A cratonic Middle Jurassic paleopole: Callovian‐Oxfordian stillstand (J‐2 cusp), rotation of the Colorado Plateau, and Jurassic North American apparent polar wander

Abstract

Paleomagnetic results have been obtained from two sequences of the upper Middle Jurassic (upper Callovian) Summerville Formation which are located on the North American craton (north central New Mexico). The thicker of the two sections (Trujillo) exhibits a well‐defined dual‐polarity remanence, forming a magnetostratigraphy of at least 12 relatively short polarity intervals. The high reversal frequency is consistent with that recorded by the oceanic crust at this time. The second section (Romeroville) is thin, gypsiferous, and poorly indurated; consequently, only a short interval (5 m) was successfully sampled. The characteristic magnetization was wholly normal polarity. In the rapid reversals of the thicker section, that thickness of normal polarity is unique and thus forms the basis of magnetostratigraphic correlation between the two sections. A cratonic late Middle Jurassic paleomagnetic pole at 148.2°E, 57.3°N (k = 30.3, α95 = 6.0°) was obtained from the larger Trujillo sequence. Despite large confidence limits, the small sample population of the Romeroville sequence displayed the same pole. Throughout the western United States, the Summerville Formation is overlain with little or no discordance by the Morrison Formation, although the two stratigraphic units are commonly reported to be separated by an unconformity, designated J‐5. The New Mexico paleopoles of the Summerville and overlying lower Morrison Formations are statistically identical at the 95% confidence level; a similar situation exists between Summerville and lower Morrison paleopoles of the Colorado Plateau. The presence of the same geomagnetic field direction in both the Summerville and the lower Morrison Formations suggests that the J‐5 unconformity which separates them may encompass little time, perhaps representing only a minor break in deposition. The two newly investigated Summerville sites are located east of the Rio Grande Rift, hence representative of the North American craton; their paleopoles are displaced counterclockwise relative to those of the Colorado Plateau, and in amounts consistent with other coeval craton‐Colorado Plateau pole pairs. Together, six coeval paleopoles indicate that the Colorado Plateau has rotated clockwise by about 9.0 ± 3° since Late Pennsylvanian time. The cratonic Summerville paleopole also removes the uncertainty over the Jurassic North American apparent polar wander path. The statistically identical Summerville and lower Morrison paleopoles form a substantial data set, five paleopoles, which indicate that NA APW traced a path roughly along 60° latitude. The five paleopoles stand in stark contrast to the paleopole from the slightly older (5–8 Ma) Moat Volcanics. Spreading rates in the central Atlantic do not allow NA plate motion that would include both paleopoles positions. Because the weight of the data, five paleopoles from widely spaced localities and different tectonic blocks, the Moat Volcanics pole cannot be representative of North America in the Middle Jurassic.