Foraminifer Biostratigraphy And Sedimentary Environment Of The Middle Permian Longge Formation, Rongma, Nyima, Tibet

The paper based on the four Gelogical Maps 1:50000 Regional Geological and Mineral Survey Project in Rongma Area, Tibet. By the means of detailed field geological investigation, indoor rock thin section analysis and identification of fossils, we systematically studied the Permian Longge Formation in Rongma Township, Nyima County, Tibet. It mainly includes foraminiferal biostratigraphy and distribution of sedimentary facies belts. We also discussed the change of types and individual characteristics of small foraminifera in different sedimentary environments.By statistical identification, we found 15 genera and 45 species of foraminifera in the fossil samples of Longge Formation in the study area, including Pachyphloia, Geinititzina, Langella, Nodosaria, Endothyra, Glomospira, Globivalvulina, Tuberitina, Neotuberitina, Pseudoglandulina, Tetrataxis, Ammodisccus, Eotuberitina, Frondina, Hemigordiopsis。According to the distribution and quantitative change of small foraminifera, we divided two small foraminiferal combination combining the predecessors’ work from bottom to top, including Pachyphloia-Nodosaria-Geinitzina which living in late Qixian age in middle Permian and Hemigordiopsis which living in early Maokouan age in middle Permian. Therefore the era of Longge Formation was from late Qixian age to early Maokouan age in middle Permian.Eleven major rock types were identified in the carbonate rocks: lime mudstone, bioclastic wackestone, bioclastic packstone, biological grainstone, intraclast wackestone, intraclast grainstone, oolitic wackestone, oolitic grainstone, bioherm limestone, finely crystalline dolomite, and remnant grain dolomite. According to the characteristics and compositions of the rocks, the rock types can be divided into five sedimentary facies: restricted platform facies, open platform facies, platform margin shoal facies, platform margin organic reef facies, and platform margin slope facies, which together constitute two sequential transgressive–regressive cycles of the Longge Formation and overall show a facies sequence structure of regression.The study found that most of the foraminiferal species are very sensitive to the changes of sedimentary environment and there is a great difference of foraminiferal taxa in different sedimentary environments. The following conclusions could be drawn.(1)The main factors that affect the ability of the small foraminiferal to adapt to environment including shell form, shell size, shell thickness and internal structure of chamber.(2) Hemigordiopsis, Langella and Nodosaria have the strongest ability to adapt to the environment. Hemigordiopsis was only found in the platform margin shoal facies. Both of Langella and Nodosaria were developed in open platform facies, platform margin shoal facies and platform margin organic reef facies. Pachyphloia, Geinititzina, Endothyra, Glomospira, Globivalvulina, etc. were only found in open platform facies.(3) Open platform was the most suitable environment for foraminifera. Only a few foraminiferal genera were found in platform margin shoal facies and platform margin organic reef facies and few was found in restricted platform facies.