扬子区下、中奥陶统大湾组及其同期地层

THE LOWER TO MIDDLE ORDOVICIAN DAWAN FORMATION AND ITS COEVAL ROCKS IN THE YANGTZE REGION

华南扬子区不同地点奥陶纪的岩相、生物相变化显著。下、中奥陶统大湾组及其同期地层的总体走向为南西—北东向,横向上呈带状分布,从近岸到远岸呈渐变过渡。对四川长宁大官山组、贵州桐梓和沿河湄潭组以及湖北宜昌大湾组的比较研究发现,扬子区下、中奥陶统阿仑尼格期(Arenig)的地层按照其离岸的远近,在地层厚度、沉积相以及腕足动物分异度上呈现出规律性的变化。华南早、中奥陶世腕足动物的辐射首先发生在处于正常浅海底域环境的桐梓和沿河一带。

The Ordovician lithofacies and biofacies change greatly at different localities of the Yangtze Region, South China. The Lower to Middle Ordovician Dawan Formation （Arenig） and its coeval rocks are generally striking southwest-northeast, and are belted in horizontal distribution meanwhile transitional between the two neighboring rock units. Detailed correlation between the Daguanshan Formation （Changning of Sichuan）, the Meitan Formation （Tongzi and Yanhe of Guizhou） and the Dawan Formation （Yichang of Hubei） based on different graptolitic biozones from the Acrograptus filiformis Biozone to the Undulograptus austrodentatus Biozone, shows that the Arenigian rocks of Yangtze Region are regularly changing in thickness, sedimentological facies and the brachiopod diversity from nearshore to offshore localitites. The Early to Middle Ordovician brachiopod radiation of South China first happened within the normal marine shallow water benthic regimes, such as the area of Tongzi and Yanhe, northern and northeastern Guizhou Province.