CATASTROPHIC KARST-SUFFOSION SINKHOLE IN THE KHADAKHAN-MELKHITUI KARST MASSIF, SOUTHERN PRIANGARIE: FACTORS, ENVIRONMENT, MECHANISM

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Abstract
This paper presents the results of a comprehensive study of a catastrophic karst-suffosion sinkhole that occurred on September 19, 2024, in the Khadakhan-Melkhituy karst massif (Southern Angara region) and resulted in a human fatality. Using UAV‑derived data, the morphometric parameters of the sinkhole were determined (depth 9.7 m, volume 412 m³), and a comparison was made with historical monitoring data from the 1980s. Laboratory tests performed on samples collected from the sinkhole walls revealed that the 9.7‑m thick cover sequence consists of structurally unstable loess‑like loams and sandy loams, with anomalously high silt content (up to 84 %) and porosity (up to 64 %). The interaction between karst and suffosion processes plays a key role in the sinkhole formation. The long‑term (57‑year) backwater effect of the Bratsk Reservoir and its cyclic level fluctuations caused leaching of sulfate‑carbonate rocks, promoting mechanical removal of silt‑sized particles from the cover sequence by percolating water. A two-stage mechanism of karst-suffosion sinkhole development was identified: from the formation of a loosening zone above an ancient karst cavity to the instantaneous (brittle) roof collapse triggered by a technogenic factor – the weight of heavy agricultural machinery. The study of the reactivation of karst‑suffosion processes in the backwater zone of the Bratsk Reservoir contributes to the understanding of modern geodynamic processes and ground surface deformation mechanisms, including those induced by technogenesis.

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