Origin Of Polygonal Cracks In Shicheng National Geopark Of Jiangxi Province

Shicheng National Geopark is located in Ganzhou City of Jiangxi Province,and is representative of the Danxia landform in Jiangxi Province.This geopark is featured by a variety of polygonal cracks,which are mainly observed in the red sandstones of the Hekou Formation of the Late Cretaceous Period.The polygonal cracks are diverse in shape,and the rock surfaces have various colors,but they are mostly reddish in color,such as brick red,fuchsia,gray-purple,brownish yellow,etc.The polygonal cracks have unique and diverse shapes,forming various pictographic landscape,such as Guishou stone,pine cone stone,crocodile stone,fairy plow field,etc.,which have high ornamental value and research value.However,at present,there is insufficient research on polygonal cracks at home and abroad,and there is also no systematic and in-depth explanation of its causes.In this paper,Tongtianzhai scenic area with more polygonal cracks is the main research area,combined with field investigation and indoor experimental analysis to analyze the cause and evolution process of polygonal cracks in the Shicheng National Geopark.Field observations revealed that polygonal cracks in Shicheng were mainly developed in thick and fine-grained sandstones with relatively uniform particles.They can be on the sandstone level or other exposed surfaces.The developed rock layers often have a black film,and the fissures developed on the surface generally extend downward.Does not exceed 30 mm.In the field observation process,the compass is used to measure the direction of the crack,and then the dominant direction of the crack is analyzed indoors,and the relationship between the compressive strength of the rock surface color is analyzed by the rebound meter.Observed by electronic probe experiment,the microscopic characteristics of sandstone with polygonal cracks were analyzed.Under the polarizing microscope,the debris particles of the sandstone sample are mainly sub-angular to sub-rounded,with good sorting.The cement between sand grains consists dominantly of calcite and iron oxide.Scanning electron microscopy experimental analysis results show that the particles under the microscope are loosely cemented,and the erosion pores and cracks on the surface of the particles are relatively common.The voids are often filled with foreign materials.Electronic probe images show that the rock particles are broken,and the minerals are often filled by other materials.Macroscopically analyze the impact of rock lithology,sedimentary structure and climate on the formation of polygonal cracks,study the effect of crust hardening on the development of polygonal cracks,and discuss the early stages of the formation of polygonal cracks to mature and finally disappear Evolution process.Combining similar research at home and abroad and comparing biogenetic structures and mud fissure structures similar to polygonal cracks,this paper believes that lithology is the basic element for the development of polygonal cracks.The rose diagram of the fracture trend shows that the faulted structure controls part of the polygonal cracks Formation.During the weathering process,thin crusts formed on the surface of the sandstone due to the precipitation of silicon,iron,and manganese oxides,and then the crusts were broken due to stress tension,forming small fissures.Finally,the four-stage evolution model of polygonal cracks is proposed.