INTEGRATED DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF THE PALEOGENE MISSOURI RIVER HEADWATER SYSTEM IN SW MONTANA

Cenozoic intermontane valleys in SW Montana (SWMT) lie within the Sevier-Laramide orogenic belt and constitute many of the basins within the Northern Basin-and-Range province. Recent work on the Paleogene Renova Formation in the Divide, Beaverhead, Jefferson, Gallatin, and Radersburg-Townsend intermontane valleys has documented an early Cenozoic (>50 Ma) basin system and drainage network similar in configuration to that of the modern network. However, the ultimate source area and sediment routing pattern of these deposits remain contested. As a means of assessing Paleogene basin models, we present sedimentological and detrital zircon (DZ) U-Pb age data from key Eocene-Oligocene fluvial and alluvial facies exposed throughout eight intermontane valleys in SWMT and central Idaho.

An Eocene fluvial conglomerate in the Beaverhead basin demonstrates northward paleoflow and contains DZ U-Pb spectra peaks consistent with documented zircon U-Pb ages from Proterozoic Belt Supergroup strata (~1400-2800 Ma), Late Cretaceous plutonic rocks (~70-80 Ma), early Paleogene volcanics (~45-52 Ma). Together, paleocurrent and provenance data serve as evidence for a major influx of detritus to the paleo-Beaverhead drainage system from previously undocumented tributaries to the southwest, including near the MT-ID border. Paleocurrent and DZ U-Pb detrital-tracer data document that the Grasshopper and Medicine Lodge Basins were contiguous with the Horse Prairie Basin, similar to the modern, with net outflow eastward toward the modern Beaverhead canyon. DZ U-Pb ages from trunk fluvial outcrops suggest a paleodrainage link between the paleo-Beaverhead Basin and the southwestward Horse Prairie and Medicine Lodge paleobasins. Divergent N-S paleocurrent data from alluvial fan facies in the southern Horse Prairie basin suggests an ancient drainage divide along the MT-ID border. Overall, provenance data reveal that Paleogene basin-fill sediments in SWMT can be traced to source rocks present within SWMT and central ID, and were routed along pathways that were essentially identical in configuration to the modern systems. This sediment dispersal pattern is similar to that which is typical of modern fold-thrust belts, suggesting that paleodrainage was largely controlled by inherited Sevier-Laramide fabrics.