Stratigraphic characteristics and geological significance of Qingshankou Formation in Songliao Basin

Abstract

The Qingshan Formation is the first large-scale lake invasion in the geologic evolution history of the Songliao Basin, the central depression area of the basin is widely distributed with black mudstone and shale, and the coarse clastic rocks in the western slope area and the northern dumping area are obviously increased, while the large sets of green-gray and purplish-red mudstone appear in the eastern uplift area and the southeast uplift area. By chronological analysis of the zircon minerals in the lacustrine sedimentary layers of the lake facies of the Qingshan formation, the U-Pb isotope harmonic ages were 91.35Ma, 91.06±0.69 Ma, it is confirmed that the bottom of Qingshankou Formation is proven to belong to the lower part of Turonian period; the ⁴⁰Ar/³⁹Ar isotope plateau age and the age of equal time line obtained from two or three section of Qingshankou Formation in Jin 6 well are 88.0±0.3 Ma, confirming that the top boundary of Qingshankou Formation has been extended to Coniacian period. At the same time, the appearance of Borealipollis, Plicapollis and Complexiopollis, which has important geologic age significance, indicates that the bottom boundary of Qingshankou has already entered to the late.