Pedogenic features of the Chinle Group, Four Corners region: Evidence of Late Triassic aridification
— Lawrence H. Tanner

Paleosols and pedogenic features in formations of the Upper Triassic Chinle Group preserve a record of changing paleoclimate in the Four Corners region. The Carnian mottled strata and Shinarump formations contain strongly gleyed and bioturbated paleosols with a prismatic fabric that formed under the infl uence of a subhumid but (likely) strongly seasonal climate. Thick argillic horizons with epipedons in the overlying Cameron Formation and the Blue Mesa Member of the Petrifi ed Forest Formation, and the prominence of vertic features and calcrete horizons in the Blue Mesa and Painted Desert members suggest that the climate became less humid, but remained strongly seasonal, from late Carnian through early Norian time. Mature calcrete profi les in the middle Norian Owl Rock Formation document continued aridifi cation leading to semiarid conditions. Strata of the Rhaetian Rock Point Formation contain few paleosols other than immature calcretes, but the abundance of eolian and playa facies in this formation suggest increasing aridity through the end of the Triassic. Interformational variations in the carbon-isotope composition of pedogenic carbonate are consistent with this trend of aridifi cation at a time of elevated atmospheric CO₂. Although not world-wide, aridifi cation was widespread across Pangea, and possibly resulted from the increasing strength of monsoonal flow and the weakening of zonal circulation, particularly at lower latitudes, as the continent drifted northward.