The biostratigraphy of the 2^nd Member of the Nenjiang Formation from the Yuewangcheng section of the eastern Songliao Basin and its response to the lake transgression event

Large-scale lake transgressions occurred in the lower part of the 2^nd Member of the Nenjiang Formation in the Songliao Basin, which deposited a set of dark mudstone and black shale with intercalation of oil shale throughout the basin. However, previous researches on strata and related geological events in the 2^nd Member of the Nenjiang Formation were mainly based on the cores, with very insufficient attention paid to outcrops. A new section called Yuewangcheng section was found along the Di'er Songhua River in Qingshankou, Nong'an County, eastern margin of Songliao Basin, which successively exposes the top of the 1^st Member to the lower parts of the 2^nd Member of the Nenjiang Formation. In this study, the strata were described in centimeter-level and divided in detail into 24 layers with a cumulative thickness of 31.54m. The first layer is the top of the 1st Member of the Nenjiang Formation, which deposited bluish gray sandy mudstone with a small quantity of ostracods. 2^nd-24^th layers constitute the lower part of the 2^nd Member of Nenjiang Formation, which is composed of dark shale with oil shale and marl, characterized by rich ostracods, phyllodes and fish, with a small amount of foraminifera and other organisms. In addition, a layer of volcanic ash and phosphorite was found at the bottom of Nenjiang Formation. 18 species of 11 genera of ostracods were preliminarily identified. On the basis of sedimentology and ostracod palaeoecology combined with other evidence, the lake environment is preliminarily discussed. It is suggested that a large-scale lake transgression occurred in the lower part of the 2^nd Member of Nenjiang Formation. The shore-shallow lake environment at the top turned to a deep lake environment at the bottom, and then the lake level was gradually declined in fluctuation.