Abstract: This paper describes in detail a Jurassic section at Maliao Mountain in the Ulan Ul Lake area,northern Qiangtang basin.The Jurassic strata of the section are divided into the Qoima Co ,Biqu,and Xiali Formations ,of which the Biqu Formation is completely exposed and contains abundant fossils.Ac-cording to the sedimentary environment and relative sea-level changes,the Biqu Formation is subdivided into two depositional sequences and the characteristics of various system tracts are discussed.According to the main biotic assemblages and distribution characteristics of the Biqu Formation at the Maliao Mountain section,a bivalve assemblage zone,i.e.the Chlamys tipperi-Protocardia stricklandi Assemblage Zone,is established,and three subzones are distinguished.Its time range is Middle Jurassic Bathonian.