Abstract: Recognizing Tectonic and Compaction Driven Quartz Grain Fracturing and Annealing in the Almond Formation, Green River Basin, Wyoming

Thomas L. Dunn

Fracturing and healing of quartz grains in the Almond Formation are visible using scanning electron cathode luminescence microscopy of polished thin sections. These features record both the grain packing adjustments of compactional porosity loss due to brittle deformation and the strain related to the basin's tectonic history. Evidence for these processes includes the orientation of the annealed quartz-grain fractures, traces of annealed fractures that extend through adjacent grains, the emanation of annealed fractures from grain point contacts, and microstylolite orientation. Increases in grain volume and in the relative amount of effected grains indicate that grain fracturing and annealing are important processes in compactional porosity loss, and may provide documentation of the ev lution of microfracture process zones that form prior to fracturing.

Quartz-grain fracturing and annealing creates an increase in polycrystalline and undulose quartz grains. Both of these silica forms have been found by others to support less quartz cementation than monocrystalline and nonundulose grains. However, this brittle deformation process may in some cases promote quartz cementation by providing fresh nucleation surfaces. The formation of early and extensive secondary quartz within detrital quartz grains has implications for quartz-cementation-based porosity prediction modeling, intergranular-volume calculations, grain shape and size analysis, geochemical modeling, stable isotope studies of overgrowth formation, and fluid inclusion studies.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994