FACIES AND PROVENANCE ANALYSIS OF EOCENE DEPOSITS IN THE RADERSBURG-TOSTON REGION: UNDERSTANDING THE ROLE OF CENOZOIC TECTONICS ON CHANGING INTERMONTANE BASIN GEOMETRY IN SOUTHWESTERN MONTANA

Facies, paleocurrent and composition data from Eocene clastic formations (Renova Formation-equivalent) in the Radersburg-Toston basin of southwestern Montana document a north-south elongate paleobasin bound by high relief, lithologically diverse uplifts. Along the western basin margin, breccias choked with Cretaceous igneous clasts document eastward dispersal off of the Elkhorn Mountains, an uplift that persists today. In the modern basin center, polymictic alluvial fan deposits document westward drainage off of a topographically high eastern paleobasin margin. Sub-angular, blocky limestone boulders suggest an especially proximal eastern uplift. Scattered, low relief outcrops of limestone in the southeastern region of the basin are the only modern remnants of this proposed uplift.

These paleobasin margin deposits not only indicate considerable topographic relief during the Eocene, but also delineate a much narrower paleobasin than exists today. At its narrowest point, the modern basin is approximately 20 km wide, bordered to the west by the Elkhorn Mountains and to the east by the Townsend basin. In contrast, the distance between the western and eastern paleobasin margin deposits indicates that the Eocene paleobasin was approximately five kilometers wide, only a quarter of the modern width.

To explain the paleobasin geography, we propose that a topographically high Sevier thrust sheet of diverse lithologies sourced the polymictic fan deposits of the eastern paleobasin margin. The fault trace for this uplift was likely ancestral to the Rattlesnake Creek Fault, an eastward-convex normal fault that crosscuts the modern basin in the region of the proposed Eocene eastern margin. Cretaceous thin-skinned thrust movement along this trace likely resulted in a basin-bounding uplift. Subsequently, normal reactivation of this fault, possibly driven by Basin and Range-style extension, down-dropped this eastern margin thrust sheet, considerably reducing the topographic relief and widening the basin to its modern dimensions. This suggests that extensional tectonics in southwestern Montana were post-Eocene and did not immediately follow the end of the Sevier Orogeny.