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

Fracture Network Patterns from the Brejões Outcrop, Irecê Basin, Brazil

Fracture patterns in the Neoproterozoic carbonates of the Salitre Formation from the Brejoes outcrop, Irece Basin,Brazil, digitised from drone photogrammetry using an automated fracture extraction tool. The fracture traces are georeferenced to a local coordinate system (EPSG:32724, WGS 84 / UTM zone 24S). The data contains: - shapefile of the fracture network, basemap orthomosaic and P21 fracture intensity raste

The Use Of Apatite Fission Track Thermochronology To Constrain Fault Movements And Sedimentary Basin Evolution In Northeastern Brazil

An apatite fission track study of crystalline rocks underlying sedimentary basins in northeastern Brazil indicate that crustal blocks that occur on opposite sides of a geological fault experienced different thermal histories. Samples collected on the West block yielded corrected fission-track ages from 140 to 375 Ma, whereas samples collected on the East block yielded ages between 90 and 125 Ma. The thermal models suggest that each block experienced two cooling events separated by a heating event at different times. We concluded that the West block moved downward relative to the East block ca. 140 Ma ago, when sediments eroded from the East side were deposited on the West side. This process represents the early stage of sedimentary basin formation and the opening of the South Atlantic Ocean in the region. Downward and upward movements related to heating and cooling events of these crustal blocks at different periods until recent times are proposed. © 2005 Elsevier Ltd.396627633Bigazzi, G., Guedes, S., Hadler Neto, J.C., Iunes, P.J., Paulo, S.R., Tello Saenz, C.A., Application of neutron dosimetry by natural uranium and thorium thin films in fission-track dating (2000) Extended Abstract of Ninth International Conference on Fission Track Dating in Thermochronology, pp. 33-35. , Lorne, AustraliaFleischer, R.L., Hart, H.R., Fission track dating: Techniques and problems (1972) Calibration of Humanoid Evolution, pp. 135-170. , W.W. Bishop J.A. Miller S. Cole Scottish Academic Press EdinburgFrançolin, J.B.L., Szatmari, P., Mecanismo de rifteamento da porção oriental da margem norte brasileira (1987) Rev. Bras. Geoc., 17, pp. 196-207Galbraith, P.F., On statistical models for fission track counts (1981) J. Math. Geol., 13, pp. 471-488Gleadow, A.J.W., Fission-track dating methods - What are the real alternatives? (1981) Nucl. Tracks Radiat. Meas., 5, pp. 3-14Green, P.F., A new look at statistics in fission-track dating (1981) Nucl. Tracks Radiat. Meas., 5, pp. 77-86Green, P.F., The relationship between track shortening and fission track age reduction in apatite: Combined influence of inherent instability, annealing anisotropy, length bias and system calibration (1988) Earth Planet. Sci. Lett., 89, pp. 335-352Guedes, S., Hadler Neto, J.C., Iunes, P.J., Paulo, S.R., Zuñiga, A., The spontaneous fission decay constant of 238U using SSNTD (2000) J. Radioanal. Nucl. Chem., 245, pp. 441-442Guedes, S., Hadler Neto, J.C., Iunes, P.J., Zuniga, A., Tello, C.A., Paulo, S.R., The use of the U(n,f) reaction dosimetry in the determination of the λf value through fission-track techniques (2003) Nucl. Instrum. Meth. A, 496, pp. 215-221Guedes, S., Hadler Neto, J.C., Sarkis, J.E.S., Oliveira, K.M.G., Kakazu, M.H., Iunes, P.J., Saiki, M., Paulo, S.R., Spontaneous-fission decay constant of 238U measured by nuclear techniques without neutron irradiation (2003) J. Radioanal. Nucl. Chem., 258, pp. 117-122Guedes, S., Hadler Neto, J.C., Iunes, P.J., Tello Saenz, C.A., Kinetic model for the relationship between confined fission-track length shortening and fission-track age reduction in minerals (2004) Nucl. Instrum. Meth. B, 217, pp. 627-636Hackspacher, P.C., Legrand, J.M., Deformation and metamorphism on the Portalegre Shear Zone, Northeastern Brazil (1989) Rev. Bras. Geoc., 19, pp. 63-75Hadler Neto, J.C., Paulo, S.R., Iunes, P.J., Tello, C.A., Balestrieri, M.L., Bigazzi, G., Curvo, E.A.C., Hackspacher, P.C., A PC compatible Brazilian software for obtaining thermal histories using apatite fission track analysis (2001) Radiat. Meas., 34, pp. 149-154Holden, N.E., Hoffman, D.C., Spontaneous fission half-lives ground-state nuclides (Technical report) (2000) Pure Appl. Chem., 72, pp. 1525-1562Hurford, A.J., Standardization of fission track dating calibration: Recommendation by the Fission Track Working Group of the I.U.G.S. Subcommission on Geochronology (1990) Chem. Geol., 80, pp. 171-178Iunes, P.J., Hadler Neto, P.C., Bigazzi, G., Tello Saenz, C.A., Guedes, S., Paulo, S.R., Durango apatite fission-track dating using length-based age corrections and neutron fluence measurements by natural Thorium thin films and natural U-doped glasses calibrated through natural uranium thin films (2002) Chem. Geol., 187, pp. 201-211Iwano, H., Danhara, T., A re-investigation of the geometry factors for fission-track dating of apatite, sphene and zircon (1998) Advances in Fission-track Geochronology, pp. 47-66. , P. Van den haute F. De Corte Kluwer Academic Publishers DordrechtJonckheere, R., On the densities of etchable fission tracks in mineral and co-irradiated external detector with reference to fission-track dating of minerals (2003) Chem. Geol., 200, pp. 41-58Matos, R.M.D., The northeast Brazilian rift system (1992) Tectonics, 11, pp. 766-791Morais Neto, J.M., Hegarty, K.A., Karner, G.D., Alkimin, F.F., Matos, R.M.D., Uplift and erosion in the Borborema Province, northeast Brazil: Insights from apatite fission track analysis (2000) XXXI International Geological Congress, , Abstract Volume, IUGS/SBG, Rio de Janeiro, Brazil. GS 18.1Tello, C.A., (1998) Estudo de Annealing de Traços de Fissão em Apatitas, Tanto em Seções Basais Quanto em Seções sem Orientação Preferencial, Análise dos Comprimentos dos Traços de Fissão, 111p. , UNICAMP, Campinas, Brazil, Ph.D. Thesi

Hydrothermal activity along a strike-slip fault zone and host units in the São Francisco Craton, Brazil – Implications for fluid flow in sedimentary basins

This study combines multiscale analyses of geological, fault, fracture, and stable isotope data to investigate strike-slip deformation and channeling of hydrothermal fluids along the Cafarnaum fault and calcite veins at different distances from the fault, which is a structure in the São Francisco Craton, northeastern Brazil. Meteoric fluids with δD values near −45‰ and δ18O values near −6.5‰ and temperatures at 40–70 °C precipitated as calcite veins in the host carbonate units. The Cafarnaum fault, a N-S-striking vertical, ~170 km long fault zone, juxtaposes Neoproterozoic carbonate rocks in the western block and Mesoproterozoic siliciclastic rocks in the eastern block. A zone of restraining bends occurs at the central part of the fault, whereas termination zones of horsetail geometry occur at both ends of the Cafarnaum fault. These zones are marked by NW-SE-striking extensional faults that are oblique to the main N-S-striking fault zone, where hydrothermal deposits occur. The zone of influence of the Cafarnaum fault is ~ 20 km wide around the main fault. The fault formed during the Brasiliano orogeny (740–560 Ma) after Neoproterozoic carbonate platform deposition. In contrast with the host units, fluids along the fault zone originated in deeper levels of the crust and show much lower δ18O values, indicating higher crystallization temperatures. These fluids caused brecciation in the Neoproterozoic carbonate host rocks, whereas a subsequent decrease in fluid pressure and cooling near the surface resulted in the precipitation of a hydrothermal paragenesis in veins, also affecting the host rock