Nonmarine Jurassic-Cretaceous boundary in Sichuan and Yunnan, China

A whole-rock 40Ar-39Ar age of 0.81±0.21Ma was obtained for the North Pulu potassic volcanic rocks in the northwestern margin of Tibetan Plateau. The rocks belong to shoshonite series and are mainly composed of shoshonite and trachyandesite. Geochemically, they have coherent content of SiO2 53.08%~55.12%, total alkali 7.36%~8.04% and K2O/Na2O ratios 1.17~1.2. They are significantly enriched The polarity zone M19 of the J/K mixing polarity superzone in the Chuanjie section of Lufeng, Yunnan, is located in the upper part of the lower member of the Anning Formation and the polarity subzone M18r in the Naxi section of Luzhou, Sichuan is located in the upper part of the second member of the Penglaizhen Formation. The paleogeomagnetic boundary coincides with the biological boundary marked by the distribution of Mamenchisaurus, appearance of new genera and species and conspicuous changes in abundance and diversity of bivalves and conchostracans in the Anning Formation, the upper member of the Tuodian Formation and the second member of the Penglaizhen Formation. Therefore the nonmarine Jurassic-Cretaceous boundary is located between the upper and lower members of the Anning Formation and between two submemebers of the upper member of the Tuodian Formation in central Yunnan; whereas in Sichuan the boundary is placed between the middle and upper parts of the second member of the Penglaizhen Formation.