Structural Relationship of the Beartooth Mountains and Big Horn Basin in South-Central Montana and Northwestern Wyoming

CLARK, DOUGLAS M., Exodus-Exploration Inc., Billings, MT

Geologic structure along the Beartooth Mountain front and adjacent Big Horn basin in south-central Montana and northwestern Wyoming has been the subject of considerable debate for many years. Public drilling records and Laramide fault movements have produced additional clues to the complex structure of the Beartooth Mountain front.

Directional drilling by Amoco Production Company, located on the northeast corner of the Beartooth block, indicates three things. (1) Western Big Horn basin Paleozoic and Mesozoic sections recumbently folded 2 mi (3.2 km) under the northeast corner of the Beartooth block, measured horizontally from the surface exposure of the Beartooth fault. (2) The main Beartooth fault dips 19 degrees northwest at a depth of 8300 to 8400 ft (2530 to 2560 km), 1 mi (1.6 km) from the surface exposure of the main Beartooth fault. (3) The main Beartooth fault appears to be a component of a complex fault system which horizontally displaces and faults-out formations in both the Paleozoic and Mesozoic section on the upper limb of the recumbent fold.

In addition to the drilling data, interior Beartooth Mountain faults, with dominant northeast and east-west structural orientation, show reactivation of left-lateral movement, intersecting the Beartooth front and offsetting Paleozoic and Mesozoic sections 1-2 mi (1.6-3 2 km) horizontally. Reactivation of some of these faults seems to coincide with the intrusion of Tertiary (Eocene?) igneous bodies along some of the same structural trends.

Beartooth front fold orientation and fault movement correlated with a reexamination of internal block fault systems lend additional weight to the argument of horizontal compression as a major factor in late Laramide formation of geologic structure along the northeast face of the Beartooth Mountains.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)