New advances in the study of Meishucun section of Jinning, Yunnan
Luo Hui-lin, Jiang Zhi-wen, Wu Xi-che, Song Xue-liang and Ouyang Lin
New advances in the study of petrostrata, biostrata, isotope age, stable isotopic dating as well as palaeomagnetism are briefly summarized in the present paper. In order to justify the precise position of boundary point of Precambrian-Cambrian (Point "B"), an additional section was measured at the southern sector of Xiaowaitoushan. In so doing, the contents of the small shelly fossils near the Point "B" were enhanced. Our studies were emphasized oa the Tannuolina, Lapworthella, Chancellorida and Sachites occurring at the bottom of Unit 13 of the section. For the sake of showing the primitive composition and texture of shell as well as its evolutional patterns, by means of polarization microscope and SEM, the components and microtextures and microstructures of small shelly fossils collected from the strata between late Sinian and Meishucunianian age have been carefully studied. The isotope age for the black shale in the Lower Badaowan Member is determined at 570.7±4.7Ma, and the phosphorite of the Upper Zongyicun Member at 596.9±4.6Ma. According to the recently measured data on palaeomagnetism, the palaeolatitude of Meishucun district during late Sinian to early Cambrian should not be over 18° beyond equator northward and southward. The new data have also demonstrated that there must be at least six reversed magnetized layers appearing in the column of Meishucun section.