The Research Of Oligocene Cyprinoid Fossils And Their Depositional Environment In The Northwestern Qaidam Basin,Northwest China

According to the detailed field observation and descriptions,the paper focus on the systemic study of a Cyprinidae fossil found in a lithological profile of early Oligocene(about35Ma)Upper Ganchaigou Formation and Lower Ganchaigou Formation in northwestern margin of Qaidam Basin,and discussed the sedimentary facies,and environment evolution of the profile by using the theories and methods of petrology,paleontology and geochemistry.Based on the regional geological and lithological features,Upper and Lower Ganchaigou Formation in research profile were divided into two parts separately.The lower part of Lower Ganchaigou Formation is dominated by pebbly sandstones,sandstones,microcrystalline limestones,and dolomite with trough cross beddings,parallel beddings,horizontal beddings and ripple marks.Such features reflect the meandering river facies including microfacies of beach and flood fan,fan delta facies and then transition to lake beach with time;The upper part of Lower Ganchaigou Formation was mainly composed of coarse-grained sandstone with calcareous coatings,gray-white calcerous(and or dolomitic)mudstones with abundant fossil plant fragments,and gray to gray-green fine-grained sandstones to siltstones.The common sedimentary structures include parallel beddings,horizontal beddings,ripple marks,and rain marks.Ostracods Hemicyprinotus were found under lithological thin sections.These features indicate the changes from the lake beach to shallow lake subfacies.The lower part of Lower Ganchaigou Formation was mainly composed of Light gray-green calcareous mudstones,mudstones,and calcareous sandstones with parallel beddings and horizontal beddings.These features indicate the sedimentary facies changed from shallow lake to beach subfacies.The upper part of Upper Ganchaigou Formation was mainly composed of dolomite,shell limestones,oolitic limestones,calcareous sandstone,and conglomerate sandstones.These features indicate the environmental transition from the lake beach to a distal fan of alluvial fan.There is one complete fish fossil found in gray green calcareous mudstone of upper Lower Ganchaigou Formation.The body is long,flat,round snout,with the pharyngeal bones shape 5/3/2,that was identified as the features of a Barbinae(Pisces,Cypriniformes)of thesubfamilies of freshwater fish.The structure and living habits of this type of fossil reflect the relatively stable semi-deep lake environment in the early Oligocene research period.The salinity of the lake is low,the temperature is between 15? and 30?,and aquatic plants are flourishing.The Sr/Ba raitoes of profile samplesgeochemical data change in stages and it shows the appearance and disappearance of plant fossils and animal fossils.The data of Carbon and oxygen isotope shows that a cold-dry to warm and humid climate to cold-dry pattern appeared in the northwestern edge of Qaidam Basin from the end of Eocene to the end of Oligocene and the salinity of water bodies from fresh water to fresh water or brackish water to salt water,and the water body gradually shifts from closed to open.We know that the Qinghai-Tibet Plateau may have started to ascend during this period according to the ancient environment of the Qaidam Basin according to the the fossils,sedimentary facies,paleo-water salinity,temperature,and water oppening.