Abstract: Back-Thrusting and Triangle Zone Development Associated with Laramide Basement Uplift, Bridger Range, Southwestern Montana

D. R. Lageson, Betty Skipp

The Bridger Range represents a geologic microcosm of the major tectonic events that have shaped the northern Rocky Mountains. More than three billion years of history are represented by the stratigraphic and structural relationships within the range. Major tectonic events include Late Archean metamorphism of the basement complex, Middle Proterozoic extensional faulting associated with the south margin of the Belt basin (Helena embayment), Late Cretaceous-Paleocene "thin-skinned" folding and thrusting at the south margin of the Helena salient, Laramide-style uplift in the latest Paleocene to earliest Eocene, and late Tertiary-Quaternary basin-and-range faulting. Fault zones that originally defined the Belt basin were reactivated by subsequent tectonic events, thus producing complex "st uctural inversion."

Three bed-length balanced cross sections drawn through the Bridger Range and western Crazy Mountains basin (Sedan Quadrangle, 1:48,000) are tied to limited well control and seismic data in order to define the deep geometry of the structural and tectonic elements. The most prominent structural feature is the "sub-Bridger thrust zone," a Laramide, basement-involved imbricate fan and duplex complex that dips west beneath the range. The sub-Bridger thrust zone has approximately 25,000 ft (7620 m) of throw and 9840-16,400 ft (3,000-5,000 m) of heave or "basement overhang." Early Eocene late-stage motion on the sub-Bridger thrust zone delaminated the Upper Cretaceous Livingston Group within the Billman Creek Formation to form a major back-thrust and footwall triangle zone. The triangle zone crops out up-plunge along the northern third of the Sedan quadrangle and is one of the best documented examples of a back-thrust/triangle zone associated with a Laramide-style uplift in the Rocky Mountains.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994