--> The Record of Changing Accommodation and Provenance in Early Cretaceous Fluvial Channel Deposits of the Western Interior Basin: Lower Kootenai Formation, Great Falls, Montana

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The Record of Changing Accommodation and Provenance in Early Cretaceous Fluvial Channel Deposits of the Western Interior Basin: Lower Kootenai Formation, Great Falls, Montana

Abstract

Early Cretaceous strata of the Lower Kootenai Formation exposed near Great Falls in west-central Montana provides critical insight into foreland basin accommodation development and sediment input in response to Cordilleran tectonism. The stratigraphy is divided into three lithostratigraphic units: the basal Cut Bank Member, medial Sunburst Member, and informal upper Red Sandstone Member. This 70 m thick package of strata accumulated over approximately 20 m.y. (Aptian-Albian). Measured sections, scaled outcrop sketches, paleoflow measurements, thin sections, and detrital zircon ages characterize changes in the depositional systems and provenance. Channel-fills in the Cut Bank Member consist of medium- to coarse-grained sandstone with rare chert cobbles, and inclined heterolithic stratification (IHS). They occur in sandstone bodies up to 25 m thick, and are typically laterally amalgamated with other channel-fills. Overbank facies are dominated by red mudstone with limestone concretions. The mean paleoflow is 353°. Sunburst Member channel-fills range from mudstone, to fine- to medium-grained sandstone with common lateral accretion surfaces, to IHS. The channel-fills are up to 30 m thick and exhibit a high degree of lateral amalgamation. Overbank facies are dominated by gray mudstone beds. The mean paleoflow is 256°. Red Sandstone Member channel-fills include IHS and structureless, fine-grained sandstone. The thickest channel fill is 34 m. Channel-fills at this level are typically isolated, encased in red and gray overbank mudstone with thin, tabular sandstone interbeds. The mean paleoflow is 20°. The units are interpreted to record meandering river systems; the shift from more laterally connected sandstone bodies to more isolated sandstones in the Red Sandstone Member is attributed to increased accommodation due to thrust-belt propagation. The transition from low-accommodation, quartz-rich Cutbank and Sunburst members to the lithic-rich, high-accommodation Red Sandstone Member coincides with a shift from a diverse spectra of detrital zircon ages to one dominated by Mesozoic ages. These observations, coupled with shifting paleoflow directions, are interpreted to record significant introduction of Cordilleran detritus. The coincidental timing of accommodation development and increased western provenance is plausibly tied to a more active phase of orogenesis and foreland basin development by the middle Albian.