The distribution and origin of silcrete in the ogallala formation, Garza County, Texas



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Silica-cemented sandstone, conglomerate, and caliche intervals (“silcretes”) occur at multiple levels within the Ogallala Formation in Garza County, Texas. The distribution, stratigraphic position, petrology, and isotopic geochemistry of Ogallala silcretes are of interest because stratigraphic constraints indicate that they must have formed very recently, and at low temperature (2 to 17˚C) under groundwater conditions. Silicification of these silcretes, mainly by opal-CT, began with: 1) precipitation within intergranular pore spaces; 2) silicification by replacement of microcrystalline calcite; and 3) silica precipitation within remaining voids. The sequence of silica precipitation within voids was from opal-CT to lussatite to chalcedony to mega-quartz. The Ogallala silcretes likely formed through the mixing of groundwater and lake water (Pleistocene Spring Creek Lake) that created an environment conducive to the precipitation of opal-CT.