EVIDENCE FOR A HIGHLY DISSECTED SEVIER-THRUST-WEDGE AND INTERMONTANE BASIN DEPOSITION IN SOUTHWEST MONTANA BY THE LATE EOCENE

The intermontane Radersburg-Toston Basin (R-TB) and northeastward-adjacent Townsend Basin (TB) lie within the eastern end of the Helena Salient thrust system. The basin complex is flanked along the western margin by the Elkhorn Mountains and along the east by the Big Belt Mountains. Late Eocene to early Miocene age (Chadronian-Arikareean) strata within these basins mark the earliest record of post-compressional sedimentation. Facies assemblages, paleocurrent data, and compositional analysis argue for a highly dissected Sevier-thrust-wedge of significant topography, convergent alluvial transport into the basins, and a northward interbasinal drainage network in the same direction as the modern Missouri River.

During the Eocene, alluvial fans along the western margin of the R-TB transported predominantly Cretaceous volcanic and plutonic igneous clasts eastward, off a paleouplift in the same location as the modern Elkhorn Mountains. However, within the modern basin center, Eocene age polymictic fan deposits, including angular limestone boulders, document westward dispersal from a nearby paleotectonic feature that is no longer present. Today, scattered, low relief exposures of Paleozoic limestone to the southeast of the R-TB basin are the only remnants of this proposed paleouplift.

In contrast, along the eastern margin of the TB, Eocene sedimentation was characterized by fine-grained deposition in overbank and lacustrine environments and a small-scale, compositionally limited, fluvial network that drained paleouplands to the east. Subsequently, during the Oligocene and early Miocene, higher energy deposition dominated with alluvial fan and colluvial systems dispersing Proterozoic through Mesozoic siliciclastic and carbonate sediment westward, away from the same paleouplift. Today, the same lithologies are actively being eroded off of the Big Belt Mountains and transported westward across the Sixmile Creek Fault, into the basin. In addition to the alluvial/colluvial facies, an axial fluvial system transported both locally derived Big Belt-related and granitic detritus, northward. The quartzofeldspathic detritus was contributed to the Paleogene trunk-fluvial system from exorheic sources to the south and west.