Burial diagenesis of illite/smectite in shales and the origins of authigenic quartz and secondary porosity in sandstones
- Univ. of Texas, Austin, TX (United States)
Several gram-size samples of <0.1 {mu}m, 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 {approximately}7,000 ft (2133 m) and {approximately}15,000 ft (4572 m) burial depth I/S changes from random-interstratification (R = 0) with low %1 to ordered-interstratification (R = 3) with high %I. The accompanying chemical changes are loss of Si, Mg, and Fe{sup +3}, gain of K, Al, and Fe{sup +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 {approximately}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{sub 2}O and Al{sub 2}O{sub 3} and results in loss of SiO{sub 2}. The amount of SiO{sub 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. 68 refs., 12 figs., 4 tabs.
- DOE Contract Number:
- FG05-92ER14249
- OSTI ID:
- 543116
- Journal Information:
- Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 10 Vol. 61; ISSN GCACAK; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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