DEPOSITIONAL ENVIRONMENTS AND CYCLOSTRATIGRAPHY OF LOWER PERMIAN MARINE TO FLUVIAL DEPOSITS IN WUDA COALFIELD, SOUTHWESTERN INNER MONGOLIA, NORTH-CENTRAL CHINA

Two outcrop sections of the lower Permian sedimentary rocks in Wuda Coalfield, northwestern Ordos Basin, China, record cyclic sedimentation of shallow marine and fluvial siliciclastic sediments. They were deposited in shelf, shoreface, beach, wave-dominated delta, coastal plain and swamp, and coarse-grained meandering stream environments. Two types of sedimentary cycles are defined by environmental changes related to shoreline transgression and regression and fluvial erosion and deposition, respectively. Cycle stacking forms four long-term trends of marine flooding and progradational infilling and major advancement of fluvial systems. The tuff that entombed the “vegetational Pompeii” between coals 6 and 7 is an ash-flow deposit in a coastal peat swamp and temporarily disrupted the peat accumulation. The Histosols and Gleysols throughout the sections suggest an ever-wet paleoclimatic condition. The coal-bearing cyclic record in the study area at the northern coast of Paleo-Tethys Ocean at the mid-paleo-latitude is similar to the other coeval records in the North China Plate and the Appalachian-type cyclothems in the paleo-equatorial region of North America during early Permian.