山西二叠系河流沉积特征

Characteristics of Fluvial Deposits in Permian, Shanxi

山西西部二叠系沉积环境以河流为主,除曲流河外,还有为数不多的辫状河与交织河。曲流河以发育曲流沙坝、洪泛平原和决口扇为其特征。辫状河发育河道沙坝沉积,洪泛平原沉积不太发育。交织河以河道稳定为特征,伴有沼泽沉积,是理想的成煤环境。

The Permian System of Western Shanxi has now recognized that it mainly consists of fluvial sediments, and the ancient rivers in this area can be divided into three main types as meandering, braided and anastomosed. The depositional sequences of meandering river are typified by the development of point bars, flood plains and crevasse-splays. Lower and upper point bars with erosive contacts between them have been found in the Permian.In extreme circumstances deposits of crevasse-splays can be found as many as 11 layers within a single depositional sequence of the meandering river which indicates a very frequent occurrence of floods.Possible sand dune deposits have been suggested in some of the meandering rives. The depositional sequences that are rich in sandstones are believed to form in braided rivers.Flood plain sediments are much thinner than the channel's and their thickness is in the ratio of 0.25-0.5.The sequences are normally poor in crevasse-splays and levee deposits. Braided-meandering rivers are proposed the environment for those depositional sequences which are both rich in channel and flood plain sediments, but lack of natural levee and crevasse-splay deposits.Various scales of trough cross bedding prevail in channel sediments in these sequences. The repeatedly overlapping of channel sand bodies and the development of coal in marshes or bogs are used as the evidences of asastomosed river in the area. While the sediments overlaying just above coastal deposits, indicate the once existed flat plain when anastomosed channel was developing.