Genesis and karst characteristics of marble agglomerate in volcanic rocks on the eastern slope of Wutong Mountain,Shenzhen

WANG Xiaohu; WANG Ping; WANG Xianneng; LAI Anfeng

doi:10.11932/karst20250603

Abstract：Karst phenomena,such as fissures and caves,occur in the marble agglomerate of pyroclastic rocks on the eastern slope of Shenzhen's Wutong Mountain.These features have affected the engineering projects such as subway construction and pile foundation engineering in the study area.This study collected numerous borehole histograms and data on multi-phase geological surveys from the Wutong Mountain area,interpreted volcanic structures using airborne LiDAR data from the middle and eastern parts of Wutong Mountain,and conducted engineering geological ground surveys in the study area. The study shows:(1)Marble agglomerate volcanic rocks on the eastern slope of Wutong Mountain are distributed in the explosive collapse phase rocks of the second eruptive cycle of Wutong Mountain's volcanic activity.It is inferred that the karst area within these volcanic rocks is located in the Wangjihu-Xiao'ao volcanic structural depression,covering total area of about 1.826 km2.(2)It is further inferred that the marble agglomerate in the volcanic rock,originated from the limestone in the lower Carboniferous Shidengzi Formation(C1s)within the volcanic channel,formed during early intense eruptions.Prior to the volcanic eruption,magma compression and gas explosions caused the surrounding rock to fracture and collapse.During the volcanic eruption,the release of magma pressure propelled the fragmented and collapsed surrounding rock material to the surface,where it mingled with magma and solidified into the volcanic rock.(3)Due to the favorable groundwater circulation and alternating conditions in the study area,the marble agglomerates have undergone erosion,resulting in karst morphology primarily characterized by small and medium-sized caves.The connectivity between these caves is poor,and the degree of karst development ranges from weak to moderate.(4)The development of karst is influenced by fault structures,depth,topographic features,and the degree of weathering,exhibiting strong zoning and heterogeneity.Karst development pronounced in the NE direction,followed by the EW and NW directions,which aligns with the main orientations of regional faults.Karst is more developed in sections with burial depths of 10 to 20 m and 50 to 60 m,and at elevations of 30 to 40 m and 50 to 60 m.Karst development is stronger in valley areas and weaker on hillside.It is most developed in strongly to moderately weathered zones,followed by the residual soil to completely weathered zones,while karst development is weak in slightly weathered area. This study innovatively analyzes the formation of marble agglomerations based on the history of volcanic eruption.Through DEM data interpretation,Wutong Mountain is shown to have evolved from early"central eruption+peripheral multi-point eruption"phase to a later"central eruption"phase.It is inferred that early intense eruptions formed marble-bearing agglomerated volcanic rocks.Due to the weakening of eruption intensity and the lengthening of eruption path during the middle and late stages,the limestone blocks failed to integrate into the volcanic rock mass.Most of Wutong Mountain's large block-bearing volcanic rocks were subsequently covered by later eruption materials,while the eastern periphery was exposed or shallowly buried due to fewer eruption events,providing favorable conditions for the subsequent dissolution of marble agglomerations. The research findings will provide a foundation for the prevention and control of karst environmental geology and engineering geological problems in the volcanic rock area of Wutong Mountain as well as provide a reference for karst research in similar volcanic rock areas.