Structural control on the formation of iron-oxide concretions and Liesegang bands in faulted, poorly lithified Cenozoic sandstones of the Paraba Basin, Brazil
F. Balsamo et al., Dept. of Physics and Earth Sciences, Parma University, Campus Universitario, Parco Area delle Scienze 157/A, I-43124, Parma, Italy. Posted online 7 March 2013; http://dx.doi.org/10.1130/B30686.1.
In this contribution, F. Balsamo and colleagues describe the occurrence and geometry of different types of iron oxide deposits, which are significant indicators of the mobility of Fe2+ and O2 in shallow groundwater, associated with strike-slip faults developed in the vadose zone in quartz-dominated sandstones of the Paraba Basin, NE Brazil. The development of highly permeable and low-permeability domains along isolated fault segments promoted the physical mixing of Fe2+-rich waters and oxygenated groundwater. This arrangement favors O2 diffusion in flowing Fe2+-rich waters and, consequently, iron oxide precipitation as sand impregnations, small nodular concretions, and well-cemented mineral masses. The formation of hydraulically isolated compartments along more complex strike-slip fault zones promoted the development of Liesegang bands (a classical example of spontaneous self-organization process) in a reaction zone dominated by pore-water molecular diffusion of O2 into Fe2+-rich stagnant water. The structural-diagenetic coupling described in this paper support the role of tectonic activity on near-surface sandstone diagenesis in determining preferential hydraulic pathways for the physicochemical interaction between oxygenated groundwater and iron-rich fluids. Structural setting, fault zone architecture, and related grain size-permeability structures determine the dominant mode of solution
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