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

The Hell Creek region of Montana is an excellent study system to explore biotic recovery from the Cretaceous-Paleogene (K-Pg) mass extinction. The Tullock Member of the Fort Union Formation (Tullock, hereafter) was deposited 1.2 million years after the mass extinction. Mammalian local faunas from the lowermost Tullock are ‘disaster’ faunas, dominated by a few abundant, mostly insectivorous survivors. Mammalian faunas from the middle Tullock are ‘recovered’ faunas: they exceed pre-K-Pg levels of taxonomic and ecological richness. Middle Tullock fossil assemblages, however, have been interpreted as deposited in one large channel complex—the Garbani Channel—that is coarsely constrained stratigraphically. Ashes bracketing the Garbani Channel constrain deposition to an interval of ~623 k.y., but the stratigraphic relationships of localities within this interval are largely unknown. Thus, our view of the local recovery is temporally aggregated, and the record of biotic recovery in the interval between ‘disaster’ and ‘recovered’ faunas is unclear. Here, we present a new stratigraphic model for the middle Tullock resulting from a combination of field mapping, stratigraphic logging, and petrography.

We find that middle Tullock fossil localities are not derived from one channel complex but two temporally and lithologically distinct sedimentary units: Biscuit Springs Beds (BSB) and Garbani Channel (GC). The BSB are ~10 m thick and laterally continuous, cropping out across much of the study area; whereas the GC is ~20–30 m thick and has a limited lateral extent (~100 m across). The top of the GC is stratigraphically ~10–15 m above the top of the BSB, and in some places the GC cuts through the entirety of the BSB, a relationship that previously complicated interpretations of their relative age. This cross-cutting relationship reveals that the BSB, and the mammalian fossils they host, are likely older than the GC. Thus, the BSB now represent a potential intermediate fauna between the older, post-K-Pg ‘disaster fauna’ and the younger GC local fauna. Further, the BSB local fauna can now be temporally constrained by two ashes dated to 65.741 and 65.540 Ma. Studying the succession of Tullock local faunas in light of these new findings may help clarify the tempo and mode of mammalian recovery in the aftermath of the K-Pg mass extinction.