PALEOSOLS AS AN INDICATOR OF CLIMATIC-CHANGE AT THE EARLY-LATE CRETACEOUS BOUNDARY, SOUTHWESTERN NEW-MEXICO

Cretaceous (Aptian to Lower Campanian) alluvial paleosols in southwestern New Mexico suggest that a paleoclimatic change from arid or semi-arid to humid or subhumid occurred near the boundary between Early and Late Cretaceous. Lower Cretaceous paleosols of the Hell-to-Finish (Lower Aptian) and Mojado (Upper Albian) formations are primarily Aridisols, characterized by a dark reddish brown, pedoturbated Bw horizon underlain by a Bk horizon composed of calcareous nodules and tubules or a massive calcareous Bkm horizon. Retention of carbonate in the paleosol profiles suggests that annual precipitation was less than 60 cm. In contrast, Upper Cretaceous paleosols consist of non-calcic Alfisols and Inceptisols. Inceptisols consist of two types: 1) a gleyed variety displaying A, Bg, and BC or Cg horizons in the Sarten Formation (uppermost Albian to lowermost Cenomanian) and in the Crevasse Canyon Formation (Upper Coniacian to Lower Campanian); and 2) a non-gleyed variety consisting of A, Bw, and C horizons in the Moreno Hill and Tres Hermanos formations (Middle Turonian) and in the Crevasse Canyon Formation. In addition, Alfisols in the Tres Hermanos and Crevasse Canyon formations are composed of A, E, Bt, BC, and C horizons. The argillic B horizons (Bt) contain kaolinite and mixed-layer clay and display blocky peds, vertical, clay-filled root traces, and argillans, indicative of relatively well-drained woodland soils. Modern counterparts of the Upper Cretaceous Alfisols exist in humid and subhumid regions receiving greater than 75 cm/yr precipitation. Comparison of the data presented in this study with paleosol and paleobotanical data from other locations in the Western Interior of the U.S. suggests that the paleoclimatic change proposed for southwestern New Mexico may have been of regional extent.