New occurrence of Neurankylus sp. (Testudines: Paracryptodiria: Baenidae) in the Upper Cretaceous Frontier Formation of south western Montana (USA)

Panascí, Giulio; Varricchio, David J.; Hirayama, Ren

doi:10.1016/j.cretres.2019.104318

Cited by 2 publications

(1 citation statement)

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“…The Frontier Formation in southwestern Montana was deposited in a mainly nonmarine depositional system as a broad delta plain containing anastomosing fluvial channels (Dyman & Tysdal, 1998;Panascí et al, 2021). This interpretation is supported by the occurrence of carbonaceous plant material, root structures, freshwater turtle fossils, dinosaur tracks and other ichnofossils, and rare in situ small tree trunks in siltstone and mudstone; fine-grained siltstone and mudstone constitute ∼75%-85% of the formation (Dyman & Tysdal, 1998;Dyman et al, 2008;Panascí et al, 2020Panascí et al, , 2021. Prior palynological analysis yielded dominantly nonmarine to coastal spore and pollen assemblages (Dyman et al, 2008).…”

Section: Frontier Formationmentioning

confidence: 99%

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

et al. 2023

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The Upper Cretaceous Frontier Formation in southwestern Montana is coeval with the transition from Sevier‐ to Laramide‐style tectonism in the Idaho‐Montana sector of the North American Cordillera. To better constrain the timing of initial exhumation above the Laramide‐style Blacktail‐Snowcrest arch, we use biostratigraphic data, sandstone petrography, and detrital zircon (DZ) geochronology to determine the provenance and depositional age of the Frontier Formation. Near Lima Peaks, erosion of Lower Cretaceous strata from the frontal Sevier‐style Tendoy thrust sheet provided sediment to the foreland basin. In the Western Centennial Mountains, sediment sources included those same sources as well as Neoproterozoic and Cambrian strata and Mesoproterozoic plutons in the Belt basin. In contrast, near the Gravelly Range, sediment eroded from Pennsylvanian‐Upper Cretaceous strata atop the Blacktail‐Snowcrest basement‐cored uplift, documenting unroofing to Pennsylvanian‐Permian stratigraphic levels by 87–85 Ma. Palynology corroborates recycling into the Frontier Formation, including from the underlying Blackleaf Formation. The ubiquitous presence of 100–85 Ma DZ ages coupled with different interpreted source regions suggests that an ash‐fall source contributed young zircon grains to the Frontier Formation. The timing of exhumation above the Blacktail‐Snowcrest arch provided by these new data presented herein suggest that Laramide‐style tectonism in Idaho‐Montana may be unrelated to shallow‐angle subduction within a narrow corridor as envisioned by current models. Instead, upper plate controls, such as the locations of inherited faults and basement highs, the distribution and thickness of pre‐orogenic sedimentary cover, and the availability of detachment surfaces, may be responsible for Laramide‐style tectonism during early Late Cretaceous time.

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