LATE WISCONSIN STRATIGRAPHY OF THE GLACIAL LAKE OSHKOSH BASIN, WISCONSIN

Sediments in the glacial Lake Oshkosh basin record late Wisconsin ice-margin fluctuations during deglaciation of the part of east-central Wisconsin covered by the Green Bay Lobe. As part of a USGS STATEMAP project to map the surficial geology of the lake basin, the Wisconsin Geological and Natural History Survey (WGNHS) drilled a series of rotosonic core holes targeting the deepest parts of the basin. Core RS-3, which is 200 feet long and located in the north-central part of the basin, contains the most complete and the best-dated stratigraphic section, including two tills and three separate sequences of varved lake sediment representing deep-lake phases of glacial Lake Oshkosh. Except for Holocene lake sediment and peat at the top of the core, the sediments are part of the Kewaunee Formation. At the base of the core, sandy red till (Silver Cliff Member) overlies Cambrian sandstone. Overlying this till are ca. 25 feet of couplets (>257 of them; couplet counts represent minima because sediment deformation prevents counting in some core segments) beneath a clayey red till (Kirby Lake Member), which, in turn, is overlain by ca. 40 feet of couplets (>213 of them). A third sequence of couplets (>32 of them), ca. 20 feet thick, is separated from the couplets beneath it by nearshore lacustrine sand, which represents deglaciation and partial draining of the lake. This deglaciation presumably correlates with the Two Creekan interval (ca. 11,850 C-14 yr BP), which is represented by a forest bed in nearby localities. Though this correlation is contradicted by a C-14 date of 12,830 +/- 60 (Beta- 167191) from the top of the sand, pollen analysis supports correlation of the sand with the Two Creeks forest bed. The uppermost couplets represent a glacial advance to within a few miles of the site where till of the Middle Inlet Member was deposited. A succession of marl-peat-marl- peat in the top 10 feet of the core, that dates to the late Wisconsin-Holocene transition ca. 10,000 C-14 yr BP, may reflect hydrologic changes occurring at that time.