Deep Basin Overpressure Resulting From Fluid Migration and Hydraulic Head in the Uinta Basin: Insights From Beef Calcite in the Green River Formation

Beef calcite veins in the Green River Formation of the Uinta Basin, Utah, were geochemically characterised to test two hypotheses: (1) that beef calcite veins can form during extensional tectonism and (2) that fluid overpressure can develop in open or partially restricted hydrologic systems. Laser ablation U–Pb geochronology yielded three precipitation ages, with the most precise at 24.8 ± 4.8 Ma (2σ), consistent with maximum burial of the formation and coinciding with uplift of the Uinta Basin segment of the Colorado Plateau. Clumped isotope thermometry indicates precipitation temperatures between 55°C and 72°C—substantially lower than the estimated host rock temperatures of 110°C to 140°C based on a ~30°C/km geothermal gradient. δ 13 C and δ 18 O values of beef calcite range from 1.6‰ to −1.2‰ and −10.7‰ to −11.5‰ (VPDB), respectively, with calculated δ 18 O of the precipitating fluid (VSMOW) ranging from −3.3‰ to −5.2‰. These values are consistent with a mixed meteoric and shallow connate water source, suggesting the downward invasion of cold, evolved meteoric fluids along faults and fractures during post‐Laramide extensional tectonic deformation. The overpressure required for beef calcite formation may have been generated by hydraulic head associated with these downward‐migrating fluids and the subsequent lateral displacement of basin brines along stratigraphic interfaces beneath regionally continuous mudstone and evaporite seals.