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Abstract: Ordovician Carbon Isotope Stratigraphy: Some Lessons from Detailed Unit Characterization

TOBIN, K.J., Department of Geosciences, Guyot Hall, Princeton University, Princeton, NJ, 08831; K.R. WALKER, Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996

We have characterized the stable isotopic composition of Previous HitmarineNext Hit cements from four Middle Ordovician units from Laurentia and Baltica. A common working assumption of isotope stratigraphy is that Previous HitmarineNext Hit d13C values are much more resistant to diagenetic resetting and therefore should retain isotopic compositions indicative of ambient Previous HitmarineNext Hit DIC d13C. However, recent studies of Cenozoic shoal carbonates have demonstrated the ability of microbial activity to significantly affect Previous HitmarineNext Hit carbon values. Consequently, it is important to develop criteria that can be applied to ancient carbonates to delineate early diagenetic microbial activity that can cause deviations in the Previous HitmarineNext Hit carbon record. Low-Mg calcite Previous HitmarineNext Hit cement from the Chickamauga Formation (Alabama) provide an example of this pitfall; this cement was associated with Mn reduction. Chickamauga cement has Previous HitmarineNext Hit d18O values and yet is brightly luminescent with Mn values approaching 1 wt.%. This cement has more negative d13C values than most of the Previous HitmarineNext Hit cements from the nearly coeval Holston (Tennessee), Kullsberg (Sweden), and Effna (Virginia) formations. Another example comes from Previous HitmarineNext Hit cement of the Effna Formation that is interpreted to have formed during sulfate reduction. This cement has more negative d13C values compared with other Previous HitmarineNext Hit cements from this unit. In conclusion, it is recommended that when possible Previous HitmarineTop cements should be examined for early diagenetic microbial effects and that if present metazoan skeletal components (e.g. brachiopods) that were likely to have formed under aerobic conditions should be utilized for d13C chemostratigraphy.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah