Influence of fracture stratigraphy on hypogene cave development and fluid flow anisotropy in layered carbonates, NE Brazil

Abstract

Abstract Fractured and karstified carbonates are often major aquifers and hydrocarbon reservoirs. The Toca da Boa Vista (TBV) and Toca da Barriguda (TBR) caves in NE Brazil, which form the longest cave system in South America, developed in a gently folded, layered and fractured Neoproterozoic carbonate sequence made of different lithological units (limestones versus siliciclastic rocks and marls) with variable bed properties (bed thickness, surface roughness of bed-to-bed interlayers) and fracture attributes (spacing, height). The functional organization of the hypogenic cave system has been recently revealed, with subdivision in three speleo-stratigraphic storeys which, from bottom to top, acted as recharge, lateral redistribution and outflow components of the rising fluid flow system, respectively. Structural analysis and new fracture data from TBV and TBR indicate that the stratigraphic setting controlled the fracture pattern development during two tectonic phases, which determined the fluid flow anisotropy during speleogenesis. We found that mechanical and fracture stratigraphy, in conjunction with bed thickness and bed-to-bed interlayer characteristics, controlled the spacing and height of fractures and, as a result, strongly influenced the fluid flow pattern and functional organization of the cave system. In particular, bed-normal rising fluids localized along m-to tens of m-spaced throughgoing fractures developed in the lower and upper storeys, whereas horizontal and bed-parallel fluid flow was focused in a package of thin carbonates, siliciclastics and marls with cm-spaced, strata-bound fractures. This fluid flow anisotropy is attained also due to the presence of rough-walled, bedding-parallel interlayers (i.e., burial stylolites) in the lower massive grainstone which hampered bed-parallel flow, and smooth, planar bedding-parallel interlayers which enhanced lateral fluid movements in the well-layered and fractured middle storey units. Therefore, we conclude that in heterolithic layered carbonate packages, the intricate hypogenic karst pattern is related to heterogeneous fluid flow behaviour associated with the structural complexity imparted by the mechanical versus fracture stratigraphy.