ABSTRACT

The Cat Creek Anticline, scene of Montana's first major oil discovery, is a Laramide foreland structure. The anticline is a fault-propagation fold that overlies a south-dipping, slightly listric high-angle fault in crystalline basement. The Cat Creek fault has undergone several reversals of displacement through geologic time. It likely originated as a normal fault on the northern margin of a Proterozoic failed rift (Montana aulacogen) during deposition of the Belt Supergroup. Probable reactivation as a reverse fault, south side up-thrown, occurred during Late Cambrian time. About 300 ft (90 m) of reverse displacement took place near the end of the Devonian Period. The southern block gradually subsided during Late Mississippian time, creating the Big Snowy trough. Between Middle Pennsylvanian and Middle Jurassic time the Cat Creek fault underwent as much as 1,500 ft (460 m) of normal displacement. Finally, during the Laramide orogeny (early Tertiary) the Cat Creek fault underwent up to 4,000 ft (1,220 m) of reverse dip-slip displacement, along with left-lateral displacement that produced en echelon domes and faults. En echelon faults are listric and mostly confined to Cretaceous strata.

Only 4 of 10 domes along the Cat Creek Anticline have yielded oil. Hydrocarbons probably were generated during the Cretaceous Period, before the domes formed. Hence, oil escaped up-dip except in those areas having pre-Laramide structural or stratigraphic traps.