Role of Quartz Type and Grain Size in Silica Diagenesis, Nugget Sandstone, South-Central Wyoming

Abstract

ABSTRACT Silica-cement-dominated intervals within the Nugget Sandstone illustrate several relationships involving pressure solution and initial silica cementation as influenced by clast size and quartz grain type. For medium-sand-size, nonundulose, undulose, and polycrystalline quartz, the portion of grains with well-developed silica overgrowths (cement occupying > 50% of free grain surface) is 26%, 14% and 4%, respectively. Overall, monocrystalline grains are about three times as likely to have well-developed silica overgrowths as polycrystalline clasts. Well-developed silica overgrowths on monocrystalline quartz are about seven times more common in coarse-sand-size grains than fine-sand-size grains. No apparent grain size-initial overgrowth development relation is characteristic of the polycrystalline quartz population. This may be due to a predominant overgrowth-inhibiting influence related to differences in c-axis orientation between adjacent crystal domains within polycrystalline quartz grains. Pressure solution at grain contacts accounts for slightly more silica generated than precipitated as overgrowths for silica-cement-dominated intervals within the Nugget. The volume of silica produced along more extensive, stylolitic contacts is approximately equal to 25 to 30% of the remaining volume of pore space. Thus, the portion of the Nugget studied is an overall exporter of silica due to the pressure solution process.