Geological Characteristics And Origin Of The Beaded Limestone In Yelang Formation In The Dejiang Area,Guizhou Province

The Dejiang area in Guizhou Province is located in the uplift of the Upper Yangtze Block-Cangbei Uplift.It retains relatively intact remains of the early Triassic shallow water deposits,and it has preserved the evolutionary process from late Permian to early Triassic biological evolution from extinction to recovery.The microbiological rocks are also well-preserved and abundant.In the early triassic formations,there are commonly known as beadlike limestone,which are in fact oncoite.The geological characteristics of the oncoite in the area are analyzed to explore their causes.This article through the use of microscope observation and scanning electron microscopy data analysis to determine the oncoite inside the material composition,particle size,structure and structure.Round bead is a kind of nucleus with the core of calcite as the core,and the grain size of the bright grain layer gradually increases from the inside to the outside.It is divided into two categories according to whether there is a bottom structure:The first type,which has a bottom-structured oncoite,is formed by its algae and shell organisms,and its shell organism lives under the nucleus and consolidates during the later diagenesis;In the second category,there are no round-bottom structures with different shapes,which are mainly formed by rolling growth and swing growth.Their morphology is mainly affected by the hydrodynamic strength and the concomitant squeezing effect of vein penetration.Based on field surveys and profile surveys,from the analysis of macroscopic characteristics,the oncoite emerged from the top of the section of the Yelang Formation.The oncoite was convex,the weathered surface was yellow-brown,and the fresh surface was gray-white and yellow-brown and gray.Species,core and circle obviously.In dense areas,"round oncoite"account for about 45%-60%of the whole rock and 35-50 in 10~2cm.Under normal circumstances round oncoite account for about 20%-35%,10-16 in 10cm~2;diameter is 0.8-2cm,individual diameter is 3-5cm,and a few diameters are 0.2-0.6cm.The study of the back profile shows that the oncoite mainly appears in the 23th to 29th layers of the section,and the three cycles are divided according to the distribution of the oncoite.Among them,the 24th,26th,and 28th floors have three peaks.From the study of microstructural features,the composition of the bead core is two types of muddy crystal calcite and micro bright crystal calcite.From the core form,it is divided into four types:spherical,ellipsoidal,elongated,and irregular.The inner circle of the oncoite is divided into two types:muddy layer and bright grain layer;the muddy layer is divided into regular muddy layer and irregular muddy layer.Its particle size is At 1-4um,the bright crystal calcite is divided into a regular bright grain layer and an irregular bright grain layer with a particle size of 20um-600um.The bright grain layer gradually increases in size from the inner layer to the outer layer,which indicates a growth process of the algae.Among them,long and irregular round beads are formed by the swinging growth of algae,which is a kind of near-in-place deposition.When the formation is limited,the hydrodynamic force is relatively strong.When the sediment is formed,the hydrodynamic force is relatively weakened due to surface changes.The evaporation of the environment.The round oncoite and ellipsoidal oncoite are formed by rolling growth.The round oncoite shell is smooth and smooth.It is carried over a certain distance,but its core material composition is similar to the matrix outside the outer shell,indicating that it grows in a strong hydrodynamic environment.The restricted platform is deposited on the environment of the evaporating platform where the hydrodynamic forces are relatively weakened.Studies have shown that the formation process of the oncoite is mainly four stages:the core formation stage-the formation of the shell-the deposition process at close range-diagenesis,and later by the biological effects,tectonic effect and the hydrothermal activity of the veins and differences stage.