Characteristics And Geomophogensis On Sandstone Caves In Fangshan Area Of The Northern Of Northeast Sichuan

The sandstone caves in the low mountain and Fangshan landform area in the northeast of Sichuan Province are typical.As a kind of precious geological relic resource,it records the local geological structure and geological process.Meanwhile,as an important green resource for the sustainable development of local economy,it plays an important role in the new era.But up to now,few people have paid attention to the types,characteristics and genesis of sandstone caves.On the basis of data collection and field investigation,this paper takes the sandstone cave of Penglaizhen in the north of Yingshan as an example,carry out joint test,polarizing microscope analysis and major element analysis of rock samples.At the same time,combined with the method of GIS analysis,this paper studies the classification,characteristics,development conditions and development process of sandstone caves in the study area,and finally obtains the following understanding.?1?There are more than 40 sandstone caves in the study area with complete types:According to the scale and form,there are many domed or arched large sandstone caves;According to the origin classification,the majority caves are those formed mainly by differential weathering.According to hydrological conditions,there are many wet type sandstone caves and dry type sandstone caves.?2?The distribution of sandstone caves in the study area has certain characteristics.The sandstone caves are located on the cliff of the long beam stepped Fangshan and at the junction of the cliff and the steps,showing the characteristics of"large dispersion and small concentration";the sandstone caves are mainly developed in the Penglaizhen formation,and are concentrated in three layers of 310-330m,380-520m and 670-780m;in this area,the sandstone cave mouths are mainly distributed in the north and east of the mountain area,and in the northeast and south;Peacock Cave and Tangfang Cave are greatly affected by the axis of Tiedingya syncline,and the caves on Taipeng Mountain are affected by Shuikouchang anticline;the caves are also affected by the river,and water storage type sandstone caves and humid type sandstone caves are mostly developed in the small valley catchment area of Fangshan Mountain.?3?In terms of the development conditions of sandstone caves,the gentle fold structure,structural fracture and weathering fracture are the structural conditions for the formation of Fangshan landform and even sandstone cave groups in this area under the background of lotus shaped structure;the calcareous feldspar sandstone interbedded with shale stratum in Penglaizhen formation is the material basis for the development of sandstone caves;the fracture water developed in sandstone is the important hydrogeological conditions for the formation of water caves;The exposed cliff of Long-ridge shaped Fangshan landform is an important condition for cave development;in addition,the formation of sandstone caves is also affected by climate,hydrology,soil and vegetation.?4?According to the analysis of thin section and major elements of rock,there are significant differences in lithology between the top and the wall of sandstone caves.The thick caprock at the top of the sandstone cave is rock-debris sandstone,and there are many argillaceous or calcareous components in the cave wall of the sandstone cave.The average content of SiO₂ in the concave part is lower than that in the convex part,while the average content of Al₂O₃ in the concave part is higher than that in the convex part.The average content of Cao is about twice that of protruding part.After UCC standardization,the rock samples in the concave part are obviously enriched in CaO.SiO₂,Al₂O₃ and Cao are the main oxides that cause the difference of lithology and differential weathering between the top and the wall of the tunnel1.?5?In this paper,the index of compositional variabitlity?ICV?,chemical index of alteration?CIA?,chemical index of weathering?CIW?and plagioclase index of alteration?PIA?are selected to evaluate the weathering degree of sandstone cave rocks.The ICV values indicates that the rock of sandstone cave sample is the primary sedimentary product.The results of CIA,PIA and CIW indicate that the weathering degree of sandstone cave wall is greater than that of cave top,and the differential weathering phenomenon is obvious.The CIA also indicates that the rocks in the study area are mainly physical weathering,and the chemical weathering degree is relatively low.?6?The wet type and dry type sandstone caves are mainly affected by physical weathering,which is mainly caused by the differential weathering caused by the difference of lithology between the roof strata and the wall strata.The small groove extending along the layer is formed by differential weathering,and the groove cavity is further developed to form dome like cave without obvious cave wall,or horizontally connects to form a rock groove along the layer;the cave space continues to increase to form a sandstone cave with obvious slightly arched wall,and further development may also form a through hole and natural bridge;finally,under the effect of gravity,the cave collapses and the Fangshan cliff retreats as a whole.The newly exposed cliff began to form new caves because of different weathering.?7?Water storage sandstone caves are mainly affected by chemical weathering.Sandstone is a permeable rock layer,fracture is an important water channel,which makes groundwater erode vertically.Water and rock have karstification,hydrolysis and hydration.On the one hand,they destroy rock structure,accelerate weathering and form small space.On the other hand,they form aquiclude at the bottom of the tunnel.Water collects at the bottom of the tunnel and begins to erode the wall laterally.Under the influence of gravity,the rock at the top of the cave collapses,causing the water level in the cave to rise suddenly,causing waves to bring part of the sedimentary collapses out of the cave;the accumulated collapses at the bottom of the cave block the water in the cave;on the other hand,the internal space of the cave increases,resulting in the formation of water storage sandstone cave.