BEDROCK GEOLOGY OF BROWN COUNTY, WISCONSIN

A two-year project to map the bedrock of Brown County (1:100,000 scale) was completed by the Wisconsin Geological and Natural History Survey as part of a USGS-NCGMP funded STATEMAP project. Brown County is in northeastern Wisconsin along the western edge of the ancestral Michigan basin in an area glaciated by the Green Bay Lobe of the Laurentide Ice Sheet. Complex glacial and proglacial Pleistocene sediments up to 100 m (330 ft) thick cover parts of the county, concealing much of the underlying bedrock and buried bedrock river valleys. Between 225 and 450 m (730 – 1,475 ft) of eastward-dipping Cambrian, Ordovician, and Silurian rocks consisting primarily of sandstone, dolostone, and shale are present. Middle Ordovician through Lower Silurian rocks are exposed at the surface along the Niagara escarpment and in quarries and isolated exposures. Exposures were supplemented with several deep cores and boreholes and complimented by several thousand water-well driller’s logs.

Lower Silurian (Llandovery) dolostones were mapped regionally as four stratigraphic units: Mayville Dolostone, Burnt Bluff Group, Manistique Dolostone, and the Engadine Dolostone. The exposed Ordovician rocks were divided into three main units: the Platteville and Galena Dolostones (Sinnipee Group) and the Maquoketa Shale.

The most surprising result of the study is the discovery of several previously undocumented faults, based mainly on subsurface data. At least three unnamed east-west and southeast-northwest trending faults in the Green Bay Metropolitan area converge along a southeast-trending buried bedrock valley located south of Bellevue, Wisconsin. Although inactive, vertical displacements of at least 18-30 m (60-100 feet) have been inferred from water well logs and a deep borehole drilled during the study. The fault zone appears to have influenced the location and orientation of the Niagara escarpment in the county.

A separate east-west trending fault in southern Brown County has at least 30 m (100 feet) of post-Silurian vertical displacement. Based on aeromagnetic data, this fault appears to be the eastern extension of the Spirit Lake Tectonic Zone, which trends across Wisconsin into Minnesota and represents the boundary between the Proterozoic Yavapai and Penokean basement provinces.