Structural control on silicification and hypogenic karst: insights from Morro Vermelho cave, Irecê Basin, Brazil

Fault-related karst systems and silicification processes are important factors in controlling permeability heterogeneities in shallow crusts. In recent years, the interest in these processes has become more important since they significantly modify the texture, mineralogy, and petrophysical properties of carbonate reservoirs.

The hypogenic Morro Vermelho Cave, in the Irecê Basin, Bahia (Brazil), is a key study area for understanding the development of fault-related silicification and subsequent karstification along fault networks in dolomitized carbonate rocks of the Neoproterozoic Salitre Formation.

This contribution focuses on a tridimensional digital cave model analysis and a detailed outcrop and cave investigation to constrain fracture attitude, type, geometry, and kinematics. Field data show the presence of regional-scale E-W thrust data.

The Morro Vermelho cave is developed in the proximity of one of these thrusts and is mostly developed along silicified carbonates with bedding dipping 40° toward SE.

The analysis of the lidar model reveals that the cave has an irregular morphology with several branching passages controlled by major N-S to NNE-SSW thoroughgoing fractures.

Both 3D model and high-resolution structural mapping in the cave highlight the presence of high-angle N-S-oriented normal faults and the following fracture sets: E-W-striking sheared veins and joints, bed-parallel NE-SW-striking veins, fault-parallel N-S-striking veins and joints, and bed-perpendicular NW-SE-striking veins and joints.

The obtained results indicate that the silicification process was mostly controlled by regional-scale E-W-striking thrust, associated with the N-S shortening of the Brasiliano orogeny, whereas the Morro Vermelho cave developed mostly along bedding layers and small-scale N-S-oriented normal faults, system and related fractures. The fractures also controlled the occurrence of late-stage silica crusts that post-date the cave development.

We propose a preliminary conceptual model that includes different stages of silicification events and karstification related to fault systems and fracture networks, in alignment with the structural regional-scale evolution of the study area.

The findings might be highly significant for understanding the permeability characteristics of deeply buried pre-salt fractured carbonate reservoirs in offshore Brazil and other similar settings.