| Summary: | The exploitation of coal resources in the Huaibei mining area has provided crucial energy support for the industrial production and development of East China. The mining area is located on the southern margin of the North China Coalfield. The limestone of the Carboniferous Taiyuan Formation (referred to as "Taihui") occurs in the lower part of the Permian coal-bearing strata, characterized by "high water pressure and medium water abundance", and maintains a close hydraulic connection with the underlying Ordovician karst aquifer. With the gradual depletion of shallow resources, the threat posed by high-confined limestone water to mining safety under complex hydrogeological conditions in deep coal seams has become increasingly prominent. Historically, multiple "mine flooding" accidents occurred. To improve the technical system for preventing and controlling karst water hazards in the "North China-type" coalfields, this paper conducts a systematic investigation on the morphological structure characteristics and genetic evolution of karst features in the first to fifth limestone layers (L1—L5) of the Taihui. Methods such as hydrogeological tests and high-pressure mercury intrusion experiments were employed to systematically examine the distribution differences of karst pores, fractures, and cavities in both vertical and horizontal dimensions at macro- and micro- scales, and to evaluate the developmental characteristics of dissolved pores, cavities, and high- (low-) angle dissolved fractures.The results indicate that the Taihui in the northern Zhahe mining area is characterized by the development of karst fracture zones and cavities, whereas the southern Linhuan and southeastern Sunan mining areas are dominated by dissolved pores and fractures. Layers L1—L2 primarily contain mesopores and small-aperture macropores, while layers L3—L5, in addition to mesopores, also feature large-aperture macropores (approximately 100 μm). Based on an analysis of structural control over groundwater and strata, it was determined that the key reason for the vertical and horizontal differences in the karstification of L1—L5 in the Huaibei mining area is multi-phase tectonic movement. Horizontally, with the Subei Fault as the boundary, the northern mining area has undergone more intensive karstification. Vertically, the strongly karstified layers in the upper section of the Taiyuan Formation range from L3 to L5, which can serve as ideal target layers for regional grouting reinforcement and modification of the floor limestone. By integrating the regional tectonic evolution background and karst development characteristics, the transformative effects of regional tectonic evolution during various karst periods on the Taihui karst were discussed, and a genetic evolution model for the karst in L1—L5 of the Huaibei mining area was proposed: (1)Sedimentary Karst Period (302−295 Ma): During the deposition of L1—L5, frequent sea-level fluctuations occurred. At sedimentary hiatus interfaces, freshwater mixing and modification took place, leading to meteoric water leaching karst or interlayer karstification. Intergranular dissolved pores gradually expanded and interconnected, forming dissolved pores and interlayer dissolved fractures.(2)Indosinian-Yanshanian Karst Period (251−52 Ma): The high-angle and low-angle dissolved fractures formed in L1—L5 during this stage were stretched and further expanded by karst processes in the late Yanshanian period. The upper Taihui developed into a fracture network of considerable scale, marking the beginning of karstification differentiation on either side of the Subei Fault.(3) Himalayan Karst Period (52 Ma−present): The Huaibei mining area subsided into a new sedimentary depression phase, and L1—L5 underwent deep secondary burial karstification. The previously formed high- and low-angle dissolved fractures were filled with calcareous material after dissolution and expansion. The unfilled pores and fractures, along with the burial karst processes, developed into the observable karst pore-fracture-cavity system seen today.This study, through macro- and micro-scale investigations of the upper Taihui karst, has revealed the characteristics and genetic evolution model of pores, fractures, and cavities in the limestone of the study area. The main conclusions are as follows.(1) At the micro-scale, in the L1 limestone, the Sunan mining area exhibits the highest contribution from macropores with apertures <1 μm and mesopores, while other mining areas are dominated by mesopores with apertures <20 nm. In the L2 limestone, mesopores and macropores with apertures of 10 to 5,000 nm are the main pore types. In the L3 limestone, mesopores (<50 nm) dominate.(2) At the macro-scale, the L1 limestone across the Huaibei mining area is relatively intact, with rare fracture zones. In the Linhuan mining area, the L2 limestone develops fracture zones approximately 10 cm in length, with fracture surfaces mostly aligned parallel to the sedimentary bedding planes. In the Zhahe mining area, the L3 limestone exhibits fracture zones longer than 10 cm, with vertically developed fracture surfaces. The core is relatively fragmented, and fractures are often filled with calcareous material. In the Sunan mining area, the L4 limestone contains a fracture zone about 20 cm in length in its middle section.(3) The present-day karst characteristics in the Huaibei mining area represent the product of superimposed multi-phase tectonic movements. The activation of structures such as the Xusu arcuate thrust nappe and the Subei Fault has initiated erosive fluid circulation conditions, governing the horizontal and vertical paleo-karst differentiation within the mining area.(4) Four karst formation stages have been delineated: The Sedimentary Karst Period, Indosinian Period, Yanshanian Period, and Himalayan Period, which are divided into three developmental phases. The genetic evolution model of the upper Taihui karst in the Huaibei mining area comprises stages of primary porosity development, secondary modification, opening-closing of dissolution fractures, and differential burial.
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