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

Key words: 
• geomicrobiology /
• carbonate micro-shapes /
• Zoophycos /
• Taiyuan Formation /
• North China

Abstract: 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.