Palaeogeography of the Permian Shanxi Formation in northwestern Ordos Basin

Six medium-term cyclic sequences have been distinguished for the Shanxi Formation in the light of high-resolution sequence stratigraphy,and associated maps are constructed,including the isopach map of sandstones,isopach map of strata and isogram of total sandstone thickness to stratigraphic thickness ratios.The palaeogeographic interpretation of the Permian Shanxi Formation is presented on the basis of base levels and accommodation spaces.The deposition of progradation and aggradation resulted in the formation of larger deltas and thicker sandstones during the deposition of the third submember to the first submember of the second member of the Shanxi Formation,whereas the deposition of progradation,aggradation and retrogradation led to the northward migration of the lake strandline and the extension of the lake area during the deposition of the third submember to the first submember of the first member of the Shanxi Formation.The delta plain was larger in scale than the delta front during the deposition of the second member of the Shanxi Formation,and the delta front became progressively smaller during the deposition of the first member of the Shanxi Formation.