T71 The Changing Cordilleran Margin: Permian through Eocene Tectonism, Magmatism, and General Geology from Western Mexico through Northern California and Eastward

Sunday, 22 September 2024
1:30 PM - 5:30 PM
304A

DATING DINOSAURS IN THE LARAMIDE FORELAND: U-Pb GEOCHRONOLOGIC CONSTRAINTS ON A STRATIGRAPHIC SECTION CONTAINING ALAMOSAURUS AND THE POSTULATED TYRANNOSAURUS MCRAEENSIS IN THE LOVE RANCH BASIN, NEW MEXICO

Paper number: T71-11
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

Abstract

An important dinosaur-bearing interval of fluvial strata occurs in the Upper Cretaceous McRae Group of the Laramide Love Ranch Basin in south-central New Mexico. The group in ascending order consists of the Jose Creek Fm., Hall Lake Fm., and Double Canyon Fm. Dinosaur fossils, including Alamosaurus and a potentially new species of Tyrannosaurus (T. mcraeensis; older than T. rex: Dalman et al., 2024), have been found in the HLF. We used LAICPMS to obtain U-Pb ages from detrital and igneous zircons from sandstone and tuffs of the McRae Group below, at, and above the tyrannosaurid locality. Herein we report new 238U/206Pb zircon dates (±2s) to establish the age of the strata containing the fossils, evaluate duration of Laramide deformation, and determine whether the K-Pg boundary is preserved in this section. Sandstones have abundant volcanic lithic grains and yielded many young detrital zircons interpreted to be approximately syndepositional. We used weighted means of the youngest grain clusters for max depositional ages (MDA).

The Hall Lake Fm. (HLF), ~700 m thick, consists of mudstone and sandstone; a tuff ~15 m above the unit’s base yielded an age of 73.6 ± 0.5 Ma. Strata 30 m above the base, in a different measured section that may be separated from the other section by a fault or unconformity, contained the postulated new Tyrannosaur, and a volcaniclastic sandstone at this fossil locality yielded an MDA of 69.0 ± 0.4 Ma (n=29, MSWD=1.0). A sandstone 2 m above this locality has an MDA of 69.5 ± 0.7 Ma. An Alamosaurus fossil occurs ~90 m above the base of the HLF, where zircons from a sandstone give an MDA of 69.5 ± 0.5 Ma. A sandstone ~550 m above the base of the HLF and ~150 m below the Double Canyon Fm. has an MDA of 63.2 ± 1.0 Ma, showing that the K-Pg boundary lies in the HLF, though strata at the boundary may not be exposed.

Sandstone at the base of the Double Canyon Fm. (DCF) yielded only two young zircons which have a mean of 61.5 Ma. Two tuffs ~80 m above the base of the DCF each yielded a mean age of 60.6 ± 1.2 Ma. Two sandstones higher in the DCF yielded MDAs of 57.6 Ma and 56.0 Ma, indicating that this formation spans the Paleocene and that the Love Ranch basin accommodated deposition well into Paleogene time.

Currently we infer that the key stratigraphic layer of T. mcraeensis is younger than 69.0 ± 0.4 Ma and older than the accepted K-Pg boundary age of 66.0 Ma.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-405342

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