REGIONAL CONNECTIVITY OF PALEOGENE FLUVIAL SYSTEMS IN THE VICINITY OF THE BOULDER BATHOLITH: EVIDENCE FROM DETRITAL ZIRCONS, RENOVA FORMATION, SOUTHWEST MONTANA

Detrital zircon U-Pb ages, paleoflow, and facies data indicate a regionally integrated fluvial system connected portions of the intermontane basin system during the Paleogene. Detrital zircon U-Pb ages from arkosic sandstones of the Climbing Arrow member of the Renova Formation determined using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) reveal complex zircon age spectra generated by detrital mixing in a distally and locally sourced paleodrainage network that drained eastward between the spatially segregated Idaho and Boulder batholiths.

Sandstones from a south draining trunk fluvial system in the Divide basin on the western margin of the Boulder batholith yield locally derived Elkhorn Mountain Volcanics and Belt Supergroup zircon as well as 78.5 ± 0.5 Ma zircon likely sourced by the Climax Gulch and/or Burton Park lobes of the Boulder batholith. Distally sourced zircon populations include 71-73 Ma zircon and c. 51 Ma zircon, sourced by the Idaho batholith and plutonic rock in the Anaconda Range, respectively. Detrital mixing of locally and distally sourced zircon indicate regional fluvial connectivity between the Divide drainage and drainages in the vicinity of the Idaho batholith to the west.

Trunk fluvial sandstones east of the Boulder batholith are dominated by locally sourced zircon populations and do not conclusively yield distally derived zircon. Sandstones in the Jefferson, North Boulder, and Three Forks basins yield locally sourced 73-80 Ma Boulder batholith-Elkhorn Mountain Volcanics zircon. Sandstones in the Harrison basin yield 74.6 ± 0.8 Ma and Archean zircon of the basin-bounding Tobacco Root batholith and uplift. The source of a small population (n=14) of 71 Ma zircon in the Three Forks basin has yet to be resolved and may reflect local input from the Tobacco Root batholith or eastern lobes of the Boulder batholith, or distal input from the Idaho batholith. These data do not conclusively reveal if the regionally integrated Idaho Batholith-sourced fluvial system drained into basins east of the Boulder batholith.

Overall, data indicate a regionally integrated Paleogene fluvial system drained the vicinity of the Idaho batholith and flowed east into the Divide basin. It is inconclusive whether that fluvial network drained into basins east of the Boulder batholith as well.