New Mexico Geological Society Annual Spring Meeting — Abstracts

Situated within the Laramide foreland province, the Love Ranch Basin hosts the Upper Cretaceous to Paleogene McRae Group. This group comprises of three Formations, listed in ascending stratigraphic order: the Jose Creek Formation, the Hall Lake Formation, and the Double Canyon Formation. Strata in the McRae Group are mainly fluvial with abundant volcaniclastic input. The Campanian Hall Lake Formation contains several dinosaur fossils, including Ceratopsians, Alamosaurus, and a postulated new species of Tyrannosaurus, dubbed Tyrannosaurus mcraeensis (Dalman et al., 2024). This work presents new high-precision ²³⁸U/²⁰⁶Pb zircon dates (±2s) aimed at establishing the maximum depositional age (MDA) of fossil-bearing rocks, improving our understanding of the duration of Laramide deformation, and testing for the presence of the K–Pg boundary in this basin. We used weighted means of the youngest grain clusters for max depositional ages (MDA).

Thus far seventeen samples have been analyzed using LA-ICPMS, including tuffs and sandstones with abundant volcanic lithic grains. These samples yielded many young detrital zircons interpreted to be approximately syndepositional. Key sample localities were within the Double Canyon and Hall Lake Formations. To constrain the age of a postulated T. mcraeensis, samples were collected from the fossil-bearing shale and overlying sandstone, yielding MDAs of 69.2 ± 0.4 Ma and 69.0 ± 0.8 Ma respectively. These ages make it unlikely that T. mcraeensis predated T.rex by seven million years. An Alamosaurus-bearing unit produced an MDA of 69.7 ± 0.3 Ma, which we suggest indicates a correlation between these stratigraphic units.

A tuff located stratigraphically below the Tyrannosaur fossil and previously dated by Amato et al. (2019) was reanalyzed and yielded a consistent age of 73.6 ± 0.5 Ma. Four newly dated tuffs from the Double Canyon Formation span 60.1 ± 1.1 to 60.8 ± 1.5 Ma, marking the first geochronological constraints for this unit. MDAs from its base (61.5 ± 1.3 Ma) and top of the exposed section (56.0 ± 0.1 Ma) further constrain the timing of deposition and indicate that the Double Canyon is entirely Paleogene in age, possibly extending into the Eocene.

Our work indicates that the K-Pg boundary, if exposed, is located in the upper Hall Lake Formation. This boundary has been confined to the field area between the Alamosaurus bearing strata and the base of the Double Canyon formation. Finding this boundary is the focus of ongoing research.

References:

1. Amato, J.M., Mack, G.H., Jonell, T.N., Seager, W.R., and Upchurch, G.R., 2017, Onset of the Laramide orogeny and associated magmatism in southern New Mexico based on U-Pb geochronology: Geological Society of America Bulletin, p. B31629.1, doi:10.1130/B31629.1.
2. Dalman, S.G., Loewen, M.A., Pyron, R.A., Jasinski, S.E., Malinzak, D.E., Lucas, S.G., Fiorillo, A.R., Currie, P.J., and Longrich, N.R., 2024, A giant tyrannosaur from the Campanian–Maastrichtian of southern North America and the evolution of tyrannosaurid gigantism: Scientific Reports, v. 13, p. 22124, doi:10.1038/s41598-023-47011-0.