Study On The Formation And Water Protection Of Sandstone Water Cave In Penglaizhen Fm,Fangshan,Northeast Sichuan

Kongque Sandstone Water Cave is located in Yingshan County,Nanchong City,northeast of Sichuan Province.It is a typical geological relic landscape in the Yingshan Geological Park.It is developed in the lower strata of the Upper Jurassic Penglaizhen Formation（J₃p¹）.The lithology is thick sandstone with thin glutenite and mudstone unequal thickness interbed.Caves are widely developed in the red beds of northeastern Sichuan,but they are small in scale and mostly dry caves.However,there are few reports on the genesis of typical sandstone water holes and their water resources protection.The sandstone cave extends hundreds of meters in length and the water resources in the cave are abundant,which is rare in China.This paper takes the Kongque Cave in the Yingshan Provincial Geological Park of Sichuan as the research object,through the field investigation,physical and chemical analysis of rock,soil and water samples and PHREEQC hydrogeochemical simulation,the purpose is to explore the genesis of sandstone caves and the process of water resources formation,and to discuss the tourism development and protection strategies of sandstone cave,the main understanding is as follows.1.In terms of morphological characteristics of the cave,influenced by the Tiedingya syncline and joint,the development direction of Kongque Cave is mainly northwest-southeast.The cave consists of a main tunnel.Its plane form is hall-corridor style.The hall is a flat-shaped dome.The top of the cave is mostly smooth and flat.and the narrowest part of the corridor is only for a person to pass sideways.The depth of the cave reaches hundreds of meters and is in the process of continuous development,accompanied by local collapse,the tunnel continues to extend and widen.And there are various types of sediments in the cave,including collapse deposits,flowing water deposits,chemical deposits and biological deposits.2.Regarding the causes of cave formation,the conditions of cave development and formation are discussed from the aspects of geological structure,stratigraphic lithology,groundwater hydrogeochemistry and geomorphology.Lotus-like structures and structural joints in Yilong-Bazhong are the controlling conditions for cave formation.Tiedingya syncline is a typical confined water structure with a dip angle of2-5°,which is conducive to the convergence of confined water in caves along the inclined axis and provides favorable conditions for the convergence of water in Kongque Cave,which leads to different weathering within the cave.It is developed in the lower strata of the Upper Jurassic Penglaizhen Formation（J₃p¹）.The lithology is a combination of grey-white calcareous lithic feldspar sandstone,purple-red calcareous silty mudstone and conglomerate,which provide material basis for the formation of the cave.The interlayer fissures and intra-layer fissures are also quite developed.In addition,the sandstone has high porosity,high water-rich and water-permeable properties,and strong water mobility,which is beneficial for chemical weathering.Subtropical monsoon climate brings abundant precipitation to provide sufficient water source for groundwater,which promotes water-rock interaction,expands of fissures and pore.3.The water sample data in the Gibbs diagram confirmed that the ions in the water mainly originated from the weathering of the rock;The mineral source of the main components in the groundwater was obtained by calculating the ion ratio.By calculating saturation index and reverse hydrogeochemical simulation,biochemical reactions and mass changes in flow process are deduced and quantified.A series of chemical weathering makes cracks and pore develop and expand continuously.And physical difference weathering,gravity collapse and so on promote the further formation of peacock cave.4.For the sustainable development of sandstone caves,the protection of the Kongque Cave must be strengthened because of their typicality,rarity and non-renewability.We should improve laws and regulations,set up supervision institutions,strengthen supervision,raise people’s awareness of protection,prohibit large-scale development and large-scale animal husbandry activities,and avoid damage to the geological environment and water pollution around the Kongque Cave.On the basis of not affecting the cave landscape,appropriate water diversion infrastructure will be added to the top area where water resources are scarce,which will be used for living,agricultural irrigation,scenic area operation,and assist local residents to get rid of poverty and become rich.