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
13Ccarb
-
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