Lithological Characteristics And Geological Resources Significance Of The Red Beds In Longxi Section Of The Northern Part Of The Western Qinling Mountains

The Longxi section of the northern margin of the Western Qinling Mountains is located in the northeastern margin of the Qinghai-Tibet Plateau.The formation and evolution of the red beds are influenced by the uplift of the Qinghai-Tibet Plateau,which contains abundant geological and historical information.At the same time,several typical Danxia landforms have been developed in the red beds.In this paper,a detailed geological survey is carried out in Longxi County and its surrounding areas,including geological structure and stratigraphic lithology.The structure,composition and mechanical properties of red beds are systematically studied by field gravel statistics,laser particle size analyzer,thin section polarizing microscope,scanning electron microscopy(SEM)and point load test.The slope,aspect and mechanical properties of the red beds are extracted by ArcGIS.Geomorphological parameters,such as surface fluctuation and cutting depth,are calculated.The relationship between lithology and geomorphology is analyzed by calculating river network density and fractal dimension of geomorphic elements.The main conclusions are as follows:(1)Through field geological survey,the strata in the study area are mainly Paleogene and Neogene red beds,the lower part of Paleogene strata is thick conglomerate-bearing sandstone and silty mudstone,and the upper part is conglomerate,sandstone-conglomerate interbedded rock;the Neogene strata are mainly composed of silty mudstone,sandstone and part of sandy conglomerate.(2)According to the statistical analysis of field gravels,the sphericity and flatness of gravels from south to north are getting larger and smaller;in composition,there are more granites and shallow metamorphic gravels in the red beds of southern conglomerates,while quartz and limestone gravels account for the largest proportion in the sandy conglomerates of northern conglomerates;because of the large area ofgranite in Xujiawan area,there are more granites in the conglomerates east of Weihe River,and more granites in the conglomerates east of Weihe River.The conglomerate gravel to the west contains almost no granite,which indicates that the red beds are near-source deposits.Sandstone is mainly composed of quartz,feldspar,calcite and a small amount of dark minerals(mainly iron-bearing minerals).The cements are mainly argillaceous and calcareous,with more fissures and loose structure.(3)When other factors are similar,the maximum density of the Cretaceous strata,Neogene strata and Quaternary loess gradually decreases,and the maximum slope,surface cutting depth and meta-fractal dimension gradually increase.Among them,the maximum density of the Paleogene strata is less than that of the Neogene strata,and the slope and meta-fractal dimension of the Paleogene strata are more than that of the Cretaceous and Neogene strata.In contrast,the river network density of Paleogene strata is small and the valleys are narrow.(4)The relationship between Danxia landform and lithology in the study area is analyzed.Paleogene glutenite is beneficial to the formation of Danxia landform.It is a rich geological heritage resource with high significance of geological resources.