DETRITAL ZIRCON AND PETROGRAPHIC EVIDENCE FOR PALEOGENE BASIN DEVELOPMENT ATOP THE CORDILLERAN OROGENIC WEDGE: PROVENANCE OF THE RENOVA FORMATION, SOUTHWEST MONTANA

Detrital zircon U-Pb ages from granitic clasts determined using the Sensitive High Resolution Ion Microprobe-Reverse Geometry (SHRIMP-RG), combined with petrographic and facies data indicate that local plutons and uplifts served as major sediment sources to the upper Eocene basal Renova Formation.

In the North Boulder basin, granitic pebbles from a fluvial sandstone body in the (Chadronian) Red Hill map unit of the Renova Formation yield 73.9-79.2 Ma zircon (n=13), with an average of 77.4 ± 0.4 Ma from a coherent group of four ages. Rare detrital zircons contain inherited discordant late Proterozoic cores. Feldspars within these fluvial deposits are consistent with a potassium feldspar-rich plutonic source. Zircon and petrographic data, coupled with southward paleoflow, indicate a Boulder batholith source, possibly the Unionville lobe. Exposure of the Boulder batholith is also supported by abundant grains of quartz monzonite, plagioclase, and biotite in the Jefferson basin to the south.

A biotite tonalite cobble from a fluvial sandstone within the (Chadronian) Antelope Creek map unit of the Renova Formation in the Harrison basin yields 70.9-81.7 Ma zircon (n=16), with an average of 75.5 ± 0.5 Ma from a coherent group of five ages. Rare detrital zircons contain inherited Archean to late Proterozoic cores. Zircon data, northeastward paleoflow, and an abundance of microcline, myrmekite, and metalithic clasts suggest a source of plutonic and Archean metamorphic rocks within the Tobacco Root uplift.

A granitic cobble sampled from a trunk fluvial body in the (late Chadronian to early Orellan) Climbing Arrow member of the Renova Formation in the Divide basin yields 51.8-60.1 Ma zircon (n=10), with an average of 51.9 ± 0.9 Ma from a coherent group of four ages. U-Pb ages, two-mica granite clasts, and southward paleoflow suggest a north-northwest plutonic source, possibly within the Anaconda complex. Alluvial fan facies contain granitic, Belt Supergroup, and Paleozoic sedimentary cobbles that record westward dispersal from the Boulder batholith-Highland uplift.

Overall, the data indicate a complex paleotopography, with localized Eocene basins receiving material shed from intervening crystalline highlands. This contradicts the presently favored model of a broad, contiguous basin sourced by the Idaho batholith.