DIFFERING IMPACTS OF METEORIC DIAGENESIS IN THE PERMIAN SAN ANDRES FORMATION (Invited Presentation)

In ancient rocks, tantalizing records of climate change and basin restriction are captured through facies variations and carbon isotope trends. However, the influence of diagenetic fluids makes primary interpretation enigmatic. In this study, we use a multi-isotope approach, in combination with thin section analysis, to understand how secondary meteoric water is recorded in a carbonate platform sequence.

The Middle Permian San Andres Formation of West Texas is comprised of intercalated dolomite, limestone and siltstone rocks. The δ¹³C records of these sequences show positive excursions, similar to recent work in the adjacent Delaware Basin. At first glance, these shifts look to be driven by facies differences, consistent with changes in basin restriction. When we overlay δ³⁴S in both carbonate associated sulfate (CAS) and water soluble sulfate (WSS) we see facies-dependent differences reflecting secondary precipitation. The limestone intervals resemble the Middle Permian seawater sulfate, implying open water formation. In contrast, the dolomitic layers have enriched CAS and WSS δ³⁴S, reflecting post-depositional carbonate recrystallization with microbial sulfate reduction. Additionally, intervals with microcrystaline dolomite have no measurable CAS. We hypothesize this to result from low-sulfate, meteoric water diagenesis. The meteoric water brought oxygenated water into the system, resulting in the precipitation of elemental sulfur, along with the oxidation of pyrite in select intervals. Finally, the ratio between δ¹⁸O and δ³⁴S in the CAS show different reprecipitation events.

Overall, meteoric diagenesis is only apparent when sulfur is included in the isotope suite, constraining the observed recrystallization to early dissolution and reprecipitation processes. Even with this overprinting, however, much of the primary δ¹³C record is preserved. By constraining the type and extent of diagenetic processes in this system, we gain confidence for larger basin and climatic reconstructions.