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

Abstract

Abundant early diagenetic siderites occur as spherulites and rhombohedral microcrystalline and macrocrystalline 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 relationships with the other constituents and with the host sediments. Spherulitic to macrocrystalline siderites from group 1 are almost pure (average: 94.7mol% FeCO3; 1.2mol% MgCO3; 2.3mol% CaCO3; 1.8mol% MnCO3) and precipitated from meteoric porewaters in continental siliciclastic rocks under suboxic conditions (δ18Ovpdb values range in −10.28 to −5.57‰ and the δ13Cvpdb values in −12.68 to −4.33‰). Microcrystalline rhombohedral siderites from group 2 have zonation due to substantial Ca and Mg substitution (core average: 78.5mol% FeCO3; 4.2mol% MgCO3; 15.7mol% CaCO3; 1.6mol% MnCO3; edge average: 74.0mol% FeCO3; 9.2mol% MgCO3; 15.6mol% CaCO3; 1.1mol% MnCO3), and δ13Cvpdb and δ18Ovpdb values of +0.17‰ and −1.96‰, 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 (average: 80.2mol% FeCO3; 7.9mol% MgCO3; 11.3mol% CaCO3; 0.6mol% MnCO3), with δ18Ovpdb values ranging from −5.96 to −7.61‰ and δ13Cvpdb values ranging from −5.15 to −10.41‰. The group 4 microcrystalline siderites are magnesium-rich (average: 57.3mol% FeCO3; 31.4mol% MgCO3; 9.6mol% CaCO3; 1.7mol% MnCO3; δ13Cvpdb +1.43‰ and δ18Ovpdb −14.09‰). 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.