Stratigraphy and paleoenvironment of the Quemo Co Formation in Shengli River area, northern Tibet

Abstract：Recently abundant silicified wood was discovered in the Quemo Co Formation along the Shengli River section in Shengli River area, northern Tibet. In addition, certain amounts of bivalves (e.g., Astarte delicata Fan, A. cf. voltzii Goldfuss, Pleuromya sp., Protocardia sp., Modiolus sp.，Neomiodon sp., and Chlamys sp.), spores and pollen grains (e.g., Cyathidites, Deltoidospora, Neoraistrickia and Classopallis) were also found in this section. A detailed investigation along the section was performed to reconstruct the paleoenvironment during Quemo Co deposition. The magma emplacement age of the Nadi Kangri volcanic rocks underlying the Quemo Co Formation is ca. 205-220 Ma, representing the early sedimentary age of the Quemo Co Formation. The Quemo Co strata are overlain conformably by the middle Jurassic Buqu strata. Therefore, the sedimentary age of the Quemo Co Formation should be early-middle Jurassic. The Quem Co Formation is composed chiefly of quartz sandstone and conglomerate intercalated with minor micritic limestone. The basal units are typically conglomeratic with the characteristics of alluvial and fluvial sedimentation. These units are overlain by quartz sandstone intercalated with micritic limestone containing salt, brackish and fresh water bivalve fossils, thus considered to be a delta environment. Subsequently, these strata were overlain by restricted platform limestone, indicating a progradational sequence of transition from continental to marine facies. The composition of palynofloras from Quemo Co strata suggests that the climate of the Shengli River region belonged to a sub-humid tropical to sub-tropical zone during the deposition of silicified wood.