Correlation of Parasequence and Short-Term Base Level Cycles in River Facies: A Case of the Xujiahe Formation in Central Sichuan
GAO Zhiyong Central Laboratory of PetroChina Exploration and Development Research Institute, CNPC Key Laboratory for Oil and Gas Reservoirs, Beijing, 100083
Through analyzing the sedimentary dynamics in meandering stream, a model of the Xujiahe Formation parasequence in river facies was established in central Sichuan. Fining-upward and coarsening-upward parasequences occurred usually in meandering stream sediments. There was fining-upward parasequence in braided stream, but coarsening-upward parasequence was not in braided stream sediments. The fining-upward parasequence boundary of meandering stream and braided stream is the ravinement surface of river course. The coarsening-upward parasequence boundary of meandering stream was the exposure mudstone of flooding plain and root clay. By the correlation of parasequence and short-term base level cycles in river facies, they had the same time limit. The time limit was 0.01~0.05 Ma. And the boundary of them has the same lithologic character: the ravinement surface of river course or exposure mudstone of flooding plain and root clay. The sediment was of the same age in parasequence and short-term base level cycles, and it was just one thing. From different analyses, the differences of them are revealed: one is the classic sequence stratigraphy, and the other is the high-resolution sequence stratigraphy. The focus of parasequence and short-term base level cycles in river facies is that, when the base-level in meandering stream changes, the sediment relative to the stream stage gets shallow upward. The sediments in the stream stage was getting shallower to the natural levee and the flooding plain.