寒武系第二统浅水混合沉积发育特征及影响因素——以陕南朱家坝剖面仙女洞组为例

Factors Influencing the Composition of Shallow-Water Mixed Siliciclastic-Carbonate Sedimentation from the Cambrian Series 2:A case study on the Xiannüdong Formation of the Zhujiaba section(southern Shaanxi)

【目的】混积岩同时具有指示水体化学条件和生物活动的碳酸盐组分以及指示物源区性质和风化过程的碎屑组分,因而在古环境和古气候研究方面具有独特价值。【方法】寒武纪早期上扬子克拉通北部汉南—米仓山古陆周缘混合沉积非常发育,以陕西南郑朱家坝剖面仙女洞组为例,对研究区混积岩特征开展了系统的岩相学和沉积学研究。【结果与结论】混合沉积形成于海退背景下由中缓坡向内缓坡转变的过渡环境,且混积程度具有规律性,即在多个向上变浅序列的中部最为发育,在每个序列底部和顶部分别为碎屑组分主导和碳酸盐组分主导(或清水碳酸盐沉积)。中缓坡环境下以碎屑组分沉积为主,碳酸盐组分除小型原地生物礁外,主要来自浅水环境搬运的细粒沉积物;内缓坡潮下带相对低能环境仍以碎屑组分沉积为主,但原地生物礁以及礁角砾和内碎屑搬运沉积常见;内缓坡潮下高能带以鲕粒滩和生物礁等原地碳酸盐沉积为主,混入的陆源组分主要为砂级石英和岩屑颗粒;潟湖环境以白云石化颗粒和陆源细粒物质的混合为特征。研究区寒武纪早期混积体系的发育与当时浅水环境较高的碳酸盐产率以及沿岸流、波浪、风暴等搬运作用有关。本实例可为了解深时混合沉积成因提供一个思路,也可为上扬子地区寒武系混积岩油气勘探提供参考。

[Objective]The mixed siliciclastic-carbonate sediments(MSSs)contain both carbonate and detrital components that can concurrently carry aqueous and provenance signatures in their compositions.During the early Cambrian,the MSSs were widely distributed in the Hannan-Micangshan area,western Yangtze Craton,but the factors influencing their development have not been well known.[Methods]In this study,we performed a combined petrological and sedimentological analysis on the compositions of MSSs in the Xiannüdong Formation of the Zhujiaba section,southern Shaanxi.[Results and Conclusions]The results indicate that MSSs formed from the middle to the inner ramp settings during a regressive depositional cycle,and the highest level of admixture is generally distributed in the middle part of each upward-shoaling sequence compared to that of the lower and upper parts.Fine-grained detrital sediments and low-relief reefs developed in the middle ramp environment,and other carbonate fractions were mainly transported from shallow-water carbonate depositional settings.In contrast,ooid shoals and reefs developed in inner ramp environments with a small quantality of terrigenous sand-sized particles(quartz and lithic fragment).In addition,the petrological evidence indicates that transported(dolomitized)carbonate grains and fine-grained detritus were concurrently mixed in the lagoon setting.The development of early Cambrian MSSs in the study area may be related to the high carbonate productivity of shallow-water environments,as well as the active onshore-to-offshore transportation mainly induced by storm currents.This study may provide a case for understanding the factors influencing the development of deep-time MSSs and a reference for oil and gas exploration of Cambrian MSSs in the western Yangtze area.