BIOSTRATIGRAPHY AND PALEOENVIRONMENTAL CHANGE OF THE LOWER NENJIANG FORMATION IN THE QINGSHANKOU AREA,SONGLIAO BASIN

Ostracods and sporopollen samples were collected from the Houjingou section in the Qingshankou area,Songliao basin.The authors have recognized two ostracod assemblages:the Cypridea gunsulinensis-Cypridea ardua assemblage and the Ilyocyprimorpha netchaevae-Periacanthella portentosa-Cypridea ordinate assemblage.They have also identified two sporopollen assemblages:the Proteacidites-Cyathidites-Dictyotriteles assemblage and the Lythraites-Aquilapollenites-Schizaeoisporites assemblage.Based on lithostratigraphic and biostratigraphic features,the Houjingou section has been correlated to the upper part of Member 1 to the lower part of Member 2 of the Nenjiang Formation.Based on ostracod data(such as morphology,abundance,diversity,and preservation),sporopollen data(such as climate distribution,and humidity type),concostracan data,and sedimentary and mineralogical features,the paleoeclogy,paleoenvironment,and paleoclimate of the Houjingou section have been constructed.It is inferred that a lake trasgression occurred in the upper part of Member 1 and lower part of Member 2 of the Nenjiang Formation.Ostracods underwent a diversity decline,disappearance,and recovery during this transgression.Statistical analysis of sporopollen fossils indicates that the Houjingou section was deposited in tropical to subtropical and humid to subhumid paleoclimates.The paleoclimate was probably more humid and warmer during the deposition of the black and oil shales in the lowermost Member 2 of the Nenjiang Formation,followed by drier and cooler paleoclimate.