浅谈华南上扬子区中奥陶统铁质鲕粒沉积

A BRIEF DISCUSSION ON THE MIDDLE ORDOVICIAN FERRUGINOUS OOIDAL DEPOSITS IN THE UPPER YANGTZE REGION,SOUTH CHINA

中奥陶统铁质鲕粒(简称铁鲕)沉积在华南上扬子区广泛分布,但迄今仅有零星报道,缺乏系统的研究。本文在前人工作的基础上,通过开展广泛的野外勘察和分析测试,全面阐述了这套铁鲕沉积的分布规律、发育特点及其控制因素与环境背景。该铁鲕沉积主要见于华南上扬子区中—上奥陶统华弹组、十字铺组和牯牛潭组,在川滇交界至川西南以鲕铁岩、砂页岩、灰岩形式产出,在滇东北—川南—黔北及陕南—川北多为灰岩,含铁矿物主要包括赤铁矿、鲕绿泥石、针铁矿等。铁鲕沉积的时代主要为达瑞威尔中—晚期,在多数地点发育1~2层,个别可见3层,可归属于两期。铁鲕的形成主要受沉积或早期成岩环境的影响或控制,如区域构造运动、古海洋水物理化学条件、微生物活动等。华南的铁质核形石、叠层石和海相红层等独特沉积记录同样发育在大致同期的台地上,共同构成一套含铁质“时征相”。进一步系统研究这套铁鲕沉积,将为全面认识铁质沉积、深入理解华南乃至全球奥陶纪古地理与古环境演变、探索地史时期重大事件等奠定基础。

The Middle Ordovician ferruginous ooidal deposits of South China received less attention,despite the widespread distribution in the Upper Yangtze region.In the present paper,their tempo-spatial distribution,developmental characteristics,controlling factors and depositional backgrounds are discussed based on literatures reviewing,field investigations and sample analyses.The Middle-Upper Ordovician Huadan,Shihtzupu and Kuniutan formations are major ferruginous ooids-bearing units in the Upper Yangtze region.The lithology is characterized by ooidal ironstones,and siliciclastic or carbonate rocks in southwestern Sichuan and its border area with Yunnan,and mostly the carbonate rocks in northeastern Yunnan,southern Sichuan,northern Guizhou,southern Shaanxi and northern Sichuan,respectively.The iron minerals of ferruginous ooids are mainly hematite,chamosite and goethite.The ferruginous ooidal deposits,occurring at one to two levels,or rarely three levels in the successions,are of two developing period with their age roughly falling the Middle-Late Darriwilian.Those deposits were mainly controlled by syn-sedimentary and/or early diagenetic processes,such as tectonic movements,paleo-oceanic water conditions and microbial activities.The coeval iron-rich microbialites(oncoids and stromatoids) and marine red beds are recorded in the platform environment of South China,in association with these ferruginous ooidal deposits as a set of “time-specific facies”.A further comprehensive study in future on these ferruginous ooidal deposits may contribute to the understanding of ferruginous deposits and Ordovician paleo-geography and paleo-environments,as well as providing new insights into the major events during geologic time.