REVISION OF THE LITHOSTRATIGRAPHIC UNITS OF THE ORDOVICIAN IN NORTHERN SHAANXI

Ordovician strata are well developed in Longxian, Yongshou, Tongchuan and Hancheng of northern Shaanxi. According to the principle of the “study of multiple lithostratigraphic division and correlation", nine formations have been distinguished. The Early-Middle Ordovician is divided in ascending order into the Sanshanzi Formation, Jiawang Formation and Majiagou Formation, in which carbonate platform facies deposits predominate and lithology and lithofacies are persistent with scanty fossils. The lithofacies of Middle Late Ordovician deposits are very complex. There is marked difference in lithofacies of the eastern and western parts. The Taoqupo and Zhaolaoyu Formations in the eastern part are dominated by argillaceous carbonate deposits of slope facies, ranging in thickness from 100 to 400 m. In the western part the Pingliang, Beiguoshan, Duanjiaxia and Tangwangling Formation are distinguished. They consist dominantly of clastic rocks and argillaceous carbonate rocks with a small amount of pyroclastic rocks. The lithofacies are highly varied and fossils are abundant. The thickness may be up to 3 000 m.