Redefinition of the Cambrian-Ordovician Strata in the Olongbuluck Area, Northern Margin of the Qaidam Basin

We have carried out field investigations and precise profile measurements in the Olongbuluk area,northern margin of the Qaidam basin.In combination with rock assemblages,sedimentary structure,stratigraphic contact relationship and newly discovered fossils,we made a detailed division,and comparison,and redefined the Cambrian-Ordovician strata.The results show that:(1) The Olongbuluck Group only contains the third series and Furongian series of Cambrian,and lacks Terreneuvian and second series.(2) The Duoquanshan and Shihuigou Formations are of the lower and middle Ordovician,respectively.(3) The original division of the middle Ordovician Datouyanggou Formation should be fragments of structural reformation,which does not match the principle of establishing formation and should be abandoned.(4) Through regional stratigraphic comparison of those blocks around the Qaidam basin,we suggest that the Cambrian-Ordovician strata are exposed over the Olongbuluck block(marine carbonaterock formation),Tanjianshan Group,Qimantage Group,and Naijtal Group(intermediate-acid volcanic construction),which represent the products of different structural settings and sedimentary environments.This study provides an important basis not only for establishing rational and effective lithostratigraphic and chronostratigraphic systems,but also for the evolution of stratigraphic sequence-lithofacies palaeogeography and hydrocarbon resource evaluation of Cambrian-Ordovician in the northern Qaidam basin.Furthermore,it also provides evidence for better understanding tectonic evolution of blocks in the southeast margin of the Tarim basin.