Calcareous paleosols of the Upper Triassic Chinle Group, Four Corners region, southwestern United States: Climatic implications

Paleosols are prominent features of the Upper Triassic Chinle Group. The oldest (Carnian-age) formations of the Chinle Group (Zuni Mountains and Shinarump Formations) contain kaolinitic paleosols that display gley features but generally lack calcretes. Paleosols of the (Upper Carnian) Blue Mesa Member of the Petrifi ed Forest Formation are mostly mature Alfi sols that have distinctive horizonation and commonly host stage II to III calcretes. Mudstones of the Jim Camp Wash Bed of the overlying Sonsela Member host similarly mature paleosols with abundant stage II to stage IV calcretes. The (Lower Norian) Painted Desert Member of the Petrifi ed Forest Formation is characterized by paleosols that lack well-developed A horizons but display thick, red B horizons in which pedogenic slickensides, rhizocretions, and stage II to III calcretes are locally abundant. Immature paleosols hosting stage II to stage III calcretes characterize the lower part of the (Middle Norian) Owl Rock Formation. The upper Owl Rock Formation contains stage III to IV calcretes and laterally persistent limestone ledges that formed as palustrine limestones and groundwater calcretes. The (Norian-Rhaetian) Rock Point Formation generally lacks pedogenic features in most of the study area, but the uppermost strata in some locations host multiple pedogenic horizons that display drab root traces, desiccation cracks, and stage II to III calcretes. Interformational variations in the types of paleosols and the maturity of calcretes in Chinle Group strata refl ect gradual aridifi cation across the Colorado Plateau during the Late Triassic. This climatic change overprinted variations in basin sedimentation rate that were potentially controlled by base level and tectonics.