New Mexico Geological Society Annual Spring Meeting — Abstracts

The Upper Cretaceous McRae Formation was deposited in the Laramide (latest Cretaceous-Eocene) Love Ranch basin and is most widely exposed in the Cutter Sag, northeast of Truth or Consequences, south-central New Mexico. Primarily fluvial in origin, the McRae Formation is divided into the lower Jose Creek Member, which contains leaf fossil sites and at least 50 petrified stumps in growth positions, and the upper Hall Lake Member. A Maastrichtian age for the McRae has been based on the presence of two dinosaur fossils, which have poorly constrained stratigraphic positions in the Hall Lake Member because of faulting and Quaternary cover. Presented here are U-Pb-zircon dates from previously undated ash fall tuffs, including three in the middle and upper part of the Jose Creek Member and the “pink” tuff located nine meters above the base of the Hall Lake Member. The fine-grained, felsic tuffs were dated using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). Between 11 and 17 zircons in each sample were dated, and no inherited cores were observed or analyzed. The stratigraphically lowest tuff yielded the oldest ²⁰⁶Pb/²³⁸Pb age (75.2 Ma) and the highest tuff yielded the youngest age (74.0 Ma), with the intermediate-level tuffs yielding ages of 74.7 Ma and 74.9 Ma. However, all four tuffs have the same age within the two-sigma uncertainty level (0.5-1.5 Ma). Taking into consideration the maximum uncertainties, the tuffs erupted between 73.0 and 76.7 Ma, making them late Campanian in age. These new dates for the lower McRae Formation indicate that the onset of Laramide sedimentation in the Love Ranch basin was contemporaneous with that in the Laramide Ringbone and San Juan basins in New Mexico, and they provide a more accurate comparison of the Jose Creek paleoflora with coeval paleoflora elsewhere in the Western Interior of North America.