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Greg A. Ludvigson1, Brian J. Witzke1, Scott J. Carpenter2, Matthew R. Saltzman2, Luis A. Gonzalez2, Chris L. Schneider3
(1) Iowa DNR Geological Survey, Iowa City, IA
(2) Univ. of Iowa, Iowa City, IA
(3) Univ. of Texas, Austin, TX

Abstract: Use of carbon isotope chemostratigraphy for refining the sequence stratigraphic architecture of offshore subtidal carbonates: An example from the Ordovician

A positive ^dgr13C excursion in carbonate and organic carbon fractions of the Chatfieldian (Middle Ordovician) Decorah Fm of the Upper Mississippi Valley, USA (Ludvigson et al., 1996, GSA SP306:67-86) is recognized in correlative strata in Pennsylvania (Patzkowski, et al., 1997, Geology 25:911-914) and Estonia (Ainsaar et al., 1999, Geol. Mag. 136:49-62). Widespread appearance of this geochemical event and coincident decreases in ^dgr13Ccarb  - ^dgr13C OM values during the excursion suggest that the event resulted from a brief drawdown in atmospheric pCO2.

To test the areal extent of the excursion, we analyzed a cored section of correlative nearshore siliciclastic facies of the Decorah Shale in NW Iowa. A positive carbonate  ^dgr13C  excursion is present (~2%) and is not associated with significant organic carbon burial (%TOC ^le 1); it occurs in a stratigraphic section of about 70 ft (21 m) in thickness. This compares with an excursion interval of similar  ^dgr13C  magnitude, but with a thickness of only 20 ft (6 m) in more offshore organic-rich (%TOC to 40) carbonate facies in eastern Iowa. Our results suggest: 1) the  ^dgr13C   excursion is not limited to carbonate or organic-rich facies; 2) shoreward thickening of the excursion interval resulted from higher rates of sedimentation via clastic dilution; and 3) the  ^dgr13C   excursion may be used as a chronostratigraphic tool to test the hypothesis that portions of the interval are starved out over much of the eastern United States (Kolata et al., 1998, GSA Bull. 110:723-739).

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana