During the Jurassic to Cretaceous periods, a set of continental red basin sediments were deposited in front of Longmen Mountain on the western margin of the Sichuan Basin. The predominant lithology consisted of mud-calcium cemented sandstone (conglomerate). Under the action of groundwater dissolution, the rock layer has been developed with large-scale karst phenomena, including depressions and extensive horizontal karst caves that extend thousands of meters. These features pose a major threat to the safety of constructing reservoirs and tunnels in the study area. Based on existing geological and hydrogeological investigations, combined with ground surveys, and identification of rock and mineral types, this study examines the development characteristics and controlling factors of sandstone (conglomerate) dissolution from multiple perspectives. These perspectives include macroscopic structural evolution, sedimentary environment, hydrodynamic conditions, microscopic mineral composition, and cementing characteristics. The main conclusions of this study are as follows.

The karst forms developed on the surface of the study area mainly include karst depressions, karst hills, sinkholes, stone teeth, karst valleys, karst fissures, and karst gullies. In the conglomerate area, circular depressions resulting from collapses are often formed on the surface, with sinkholes frequently developing at the base. In contrast, the sandstone area is typically eroded by surface water along joints and fissures, leading to the formation of long troughs or irregular corrosion funnels. The underground karst phenomenon is characterized by karst caves, which contain deposits such as stalactites, stone mantles, and stone flowers. These karst deposits are relatively rare and often impure, primarily consisting of gray muddy calcareous mixture that is mainly composed of calcium carbonate mixed with clay materials. The entire karst phenomenon exhibits the features typical of early-stage karst landforms, which can be classified as either red-bed karst landforms or sandy gravel karst landforms.

The karst phenomenon is mainly developed along the bedding plane in the trend of rock formations, and secondarily in the dip direction. The local extension direction is controlled by the joints and fissures. In the areas with strong karst development, the lithology of the strata is dominated by calcareous conglomerate of the Guankou formation (K₂g) and the Tianmashan formation (K₁t). The karst morphology is diverse and of large scale, often featuring a complete set of karst systems, developed with vertical recharge and horizontal discharge. In the areas with moderate karst development, the lithology consists of calcareous conglomerate of the Guankou formation (K₂g) and the Tianmashan formation (K₁t), conglomerate of the Mingshan formation (E_1-2m), and sandstone of the Shaximiao formation (J₂s) and the Penglai formation (J₃p). In this area, karst fissures and pores are widely developed, with visible karst caves and small-scale karst pipelines. The depth of horizontal karst development is generally less than 100 meters. In the areas of weak karst development, the main lithology is dominated by conglomerate of the Mingshan formation (E_1-2m), and sandstone and mudstone of the Jiaguan formation (K₂j), the Shaximiao formation (J₂s), and the Shaximiao formation (J₃p). The main karst phenomena are karst fissures and pores developed along the bedding plane or dominant joints.

The sedimentary environment is the fundamental internal factor controlling karst development. The complex and variable sedimentary environment has led to differences in lithology and structures of rock groups in both the planar and vertical directions within the study area, which in turn control the location and intensity of red-bed karst development. The provenance of Mesozoic and part of Paleozoic carbonate rocks in the north–west of the study area ensured the solubility of sandy conglomerate in the area. The alluvial fan sediments corresponding to the three intense tectonic phases during the Yanshanian Period are manifested as areas with strong karst development. Differences in lithology lead to variations in soluble mineral contents, showing a trend of gradually decreasing solubility from calcareous conglomerate to conglomeratic sandstone, then to siltstone, and finally to mudstone, accompanied by a corresponding weakening of karst action. Hydrodynamic conditions under the control of climate, topography, and geological structures are important external factors affecting karst development. The warm and humid climate in the study area provides abundant recharge and erosion capacity for groundwater. The landform and strong tectonic action control the scale and extension direction of red-bed karst development. During the Neotectonic Movement, intermittent uplift and stability have resulted in a multi-layered karst system in the study area, which corresponds, to some extent, with the multi-tiered river terraces.