CONTROLES COMPOSICIONAIS NA DIAGÊNESE DE ARENITOS

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Uppsala University, Sweden

DATA DE DEFESA: 1996
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COMPOSITIONAL CONTROLS ON SANDSTONE DIAGENESIS

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Uppsala University, Sweden

DATE OF ORAL PRESENTATION: 1996
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ABSTRACT

Aiming to address the role of compositional variables on sandstone diagenesis, detailed petrologic and geochemical studies were carried on twelve sedimentary clastic units from different basins in Brazil, Norway, Tunisia, Spain and the North Atlantic, ages ranging from Ordovician to Miocene, depositional environments from alluvial-fan to deep-sea turbidites, burial depths from surface to more than 5000 m and composition from quartzarenites to arkoses and diverse litharenites. Geochemical models developed to explain the patterns of evolution of sandstone diagenesis and porosity usually stress the control of temperature, pressure, time and fluid-flow parameters, regarding compositional variables to a subordinate role. The results of this thesis yielded however ample evidence that realistic diagenetic models must incorporate substantial data input on the composition of detrital grains, pore fluids and early diagenetic constituents, as these variables exert a consistent and basic control on the diagenetic evolution of sandstones.

     Interactions involving fluids, detrital and diagenetic constituents during early, near-surface diagenesis, as well as during the progressive burial and the uplift and exposure of the clastic sequences were investigated through the examination of cases selected to represent specific detrital compositional classes and diagenetic environments. The investigated processes comprised the dissolution and replacement of detrital and authigenic components, and the precipitation of authigenic carbonates, silicates, sulfides, oxides and sulphates, as well as the mechanical deformation and pressure dissolution of detrital constituents. The diagenetic evolution of most clastic sequences is concluded to be strongly imprinted by paleoclimatic controls and the influence of labile detrital components such as volcaniclastic grains on the chemical and isotopic composition of eodiagenetic phases, which in turn influence mesodiagenetic processes, and by patterns of fluid circulation through faults and fracture systems.