Preliminary study on the evolutionary process and genetic mechanism of the contact karst tiankeng in Qianxi, Guizhou

Contact karst tiankengs develop at the contact zone between soluble and insoluble rocks, yet research on their evolutionary mechanisms and spatial differentiation patterns remains relatively limited. This study focuses on the newly discovered Dadongkou tiankengs(central coordinates at 27°07′18.96″N, 105°53′59.63″E)in Qianxi, Guizhou Province. These tiankengs primarily developed at the contact zone between the clastic and carbonate rocks of the Lower Triassic Jialingjiang Formation. Utilizing high-precision geomorphological survey and structural analysis, it is ascertained that the maximum depth of the largest tiankeng(Luojingwei)reaches 150 m, with a projected area of 26.38×104 m2. This study identifies the distinctive "karst window exposure" phenomenon characteristic of this contact karst type. A multi-stage evolutionary model was established: "Concentrated dissolution at the contact zone—deep dissolution after karst window exposure—differential collapse." The research reveals that tiankeng development is primarily driven by the coupled interaction of three mechanisms: structural joint control, hydrological convergence at the contact zone, and lithological differentiation. Furthermore, regional tectonic uplift drove a multi-stage, pulse-like descent of the paleo-groundwater table. During this process, the regional discharge base level underwent three sequential shifts: from the 1500~1560 m contact zone, down to the 1470 m horizontal cave system, and finally to the sinkhole at the tiankeng floor(incised to 1340 m). Based on these findings and mechanistic analysis, this paper proposes a supplementary classification scheme for karst and contact tiankengs, further subdividing contact karst tiankengs into thin-cover and thick-cover subcategories. This classification framework provides a scientific basis for refining the tiankeng classification system, guiding engineering geological risk assessment and conservation in contact karst regions.