渝东南-黔北地区赫南特期冰川事件的沉积响应

奥陶纪末的赫南特期爆发全球冰川事件,渝东南-黔北地区赫南特期沉积的观音桥组,从岩相、古生物组合、古环境等方面表现出该地区对赫南特期冰川事件的响应。通过对观音桥组沉积特征的分析,发现其主要分为3类:一类为颗粒灰岩,细分为泥晶生屑砂屑灰岩和亮晶鲕粒灰岩、亮晶砂屑灰岩等,含珊瑚等化石,代表了中高能量的浅滩环境;一类主要为粉砂质灰岩、粉砂质泥灰岩组合,含腕足类及三叶虫动物群,沉积环境为潮坪。这两类是近岸沉积,主要受相对海平面下降的影响。另一类为形成于浅海陆棚环境的混合岩相,包括泥灰岩与钙质泥岩组合、粉砂质泥岩与泥质粉砂岩组合及含炭泥灰岩;生物包括三叶虫、笔石及腕足等,尤其是标志性冷水型Hirnantia动物群的短暂繁盛,表明其受海水温度变凉的影响较大。潮坪靠近黔中古陆分布,局部发育浅滩,远离古陆变为浅海陆棚环境。总体上,观音桥组沉积环境比五峰组与龙马溪组黑色笔石相页岩的缺氧滞留深水陆棚环境更浅、相对海平面及水温更低,是赫南特期冰川事件直接影响的结果。

The Sedimentary Response to Gondwana Glaciation in Hirnantian (Ordovician) of the Eastern Chongqing and the Northern Guizhou Region,South China

The global glaciation happened in Hirnantian of the Late Ordovician, the lithofacies, biologic facies and sedimentary environment of Hirnantian Kuanyinchiao Formation in the eastern Chongqing and northern Guizhou region, were the response to the glacial event. There were 3 types of lithofacies as a whole. The first type was composed of grainstone with coral. could be subdivided into bioclast limestone, calcarenite, oolitic sparite limestone and spararenite. Which formed in the shoal with medium to high energy. The major components of the second one were the silty limestone and silty argillaceous limestone that formed in tidal flat, where the plentiful brachiopoda and a few trilobites lived. These two types of lithofacies were both the inshore carbonate sediments resulted from sea level drop. The associations of marlstone and calcareous shale, argillaceous siltstone and silty mudstone, and the carbonous marlstone, constituted the last type. And its mixed biofacies containing brachiopoda, trilobites, graptolite, especially the bloomed Hirnantia fauna, indicated the cold water environment and temperature drop of seawater in Principal Glacial Age (Hirnantian). The tidal flat distributed near the Qianzhong(Central Guizhou)Uplift, and the shoal also occupied local area upon the tidal flat, and shallow shelf distributed far away from uplift. As a whole, the sedimentary environment of Kuanyinqiao Formation was shallower than the black shales of Wufeng Formation and Longmaxi Formation formed in anoxic deep-water shelf, the sea-level and seawater temperature was much lower in Hirnantian. All of which were the results influenced by the glacier events in Hirnantian.