LANDFORM ASSEMBLAGES, QUATERNARY STRATIGRAPHY, AND PRELIMINARY QUATERNARY GEOLOGIC MAP, JEFFERSON COUNTY, WISCONSIN

A new 1:100,000-scale Quaternary Geology map of Jefferson County, Wisconsin has revealed or highlighted previously un- or under-recognized details and features of the southern Green Bay Lobe (GBL). Jefferson County is situated wholly within the southern GBL of the Laurentide Ice Sheet. While Jefferson County has been included on previous, smaller-scale maps of the GBL, there has been no comprehensive study of the county’s surficial geology to date. The map was created using field observations, DEMs and hill-shades derived from 5’ LiDAR data, soil survey data, pre-existing well-construction reports, and new geoprobe cores.

Jefferson County has long been recognized for its significant drumlin fields and lacustrine sediments of glacial Lake Scuppernong, which formed behind the Johnstown (MIS2 terminal) Moraine. New observations offer insight into the glacial and post-glacial history of the southern GBL. High-relief bedrock topography and variable lithology likely placed strong controls on the deposition of glacial and non-glacial sediments. Glaciogenic deposits are composed of till, sand, and gravel of the Holy Hill Formation’s Horicon Member. Tills were primarily shaped into streamlined bedforms, which occur in a wide range of sizes and morphologies. Lacustrine sediments are largely, but not wholly, confined to bedrock valleys, where they can be up to 400’ thick. Lacustrine deposits are more extensive at the surface in the southern part of the county, indicating that Lake Scuppernong may have been more temporally and spatially continuous there. Moraine morphology within the recessional Lake Mills Morainic System (LMMS) is highly variable, including highlands resembling the Kettle Moraine and abundant outwash.

Bulk organic material from a geoprobe core in from the northern part of the county yielded a radiocarbon age of 17860 ± 160 cal yr BP. The material was collected from 15’ below the surface in laminated, lacustrine sediments. This date provides a new minimum age for the Holy Hill Fm and the LMMS in Jefferson County, and suggests that lake deposits of the southern GBL are potential environmental archives for the period between the retreat of the GBL (by ~20 ka) and the Two Creeks re-advance (~14 ka).