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Structural and Diagenetic Evolution of Deformation Bands in Contractional and Extensional Tectonic Regimes & Implications for Sandstone Diagenesis

Abstract

Mechanisms of brittle deformation of sediments and sedimentary rock change with burial because of increasing confining stress, change in pore fluid chemical and temperature conditions, and diagenetic state. Quartz arenite and subarkosic eolian sandstones in the San Rafael monocline and adjacent San Rafael Desert region, Utah, allow comparison of deformation band structures and their diagenetic attributes in contractional and extensional tectonic settings in close proximity. Cataclastic bands in the San Rafael monocline are interpreted to form as reverse faults during progressive rotation of the steeply dipping fold limb, resulting in an array of bands of varying dip. Bands in the San Rafael Desert form as normal faults with a narrower dip range. Although structural characteristics of bands differ in extensional and contractional settings, cataclastic bands in either regime have comparable amounts of porosity loss and quartz cementation indicating that tectonic regime does not influence band diagenesis. Furthermore, investigations of up to six generations of bands within a single sandstone formation allow for analysis of structural-diagenetic interactions with increasing deformation, time, and burial. Abundance of quartz cement in bands, determined by point counting of SEM images, increases from earlier to later generations of bands and, within a single generation, with increasing slip along the band, reaching up to 24% of band volume. This trend is attributed to an increase in cataclasis with increasing host rock cementation and confining stress during burial, and, within the same generation, with increasing slip. To assess effects of mechanical grain deformation on quartz cementation, we compare grain size distributions and grain surface area (circumference) with quartz cement content for cataclastic bands. We find that intergranular volume (IGV), band grain size distribution (% of band composed of grains <5µm), and amount of grain size reduction positively correlate with quartz cement content. Bands have comparable amounts of remnant porosity and quartz cement regardless of host rock composition, texture, or tectonic setting. These findings demonstrate that quartz cementation, and thus band permeability, are primarily controlled by the degree of cataclasis in the bands, and highlight the interdependence of mechanical deformation and chemical diagenetic processes in deformation bands and other faulted and sealed structures.