湖南晚奥陶世五峰期的古地理

湖南晚奥陶世的沉积,由于受区域构造控制,其沉积类型计有地台型(湘西)北区);过渡型(湘中区);地槽型(湘南区)等三种。它们以江南列岛和白马山—龙山东西向构造带为分界线。五峰早期除了江华汝城隆起和江南列岛外均接受海侵,海水自北向南逐渐加深。在湘南沉积了巨厚的砂页岩复理石建造。在湘中、湘西北区沉积厚度逐渐变薄。生物群在湘南,湘中为纯笔石相,湘西北为以笔石为主有三叶虫和腕足类等构成的混合相。五峰晚期湖南境内大部分抬升为陆,仅在东安县苏水冲和桃源县九溪等地形成滞流海盆。

PALEOGEOGRAPHY OF HUNAN PROVINCE IN WUFENGIAN AGE OF LATE ORDOVICIAN

The Late Ordovician sedimentation in Hunan province, due to the influence of the regional structural pattern, has three sedimeatary types, i. e., platform type in Northwestern Hunan, the geosynclinal one in Southeastern Hunan and the transitional one in Central Hunan. Accordingly, three paleogeographie areas can be distinguished, which are Southern Hunan area, Central Hunan area and Northwestern Hunan area. Their boundaries are Baimasan-Longsan EW trend tectonic belt and Jiangnan archipelago seperately. The Southern Hunan area was epicontinental sea exclusive of the Jianhua-Ru-cheng uplift in the south end in early Wufengian. The sedimentary facies is Flysch facies which consists of sandstone intercalated with shale. The total thickness of the sediment reaches 1500-3000m. The paleobiologic feature is characterised by Graptolites facies(Southchina type); In the late Wufengian, the Jianhua-Rucheng uplift extends northwestward, the whole area became uplift except in Sushuichonh survived a sluggish sea, where sedimented a set of blackish-grcy thin-laminated organic shale intercalated with sandstone, 46. 3m thick, characterised paleobiologicaly by Graptolites. The Central Hunan area was a badly-fed epicontinental sluggish sea since Cambrian, where deposited a set of blackish thin-bedded siliceous rock and blackish thin-laminated siliceous shale, only 25-60m thick. The Graptolites are transitional type. The northwestern Hunan area was a neritic sea in Wufengian, in the early Wufengian, where developped blackish siliceous rock and carbonaceous-siliceous shale or sandy shale intercalated with siliceous rock, 0.7-28m thick, in which yields Centralchina type Graptolites; and in late Wufengian, where developped blackish carbonaceous shale intercalated with blackish-grey thin-laminated microlitic siliceous rock and yellowish-brown or light purplish-grey hydromica clay rocks in the top, 15m thick, in which yields Graptolites, Trilobites and Brachiopods.