Abstract: Structural Geology of the Northern Pryor-Bighorn Uplift, South-Central Montana

David A. Lopez

Moulton, in 1924, described a northeast-trending zone of faulting just south and west of Billings, Montana and named it the Fromberg Fault Zone. This fault zone extends from the Nye-Bowler Lineament to the vicinity of Billings and exhibits small, vertical and lateral offsets in Cretaceous rocks of the Colorado and Montana Groups. The Fromberg Fault Zone represents the northwest edge of the Pryor-Bighorn uplift. Immediately south of Billings, these faults end abruptly. Recent geologic mapping has identified a zone of northwest-striking faults that intersects the Fromberg Fault zone and causes its termination. These northwest-striking faults appear to mark the northeast edge of the Pryor-Big Horn uplift. To the southeast this fault zone dies out and is expressed as a surface monocline; he fault zone is probably present at depth. Together, the northwest-striking faults and monocline, are the northwest extension of the fault system along the northeast margin of the Big horn Mountains. These two fault zones are interpreted as boundaries of an Archean basement block uplifted to form the Pryor-Big Horn Uplift.

Within the uplift, the structures are best characterized as basement-involved fault-propagation folds at various stages of development.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana