ANCESTRY OF THE MISSOURI RIVER HEADWATERS: INSIGHTS FROM ZIRCON PROVENANCE ANALYSIS OF LATE PALEOGENE PALEOVALLEY DEPOSITS

In the vicinity of the Helena Salient of the Cordilleran fold-and-thrust belt of southwestern Montana, provenance data and paleogeographic reconstructions reflect development of an early Cenozoic paleovalley system ancestral to modern intermontane basins and the extant drainage network of the Missouri River headwaters. Deposits of the Paleogene Renova Formation are preserved within the paleovalleys, which are delineated by relict erosional topography developed upon the crustal infrastructure of Sevier-Laramide orogen basement. Renova Formation alluvial fan facies preserved within the paleovalley system reflect sediment derivation from upland areas that coincide with the location of modern highlands. Similarly, sediment composition and detrital zircon U-Pb ages of alluvial deposits reflect erosion of source rock suites that currently compose modern highlands, including Mesoproterozoic Belt Formation strata, Archean basement, Mesozoic strata (i.e., Blackleaf Formation, Beaverhead Group), fully unroofed Late Cretaceous granitic plutons (including the Boulder batholith, Tobacco Root batholith, McCartney Mountain pluton), partially unroofed plutons (including the Idaho batholith and plutons in the Anaconda range), and Paleogene volcanic rocks (Dillon, Elkhorn Mountain and Lowland Creek volcanics). Trunk fluvial sandstones show similar detrital zircon age signatures to nearby alluvial fan sandstones, and downstream along the trunk fluvial flow path, sediment composition and detrital zircon ages evolve reflecting point-sourced alluvial input from erosion of flanking highlands. Overall, integrated provenance analysis demonstrates that Renova Formation deposits record erosion of unroofed highlands and backfilling of an incised landscape, and indicates that ancestry of the Missouri River headwaters, including tributaries that extend westward to the Montana-Idaho border, likely predates mid-Eocene time. Although the ultimate sink for the detrital remnants of the excavated paleovalley system and unroofed highlands has not been identified, Cordilleran-derived zircon grains documented in Paleocene deposits of the Mississippi Embayment point toward a long-lived (≥ 45 Ma) connection between the uppermost Missouri and lower Mississippi drainages.