Sedimentary characteristics and age of Amushan Formation in Ejin Banner and its adjacent areas, western Inner Mongolia

The westernmost end of the Amushan Formation is Ejin Banner area. The Amushan Formation was formed in an extended setting of rift, with abundant volcanic eruption. It comprises three units, namely volcanic rocks, limestone, and clastic rocks. Abundant fusulinids were found in units of volcanic rocks and limestone. In addition, abundant coral fossils indicating the age of Late Carboniferous were found in the lower part of limestone. The fusulinids from units of volcanic rocks and limestone of Amushan Formation belong to fusulinid Triticites and Pseudoschwagerina assemblages, suggesting that the Amushan Formation should be assigned to the Late Carboniferous-Early Permian period; in addition, the unit of volcanic rocks should be assigned to the Late Carboniferous Xiaodushanian stage or even earlier, the lower part of limestone unit to Xiaodu Shanian stage, the upper part of limestone unit to Zisongian stage and the clastic rocks unit to the Early Permian Zisongian stage or even to later Longlinian stage.