Provenance of Synorogenic Foreland Basin Strata in Southwestern Montana Requires Revision of Existing Models for Laramide Tectonism: North American Cordillera

Abstract

The Laramide province is characterized by foreland basin partitioning through the growth of basement arches. Although variable along the western U.S. margin, the general consensus is initiation of this structural style by the early Campanian (~80 Ma). This has been linked to flat-slab subduction beneath western North America, but the extent and cause for a flat slab remain debated, invoking the need for better constraints on the regional variations in timing of Laramide deformation. We present new conglomerate clast composition, sandstone petrographic, and detrital zircon U-Pb geochronologic data from the Upper Cretaceous Beaverhead Group in southwestern Montana that suggest a pre-Campanian history of basement-involved deformation. During the early stages of deposition (~88-83 Ma), two separate depositional systems derived sediment from the Lemhi subbasin and distal thrust sheets to the west as well as Paleozoic strata eroding off the exhuming Blacktail-Snowcrest arch to the east. Our data provide the first conclusive evidence for the longitudinal transport of gravel via Cordilleran paleorivers connecting sediment sources in east central Idaho to depocenters in southwestern Montana and northwestern Wyoming. Furthermore, erosion of Paleozoic strata by this time requires that the Blacktail-Snowcrest arch was exhuming prior to~88 Ma in order to remove the Mesozoic overburden. Later (~73-66 Ma) sediment flux was entirely from the foreland-propagating fold-thrust belt to the west. These results suggest that Laramide-style deformation in southwestern Montana preceded initiation elsewhere along the margin, requiring revision of existing models for Laramide tectonism.

New Timing Constraints for the Onset of Laramide Deformation in Southwest Montana Challenge our Understanding of the Development of a Thick‐Skinned Structural Style During Flat‐Slab Subduction

“…Data sets for this research are published in the literature, with the provenance data of Garber et al (2020).…”

“…Clast rock types and detrital zircons ages of the Antone Peaks Formation exposed near Lima, Montana, suggest derivation from Triassic and Paleozoic strata, but no contribution from the Belt Supergroup and few or no Cordilleran magmatic arc‐derived grains. Garber et al (2020) link this detritus to the Triassic Thaynes, Woodside, and Dinwoody Formations and Paleozoic strata, namely, the Permian Phosphoria Formation, Mississippian‐Pennsylvanian Quadrant, and Phosphoria Formations, the Mississippian Snowcrest Range Group, and possibly the Mississippian Madison Group (Figure 1b). These sources, and in particular the lack of Belt Supergroup detritus, are consistent with derivation from the Blacktail‐Snowcrest uplift.…”

“…Queries indicate age uncertainties. A: Garber et al (2020); B1: Wilson (1970), Ryder and Scholten (1973), Nichols et al (1985), and Haley (1986); B2: DeCelles (1986) and Perry (1983); C: Tysdal et al (1986), DeCelles (1986), DeCelles et al (1987), and Kellogg and Harlan (2007); D: Dickinson et al (1988); E: Guiseppe and Heller (1998) and Lawton (1983); F: Leary et al (2014), Devlin et al (1993), and Mederos et al (2005); G: Tindall et al (2010); H: Clinkscales and Lawton (2015) and Amato et al (2017); I: Schwartz and DeCelles (1988) and Suttner et al (1981); J: Liu and Nummedal (2004), Cross (2009), Painter and Carrapa (2013), and Lopez and Steel (2015); K: Schmidt and Garihan (1983) and Kellogg et al (1995); L: Jackson et al, 2019; M: Bird (1988); N: Brown (1988), Lageson (1987), and Craddock et al (1988); O1 and O2: Carrapa et al (2019); P: Fan and Carrapa (2014) and Stevens et al (2016); Q: Cerveny and Steidtmann (1993) and Omar et al (1994); R: Crowley et al (2002) and Peyton et al (2012); S: Lageson et al (2001) and Lund et al (2002); and T: Coney and Reynolds (1977) and Copeland et al (2017).…”

Determining the source of placer gold in the Anaconda metamorphic core complex supradetachment basin using detrital zircon U-Pb geochronology, western Montana, USA

“…With a KDE bandwidth of 20 m.y., the source inputs (see Methods section) yield a cross-correlation coefficient (R 2 ) of 0.873 ± 0.014 between the model and mixed sample. Studies that consider an R 2 >0.70 to indicate a reasonable representation of all potential sources (e.g., Sundell and Saylor, 2017;Garber et al, 2020) give us confidence that we are not missing a prominent source in our modeling. Figures 6C and 6D show the relative zircon contributions from each source input calculated by the cross-correlation coefficient and Kuiper test, respectively.…”

“…However, we deem the former interpretation more plausible due to the time gap between Flint Creek Basin and Beaverhead deposition, given that the Flint Creek Basin sedimentation occurred largely during the mid-Oligocene to late Miocene (Loen, 1986;Portner et al, 2011), while main-phase deposition of the Beaverhead Group occurred during latest Cretaceous and Paleocene time (Haley and Perry, 1991;Schwartz and Graham, 2017). Furthermore, the Beaverhead Group is a synorogenic unit that was locally sourced from the frontal fold-thrust belt and various Laramide intraforeland uplifts, resulting in spatially variable provenance trends (e.g., DeCelles et al, 1991;Haley and Perry, 1991;Garber et al, 2020). The differing provenance dependent on geographic location for the Beaverhead Group further discounts the hypothesis that the Flint Creek Basin had a shared source.…”