--> Early Diagenetic Evolution of Fabric in the Barra Velha Formation, Santos Basin, Brazil

AAPG ACE 2018

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Early Diagenetic Evolution of Fabric in the Barra Velha Formation, Santos Basin, Brazil

Abstract

In order to provide insight into the controls on formation and diagenesis of largely non-marine carbonate successions deposited along the South Atlantic margin, we present combined petrographic, geochemical, mineralogical, and stable isotopic analyses of carbonate rocks from the Barra Velha Formation of the Santos Basin. These are used to highlight the key factors that contribute to the evolution of mineralogy, as well as controls on early diagenesis, which create the final carbonate depositional fabrics. In turn, these create testable hypotheses for the general controls on formation and evolution of carbonate deposits of the Barra Velha Formation.

Spherulitic facies records a complex series of sedimentary and diagenetic events strongly influenced by chemical evolution of parent waters. Spherulitic growth of fibrous calcite preceded contemporaneously with the deposition of extremely fine-grained Al-rich dust. Changes in fluid chemistry triggered dissolution of spherulite components, contemporaneous with, or shortly followed by, silica cementation. Mg-silicate precipitation engulfed spherulitic components, which was syndepositional with microcrystalline dolomite crust formation around the spherulites, in spherulite cores, and within the Mg-silicate matrix. Magnesite developed at the interface between Mg-silicates and dolomite, and euhedral dolomite overgrowths developed during Mg-silicate de-watering and shallow burial.

Shrubby boundstone facies also record a complex series of depositional and diagenetic events that reflect a strong chemical control on their formation and alteration. Deposition of shrubby boundstone facies began with upward divergent growth of fibrous high-Mg calcite crystal bundles, which were periodically terminated by conditions that halted crystal growth, or, less commonly, dissolution of crystal edges. Shrub growth was closely followed by recrystallization, and partial obliteration of internal fibrous to prismatic calcitic fabric, as well as local dissolution and precipitation of silica cements (as microcrystalline quartz). This was followed by the development of microcrystalline dolomite crusts and euhedra, as well as the deposition of Fe-rich glauconitic clay, organic matter, and sparry calcite cements occupying intra-shrub voids.