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. • Five subclasses stromatolites were recovered from the Ma-5 member in the Ordos Basin, which mainly distributed in three blocks, and near the boundary between the gypsum-bearing dolomite tidal flat and gypsum salt lake. • Abundant calcified cynobacteria are retrieved in Stromatolites at the middle Ordovician Ma-5 Member, which represents a substantial addition to diversity of calcified cynobacteria fossil record in this period. • The hypersalinity conditions at the north-central part in the Ordos Basin was most favorable for the stromatolite flourishing. [ABSTRACT FROM AUTHOR]