Abstract

The Devonian-Mississippian Sappington Formation in the Bridger Range, Montana was investigated for its paragenetic sequence and thermal history. These results were used to establish a burial history for the area and compared to data from nearby oil and gas wells.

The paragenetic evolution of the Sappington includes early diagenetic feldspar dissolution, formation of quartz overgrowths, and illite precipitation during early diagenesis at temperatures < 50 °C. Subsequent burial diagenesis resulted in the precipitation of non-ferroan and ferroan dolomite, followed by calcite cementation and replacement, pyrite replacement, and hydrocarbon generation and expulsion at temperatures > 130 °C. Devonian formations were the source of the non-ferroan dolomite cement and began precipitating in the latest Mississippian. Subsequent growth of ferroan dolomite resulted from clay transformation reactions in the Upper and Lower Sappington Members and was initiated during rapid burial in the late Cretaceous.

The Bridger Range and the adjacent Western Crazy Mountain Basin underwent similar Paleozoic and Mesozoic burial histories. Vastly different Cenozoic burial histories resulted from movement along the Cross Range and Pass thrusts that caused the Bridger Range to begin uplift prior to the cessation of deposition of the Livingston Group in the early Paleocene. The discrepancies in burial history caused the Sappington Formation to reach a maximum temperature of ∼135 °C in the Bridger Range and ∼230 °C in the western Crazy Mountain Basin.