Abstract
Extension is a fundamental tectonic process responsible for the breakup of supercontinents, formation of oceanic basins, and the collapse of orogenic belts. Extensional tectonics has dominated the western United States following Mesozoic contraction but it has proven difficult to reconstruct the timing and distribution of early extension. Eocene sedimentary strata in southwest Montana record a period of widespread extensional basin formation associated with metamorphic core complex exhumation (Deer Lodge Valley) and collapse of structural culminations (Muddy Creek). This study will outline the evolution of both basins through detailed stratigraphy and detrital zircon U-Pb analyses to reconstruct the earliest record of extension and paleodrainage reorganization during Eocene-Oligocene time. Deer Lodge Valley stratigraphy proximal to the detachment coarsen up from organic-rich mudstone and siltstone to limestone-clast-rich breccia. Distal strata consist of pebble-cobble cross-bedded conglomerate interbedded with organic rich mudstone. Detrital zircon grains are dominated by grain age populations overlapping with the locally exposed Lowland Creek volcanics, the Idaho Batholith, sedimentary Pennsylvanian-Triassic strata, and the Belt Supergroup. Basal strata have a maximum depositional age of 70 Ma. Muddy Creek basin stratigraphy consist of basal Eocene Challis ignimbrites overlain by a lower interval of volcaniclastic sandstone with lenses of granule conglomerate. The lower interval is overlain by organic-rich lacustrine mudstone and limestone interbedded with coarse grained sandstone and pebble conglomerate. Basal detrital zircon signatures are dominated by grain age populations overlapping with the Challis volcanic group and the Idaho Batholith, with a maximum depositional age of 44 Ma. Upper strata consist of grains sourced from locally exposed Pennsylvanian-Permian sedimentary strata and Belt Supergroup. The Deer Lodge Valley and the Muddy Creek Basin both record fluvial lacustrine deposition during active extension with vastly different depositional histories and provenance patterns, suggesting a possible southward sweep of extension in southwest Montana during Eocene time, with extension starting earlier in the north due to Farallon slab rollback.