RADERSBURG-TOSTON INTERMONTANE BASIN OF SOUTHWEST MONTANA: FACIES AND DRAINAGE PATTERNS DEFINE STRUCTURALLY PARTITIONED PALEOGENE SUB-BASIN AND SEVIER-UPLIFT SOURCE AREAS

The Radersburg-Toston basin lies within the eastern part of the Helena thrust-belt salient in between the Elkhorn Mountains and the southern Townsend intermontane basin. Late Eocene through possibly early Oligocene stratigraphic units in the basin (Chadronian to Orellan North American mammal ages) are age equivalent to the Renova Formation in intermontane basins to the south and west and mark the earliest post-contractile record of sedimentation in the basin.

Facies, paleocurrent, and compositional data document a north-south elongate paleo-basin within the western half of the larger modern intermontane region. Along the western margin of the paleobasin, at the base of the Elkhorn Mountains, east-directed mass flow and fluvial conglomerate bodies containing solely Cretaceous Elkhorn Mountains Volcanics clasts document dispersal away from the mountain front. Other fluvial sites with added Mesozoic and Proterozoic clasts signify derivation from paleovalleys, coincident with modern valleys, reaching westward into the range interior. Along the northwestern margin of the basin, subangular boulders and cobbles of Proterozoic siliciclastics, Paleozoic limestone, Cambrian sandstone, and Cretaceous intrusives and volcanics indicate adjacent sources in northward- and northwestward-located locales. Fluvial dispersal was along a paleovalley system coincident with the present-day Crow Creek drainage.

On the east-central side of the paleobasin, fluvial conglomerate and sandstone bodies document westward- to southwestward-directed drainage around the southern perimeter of Lone Mountain, an isolated, fault-bound uplift, into the central paleobasin. Subangular, block-shaped boulders and cobbles in the conglomerates were derived from nearby limestone, volcanic, and feldspar-rich plutonic source rocks. The most likely source areas, in addition to Elkhorn Mountains Volcanics in Lone Mountain, were less than several km to the east, within a Sevier thrust sheet that was more recently down-dropped by Cenozoic faults.

Overall, drainage was convergent into the paleobasin where polymictic and Tertiary-volcaniclastic fluvial channels flowed southward before rotating southeastward near the merger with eastward-flowing quartzo-feldspathic fluvial systems in the adjacent Three Forks basin.