EOCENE BASIN RECORD OF EXTENSIONAL DEFORMATION IN SOUTHWEST MONTANA

Extension is a fundamental tectonic process in the formation and subsequent collapse of orogenic belts, the development of ocean basins, and the breakup of supercontinents. While extensional tectonics has dominated the western United States Cordillera for most of the Cenozoic following Mesozoic contraction, it has proven difficult to reconstruct the earliest record and distribution of extension. Eocene sedimentary strata in southwest Montana record a period of widespread basin formation and filling linked to metamorphic core complex exhumation (Deer Lodge Valley) and episodic collapse of structural culminations (Muddy Creek Basin). The timing and structural style of basin formation are fundamental to our understanding of the processes that drove extension. We address these questions through stratigraphic and geochronologic data. Deer Lodge Valley strata adjacent to the detachment fault coarsen up from basal mudstones to sandstones to conglomerate with quartzite and limestone clasts, likely derived from the footwall. Distal strata consist of traction-structured pebble to cobble conglomerate with interbedded organic-rich mudstone and siltstone. Detrital zircon signatures are dominated by locally exposed sedimentary Jurassic and Cretaceous strata, Belt Supergroup, and Boulder Batholith. Muddy Creek strata consist of Eocene Challis volcanic rocks overlain by a lower interval of volcaniclastic sandstone with granule-pebble conglomerate lenses and limestone beds. This lower interval is overlain by a thick section of freshwater lacustrine mudstone containing varying proportions of organic matter and fossils, including gastropods, leaves, and sticks. Detrital zircon grains in the lower Muddy Creek are predominantly Challis grains, with fewer than 10% of grain ages between 1350-1500 Ma. Stratigraphy within these two basins both record fluvial-lacustrine deposition within extensional basins with different provenance patterns. The Deer Lodge Valley has extra-regional detrital zircon signatures, and formed adjacent to higher magnitude extension, whereas Muddy Creek Basin has zircon of predominantly local origin, having formed due to lesser-magnitude extension. This may indicate a link between the magnitude of extension and the catchment size, possibly due to the degree of surface lowering.