COUPLING SIMS ANALYSES, FLUID INCLUSIONS, AND 1-D BURIAL MODELING TO CONSTRAIN POREWATER δ18O EVOLUTION IN SANDSTONES OF THE CRETACEOUS TRAVIS PEAK FORMATION IN EAST TEXAS

The δ¹⁸O of a mineral precipitating in equilibrium with water is controlled by the ambient temperature and the δ¹⁸O of the water from which the mineral precipitates, making oxygen isotopes a powerful tool for unraveling the chemical and thermal evolution of lithifying rocks in sedimentary basins. However, δ¹⁸O_porewater can change by >10‰ during prolonged water-rock interaction over geologic time scales and it is imperative that a better predictive understanding be developed describing how δ¹⁸O_porewater changes in response to burial and heating. Fracture-bridging (0.5-1 mm) quartz cements in mudstone-bounded channel sandstones of the Cretaceous Travis Peak formation in the East Texas basin offer one such opportunity. These features contain fluid inclusion records indicating progressive increasing temperature trends from 130°C to 150°C during fracture opening, equivalent to roughly 500 m of additional burial under the regional geotherm (Fall et al., 2016).

In honor of Robert H. Dott Jr.’s pioneering research into sedimentary rocks and sandstones, we present coupled Qtz-bridge data for fluid inclusion temperatures with δ¹⁸O_qtz (22-26‰ VSMOW) obtained with a Secondary Ion Mass Spectrometer (SIMS) to reveal a progressive increase in δ¹⁸O_porewater by about 2‰ (from ~5 to ~7‰) during this burial period. To test whether this trend in δ¹⁸O_porewater could be reproduced numerically, a 1-D burial model was developed for a quasi-closed system (fluids are allowed to leave the system, a reasonable approximation for faulted and vertically confined units, but externally derived fluids can’t infiltrate the system) using the R programming language. The model takes as inputs estimates of porosity change and rates of mechanical compaction, and then calculates an expected δ¹⁸O_porewater for every stage of burial. Based on published cementation rates and porosity changes in the Travis Peak formation, the magnitude and rate of change in δ¹⁸O_porewater that we infer from quartz bridge geochemistry places important restrictions on initial δ¹⁸O_porewater, cementation rates during burial, and silica mass balance within the Travis Peak.