Abstract: Detailed stratigraphic and depositional facies analyses for the Qixia to the Lower Maokou Formations from several sections in western Hubei-northwestern Hunan Provinces indicate an intra-craton carbonate ramp environment，including inner，middle and outer ramps，and basinal facies.The inner ramp facies consists of thick-bedded to massive bioclastic limestone，characterized by bioclastic grains of green algae and benthic foraminifera and lack of high-energy bioclastic shoals.The middle ramp facies consists of medium- to thick-bedded limestone bearing bioclasts and thick-bedded lime mudstone.This facies is characterized by skeletal grains of crinoids，bryozoans and small foraminiferas，but is lack of the hummocky cross-stratification.The outer ramp facies consists of marlstone interbedded with thin-bedded lime mudstone bearing bioclastic grains of thin-shelled brachiopods.Basinal facies is characterized by dark-grey marlstone intercalated with black calcareous shales.These features suggest a low energy protected carbonate ramp(or shelf)environment.Three third-order sequences(S1，S2 and S3)were identified in the Qixia and Lower Maokou Formations，each spanning about 2 Ma.These sequences are dominantly composed of the lower transgressive and upper highstand systems tracts.The basal sequence S1 was deposited during the initial transgression of Qixia Age，during which the carbonate ramp was established，developing intrashelf basin in the western Hubei area.Then an overall mild shallow carbonate ramp(or shelf)environment predominated over as the intrashelf basin was filled up during S2 deposition.Sequence S3 was deposited during the largest-scale transgression in the Late Paleozoic，resulting in an apparent deepening in water depth.The deep outer ramp was then established once again and expanded in a mushroom-shaped area in Sangzhi-Changyang-Jingzhou-Changde area.

Key words: lithofacies palaeogeography, sequence stratigraphy, carbonate rock ramp, Qixia Age, western Hubei-northwestern Hunan

摘要： 对鄂西—湘西北地区多个沉积剖面的地层及沉积相进行了详细分析，结果表明，该区二叠纪栖霞期至茅口初期主要为内克拉通碳酸盐岩缓坡环境，发育内缓坡相、中缓坡相、外缓坡相和盆地相。内缓坡相以厚层至块状生物碎屑石灰岩为主，生物颗粒以绿藻和底栖有孔虫为主，缺乏高能沉积的生物颗粒。中缓坡相以中厚层含生物碎屑颗粒石灰岩以及厚层灰泥石灰岩为主，生物颗粒以海百合、苔藓虫和小型有孔虫为主，缺乏指示风暴沉积的丘状层理。外缓坡相以深灰色泥质灰岩与薄层灰泥石灰岩互层为主，生物颗粒主要为薄壳腕足类。盆地相以深灰色泥质灰岩夹钙质页岩为主。碳酸盐岩缓坡为低能受保护的沉积环境。栖霞组至茅口组下部可以分为3个三级层序，从下至上分别为层序S1、S2和S3。每个三级层序平均时限为2 Ma，层序内发育海侵体系域和高位体系域。层序S1沉积时期是栖霞期海侵的开始，形成碳酸盐岩缓坡环境，在鄂西地区形成较深水的局限台内盆地。S2时期海侵强度降低，“碳酸盐工厂”逐渐建立，将原先的台内盆地填平，研究区整体为中缓坡环境。S3时期发生晚古生代最大的海侵，研究区水体明显变深，在桑植—长阳—荆州—常德一带形成一个蘑菇状的较深水外缓坡环境。

关键词: 岩相古地理, 层序地层, 碳酸盐岩缓坡, 栖霞期, 鄂西—湘西北

CLC Number: 

• P531