广西六景泥盆系弗拉斯阶-法门阶界线层牙形石生物地层及碳同位素组成

广西六景泥盆系剖面是中国泥盆系标准剖面之一,通过对其弗拉斯阶-法门阶(F-F)界线附近地层进行详细的牙形石生物地层研究,自下而上识别出3个牙形石带:晚rhenana带、linguiformis带和triangularis带。F-F界线位于融县组下部(第7号层与第8号层之间),在谷闭组顶界之上3.32,m处。碳同位素的分析结果表明,F-F之交δ13C具有显著正偏移,增幅为2.0‰,与湖南老江冲、广西垌村和杨堤以及欧洲、美洲、非洲和澳洲等地的F-F界线附近的碳同位素记录一致,且具有相近的变化幅度。δ13C正异常与F-F界线上的生物灭绝有关,由于食微生物的高等生物灭绝,微生物大量繁盛,诱导海水缺氧,导致海洋有机碳埋藏速率增加,从而形成δ13C的正偏。F-F界线层发育一套以碎屑灰岩为特色的事件沉积,该事件沉积在广西乃至全球具有等时性,可能与小行星碰撞地球引起的全球性海啸有关。

Conodont biostratigraphy and carbon isotopic composition across the Frasnian-Famennian boundary in Liujing, Guangxi

The Liujing section(Guangxi, South China) is one of the Devonian standard sections in China. The conodont biostratigraphy of the upper part of the Frasnian to the lower part of the Famennian was studied in detail in this paper, and three conodont zones were recognized in ascending order: Late thenana Zone,linguiformis Zone,and triangularis Zone. The F-F boundary lies between the 7th layer and the 8th layer in the lower Rongxian Formation,3.32 m above the top of the Gubi Formation. The carbon isotopic result shows the positive excursion in δ~(13)C during the F-F transition, and the amplitude is 2. 0‰.Compared with other records from different palaeogeographic units in the world, the F-F carbon isotope excursion occurring in transition sequences has comparable shapes and amplitudes. δ~(13)C positive anomaly is related with the mass extinction of F-F boundary. The extinct microbes feeders would have stimulated blooms of microbes, in turn, this scenano would have created anoxic sea waters and subsequendy enhanced the burial rate of organic carbon which led to the positive δ~(13)C excursion. A suite of event deposits occurred near the Frasnian-Famennian boundary. The event deposits are widely developed in South China and other areas of the world. The event deposits could be caused by a violent tsunami related to bolide impacts into ocean.