陕南勉县寒武系仙女洞组生物礁岩相学及古环境分析

Petrographic Features and Paleoenvironmental Significance of the Lower Cambrian Reef in the Xiannüdong Formation,Mian County,Southern Shaanxi

【目的】生物礁演化史上一个关键节点是寒武纪早期广泛出现了后生动物古杯与微生物群落的联合造礁,该造礁群落形成环境和发育条件值得探讨。【方法】以华南板块西北缘汉南—米仓山地区的勉县大河坝剖面为例,对寒武系仙女洞组沉积序列和生物礁临近层位开展了系统的岩相学、沉积学和元素地球化学分析。【结果】岩相学特征显示礁体形成于海退背景下的临滨带环境,建造过程可依次分为钙质微生物主导、古杯动物大量繁盛以及古杯与钙质微生物联合建造三个阶段。生物礁灰岩中的陆源碎屑组分含量向上增多,且颗粒变粗,顶部被中—细砂岩覆盖。【结论】古杯—钙质微生物礁生长过程中对低强度的陆源碎屑输入具有一定的耐受度,当粗粒陆源碎屑组分的持续、高强度输入时将造成生物礁发育终止。本实例揭示了古杯—钙质微生物造礁群落生长过程与海底生态指标的相关性,可为古环境学、沉积学和储层地质学相关研究提供参考。

[Objective]A key point in the evolutionary history of reefs is the widespread occurrence of associated archaeocyath-calcimicrobial communities in the Early Cambrian.The developmental environments and conditions for these reef-building communities deserve more attention.[Methods]We present a case study of the Daheba section in Mian county,located in the Hanzhong-Micangshan area along the northwestern margin of the South China Block.In this region,we conducted comprehensive petrographic,sedimentological,and geochemical analysis of sedimentary successions to investigate reef development in turbid environments.[Results]A detailed petrographic study indicates that the Daheba reef was formed in a near-shore,high-energy condition.Calcified microfossils,archaeocyaths and archaeocyath-calcimicrobial communities dominated reef construction from the early to late phases of reef growth.Small amounts of terrigenous particles were found within the inner reef framework,increasing from the base to the top.[Conclusions]This indicates that the reef was tolerant of a small input of terrigenous silts and fine sands during growth.These terrigenous materials material may have provided a source of nutrients and facilitated reef growth;however,an excess of siliciclastics would have abraded reef-building organisms and blocked their feeding passages,perhaps even buried the reef architecture completely.This study has clarified the growth process and environment of an Early Cambrian archaeocyathan-calcimicrobial reef in China,and may provide a reference for further study of Early Cambrian reef systems in paleoenvironmental reconstruction,sedimentology and petroleum geology.