REFINING THE UPLIFT HISTORY OF SEVIER AND LARAMIDE-STYLE STRUCTURES USING DETRITAL ZIRCON GEOCHRONOLOGY, SOUTHWEST MONTANA OVERLAP ZONE

Structural interaction between thin-skinned, Sevier-style and thick-skinned, Laramide-style deformation strongly influenced basin evolution and sedimentation in the southwest Montana overlap zone. This interaction juxtaposed two structural styles in close proximity, each of which sourced sediment of unique age signatures. However, the chronology of Rocky Mountain uplift in southwest Montana is only loosely constrained by crosscutting relations and palynostratigraphic evidence—limiting our understanding of the chronological relationship between Sevier and Laramide deformation. Many of these crosscutting relationships involve the syntectonic Beaverhead conglomerate, which crops out to the west and southwest of Dillon, Montana. However, this unit’s isolated members are difficult to correlate at a regional scale—limiting the significance of existing field relations. This study refines the uplift history of the Pioneer and Tendoy Ranges of the Rocky Mountains by performing detrital zircon geochronology on 16 samples from clastic strata of southwest Montana’s Paleozoic and Mesozoic section. In addition, detrital zircon geochronology of four clastic samples and U-Pb geochronology on an ash flow tuff within the Beaverhead Formation correlated this unit’s isolated members. When coupled with provenance analysis to distinguish Laramide and Sevier-style uplift, this chronostratigraphic data provides new information on the nature of structural interaction in the southwest Montana overlap zone.