扬子台地西南部奥陶系宝塔组底部含鲕绿泥石灰岩成因意义

对四川省兴文县上奥陶统宝塔组底部含鲕绿泥石灰岩开展成因研究,有助于了解扬子台地西南部同时期的沉积环境演化过程.通过沉积学分析,并辅助以电子探针和扫描电镜等矿物学研究方法,发现富鲕绿泥石鲕粒及球粒为同生期沉积物,共生的生物碎屑组合指示海水为氧化性水体.丰富的微生物相关组构表明同期微生物席发育,且其代谢活动可致使水岩界面附...

Chamosite-Ooidal Limestones at the Bottom of Ordovician Pagoda Formation in the Southwestern Yangtze Platform: Genesis and Paleoenvironmental Implications

A set of the chamosite-ooidal limestones was discovered at the bottom of the Pagoda Formation (Upper Ordovician) in Xingwen County, Sichuan Province, and its genesis can facilitate the understanding the evolution of the depositional environment in the southwestern Yangtze platform during the same period. Based on a detailed sedimentological analysis, this paper presents a study on the genesis of the chamosite-ooidal limestones, combined with mineralogical characterization via electron microprobe and scanning electron microscope studies. It is found that chamosite-rich ooids and peloids are syngenetic sediments, and symbiotic bioclastic assemblages indicate that the seawater was oxidizing. The abundant microbial-related fabric indicates that microbial mats develop during the same period, and their metabolic activities can lead to the formation of reducing water bodies near the water-sediment interface, which is a necessary condition for the formation of syngenetic chamosite. The Fe and Al elements needed for chamosite formation are derived from large-scale inputs of the early weathering crust during the transgression stage. The chamosite-ooidal limestones at the bottom of the Pagoda Formation are the concrete manifestation of global sea-level evolution in the Yangtze platform at same time. It is an important indicator of sea level fall in the Early Sandbian and rapid transgression in the Late Sandbian in the Yangtze platform.