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 ¹³C 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 ¹³C_carb  - ¹³C _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  ¹³C  excursion is present (~2%) and is not associated with significant organic carbon burial (%TOC  1); it occurs in a stratigraphic section of about 70 ft (21 m) in thickness. This compares with an excursion interval of similar  ¹³C  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  ¹³C   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  ¹³C   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