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
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.