EVIDENCE OF PALEOTOPOGRAPHY AND EVOLUTION OF THE DEPOSITIONAL SYSTEMS WITHIN THE SOUTHERN TOWNSEND BASIN, SOUTHWEST MONTANA FROM THE LATE EOCENE TO EARLY MIOCENE

Late Eocene to early Miocene age (Chadronian-Arikareean) mudstones, sandstones and conglomerate/breccias in the southern Townsend Basin mark the earliest record of post-Laramide sedimentation within the basin. Facies assemblages and paleocurrent indicators in these deposits suggest the existence of an ancestral basin in approximately the same location as the modern Townsend Basin that was bounded on the eastern margin by a topographic high containing the same Proterozoic through Mesozoic lithologies as are presently exposed in the Big Belt Mountains. Within the basin fill are basin margin alluvial and colluvial facies, as well as a basin-axial fluvial facies. Overall, the succession of deposits indicates a transition from deposition by low energy systems in the late Eocene and early Oligocene to higher energy systems in the mid-Oligocene to early Miocene.

Late Eocene fluvial and overbank facies indicate the existence of a distributary fluvial fan with westward dispersal off of a paleo-Big Belt Mountain topographic high. While volumetrically minor, conglomeratic distributary fan channel bodies composed solely of Mesozoic siliciclastics indicate either limited headward erosion of the fan catchment area or the limited unroofing of the Paleo-Big Belt Mountains. The deposition of about 300 meters of sediment in dominantly low energy environments in the late Eocene and early Oligocene suggests that the basin margin was relatively stable during this time.

By the mid-Oligocene, alluvial fan facies document westward dispersal of Proterozoic through Mesozoic siliciclastic and carbonate sediment away from a relatively high-relief basin margin. Undeformed Arikareean limestone breccia and conglomerate colluvium mark the paleo-range front in the approximate same location as today. Additionally, paleocurrent data from feldspathic litharenite fluvial bodies document that a northward-flowing basin-axial system transected the toes of the alluvial fans. The absence of a quartzofeldspathic plutonic source in the adjacent Big Belt Mountains suggests sediment contribution from an exorheic source. Overall, the upward coarsening succession may indicate that reactivation of the range front fault began during the mid-Oligocene.