下扬子地区幕府山组古环境和构造背景：来自细粒混积沉积岩系元素地球化学的证据

扬子地区寒武纪纽芬兰世广泛发育富含有机质的黑色细粒混积沉积岩系,开展细粒混积沉积岩的元素地球化学研究,对于恢复沉积期物质来源、源区构造背景、古气候、沉积环境及演化,以及泥页岩储层评价和页岩气勘探具有重要理论和现实意义.本文利用下扬子地区最新完钻的XYZ-1钻井资料,综合钻井岩芯、岩石薄片、XRD分析、ICP-MS分析等手段,开展寒武系纽芬兰统幕府山组细粒混积沉积岩系的地球化学研究.结果表明,幕府山组为一套形成于局限海湾—碳酸盐岩台地—潮坪体系的细粒混积沉积岩系,岩性以深灰色、黑色(含钙质碳质)泥岩、浅灰色泥质灰岩为主,夹云灰岩、泥晶灰岩、白云岩、含陆屑碳质灰岩、安山岩和角砾岩.幕府山组沉积期,具有干旱炎热-温暖潮湿交替的古气候,总体指示正常盐度的浅水-较深水古海洋环境,垂向上显示从贫氧—缺氧到滞留硫化环境的演变.幕府山组细粒混积岩物源来自江南造山带和南京台地,源区岩石未曾经历沉积再循环作用,形成于大陆岛弧和活动大陆边缘构造背景.具有相对高海平面的近岸局限海湾、半封闭滞留还原环境、较高的有机碳含量和古生产力水平,是形成富有机质黑色细粒混积沉积岩系的有利条件.

Paleoenvironment and tectonic background of Mufushan Formation in Lower Yangtze area: evidence from geochemistry of fine- grained mixed- siliciclastic- calcareous deposits

The black fine- grained mixed- siliciclastic- calcareous deposits series rich in organic matter are developed extensively in the Yangtze region in early Cambrian. The element geochemical study of the fine- grained mixed deposits series is of great theoretical and practical significance for restoring the provenance in depositional stage, background of source area, palaeoclimate, sedimentary environment and evolution, as well as the reservoir evaluation of the shale and shale gas exploration. Using the newly drilled XYZ- 1 well data in the Lower Yangtze area, based on systematic coring identification of 442. 5 m of Lower Cambrian Mufushan formation, rock thin section, XRD (X- Ray Diffraction), and ICP- MS analysis, the geochemistry of the fine- grained deposits series in Mufushan formation in the early Cambrian is studied in this study. The results show that the lithologic composition of the Mufushan formation is dominated by dark, black calcareous- carbonaceous mudstone, light gray argillaceous limestone, interbeded with dolomitic limestone, cryptite, dolomite, terrigenous carbonaceous limestone, andesite, breccia, etc., mainly composed of siliceous clay- mixed shale facies and siliceous clay- like shale facies, which formed in environment of limited gulf- carbonate platform- tidal systems. During the sedimentary period of the Mufushan formation, there was an alternate paleoclimate of arid- hot and warm- humid, shallow water and relatively deep water paleoceanographic environment indicating normal salinity in general, and showed the vertical direction evolution from oxygen- hypoxia to retained sulfide environment. The provenance of fine grain deposits of the Mufushan formation is derived from the Jiangnan orogenic belt and the Nanjing platform. The sediments in the source area have not undergone sedimentary recirculation and were formed in the tectonic setting of the continental island arc and active continental margin. The offshore confined bay with relatively high sea level, semi- enclosed and reductive environment, and richer TOC content and higher ancient productivity are favorable conditions for the formation of black fine grain mixed sedimentary rock series rich in organic matter.