Paleomagnetism of the Late Triassic Petrified Forest Formation, Chinle Group, western United States: Further evidence of “large” rotation of the Colorado Plateau

Paleomagnetic poles and magnetostratigraphy were obtained from the late Carnian and early Norian Petrified Forest Formation. The remanent magnetization characteristics of this variegated claystones formation are correlated closely with stratal color; red strata display univectorial magnetizations directed to the origin of orthogonal axes plots between 300° and 630°C, whereas progressively greater overlap in magnetization component stabilities is exhibited by purple, blue, and green strata. Rock‐magnetic, petrographic, and microprobe investigations indicate that detrital titanohematite is the carrier of much of the characteristic magnetization, with a smaller contribution from magnetite in green and portions of the red strata. Eight magnetic polarity intervals were observed in three quarters of the formation (155 m) good correlation is observed with the coeval portion of the Newark Supergroup. Paleopoles from the late Carnian Blue Mesa and the early Norian Painted Desert Members of the Petrified Forest Formation are identical and agree well with other Chinle Group paleopoles from the Colorado Plateau; the tight cluster formed by all poles indicates that little or no apparent polar wander (APW) with respect to North America occurred during the Late Triassic (late Carnian through Rhaetian time). In addition, Chinle Group paleopoles derived from strata located on the North American (NA) craton also are tightly grouped and indicative of a paucity of significant APW through the Late Triassic. With that same observation from the coeval Newark Supergroup, three independent Late Triassic data sets from different tectonic settings in NA indicate minimal or no APW during more than 20 m.y. The combination of the vertebrate fauna and magnetostratigraphy within the Chinle Group provides a high degree of age control, allowing comparisons among coeval Colorado Plateau (CP) and NA craton paleopoles at about a 250–500 kyr level. CP Chinle Group paleopoles are displaced 9±3° clockwise from cratonic paleopoles. The Chinle Group is one of five sets of paleomagnetically sampled Mesozoic strata shown by magnetostratigraphy and/or biostratigraphy to have been deposited coevally on both the CP and the NA craton; paleopoles from the five sets indicate that CP paleopoles have been displaced a total of 10±3° relative to NA cratonic counterparts. Furthermore, the wealth of paleomagnetic and biostratigraphic data now available from both the Chinle and Newark strata demonstrates that nothing is sacred in NA tectonics: Newark Basin paleopoles are displaced 9±3° counterclockwise from western cratonic poles.