广西来宾二叠系瓜德鲁普统-乐平统界线蓬莱滩剖面沉积微相演变及其对生物灭绝事件的响应

Sedimentary microfacies and sedimentary responses to the biotic extinction events within the Penglaitan section at the Guadalupian-Lepingian (Permian) boundary in Laibin, Guangxi

关键词:  广西来宾  沉积微相  中—晚二叠世  沉积响应  生物灭绝事件

Key words:  Laibin in Guangxi  sedimentary microfacies  Middle-Late Permian  sedimentary response  biotic extinction event

摘要:

在中二叠世（瓜德鲁普世）末期发生了一次全球范围的生物灭绝事件，大约有34%的海洋无脊椎动物消失，同时发生了全球规模的海退事件。该事件在中国反应为东吴运动，致使华南大部分地区瓜德鲁普统-乐平统之交产生不整合。广西来宾地区位于扬子地台南缘，在中-晚二叠世时期处于滇黔桂盆地的东部。由于其独特的古地理位置，来宾地区发育了连续的中-晚二叠世海相沉积，是研究此次生物大灭绝和环境演变的绝佳位置。本文对广西来宾蓬莱滩瓜德鲁普统-乐平统（G-L）界线剖面沉积微相和生物化石进行了综合研究。研究表明，该剖面1~7层共发育5种主要的沉积微相组合，既礁基相组合、礁核相组合、覆礁相组合、礁滩相组合和深水斜坡相组合，生物礁发育于一个海退序列中。生物碎屑的丰度和类型在7a层突然急剧降低，与该剖面碳同位素的负偏和汞异常的出现一致，但实际上在这一层位只有个别牙形石和菊石消失，生物屑丰度的剧变并不能代表灭绝线，而是对海平面剧变的沉积响应。

Abstract:

A global biotic extinction event once took place at the end of the Middle Permian (Guadalupian), and directly led to the disappearance of approximately 34% of marine invertebrates. The marine regression also occurred on a global scale during the periods. This event is responsible for the initiation of the Dongwu movement in China, which resulted in the widespread unconformities at the Guadalupian-Lepingian (Permian) boundary in most parts of China. Meanwhile, the Middle-Late Permian marine deposits were developed in the Laibin region located on the southern margin of the Yangtze platform. The emphasis in this study is placed on sedimentary microfacies and organic fossils in the Penglaitan section, a GSSP section in the eastern part of the Laibin geosyncline. Five types of sedimentary microfacies associations are recognized in the 1-7 beds of the section, including reef base, reef core, reef cap, shoal and deep-water slope microfacies associations. The abundance and diversity of fossil fragments suddenly decreased in Bed 7a, which agrees with the observations of negative carbon isotope excursion and large mercury content anomaly in the Penglaitan section. However, only a few of conodont and ammonoid species appear in this horizon. The authors contend that the sharp decrease of fossil fragments represent the sedimentary responses to sea-level changes rather than the biotic extinction horizon at the Guadalupian-Lepingian (Permian) boundary.