TOWNSEND INTERMONTANE BASIN IN THE HELENA THRUST SALIENT OF SOUTHWEST MONTANA: MOUNTAINOUS EARLY CENOZOIC LANDSCAPE AND TRUNK-FLUVIAL DISPERSAL SIMILAR TO THE MODERN MISSOURI RIVER

The Townsend Basin is a wide, arcuate valley located between the Elkhorn Mountains and southern Big Belt mountains, paralleling the fabric of the Sevier salient. Middle Eocene through early Miocene stratigraphic units in the basin (Chadronian-Arikareean 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 within the basin.

Paleocurrent and compositional data from mountain-side colluvial and alluvial fan facies in the southern part of the basin document westward dispersal away from relatively high-relief source areas in the Big Belt Mountains. Sand-to-boulder size Proterozoic siliciclastics and diabase, and Paleozoic siliciclastics and limestone in debris flow, fan-channel and sheet-flow deposits within an alluvial fan complex document long-term headward erosion and down-cutting into Sevier folds within the mountains along paleovalleys coincident with major present-day valleys (e.g., Dry Creek). Undeformed Arikareean limestone breccia and conglomerate colluvium mark the paleo-range front, same as today, whereas drape folding of the colluvium in other locations along the range front marks post-Arikareean uplift. Combined with paleoflow data, net upward coarsening from distal to more proximal facies document westward fan progradation across a narrow syn- to post-depositional graben within the basin. In contrast, paleocurrent data from arkosic and granite-pebble-bearing fluvial bodies document that northward-flowing longitudinal trunk systems transected toes of the fans. The absence of a quartzofeldspathic source in adjacent source areas and the alignment with compositionally similar, northward-flowing, Paleogene trunk-fluvial systems in the south-adjacent Madison-Gallatin and easternmost Three Forks Basins suggest through-going transport with a drainage pattern similar to that of the modern Missouri River headwater system.

Overall, net sediment dispersal from interconnected intermontane basins of the Helena salient may have been northward, longitudinally out of the salient, into the paleo-Lower Missouri River tract, instead of directly eastward into other intermontane basins and the distant Crazy Mountains Basin.