Upper Cretaceous-Paleocene Synorogenic Conglomerates of Southwestern Montana

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

“…Tectonics ORME Previous work on the Beaverhead Group broadly estimated the timing of the Blacktail-Snowcrest uplift to Santonian-Campanian, based on the palynomorphs (Nichols et al, 1985), lithologic composition of sandstones and conglomerates, and structural and stratigraphic relationships ( Figure 1, Line B1; Haley, 1986;Haley & Perry, 1991;Ryder & Scholten, 1973;Wilson, 1970). In addition, paleocurrent data of Ryder and Scholten (1973) show a contemporaneous radial pattern away 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), andHaley (1986); B2: DeCelles (1986) andPerry (1983); C: Tysdal et al (1986),DeCelles (1986),DeCelles et al (1987), andKellogg and Harlan (2007); D:Dickinson et al (1988); E:Guiseppe and Heller (1998) andLawton (1983); F: Leary et al (2014),Devlin et al (1993), andMederos et al (2005); G:Tindall et al (2010); H:Clinkscales and Lawton (2015) andAmato et al (2017); I:Schwartz and DeCelles (1988) andSuttner et al (1981); J:Liu and Nummedal (2004),Cross (2009),Painter and Carrapa (2013), andLopez and Steel (2015); K:Schmidt and Garihan (1983) andKellogg et al (1995); L:Jackson et al, 2019; M: Bird (1988); N: Brown (1988),Lageson (1987), andCraddock et al (1988); O1 and O2: Carrapa et al (2019); P:Fan and Carrapa (2014) andStevens et al (2016); Q:Cerveny and Steidtmann (1993) andOmar et al (1994); R:Crowley et al (2002) andPeyton et al (2012); S:Lageson et al (2001) andLund et al (2002); and T:Coney and Reynolds (1977) andCopeland et al (2017).…”

Timing of the Transition From Sevier‐ to Laramide‐Style Tectonism in Southwestern Montana Based on the Provenance of the Frontier Formation, North American Cordillera

“…The overlying Beaverhead Group in this same region, several kilometers to the west on the eastern flank of the Snowcrest Range, displays an up‐section pattern that records unroofing of Permian through Mississippian strata from atop the Blacktail‐Snowcrest Arch as early as ∼81 Ma (Garber et al., 2020). These sediments were sourced entirely from the Blacktail‐Snowcrest uplift through a flanking system of braided‐stream‐dominated alluvial fans (Haley, 1986; Ryder & Scholten, 1973; Wilson, 1970). Therefore, we infer that 18MT‐52 records erosion of both Cretaceous strata located in both the fold‐thrust belt to the west and Pennsylvanian‐Cretaceous strata from atop the adjacent basement uplift.…”