220-8 Depositional environments of vertebrate microfossil sites change abruptly from the “Mesaverde” to Meeteetse formations (Campanian–early Maastrichtian) in the Bighorn Basin, Wyoming

Session: Life and Environments Through Time and Space: Multi-Record Approaches to Stratigraphic Paleobiology (Posters)

Poster Booth No.: 134

Presenting Author:

Rory Sweedler

Authors:

Sweedler, Rory Elizabeth¹, Tobin, Thomas S.², Loughney, Katharine M³, Woodmansee, Benjamin J⁴, Farke, Andrew A.⁵, Weaver, Lucas N⁶

(1) Museum of Paleontology, University of Michigan, Ann Arbor, MI, USA; Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA, (2) Department of Geological Sciences, University of Alabama, Tuscaloosa, AL, USA; Alabama Museum of Natural History, Tuscaloosa, AL, USA, (3) Department of Earth and Environmental Sciences and Watershed Studies Institute, Murray State University, Murray, KY, USA, (4) Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA, (5) Alf Museum of Paleontology at The Webb Schools, Claremont, CA, USA, (6) Museum of Paleontology, University of Michigan, Ann Arbor, MI, USA; Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA,

Abstract:

The Campanian–early Maastrichtian interval (ca. 80–68 Ma) in the Western Interior of North America is characterized by the last major pulse of Western Interior Seaway transgression before the seaway’s retreat approaching the end-Cretaceous mass extinction. That marine transgression limits the availability of nonmarine strata which document the evolution of terrestrial ecosystems from this poorly understood interval. In the Bighorn Basin, northwest Wyoming, this interval is represented by the “Mesaverde” and overlying Meeteetse formations. The “Mesaverde” Formation is roughly coeval with other well-known Late Cretaceous units, including the Belly River Group (Alberta) and the Judith River (Montana), Wahweap and Kaiparowits (Utah) formations; the Meeteetse Formation with the Horseshoe Canyon (Alberta) and Williams Fork (Colorado) formations. We present the stratigraphy, sedimentology, and taphonomy of three new vertebrate microsites from the Meeteetse Formation, which we compare to microsites from the “Mesaverde” Formation.

 

Historic “Mesaverde” Formation localities near the eastern margin of the Bighorn Basin are known for a mix of marine and terrestrial taxa including mammals and sharks, which we observe preserved along troughs and scours throughout the “Teapot Sandstone,” the uppermost sandstone of the “Mesaverde” Formation. In newly identified “Mesaverde” Formation localities near the western margin of the Bighorn Basin, vertebrate microfossils are preserved in sandstone channel lags, in intervals rich with mud rip-ups, plant debris, and bivalves. In the northcentral exposures of the Meeteetse Formation, vertebrate microfossils are preserved in a wider variety of depositional environments: the base of sandstones with mud rip-ups and plant debris, pedogenic claystone-filled scours with abundant unionid bivalves and gastropods, and siltstones and claystones with plant debris, abundant unionid bivalves and gastropods, and pedogenic overprinting, which we interpret as channel lags, oxbow lakes, and ponds, respectively.

 

Although refinements to the chronostratigraphic constraints on the “Mesaverde”–Meeteetse transition are in progress, this marked change in depositional environment heterogeneity was likely related to changes in accommodation space and proximity to the Western Interior Seaway. Future work documenting stratigraphy, taphonomy, and fossil occurrences will set the stage for studies of terrestrial ecosystem responses to landscape change across the Campanian–Maastrichtian interval and approaching the K-Pg boundary, allowing us to evaluate whether stratigraphic faunal turnover merely reflects changes along depositional dip and thus ecological gradients, which may have been misinterpreted as evolutionary change.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11147

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