DETRITAL ZIRCON STRATIGRAPHY OF PERMIAN TO MIOCENE STRATA FROM DILLON AREA, SW MONTANA, AND IMPLICATIONS FOR REGIONAL CORDILLERAN TECTONICS

The landscape of southwest Montana is dominated by contractional structural culminations and adjacent sedimentary basins that resulted from complex interactions between Ancestral Rocky mountain uplifts, Sevier- and Laramide-style structures. Stark changes in drainage patterns and depositional environments from Permian to Miocene time have been described from thick sedimentary sequences that have been exposed and uplifted by deformation and thrusting. Understanding the chronology of deformation, the evolution of drainages and changes in sediment migration patterns is critical to reconstruct the complex tectonic history of this region. Despite numerous studies, the precise timing of eastward propagation of the Cordilleran fold‐thrust belt and its impact on drainage patterns, as well as the timing of foreland basin partitioning into smaller intraforeland basins, remain unclear.

We propose to use detrital zircon U/Pb geochronology and sandstone petrography data to highlight changes in drainage patterns and denudation. In this project, we present preliminary data from Permian through Miocene strata exposed near Dillon, southwest Montana in order to address the regional chronology of deformation. Preliminary data from samples of the Quadrant, Morrison, Kootenai, and Blackleaf Formations highlight changes of sedimentary provenance through the Mesozoic that remain to be interpreted in a broader regional context.