A RECORD OF DIAGENESIS AND SEVIER CRUSTAL DEFORMATION WITHIN BEDDING PARALLEL FIBROUS CALCITE VEINS OF THE HEATH FORMATION, CENTRAL MONTANA TROUGH

Bedding parallel fibrous calcite veins (BPFV), also known as beef calcite, precipitate almost exclusively in organic-rich mudstones. Precipitation occurs in fractures opened due to fluid overpressure and, sometimes, contractional tectonic strain. Carbonate material is sourced from depth-related diagenetic processes including bacterial consumption of organic matter, petroleum generation, or the dissolution of depositional carbonates. To comprehensively understand the genesis of BPFV, source of carbonate, vein precipitation age, and temperature of formation must be studied together and related to basin history (i.e. burial and tectonic).

The Mississippian Heath Formation of the Central Montana Trough is an organic rich mudstone that sources most of the oil in the region. Laser ablation ICPMS U-Pb geochronology, clumped isotope thermometry, and conventional carbon and oxygen geochemistry were performed on three samples of BPFV found within the Heath Formation. BPFV were also examined petrographically via optical microscopy, cathodoluminescence, whole-rock and clay mineralogy by XRD, and mineral geochemistry by electron microprobe.

Geochronology produced an age coincident with the Sevier Orogeny and, when related to burial history diagrams, a burial depth of 1-1.5 km. Large errors in geochronologic ages are a result of contamination of vein material by inclusions of host rock material throughout BPFV. Carbon and oxygen stable isotopes analyzed by IRMS in several discrete micro-drilled samples across vein margins indicate symmetrical vein growth across a median suture line and two distinct populations of calcite growth. Carbon and oxygen isotopes of the main population are located on the interior of the vein and are consistent with a carbonate fluid source evolving from original seawater because of bacterial sulfate reduction and methanogenesis. Clumped isotope thermometry of this vein produced a formation temperature of 54.1 ± 4.3 °C, consistent with a burial depth of ~1.1 to 1.3 km assuming a normal geothermal gradient. The second generation of calcite is found at the margin of the veins. It has significantly more negative δ¹³C (PDB) values and likely precipitated from carbonate generated during hydrocarbon maturation.