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