LATEST CRETACEOUS (MAASTRICHTIAN) ARIDITY INDICATED BY PALEOSOLS IN THE MCRAE FORMATION, SOUTH-CENTRAL NEW-MEXICO

The McRae Formation in south-central New Mexico contains paleosols tha t indicate a significant increase in aridity occurred during the lates t Cretaceous (late Maastrichtian). The McRae Formation is approximatel y 420 m thick and consists of interbedded fluvial channel sandstone an d conglomerate, floodplain mudstone and fine sandstone, and siliceous ash-fall tuff. It has been divided into a lower Jose Creek Member and an upper Hall Lake Member. A Lancian dinosaur fauna is present in both members, indicating a late Maastrichtian age for the McRae Formation. Jose Creek paleosols are classified as argillisols and probably forme d under humid to subhumid conditions. Paleosols in the Jose Creek Memb er consist of A, +/-E, and Bt horizons and are associated with numerou s in situ petrified stumps, the largest of which is 1.7m in diameter. The argillic B horizons (Bt) contain vertical clay-filled root traces, rhizoliths, blocky peds, embedded-grain argillans, and ped argillans. In contrast, paleosols in the Hall Lake Member are calcisols and vert ic calcisols. These paleosols are characterized by Bw, Bt, Btk, Bk, an d K horizons. The calcic horizons contain vertical calcite-filled tubu les and carbonate nodules, in addition to calcans, spar coats, breccia ted zones, alveolar-septal fabric and gypsum pseudomorphs. The vertic calcisols contain wedge-shaped peds and slickensides. The presence of calcic horizons in the Hall Lake Member suggests that the paleoclimate was significantly drier than during deposition of the Jose Creek Memb er. In addition, the vertic features indicate periods of wetting and d esiccation (seasonality of precipitation!) during Hall Lake rime. Late Maastrichtian aridity, based on paleosols, has also been interpreted for the Big Bend region of Texas, central Utah, and for Alberta, Canad a, suggesting that the increase in aridity found in south-central New Mexico occurred throughout the Western Interior of North America. (C) 1995 Academic Press Limited.