SE Iowa Stratigraphy

Stratigraphy refers to the order in which sedimentary bedrock layers are deposited and the interpretations based on their characteristics. Variations in lithologic facies, structural features, unconformities, and erosion all serve to complicate the effort of correlating these rocks units over mappable areas. The Iowa Geological Survey (IGS) has been constructing detailed geologic maps (1:24,000 scale) in southeastern Iowa since 2016, attempting to improve upon the statewide bedrock geologic map that was compiled in 2010 (1:500,000 scale). At that scale, the 2010 bedrock map grouped many stratigraphic formations of the Mississippian Subsystem (355-325 million years old) together, such as the Warsaw, Keokuk, and Burlington formations (Augusta Group) and the Wassonville, Prospect Hill, and McCraney formations (Kinderhookian). Southeast Iowa hosts several key type sections of the Mississippian, including the Keokuk, Burlington, Wassonville, and the Prospect Hill formations. The Kinderhookian type area, including the type section of the McCraney Formation, is located in west-central Illinois. Just across the Mississippi River from Kinderhook, Illinois are the type sections of other key lower Mississippian strata, the Hannibal Formation (Hannibal, MO) and the Louisiana Formation (Louisiana, MO).

Correlation of the lower Mississippian strata in the tristate area has been the subject of decades of research with many questions remaining. Geologists with the IGS are collaborating with Dr. Bradley Cramer and students with the Department of Earth and Environmental Sciences at the University of Iowa to further research the stratigraphic enigmas that punctuate the Lower Mississippian Subsystem. Currently, the use of biostratigraphy and chemostratigraphy are leading to new insights on how the Kinderhookian in particular fits within the framework of the tristate area. Preliminary results of carbon isotopic analyses confirm that the Kinderhook-Osage boundary excursion represents a regional unconformity and a significant gap in deposition. Preliminary results of conodont sampling suggest that the McCraney Formation in southeast Iowa is, at least in part, correlative with the Louisiana Formation, and that the upper “McCraney” and the Prospect Hill formations in southeast Iowa correlate with the Hannibal Formation. This research will hopefully lead to a refined stratigraphic framework of the Lower Mississippian Subsystem in the tristate area and assist IGS geologists with making more accurate geologic maps in the near future.

 

References

Clark, R.J., Stolfus, B.M., Cramer, B.D., Day, J.E., Witzke, B.J., and Tassier-Surine, S.A., 2017, Revisiting the

Devonian-Carboniferous Boundary Interval in the U.S. Midcontinent, including the Type Kinderhook Area of Illinois, Missouri, and Iowa: Geological Society of America, Abstracts with Programs, Vol. 49, No. 6.

 

Cramer, B.D., Stolfus, B.M., Clark, R.J., Hogancamp, N.J., Day, J.E., Witzke, B.J., and Tassier-Surine, S.A.,

2018, High-resolution eustatic sea-level reconstruction across the Devonian-Carboniferous boundary from the Tri-State area of Iowa, Illinois, and Missouri: Conodonts, carbon isotopes, and sequence stratigraphy of the type Kinderhookian region: AAPG Abstract

Figure Captions:

  • Map showing the edge of the Mississippian extent in the tristate region as well as the locations of several towns that host type sections. Lines of cross-section on the map are approximations of their original plot (A-A’ from Witzke et al., 2002, and B-B’ from Collinson, 1961). High resolution sampling locations are SH = Stony Hollow Road, SC = Starrs Cave State Preserve, and CP = Crapo Park. Bulk sampling locations are not labeled. Legacy samples from Straka (1968) are also shown.
  • Geologic cross-sections:
    1. Graphic northwest-southeast stratigraphic cross-section of Kinderhookian and lower Osagean strata across Iowa and down the Mississippi River Valley (type Mississippian area). Modified from Witzke and Bunker (2002).
    2. Generalized cross-section of Upper Devonian – Kinderhookian Series in bluffs of Mississippi River. Modified from Collinson (1961).