陕西省北部奥陶纪岩石地层单位厘定

REVISION OF THE LITHOSTRATIGRAPHIC UNITS OF THE ORDOVICIAN IN NORTHERN SHAANXI

陕西北部地区奥陶纪地层重新厘定后，共划分为９个组。早—中奥陶世地层由下而上划分为三山子组、贾汪组、马家沟组，均以碳酸盐岩台地相沉积为主，岩性及岩相稳定，生物化石较少。中—晚奥陶世时期沉积岩相十分复杂，东西部存在明显差别。东部区分为桃曲坡组、赵老峪组，以斜坡相泥质碳酸盐岩沉积为主，厚度在１００～４００ｍ之间；西部区分为平凉组、背锅山组、段家峡组、唐陵组，以斜坡—盆地相及深海浊积扇为主，由碎屑岩、泥质碳酸盐岩夹少量火山碎屑岩组成，岩相变化较大，厚度可达３０００ｍ。

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.