华北下二叠统太原组Zoophycos潜穴中碳酸盐岩微形体的特征及其意义

Characteristics of Carbonate Micro-Shapes in Zoophycos Burrows from the Lower Permian Taiyuan Formation in North China and Their Significance

华北地区下二叠统太原组碳酸盐岩中的Zoophycos遗迹化石十分丰富,根据其潜穴充填物颜色的不同,可将太原组Zoophycos分为四种类型,即具黑色、灰色、灰白色、红色充填物的Zoophycos。本文选取河南焦作和山西附城太原组剖面中具有代表性的Zoophycos样品,利用扫描电子显微镜和能谱仪对其潜穴中的充填物进行了超微特征观察及元素组成测定,发现了大量成因与微生物相关的碳酸盐岩微形体(单体矿物和集合体),识别出球状(表面光滑的球状体、表面具有不定形细粒结构的球状体、表面呈刺状细粒结构的球状体和球状集合体)、杆状(刺杆状分枝形网状体、刺杆状单体、螺旋杆状单体、双列杆状体、光滑直线形杆状单体、末端膨大的光滑杆状体)、簇状、网状、瓶状和似脑球状等6种形态类型,其中多数显示出不同的微生物结构。这一发现表明,Zoophycos遗迹化石并非简单的觅食迹或食物储藏所,其造迹生物与微生物很可能存在互利共生关系。此次研究有助于探索地质微生物作用对地球化学过程的影响,并为精细分析该研究区太原组碳酸盐岩的形成过程及其沉积演化提供了微观资料。

The Lower Permian Taiyuan Formation in North China,well exposed in western Henan and south central of Shanxi province,consists of littoral,neritic,lagoon and tidal flat deposits in the epicontinental sea carbonate platform. It conformably contacts with both the overlaying Middle Permian Shanxi Formation and the underlaying Upper Carboniferous Benxi Formation. The lithology of the Taiyuan Formation mainly composed of thin bedded to middle bedded,grey to dark grey biological clastic limestone,sandstone,siltstone,carbonaceous shale and coal beds. Body fossils are abundant in every layer limestone of the Taiyuan Formation,such as fusulinida,brachiopoda,gastropods,coral,crinoid,and so on. The ichnofossil Zoophycos with high abundance are discovered in carbonate rocks of this stratum,which can be divided into four types-Zoophycos with black,grey,off-white or red fillings based on the different colors of burrow fillings,and other ichnofossils associated with Zoophycos commonly are Chondrites,Nereites,Palaeophycos,Rhizocorallium,Gordia,Taenidium,Teichichnus,Planolites and Thalassinoides. In this paper,representative Zoophycos specimens are chosen from Jiaozuo cross section,western Henan province and Fucheng cross section,southeastern Shanxi province. Specimens used to test are cleaned up with distilled water,and dried in a drying box,then fixed on sample holders with conducting resin,at last coated with gold in ion sputtering apparatus. After specimen preparation,a scanning electron microscopy( SEM,model number: JSM-6390 /LV) is used to observe morphology and structure of carbonate micro-shapes in specimens,and Energy Disperse Spectroscopy( EDS,model number: INCA-ENERGY 250) is applied to test their composition. Six kinds of carbonate micro-shapes( single mineral and aggregates) related to microbes are recognized by observing Zoophycos burrow fillings,which are spheroid-like,rhabditiform,areatus,net-like,vase-like and cerebrum-like,most of which show the different microbial structures.Based on spheroids' surface features,individual size and occurrence features,carbonate spheroids can be further divided into four types,including 1Spheroid-like carbonates with smooth surface( type 1),well-preserved,and most are 2 to 15 μm in diameter; 2Spheroid-like carbonates with unshaped surface and fine grain structure( type 2),the diameters of these spheroids range mostly from 2 to 10 μm,their outside surfaces show well-organized fine grain structure,but grain shapes are irregular; 3Spheroid-like carbonates with thorn surface and fine grain structure( type 3),the their diameters are 6 to 9 μm,their outside surfaces display well-organized fine grains; 4Spheroid-like carbonate aggregates( type 4) with smaller diameter,appearing in groups,and distributing intensively,embedded within calcite crystals. The spheroids present as monomer or multi-monomer in types 1-3,but occurred as groups in type 4. Based on surface features,individual shape and branch feature,carbonate micro-shape of rhabditiform can be further divided into six types,i.e.,1net-like rhabditiform with thorn surface( type 1),with obvious branches,showing an irregular network; 2rhabditiform with thorn surface( type 2),arranging in parallel to each other and appearing in groups;3rhabditiform with spiral form( type 3),showing as a spiral,and occurring separately; 4rhabditiform with biserial form( type 4),arranging closely with smooth surface and occurring separately; 5rhabditiform in a straight line with smooth surface( type 5),displaying round cross section and appearing dispersedly; 6rhabditiform with smooth surface and expanding tail end( type 6),with tail expanding spheres and appearring independently. Areatus carbonate aggregates appear as druse,and showing granular,needle-like,sheet structures,surrounded by well-crystallized calcite. The monomers of net-like carbonate aggregates varied in shape,which interweave and overlap each other,forming irregular networks. Vase-like carbonate micro-shapes like a vase or a calabash,existing in isolation. Cerebrum-like carbonate micro-shapes,with unshaped fine grain structures on their surface,their composition mainly composed of CaCO3. Above carbonate micro-shapes found in the Zoophycos burrow filling indicates that these burrows is not a simple grazing trace or food storehouse,and it is likely to exsit a set of mutually beneficial and symbiotic relationships between the Zoophycos-maker and microbes. Microcosmic analysis showing,a part of the carbonate micro-shapes found in the Taiyuan Formation contains microbial structures,however,which is not yet enough to determine what kind of microbe species they are. On condition that combining the study of geo-microbial fossils and molecular fossils,it will be helpful to determine biological classification units and ecological characteristics of geo-microbes. This research contributes to explain the relationship between macro-organism and micro-organism,and to explore how the geomicrobial processes affect geochemical processses,and to provide new microcosmic data for finely analyzing the formation process and sedimentary evolution of carbonatite in Taiyuan Formation of the study area.