Structural control on the formation of iron oxide concretions and Liesegang bands in faulted, poorly lithified Cenozoic sandstones of the Paraíba basin, Brazil.

Iron-oxide coloration and deposits in sandstone are signifi cant indicators of the mobility of solutes (Fe2+ and O2) in groundwater, mainly controlled by host-rock porosity and permeability. We describe the occurrence and geometry of different types of iron-oxide deposits developed within the vadose zone along faults affecting poorly lithifi ed, quartzdominated, heterolithic sands in the Paraíba Basin, NE Brazil. The development of highly permeable damage zones (100–102 Darcy) and low-permeability fault-core–mixed zones (10–3–101 Darcy) promotes the physical mixing of Fe2+-rich waters and oxygenated groundwater. This arrangement favors iron-oxide precipitation as meter-scale sand impregnations, centimeter- to decimeterscale concretions, and well-cemented decimeter- to meter-thick mineral masses. The formation of hydraulically isolated compartments along hard-linked strike-slip faults promotes: (1) the development of Liesegang bands in a reaction zone dominated by pore-water molecu lar diffusion of O2 into Fe2+-rich stagnant water, and (2) the precipitation of iron-oxide impregnations and concretions in the fault-core–mixed zone boundaries, likely by O2 diffusion in fl owing Fe2+-rich waters . Late-stage fault reactivation provides preferential pathways for the circulation of gravity-driven reducing fl uids, resulting in localized dissolution of iron and bleaching along fractures and iron remobiliza tion. These relationships reveal the roles of tectonic activity and near-surface sandstone diagenesis in determining preferential hydraulic pathways for the physicochemical interaction between oxygenated groundwater and iron-rich fl uids. Structural setting, fault-zone architecture, and related grainsize– permeability structures determine the dominant mode of solution interaction, leading to the formation of iron-oxide Liesegang bands where O2 diffuses into stagnant Fe2+- rich water, and concretions when diffusion is complemented by Fe2+ advective fl ow.