MAGNETOSTRATIGRAPHY OF PERMIAN-TRIASSIC MARINE SEDIMENTS FROM SHANGSI, SICHUAN PROVINCE, CHINA

We report on a new study of Permo-Triassic (P-T) magnetostratigraphy of marine deposits near Shangsi, northern Sichuan Province, in the Longmenshan Indosinian folded zone of the Qinling fold system. We have sampled for magneto-stratigraphy in two parallel sections separated roughly 150 m apart, and at a third section located 4 km to the northwest that lies within the opposing limb of an anticline trending 40°. We sampled at roughly 1m-intervals throughout the section, and more closely near the P-T boundary, collecting a total of nearly 300 samples from the 3 sections. Earlier Permo-Triassic magnetostratigraphic studies at Shangsi indicate that the section contains at least 6 polarity chrons and that the P-T boundary likely corresponds with, or shortly precedes, a reverse-to-normal polarity transition (Steiner et al. 1989, Heller et al., 1988). Still, the exact position of the boundary with respect to this reversal is in question since these earlier studies did not sample a 3m zone of friable, silty sediments immediately above the boundary. We find that the resulting magnetostratigraphy from our three sections are in close agreement, indicating that despite that stable endpoints were not always achieved, the magnetostratigraphy can be reliably reproduced. Furthermore, section-mean directions from the two limbs pass a fold test at the 95% confidence level demonstrating that the magnetizations pre-date folding. These results provide the first concrete support for the reliability of the paleomagnetic directions from Shangsi, and are consistent with the sediment remanence representing a primary magnetization. Our results show that the onset of the Illawarra mixed interval lies below the sampled section (which extends 50m below the previous study of Heller et al., 1988) indicating that the uppermost Permian Changxingian and at least part of the Wuchiapingian stages postdate the end of the Permo-Carboniferous Reverse Superchron. Paleomagnetic results from hand samples taken from the clastic zone lying immediately above the extinction event, indicate that at Shangsi, the onset of a reverse-to-normal polarity transition is constrained to within 50cm above the litho-stratigraphic boundary. In combination with magnetostratigraphy from terrestrial sedimentary records we aim at correlating events on land and in the oceans at P-T times.