Sequence Stratigraphy Of The Middle Jurassic Formations In Xiaojihan Coal Mine

Based on the core observation, geophysic log analysis and geologic assays, the paper investigated the sedimentary facies and stratigraphic sequences of the middle Jurassic Yan’an formations. Winthin the framework of stratigraphic sequence and integrately from the aspects of thickness of areal formations, sand sediments, clay deposits, and coal layers, the paper analyzed the paleogeogaraphic, sedimentary and coal accumulating features of the middle Jurassic famations in Xiaojihan mine. The sedimentary facies can be divivied into meander, braided river, delta and lacustrine, and further subdivided the facies into 10 kinds of sub-facies and 23 kinds of micro-ficies. The framework of stratigraphic sequences consist of 6 types of stratigraphic sequences and 15 types of internal system tracts. The occurrence of stratigraphic sequences were controlled by tectonic lifting & dropping, variation of lake surface, geologic climate, sedimentary flux and containing spaces, in which tectonic lifting & dropping plays a main role. It can be concluded that the paleogeographic evolution of research area had experienced four stages of initial lake-filling, lake re-expanding, lake shrinking and post-shrinking during the middle Jurassic period. There experienced four kinds of coal accumulative modes which include meander flood-plain, lacustrine offshore peat-bug, prodelta estuary, and delta plain in the rearch area.