Differences in evaporite geochemistry of the greater Ordos Ordovician salt basin, China: evidence from the M56 submember of the Majiagou Formation

Ordovician evaporites are very rare in the global geological record. Thick salt deposits in the Ordovician Majiagou Formation are present in the eastern greater Ordos Basin, China. Potash mineralization layers and a thin industrial grade potash layer occur in the sixth submember of the fifth member of the formation, thus indicating a good potash prospect. The eastern and western salt depressions are separated by an uplift and have been divided into five secondary salt sags (East-1, East-2, West-1, West-2, and West-3). The ratio of the salt layer thickness to that of the whole succession was very low in the West-2 sag and the salt depositional center was the East-2 sag, which implied that seawater entered the basin from the western sags and led to desalination and the formation of deep-water sediments. Analyses indicated that the potash formation conditions were obviously different in the salt sags. Although a thin potash layer was present in the East-1 sag, findings indicated that this area was not favorable for potash accumulation. The geochemical and sedimentary environment of the East-2 sag was similar to that of typical potash basins globally. Rounded sylvine grains occurred between salt crystals. The preservation characteristics indicated that evaporite deposits had reached the KCl bitterns stage. An eastward input of terrestrial materials carried a large amount of clay, and the clay layer protected previously deposited sylvine particles, thereby avoiding further potassium dissolution. Sedimentary, geochemical, and petrological evidences indicate good prospects for potash exploration in the East-2 salt sag.