19-1 U-Pb Geochronologic Constraints on Laramide Basin Stratrigraphy and the Age of the Postulated Tyrannosaurus mcraeensis in the Love Ranch Basin, New Mexico

Session: The Campanian Crucible: A synthesis of vertebrate paleobiogeography and ecosystem dynamics in Laramidia

Presenting Author:

Jeffrey Amato

Authors:

Amato, Jeffrey M.¹, Schantz, Emma Elizabeth², Lawton, Timothy F.³

(1) New Mexico State University, Las Cruces, NM, USA, (2) New Mexico State University, Las Cruces, NM, USA, (3) Department of Geosciences, University of Arizona, Tucson, AZ, USA,

Abstract:

The Upper Cretaceous McRae Group of the Love Ranch basin in southern New Mexico contains critical evidence for inferred dinosaurian evolution near the end of the Cretaceous Period. The group includes three formations, Jose Creek (JCF), Hall Lake (HLF), and Double Canyon (DCF), with dinosaur fossils, including Alamosaurus, in the Hall Lake Formation. Of particular note is Tyrannosaurus mcraeensis, a postulated new species recently interpreted to predate T. rex by 6–7 m.y. and therefore be of Campanian age based on a previously published U-Pb tuff age inferred to correlate with strata 30 m below the fossil locality. We show that the published stratigraphic model miscorrelates the tuff-bearing and dinosaur-bearing sections, which are separated by 3.5 km, on the basis of new U-Pb zircon ages on tuffs and detrital zircons. Although the sections are incomplete in each area, they indicate an apparently continuous depositional history from 74–56 Ma. Sandstones have abundant volcanic grains and yielded many detrital zircons used to calculate maximum depositional age (MDA) weighted means (MDA reported at ±2s uncertainty).

Three different tuffs in the JCF dated at ca. 75 ± 1 Ma indicate rapid Campanian deposition over ~90 m of the ~>200 m total section. Mudstone and sandstone of the HLF, >700 m thick, contain a tuff near the base with an age of 73.6 ± 0.5 Ma. A new measured section in the study area, apparently with a faulted base, is 700 m thick with a 66.5 ± 1.1 Ma tuff 30 m above the exposed base. Ten sandstones spanning ~ 600 m upsection of the tuff have MDAs ranging 69.0–63.4 Ma. This section thus spans the K-Pg boundary, which remains unlocated. The overlying DCF contains sandstone MDAs and two tuffs spanning 61.5 ± 1.6 Ma to 56.0 ± 0.2 Ma. Our new data indicate that correlation of local sections is currently inadequate to constrain the age of the Tyrannosaur, but new zircon dates in the fossil-bearing strata allow us to infer that T. mcraeensis is younger than 69.0 ± 0.4 Ma and is thus mid to late Maastrichtian. Therefore, this fossil cannot be said to predate T. rex by as much as 7 m.y., a key line of evidence used to infer the new Tyrannosaur identity.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

doi: 10.1130/abs/2026RM-14441

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.