SAGINAW LOBE GLACIAL STRATIGRAPHY PRESERVED IN A DEEP (67.1 M) ROTOSONIC CORE, NORTHERN INDIANA

A recently acquired 67.1 m core (at 41.673731 N; 85.423520 W) in a proximal portion of an outwash fan contains sequences of sand and gravel broken up by three till units. The core was taken from the Lima outwash plain, a northern Indiana landform located between the Sturgis and Shipshewana moraine and fan systems which marked ice positions for the Saginaw lobe of the Laurentide Ice Sheet. This is the deepest and most complete core with recovered samples taken in LaGrange County, Indiana. A passive seismic bedrock survey suggested depth to bedrock is approximately 54.9 m, however sediment was recovered from a depth of 67.1 m without refusal. A gamma log of the borehole was taken concurrently. The sediments in the core indicate a variable flow energy for outwash units, containing large clasts as well as well-sorted fine units. We have divided the core in five units, from bottom up: Unit A (25.6-67.1 m) is a 41.5 m unit containing brown interbedded sand and gravel and massive glaciolacustrine units. At 28 m some laminations have been preserved. This unit is interpreted as a pre-Wisconsin outwash fan; (B) (24.1-25.6 m) is a 1.5 m unit of sandy gray massive diamicton that transitions to dense dark gray diamicton with striated cobbles. The diamicton is calcareous and shows no indication of carbonate leaching. Unit B is interpreted as a basal pre-Wisconsin till; (C) (15.5-24.1 m) is a 8.6 m unit of massive gravel and sands; (D) (14-15.5 m) is a 1.5 m calcareous massive till; (E) (7.3-14 m) is a 6.7 m brown, well-sorted medium-to-coarse massive sand; (F) (3.7-7.3 m) is a 3.6 m unit of dark gray, silty, organic-rich diamicton; (G) (0-3.7 m) is a 3.7 m unit of coarse, pebbly oxidized sand from which a soil horizon has developed. This core has the potential to provide the most detailed Pleistocene record of glacial advance in northern Indiana. Future work on this core will include Optically Stimulated Luminescence and AMS radiocarbon dating of well-bedded sand layers and organic-rich horizons, respectively.