Cretaceus exposures, Big Sioux River Valley north of Sioux City, Iowa
Cretaceus exposures in the Sioux City area, primarily along the Big Sioux River in Iowa and the Missouri River Valley in northeast Nebraska, were of considerable importance in the early formulation of stratigraphic nomenclature in the Western Interior Province (Tester, 1931). It was from this area that Meek and Hayden (1862) originally named the “Dakota Group,” the basal subdivision of Cretaceus rocks as used throughout much of the western U.S. A summary of Cretaceus stratigraphy and paleogeography in the Iowa area is given by Witzke and others (1983).
The east side of the Big Sioux River Valley reveals a series of Cretaceus exposures (Fig. 1) that are among the best in Iowa. Many of the exposures are along the Iowa 12 right-of-way and are publicly accessible (localities 1, 2, 3, 4, 8). Roadside exposures in Stone State Park (localities 5, 6) and old shale pit operations at Kirk Hanson Recreation Complex in Riverside (locality 9) are also publicly accessible, but collecting is discouraged. Other exposures are on private land, and permission must be secured before entering (e.g., shale pit of Siouxland Sand and Gravel, locality 7). Many of the exposures are extremely steep and potentially hazardous; please exercise caution.
A general description of Cretaceus rock units examined and measured at localities 1 through 9 (Fig. 2) is given in the Measured Section. Strata at locality 10 (Fig. 2) are poorly exposed and the illustrated section is adapted from