Multi-phased hypogene speleogenesis in a marginal horst structure of the Malé Karpaty Mountains, Slovakia

The Plavecká jaskyňa Cave on the western fault edge of the Malé Karpaty Mountains (western Slovakia) is a result of multi-phased hypogene speleogenesis. It formed in fractured Triassic carbonates by waters ascending along the Vienna Basin Transform Fault between Malé Karpaty Mountains and Záhorská nížina Lowland (the north-eastern part of the Vienna Basin) and/or the N–S-trending faults that intersect it in the cave vicinity. Morphologically, the cave is featured by (1) phreatic chimneys, cupolas, ceiling pockets, enlarged fissures with spongework cavities, upward wall channels and upward oriented large scallops, (2) epiphreatic flat corrosion bedrock floors, feeding fissures and wall water-table notches, as well as (3) vadose vents, upward half tubes and shallow cupolas formed by condensation corrosion on the cooler overlying walls and ceilings. Initial fault-controlled phreatic morphologies of the cave formed due to the dissolution of limestones caused by ascending deep-seated water. The isotopic composition (O and C) of the uppermost thin layer of limestone bedrock on the cave wall resulted from its interaction with hypogene water. Flat corrosion bedrock floors truncate fissure discharge feeders, on the edges with wall water-table notches, indicate rapid lateral corrosion by the sulfuric low-thermal waters. Four subhorizontal passages have been developed at former levels of the piezometric surface during water table stagnations corresponding with phases of erosion base level stabilization in relation to the landform evolution during the subsidence of the adjacent part of the Vienna Basin. The passage of the lowest evolution level is at about the same elevation as the recent springs of slightly warmer groundwater near the cave (11.6 to 13.6°C; about to 3°C warmer than the regional mean-annual temperature). In addition to morphological indicators (flat corrosion floors and associated wall water-table notches), the sulfuric low-temperature acid speleogenetical phases of the Plavecká jaskyňa are indicated by the presence of gypsum in association with hydrated kaolinite, illite, clinochlore and montmorillonite (XRD) in rare deposits. Subaerial calcite popcorn rims were also precipitated due to H2O evaporation and CO2 degassing from condensation water at the edges of feeding fissures that were still active as thermal vents when the water table was dropped. Hydrogen sulfide involved in the sulfuric acid speleogenesis was probably derived from hydrocarbon reservoirs of adjacent Vienna Basin. Features similar to those detected in the Plavecká jaskyňa were identified also in some other caves of the Plavecký Karst (e.g., Plavecká priepasť Shaft, Pec Cave)