Environmental and tectonic significance of Late Permian reefs in the Linxi and adjacent areas in Inner Mongolia of China

Abstract

Many plateau-edge reefs and reef mounds of the Late Permian have been discovered in Linxi County and adjacent areas of Inner Mongolia, China. These reefs verify that the Hinggan-Inner Mongolia Area was an open and unobstructed sea-trough in Late Permian. Facies and sequences of reef strata in Member 4 and Member 5 of the Linxi Formation were studied for information to reconstruct the closing process of the Linxi-Jiutai chasm in the sea-trough, i.e., to constrain subsidence, uplift, and final closing. Ages of the reef-bearing strata have been determined to be late and end of Late Permian, based on ranges or abundances of diverse fossils. Six reef-building organic assemblages were distinguished, representing different ecologic conditions, according to morphological functions and paleoecological adaptations of organisms including primarily sponges, bryozoans, and calcareous algae. Two types of shelf slopes, the gentle slope and the steep slope, have been interpreted for Member 4 and Member 5 of the Linxi Formation, based on the assemblages. The locations of outcrops, and types of these reefs delineate the north margin of Sino-Korea Platform, and define uplift and final closing of the Linxi-Jiutai chasm at the end of Late Permian.