Stratigraphic Division And Depositional System Analysis Of Jurassic Stratum In Cuoqin Area, Tibet

The Cuoqin Basin is a Mesozoic-Cenozoic basin developed on the basis of Paleozoic drape. The evolution of its Jurassic strata is mainly controlled by the tectonic evolution of the Bangong-Nujiang suture zone. Jurassic strata mainly distribute in the central and northern area of this basin. This study area is located in the southern area of Cuoqin Basin. The structural unit belongs to Geji-cuoqin-shengzha composite Back-arc basin of Gangdisi-Nianqingtanggula slab. Stratigraphic Subzone belongs to cuoqin-shengzha subzone and Bange-Basu subzone of Yunan-Tibet strata area. This paper based on stratigraphic profile, This thesis makes multi stratigraphic division on the Jurassic strata of the study area by the methods of lithostratigraphy, biostratigraphy and chronostratigraphy on the basis of sections. This thesis also analyses the depositional system of the Jurassic strata of study area.This thesis describes five sections in study area. They are Quluo section in Cuoqin County(J1q), Renduo section in Zhongba County(J2-3r), Chazaerbu section in Cuoqin County(J2-3r1-2), Zigang section in Shenzha County(J2-3j1) and Kama section in Gaize County(J2-3j2) respectively. One the basis of previous study, the Jurassic strata is divided into Quluo Formation, Renduo Formation and Jienu Group by features of rock assemblage. Both of Quluo Formation and Renduo Formation are divided into three lithological sections. Jienu Group is divided into two lithological sections. This thesis discusses the lithologic combination, contact relationship and lateral alternation of each section.According to the fossil assemblages, this thesis takes biostratigraphic division, builds five fossil assemblages and one fossil assemblage. They are Astarte cf. delicate - Astarte cf. subminima assemblages, Neocalamites sp. - Deltoispora sp. assemblages, Orbitopsella cf. dubori - Orbitopsella praecusor assemblages, Epstreptophyllum cylindratum - Parepismilia cf. andoensis assemblages, Spongiomorpha robasta - Cladocoropsis cf. lata assemblages and Parastromatopora sp. assemblage. And on the basis of that, the thesis discusses the chronostratigraphy of each lithostratigraphic units. The Quluo Formation belongs to the Lower Jurassic, and Renduo Formation belongs to Middle-Upper Jurassic.On the basis of comprehensive analysis of those sections, rock specimens, fossil specimens and the sedimentary information they reflected, the thesis builds the type of sedimentary facies by principles of sedimentology and sequence stratigraphy. Then, the Jurassic stratais identified as marine sedimentary system group, transitional sedimentary system group and three depositional systems. Among that, the sea depositional system includes carbonate platform and shore depositional system. The transitional depositional system is mainly delta system. At last, the thesis discusses Jurassic tectonic evolution and character of sedimentary basin by the analysis of depositional system in the Cuoqin Basin.