The transition from marine to terrestrial in the Early Permian : Evidence from the Early Carboniferous-Late Permian sedimentary records of the eastern margin of the Ordos Basin, North China

The sedimentary successions provide direct evidence of climate and environment, which offer clues to the cause of the great transition from marine to terrestrial in the Early Carboniferous-Late Permian in the Ordos Basin. The eastern Ordos Basin's early Carboniferous-late Permian strata are sequentially stratified in ascending order as the Benxi Formation of the early Carboniferous, the Taiyuan Formation of the late Permian, and the Shanxi Formation of the late Permian. The Benxi Formation is composed of mudstone with a large number of siderite concretions, carbonaceous mudstone, rhythmic layers of siltstone and mudstone, and coal. The rhythmic layers indicate depositional products of lagoon and tidal flat. The Taiyuan Formation comprises carbonate and clastic sedimentary record, which lithology is mainly biological detrital limestone, limestone with chert nodules, mudstone, coal, and locally sandstone, indicating a shallow-sea shelf depositional environment. The lower part of the Shanxi Formation is a lithological combination of black mudstone, and purple-red sandy mudstone rhythmic layers interbedded with sandstone and coal seams. The black shale contains both marine biological detrital and terrestrial plant fragments. The sandy mudstone rhythmic layers have developed a large number of tidal beddings (such as flaser bedding, wave bedding, and lenticular bedding). The upper part of the Shanxi Formation is mainly depositional in the delta front-plain system. The lithology is mainly fine-coarse grained sandstone, sandy mudstone interbedded with carbonaceous mudstone containing plant fossils and coal seams. Overall, the palaeoenviroment changes from humid-semihumid to arid-semiarid are recorded by the transition from shallow-sea shelf to tidal flat-lagoon to delta deposition records. This study has established the sedimentary facies sequence of the late Carboniferous-early Permian by investigating five typical outcrops on the eastern margin of the Ordos Basin, and analyzed the paleoenvironmental characteristics of the Benxi Formation-Shanxi Formation based on geochemical methods. In addition, we believes that the Shanxi Formation has not completely transformed into terrestrial deposits in the southeastern part of the basin, and it is still in a transitional marine-terrestrial depositional environment.