Characteristics of the Cretaceous supersequences in northern Songliao Basin
Lin Chunming Feng Zhiqiang Zhang Shun Zhao BoZhuo HongchunLi YanliXue Tao
1 State Key Laboratory of Ore Genesis, Department of Earth Sciences, Nanjing University, Nanjing210093,Jiangsu
2 Exploration and Development Research Institute of Daqing Oilfield Company Ltd, Daqing163712,Heilongjiang
3 China Academy of Land and Resource Economics, Mail Box 259, Beijing101149
The Cretaceous in northern Songliao Basin can be divided into three supersequence sets,eight supersequences and twentyeight sequences. Volcanic and coarse clastic successions formed during faulting stage contain three supersequences and six sequences. Mediumcoarse grained clastics formed in the interim stage from faulting stage to depression stage contain one supersequence and four sequences. During the depression stage the lithology is dominated by lacustrine and deltaic mediumfine grained clastics and can be divided into three supersequences and fifteen sequences. During the deposition of transgressive systems tract(TST)and highstand systems tract(HST)semideep and deep lacustrine dark mudstone were widely deposited. One supersequence and three sequences were identified during the shrinkage phase of lake basin when the lacustrine and deltaic mediumcoarse grained clastics were dominant.Supersequence set boundaries that were controlled by tectogenesis, include the top boundaries of Cretaceous bedrock, Yingcheng Formation and Nenjiang Formation, which correspond to regional unconformities formed by major tectonic phases of Yanshan Movement. Supersequence boundaries can be either supersequence set boundaries disconformities or erosional surfaces levels. At the supersequence boundaries, the depositional hiatus is quite long, so that channel downcutting, stratigraphic gap and exposure etc. widely exist. Supersequence boundaries are in accord with lithostratigraphic unit boundaries. Compared to supersequence boundaries, the depositional hiatus at sequence boundaries is shorter and sequence boundaries are not compatible with lithostratigraphic unit boundaries. Supersequence boundaries are characterized by truncation, onlap and toplap etc. on the seismic sections and sudden change of lithology and logging curve.