The Paleokarst On The Top Of The Mesoproterozoic Tuanshanzi Formation And Its Geological Significance In Xingcheng Area,Liaoning Province

The Xingcheng area of Liaoning Province is located on the eastern edge of Yanshan Aulacogen,where developed the typical North China platform sedimentary formations during the Mesoproterozoic.The Changcheng System is composed of Changzhougou Formation,Chuanlinggou Formation,Tuanshanzi Formation and Dahongyu Formation from the bottom to the top,and overlaid on the Archean granite.Under the supported by the project of China Geological Survey < 1: 250000 Scale(K51C004001)Regional Geological Mapping Project of Jinxi City,Liaoning Province>,this paper makes a detailed study on the lithostratigraphic characteristics of the Changcheng System in the study area,and finds paleokarst phenomena at the top of the Tuanshanzi Formation,which is probably the oldest well preserved paleokarst event in the North China platform.According the analyzing on the characteristics,types,evolution and genesis of the Tuanshanzi paleokarst,the geological significance is discussed.The Dahongyu Formation unconformable overlaid on the Tuanshanzi Formation,and through the analyzing its bottom conglomerate,this paper discusses the nature of the bottom boundary and its significance of the stratigraphic division and the tectonic movement.In this paper,the stratigraphy of the Changcheng System in Xingcheng area is measured.The deposits of Changzhougou Formation,Chuanlinggou Formation and Guanshanzi Formation are continuous and can be correlated with the standard section of Jixian in Tianjin,but the Dahongyu Formation only developed clastic rocks without carbonate strata.The Dahongyu Formation is mainly divided into three members: the first is composed of gray-white thick bedded quartz sandstone with medium-thin layers of coarse-grained feldspar quartz sandstone,occasionally with the tuffaceous siltstone and sedimentary tuff,and the bottom are siliceous cemented quartz conglomerate,granitic conglomerate and siliceous breccia;the second is composed of gray-green,black-green siltstone,silty shale,and interebedded with bluish gray thin plate,medium fine-grained quartz sandstone with parallel bedding,lamellation and occasional wavemark;the third is composed of light gray-brown thick bedded feldspathic quartz sandstone with debris feldspar quartz sandstone and grayish thin-layer tuffaceous sandstone.Based on the stratigraphic correlation analysis,this paper redefines the stratigraphic division of the Tiemashan and Mopanshan areas in the original 1:200,000 regional geological survey report.In Tiemashan area the Changzhougou Formation is revised to the first member of the Dahongyu Formation and the Chuanlinggou Formation is revised to the second and third members of the Dahongyu Formation.In the area of Mopanshan Changzhougou Formation is revised to the Dahongyu Formation,and the Dahongyu Formation is revised to the Gaoyuzhuang Formation.Paleokarst occurred at the top of Tuanshanzi formation in Xingcheng area.The paleokarst unconformity surface,fissures,dissolution trenches and corrosion pits develop obviously,and are filled with breccia of the overlying Dahongyu Formation.In this paper,four types of breccia and conglomerate related to Tuanshanzi paleokarst are divided,and the characteristics of filling materials are analyzed.According to the characteristics of the paleokarst,it occurred before the deposit of the Dahongyu Formation,which belongs to the local paleokarst in the supergene environment.The evolution of paleokarst is further divided into five stages.Based on the controlling factors of paleokarst,this paper analyzed the origin background of Tuanshanzi paleokarst.The Dahongyu Formation in Xingcheng area unconformablely covers many layers before its deposition.The composition of conglomerate at the bottom is complex,and the thickness and characteristics in each section are quite different.This paper describes in detail the petrological characteristics of the bottom conglomerate of the Dahongyu Formation on several sections,and analyses their nature.According to the characteristics and thickness of the conglomerates in the sections,this paper preliminarily restores paleocoastal zone and paleotopographic features in the early deposit of the Dahongyu Formation in the study area.In the standard section of Jixian,Tianjin,there are two different views on the division of the bottom boundary of the Dahongyu formation.That is there is no unified opinion on the basis boundary of "small white quartz rock" or "big white quartz rock".According to the paleokarst phenomenon at the top of the Tuanshanzi Formation,it is proved that there is obvious sedimentary discontinuities between the Dahongyu Formation and the Tuanshanzi Formation.In the study area,the Dayongyu formation is unconformablely overlay on the Archean granite,Changzhougou Formation,Chuanlinggou Formation and Tuanshanzi Formation,and the unconformity between the Dahongyu Formation and the underlying strata also exists in many areas through the Yanshan Aulacogen.The above phenomena indicate that there are obvious erosion discontinuities between the Dahongyu Formation and the underlying strata,which can be cooperated with the conglomerate phenomenon with 10 cm under the "small white quartz rock".Therefore,it is reasonable to regard "small white quartz rock" as the bottom boundary of Dahongyu Formation.The "Xingcheng Movement" has always been one of the controversial issues for the Mesoproterozoic strata in China.Although the unconformity between the bottom of the Dahongyu Formation and the underlying strata and the Archean crystalline basement in many areas of the Yanshan Aulacogen has been generally recognized in domestic academia,most people do not consider it to be a crustal uplifting movement.In this paper,the bottom unconformity of Dahongyu Formation is studied in detail,and two types of unconformity are classified: heterogeneous overlying unconformity and monoclinic unconformity.It is considered that the heterogeneous overlying unconformity and overlying unconformity represents the gradual expansion of the Yanshan basin in the early sedimentary period of the Dahongyu Formation,while the monoclinic unconformity represents the differential crustal movement in Xingcheng region before the Dahongyu transgression.The influence of this crustal movement does not extend to the center of the sedimentary basin,and it is evident only at the margin of the basin.The North China platform is a part Colombian supercontinent,which develop the Yanshan Aulacogen during Paleo-Mesoproterozoic and the deposition begin after the Lvliang Movement.The Mesoproterozoic Changzhougou-Tuanshanzi period is the strong fault depression stage in Yanshan Aulacogen,and from the Dahongyu period,and the it enters the relatively stable depression stage.The paleokarst events of the Tuanshanzi Formation and the Xingcheng Movement represent an important turning point in the evolution of the Yanshanian Aulacogen.