The Paleo-ocean Environment Evolution Of Early Cambrian And Its Influence On Organic Matter Enrichment Model In Northern Guizhou

The Early Cambrian is an important period that records the evolution of global palaeo-marine environment and is also one of the key periods for the formation of organic matter enrichment.In this study,sedimentology,stratigraphy and mineralogy methods were used to analyze iron geochemistry,redox sensitive trace elements and the abundance of different nutrient elements in Well ZK-402(outer-shelf facies)and Well CY-1(slope-deep water facies),which were located in northern Guizhou.The evolution of the Lower Cambrian paleo-marine environment and its influence on the accumulation of organic matter were studied.The main points are as follows:(1)Based on the correlation between the development and distribution of the Lower Cambrian strata and sedimentary facies in the northern Guizhou region,the rock assemblage rules of different facies zones are pointed out,and the chronostratigraphy of the Lower Cambrian research strata in the northern Guizhou region is classified according to the geological age of Ni-Mo metal enrichment layer(521Ma).Sedimentary facies belts in the study area mainly include outer-shelf facies and slope facies.In the outer-shelf facies,on the basis of dolomite(Dengying Formation)at the bottom,the 2nd?3rd order black shale and siliceous shale rock assemblage(Niutitang Formation)and the 4th order calcareous mudstone(Mingxinsi Formation)are developed successively.On the basis of the bottom Sinian siliceous rock(Liuchapo Formation),the Cambrian black shale of 2nd?3rd order,siliceous shale,limestone assemblage(Jiumenchong Formation)and silty shale of 4th order(Changmachong Formation)are developed successively.(2)The changes of iron composition and redox sensitive trace elements concentration in the Lower Cambrian in the northern Guizhou reflect the dynamic development process of water redox during deposition period of the second to fourth stage.In the period of the second and early third stage,the water environment extended from the shelf to the slope facies area,and the water environment was all sulfide and anoxic.At the period of the late third stage,the water environment from the shelf to the slope phase still maintains anoxic condition,but the sulfide water disappears in the shelf phase and the water environment becomes ferruginous.During the fourth stage,the oxygen-containing water expanded,the water environment in the shelf facies area changed to oxidation,the sulfide water in the slope facies area disappeared,and the water remained ferruginous.(3)The P cycle model of the early Cambrian palaeo-ocean in the northern Guizhou region is established to reflect the evolution of the palaeo-ocean environment from the perspectives of water reoxidation and reduction conditions,palaeo-productivity level and organic carbon burial.In the period of the second and early third stage,the P cycle formed a"positive feedback"mechanism to the evolution of the palaeo-ocean:it increased the level of productivity and promoted the expansion of the sulfide water,which was conducive to the burial of organic matter.In the period of the late third stage the fourth stage,the P-cycle formed a"negative feedback"mechanism,which intensified the biological competition,reduced the productivity level and the expansion of oxygen-containing water,and was not conducive to the generation and preservation of organic matter.(4)The accumulation of organic matter in the Lower Cambrian in the northern Guizhou is the result of multiple controlling factors.In the period of the second and early third stage,the strong enrichment of Mo,V,U and C_org/P indicates a continuous sulfide water environment.The high concentration of TOC,P/Al,Ba and other nutrient elements reflected the high level of productivity during the main stage of the Cambrian biological explosion.Under the influence of extensive transgression events,the sedimentation rate of marine sediments was low,which together controlled the formation and distribution of organic-rich shales of the Lower Cambrian in the northern Guizhou area.