin maximum limit where a potential loess and paleoenvironmental record could be recovered. The central part of the closed basin is permanently wet, and peat was mined historically in the basin. This area was targeted for coring and a 5.2-m-long Livingstone core was collected near the center of the basin. Additional cores were collected to characterize the fill of the closed basin. In the central core, the upper 1.5 m is composed of peat and is underlain by 3.1 m of organic-rich silt, interpreted to be dust deposited in a shallow pond environment. The basal 0.6 m of the core is dense organic-rich silt loam, interpreted to be colluvium derived from or part of the Farmdale Geosol. In addition to radiocarbon dating, the core was sampled for total organic carbon, total inorganic carbon, particle size analysis, clay and silt mineralogy, silt geochemistry, and magnetic susceptibility to characterize units. Five ¹⁴C ages, ranging from 28,690 cal yr BP to 2,550 cal yr BP, constrain the chronology. The age-depth model suggests the organic-rich silt, interpreted to be Peoria Loess, was deposited beginning ca. 27 k cal yr BP and continued until ca. 16 k cal yr BP. This chronology agrees well with the chronology of advance and retreat of the Huron-Erie Lobe into the paleo-Wabash and West Fork White River basins, based on a record of slackwater sedimentation just beyond the late Wisconsin maximum limit ca. 15 km from the core site. An increase in total organic carbon (inferred lower sedimentation rate) in the upper 1 m of the silt unit may record a change from locally derived (5 km away) West Fork White River loess to more distal (90 km away) Wabash River loess, ca. 19.9 k cal yr BP.