Research On The Formation Mechanism Of Ooids

Ooids are spherical carbonate grains with cores and carbonate crystal coverage.As a special kind of carbonate sediments,ooids hold similar environment sensitiveness with other carbonate sediments.Thus,research on the formation of ooids may reveal parameters of paleo environment such as composition of seawater,water currents,etc.which are meaningful for the reconstruction of paleo environments and the understanding of process of carbonate sediments formation.According to current study,opinions about the formation process of ooids could be divided into three parts: inorganic origin theory,organic origin theory,biological origin theory,and the research method could be divided into lithological analysis and simulation experiments.Although much efforts were spend on illustrating the formation mechanism and process of ooids,they are still unclear and the impetus is still mysterious,so further research should be made in this area.In this paper,I combine the lithological method with experiment simulation to reconstruct the sedimentary environment and illustrate the formation process,and further propose a process model for this unique carbonate sediment.Through the combination of lithological and geochemical analysis,this paper reveal the relationship between ooids type and the distribution of organic compounds,organism activity.Though experiment simulation,this paper analyzed the environment parameters of ooids formation to reconstruct the condition of ooids formation.This paper compared the results with sedimentary conditions of modern Bahamian ooids sand beach and lithological analysis results for the build of ooids formation model and further illustrate the function of organic compounds and organisms,together with the impetus of the formation of ooids.The details are as follows:Firstly,this paper perform sufficient field work and sampling for evidence to illustrate the formation process of ooids.Through fluorescence microscopic analysis,it is found that the organic membrane is not a common phenomenon in ooids.The SEM and carbon and oxygen isotope test also failed to find evidence that support the biological genesis hypothesis.Based on the information above,it is concluded that the formation of ooid is dominated by inorganic reaction process,biological activity and organic matter function resides in a secondary position.Secondly,this paper analyze the marine sedimentary conditions and extract six variables including water agitation condition,carbonate supersaturation,nuclease density,Mg/Ca ratio,algae,and humid acid concentration and perform simulation experiments based on the six variables.By using SEM,fluorescence microscope,this paper characterize the external morphology and internal structure of synthesized ooids,further the distribution of organic compounds.By using AFM,FTIR this paper characterized the structure similarities of organic compounds from different origin and their absorption features on the plate of crystals.Meanwhile,this paper use DLS to characterize the diameter of different dispersed systems for their colloid features.With all characterizations mentioned above,this paper reconstruct precipitation environment in laboratory and summarized the model of ooids formation.Further,this paper compare the reconstructed environment with Bahamian ooids sand beach to test the model.The study found that,the formation of ooids is a process which dominated by inorganic factors rather than biological activity and organic matter factors.The environment that generates ooids must contain the following factors:(1)little water agitation;(2)less impurities;(3)high degree of seawater saturation.Other factors such as high Mg/Ca ratio,the existence of organisms and organic matter enrichment are secondary factors.For the formation mechanism analysis,the formation process of ooids is under kinetic control,which is a step by step reaction process rather than a typical thermodynamic one-step process.Ooid is metastable,the reason of its existence is that the transformation to next step requires too much energy.In addition,in order to prevent the dissolution of ooid,sedimentary environment should be above shallow depth,as a preservation condition for ooid.Combining the experimental results with field knowledge,this paper argues that the best environment where ooids are formed is below the wave base and aroud the transitional environment between lagoon and reefs or sand bar barrier.This conclusion is consist with previous knowledge and modern oolitic shoal sedimentary environment as well as the paleogeographic evolution of Zhangxia formation in North China region,which further illustrates the correctness of the model.In conclusion,we use combination of lithological analysis and experiment simulation to rebuild the sedimentary environments of ooids formation from the simple to the complex and summarize a model for the precipitation and further illustrate the effects of factors in the process.This study provide information for further understanding the formation of ooids and other carbonate sedimentary rocks.