Implications for Tectonic Control on Paleogeography and Sediment Dispersal Pathway: Integrated U-Pb Detrital Zircon Age-analysis of the Paleogene Missouri River Headwater System, SW Montana
Barber, Douglas E.; Schwartz, Robert K.; Weislogel, Amy; Schricker, Lauren; Thomas, Robert C.
Cenozoic intermontane basins in southwest Montana lie within the Sevier-Laramide orogenic belt and constitute many of the basins within the Northern Basin and Range (B&R) province. Paleogene fluvial deposits within the intermontane basins record basin evolution and sediment dispersal pathways across the post-contractile Cordilleran fold-thrust belt. Recent work has demonstrated that structurally controlled fluvial incision served to establish an interconnected drainage system and develop intermontane-scale paleovalleys prior to B&R extension. In adjacent regions to the south and west of the uppermost modern Missouri River, paleotributaries to the paleoMissouri River were essentially identical to those of the modern system. The sediment dispersal pattern is similar to that which is typical of modern fold-thrust belts. Resultant outflow was northward into the upper paleoMissouri River and beyond, into the northeastern continental basin region.
A newly documented large-scale Eocene fluvial conglomerate (cobble to boulder) in the Beaverhead Basin demonstrates northward paleoflow and serves as evidence for a major influx of detritus to the paleoMissouri River system from previously undocumented tributaries in the far southwest, near the Montana-Idaho border. Paleocurrent data document that the Grasshopper and Medicine Lodge Basins were contiguous with the Horse Prairie Basin, similar to the modern, with net outflow eastward toward the modern Beaverhead canyon. That paleodrainage configuration is further supported by using detrital zircon U-Pb ages and conglomerate clasts as provenance indicators and tracers. Composition of conglomerate constituents and detrital zircon ages from trunk fluvial outcrops suggest a paleodrainage link between the paleoBeaverhead Basin and the southwestward Horse Prairie and Medicine Lodge paleobasins. Furthermore, these data indicate erosion of Mesoproterozoic Belt Fm strata as a primary sediment source, with a minor to moderate supply of sediment from Archean basement, Cretaceous plutons, and early Paleogene volcanics. In addition, detrital zircon ages serve as an effective tracer of sediment derived from Cretaceous plutons in the southern Pioneer Mountains that was routed through separate drainages in the Grasshopper and Frying Pan Basins. The Frying Pan Basin discharge was eastward, directly into the Beaverhead Basin, whereas discharge from the southern basin system was most likely northward through the paleo-Clark River canyon.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013