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Abstract: Incipient Diagenesis of Arkosic and Vitric Sandstone, Cenozoic High Plains Sequence, Wyoming and Nebraska

Kenneth O. Stanley, L. V. Benson

The Cenozoic High Plains sequence is an alluvial and eolian complex in which diagenesis is limited to a record of chemical reactions of sediment and groundwater. These processes were active in sediment unaffected by deep burial or high temperatures, and were primarily the result of reactions of dilute solutions of interstitial fluids. The hydrographic basin, the climate, and the composition of rocks are constants in the hydrochemical system. Time and flow rates of fluids through the sandstone are the major variables in the system; sedimentary rocks controlled composition of the fluids. Postdepositional modifications of arkosic and vitric sandstone are the result of intrastratal alteration and solution of chemically unstable grains and the precipitation of cement in pore s ace. These processes have only slightly altered the fabric and mineralogy of High Plains sandstone, although chemically unstable opaque heavy minerals and volcanic glass are the major components. The major aquifer of eastern Wyoming and adjoining Nebraska is in the High Plains sequence. Calculations of activities of mineral phases, using ideal-site theory, indicate that this groundwater is in possible equilibrium with calcite, smectite, kaolinite, interstratified smectite-illite and kaolinite, and amorphous silica. Cement in sandstone is commonly smectite, but can be calcite, opal, chert, or clinoptilolite. Most sandstone is friable and cemented with smectite. Interspersed in this sandstone are calcite concretions; opal- and chert-cemented sandstone is present where the High Plains rocks overlie Cretaceous shale or are adjacent to faults. Commonly, the order of precipitation of cement is calcite, smectite, and opal. All cemented sandstone shows some solution of grain surfaces, but only the friable sandstones show extensive intrastratal solution of grains, peripheral alteration of pyroxene and amphibole grains, and hollow smectite coatings where chemically unstable grains were dissolved. Smectite coatings of grains are up to 100µ thick and consist of a reticulate arrangement of relatively porous clay crystals oriented normal to the grain surface. Soil genesis and evaporative concentration of groundwater in the capillary zone are mechanisms believed responsible for the formation of most cements.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado