New Mexico Geological Society Annual Spring Meeting — Abstracts

A significant increase in aridity in south-central New Mexico in latest Cretaceous (Maastrichtian) time is indicated by paleosols and paleo flora of the McRae Formation. Subdivided into lower Jose Creek Member and upper Hall Lake Member, the McRae Formation is 350+ m thick and consists of interbedded fluvial channel sandstone and conglomerate, floodplain mudstone and fine sandstone, and siliceous ash-fall tuff. A late Maastrichtian age for the McRae is indicated by a Lancian dinosaur fauna, which is present in both members.

Alluvial paleosols in the Jose Creek Member consist of A, ±E, and Bt horizons and are associated with in situ petrified stumps up to 1.7 m in diameter. The argillic B horizons (Bt) contain vertical clay-filled root traces, blocky peds, embedded-grain argillans, and ped argillans. Jose Creek paleosols, which resemble modern Alfisols, probably formed under humid to subhumid conditions. Jose Creek leaf megaflora consists of conifers, cycads, palms, and other broadleaf angiosperms. This leaf flora is similar in physiognomy and taxonomy to other Maastrichtian assemblages in North America that indicate subhumid conditions. Furthermore, the absence of distinct continuous growth rings in conifer wood suggests low seasonality in Jose Creek time.

In contrast, the Hall Lake Member contains calcic paleosols, which display Bw-Bk, Btk-Bk, and Btk-K horizons, and few megascopic plant fossils. Calcic paleosols in the Hall Lake Member suggest that the paleoclimate was significantly drier than during deposition of the Jose Creek Member. Maastrichtian dry periods, based on paleosols, have also been
suggested for the Big Bend region of Texas and for Alberta, Canada.