SEDIMENTARY SYSTEMS IN MAXIMUM FLOODING PERIOD OF CARBONIFEROUS IN ZHONGWEI AREA AND ADJACENT REGIONS

The Zhongwei area and adjacent regions are surrounded by the Alxan block, North China Plate and North Qilian. Stratigraphic classification in the study area is different from the North China Plate because of regional tectonic and eustatic sea level changes. Field investigations,in addition to previous studies, were carried out in order to study the depositional systems during the maximum flooding period. According to lithology, depositional fabric, sedimentary structures, sedimentary cyclicity of the deposits. a detailed sedimentary facies division for Carboniferous has been made for the research area. They are grouped into 4 depositional systems, i.e. the deltaic system, barrier-tidal flat-lagoon system, carbonate platform system, and shallow sea system. In the early period of early Carboniferous, seawater invaded into the study area surrounded by the Alxa, Erdos, and Northern Qilian, and formed the carbonate platform and the barrier-tidal flat-lagoon systems. In the period of late Carboniferous, transgression was extended and the research area was then connected with the seawater from North China in Otog and Otog Front. In the early period of early Permian, the sea was further expanded to be an epicontinental sea. Sea level frequently changed in the period of late Carboniferous, and the barrier-tidal flat-lagoon, deltaic and shallow sea depositional systems were developed in the study region