Karst porosity development in layered and fractured carbonates: field evidences of structural control on sulfuric acid speleogenesis (Majella Massif, Italy)

Sulfuric acid speleogenesis (SAS) has been widely recognized as one of the most aggressive processes involving carbonate dissolution and rapid formation of karst porosity under hypogenic conditions. Italian carbonate sequences, and especially those outcropping in the Central Apennines, host some of the best studied hypogenic SAS caves of Italy (such as Frasassi, Monte Cucco, Acquasanta Terme, just to mention the most famous).

The Cavallone-Bove cave system (CBS) is one of the longest natural caves in Abruzzo region (over 1 km of length) and opens at ca 1470 m asl in the Taranta Gorge, Majella Massif. The sulfuric-acid origin of this inactive hypogenic system has been previously proven by D’Angeli et al. (2019) using field evidences, secondary minerals and stable isotopes analysis. ⁴⁰Ar/³⁹Ar dating of alunite deposits suggested the SAS process occurred about 1.52 ± 0.28 Ma.

Both caves are characterized by a main sub-horizontal rounded or trapezoidal passage with only minor secondary branches and sub-vertical rift-conduits (feeders). Spatial geometry and arrangement of CBS conduits differs significantly from typical SAS water table caves, where complex anastomotic or maze network patterns are observed. Combining classical geological surveys, fracture stratigraphy and cave morphogenetical analysis we characterized the speleogenesis of the CBS. Field observations, remote sensing, detailed geological and geomorphological surveys were performed to characterize the structural evolution of the carbonate sequence hosting the caves, and to explain the relationship with the peculiar spatial and functional organization of CBS.

Our work highlights that lithostratigraphy and fractures pattern guide the development of karst macro-porosity in a specific stratigraphic interval within the Santo Spirito Formation, consisting mainly of layered micritic limestones, confined at the top by a chert interlayers dominant member. Through-going faults and fracture-clusters zones are identified as the main permeability pathways for ascending and laterally spreading H₂S fluids, influencing the spatial localization of conduits. These fluids reacted close to past water table in oxi-conditions, creating aggressive H₂SO₄. Sulphur stable isotopes signatures of secondary minerals suggest an origin for these H₂S bearing fluids from deep-seated Triassic evaporites interacting with hydrocarbons, thus migrated upward through a network of interconnected fractures. Permeability pathways for this vertical ascending flow were provided by NW-SE persistent strike-slip fault zones segmenting the eastern front of the Majella anticline structure and NNE-SSW striking fracture-clusters localized in the hinge zone of the fold.