New Mexico Geological Society Annual Spring Meeting — Abstracts

The Upper Cretaceous McRae Group, situated in the Laramide foreland basin known as the Love Ranch Basin in south-central New Mexico between Engle and Truth or Consequences, consists of the José Creek (oldest), Hall Lake (middle), and Double Canyon (youngest) formations. However, there are limited geochronological data available for these units. The Hall Lake formation has several dinosaur fossils, including Alamosaurus and Tyrannosaur bones previously identified at T. rex but later postulated by Dalman et al. (2024) to be a new Tyrannosaur species (T. mcraeensis), predating the late Maastrichtian T. rex. We used laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to obtain U-Pb ages from detrital and igneous zircons from sandstone and tuffs within the McRae Group, below, at, and above the fossil locality. All ages are ²³⁸U/²⁰⁶Pb zircon ages with uncertainties reported at 2s. We seek to establish the likely age for the strata containing the fossils, evaluate when Laramide deformation ended, and determine whether the K-Pg boundary is exposed in this section.

The Hall Lake Formation is approximately 700 m thick (Lucas et al., 2019) and consists of primarily mudstone, sandstone, and a 73.1 ± 0.7 Ma tuff layer (U-Pb zircon; Amato et al., 2017) that is 10 meters above the base of the Hall Lake Formation. We redated this tuff and it has a weighted mean U-Pb zircon age of 74.1 ± 0.9 Ma (n=17, MSWD=1.4; Campanian). An Alamosaurus fossil is ~150 m above the base of the Hall Lake strata (Lucas et al., 2019). Detrital zircons from a sandstone at this fossil locality stratigraphic level yielded a maximum depositional age (MDA) of 69.8 ± 0.7 Ma (Maastrichtian).

The Double Canyon Formation consists of sandstone, conglomerate, shale, and two newly identified silicified tuff layers previously described as chert. These tuffs are approximately 80 m above the Hall Lake contact. The lower tuff is approximately 5 meters thick and contains phenocrysts in an ashy matrix. The upper tuff is 1-2 m above the lower tuff and lacks phenocrysts. Igneous zircons from two samples of the lower tuff in the Double Canyon Formation yielded a combined age of 60.6 ± 1.3 Ma (n=7, MSWD = 0.5). The two samples of the upper tuff yielded a combined age of 60.6 ± 1.2 (n=11, MSWD=1.1); both ages are Paleocene. Sandstone near the base of the McRae Forest section of the Double Canyon Formation has an MDA of 57.6 ± 0.5 Ma, and sandstone near the top of this section has an MDA of 56.0 ± 0.6 Ma (at the Paleocene/Eocene boundary).

The Double Canyon Formation now known to be partly Paleocene and likely extends into the Eocene. The Alamosaurus strata are no older than 70 Ma. This means the K-Pg boundary, if exposed in this area, is located either in the 80 m of section below the 61 Ma tuffs in the Double Canyon Formation or, more likely, in the 550 m of section above the Alamosaurus strata in the Hall Lake Formation. The stratigraphic location of this boundary is the focus of ongoing investigation, as is the MDA of the Tyrannosaurus-bearing strata which will bear on the hypothesis that T. mcraeensis is a predecessor of T. rex.

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.
3. Lucas, S.G., Nelson, W.J., Krainer, K., and Elrick, S.D., 2019, The Cretaceous System in central Sierra County, New Mexico: New Mexico Geology, v. 41, p. 3–39, doi:10.58799/NMG-v41n1.3.