Chronostratigraphy and Stratigraphic Classification of the Cretaceous of the Songliao Basin

Isotopic ages of synsedimentary clay minerals were directly determined with the ultrasonic scattering—settlement separation—K—Ar dilution technique. The apparent age of black mudstone is 123 Ma for the Quantou Formation, 111.9—89.0 Ma for the Qingshankou Formation and 77.6—76.8 Ma for the upper part of the Nenjiang Formation. The Rb—Sr isochrone age of the muddy limestone—mudstone sequence of the lower part of the Nenjiang Formation is 81 Ma and that of the muddy evaporite of the middle part of the Quantou Formation is 122.2 Ma. On the basis of the above isotopic ages, the authors propose a modified scheme of stratigraphic classification of the Cretaceous of the Songliao basin.