THE ORIGIN OF THE BLACK SHALES AND THE BOTTOM CURRENT MODEL FOR SEAWATER ENCROACHMENT IN THE CRETACEOUS QINGSHANKOU FORMATION,SONGLIAO BASIN,NORTHEAST CHINA

The study of sedimentary sequences,mineralogy and geochemistry of the black shales in the Cretaceous Qingshankou Formation has revealed that the Songliao nearshore continental basin was affected by periodic transgression/regression of the adjacent marine water from the Pacific Ocean during the deposition of the Qingshankou Formation (88.5-100 Ma).On the basis of the palaeoenvironmental analysis of the black shales,the authors conclude that the bottom current resulted from periodic transgressions may be the main factor which gives rise to the stratification of lake water and the deposition of the black shales in the study area,and a simplified model showing the marine incursions in this area is also proposed.