A Middle Ordovician reef complex on the carbonate ramp of the NW Tarim,NW China
作者单位：Wuhan Center of Geological Survey Nanjing Institute of Geology and Palaeontology
会议名称：《2015年全国沉积学大会》
会议日期：2015年
学科分类：070903[理学-古生物学与地层学(含：古人类学)] 0709[理学-地质学] 07[理学]
关 键 词：Calathiμm reef complex Yijianfang Formation Middle Ordovician Tarim Block NW China
摘 要：Ordovician carbonates are well outcropped along the Lianglitag Mountains with distance about 35 km in Bachu County, NW Tarim. It was an epicontinental sea closed to the northwest margin of the Central Tarim Uplift palaeogeographically. Darriwilian(upper Middle Ordovician) Yijianfang Formation is about 70 meters in thickness completely composed of shallow marine limestones. Patch reefal and biostromal units constructed dominantly by Calathiμm from the middle member of the formation are respectively concentrated from northern and southern regions along the Lianglitag Mountains. Bigger patch reefs more than 10 m in thicknesses and 50 m in diameters are prevailed in the northern region. Some of them are stacked by smaller mounds inserted by bioclastic units. Reef-attached shelly fauna are diverse identified as elements such as trilobites, nautiluses, bivalves, brachiopods etc.. Dilapidated Calathiμm together with coarse organism fragments illustrate a high-energy belt. The sizes of patch reefs are gradually shrinkable to meters in thicknesses and weak relieves southward. Coeval autobiostrome formed by in situ Calathiμm frameworks baffling matrixes is only about 3 m in thickness at the southernmost section. Other shelly elements are quite low both in diversity and abundance herein. It was likely to be a calmer environment favorably of a back reef setting. Moreover, the sedimentary unit equivalent to the Yijianfang Formation is completely lacked toward the southern Mazatag Well Block, where was an oldland exposed above the sea-level during this time-interval. Therefore, such ecologicl, morphological and taphonomicdifferentiations of subfacies on the carbonate ramp enhances a reef complex with more turbulence leeward from the northern region and calmer back reef belt southward. The northern margin of the carbonate ramp further extends to the Yangjikan section of Kelpin County. The Darriwilian graptolitic black shales of the Sargan Formation were formed in typical stagnated basin. Disappearance of the reef complex was essentially caused by the transgressional tract occurred from the upper member of the Yijianfang Formation.