The Early Ordovician System is composed mainly of a series of carbonate platform deposits interbedded with shale and is especially characterized by a large number of organic reefs or buildups that occur widely in the ...The Early Ordovician System is composed mainly of a series of carbonate platform deposits interbedded with shale and is especially characterized by a large number of organic reefs or buildups that occur widely in the research area.The reefs have different thicknesses ranging from 0.5 m to 11.5 m and lengths varying from 1 m to 130 m.The reef-building organisms include Archaeoscyphia, Recepthaculitids,Batostoma,Cyanobacteria and Pulchrilamina.Through the research of characteristics of the reef-bearing strata of the Early Ordovician in the Yichang area,four sorts of biofacies are recognized,which are(1) shelly biofacies:containing Tritoechia-Pelmatozans community and Tritoechia-Pomatotrema community;(2) reef biofacies:including the Batostoma,Calathium-Archaeoscyphia, Pelmatozoa-Batostoma,Archeoscyphia and Calathium-Cyanobacteria communities; (3) standing-water biofacies:including the Acanthograptus-Dendrogptus and Yichangopora communities;and(4) allochthonous biofacies:containing Nanorthis-Psilocephlina taphocoense community.The analysis of sea-level changes indicates that there are four cycles of sea-level changes during the period when reef-bearing strata were formed in this area,and the development of reefs is obviously controlled by the velocity of sea-level changes and the growth of accommodation space.The authors hold that reefs were mostly formed in the high sea level periods.Because of the development of several subordinate cycles during the sea-level rising,the reefs are characterized by great quantity, wide distribution,thin thickness and small scale,which are similar to that of Juassic reefs in northern Tibet.The research on the evolution of communities shows that succession and replacement are the main forms.The former is favorable to the development of reefs and the latter indicates the disappearance of reefs.展开更多
The Early Ordovician stromatolites in the study area are mainly exposed at the bottom and middle and lower parts of the Second Member of Nanjinguan Formation,the top of the Fourth Member of Nanjinguan Formation,the mi...The Early Ordovician stromatolites in the study area are mainly exposed at the bottom and middle and lower parts of the Second Member of Nanjinguan Formation,the top of the Fourth Member of Nanjinguan Formation,the middle of the Second Member of Fenxiang Formation and the bottom of the First Member of Honghuayuan Formation.In order to find out the controlling factors of Lower Ordovician stromatolites development in the study area,the analysis data of carbon,oxygen isotope samples,macro element samples as well as their variation characteristics are studied and then combined them with previous research results,which is eventually beneficial for reaching the conclusion:sea level change is one of the main factors controlling the formation of stromatolites.There are five sea level change cycles in the Early Ordovician period.The analysis of sea level change and accommodation space variation characteristics show that when the growth rate of the accommodation space is approximately equal to that of carbonate,the circulation of seawater is well,and the amount of light and oxygen is sufficient,so that the cyanobacteria organism can be multiplied in large quantities.The growth and development provide favorable conditions,and favorable conditions are provided for the growth and development of stromatolites;The peak changes in CaO content and CaO/MgO ratio indicate that the formation period of stromatolites is arid climate environment with high water temperature and large evaporation.Prokaryote reproduction is suitable for the environments with high seawater cleanliness.When the amount of land-based debris injected into the sea increases,the turbidity of water will lead to a large number of deaths of algae microorganisms that form stromatolites,and the stromatolitic microbial mats cannot be preserved.In addition,the growth and predation of macro-organisms play a restrictive role in the development of stromatolite-forming micro-organisms blue-green algae.According to the actual situa-tion of the development of the laminated rocks in the study area,what is the dominant function in the formation and decline of stromatolites between the evolution of metazoans and changing environments is discussed in the study.展开更多
基金supported by the National Natural Science Foundation of China(No.40972019)the S&T plan projects of Hubei Provincial Education Department(No. 03Z0105)+1 种基金the Project of Scientific and Technologic Development Planning of Jingzhou(No.20101P031-5)the Innovative Experimenting Plan of Undergraduate Students of China(No.091048934)
文摘The Early Ordovician System is composed mainly of a series of carbonate platform deposits interbedded with shale and is especially characterized by a large number of organic reefs or buildups that occur widely in the research area.The reefs have different thicknesses ranging from 0.5 m to 11.5 m and lengths varying from 1 m to 130 m.The reef-building organisms include Archaeoscyphia, Recepthaculitids,Batostoma,Cyanobacteria and Pulchrilamina.Through the research of characteristics of the reef-bearing strata of the Early Ordovician in the Yichang area,four sorts of biofacies are recognized,which are(1) shelly biofacies:containing Tritoechia-Pelmatozans community and Tritoechia-Pomatotrema community;(2) reef biofacies:including the Batostoma,Calathium-Archaeoscyphia, Pelmatozoa-Batostoma,Archeoscyphia and Calathium-Cyanobacteria communities; (3) standing-water biofacies:including the Acanthograptus-Dendrogptus and Yichangopora communities;and(4) allochthonous biofacies:containing Nanorthis-Psilocephlina taphocoense community.The analysis of sea-level changes indicates that there are four cycles of sea-level changes during the period when reef-bearing strata were formed in this area,and the development of reefs is obviously controlled by the velocity of sea-level changes and the growth of accommodation space.The authors hold that reefs were mostly formed in the high sea level periods.Because of the development of several subordinate cycles during the sea-level rising,the reefs are characterized by great quantity, wide distribution,thin thickness and small scale,which are similar to that of Juassic reefs in northern Tibet.The research on the evolution of communities shows that succession and replacement are the main forms.The former is favorable to the development of reefs and the latter indicates the disappearance of reefs.
文摘The Early Ordovician stromatolites in the study area are mainly exposed at the bottom and middle and lower parts of the Second Member of Nanjinguan Formation,the top of the Fourth Member of Nanjinguan Formation,the middle of the Second Member of Fenxiang Formation and the bottom of the First Member of Honghuayuan Formation.In order to find out the controlling factors of Lower Ordovician stromatolites development in the study area,the analysis data of carbon,oxygen isotope samples,macro element samples as well as their variation characteristics are studied and then combined them with previous research results,which is eventually beneficial for reaching the conclusion:sea level change is one of the main factors controlling the formation of stromatolites.There are five sea level change cycles in the Early Ordovician period.The analysis of sea level change and accommodation space variation characteristics show that when the growth rate of the accommodation space is approximately equal to that of carbonate,the circulation of seawater is well,and the amount of light and oxygen is sufficient,so that the cyanobacteria organism can be multiplied in large quantities.The growth and development provide favorable conditions,and favorable conditions are provided for the growth and development of stromatolites;The peak changes in CaO content and CaO/MgO ratio indicate that the formation period of stromatolites is arid climate environment with high water temperature and large evaporation.Prokaryote reproduction is suitable for the environments with high seawater cleanliness.When the amount of land-based debris injected into the sea increases,the turbidity of water will lead to a large number of deaths of algae microorganisms that form stromatolites,and the stromatolitic microbial mats cannot be preserved.In addition,the growth and predation of macro-organisms play a restrictive role in the development of stromatolite-forming micro-organisms blue-green algae.According to the actual situa-tion of the development of the laminated rocks in the study area,what is the dominant function in the formation and decline of stromatolites between the evolution of metazoans and changing environments is discussed in the study.
