--> Abstract: Carbon Isotopic Stratigraphy of the San Andres Formation--A Possible Correlation Tool?, by R. E. Colgan and P. A. Scholle; #91004 (1991)

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Carbon Isotopic Stratigraphy of the San Andres Formation--A Possible Correlation Tool?

COLGAN, R. EUGENE, ARCO Oil and Gas Co., Houston, TX, and PETER A. SCHOLLE, Southern Methodist University, Dallas, TX

Exposures along the Algerita Escarpment record the depositional history of the San Andres Formation on the northern shelf of the Permian basin. Deposition began with a transgression over a restricted shelf and continued until "outer shelf" water depths were reached. The upper San Andres, however, marked an abrupt basinward shift of nearshore environments with multiple progradational cycles. Field analysis of the San Andres yielded detailed correlations based on such shallowing-upward cycles plus previously interpreted boundaries.

Carbon and oxygen isotopic values were determined for 222 whole-rock samples (generally fine-grained dolomites). These data, plotted stratigraphically, were compared to the field correlations developed along the escarpment. The results delineate two major shifts in the carbon isotopic compositions of the San Andres carbonates. The lower of these changes is small enough to be explainable by local depositional or diagenetic factors. The upper zone of change, however, is of greater magnitude (+2.5-3%), occurs across the entire study area, is independent of rock constituents, and continues well into the overlying strata. This upper San Andres isotopic shift may be the result of a regional or global excursion of seawater C(13) and total dissolved carbon, as neither depositional nor diagene ic processes can account for such a large change in this interval. This interpretation is supported by (1) low organic carbon levels; (2) high sulfate levels; and (3) apparent early dolomitization preserving "primary" carbon values. All imply a rock-dominated carbon isotopic system. These results indicate that fluctuations in the carbon isotopic values of San Andres shelf dolomites may be a useful chemostratigraphic tool in the Permian basin.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)