Neoproterozoic Windermere Supergroup Near Bayhorse, Idaho: Late‐Stage Rodinian Rifting Was Deflected West Around the Belt Basin

Abstract

Conflicting models of Rodinian rifting have been proposed to explain the recognized variation in the Neoproterozoic and early Cambrian tectonostratigraphic architecture of the western Laurentian margin. However, discrimination among rift models is hampered by limited exposure and metamorphism of the rocks. Southeastern Idaho preserves more than 6 km of Neoproterozoic and Cambrian strata. In contrast, along the inferred continuation of the margin in east central Idaho, correlative rocks are missing across the Lemhi arch. Our field mapping and U‐Pb dating studies, located approximately 50 km west of the Lemhi arch unconformity, focused on a succession of regionally extensive rocks that were previously assigned an Ordovician age. We show that ~1.5 km of strata here overlies a ~667 Ma reworked felsic tuff and was intruded by a 601 ± 27 Ma gabbro sill; we thus redesignate these rocks as Cryogenian and Ediacaran in age. These rocks are overlain by a ~1 km thick Ediacaran to middle Cambrian quartzite. Middle Ordovician quartzites overlie these middle Cambrian strata, indicating that though Neoproterozoic and lower Cambrian rocks are present west of the Lemhi arch, upper Cambrian and Lower Ordovician rocks are thin or absent. Comparison of this redesignated section to the closest correlative sections suggests an initial stage of symmetric rifting followed by later asymmetric rifting. We suggest that prerifting ~1,370 Ma magmatism within the Belt basin produced lithospheric rigidity that influenced the final stage of rifting and produced heterogeneity in the geometries of structural domains similar to those documented in other well‐defined, modern rift margins.

Early Cretaceous Provenance, Sediment Dispersal, and Foreland Basin Development in Southwestern Montana, North American Cordillera

“…These results are consistent with visual comparison of detrital zircon spectra (Figure 9), from which distinctive age peaks of 1,840, 1,920, 2,080, and 2,700 Ma can be clearly recognized. These same age peaks are present in detrital zircon data from Ordovician, Devonian, and Mississippian strata located in east-central Idaho (Baar, 2009;Beranek et al, 2016;Brennan et al, 2020;Ma et al, 2016). Specifically, the Ordovician Kinnikinic Formation, Devonian Milligen Formation, and Mississippian Copper Basin Group have strikingly similar abundant Archean-Paleoproterozoic ages compared to the Type II samples.…”

“…The Neoproterozoic strata composite (N = 6, n = 964) is multi-modal and has four main age peaks at 1,175, 1,460, 1,775, and 2,685 Ma; a local ∼667 Ma tuff within the Neoproterozoic strata was not included in our source composite (Brennan et al, 2020). The Cambrian strata composite (N = 6, n = 923) is identifiable by a strong unimodal peak at 1,770 Ma (Brennan et al, 2020); data from Upper Cambrian strata (Link et al, 2017) were not included because they exhibit a prominent ∼500 Ma peak that is not present in the Kootenai Formation and are otherwise indistinguishable from Lower Cambrian data. The Ordovician-Devonian-Mississippian strata composite (N = 28, n = 2,592) has distinctive age peaks of 1,840, 1,920, 2,080, and 2,700 Ma (Baar, 2009;Beranek et al, 2016;Brennan et al, 2020;Ma et al, 2016).…”

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

“…Thick‐skinned thrusts beneath overlying older thin‐skinned thrusts provide a way to thicken the wedge with minimal horizontal shortening. The initial geometry of the basement‐cover contact, which in the current study is a result of heterogeneities during prior rifting (Brennan et al., 2020), determines where structural style will transition from thin‐skinned to thick‐skinned. Thick‐skinned thrusting initiates as a more effective way to build structural relief, without large‐magnitude horizontal shortening.…”

“…In central Idaho (Figure 2), the Pioneer and Copper Basin thrusts form the orogenic interior of an east‐verging to northeast‐verging, leading imbricate fan (Dover, 1981; Rodgers & Janecke, 1992; Skipp & Hait, 1977). Recent work (Brennan et al., 2020) demonstrates that these thrusts deformed a relatively thick succession of Neoproterozoic and Paleozoic rift and passive margin rocks southwest of the Lemhi arch basement high.…”

A tectonic model for the Transcontinental Arch: Progressive migration of a Laurentian drainage divide during the Neoproterozoic–Cambrian Sauk Transgression

“…Palaeogeographic reconstructions are modified from Gehrels and Pecha (2014) domain (e.g. Brennan et al, 2020;Yonkee et al, 2014). While precise timing is often difficult to determine, in proximal margin domains, the lithospheric breakup surface (i.e.…”