Abstract: During the 1:50 000 regional geological survey in Chaganaobao area of Sonid Left Banner in Xilinguole League, Inner Mongolia, the newly discovered fluvio-lacustrine sedimentary formation are mainly composed of marl, limestone, conglomerate, sandy conglomerate and fine sandstone. Besides, a lot of gastropods, bivalve, ostracods, and a large number of plant debris and animal bones fossils are discovered in the study strata. The sedimentary formation is part of the Tongguer Formation of Miocene of Neogene according to the analysis of lithological assemblage and fossil identification. It is proposed that the northern margin of Erlian basin locates at the Wurigentalabaga area of Chaganaobao town, and the Erlian depression form in Miocene according to the existence of Tongguer Formation in the study area.