REED, Robert M., and LAUBACH, Stephen E., Bureau of Economic Geology, The University of Texas, University Station, Box X, Austin, TX 78713-8924

SEM-based cathodoluminescence studies of several Paleozoic and Mesozoic sandstones of the western U.S. reveal diagenetic features not visible by other methods. Differences in intensity of cathodoluminescence reveal both distinct zones within quartz overgrowth cements (CL zones) and quartz-filled microfractures that cut grains and/or cement. Cement textures include both zoned and featureless concentric overgrowths around grains, and nonconcentric cement marked by elongate euhedral crystals, partial euhedral overgrowths and zoned bulbous masses.

Textural evidence provides clues to the role of microfracturing in diagenesis. Microfractures crosscut some CL zones, but commonly merge with other CL zones. Some localized and/or nonconcentric CL zones are clearly spatially associated with microfractures. These CL zones are generally centered on the microfracture. Another observation is the development of a thick CL zone continuous with fracture fill but developed only on the side of a pore adjacent to the microfracture.

Microfractures should be considered as one factor influencing the development of quartz cement. Microfractures may localize development of quartz cement by providing pathways for silica-bearing fluids. Pre- to syn-cementation microfractures significantly increase permeability in some sandstones during diagenesis. Some localization of quartz cement could be due to preferential nucleation on the fresh fracture surfaces. Crosscutting relations among quartz cements and fractures, as well as continuity of some fracture fills with cement layers, suggest that fracturing was synchronous with cementation. This interpretation is consistent with crack-seal textures observed in quartz-lined macrofractures which occur in many of these sandstones. Complex cement patterns and crosscutting quartz-filled fractures should also be considered during isotopic and fluid-inclusion studies of quartz cement.