| Oncoid is a kind of calcareous coated grain mainly constructed by cyanobacteria.It can be found in many environment like shallow sea,lake,river and even marsh.Oncoid can be affected by different environment parameters and record these parameters.So,it is useful for us to study oncoid if we want to investigate palaeoenvironment and paleoclimate.There are many oncoids in Gucheng Formation and Bianqiao Formation,Pingyi Basin,Shandong province.Most of the oncoids are well preserved,which makes great convenience for us to observe and test oncoids.We used polarizing microscope,fluorescence microscope,scanning electron microscope,isotope mass spectrometer and X-ray micro beam to investigate the structure and environment significance of oncoid.Finally we conclude three opinions as follows:Oncoid has dark and light lamina which appear alternately.Light lamina is with bigger calcite and less organic matter,while dark lamina is with smaller calcite and more organic matter.Dark lamina is formed in summer and autumn when the temperature is relatively high,while light lamina is formed in winter and spring when the temperature is relatively low.There are many blue-green algae in oncoids.Some are restricted to a single lamina,while others can grow in different lamina.From the structural features of the oncoids we observed that comprehensive analysis on many aspects,such as shape,lamellar patterns,size of oncoids and cementation,can provide detailed information of the hydrodynamic conditions of oncoids.Particularly,the lamina structure is an important feature to determine the sedimentary environment of oncoid.The thin and continuous laminae with laterally unchangeable thickness of the oncoids is a reliable representative of high hydrodynamic conditions.Through impersonal statistical analysis,we realized that oncoid is related to some major geological events like mass extinction.It is possible for us to investigate these geological events through oncoid. |
PDF Full Text Request