COMPACTION BANDING AS A DEFORMATION MECHANISM IN POROUS, WEAKLY CEMENTED QUARTZ SANDSTONE AT LOW PRESSURE, TEMPERATURE AND STRAIN

Discrete tabular bands of brittle granular deformation and compaction comprise a systematic, fabric throughout the upper half of the 1,400-m-thick Jurassic Aztec sandstone of southeastern Nevada. On the order of a cm thick and up to 100+ m in planar extent, these sharply defined compaction bands (CBs) are characterized by quartz-grain microfracturing and collapse, and the consequent loss of porosity. A dominantly uniaxial strain related to closing-mode displacement across the bands, coupled with clear outcrop evidence of in-plane propagation, defines them kinematically and mechanically as anticracks—analogous to pressure solution seams, but with porosity accounting for the volume reduction (~10%). Subparallel, anastamosing arrays of these CBs comprise the oldest structural fabric present in the Aztec, having formed at high angle to bedding and generally perpendicular to the direction of regional shortening associated with the Cretaceous Sevier orogeny. Geological and geochemical observations further indicate that the CBs formed at shallow depth (<2 km) and low temperature (<80°C) in a medium-grained, quartz-rich sandstone with ~25% porosity and ~1% hematite grain coating cement.

We conclude that elastic bulk material behavior prevailed in this porous, weakly cemented quartz sandstone even at low to moderate confining pressures, and that stress concentrations generated at abundant grain-scale flaws in response to tectonic loading provided the seeds of collapse from which CBs then propagated and interacted as anticracks to form the striking fabric of tabular fins now observed in outcrop. The highly localized nature of this compaction event, which represents no more than 0.5% total shortening across the Aztec, stands in contrast to the homogeneous compaction generally attributed to such materials, and observed in cases of simple burial, up to much higher levels of bulk strain. Given the potential for low porosity and permeability CB fabrics to exert profound impacts on fluid flow in sandstone aquifers and reservoirs, we suggest that this deformation mechanism, only recognized over the past decade, warrants further scrutiny—in the field, in the lab and in theory.