PROVENANCE OF EOCENE TO OLIGOCENE SANDSTONES OF SOUTHWEST MONTANA: EVIDENCE FROM DETRITAL ZIRCONS

Late Paleogene sedimentary rocks in southwest Montana and eastern Idaho were deposited in syn-extensional basins of a north-trending ‘Paleogene rift system' (Medicine Lodge and Cabbage Patch beds) and a broad depositional basin (Renova Formation) to the east of the rift. Amagmatic Paleogene rifting is attributed to gravitational collapse of the Sevier fold-and-thrust belt.

Detrital zircon SHRIMP geochronology and LA-MC-ICPMS Hf isotope geochemistry provide evidence for several distinct sources of sediment and complex dispersal patterns. Conspicuous two-mica feldspathic sandstones of the syn-extensional Medicine Lodge beds and Cabbage Patch beds within the rift were derived from two-mica sources in the Anaconda range. U-Pb zircon ages from modern alluvium and Paleogene sandstones provide evidence for a ca. 75 Ma age for this source, 20 m.y. older than available K-Ar cooling ages for these granites which may record exhumation of the Anaconda core complex rather than emplacement. We suggest persistent basin-axial SSE-ward flow fed the Medicine Lodge beds and north- and east-flowing fan systems off the Anaconda Range fed the Cabbage Patch beds. Similar detrital zircon age spectra from the southern Medicine Lodge beds and the southern Renova Formation imply sediment contribution from the southern Atlanta Lobe of the Idaho batholith in central-Idaho. We suggest that NE-directed drainage with headwaters in central Idaho, far west of the modern continental divide, fed the southern Medicine Lodge beds and the southern Renova Formation.

The type Renova Formation south of Whitehall, MT contains almost exclusively 65 and 80 Ma grains with distinct Hf signatures likely derived from the Boulder batholith and associated volcanic rocks immediately to the west.

Anomalous 90-110 Ma zircon grains are present in some Paleogene sands in the area. We identified the mid-Cretaceous (Cenomanian) Vaughn Member of Blackleaf Formation in Montana as a possible proximate source. The ultimate source of these grains may be an early phase of the Bitterroot Lobe of the Idaho batholith yet to be identified or eroded away. Grains of this age may have been recycled through the Cretaceous foreland basin and back-arc deposits. Highly variable Hf isotope values distinguish these grains from grains of the southern Atlanta Lobe of the Idaho batholith.