DOES THE ORDOVICIAN "GUTTENBERG EVENT" CARBON ISOTOPE EXCURSION EXTEND SHOREWARD INTO THE DETRITAL SHALE BELT OF THE DECORAH FORMATION?
G.A. Ludvigson, M.R. Saltzman, T.S. White, and L.A. González
The Geological Society of America
33rd Annual North-Central Section Meeting
Clarion Hotel and Convention Center, Champaign, IL, April 22-23, 1999
1999 Abstracts with Programs, v. 31, no. 5, p. A-32
A positive 13C excursion first recognized in the organic-rich Guttenberg Limestone Member of the Chatfieldian Decorah Formation in the Upper Mississippi Valley (Hatch et al., 1987, AAPG Bull. 71:1342-1354) has also been recognized in correlative carbonate strata of the Salona Formation of Pennsylvania (Patzkowski et al., 1997, Geology, 25:911-914). Interbedded shales and carbonates of the Decorah Formation thin to the southeast away from detrital sources on the Transcontinental Arch in Minnesota, and Ludvigson et al. (1996, GSA SP306:67-86) proposed that portions of the Decorah Formation (Spechts Ferry and Guttenberg members) completely starve out in offshore sections to the southeast in central Illinois. Given current uncertainties about the geographic extent of paleoceanographic processes responsible for the carbon isotope excursion, we are evaluating chemostratigraphic profiles of d13Ccarbonate, d13Corganic, TOC, and HI of a 31.4 m section of Decorah Shale in the Iowa Department of Natural Resources -- Geological Survey Bureaus Camp Quest (D-21) core (W-25498) in Plymouth County, northwest Iowa. Decorah strata in the Camp Quest core were deposited in the shoreward detrital shale belt, inboard from the depositional limits of the organic-rich Guttenberg Limestone Member. The chemostratigraphic details of this succession may help to clarify whether the carbon isotope excursion was driven by processes operating on a basinal or global scale.