Paleomagnetism of the Chinle Formation at Ghost Ranch, New Mexico: age constraints for Coelophysis bauri

Argues that the Ghost Ranch Coelophysis quarry is stratigraphically lower and older than many previous workers had concluded.

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The "siltstone member" of the Late Triassic Chinle Formation at Ghost Ranch, New Mexico has not been adequately correlated to formal members of the Chinle Formation in Arizona and New Mexico. This lack of correlation has prevented the establishment of an accurate age for Coelophysis bauri, one of the oldest dinosaurs in North America. I report paleomagnetic data that suggests correlation of the "siltstone member" of the Chinle Formation at Ghost Ranch, and, thus, provides age constraints for Coelophysis.

Characteristic remanent magnetizations (ChRMs) isolated during thermal demagnetization yield a high unblocking temperature, single-component, primary magnetization. I argue that the ChRM is carried dominantly by specular hematite that formed during rapid oxidation of the "siltstone member" sediments soon after deposition. Six sites result in a mean paleomagnetic pole at 54.9°N, 78.1°E (K = 120.5, a95 = 6.1°). Correction for 5° of clockwise Colorado Plateau rotation results in a pole for the "siltstone member" above the "orange" bed at 54.2°N, 86.6°E. Comparison to other Late Triassic data suggests that this portion of the "siltstone member" correlates to the upper part of the Petrified Forest Member (early Norian) of the Chinle Formation. The magnetostratigraphy of the upper 40 m of the "siltstone member", when compared to other Chinle Formation polarity sequences, corroborates this interpretation and further indicates that the uppermost portion of the "siltstone member" is equivalent to the Redondo Member (middle to late Norian). The location of the Coelophysis quarry, over 10 m below the base of our sampled section, suggests that the age of Coelophysis is no younger than early Norian and is more likely near the Carnian/Norian boundary (~223 Ma).