PALEOMAGNETIC INVESTIGATION OF UPPER PERMIAN SEDIMENTS IN THE SOUTH JUNGGAR BASIN, CHINA

Lacustrine organic-rich mudstones of the Upper Permian Lucaogou Formation are a significant petroleum source bed throughout most of the Junggar basin in northwest China, and paleomagnetic data may provide insight into tectonic conditions prevalent during their deposition. Secondary magnetizations were removed from 180 samples from 29 sites within the South Junggar basin. Characteristic magnetizations of these samples were either revealed directly during thermal demagnetization or were calculated from great circle trajectories. The primary nature of the characteristic magnetization is attested to by a positive fold test, by exclusively reversed magnetizations which are consistent with the mid-Carboniferous to Late Permian Kiaman reversed interval for the geomagnetic field, and by relatively low levels of thermal maturity found in the mudstones. Tectonically corrected remagnetization circles and direct observations combined give a steep direction (I = -61.9-degrees, D = 163.0-degrees, k = 17, n = 78 specimens). The steep mean inclination recorded within the lacustrine organic-rich mudstones suggests that Junggar was situated near its present-day latitude during deposition of the mudstones in Late Permian times, consistent with the marginal ocean basin origin for Junggar that has been proposed based upon stratigraphic relationships. Our data suggest that Junggar may always have been associated with the northern continents of Laurasia, initially as a pre-Permian marginal ocean basin north of the Tethys and subsequently as a Permian, Mesozoic, and Cenozoic nonmarine foreland basin.