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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Channel sands and sand lenses in the Catahoula Formation, an Oligocene-Miocene fluvial unit of the Gulf Coast, are in places cemented by an opaline material. Petrographic, X-ray diffraction, and electron microscopy studies indicate that the cement (1) shows poorly developed banding of varying birefringence (or degree of crystallinity); (2) has shrinkage cracks apparently resulting from ordering and dehydration; and (3) consists of opal and crypto- to micro-crystalline cristobalite which occurs as lephispheres about 2 µm in diameter. The cement developes in sands that are encased in tuffaceous silty mudstones. The development is apparently restricted to outcrops and near-surface zones of high permeability and appears to be pedogenic in origin.
Pervasive alteration of the rhyolitic tuffaceous mudstones to clay minerals gives rise to excessive free silica, which is carried by groundwater to the permeable sandy zones. The silica was initially precipitated in sand interstices as a silica gel. Subsequent dehydration and ordering produces the opal-cristobalite observed. Further crystallization of opal-cristobalite during burial diagenesis may result in chalcedony or quartz cements.
Calculations show that for a rhyolite ash to alter completely to 2:1 layer clay minerals (smectites), as much as 35 wt. % of the ash may be released as hydrogen silicate. This suggests that ash beds are significant sources of silica cement in sandstones.
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