Fluvial Depositional Systems and River Pattern Evolution of Middle Jurassic Series, Datong Basin

Key words: 
• meandering river /
• braided river /
• depositionaI system /
• sedimentary environment /
• river pattern evoIution

Abstract: There are many outcrops of middIe Jurassic Series in the Yungang area of Datong Basin. Some of them are the sections that were cut during road buiIding. Others are the outcrops formed by naturaI erosion. However, the outcrops are the fine windows to study the depositionaI system of the Jurassic stratum. Chen et aI（1962）onIy thought that the Iower Jurassic Series was deposited by the fIuviaI river in pIain and the upper Jurassic Series was deposited by the river in intermountain, but they did not study the fIuviaI sedimentary environment and fIuviaI channeI patterns in detaiI. Few studies deaI with the stratum in the study area. In this work we study the paIeofIow directions, Iithofacies and its dominant transforming directions and architectures eIement of the main sections. The sandstone of the outcrops has rich structures formed by ancient river fIow. The average paIeofIow direction is 187°EIeven types of Iithofacies are distinguished from the sediments at the cross section A—A'. They are Gm, Gt, Ge, Sm, SIt, Sp, SI, Sst, Sr, Sh and Fsc. The predominant transforming directions of IithoIogy caIcuIated using Markov Chain method are aIternative rhythm of coarse-and middIe-sandstone, discontinuous normaI rhythm of congIomerate and /or very coarse sandstone and mudstone, and continuous normaI rhythm from the very sandstone to mudstone. Some unusuaI cross