四川广元清风峡剖面上泥盆统风暴沉积特征及其古环境意义*

泥盆纪吉维特期—弗拉期(G-F)之交的环境突变拉开了晚泥盆世生物大灭绝的序幕。然而,不同学者对于该时期环境变化的认识仍存在争议。为了解泥盆纪G-F之交的环境变化特征,通过对四川广元清风峡剖面泥盆系观雾山组中6套风暴岩的详细沉积学研究,识别出了典型的风暴沉积标志,如底面侵蚀构造、粗粒滞留沉积、粒序层理、丘状交错层理等。根据风暴沉积构造的组合特征,建立了3种近源风暴沉积序列: 序列Ⅰ,底部为具侵蚀构造的砾屑灰岩,顶部为块状泥晶灰岩;序列Ⅱ,底部为具侵蚀构造的砾屑灰岩,顶部为具粒序层理的砂屑灰岩;序列Ⅲ,底部为具侵蚀构造的砾屑灰岩,中部为具粒序层理的介壳灰岩,上部为具丘状交错层理的颗粒泥晶灰岩,顶部为具波状层理和水平层理的泥晶灰岩。牙形石生物组合分析显示,风暴岩形成于晚泥盆世弗拉期初期。基于现代风暴成因分析,认为弗拉期初期全球低纬度地区广泛分布的风暴岩与气候变暖和快速海侵密切相关。研究区风暴岩的发现对认识上扬子地区晚泥盆世沉积环境、古地理、古气候均具有重要意义。

Sedimentary characteristics and its palaeoenvironmental significance of the Upper Devonian storm deposits at Qingfengxia section in Guangyuan City,Sichuan Province

It is well known that environmental changes during the Devonian Givetian-Frasnian(G-F) transition initiated the beginning of the Late Devonian extinction. However,it is still controversial as to the understanding of environmental changes in this period. For that,this study conducted detailed sedimentological study on six sets of tempestites in the Devonian Guanwushan Formation at the Qingfengxia section in Guangyuan City,Sichuan Province,China. Some typical storm depositional indicators,including erosional structures,coarse lag deposits,graded bedding,and hummocky cross-stratification were identified. Three proximal storm sedimentary sequences have been established based on the combined characteristics of storm sedimentary structures in the Guanwushan Formation. Storm sequence Ⅰ consists of calcirudites with erosional surface at the bottom and homogeneous mudstones at the top. Storm sequence Ⅱ is characterized by calcirudites with erosional surface at the bottom and calcarenites with graded bedding at the top. Storm sequence Ⅲ is composed of calcirudites with erosional surface at the bottom,coquinas with graded bedding in the middle interval,hummocky cross-stratificated wackestones in the upper part,and mudstones with wavy and horizontal beddings at the top. The tempestites were formed in the early Frasnian age according to the biochronological analysis of conodont assemblages. Based on the analysis of origin of modern storms,the widely distributed tempestites in early Frasnian age at the low-latitude areas might have resulted from climate warming and rapid transgression. Discovery of tempestites in the study area is of great significance in understanding of the Late Devonian sedimentary environment, palaeogeography,and palaeoclimate in the upper Yangtze region.