Karstification and fluid flow in carbonate units controlled by propagation and linkage of mesoscale fractures, Jandaíra Formation, Brazil

Abstract

This study examines how centimeter- to meter-long fracture sets propagate, link and form m-long structures, which are then karstified in the vadose (aerated) zone in the upper parts of exposed layered carbonate units. We characterize the fracture patterns, including both mesoscale extensional joints, veins, stylolites and macroscale faults, within the Rosario pavement, which is a 1100-m-long, 340-m-wide outcrop in the Jandaíra Formation, Potiguar Basin in semiarid Brazil. The work compares the directions of the mesoscale structures with the strikes of surface collapse dolines and cave conduits at depths of 10–20 m. The main results indicate that the background (diffuse) deformation consists mostly of bed-perpendicular, stratabound N-S-striking veins and extensional (opening mode-I) fractures and E-W- to ENE-WSW-striking stylolites, which are consistent with the same stress field. Both sets of structures are laterally continuous and are commonly traceable across the pavement. Linkage of stylolites and stratabound fractures formed via throughgoing fractures that affected several layers, as deep as 15 m. The throughgoing fractures were favorable structures for fluid pathway formation and karstification in the epigenetic environment. We conclude that sets of mesoscale structures fit with the orientations of the collapse dolines and subhorizontal cave passages, and induced hydraulic anisotropy within the vadose level. These findings indicate that the fractures, veins and stylolites grow, link and form localized vertical conduits for water percolation and karst development. These findings can constrain future models and numerical simulations of karst conduits in investigations of groundwater and oil reservoirs.