Heterogeneous Packing and Quartz Cementation of the Eureka Quartzarenite (Middle Ordovician), Utah and Nevada, U.S.A

Abstract: The Eureka Sandstone is a quartzarenite 200 m thick that was deposited on the eastern shelf of the Cordilleran foreland basin from Canada to California. It has many characteristics of other lower Paleozoic quartzarenites in terms of its sheet-like geometry, mineralogical and textural maturity (99.5% detrital quartz, almost free of detrital clay), shallow-marine environment of deposition, and abundance of quartz cement (10–26%). However, it displays wide variations in compaction as measured by intergranular volume (IGV range of 16%) that are not all related to differences in burial depth plus IGV values as low as 10%. Sutured grain contacts and fractured grains cannot account for such abnormally low IGV values, which suggests that dissolution of the edges of grains where they overlapped their neighbors (the Füchtbauer effect), leaving minimal evidence, was a major process of compaction. The normal evolution of microquartz overgrowths (< 10 µm) to meso- and macroquartz overgrowths (> 10 µm) with time was retarded; consequently microquartz and mesoquartz cement (80%) dominate over macroquartz (20%) and are the only cements in the poorest-cemented beds and laminations. Illite co-precipitated with micro- and mesoquartz and impeded quartz cementation by coating quartz crystal faces. Typical macrocrystalline quartz was the final cement only in illite-poor samples. Heterogeneous illite distribution may reflect heterogeneous abundance of K-feldspar or detrital illite that possibly served as seed crystals for authigenic illite. In spite of reaching temperatures > 135° C for ∼ 100 My, many Eureka sandstones, especially in Utah, remain incompletely and unevenly cemented. Consequently, beds range from weakly friable to pervasively quartz-cemented “quartzite” in the same outcrop. Porosity was generally reduced to less than 2% except in rare laminations where it reaches 22%. Grain size had some influence on the amount of quartz cement locally, but the chief cause of cement heterogeneity appears to have been illite abundance. The most likely major source of silica for quartz cement was quartz grain dissolution at shale beds and clay drapes, although documentation is absent. Intergranular grain dissolution was a minor source of silica; overlap quartz averages less than 4%. Oxygen isotopic data are compatible with the precipitation of quartz either from diagenetically evolved meteoric water that invaded the sandstones from tectonic highlands to the west or evaporative brines.

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