Sedimentary Fades Of Ordovician Majiagou M₅⁶-M₅⁸ Formation In Bahannao Area South Ordos

The object of this paper are microfacies types and characters of Majiagou Formation in Bahannao area, Ordos basin. By using advanced theories of sedimentary, geophysics, and geochemistry, combining with outcrops, drilling and logging data, the stratigraphic characteristics of M58-M56, paleotopography and the changing of sea were identified; through the facies markers prejudged the facies; reconstructed protolith and classified sedimentary facies.Bahannao’s formation of M58, M57, M56 pinching out from the southeast to the northwest, the average thicknesses are about 25m. Paleotopography lying in the northwest, southeast low.δ13C and δ18O firstly rises, then declines from M58 to M56, which presents the climate from wet to warm-wet then return to wet.Dolomite is very development in all of the targets, M58 and M56 have a few mud, gypsum, and javascript void; content less limestone than M56, noteworthily most of it is secondary limestone. Although M57’s limestone is more than others, but the secondary limestone is less. besides there is no mud and gypsum. By studying, M56 and M58’s secondary limestone is from anhydrtic dolomite, M57’s secondary limestone is from dolomitic limestone.In conclusion, M58, M57 and M56 all belong to restricted sea platform facies, M58 and M56 are close to the supratidal zone with the wet climate; M57 is located in the lower supratidal-upper intertidal zone with the wet-warm climate.