宜昌奥陶纪的古环境演变

<正> 宜昌黄花场的奥陶系剖面,1972年以来逐渐作为我国研究奥陶系的标准剖面。宜昌王家湾的奥陶系—志留系界线剖面在国际上也被认为是界线对比的标准。本文仅就古环境方面的研究进行初步探讨,研究的标本(编号ACC)采用穆恩之等1972年测制的剖面。

Ordovician palaeoenvironmental reconstruction of Yichang area, W. Hubei

The Ordovician faunal and depositional characters indicate that the Nantsinkuan and Fen-hsiang Formations (Tremadocian) are mainly high subtidal with partially intertidal deposits; the Hunghuayuan and Dawan Formations (Arenigian) are from high subtidal to low subtidal deposits; the Kuniutan, Miaopo, Pagoda, Linh siang, and Wufeng Formations are all subtidal deposits. The Dawan Formation and the Miaopo Formation indicate the Arenigian and Ca-radocian sea transgressions and the top of the Kuniutan Formation as well as the top of the Wufeng Formation (the Dalmanitina-Hirnantia bed) indicate the sea regressions. These two transgressions and two regressions are coincided with the eustatic sea level changes. The Tremadocian and Arenigian trensgressions increased and brought the Kiaerograptus and the Glyptog-raptus austrodentatus graptolite faunas to the Yangtze Platform.The secondary dolomitization during the early Tremadocian (the Nantsinkuan Formation) is discussed. The so-called cracked Pagoda limestone is considered to be and under-seawater shrinkage limestone. The nodular limestone of the Linhsiang Formation was formed when the under-seawater shrinkage highly developed.