Structure, burial history, and petroleum potential of frontal thrust belt and adjacent foreland, southwest Montana

The frontal thrust belt in the Lima area of southwestern Montana consists of blind (nonsurfacing) thrusts of the Lima thrust system beneath the Lima anticline and the Tendoy thrust sheet to the west. The Tendoy sheet involves Mississippian through Cretaceous rocks of the southwest-plunging nose of the Mesozoic Blacktail-Snowcrest uplift that are thrust higher (northeast) onto the uplift. The front of the Tendoy sheet west of Lima locally has been warped by later compressive deformation which also involved synorogenic conglomerates of the structurally underlying Beaverhead Formation. To the north, recent extension faulting locally has dropped the front of the Tendoy sheet beneath Quaternary gravels. Rocks of the exposed Tendoy sheet have never been deeply buried, based on itrinite reflectance of