CATACLASITE BANDS IN WELL CEMENTED QUARTZ ARENITE: A PRODUCT OF SHEAR OR RAPID DILATION?

Tabular bands 20 µm - 1 cm wide filled with quartz 5-10 µm in grain size are common in well cemented, quartz arenites in the Appalachians and elsewhere, particularly in the vicinity of faults. The bands have microstructural textures that are similar to cataclasite, so they have been ascribed to a shear origin involving wall-rock comminution. Yet, our investigation of these structures using CL, TEM, FTIR, and stable isotope geochemistry suggests otherwise.

Although shear offset is demonstrable for some bands, most show only dilational morphologies. Many are zoned with fine-grained margins and a coarse-grained, sometimes vuggy core containing euhedrally-terminated crystals. In CL, the band fill has a uniformly dark luminescence unlike wall-rock fragments or overgrowths. In TEM, quartz in the band consists of equant, dislocation-free grains. FTIR traverses across the bands show pronounced drops in water content of ~6,000 H/10⁶ Si relative to deformed wall-rock grains. Isotopically, the Δ¹⁸O of quartz in the band is ~10 per mil heavier than the wall rock. Taken together, these observations argue for a dilational origin of the tabular bands and that they contain quartz cement, not comminuted wall rock. If true, the fine-grained texture of the cement is unusual and suggests a different origin than found with other abundant extensional microstructures in the sandstones, such as fluid inclusion planes (FIP) and microveins. We propose that the difference relates to the rate of dilation and aperture. FIP and microveins dilate slowly, having apertures that are small enough to favor quartz growing syntaxially on wall-rock grains. In contrast, we argue that tabular bands of fine-grained quartz opened rapidly and with an aperture large enough that syntaxial quartz cement could not fill the void. The fine grained texture of the cement would be the product of rapid crystallization where the rate of nucleation >> the rate of crystal growth. This condition can be caused by a rapid (seismic?) stress drop leading to supersaturation of quartz in the fluid, thus promoting rapid crystallization. Also, these bands would constitute outcrop-scale equivalents to the dilational microstructures, such as FIP's and microveins.