Analysis Of Sedimentary Microfaces And Microfabics Of Mesoproterozoic Gaoyuzhuang Formation In Jixian,tianjin

In this paper, the Mesoproterozoic Gaoyuzhuang Formation(1.6-1.53Ga) cropped at Jixian, Tianjin was studied in terms of micro-facies analysis, with an emphasis on the fabrics of some special sediments. Based on the study of faci。es succession analyses in field and observation of the thin sections from 81 collected samples, three types of parasequences and 11 kinds of microfacies were identified in this formation. Using polarizing microscopy, field emission scanning tunneling microscopy and EDS spectrum analysis, this paper is focused on revealing the environmental conditions of these microfaces and the potential microbial impacts on their formation during early diagenesis.Thrombolites is one of the several microblites that were common and significant in Precambrian times, with their earliest records in Paleoproterozoic(ca 1.92Ga), when they were suggested to have been of inorganic origin predominantly. So far the earliest known biogenic thrombolites were those reported from the Wumishan formation. In the Wimishan Formation, most clots in thrombolites have their outer rings quite similar to the crystalline inorganic light cementation found in Archean. However, the emergence of thrombolites in Gaoyuzhuang may have different mechanisms. The interiors of the clots in the Gaoyuzhuang thrombolites are typically composed of mineralized microbial mats, which are commonly surrounded by aragonite fibers radiating outwards. Based on their structures and composition architecture, it is reasonable to deduce that these fibers were resulted from early diagenesis during which the bacterial filaments were mineralized rapidly through biochemical processes under specific ocean chemical conditions. During most of Precambrian time, prior to microbial calcification occurred, once existed microbes could harly be preserved as fossils, except in some specific conditions. It was commonly suggested that during Proterozoic period the atmosphere had a high CO2 concentration, this would prevent calcification of bacterial sheaths, and thus result in less chance for microbes to be preserved as fossils. In the Gaoyuzhuang thrombolites, there existed a lot of filament remnants, some of them were well preserved with recognizable calcified sheaths. This could be used as an indirect indicator to infer the atmospheric CO2 level of that time.In the upper part of the third member of Gaoyuzhuang Formation, a set of well preserved aragonite seafloor precipitation sequences are observed, although they have been seriously recrystallized and compacted. In these seafloor precipitates, fibrous aragonite crystals were clearly observed around the organic cores, forming well developed aragonite splays. In vertical sections, these splays would show as weaved laminations. In each of the aragonite splays, an organic core composed of microbial mats and a fringe composed of radially arranged aragonite fibers are clearly recognizable. The formation of aragonite fibers were likely derived from a chemical precipitation process, but microbial activity must have played an important role in the initial nucleation and precipitation. The degradation of organic matter would release HCO3-, resulting in supersaturated seawater in the micro-environments. In these cases, large accommodation space and rapid sedimentation rate would ensure fast growth of aragonite splays. In vertical sections, it can be seen that light-color laminae are regularly intercalated with dark laminae, the former likely resulted from chemically precipitated cements, while the later possibly formed by calcified microbial mats. The reason for this change process may be the periodic changes in water temperature or calcium carbonate saturation. Between the flattened aragonite fiber layers a thin bituminous layer often existed, which was most likely resulted from calcification of microbial mats. It was likely that the coupling of organic and sulfate supply had exerted a great influence to the calcium carbonate saturation of seawater. If sulfate-reducing bacteria(SRB) activity remained persistent, increased aerobic weathering on landmass would supply more sulfate to the ocean, thus exerting some influences to the ocean sedimentation. If the alternating laminate phase was mainly controlled by fluctuation of seawater calcium carbonate saturation, the most likely reason was that such periodic changes in ocean primary productivity had resulted in changes in the total amount of organic matter.If the thrombolites and aragonite aragonite splays in the Gauyuzhuang Formation can be used as an indicator to reflect the microbial activity and physicochemical conditions for the microfacies, the upper third member of the Gaoyuzhuang formation sequence and the well developed pseudo-oncolite horizons may represent a transition state between them. Each of the pseudo-oncolites is commonly composed of a clear core and a rim, with their diameters mostly 1: 1 to1: 2. Composionally, the core consists of micrites, and organic matter are abundant in particle interstices, which may reflect the bloom of microbes and resulted from organic mineralization. Therefore, formation of the core was largely controlled by biological factors, while the rims show the characteristics of light grain and cement, high recrystallization and rare organic matter in the particle interstices. Furthermore, the rims show regular extinction character under polarizing microscope. Thus, the formation of rims may be mainly controlled by the physicochemical conditions of seawater or the local environment changes during early diagenesis.