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.

Carbon, oxygen and strontium isotopic composition of low-latitude Permian carbonates (Venezuelan Andes): climate proxies of tropical Pangea

“…If this is correct, then the long-term aridification of Pangea could have been caused by plate movement rather than by a change in atmospheric conditions. However, palaeomagnetic studies (May & Butler 1986;Bazard & Butler 1991;Steiner & Lucas 2000;Steiner 2003) have suggested that western Pangea remained at the same latitude until the Late Triassic. The rapid change from humid to arid conditions and the persistent humid climate in eastern tropical Pangea are arguments against this hypothesis of plate movements.…”

Paleomagnetism of the Todos Santos and La Silla Formations, Chiapas: Implications for the opening of the Gulf of Mexico

“…To further complicate this analysis, the Jurassic is a time when there is rapid motion of the North American (NA) plate and a fast rate of appar-ent polar wander. The fi rst issue arises from the contrasting paleomagnetic results from northeast and southwest North America (e.g., Van Fossen and Kent, 1990;Bazard and Butler , 1991;Steiner, 2003). Middle Jurassic poles from the northeast part of the continent lie at high latitudes, near the present-day North Pole, which in turn predicts higher paleolatitudes for the NA plate.…”

“…The effect of the reference poles from both regions is smaller in the relative orientation of the plate with respect to the paleomeridian (and thus expected declinations and rotation estimates). The second point relates to contrasting estimates of clockwise rotation of the Colorado Plateau with respect to the craton (Molina-Garza et al, 1998;Kent and Witte, 1993;Steiner, 2003;Kent and Olsen, 2008). These estimates range from ~4° to over 15°.…”

“…Taking these facts into consideration, we calculated estimates of R and F based on the Pliensbachian to Callovian age of the magnetization in the Todos Santos and La Silla formations. We make direct comparison with poles for the Pliensbachian Kayenta Formation (Bazard and Butler, 1991), late Callovian Summerville Formation (Steiner, 2003), the ca. 169 Ma Moat Volcanics from the White Mountains (Van Fossen and Kent, 1990), and the synthetic pole for North America for 170 Ma of Besse and Courtillot (2002).…”

Burrows Dug by Large Vertebrates into Rain‐Moistened Middle Jurassic Sand Dunes

“…1) is one of the youngest. It accumulated as the supercontinent was breaking up, when the center of the plateau had reached about 20ЊN (Steiner 2003). Although some of the eolian sandstones on the plateau preserve few, if any, signs of life (e.g., the Permian White Rim Sandstone), others, including the Permian Coconino and Cedar Mesa Sandstones (McKee 1934;Loope 1984) and the Jurassic Navajo Sandstone Loope and Rowe 2003;Loope 2006), preserve abundant rhizoliths, invertebrate burrows, and vertebrate tracks.…”