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Abstract: Carbon and Oxygen Isotopic Evolution of Whole Rock and Cements from the Stuart City Trend (Lower Cretaceous, South-Central Texas)
D. R. Prezbindowski (1)
The Stuard City Trend consists of a shelf-margin buildup of Middle Cretaceous carbonates, now buried to depths of 10,000 to 18,000 ft in south central Texas. Whole rock analyses of 92 samples from 16 wells along a 250 mile stike section show a 018 range of -5.9 to -2.7 and a C13 range of -.7 to +5.1 relative to PDB. Oxygen isotopes become lighter toward the southwest. Whole rock values of c13 indicate that vadose diagenesis was not volumetrically important.
Individual cements were also analyzed. The two predominate cement sequences are: 1) fibrous crust, 2) inclusion-rich radiaxial, and 3) clear spar; or 1) fibrous crust, 2) inclusion-rich spar, 3) clear spar. Inclusion-rich radiaxial cements show 018 values closely grouped about a mean of -2.6 PDB and C13 values between -29.1 to +3.2 PDB. Inclusion-rich spar cements likewise show 018 values closely grouped about the mean of -2.8 PDB and C13 values ranging from -7.4 to +3.8 PDB. In contrast, the clear blocky spars exhibit a wider range of 018 values, from -6.6 to -2.3 with a mean of -5.2 PDB; C13 values range from -5.5 to +4.5 PDB. No significant isotopic differences were observed in the final generation of clear blocky spar cement, between depths of 10,300 and 20,400 ft.
The whole rock and cements are not in oxygen isotopic equilibrium with sampled formation fluids. Individual cements maintain an isotopic memory of successive cementation events during burial.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
(1) Department of Geological Sciences, The University of Texas at Austin
Copyright © 1999 by The Gulf Coast Association of Geological Societies