A record of overpressure and Sevier tectonics within beef calcite of the Heath Formation, Central Montana Trough

Abstract

The Central Montana Trough is an important sedimentary basin for petroleum resources and as a record of North American tectonism. The thermal, overpressure, and tectonic histories of the trough are investigated through a study of beef calcite veins taken from the Heath Formation, the primary hydrocarbon source rock of the basin. Results from beef calcite analysis include petrographic characterization, U-Pb geochronology via LA-ICP-MS, clumped isotope thermometry, and conventional carbon and oxygen stable isotopes. U-Pb ages and clumped isotope temperatures are 121.8 ± 19.2 Ma at 43.1 ± 2.3 °C; 96.2 ± 25.9 Ma at 52.1 ± 3.7 °C; and 101.6 ± 24.2 Ma at 50.2 ± 2.4 °C. Large errors on U-Pb ages result from very low U contents (<0.03 ppm). Ages and temperatures indicate staged vein precipitation at burial depths between 600 m to 1.3 km during the major burial event of the basin and a normal geothermal gradient of 25 °C/km. Beef calcite emplacement and basin burial are the direct result of regional tectonism of the Sevier Orogeny. Calculated fluid δ¹⁸O using clumped isotope temperatures revealed δ¹⁸O values of 3–7 ‰ (SMOW), indicating beef calcite precipitation from connate formation brine. Analysis of median-symmetrical beef carbonate δ¹⁸O and δ¹³C values indicates precipitating fluids were diagenetically modified by bacterial sulfate reduction and methanogenesis of organic matter. A thin (0.1 mm) second generation of growth is found on the outermost vein margin by a sharp chemical boundary and a trail of inclusions. This second growth stage could not be dated separately, but significantly lower δ¹⁸O and δ¹³C values of −2 ‰ and −5.5 ‰, respectively, indicate precipitating fluids were hotter and further diagenetically altered by thermal organic matter breakdown.

Supplemental data: https://osf.io/39znd/.