Paleoenvironmental implications of early diagenetic siderites of the Paraíba do Sul Deltaic Complex, eastern Brazil

Journal of Sedimentary Geology 323 (2015).

Abstract

Abundant early diagenetic siderites occur as spherulites and rhombohedral microcrystalline andmacrocrystalline crystals in the cores of the 2-MU-1-RJ well, drilled in the Paraíba do Sul Deltaic Complex, Rio de Janeiro (Brazil). The host sediments of the siderites are siliciclastic, hybrid, and carbonate deposits. Intense pedogenetic processes affected the siliciclastic sediments immediately after deposition, comprising clay illuviation, plants bioturbation, feldspar dissolution, and iron oxide/hydroxide precipitation. Siderite and pyrite are the main diagenetic constituents. The other diagenetic products are kaolinite, smectite, argillaceous and carbonate pseudomatrix, quartz overgrowths, diagenetic titanium minerals, jarosite, and iron oxides/hydroxides. Early diagenetic siderites were separated into four groups based on their elemental and stable isotopic composition, as well as on their paragenetic relationshipswith the other constituents andwith the host sediments. Spherulitic tomacrocrystalline siderites from group 1 are almost pure and precipitated from meteoric porewaters in continental siliciclastic rocks under suboxic conditions. Microcrystalline rhombohedral siderites from group 2 have zonation due to substantial Ca and Mg substitution, precipitated from marine porewaters in packstones/wackestones under methanogenic conditions. The group 3 is represented by irregular spherulitic siderites with moderate Ca and Mg substitutions. The group 4 microcrystalline siderites are magnesium-rich. The group 3 and 4 siderites were formed from brackish porewater under suboxic conditions in hybrid and siliciclastic rocks. These variations in siderites are probably related to the Paraíba do Sul River dynamics, to sea level changes and to climatic variations that took place during the Quaternary.

Authors: Amanda Goulart Rodrigues (1), Luiz Fernando De Ros (2), Reiner Neumann (3), Leonardo Borghi (1)

(1) Laboratório de Geologia Sedimentar, Instituto de Geociências, UFRJ, Rio de Janeiro, RJ, Brazil
(2) Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil
(3) Centro de Tecnologia Mineral, Rio de Janeiro, RJ, Brazil