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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Vol. 27 (1977), Pages 443-443

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 ^dgr018 range of -5.9^pmil to -2.7^pmil and a ^dgrC13 range of -.7^pmil to +5.1^pmil relative to PDB. Oxygen isotopes become lighter toward the southwest. Whole rock values of ^dgrc13 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 ^dgr 018 values closely grouped about a mean of -2.6^pmil PDB and ^dgrC13 values between -29.1^pmil to +3.2^pmil PDB. Inclusion-rich spar cements likewise show ^dgr018 values closely grouped about the mean of -2.8^pmil PDB and ^dgrC13 values ranging from -7.4^pmil to +3.8^pmil PDB. In contrast, the clear blocky spars exhibit a wider range of 018 values, from -6.6^pmil to -2.3^pmil with a mean of -5.2^pmil PDB; ^dgrC13 values range from -5.5^pmil to +4.5^pmil 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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(1) Department of Geological Sciences, The University of Texas at Austin

Copyright © 1999 by The Gulf Coast Association of Geological Societies