Geologic units in Grant county, Wisconsin

Additional scientific data in this geographic area

Sinnipee Group (Ordovician) at surface, covers 53 % of this area

Dolomite with some limestone and shale; includes Galena, Decorah, and Platteville Formations

Ancell Group (Ordovician) at surface, covers 23 % of this area

Orthoquartzitic sandstone with minor limestone, shale and conglomerate; includes Glenwood and St. Peter Formations

Prairie du Chien Group (Ordovician) at surface, covers 14 % of this area

Dolomite with some sandstone and shale; includes Shakopee and Oneota Formations

Cambrian, undivided (Cambrian) at surface, covers 8 % of this area

Sandstone with some dolomite and shale, undivided; includes Trempealeau, Tunnel City, and Elk Mound Formations

Maquoketa Formation (Ordovician) at surface, covers 2 % of this area

Shale, dolomitic shale and dolomite; includes overlying Neda Formation (age uncertain) consisting of oolitic iron oxides and shale

Silurian, undivided (Silurian) at surface, covers 0.2 % of this area

Dolomite, undivided; includes Cayugan, Niagaran, and Alexandrian series

Prairie Du Chien Group and St. Peter Sandstone (Prairie du Chien, Lower Ordovician, Ibexian (Tremadocian-lower Floian); St. Peter, Middle-Upper Ordovician, Whiterockian (upper Darriwilian-lower Sandbian).) at surface, covers 0.1 % of this area

Prairie du Chien Group includes Oneota Fm (Coon Valley, Hager City members) and Shakopee Fm (New Richmond Sandstone, Willow River members); St. Peter Sandstone includes Readstown and Tonti members. Sub-St. Peter erosional surface bevels and locally truncates Prairie du Chien strata. Primary lithologies: dolomite, part cherty, part sandy, mostly unfossiliferous, common voids and vugs (Prairie du Chien); sandstone, quartzose, very fine to medium grained (St. Peter, Shakopee). Secondary lithologies: dolomite, laminated to stromatolitic, part intraclastic; dolomite, fossil molds (primarily mollusks); interbedded very fine to fine sandstone and dolomite (Coon Valley, Shakopee); conglomerate, dolomite and chert clasts in sandy matrix (Readstown). Minor: oolitic dolomite and oolitic chert (Shakopee); shale, green, part sandy (Shakopee); shale, green-brown to gray, part laminated, silty to sandy, part fossiliferous (locally in St. Peter of Winneshiek Co.); coarse sand grains (St. Peter, Shakopee); breccia, dolomite-chert clasts; chalcedony/quartz nodules. Maximum thicknesses total interval: generally 300-400 ft (90-120 m), may reach thicknesses to 700 ft (210 m) (locally thick St. Peter in Winneshiek Co.). Unit thicknesses: Prairie du Chien Group, 225-335 ft (69-102 m), locally thinner where overlain by thick St. Peter; Oneota Fm, 130-225 ft (40-69 m), locally thinned beneath thick St. Peter; Shakopee Fm, 55-100 ft (17-30 m), locally thinned to absent beneath thick St. Peter; St. Peter Sandstone, generally 40-75 ft (12-23 m), varies 25-225 ft (8-69 m), may locally reach thicknesses to 700 ft (210 m) in Winneshiek Co.

Jordan Sandstone, St Lawrence and Lone Rock formations (Upper Cambrian, upper Steptoean-Sunwaptan-lower Skullrockian [formerly middle Franconian-Trempealeauan)) at surface, covers 0.1 % of this area

Surface exposures limited to Allamakee and northeast Clayton counties; subcrops beneath Mississippi River alluvium in southern Allamakee and Clayton counties. Primary lithologies: sandstone, very fine to coarse grained, quartzose to feldspathic (Jordan); silty dolomite to dolomitic siltstone (St. Lawrence); sandstone, very fine to fine grained, glauconitic to very glauconitic, common greensands (Lone Rock). Secondary lithologies: siltstone, part argillaceous (St. Lawrence, Lone Rock). Minor: silty shale partings (St. Lawrence, Lone Rock); dolomite, part sandy; intraclastic dolomite, sandstone, and siltstone. Maximum thicknesses of total interval: 260-310 ft (79-95 m). Formation thicknesses: Lone Rock Fm, 110-140 ft (34-43 m); St. Lawrence Fm, 60-80 ft (18-24 m); Jordan Sandstone, 80-110 ft (244-34 m).