中上扬子晚奥陶世赫南特期岩相古地理

Lithofacies palaeogeography of the Late Ordovician Hirnantian in the middle-upper Yangtze region of China

晚奥陶世赫南特期发生了全球性的气候变冷事件。中上扬子地区典型沉积为观音桥组,富产全球广泛分布的赫南特动物群,对应冰盖扩张引起的全球海平面下降阶段。根据岩性、沉积构造、生物组合及其保存状态,赫南特阶划分出潮坪相及浅海陆棚相。潮坪相主要沉积观音桥组炭泥质灰岩、钙质泥岩,底栖生物繁盛。浅海陆棚相主要沉积五峰组上部Normalograptus extraordinarius带黑色富硅质页岩、含放射虫硅质岩,并缺失观音桥组。研究表明,赫南特期延续了凯迪期晚期半局限海沉积格局。区域上,上扬子北部-中扬子大部地区、上扬子西部洪雅-康定等地继承了凯迪期晚期基座的浅海陆棚环境,仅有黄陵背斜-神农架、重庆城口、鄂东南宋溪等地随着海平面下降沉积了观音桥组壳相灰岩。川中古隆起-黔中古隆起-康滇古陆间的潮坪沉积区,观音桥组内碎屑浅滩沉积主要分布于黔中古隆起北侧边缘,而局部相对低洼的地带则缺失了观音桥组。赫南特末期海平面下降至最低位,推测湖北五峰、汉南古陆边缘、黄陵背斜-神农架、赣西北修水地区存在小范围的暴露区或剥蚀区。

The beginning of Hirnantian in Late Ordovician was followed by a global episode of Gondwana glaciation. Typical deposits were Guanyinqiao Formation deposits in the middle-upper Yangtze region of China during the period of the global sea-level fall caused by the ice-sheet growth. There, the worldwide spread Hirnantia-Dalmanitina fauna was abundant. According to lithologic characters, sedimentary structures, and biological assemblages and their preserved states, Hirnantian can be mainly classified as tidal flat facies and neritic shelf facies. There are several subfacies and microfacies in this study area. In the tidal flat facies, the Guanyinqiao Formation is mainly composed of carbon-argillaceous limestone and calcareous mudstone, including benthos of brachiopodas and trilobites of Hirnantia-Dalmanitina fauna. In the neritic shelf facies, however, the Hirnantian stage is composed of black siliceous shale and radiolarian chert in Normalograptus extraordinarius zone in the upper part of the Wufeng Formation and the Guanyinqiao Formation is absent. The results show that the Hirnantian, in succession of the paleogeographic framework in the late Katian stage, also has the characteristics of the semi-restricted shallow sea. Regionally, it succeeds the environment of the neritic shelf facies in the late Katian stage in the north of the upper Yangtze and the most part of the middle Yangtze region, as well as the tidal flat facies among the middle Sichuan uplift, middle Guizhou uplift and Kangdian old land. In the neritic shelf facies, the Guanyinqiao Formation shelly facies limestone deposited only in the Huangling anticline-Shenlongjia in the middle Hubei, Chengkou in Chongqing and Songxi in the southeast of Hubei as sea level was falling. In the tidal flat facies, the intraclast beach deposits distributed mainly over the north margin of the middle Guizhou uplift, but those deposits were absent in some lower areas. Within the process of sea-level falling to the lowest, in the Wufeng of Hubei, the marginal areas of the Hannah old land, the Huangling anticline- Shenlongjia in the middle Hubei and Xiushui in the northwestern Jiangxi, it is speculated that exposed or eroded areas existed in short time.