Middle Albian provenance, sediment dispersal and foreland basin dynamics in southwestern Montana, North American Cordillera

Abstract

The Lower Cretaceous Blackleaf Formation in southwestern Montana records sedimentation in the Idaho‐Montana retroforeland basin of the North American Cordillera. Regional‐scale sedimentology suggests that during Albian time southwestern Montana was partially flooded by an early marine incursion of the Western Interior Seaway during deposition of the Blackleaf Formation. We use sandstone petrography, large‐n (n = 600) U‐Pb detrital zircon geochronology and mixture modelling to determine the provenance of these strata. Our analysis suggests three distinct provenance groups: Group 1 sandstones occur in the eastern region of the study area, are quartz‐rich and have zircon age‐probability peaks of ca. 110, 160, 420–450, 1050 and 1160 Ma; these sandstones match with a primarily Appalachian provenance. Group 2 sandstones occur in the western region of study area, are lithic‐rich and have peaks of ca. 110, 160, 1780, 1840, 1920, 2080 and 2700 Ma; the primary source for these sandstones was exhumed lower‐middle Palaeozoic strata from the Idaho sector of the Sevier belt. Group 3 sandstones occur in the western region of the study area, are lithic‐rich and have prominent peaks of ca. 115, 170, 430, 600, 1085, 1170, 1670 and 1790 Ma; the primary source for these sandstones was exhumed Triassic‐upper Palaeozoic strata from the Idaho sector of the Sevier belt. Our provenance data record a sharp change that coincides with the western shoreline of the seaway, and we infer that it may indicate the position of an irregular, submarine forebulge depozone influenced by dynamic subsidence during a period of reduced thrusting in the adjacent fold‐thrust belt. Albian‐aged sediments in southwestern Montana were delivered by rivers with headwaters in the Sevier belt as well as transcontinental river systems with headwaters in eastern North America. In southwestern Montana, west‐flowing transcontinental fluvial systems were flooded by the Western Interior Seaway as it encroached from the north.

Supplemental Material: Foreland basin response to middle Cretaceous thrust belt evolution, southwestern Montana, USA

“…Unroofing of Triassic strata is documented to have occurred in southeastern Idaho during Blackleaf time in the Wyoming foreland (Gentry et al, 2018). Detrital zircon age modes in the Triassic strata are typified by multiple 400-600 Ma age peaks and a 1640 Ma peak seen in both Triassic zircon data from this study area (Rosenblume et al, 2021a) and in southern British Columbia (Gehrels and Pecha, 2014).…”

“…97.39 +1.49 −0.46 Ma, and Quinn et al (2018) reports an MDA for the upper sandstone of the Flood Member (probably equivalent to Blackleaf-C based on their stratigraphic section) of 103 ± 1.7 Ma. Rosenblume et al (2021a) provide multiple MDAs for the underlying Blackleaf-A in southwestern Montana, some from the same stratigraphic sections as this study, with ages ranging from ca. 113 Ma to ca.…”

“…Overall, the A-B series records fluvio-deltaic sedimentation followed by anoxic marine conditions as the Western Interior Seaway flooded the basin (Suttner et al, 1981). Rosenblume et al (2021a) provide detrital zircon U-Pb data from Blackleaf-A that indicate continued sedimentary recycling from the Idaho fold-thrust belt into the western basin but a primarily Appalachian contribution from transcontinental river systems in eastern basin sandstones.…”

“…100 Ma) nature of the grains necessitating a higher-n MDA calculation to reduce the impacts of potential lead loss that is difficult to account for in young (<600 Ma) grains. High-resolution zircon sampling throughout the basin between this study and the underlying Blackleaf-A and Kootenai formations (Rosenblume et al, 2021a;Rosenblume et al, 2021b) and the overlying Frontier Formation (Rosenblume et al, 2018) requires a method of calculating MDAs that prioritizes the preservation of stratigraphic relationships in sampled sections. Honoring age constraints from below and above in the same stratigraphic section also ruled out methods such as YPP and YSCG that occasionally produced ages significantly older than MDAs from other methods or from underlying strata.…”

“…2080 modes (Baar, 2009;Beranek et al, 2016;Ma et al, 2016;Brennan et al, 2020). Lower Paleozoic strata were a significant sediment source for the Kootenai Formation and Blackleaf-A (Rosenblume et al, 2021a;Rosenblume et al, 2021b). Notably, Mississippian strata grade from carbonate-siliciclastic Antler-derived turbidite flysch in the western foldthrust belt (Link et al, 1996) to a carbonate bank on the Lemhi Arch (Sandberg et al, 1975;Skipp et al, 1979b;Link et al, 1988), with Upper Mississippian units comprising almost 1000 m of outer carbonate bank in eastern Idaho (Skipp et al, 1979b).…”