塔里木柯坪地层区中-上奥陶统萨尔干组碳酸盐岩微相和古地理

塔里木板块西北柯坪地层区中-上奥陶统萨尔干组的分布和相变受控于当时西浅东深的海底深度差异。萨尔干组的厚度10米左右,为富含有机物的黑色页岩并夹少量灰岩薄层或透镜体。位于研究区西南的伽师西克尔剖面缺失大部分奥陶纪地层,上奥陶统铁热克阿瓦提组碎屑岩不整合于下-中奥陶统鹰山组灰岩之上,中-晚奥陶世的大部分时段属于暴露海面的剥蚀区;北部乌什的亚科瑞克剖面无萨尔干组岩性单元,为红色居多且粒度偏粗的碎屑岩夹白云岩薄层,属于近岸带碎屑岩为主的沉积区。西克尔向东北延伸150km至柯坪羊吉坎亦无萨尔干组,与萨尔干组同期沉积的地层在此表现为大湾沟组上部浅海灰岩的相变,且与上奥陶统桑比阶坎岭组偏深水相的红色薄层含泥瘤状灰岩之间呈整合接触;由西向东出露于柯坪苏巴什沟、柯坪大湾沟、阿克苏四石厂3个剖面的萨尔干组黑色泥页岩指示典型的滞流盆地相,而夹含于萨尔干组下部和上部的薄层灰岩之微相特征差异甚为显著,表现为该组下部可见数层密集砂屑颗粒形成泥粒状灰岩或颗粒灰岩,可解释为西部相邻的浅海碳酸盐岩台地区灰岩经重力流搬运后的再沉积;而该组上部含生屑泥状灰岩则属滞流盆地相区类似于黑色页岩环境的正常沉积。从羊吉坎灰岩台地到苏巴什沟滞流盆地相之间距离仅20余千米,坡度偏大是导致重力流形成的主要原因。

MICROFACIES OF THE CARBONATES AND PALAEOGEOGRAPHY OF THE SAERGAN FORMATION(MIDDLE-UPPER ORDOVICIAN),KALPIN STRATIGRAPHIC REGION,TARIM,NW CHINA

Palaeogeographic patterns,especially of eastern basinward extension,controlled the distribution of the Saergan Formation(Middle-Upper Ordovician transition)and its spatial lithofacies development.This unit has a thicknesses of about ten metres in the Kalpin stratigraphic region and is typically black shale rich in organic matter intercalated with thin-bedded limestones.A large stratigraphic gap at the southwesternmost Xiker section of Jiashi shows that the Lower-Middle Ordovician Yingshan Formation is unconformably overlain by the uppermost Ordovician Tierekeawati Formation.This could be considered as an exposed and eroded landmass during most of the Middle-Late Ordovician.The Saergan Formation can't be recognized at the Yakeruike section in Wushi,located in the northern part of study region.In this area,red coloured silts and sandstones with subordinate thin-bedded dolostones are likely indicators of nearshore conditions dominated by coarse terrigenous sediments.The Saergan Formation is also absent at the Yangjikan section in Kalpin,which is 150km northeast of the Xiker section.In that region,shallow marine limestones from the upper part of the Dawangou Formation are coeval with the Saergan Formation and are conformably overlain by transgressive red argillaceous nodular limestones of the Kanling Formation(Sanbian,Upper Ordovician). The black shales that characterize the Saergan Formation are typical stagnant basin deposits.Eastward they are spatially limited in occurrence to three sections in the Subashi and Dawangou areas of Kalpin County and at Sishichang in Aksu.However,the microfacies of the thin-bedded limestones within the black shales show diverse features.Several layers of packstone or grainstone composed of sand-size debris are commonly found in the lower part of the formation. These grains originally formed in shallower platform and were then redeposited basinward by gravity flows.Bioclastic mudstones,mostly in the upper part of the formation,represent normal stagnant basin sediments.The shortest distance between the carbonate platform of the Yangjikan section to the stagnant basin of the Subashigou section is only about 20km.This suggests that a narrow and steep sloping belt between them promoted the gravity flows.