Stromatolites in Middle Ordovician carbonate-evaporite sequences and their carbon and sulfur isotopes stratigraphy, Ordos Basin, northwestern China

At the present day, stromatolites are very rare and limited to high-salinity settings. However, abundant and variable stromatolite fossils occur in thick evaporite sequences, the Middle Ordovician Majiagou Formation of the Ordos Basin, northwestern China. The setting and fossil assemblage imply that the high salinity prevented metazoan grazing and allowed stromatolites to flourish. The carbon isotope curve based on new data (average data range from -4.52 to +0.56 parts per thousand) from drilling cores in Majiagou Formation is in good agreement with Middle Ordovician (Darriwilian), and supports the Middle Ordovician age inferred from biostratigraphy. Sulfur isotopes values (range from +27.1 to +28.0 parts per thousand) are higher than those of the Upper Ordovician but much lower than those of the Lower Middle Cambrian; thus, the sulfur values imply that the Majiagou Formation is of Middle Ordovician age. New carbon and sulfur isotope data imply that these strata are of Middle Ordovician age, in good agreement with previous paleontological dating.