Late Permian to Early Triassic Non-Marine Magnetostratigraphy, Junggar Basin, NW China

We determined a magnetostratigraphic profile for non-marine late Permian to early Triassic sediments in the southern Junggar basin of Xinjiang province, northwest China. Correlation of this magnetostratigraphy to marine P-Tr magnetostratigraphies suggests an incorrect placement of the P-Tr boundary in these sections or perhaps a mismatch between the marine and non-marine P-Tr boundary events. We sampled three sections of this continuous fluvio-lacustrine sequence: two sections from the Dalongkou anticline, 100 km east of Urumqi, and the third at the Lucaoguo monocline, 20 km east of Urumqi. We stepwise thermally demagnetized over 290 samples. Although sample magnetizations were heavily overprinted by the present field, we were able to isolate a characteristic magnetic component in 15% of the samples; we applied great circle analysis to objectively determine polarity in 50% of the samples. The remainder of the sample data were discarded due to complete present field overprinting, unstable magnetizations, or unclear polarity. The remanence data suggest that the polarities are primary: a fold test performed on the directions is positive and the sample magnetizations agree with previously published P-Tr directions from Junggar. Rock magnetic data suggest that the predominant magnetic mineral is magnetite, with some hematite. The three sampled sections are easily correlated based on lithology, and the three magnetic stratigraphies, where they overlap, are consistent. The reverse to normal magnetic transition that is commonly seen very near to the marine P-Tr biostratigraphic boundary is not found until at least 80 meters below the biostratigraphic boundary in this non-marine sequence. While a definitive explanation for this marine/non-marine mismatch awaits new palynologic and chemostratigraphic data, our results indicate either the misplacement of the biostratigraphic boundary, significant differences in the stratigraphic resolution of the marine and terrestrial sections, or transitional marine and non-marine boundary events.