PETROGRAPHIC AND ISOTOPIC EVIDENCE FOR MICROBIAL INFLUENCE IN THE ORIGIN OF THE BOLING SALT DOME CALCITE CAP ROCK, HOUSTON DIAPIR PROVINCE, TEXAS

Boling dome is a large Gulf of Mexico Basin onshore salt structure developed from the Mid-Jurassic Louann Salt. The Boling dome cap rock hosts one of the world's largest native sulfur concentrations with more than 80 million tonnes of production. Extensive cap rock drilling provides subsurface data and samples to systematically investigate the cap rock nature and origin. Conventional and cathodoluminescent petrography of Boling cap rock cores reveals a complex history of calcite paragenesis. The dome has a complex upper calcite cap rock zone that grades into an underlying anhydrite zone that rests on and is residual from the salt diapir. The upper cap rock is dominated by gray fine-crystalline calcite with irregular segregations and complex veins of pale to amber coarse-crystalline calcite. Native sulfur, and locally sulfides, is associated with veins and late-stage pores commonly filled with scalenohedral pale calcite. Historic and new cap rock calcite δ¹³C data are isotopically depleted with a large range of values from –3 to –32‰, reflecting a mixture of various carbon sources potentially including a substantial methane component. These depleted carbon isotope compositions and the presence of abundant sulfur minerals have led to interpretations that invoke microbial sulfate reduction as an important calcite cap rock process. Sulfur isotope analysis of carbonate-associated sulfate (CAS) provides a means to directly identify sulfate reduction. Reconnaissance CAS analyses yield δ³⁴S_CAS values to 42‰, significantly higher than mother salt sulfates (δ³⁴S ~ 20‰), suggesting cap rock carbonate formation via microbial sulfate reduction under closed-system conditions. The most depleted carbonate δ¹³C values combined with the enriched δ³⁴S_CAS values may reflect sulfate-dependent anaerobic oxidation of methane, particularly for the early generation gray calcites. Fluid inclusions in early coarse-crystalline calcite commonly have positive melting temperatures, suggesting the presence of methane within low-salinity inclusion fluids; later calcite veins contain high-salinity brines with local oil inclusions. These data collectively suggest that a prolonged history of fluid and microbial processes produced the current calcite cap rock and its sulfur concentrations.