上奥陶统五峰组海底麻坑沉积的首次识别及其地质意义

First identification and its geological significance of seafloor pockmarks sediments in the Upper Ordovician Wufeng Formation in China

尽管海底麻坑普遍发育在现代海洋的多种环境,但关于深时、特别是古生代麻坑沉积的研究却极为罕见,研究者对其特征、成因、鉴别标志所知甚少。本次研究在重庆石柱地区柑子坪及歇步头剖面识别了发育在上奥陶统五峰组暗色细粒岩中的海底麻坑沉积,在野外观察、采样的基础上,利用显微薄片及扫描电镜观察了其微观特征,并测试了麻坑与围岩的主量元素和微量元素。研究显示麻坑沉积总体呈楔状或顶平底凹的透镜状,麻坑内岩层紊乱并发育上拱的岩层,被硅质脉分割的角砾普遍发育,麻坑沉积中常见重晶石、沥青及赤铁矿,主量及稀土元素测试结果显示麻坑的形成可能与热液及冷泉无关,推测五峰组自生的低熟油气是麻坑形成的基础。总结认为深时麻坑的识别标志有:地震剖面的负地形形态、碳同位素负偏明显的不规则形态的碳酸盐岩岩体、楔状或透镜状宏观形态、麻坑内杂乱的沉积填充、局部地层的上拱形态及古生物化石显示的沉积间断或时序混乱等。五峰组海底麻坑沉积具岩石学、地层学、古环境及油气地质学意义。

Seafloor pockmarks are commonly seen in various marine environments.The study on the pockmark sediments,however,have been hardly involved in the literature,causing few knowledges of their characteristics,origin and identification.In this study,we recognized the pockmark sediments from the Paleozoic fine-grained rocks for the first time in China.Based on the field observations and sampling of Upper Ordovician Wufeng Formation at Ganziping and Xiebutou in Shizhu,Chongqing,we tested pockmark rocks with microscope,scanning electron microscope(SEM),and major and trace element analyses.We found the bed chaos and arched beds in the lentoid or cuneal pockmark sediment body.Breccia carved by siliceous veins,are commonly seen,together with asphalt and barite,hematite minerals in the pockmark sediments.Observations and results of major element and REE suggest that the formation of pockmarks were unrelated with hydrothermal liquid and cold seeps.All the geological facts indicate the low-matured oil and gas generated from the Wufeng Formation as the first essential for the formation of pockmarks.The identification marks of deep-time pockmarks are thought as:(ⅰ) negative relief in seismic sections,(ⅱ) lentoid or cuneal sediment body,(ⅲ) chaos beds,(ⅳ) arched beds,and(ⅴ) hiatus and temporal chaos of sediments.Meanwhile,petrological,stratigraphical,paleoenvironmental and petrogeological significances of the pockmarks in the Wufeng Formation are discussed in this study.