Study on the Micritization of Carbonate Grains By Bacteris and Algae-the Example of the Early Carboniferous in Baoshan, Western Yunnan

Key words: 
• Carbonate grain /
• Ooid /
• Puncture of becteria and algae /
• Micritization /
• Sedmentary environment /
• Diagenesis

Abstract: The sedimentary environment in Baoshan, western Yunnan was a shallow-sea carbonate platform in the Early Carboniferous, in which carbonate grains were generally micritized. The micritized grains are mainly colds, crinoides and brachiopoda, as well as a little calcirude and oncoid. The micritization was resulted by the puncture of bacteria and algae, and generally developed towards the center. However, some micritization was only in the central hole and joint of crinoide stalks. The micritization in a single grain shows periodicity. According to the shape and degree of micritization, it can be devided into 4 stages with 6 types. preP8ring stage-puncture of bacteria and algae, occurring stage-micritic pit. micritic net and radiation lines, developing stagemicritic envelope and maturating sarge-casting mould of micrtization. After puncture, the ways of micritization were not only biochemical precipituion (i. e. filling), but also biomechanical fragmentation and biochemical erosion(i. e. soilization occuring on the ocean bottom).The micritized grains of the Lower in Baoshan, Western Yunnan Chrboniferous occured mainly in shoal, secondly in platform, A few grains which belong to allochthonous deposits can also be found from platform-basin. Generally types of micritized grains are abundant and widely developed. The micritized rocks are mainly spar-oolite limestone and spar-allochemical limestone. Abundance of mieritized grains, especially micritic envelopes and casting mould can be considered as marks of shoal; The micritized grains gradually decrease from the central of the shoal; The degree of micritized depth shows an inverse ratio to the velocity of dePOsition: The homogeneity of micritization shows a direct ratio to the seawater turbulence, which is the rolling time of the grains. The complexity of the combination types of micritization was related to storm sedimentation.Corrosion holes and negative colds are the mark of an exposure environment, i. e. leaching mark of fresh water. The grain micritization would speed up the mechanical fragmentation by seawater and the buried diagenesis by compaction to fine carbonate grains and to produce intraclast and micrite. Concentric colds could be transformed into radiant ooids and pseudooids by micritization.