Revised Stratigraphic Relationships Within the Lower Fort Union Formation (Tullock Member, Garfield County, Montana, U.S.A.) Provide a New Framework for Examining Post K–pg Mammalian Recovery Dynamics

Abstract

The Hell Creek region of northeastern Montana is an excellent study system to explore the rise to dominance of mammalian faunas after the Cretaceous–Paleogene (K–Pg) mass extinction. The Tullock Member of the Fort Union Formation exposed in that region was deposited during the first 1.2 Ma after the Chicxulub bolide impact. Some aspects of post-K–Pg mammalian succession remain obscure, however, due to a lack of finer stratigraphic resolution between vertebrate fossil localities. Here, we present a new stratigraphic model for the lower and middle Tullock and identify a stratigraphic succession of five mammal-bearing sedimentary units that span the first ∼ 900 ka of the Paleocene. Most notably, we find that middle Tullock fossil localities, which were previously thought to be deposited by a single, large fluvial channel complex, are derived from two temporally and lithologically distinct sedimentary units: the Biscuit Springs unit (BS) and the Garbani channel (GC). The top of the GC is stratigraphically above the top of the BS, but in some places cuts through the entirety of the BS, a relationship that previously complicated interpretations of their relative age. This cross-cutting relationship reveals that the BS is older than the GC. Thus, the BS local fauna represents a potential intermediate between the older local faunas from the post-K–Pg ‘disaster' interval and the younger, more taxonomically/ecologically diverse GC local fauna. This new stratigraphic framework sets the stage for future studies focused on the pattern and timing of biotic recovery in the aftermath of the K–Pg mass extinction.

Microstructural and crystallographic evolution of palaeognath (Aves) eggshells

“…An eggshell of Paleocene palaeognath Lithornis celetius was analysed to acquire the data of fossil palaeognath eggshell. This material was excavated from the Fort Union Formation, Montana ( Weaver et al, 2022 ), and its polarized light microscopic and scanning electron microscopic micrographs were presented in Houde, 1988 and Grellet-Tinner and Dyke, 2005 , respectively. The material (YPM 16961) was provided by Yale Peabody Museum of Natural History (New Haven, CT, USA).…”

New mammalian local faunas from the first ca. 80 ka of the Paleocene in northeastern Montana and a revised model of biotic recovery from the Cretaceous–Paleogene mass extinction

“…Previous work in the Hell Hollow area has mainly focused on the description of two fossil localities, Hell Hollow Channel and Worm Coulee 1 (UCMP locs. V74110 and V74111, respectively), their mammalian assemblages (Archibald, 1982;Wilson 2014), and, more recently, the sedimentology, stratigraphy, and geochronology of this area (Sprain et al, 2015(Sprain et al, , 2018Weaver et al, 2022a). In contrast, the mammalian assemblages from the Hauso Flats area have not previously been described; rather, attention has focused on geological descriptions of KPB sections (Alvarez et al, 1983;Smit & van der Kaars, 1984) and related geochronology and geochemistry (e.g., Arens & Jahren, 2000;Arens et al, 2014;Renne et al, 2013;Tobin et al, 2021).…”

“…Thus, we bracket the age of the deposition of the Herpijunk FIGURE 4. A, a south-facing view of the outcrop containing the Herpijunk Promontory vertebrate microfossil locality (white star); the ∼6.4 m-thick sandstone unit that is ∼5.5 m above the locality is interpreted as the Carrie Padgett channel complex; the Herpijunk microfossil locality is within the Hauso Flats channel deposit that crosscuts both the MCZ and IrZ coals and sits below a sandstone bed that is lithologically nearly identical to the sandstone characteristic of the Carrie Padgett channel deposit (Weaver et al, 2022a). B, close-up, east-facing view of the section highlighted with a box in A, showing the Hauso Flats channel deposit that hosts the Herpijunk vertebrate microfossil locality scouring the MCZ coal; although not shown, the Hauso Flats channel also crosscuts the IrZ coal to the right.…”

“…The diagonal hatching of the Hauso Flats channel represents the lateral accretion beds. The overlapping of the Carrie Padgett channel on the Hell Hollow channel represents the temporal sequence of the deposition of Hell Hollow channel sediments first, followed by an erosive regime, and then the deposition of the Carrie Padgett channel sands (Weaver et al, 2022a). The KPB carbonaceous shale is a ∼10 cm layer that coincides with a negative carbon isotope excursion that was interpreted by Arens et al (2014) as marking the KPB.…”

“…Geological studies have constructed a high-resolution temporal framework from magneto-, litho-, chemo-, and biostratigraphy and radioisotopic ages (Archibald et al, 1987;Hartman et al, 2014;Ickert et al, 2015;LeCain et al, 2014;Moore et al, 2014;Renne et al, 2013;Sprain et al, 2015Sprain et al, , 2018Swisher et al, 1993). With careful stratigraphic correlation, researchers have integrated the fossil assemblages from the region into that framework (Archibald, 1982;Lofgren, 1995;Clemens, 2002;Wilson, 2005Wilson, , 2014Smith et al, 2018;Weaver et al, 2022a).…”