Genetic mechanism and comparative significance of the"double-dome structure"of the Baimadong-Xiaodong Cave System in Gulin county

ZHOU Wenlong; ZHAO Shiqi; MO Guifen; TAN Ming

doi:10.11932/karst20250605

Abstract：Since the concept of the"double-dome structure"was first proposed following its discovery in the Ziyun Miaoting Chamber of Guizhou in 2021,no similar structures had been reported in other karst caves,leading to its initial classification as a unique occurrence.This perception changed when a suspected"double-dome structure"was identified in the Baimadong Cave-Xiaodong Cave System in Gulin county,Sichuan.This new finding provides an excellent opportunity for comparative analysis,significantly advancing the scientific understanding of this distinctive speleological form.The Baimadong Cave-Xiaodong Cave System is situated in a mountainous valley along the periphery of a polje,on the northern side of the western segment of the Dalou Mountains fold belt within the Wumeng Mountain system:a transitional zone between the Sichuan Basin and the Yunnan-Guizhou Plateau.The area features pronounced topographic relief and a well-developed hydrological network.The Baishahe River flows underground at the entrance of Xiaodong Cave,travels approximately 1.2 km in a southwestern direction,and reemerges at the exit of Xiaodong Cave,with a hydraulic gradient of 5.5％.It eventually converges with the Bajiaogou River near Baishachang Town and discharges into the Chishuihe River.Geotectonically,the region lies between the Sichuan-Yunnan and Sichuan-Guizhou meridional structural zones,bordered to the north by the Sichuan Basin-a first-order subsidence zone of the Cathaysian structural system-and adjacent to the"Central Guizhou Uplift"along the northern edge of the complex east-west trending Nanling structural belt to the south.The area widely exposes strata ranging from the Cambrian to the Cretaceous,with the Devonian and Carboniferous systems absent.The exposed formations primarily consist of carbonate rocks such as dolomite and limestone,along with sandstone,shale,and Jurassic purple sandy shale,providing favorable conditions for karst development.The Baimadong Cave-Xiaodong Cave System has developed in mid-to thick-bedded limestone of the lower Permian Maokou Formation(P1m),which is 151 to 337 m thick and offers excellent conditions for cave formation.This study combines field surveys to document dissolution features with terrestrial laser scanning to conduct precise,georeferenced 3D mapping of the cave system.Point cloud slicing techniques were applied to deconstruct the spatial data,with a focus on the suspected"double-dome structure",to analyze its morphological characteristics and genetic mechanisms.Through comparison with the Ziyun Miaoting Chamber,we aim to generalize a universal genetic model for the"double-dome structure"and to systematically examine the developmental drivers of the Baimadong-Xiaodong Cave System,thereby enhancing the understanding of the evolution and geomorphic impact of subterranean chambers.The main conclusions are as follows. (1)Three-dimensional morphological analysis confirms that the structure in the Baimadong-Xiaodong Cave System is consistent with the scientific concept of a"double-dome structure"proposed in 2021,demonstrating that such structures are not unique in karst environments. (2)The development of the"double-dome structure"is horizontally controlled by multiple sets of multi-phase joints with varying orientations and spacing,where dense joint systems play a critical role in forming internal polygonal patterns.Vertically,the structure reflects the combined effects of tectonic uplift and fluvial incision,modified by multi-phase water flow.Its position on the flank of an anticline facilitates the opening of basal fractures,promoting groundwater infiltration.The mid-to thick-bedded,massive limestone of the Maokou Formation provides the necessary lithological conditions for stress-induced collapse and long-term stability. (3)The development of the Baimadong-Xiaodong Cave System is closely linked to its regional geological and hydrological setting.The traverse of the Baishahe River creates a steep hydraulic gradient across the cave,while abundant rainfall and a large catchment area provide sustained hydrodynamic forcing,serving as the primary agent for speleogenesis.The competent limestone of the Maokou Formation offers a suitable material basis for the development of extensive cave passages and significant vertical relief.The presence of tiered cave levels reflects neotectonic uplift,and well-developed faults and folds have contributed to the structural complexity of the system. (4)A spiral structure within the cave records a shift in the flow path of the Baisha underground river between the upper and lower levels,indicating at least two distinct phases of water flow:an early phase from the Xiaodong Cave entrance toward Baimadong Cave,followed by a later phase developing downward from the Xiaodong Cave entrance to its exit. (5)Comparative analysis with the Ziyun Miaoting Chamber reveals consistent developmental patterns for"double-dome structure".They typically form in intensely jointed zones at the hinge-limb transition of folds,at specific depths below the surface,preferably in mid-to thick-bedded,massive limestone.This lithology provides a balance between collapse potential and structural stability.Their formation is controlled by tectonic uplift and river incision,and they commonly undergo modification through multi-phase water flow.