Seismic Profile Across Leading Edge of Fold and Thrust Belt of Southwestern Montana: ABSTRACT

Abstract

The fold and thrust belt of southwestern Montana and east-central Idaho is the northern continuation of the thrust belt of Utah, Wyoming, and southern Idaho. The frontal fold and thrust zone, which the seismic profile transects, is in the same structural position and exhibits similar geologic relationships as the disturbed belt farther north in Montana and the part of the Utah-Wyoming thrust belt east of the Paris-Willard thrust. The seismic profile illustrates: (1) rocks of the Archean basement complex dip gently westward beneath the thrust belt, (2) the Phanerozoic section is thickened dramatically in the western part of the profile by thrusting and folding, (3) the principal decollement horizon is probably at the top of the Mississippian section, (4) at least one thrus involves basement rocks, (5) the basement surface is also cut by steep reverse faults, (6) Tertiary basins contain numerous steep normal faults which cut basement rocks, several of which can be projected to connect with steep northwest-trending reverse faults in the Ruby Range, and (7) Tertiary rocks in the Beaverhead Valley immediately overlie either a thin Paleozoic section or Archean basement indicating that most of the stratigraphic section was removed prior to mid-Tertiary normal faulting. End_of_Article - Last_Page 855------------