T206 Records of Early Diagenesis in Modern and Ancient Sediments

Thursday, 29 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

DIFFERING IMPACTS OF METEORIC DIAGENESIS IN THE PERMIAN SAN ANDRES FORMATION

Paper number: T206-1
Program: Technical Sessions
Time: 1:30 PM – 1:45 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-352534

© Copyright 2020 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.