STRATIGRAPHIC ARCHITECTURE OF THE EARLY LATE ORDOVICIAN CARBON ISOTOPE EXCURSION, PLATTEVILLE AND DECORAH FMS, MINNESOTA, IOWA, AND ILLINOIS

Chemostratigraphic profiles of micritic d¹³C_carbonate values from sections of the Decorah and Platteville formations in drillcores and exposed sections in Minnesota, Iowa, and Illinois have been developed to assess lateral changes in the expression of a carbon isotope excursion (Hatch et al., 1987, AAPG Bull. 71:1342-1354; Ludvigson et al., 1996, GSA-SP 306:67-86) along an onshore-offshore transect. In the discovery locale in east-central Iowa, the excursion interval records a bioevent associated with high organic-carbon burial (TOC £ 50 %). More nearshore sections in the detrital shale belt of the Decorah Shale in Minnesota and northeast Iowa have baseline d¹³C values of –2 ‰ VPDB, with positive peaks in the excursion interval reaching up to +1 ‰. Farther to the southeast, baseline d¹³C values increase to –1 ‰ in northeast Iowa, -0.5 to 0 ‰ in east-central Iowa, and finally, to +0.5 ‰ in central Illinois. The highest peak excursion values of +2.7 ‰ occur in east-central Iowa and central Illinois. The structure of the excursion consists of multiple positive peaks that occur in different stratigraphic positions, separated by negative kicks below baseline values. These peaks occur below the 454±0.5 Ma Deicke K-bentonite (to +2.7 ‰ in central Illinois), between the Deicke and Millbrig (453.7±1.3 Ma) K-bentonites (to +1.5 ‰ in east-central Iowa), and above the Elkport K-bentonite (to +2.7 ‰ in east-central Iowa). The uppermost of these corresponds to the Guttenberg Limestone, and thins to a thickness of only 0.3 m in central Illinois, confirming the conclusions of Kolata et al., 1998, GSA Bull. 110:723-739. Matching profiles of TOC values will be developed for drillcore samples to determine the extent that these variations record organic carbon burial events.