Early Diagenesis of High Plains Tertiary Vitric and Arkosic Sandstone, Wyoming and Nebraska
The Cenozoic High Plains sequence is a semiarid alluvial and eolian complex in which low temperature diagenetic processes were primarily the result of reactions of unconsolidated sediment and dilute aqueous solutions. Post-depositional modifications of arkosic and vitric sandstone include intrastratal alteration and dissolution of chemically unstable grains and the precipitation of cement in pore space. These processes, however, have only slightly altered the original fabric and mineralogy of High Plains sandstone, although chemically unstable heavy minerals and volcanic glass are abundant. Thermochemical calculations suggest that present-day groundwater is in possible equilibrium with calcite, montmorillonite, kaolinite, and a silica phase. Cement in sandstone is commonly montmorillonite, but can be calcite, opal, chert, and rarely clinoptilolite. Most sandstone is friable and cemented with montmorillonite. Interspersed in this sandstone are calcite concretions; opal and chert cemented sandstone occurs sporadically in High Plains rocks on Cretaceous shale and associated with old land surfaces in the Tertiary rocks. Commonly, the order of precipitation of cement is calcite, montmorillonite, and opal. All cemented sandstone shows some dissolution of grain surfaces, but only friable sandstone shows extensive intrastratal dissolution of grains, peripheral alteration of pyroxene and amphibole grains, and hollow montmorillonite coatings where chemically unstable grains were dissolved. These montmorillonite coatings are up to 100 micrometers thick and consist of a reticulate arrangement of relatively porous clay particles oriented normal to the grain's surface. Most of the cements and intrastratal alteration features can be explained by pedogenic processes, evaporative concentration of groundwater in the capillary zone, or by groundwater reaction with sediment.
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There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.