Study on Origin of Special Nodular Limestone

Key words: 
• nodular limestone /
• rudstone carbonate concretion /
• differential compaction /
• Du-an /
• Debao County /
• Guangxi /

Abstract: There are three interpretations for the cause of formation of nodular limestones.The first interpretation is that they are the beds of carbonate concretions, the second is that they are the result of the differential compaction,and the third is that they are formed by the sedimentation or the synsedimentary faulting.However, in the Da Liang Tang Formating of lower Devenian Series in Du-an,Debao county,Guangxi,a special nodular limestone is mistaken as a rudstone by some people,because the nodular limstone similar to the normal rudstones in its figure and mode of occurrence. The study on its petrology,paleontology,paleostructurs,etc., shows that the rudstone is not simply the beds of carbonate concretions, it was formed neither by the differential compaction nor by the sedimentation and synsedimentary faulting,and the fact is that lot of carbonate nodules were first formed in the poor-oxygen,closed flute facies of shelf platform;then,the differential compaction led to the extremely concave-convex of the bedding surface or the heterogeneity of the beds; finally,the nodules were turned and also transferred by the shearing,flexural slip,flexural flow, process of the limb during forming the dome in the Indo-Chinese epoch, the "rudstone" we see today was produced. Hence,the "rudstone" in Du-an was realy the nodular limstone,and it was the result of sedimentationg,diagenesis and tectonism,etc,and it also shows a kind of sedimentary-diagenetic environment which was the poor-oxygen and closed flute of shelf platform.