混合沉积特征精细刻画:以江苏徐州寒武系馒头组为例

Datailed characterization of mixed sedimentation:a case study of the Cambrian Mantou Formation in Xuzhou,Jiangsu Province

江苏徐州地区寒武系馒头组发育极具特色的陆源碎屑组分与碳酸盐组分混合沉积现象。本研究以寒武系馒头组一套典型的混合沉积序列为例,依据剖面实测与镜下特征,对该序列中的混合沉积组合类型开展了系统的岩石学研究;通过分析不同沉积环境中的混合沉积特征,建立了陆表海近岸背景下陆源碎屑组分与碳酸盐组分混合沉积模式,探讨了各种混合沉积组合类型的混积成因类型、混积过程及主控因素等。结果表明:徐州地区寒武系馒头组混合沉积序列中主要发育含砂鲕粒灰岩、含内碎屑砂质灰岩、灰质细砂岩、含灰粉砂岩等11种混合沉积组合类型;混合沉积序列整体为陆表海海侵背景下次级海平面频繁波动且伴随多期次风暴作用的产物,主要表现为海侵作用下由近岸的混积潮坪逐渐演化为混积局限台地;混合沉积主要为相混合与间断混合,局部发育原地混合与母源混合。气候、陆源输入程度、海平面变化、碳酸盐产率是形成相混合的主控因素,同时也受到生物活动、水动力条件的调控;多期次风暴作用在主导间断混合形成的同时,一定程度上会促进相混合与母源混合的发育;在各因素叠加影响下,研究区具有多种混积成因类型组合发育的特征。厘清各种混积成因类型的分布与混积过程可为华北板块寒武纪古环境恢复与沉积演化提供参考,并为“深时”陆表海台地混合沉积相关研究提供典型范例。

The Cambrian Mantou Formation in the Xuzhou area of Jiangsu Province is characterized by mixed siliciclastic-carbonate sedimentation.In this study,a set of typical mixed siliciclastic-carbonate sequences from the Mantou Formation were systematically analyzed for their petrological characteristics based on section and microscopic features.By analyzing the mixed sedimentation characteristics in various sedimentary environments,a mixed siliciclastic-carbonate sedimentation model was established in the epicontinental sea environment.The different types of mixing,mixing processes,and controlling factors of various mixed siliciclastic-carbonate rocks were investigated.Research has shown that there are mainly 11 types of mixed sedimentary combination types in the sequence,including sandy-bearing oolitic limestone,sandy limestone with intraclast,fine-grained limy sandstone,and limy-bearing siltstone.The overall sequence is a result of frequent fluctuations in secondary sea-level variation,accompanied by multiple periods of storm activity,within the context of epicontinental transgression.It mainly evolves from an inshore mixed carbonate-siliciclastic tidal flat to a mixed restricted platform.The sedimentation in the study area consists of facies mixing and punctuated mixing,with localized occurrences of in situ mixing and source mixing.Climate,terrestrial input,sea level changes,and carbonate yield rate serve as the primary factors influencing facies mixing,with additional influences from biological activities and hydrodynamic conditions.The multi-stage storm activity can dominate the formation of punctuated mixing,and promote the development of facies mixing and source mixing to some extent.Under the superimposed influence of various factors,the study area is characterized by the development of a combination of multiple mixing types.Clarifying the distribution of various mixing types and mixing process can provide reference for the restoration of the Cambrian paleoenvironment and sedimentary evolution of the North China Plate.It also offers typical examples for the study of mixed sedimentation in the‘deep time'epeiric platform.