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A Regional View of the Early Chatfieldian (Upper Middle Ordovician) Guttenberg 13C Excursion (GICE) in the Trenton Limestone and Equivalent Strata from the Upper Mississippi Valley to the Appalachian Basin

Young, Seth A., Stig M. Bergström, and Matthew R. Saltzman
Department of Geological Sciences, The Ohio State University, Columbus, OH

The Guttenberg 13C excursion (GICE), a +3‰ excursion, has now been identified in sections worldwide including sections in the Appalachian Basin (central Pennsylvania, New York, southwestern Virginia), the U.S. Midcontinent (Iowa, Wisconsin, Missouri, Oklahoma), and the Cincinnati Arch and Nashville Dome regions (north central Kentucky, Tennessee). The shift in the Upper Mississippi Valley has beginning values ~–2‰ and peak values at +1.5‰, in Tennessee beginning values are ~0.5‰ and peak values as heavy as ~3.8‰, and in Virginia and Kentucky the beginning values are -0.1‰ and has peak values as heavy as +2.8‰. Biostratigraphically the GICE begins in the uppermost part of P. undatus Midcontinent Conodont Zone and reaches its peak values within the P. tenuis Conodont Zone. 13C values for the GICE fall back to a pre-excursion base line close to the P. tenuis/ B. confluens zonal boundary.

The GICE occurs above the thickest and most widespread volcanic ashes, the Hagan K- bentonite Complex, and begins 2-3 m above the Millbrig K-bentonite bed in the Upper Mississippi Valley and 5 m above the Millbrig at Hagan, VA. The GICE reaches its peak values at 4-5 m above the Millbrig in the Upper Mississippi Valley, and ~76 m above the Millbrig at Hagan. The Chatfieldian positive carbon isotope excursion serves as a new tool for precise stratigraphic correlation locally and regionally of Trenton strata, which are important for oil and gas exploration. Apart from the end-Ordovician positive excursion it is the most significant 13C excursion documented in the Ordovician.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004