OUTLIER OF CRETACEOUS NIOBRARA FORMATION IN LYON COUNTY, IOWA
G.A. Ludvigson, B.J. Witzke, P.H. Liu, and J.P. Pope
Iowa Academy of Science
April 25-26, 1997, Clarke College, Dubuque, IA
1997 Program Abstracts, p. 32
The presence of erosional outliers of sandy chalks of the Late Cretaceous Niobrara Formation in SW Lyon County, northwesternmost Iowa, has long been suspected. A buried bedrock upland in T98N, R48W has elevations that overlap with known Niobrara sections nearby in South Dakota, and a water well (GSB W-11204) drilled into this bedrock upland penetrates units with similar lithologic compositions. SEM inspection of molluscan fragments recovered from the 220-230' depth interval in the well shows that the surfaces of these fossils are partly coated by adhering coccolithophores, the pelagic golden-brown algal forms of which Cretaceous chalks are composed. In particular, recognition of the zonally-significant early Campanian coccolith Aspidolithus parcus, known from the Smokey Hill Member of the Niobrara Formation, confirms that the bedrock units penetrated in W-11204 are part of the Niobrara Formation-the first known occurrence in the state of Iowa. These efforts were undertaken as part of a systematic re-appraisal of subsurface information in northwestern Iowa leading to production of a new bedrock geologic map of the state, supported by the National Cooperative Geologic Mapping Program through the U.S. Geological Survey under Assistance Award No. 1434-HQ-96-AG-01486.