Diagenetic Systematics of Carbonate Associated Sulfate in Carbonate Facies of the Yates, Capitan, and Bell Canyon Formations, Guadalupe Mountains National Park, TX

Abstract

Sulfur isotope measurements of trace sulfate in carbonates (Carbonate Associated Sulfate, CAS) underpin many marine paleo-redox reconstructions. Interactions between carbonates and biogeochemically-modified fluids may confound reconstructions of ancient seawater, but signals imparted by these diagenetic processes may themselves inform paleoenvironment. We use the petrography, δ13C, δ18O and δ34S composition of samples collected along a shelf-to-basin timeline to ascertain diagenetic processes affecting CAS in late Permian platform carbonates in McKittrick Canyon, Guadalupe Mountains National Park, TX. Our new MC-ICP-MS-based technique, with a ~103 times lower detection limit, allows us to micro-drill textures in heterogeneous hand samples for their δ34S variability. The δ34SCAS range of syndepositional rocks exceeds 20‰. Fabric-retentive dolomite grainstones and rudstones deposited in peritidal environments have δ34SCAS between 7‰ and 10‰. Lime boundstones deposited at ~50m paleo-depth in a reef are enriched in 34S to about 13‰. The deepest (~600m paleo-depth), non-bioturbated basin margin limestones are enriched in 34S to as high as 17‰. Late burial diagenetic calcite incorporates a wide δ34S range, from -5 to 25‰. Calcites formed from warmer, more rock-buffered fluids (as indicated by δ13C and δ18O data) are most 34S-enriched, implicating a much younger sulfate source. The dolomite grainstones and rudstones record the δ34S of Permian Basin seawater, at about 9.5‰, and agree well with estimates from nearly coeval evaporites. Enrichment in the deeper, finer, limestone facies indicate either extreme water-column stratification and anoxia (precluded by in-situ aerobe fossils in the reef facies), or significant diagenetic overprinting of CAS. The CAS enrichment must be diagenetic and probably reflects addition of pore-water sulfate that is the residual of microbial sulfate reduction in anoxic sediments. Marine cements' δ34SCAS match their host facies, indicating that CAS preserves the isotopic composition of fluids present during stabilization of metastable carbonates or dolomitization. This study demonstrates that CAS is a robust proxy for ancient seawater sulfate if the timing and type of diagenetic processes are well-understood. Additionally, δ34SCAS altered by early diagenetic processes contains useful paleoenvironmental data indicative of the redox structure of the basin; CAS is more sensitive than carbon or oxygen isotopes to early diagenetic reactions.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017