Meteoric Diagenesis and Fluid-Rock Interaction in the Middle Permian Capitan Backreef: Yates Formation, Slaughter Canyon, New Mexico

Bishop, James W.; Osleger, David A.; Montañez, Isabel P.; Sumner, Dawn Y.

Four correlative sections through one cycle of the Middle Permian Capitan backreef in Slaughter Canyon, NM, show significant (2-4‰) carbon and oxygen isotopic variability in coeval, texturally well-preserved samples. Overall, the δ18O and δ13C values of marine cements, brachiopods, bulk carbonate, micritic matrix, and the first generation of meteoric spar (from high to low values) delineate an "inverted J-curve", suggesting variable alteration of components by meteoric fluids.

The 13C-depleted diagenetic end-member indicates a significant (up to 40%) contribution to DIC in the meteoric fluid from soil-respired CO2. However, the shallow arc of the J-Curve suggests very high DIC in the meteoric waters, inconsistent with a purely soil and atmospheric CO2 source. Numerical models indicate that the observed stable isotopic trend is most consistent with diagenetic alteration by meteoric fluids mixed with a progressively diminishing contribution of recycled marine waters. Such models reproduce the shallow arc of the J-Curve by systematically changing the diagenetic end-member (in δ13C-δ18O space) through time. It is also likely that this system was partially closed, with recycling of early pore fluids.

In the Capitan backreef, marine cements provide a more robust primary isotopic record than micritic matrix; however, neither preserves primary seawater isotopic values. This study demonstrates that common criteria used to diagenetically screen samples may be inadequate (e.g., textural preservation, staining, luminescence, depletion near sequence boundaries). Rather, diagenetic resetting is resolved by analyzing multiple, closely spaced, independently correlated sections, and by delineating trends between primary and later diagenetic components in populations of isotopic data.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013