Abstract
The Upper Maastrichtian fluvial Hell Creek Formation of the Fort Peck Lake area, Montana (and regional equivalents) is notable for its vertebrate fossils and for the K-Pg mass extinction at or near its upper contact. Despite intense study, internal stratigraphy of the Hell Creek Formation is still poorly constrained, hindering study. This work reviews the stratigraphy of the Hell Creek Formation, as currently understood, and proposes important revisions to the recently proposed type section, particularly concerning complexity of the Hell Creek Formation basal contact. This work also subdivides the Montanan Hell Creek Formation into four 4th order depositional sequences, superimposed over a 3rd order marine transgression. Sequence boundaries are defined by four, laterally continuous disconformities formed by pauses in the creation of accommodation space, marked by overlying amalgamated channel complexes, or less commonly, correlative interfluve paleosols. Cyclicity in Montana may be correlative with similar 4th order cyclicity and marine influence documented in North and South Dakota, Alberta, and Saskatchewan. Magnetostratigraphy and new biostratigraphic data support correlation of the upper Montanan sequence with the North Dakotan Cantapeta tongue (and overlying fines) and Canadian Scollard and Frenchman Formations.
Highlights
The Laramide synorogenic fluviatile sediments of the Hell Creek Formation were deposited on a coastal plain bordering the Western Interior Seaway during the Maastrichtian stage of the latest Cretaceous [1]
The Hell Creek was not designated a formation in its own right until 1952 in Cobban and Reeside’s [8] grand correlation of Late Cretaceous strata of the Western Interior
The Hell Creek Formation of the Fort Peck area, Montana can be subdivided into four depositional sequences, each consisting of an erosional scour, amalgamated channel complex, and overlying overbank fines with isolated channels
Summary
The Laramide synorogenic fluviatile sediments of the Hell Creek Formation were deposited on a coastal plain bordering the Western Interior Seaway during the Maastrichtian stage of the latest Cretaceous [1]. The Hell Creek Formation is among few geological units worldwide that record terrestrial environments at the close of the Cretaceous. It is famous for the K-Pg mass extinction which occurs at or near the upper formational contact, and for its vertebrate fossils, especially dinosaurs [3,4]. Since Brown never formalized the Hell Creek beds as a distinct formation, nor designated a type section, researchers persisted with referring to the Montana exposures as “Lance Formation” for a further 45 years (e.g., [7]). Cobban and Reeside [8] did not designate a type section, and despite much recent interest, this problem remained until 2014 when a lectostratotype was described by Hartman et al [10]
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