SLACKWATER LAKE RECORD OF SOUTHERN WABASH TRIBUTARY INDICATES ENIGMATIC CA. 40 KA GLACIATION

A recently acquired core (at 38.005409 N; 87.918129 E) from a lower Wabash River tributary valley fill contains a curious record of proglacial sedimentation. The Wabash Valley was an important meltwater pathway for the Lake Michigan, Saginaw, and Huron-Erie lobes through at least the last two glaciations. Glacial Lake Solitude was one of many slackwater lakes formed by damming of tributary valleys by aggrading outwash in the Wabash River valley. The 19 m Beste-1 core sampled about half of the fill. The core includes a succession of lacustrine and alluvial deposits: (A) a 4.6 m basal unit of gray to olive brown, loamy, bedded to laminated, lacustrine sediment; (B) 7 m pinkish brown, silt and clay, thinly laminated, lacustrine sediment; (C) 5.4 m light brown to olive brown, fine sandy, bedded to laminated, alluvial sediment; capped by (D) 1.9 m dark to olive brown, thick laminated, silt loam alluvium. The units are separated by sharp contacts, but clay mineralogy and geochemistry do not clearly suggest provenance changes. Unit D is leached, the others calcareous. Fossil assemblages indicate shallow lacustrine to floodplain environments (e.g. gastropods Valvata; Pomatiopisis; ostracodes Fabaeformiscandona rawsoni, Limnocythere ornata wabashensis) grading up to terrestrial environments (e.g. gastropod Punctum minutissimum). All fossils indicate cool, not cold, climate, with modern analogs only as far north as the Great Lakes. Seven ¹⁴C dates on plant matter >39 ka (radiocarbon dates calibrated to IntCal13) and supported by an OSL date of 41 ka on feldspar show that units A and B were deposited prior to the last glacial maximum. A ¹⁴C date of 29.0 ka on charcoal near the base of unit C coincides with the transition from lacustrine to floodplain sedimentation. Deposition was largely complete by 16.3 ka (¹⁴C date at the top of unit C). The >40 ka ages add to a small collection of old ages determined within the lower Wabash Valley. Future research will address the provenance of the >40 ka units. Lake Michigan lobe is not yet shown to be active in the watershed at that period, yet the chemical and mineralogical data are not distinctive of Huron-Erie or Saginaw Lobe sources.