Tectonic attribute of the western Ordos basin during the Late Triassic

Abstract：Most predecessors held the view that the western Ordos basin was tectonically a foreland basin during the Late Triassic. This paper questions the validity of this view mainly based on the following three lines of evidence. First, three sequences of coarse clastic sediments in the Late Triassic Yanchang Formation in the Rujigou, Shigouyi and Kongdongshan areas on the western margin of the basin were revisited. The Yanchang Formation in the Rujigou area is of marginal facies indeed, but the appearance of intraplate basalt in the horizon nearby suggests that the Yanchang Formation in the area was deposited in an extensional environment during the Late Triassic. Recent study indicates that the Yanchang Formation in the Shigouyi is not very thick, less than 1500 m in thickness. The conglomerate in Kongdongshan is not typical of the sediments in the western part of the basin because it was probably influenced by the Qinling-Qilian orogen to the southwest of the Ordos basin. Secondly, through the compilation of the isopach map of the Late Triassic Yanchang Formation and several E-W-trending stratigraphic correlation sections it is found that a N-S-trending, very thick subsidence belt, as thought previously to be present in the western part of the basin during this period, does not exist. Finally, through the analysis of the seismic profile, balanced cross section and fission track data, the authors think that the present Hengshanbu back-thrust belt and large-scale Majiatan thrust nappe belt did not form in the Late Triassic but in the Late Jurassic. Therefore, tectonically the western Ordos basin was not a foreland basin during the Late Triassic but a component part of a superposed basin in a residual and extended craton.