Objectives: The lithological association sequence of the predicted O2 m 5 6 salt layer in the Well Zhenjia-1,drilled in the North Shaanxi Ordovician Depression, is abnormal that is because the salt rock thickness reduced greatly and replaced by pure carbonate strata more than 100 m. These changes can not be reasonably explained according to sedimentary principle. The purpose of this paper is to find out the stratigraphic position of the anomalous carbonate sequence and make reasonable explanation. Methods：The comprehensive analysis of lithological association cycle, log cycle, carbon—oxygen isotope correlation and seismic profile is carried out. Results: Evidences from vertical lithologic cycles and logging cycles, as well as the data of stable isotope of carbon and oxygen revealed that the Fourth member of Ordovician Majiagou formation abbreviated as O2 m 4 was duplicated, and those carbonates more than 100 m actually resembled with the underlying autochthonous O 2 m 4 were allochthonous rocks complicated by tectonic. The seismic profile shows that the repetition of the O 2 m 4 may be related to the involvement of the plastic salt rock in the complicated thrust fault. Therefore, the so called "O 2 m 5 6 salt formation" predicted before drilling in the Well Zhenjia-1 is actually a thrust plate dominated by the O 2 m 4, and the formation of this well is duplicated. Conclusions: The so called “O 2 m 5 6 salt formation" of the Well Zhenjia-1 is an anomalous lithological association sequence which is mainly composed of O 2 m 4 as wells as other formations. The confirmation of the repetition of the O 2 m 4 in the Well Zhenjia- 1 contributes to the correct understanding of the filling sequence of the O 2 m 5 6 strata in the salt depression of Ordos basin, and will promote the further research in the respects of local tectonic and sedimentary .