Middle Jurassic depositional environments and bivalve fossils in the Chibuzhang Lake area,southern Qinghai

The Middle Jurassic strata with a maximum sediments thickness up to 6311 m in the Chibuzhang Lake area,southern Qinghai are well developed and divisible into Qoimaco,Biqu,Gyari,Sogwa and Xueshan Formations on the basis of the fossils of bivalves,ammonites and brachiopods.The tremendous thickness and horizontal variations of the Middle Jurassic strata in this area may result from the nature of the basin,early geomorphic framework and syn-sedimentary faulting.