Characteristics of Sedimentary Facies and Sequence Boundary in Upper Cretaceous Qingshankou-Yaojia Formation of Northern Songliao Basin

Qingshankou and Yaojia Formation developed delta and lacustrine facies along the long axis of the basin during the period of depression. Qingshankou Formation consists of littoral lake, shallow lake and semi-deep lake facies. Yaojia Formation consists of delta plain, delta front and prodelta facies. Base on analyses of drilling cores, from the top of Quantou Formation to the top of Yaojia Formation can be divided into 6 sequences, and corresponding to 7 sequence boundaries. One of the sequence boundaries is supersequence boundary. Sequence boundary is characterized by paleosol, erosional surface, calcareous nodules, and truncation. Generally, oolitic limestone, bioclastic limestone, worm boring and pyrite develop above the sequence boundary.