Redefinition Of The Shuangmaidi Group And Depositional Environment Analysis In Western Yunnan

Baoshan Shuangmaidi area locates in the west of Lancangjiang-Nandinghe fault, in the east of Nujiang fault and belongs to the Baoshan Block which is northern part microplate of Gondwana Scale.1:200000 regional geological survey of the Baoshan Sheet defined this area as Shuangmaidi Group, but at that time geologists have not collected fossils with chronological significance, just according to the location that conformitely underlies Hetaoping Formation of the Late Cambrian system. However, there are no more evidences to prove the geologic age of Shuangmaidi Group locating the Early-Middle Cambrian system, which geologic age it belongs to is the long-trem controversial problem.This article is one sub-topic of the general survey project of land and resources-1: 250000 regional geological survey of Wayao,Yongping County,Longjie,Banqiao County,Shanyang,Changjie,Baoshan,Jinji village. During the process of the 1:50000 regional geological survey of the Banqiao County Sheet, through researching detailedly original Shuangmaidi Group and measuring baiyanao section and shuangmaidi section, the lithologic assemblage of Shuangmaidi Group is constituted by the follows:the lower part is light gray slaty marbles, light gray-green sericite slate; the middle is light gray-green biotite-sericite slate, pelitic-silty sericite slate with some altered basalts and altered basaltic andesite; the upper is light gray slaty marbles, gray-green sericite slate. The stratotype section of the Hetaoping Formation locates between the west of baiyanao section and shuangmaidi section, which its the lithologic assemblage of Hetaoping Formation follows: The lower is gray-green silty slate, gray marl; the upper is gray limestone, micritic limestone and a little silty mudstone, argillite. Shuangmaidi Group shows similarity with Hetaoping Formation in lithologic assemblage.The author collected 6species,5genera trilobites fossils from Baiyanao section are Blackwelderia baoshanensis Luo, Blackwelderia sp., Wayaonia hetaopingensis Lo, Parashengia elongata Luo, Chatiania hetaopingensis Luo, Pseudagnostus sp., Asaphid. Blackwelderia baoshanensis Luo, Blackwelderia sp., Wayaonia hetaopingensis Lo,Parashengia elongata Luo,Chatiania hetaopingensis Luo,Pseudagnostus sp. which is an important molecule of Bergeronites-Cyclolorenzella zonation representing Upper Cambrian period Kushanian Stage in west Yunnan established by Luo Huilin.In Comparison with the lithologic assemblage, biozone, Shuangmaidi Group in the Baoshan Shuangmaidi area is same as Hetaoping Formation. Above this, it can be correlated with the Hetaoping Formation, standing on the Late Cambrian system. Hetaoping Formation in the shuangmaidi area composed the microclastic rocks and marbles with some basalts and andesitic basalts.By researching into the geochemical characteristics of constant, trace and REE, knowing is that Shuangmaidi Group and Hetaoping Formation are similar in the sedimentary environment, and that is open-platform-neritic deposit. Just the carbonate of Hetaoping Formation in He Taoping area is thicker than that in Shuangmaidi area. Accordingly, we can propose He Taoping area is slightly deeper in dept of water.Some basalts and andesitic basalts are first discovered in baiyanao section, the thickness of basalt is more than a dozen centimeters to several meters, outcropped area is not wide, extending only a few kilometers to tens of kilometers. By analysing the geochemical characteristics including constant, trace, REE of basalts, which is tholeiite which contains medium K. According to F1-F2 diagram, F2-F3 diagram:the tectonics background belongs to active continental margin island arc setting.Through the careful analysis of depositional environment, sedimentary facies, biota fossils, geochemical characteristics of sedimentary rock and basalts, it is indicated that Shuangmaidi area in Baoshan should be the active continental margin, locating in the west of Proto-Tethyan Ocean, and which of the tectonic setting is unstable and the same as the studied by the predecessor.