华北寒武纪—奥陶纪豹皮状碳酸盐岩系生物扰动成因

华北地区寒武系和奥陶系发育了大量的生物扰动白云岩/生物扰动灰岩,选择两个代表性剖面——河南登封寒武系剖面和山西兴县奥陶系剖面进行碳酸盐岩系生物扰动成因研究。河南登封寒武系第二统朱砂洞组中的遗迹化石较单一,主要为Thalassinoides、含有少量Planolites;山西兴县中奥陶统马家沟组五5亚段中遗迹化石较丰富,识别出的遗迹化石共7个遗迹属,分别为Arenicolites、Cylindricum、Helminthopsis、Lorenzinia、Planolites、Teichichnus、Thalassinoides,其中Thalassinoides占主导地位。朱砂洞组主要为滨岸碳酸盐潮坪相,中奥陶统马家沟组五5亚段主要为局限台地相。通过野外地质考察及室内相关实验手段及方法,利用偏光显微镜、扫描电镜对生物潜穴及围岩进行了宏观和微观结构分析,发现生物潜穴内部主要为白云石,基质围岩主要为方解石,生物潜穴在野外露头上表现为颜色不一的斑块状。两个剖面大量发育遗迹化石Thalassinoides(海生迹),Thalassinoides的三维潜穴系统为白云石化流体提供了良好的运移通道,为回流渗透作用提供了有利条件,促进潜穴充填物白云石化,从而形成了大量豹皮状的生物扰动白云岩。

Formation Mechanism of Cambrian-Ordovician Bioturbated Dolomites in North China

A large number of bioturbated dolomites/bioturbated limestones were developed in the Cambrian and Ordovician in North China. Two representative stratal sections, Dengfeng (Cambrian) and Xingxian (Ordovician), were selected for study. Rocks are the cause of bioturbation. The trace fossils in the Cambrian series2 Zhushadong Formation in the Dengfeng Henan are relatively simple, mainly Thalassinoides along with a small amount of Planolites. Abundant fossil traces are present in submember 5₅ of the Middle Ordovician Majiagou Formation in the Xingxian section. Seven trace fossil ichnogenera were identified: Arenicolites, Cylindricum, Helminthopsis, Lorenzinia, Planolites, Teichichnus and Thalassinoides, of which Thalassinoides is dominant. The Lower Cambrian Zhushadong Formation from the Dengfeng section mainly occurs in the carbonate tidal flat and in the 5₅ submember of the Middle Ordovician Majiagou Formation from the Xingxian deposit in the carbonate platform. Field geological investigation and laboratory analysis by polarizing microscope and scanning electron microscope established the macro-and microstructure of the bioturbation in the surrounding rocks. Dolomite was predominant in the burrows, and calcite was the main material in the matrix. The burrows are seen in the outcrops as patches of different colors. The abundant three-dimensional burrowing system of Thalassinoides in both sections benefited the transport of dolomitization fluids and created favorable conditions for reflux osmosis, thus dolomitizing the burrow infill and forming a large number of bioturbated dolomite plaques.