Sedimentary environments of the Middle Ordovician Pingliang Formation in the Qishan section, southern Ordos Basin

The Middle Ordovician Pingliang Formation strata well-developed and outcropped in the Qishan section, Baoji, Shaanxi are interpreted to be an ideal section for the study of the Ordovician marine strata in the Ordos Basin. The Pingliang Formation may be divided into seven lithologic members. The clastic rocks make up about 80 per cent, and are organized dominantly in the 1st, 3rd, 5th and 7th members of the Pingliang Formation. The carbonate rocks occupy the other 20 per cent, and constitute the 2nd, 4th and 6th members of the Pingliang Formation. The Pingliang Formation in the Qishan section is assigned to the continental slope environment and consists of a succession of polycyclic deposits. The 1st to 2nd, 3rd to 4th, and 5th to 6th members are grouped into three transgressive-regressive cycles, respectively, and laid down in the transitional environments of the upper continental and lower continental slopes. The 7th member is made up of a single transgressive cycle, and the regression in this member occurred at the onset of the deposition of the Tangling Formation.