Name: Bedrock Geology 100k

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Description: Worth County lies along the border area of the Des Moines Lobe landform region, which was the last area covered by a Quaternary glacial advance in Iowa, and the Iowan Surface landform region, which was modified by various episodes of erosion before Wisconsin glacial events (Prior, 1991). Because of the extensive glacial and erosional activities, the land surface in this area has a relatively low topographic relief.The land surface in Worth County is mostly covered by Quaternary deposits, with an average thickness of less than 15 m (50 ft) in the eastern part of the county on the Iowan Surface landform region. However, the Quaternary is significantly thicker, with a maximum thickness of 59 m (195 ft), in the western part of the county within the Des Moines Lobe region. Bedrock outcrops were only found along the Shell Rock River and the Winnebago River.The bedrock surface of Worth County is covered by Cretaceous and Devonian rocks. The Cretaceous Windrow/Dakota Formation usually occurs as a few meters thick, iron-rich, reddish, erosional outliers or shaly/silty sandstone in north-central Iowa (Witzke et al., 2010). The Devonian rocks comprise carbonates, varying between limestone and dolomite, and shale. Based on lithologic features and fossils, the Devonian strata can be subdivided into, in descending order, the Shell Rock Formation, Lithograph City Formation, and Coralville Formation. Paleogeographically, the mapping area is within the northern portion of the Devonian Iowa Basin, a region of thickened shelf carbonate and shale deposits. Middle and lower Upper Devonian rocks form the major bedrock surface and upper bedrock aquifer in this area. Because of its stratigraphic completeness, the stratigraphy and depositional environments of the Devonian Iowa Basin have been intensively studied (e.g., Belanski, 1927, 1928; Koch, 1970). Recent geological and stratigraphic studies of this basin include Witzke and Bunker (1984), Anderson (1984), Bunker and others (1986), Witzke and others (1988), Bunker (1995), Anderson and Bunker (1998), Groves and others (2008), and McKay and Liu (2012). Bedrock geology of the surrounding area was recently mapped by Witzke and others (2010) and Liu and others (2010a & b; 2011a & b). Results from these studies and bedrock mapping projects provide an important stratigraphic framework for this bedrock geologic map. The Devonian bedrock stratigraphic nomenclature and correlation for this map follows the stratigraphic framework proposed by Witzke and others (1988).

Copyright Text: Special thanks to Keith Braun, Lynn Anne Bull, Lisa Christiansen, Larry Foley, Larry King, Eric Kline, Kathryn Lutz, Dean Moretz, Brian Nelson, and Robert and Andrea Wrage for allowing us access to their properties; and to 3 H Account LLC., BMC Aggregates, L.R. Falk Construction Co., Falkstone LLC., Holcim Cement Inc., Lehigh Cement Company, Randall Ready Mix LLC., Trenhaile & Sons Inc., Ulland Brothers Inc., Weaver Construction Co., and the Worth County Conservation Board for allowing us to work in their quarries. New subsurface geologic data was mostly generated by the University of Iowa (UI) student Kyle Bracken who produced descriptive logs of water well drilling samples. Michael Bounk of the Iowa Geological and Water Survey (IGWS) provided additional descriptive logging of water wells. Jason Vogelgesang (IGWS) prepared well samples for stratigraphic logging. Ray Anderson (IGWS) and Brian Witzke (UI) provided valued information concerning the bedrock topography, geology, and Devonian stratigraphy of the area. Mary Howes and Casey Kohrt (IGWS) provided GIS mapping technical help. Paul VanDorpe (IGWS) provided proof reading.

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