Sedimentary Characteristics of the Fourth Member of Cretaceous Nenjiang Formation in the Southern Songliao Basin, Northeast of China

The lithology of the fourth member of the Cretaceous Nenjiang formation (K(2)n(4)) in southern Songliao Basin is mainly mudstone. The proportion of sandstone is about 10% which is mainly composed of lithic feldspar sandstone. The carbonate content of sandstone is very high with an average of 25%. The clay minerals are mainly illite/montmorillonite (I/M) mixed layer. In the studied area, 9 single lithofacies types and 5 combination lithofacies types were identified and described in detail. The subfacies type of K(2)n(4) in the studied area is delta front which includes four microfacies: underwater distributary channel, mouth bar, sand sheet and interdistributary. During this period, the lake basin gradually shrank, the delta continued to push south, and the sand bodies were lingual to the south with their thickness gradually decreasing.