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Burial diagenesis of illite/smectite in shales and the origins of authigenic quartz and secondary porosity in sandstones

Geochimica et Cosmochimica Acta
Publication Date
DOI: 10.1016/s0016-7037(97)00066-5
  • Chemistry
  • Earth Science


Abstract Several gram-size samples of <0.1 μm, nearly monomineralic mixed-layer illite/smectite (I/S)were isolated from nineteen whole-cores of shale of the Frio Formation from the Texas Gulf Coast. Between ∼7,000 ft (2133 m) and ∼15,000 ft (4572 m) burial depth I/S changes from random-interstratification ( R = 0) with low %I to ordered-interstratification ( R = 3) with high %I. The accompanying chemical changes are loss of Si, Mg, and Fe +3, gain of K, Al, and Fe +2, and a shift in the REE content of the mineral. There is no chemical or mineralogical evidence for a cessation of I/S reaction at ∼80% I as is commonly accepted; both chemical and mineralogical changes occur throughout burial. Continuous change in the oxygen isotopic composition of I/S implies that the conversion of smectite-layers to illite-layers is a dissolution and reprecipitation process. Mass balance calculations indicate that burial diagenesis of Frio Formation shales is an open-system process that requires addition of K 2O and Al 2O 3 and results in loss of SiO 2. The amount of SiO 2 made available by shale diagenesis is sufficient to be the source of the quartz-overgrowth cements in the associated Frio sandstones. Iron reduction in I/S is a significant source of the acid required for the diagenesis of both Frio shales and sandstones.

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