The region of southwestern Montana and east-central Idaho, north of the Snake River plain and east of the Idaho batholith, has been affected by a complex sequence of orogenic events from the Proterozoic through Holocene time.

Deposition of Proterozoic Belt Supergroup rocks and rocks of similar age in east-central Idaho occurred in basins that were clearly fault controlled. Many of these faults were reactivated repeatedly at later times and controlled or affected the development of younger tectonic features.

This study encompasses the entire width of the Sevier orogenic belt in this part of the Cordilleran fold and thrust belt. The thrust belt comprises several major eastward-transported thrust plates that are successively younger to the east. These plates juxtapose distinct stratigraphic packages that were deposited in eugeoclinal, miogeoclinal, and continental platform settings. As a consequence, the thrust plates can be distinguished on the basis of facies and thickness distribution as well as, to some extent, structural style. In southwest Montana, Sevier-type structures overlap with, and butt against, basement-involved Laramide structures. The extension of southwest Montana basement trends into Idaho suggests that this overlap may extend into east-central Idaho.

Superimposed on these older structures are mid-Tertiary to Holocene normal faults that formed present-day basins and ranges. Many of these are reactivated older fault zones, some of which can be shown to have Precambrian ancestry.

The region has excellent oil and gas potential, because reservoir and source rocks and trapping mechanisms are all clearly present. However, an understanding of the effect of overlapping tectonic elements is necessary to predict accurately where favorable rock packages are preserved.

End_of_Article - Last_Page 941------------