Fault-related Advective and Diffusive Fluid Flow from Fe-oxide Concretions and Liesegang Bands in Poorly Lithified Sands

Iron oxide coloration and deposits in sandstone are significant indicators of the mobility of solutes (Fe2+ and O2) in groundwater, mainly controlled by host 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 lithified, quartz-dominated, heterolithic sandy sediments in the Paraíba Basin, NE Brazil. Development of highly-permeable damage zones and low-permeability fault core-mixed zones, promote physical mixing of advective Fe2+-rich waters and oxygenated groundwater along soft-linked fault zones. This favors iron oxide precipitation as m-scale sand impregnations, cm- to dm-scale concretions, and well cemented dm- to m-thick mineral masses. The formation of hydraulically isolated compartments along hard-linked, strike-slip faults promotes the development of Liesegang bands, in a reaction zone dominated by pore-water molecular diffusion of O2 into Fe2+-rich stagnant water, and the precipitation of Fe-oxide impregnations and concretions in the fault core-mixed zone boundaries, due to O2 diffusion in advective Fe2+-rich waters. Fault zone architecture, permeability and geometry determine the dominant mode of solution interaction, leading to the formation of Fe-oxide Liesegang bands where O2 diffuse in stagnant Fe2+-rich water, and concretions when diffusion is complemented by Fe2+ advective flow.