PRELIMINARY ANALYSIS OF PALEOENVIRONMENTAL RECORDS FROM TWO NE INDIANA WETLANDS SINCE THE LGM

Livingstone cores were collected from two wetland sites in northeastern Indiana to determine the timing of deglaciation and to document environmental changes since the last glacial maximum (LGM). Both sites are located in DeKalb County, Indiana between the Fort Wayne and Wabash Moraines, which were formed by the retreating Erie Lobe of the Laurentide Ice Sheet. In this region, the crests of both moraines trend NE-SW and sporadic kettles and depressional wetlands are located on the moraine and intermoraine surfaces. The two moraines are separated from each other by the St. Joseph River, which is incised approximately 12 meters below the sampling sites. The cores recovered a basal diamicton overlain by finely laminated silty clay. The basal diamicton, interpreted as subglacial till, is a pebbly sandy mud, with > 60% silt, and organic matter content of < 1.3%. The overlying laminated mud contains 2-3% organic matter and is interpreted to represent lacustrine sedimentation. A radiocarbon date from plant macrofossils located just above the basal diamicton indicates that ice retreat occurred prior to 13,840 ± 70 ¹⁴C yr BP (Beta-190864). The laminated lacustrine mud transitions upward into alternating massive and laminated units for approximately 4 m. The final change from lacustrine to depressional wetland environments is indicated by the increasing abundance of plant macrofossils and increasing organic matter content, with values reaching >10%. In this unit, volume magnetic susceptibility values are invariant and lowest of all sediments in the core. Previous work suggested that St. Joseph River incised as much as 10 m into the surface till (Lagro Formation) during periods of high fluvial discharge associated with the retreating ice sheet. Our wetland core sites adjacent to the St. Joseph River lack coarse sediment, indicating the sites were not inundated by debris-rich meltwaters. The laminated sediments may represent overbank deposits associated with later flooding events or may represent deposition associated with initial the formation of Glacial Lake Maumee.