Dakota Formation widespread in western Iowa, lower sandstone-dominated Nishnabotna Member, upper mudstone/shale-dominated Woodbury Member. Correlative Windrow Formation found as erosional outliers in northeastern and north-central Iowa. Primary lithologies: sandstone, quartzose, very fine to medium grained; mudstone/shale, light to dark gray, variably silty-sandy, noncalcareous (Woodbury Mbr.). Secondary lithologies: sandstone, medium to very coarse grained, part pebbly to gravelly, locally cemented by iron oxides (Nishnabotna Mbr., Windrow Fm.), gravel, quartz and chert clasts; siltstone; mudstone, red, pink, yellow-brown, black (carbonaceous). Minor: lignite; siderite (concretions, pedogenic sphaerosiderite pellets, cemented siltstone); massive iron ore, silty to sandy (Windrow Fm.). Maximum thickness Dakota Fm. 500 ft (150 m), commonly 100-300 ft (30-90 m); Windrow Fm. 40 ft (12 m).
Interval includes “Graneros” Shale, Greenhorn Formation, Carlile Shale. Primary Lithologies: shale, medium to dark gray, variably silty, calcareous to very calcareous. Secondary lithologies: chalk, marl, argillaceous limestone, with skeletal (inoceramid) packstones; shale, gray, silty, noncalcareous. Minor: siltstone; calcite and siderite concretions. Maximum thickness 265 ft (80 m).
Conglomerate, sandstone, mudstone, shale, marlstone, siltstone, and minor lignite, deposited in marine and non-marine settings; likely Cenomanian to Campanian age. Unit outline is the product of contouring the stratigraphic top and base, from which an isopach grid was created. Because the distribution is patchy, unit boundaries were drawn from the gridded data to represent locations where more than 25 feet (8 meters) of thickness occurs. As a result, many areas outside of the unit boundaries may be overlain by thin Cretaceous strata and the unit is depicted without a contact line.
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.