塔里木板块古城4井中奥陶统一间房组的海进序列

TRANSGRESSIVE SEQUENCES THROUGHOUT THE YIJIANFANG FORMATION(DARRIWILIAN,MIDDLE ORDOVICIAN)AT WELL GUCHENG 4,TARIM BLOCK,NW CHINA

塔里木板块中央隆起区奥陶系碳酸盐岩台地东南缘的古城4井钻遇约130m厚的中奥陶统达瑞威尔阶一间房组灰岩。该组下部为表附藻-肾形钙藻-瓶筐石礁丘群落,间夹有薄层砂屑滩,海水深度推测位于10-30m的潮下带至正常波浪带之间;上部为中-低能带藻屑砂屑组合,灰泥含量增高,大致指示正常波浪带之下到最大浪基面之间30-60m的深度。古城4井与中央隆起区西北部巴楚露头区同期沉积均显示海水逐渐加深的过程,到上奥陶统桑比阶吐木休克组沉积期两地都达到了奥陶系的最大海泛面。

Well Gucheng 4 is located at the eastern margin of the Central High of the Tarim Block during the Middle Ordovician.The Darriwillian（Middle Ordovician）Yijianfang Formation limestones with thickness of 130 m from the borehole is conformably overlain by the Tumuxiuke Formation and underlain by the Yinshan Formation respectively.The ecological parameters such as the lithofacies,biotic as well as sedimentary structures are indicator of the water depth.The limestone sequences of the Yijianfang Formation consists of the reef mound community composed of Epiphyton-Renalcis-Calathium for the lower part and bank association of sandy clastic grainstones to bioclastic packstones and wackestones for the upper part.Therefore,it is reasonable to outline a transgressive succession from a subtidal fair-weather wave influence zone（10—30 m）to a storm wave base zone（30—60 m）in ascending order.Moreover,the overlain Tumuxiuke Formation（Sanbian,Upper Ordovician）shows the transgressive zenith.Such a fact controlled by the sea-level change from Well Gucheng 4 coincides with the coeval unit from the western Central High Belt in Bachu outcrops.