THE LATE TRIASSIC CARNIAN WET EPISODE IN THE WESTERN USA AND THE DURATION OF THE NORIAN STAGE

The Late Triassic Carnian wet episode was a late early Carnian (Julian) interval of humid climates, particularly well documented in western Europe. This humid climate interval is also evident in the lowermost strata of the Chinle Group in the western USA. Thus, basal Chinle strata contain fossil palynomorphs, conchostracans and tetrapods that indicate that they are of Carnian age, more specifically Otischalkian age, which means they correlate to the German Schilfsandtein (Stuttgart Formation), which is a well-known late Julian deposit of the Carnian wet episode. Chinle deposition thus began after a substantial hiatus with major river systems of the Shinarump Formation and equivalents, overlain by the only part of the Chinle section with local coal beds and paleosols indicative of a wet climate. Specifically, basal Chinle paleosols are kaolinitic, strongly gleyed, illuviated, heavily bioturbated and locally contain spodic horizons. The Chinle strata that immediately overlie these wet episode strata are deposits of smaller rivers and contain non-kaolinitic paleosols indicative of drier but strongly seasonal climates, such as vertic features and immature calcretes, as is typical of late Carnian (Tuvalian) strata in Europe. Indeed, palynological, conchostracan and vertebrate biostratigraphy correlate these Chinle strata (Blue Mesa Member of Petrified Forest Formation and correlatives) to late Carnian strata in the Germanic basin. However, identification of a “long Norian” stage has been used to place the Norian base in the Chinle Group at the top of the strata of the wet episode. If correct, this means that Norian strata are sitting directly on early Carnian strata, so there should be a substantial unconformity at the base of the long Norian interval in the Chinle Group. No such unconformity exists, so the stratigraphic position and correlation of the Carnian wet episode in the Chinle Group is yet another piece of evidence that runs counter to the concept of the long Norian.