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Petrologic and Geochemical Constrains on the Origin of Authigenic Euhedral Quartz Crystals in Edwards Formation, Central Texas


Euhedral megaquartz crystals are present in the upper part of Cretaceous Edwards Formation from the spillway of Lake Georgetown, central Texas. The siliceous deposits formed almost exclusively in a 15 cm thick horizon within the carbonate that is underlain by rudist reef buildups. Individual crystals range from about 1 mm to 1 cm in size and many show shallow evaporative anhydrite and calcite crystal inclusions. Proportions of anhydrite inclusions vary considerably between quartz crystals. Most megaquartz crystals are characterized by strongly undulatory extinction. The paragenesis and environmental conditions of silicification are of considerable interest as silicification in carbonates appears to be closely linked to reduction of porosity and permeability. The lithological association of evaporate-bearing dolomitized carbonate strata with rudist reefs suggests that the anhydrite probably developed in a back-reef tidal-flat environment. The occurrence of anhydrite and calcite inclusions implies that precipitation of quartz crystals took place after the primary calcite cementation and partial dissolution of evaporite. Silicification is largely controlled by fluctuation of pH in pore fluids due to the partial dissolution of sulfate during burial and mixing with meteoric water. In situ silicon isotopic analyses on individual quartz crystals are useful in understanding the silicification history. Detailed silicon isotopic mapping across the megaquartz crystals show that d30Si values range from −2.72 ‰ to 2.94 ‰, almost account for the entire silicon isotopic fractionation range in nature, which indicates very complex growth histories. The very negative isotopic signature can be attributed to the dissolution of sponge spicules, which probably act as the primary source of the authigenic megaquartz. The commonly used bulk analysis of dissolving entire grains overlooks the complicated silicon isotopic composition within individual quartz crystals. Thus, combined petrologic and in-situ silicon isotope measurements demonstrate that euhedral authigenic quartz in the Cretaceous Edwards Formation can originate at shallow depth by dissolution and reprecipitation processes.