Idaho Lost River Shelf to Montana Craton: North American Late Devonian Stratigraphy, Surfaces, and Intrashelf Basin

Pre‐Thrusting Stratigraphic Control on the Transition From a Thin‐Skinned to Thick‐Skinned Structural Style: An Example From the Double‐Decker Idaho‐Montana Fold‐Thrust Belt

“…Though the shortening direction is poorly constrained, reactivation of a pre‐Devonian normal fault is suggested by the juxtaposition of drastically different Ordovician to Devonian stratigraphy across the fault. Detailed sequence stratigraphic studies of the Devonian Jefferson Formation support this interpretation (Grader et al., 2017). The oblique orientation of the inherited normal fault to the regional shortening direction may have controlled the apparent change in the local shortening direction.…”

“…Map relationships and detailed stratigraphy (Grader et al., 2017) require that before Mesozoic shortening, Devonian and younger strata were laterally continuous across the study area above the intra‐Devonian Lemhi arch unconformity, but Ordovician to Devonian strata above the sub‐Ordovician Lemhi arch unconformity are confined to the southern part of the map area. The spatial overlap of both unconformities in the study area constrains the specific locations of the Lemhi arch basement high, which is locally composed of the Quartzite of Grizzly Hill and the Beaverhead pluton.…”

“…West of the study area, the thickness of Ordovician to Devonian carbonate rocks increases from <200 m in the Lemhi Range to >1 km in the Lost River Range (Figures 2 and 3) (Ross, 1947;Sloss, 1954). The area of thinner stratigraphy was termed the Lemhi arch by Sloss (1954) (Grader et al, 2017), with angular unconformities shown where appropriate. Lithologies and grain sizes for siliciclastic (Si) and carbonate (Ca) units are shown on x-axis.…”

“… Generalized lithostratigraphic columns across the Idaho‐Montana fold‐thrust belt (see Figure 2 for locations). Red lines show sub‐Ordovician and intra‐Devonian Lemhi arch unconformities (Grader et al., 2017), with angular unconformities shown where appropriate. Lithologies and grain sizes for siliciclastic (Si) and carbonate (Ca) units are shown on x ‐axis.…”

“…The sub‐Ordovician Lemhi arch unconformity is observable throughout much of the Lemhi and Beaverhead ranges, where Kinnikinic Quartzite overlies Mesoproterozoic quartzite and hypabyssal ∼500 Ma Beaverhead plutons (Link et al., 2017; Lund et al., 2010) often with an angularity of 20°–30° (Hansen & Pearson, 2016; Pearson & Link, 2017; Scholten, 1957). The intra‐Devonian Lemhi arch unconformity is exposed in the central and southern Beaverhead Mountains, where a thin (<50 m) upper Jefferson Formation rests on Mesoproterozoic quartzite (Grader et al., 2017; Lonn et al., 2019; Scholten & Hait, 1962). The thinning and pinching out of pre‐thrusting stratigraphy near the Lemhi arch basement high restricted the distribution of weak layers available for thin‐skinned thrusting during Mesozoic time.…”