从生物化石组合特征剖析黑色页岩的沉积环境——以鄂西长阳地区寒武系牛蹄塘组为例

A sedimentary environment analysis of black shales based on fossil assemblage characteristics:A case study of Cambrian Niutitang Formation in Changyang area, western Hubei

寒武系地层在鄂西地区广泛分布,湖北省长阳县天柱山以黑色炭质页岩为主要特征的寒武系牛蹄塘组下部黑色页岩中保存有丰富的高肌虫(Sunella)、大型双瓣壳节肢动物(Perspicaris)、宏体藻类、软舌螺类和可疑的海绵类((?)Sinospongia),同时,见有零星分布的黄铁矿。通过对生物化石古生态及保存情况的研究,认为鄂西长阳牛蹄塘组沉积早期宏体生物生活于海底表层贫氧、水体上层充氧、透光性较好的、较为平静的浅海环境,且海底处于氧化-还原界面附近。由于页(泥)岩沉积时具有较高水分,在快速沉积背景下将死亡生物躯体埋藏,伴随着沉积的不断进行,沉积水与外界水体交换量逐渐减少,加之生物腐烂产生的H2S气体,形成了沉积物内部的强还原环境,有机质得以保存,最终形成黑色页(泥)岩。

The Cambrian strata are widely distributed in western Hubei area. Abundant Sunella （Bradoriida）, Perspicaris （large bivalved arthropods）, macroscopic algal, hyolithes and （？）Sinospongia （Suspicious sponges） have been found in the lower black carbonaceous shale of the Cambrian Niutitang Formation from Tianzhu area of Changyang County, Hubei Province. In addition, scattered pyrite is seen in black shale. Based on a detailed study of palaeoecology and the characteristics of fossils preservation, the authors have reached the conclusion that, during the early sedimentary stage of Niutitang Formation in Changyang County of western Hubei Province, macro- organism lived in an environment of shallow water which was suboxic at the bottom, oxic in the upper part characterized by good light transmission and relative calmness, and that the sea bottom surface was located nearby the redox interface. Besides, under the background of rapid deposition, large amounts of water and dead biological-bodies were embedded in shale, the amount of water exchange between the depositional water and the external water decreased gradually, and H2S gas was generated by rotton organism in the process of deposition, forming a strong reduction environment in sediments, so that organic matter could be preserved, which eventually formed black shale.