New Mexico Geological Society Annual Spring Meeting — Abstracts

The Chama basin, north-central New Mexico, contains excellent exposures of Upper Triassic Chinle Group strata that can be used to refine Late Triassic vertebrate biostratigraphy and to develop an improved magnetic reversal chronology for the Late Triassic. Recent fossil discoveries have expanded the known fauna for both the lower Chinle Salitral Formation and the upper Chinle Mesa Montosa Member (Petrified Forest Formation). The addition of the metoposaurid Buettneria and the aetosaur Desmatosuchus to the Salitral fauna confirmes an Adamanian age for these strata. The discovery of material pertaining to Buettneria, the aetosaurs Typothorax coccinarum and Paratypothorax, the archosaur Vancleavea and other fauna confirms a Revueltian age for the Mesa Montosa Member. Paleomagnetic sampling has been concentrated on mudrocks, using a block sampling approach. Black samples typically carry a well-defined, well grouped magnetization dominated by pigment hematite that is unblocked below about 666?C (e.g., for a single horizon, with six independent samples, Decl. =185.5?, Incl. = 0.3?, α₉₅= 6.6?, and k= 102.6). Sandstones and siltstones contain pigment hematite and, based on preliminary experiments, contain both detrital hematite and some magnetite. An initial reversal chronology has been developed for Poleo and Petrfied Forest strata (middle and upper Chinle Group) in the Chama basin. The Petrified Forest Formation is characterized by fairly regular reversals, the Poleo Formation is dominated by reverse polarity.