The northern margin of Alxa block is located at the juncture of North China plate，Tarim plate，Siberia plate and its southern marginal fold belt，and Alxa block．The pre-Mesozoic sedimentary formation in the study area，affected by structural positions and volcanic activities，show a great difference in different structural locations. Influenced by subsequent tectonic alternations，the strata，outcropped in the form of tectonic slices，should belong to the limited Smith stratigraphy or the transition between the limited Smith stratigraphy and Non-Smith stratigraphy，with characteristics of internal disordered state and local ordered state. This not only hinders the confirmation of the age of the blocks and the establishment of the normal stratigraphic sequence but also restricts the understanding of the tectonic and paleogeogrphic evolutions．The authors applied the ICP-MS technology and collected sandstone samples in the Lower Member of the previously-defined Amushan Formation in Wulanaobao on the eastern margin of the study area to carry out the study of U-Pb chronology．70 valid data were obtained from 80 measuring points with 79 zircons，with a range of surface ages from 259±5 to 2623±41 Ma. These surface ages are characterized by multi-peak values，with a dominant peak between 259～308 Ma（47 data）and some smaller spikes at 401～444 Ma（12 data），1652～1750 Ma（7 data）and 2601～2623 Ma（2 data）．In addition，there are two detrital zircons with ages of 731 Ma and 1237 Ma respectively．According to petrology，CL image and U-Pb chronology of the studied samples in combination with sedimentology and regional tectonic evolution， the authors have come to the following preliminary conclusions： (1) The previously-defined Lower Member of Amushan Formation along the Wulanaobao section should not be assigned to Late Carboniferous-Early Permian Amushan Formation，and the previously-defined Amushan Formation along the Wulanaobao section should be disintegrated into two parts: the middle and upper member (the upper and lower member after revision) should be assigned to Amushan Formation，which is characterized by interbedded clastic rocks and carbonate rocks and the existence of Late Carboniferous-Early Permian fusulinida fossils，such as Triticites sp. and Quasifusulina sp．；the lower member of this section is characterized by coarse clastic rocks of continental molasse formation and should be assigned to Upper Triassic，regionally comparable with Shanhujing Formation in the west of the study area and Beishan area of Gansu； (2) The tectonic style of the overthrusting of Amushan Formation on the Upper Triassic strata is probably the reflection of the thrust nappe structure in the middle and western part of Inner Mongolia and Beishan area of Gansu Province；(3) The sedimentary paleogeographic evolution shows that the provenance of the sampled sandstone could only be geological bodies of surrounding areas，because the paleogeography in and after Mesozoic was characterized by inland faulted lake basin. The zircon CL image and the ratio of 232Th/238U suggest that most of the zircons are magmatic zircons，with minor metamorphic zircons. Based on a chronologic comparison of the geological bodies in surrounding areas，this paper considers that the provenance of the sandstone should be Hercynian and Caledonian intrusive rocks，Permian volcanic rocks and pyrocalstic rocks，and Archean and Proterozoic metamorphic rocks．