Stromatolites from the Majiagou Formation in the Ordos Basin, Northwestern China

Stromatolites declined remarkably in the Middle Ordovician, which has been speculatively related to the concomitant proliferation of metazoan reefs during the Great Ordovician Radiation Event, however, they are flourish and widely distributed in the Middle Ordovician (Darriwilian) Majiagou (Machiakou) Formation in the Ordos Basin, northwestern China. The stromatolites in the Ma-5 Member of the Majiagou Formation in the Ordos Basin exhibit significant morphological diversity, and are ready to be divided into wavy and domal stromatolites from the south and west part, columnar in the Northeast part, accompanied with rarely hemispheroid and conical ones. The stromatolites in the Ma-5 Member distribute in a subcircular ring belt, which can be subdivided into three sedimentary divisions, mostly near the boundary between the gypsum-bearing dolomite flat and gypsum salt lake, of which, the north-central division, with its thicker stromatolite-bearing strata and more complex stromatolite morphologies, was the most favorable for the establishment of stromatolites, where grazing animals were absent and burrowing animals few. Some calcified microorganisms were recovered from the stromatolites, which substantially increases their known diversity for this interval and suggestive of a severe undersampling of the middle Ordovician calcified cyanobacteria fossil record in the Ordos Basin.