The Early Triassic Microbial Fabric Characteristics And Their Geological Significance In Northeastern Sichuan

The Early Triassic is the convalescence of post-disaster ecological environment,of which the sedimentary characteristics have always been the focus of geological research.The research of microbial rocks at the bottom of the Lower Triassic has achieved remarkable results,but studies on micrite and oolitic limestone are rare.Shallow-water environment has been known as the advantageous environment for microbial activities and the formation of microbial rock.Therefore,this paper focus on the micrite and oolitic limestone of the Lower Triassic Feixianguan Formation in Kaijiang-Liangping Trough.Based on spectral analysis of high resolution scanning electron microscope,energy and element geochemistry,that microbial fabrics characteristics and microbial activities' reflections on seawater envirolment and effects on carbonate deposition and formation of pore space are studied.The main conclusions as follows:(1)Marine environment of the Early Triassic in the northeastern Sichuan basin was mainly anoxic-reducing,where were almost unaffected by terrigenous clastic matter.There was almost no macroscopic aquatic organisms in the Early Triassic Ocean,but a little conodonts,ostracods and gastropods that significant miniaturization.However,the microbes were prosperous,indicating that the marine environment in the Early Triassic had been deteriorating.(2)Many microbial fabrics are observed in the surface and the contemporaneous intergranular cement of oolitic limestone.Cavity-shaped microbial fabric with a dialeter of-2?m?5?m,is connected by a channel with a width of about 0.5?m?1.6?m.Micro nodules-shaped microbial fabric with a diameter of about 24nm?100nm can be aggregated into a tumor-like or mat-like distribution with a diameter of 0.37?m?1.7?m.Arrow-shaped microbial fabric is distributed in the form of slender filament or short cluster of divergent with a length of about 0.22?m?4?m3.the diameter of about 60nm?250nm.A large number of micropores associated with microbes have been discovered,including nicro-intrafossil pores,micro-framework pores,and micro-dissolved pores.(3)Multi-type microbial fabrics provide intuitional evidences for the study of microbial species,microbial metabolic activities,and microbial outbreaks in the Early Triassic after catastrophic events.(4)By microbial induction the mud-mounds and planktonic carbonate factory dominated carbonate precipitation process in the Early Triassic.Microbial activity can promote precipitation of large amount of carbonate in the ocean and formation of oolite.Microbial activity and deep sea carbonate dissolution increasing the environmental alkalinity that can promote precipitation of carbonate and inhibit dissolution of carbonate in the deep sea under the anoxic marine envirorunent.It is speculated that microbial activity is a main cause of the rapid carbonate deposition phenomenon in the Early Triassic,and lead to the Late Permian intracratonic rift(Kaijiang-Liangping Trough)be filled rapidly in the Early Triassic.