NEW EVIDENCE FOR AN EARLY PALEOGENE TRUNK-FLUVIAL SYSTEM IN THE HELENA THRUST-BELT SALIENT OF SOUTHWESTERN MONTANA – THROUGH-GOING TRANSPORT BETWEEN POST-LARAMIDE INTERMONTANE BASINS

Recent remapping and detailed facies analysis of the Renova Formation in the Lower Jefferson Basin near Whitehall, MT, has led to the recognition of a major, early Paleogene fluvial system flanked by paleo-uplifts and alluvial fan facies. A tripartite, upward-fining, stacked fluvial succession (>17 m thick, >500 m composite width) unconformably overlies LaHood strata of the Proterozoic Belt Supergroup, marking the erosional margin of a pre-Eocene paleovalley that is largely coincident with the modern Jefferson valley. The basal channel system (≥4 m thick) is dominantly a polymictic cobble and boulder conglomerate interbedded with thin wedge-shaped units of large-scale trough cross-stratified very coarse grained and granular quartzofeldspathic sandstone. A second, superposed channel system (~4 m thick) is similar in architectural style and composition, but contains both less and finer-grained conglomerate. Conglomerate clasts in these two lower systems include well-rounded quartzite from Belt Supergroup strata in westernmost Montana or Idaho, granitic clasts from the Boulder batholith, Archean metamorphic clasts from the southern Highland Range of western Tobacco Root Mountains, Kootenai conglomerate from the southern Highland Range and/or eastern Pioneer Mountains, and Elkhorn Mountains Volcanics from various locales. The uppermost channel system (~9 m thick) is sandstone-dominated and contains predominantly LaHood-derived breccia with minor Elkhorn Mountains Volcanics. Each system sequentially onlaps the paleovalley margin, incorporating LaHood rip-up clasts where in contact with the pre-Tertiary basement. Laterally adjacent debris flow, sheet flow, and fan-channel facies document local sources in flanking uplifts.

Overall, the channel complex represents: (1) a trunk braided system that flowed northward in the Upper Jefferson Basin turning to eastward in the Lower Jefferson Basin, between the paleo-Tobacco Root and Bull Mountains and parallel to the Southwest Montana Transverse Zone; (2) a high-energy stage of through-going fluvial transport during the earliest phase of Tertiary intermontane basin (paleovalley) backfilling; and (3) a means of fluvial delivery from westward basins to the Jefferson Basin and beyond to the eastward-adjacent Three Forks Basin.