Chemical and mechanical diagenetic evolution of deformation bands in sandstone
O'Brien, Casey Megan
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Deformation bands are zones of localized compaction, dilation, or shear in porous or granular sediments or sedimentary rock. Deformation bands are generally characterized by reduced permeability for cross-band fluid flow and increased capillary resistance for multi-phase flow, thus acting as barriers or baffles to fluid flow at an outcrop- to reservoir-scale. Deformation bands undergo increased lithification due to enhanced compaction and cementation, despite experiencing the same burial and temperature history as their host sandstone. I test the hypotheses that enhanced cementation in deformation bands is controlled by 1) the time between formation of the deformation band and onset of exhumation; 2) the degree of grain size reduction and thus the amount of fresh surface area available for quartz nucleation; 3) the amount of pore space available for pore-filling quartz cement; or a combination of these. I also evaluate possible links between mechanical deformation and chemical diagenesis in the context of tectonic setting and hostrock composition. To test these hypotheses, I reconstructed the structural and diagenetic evolution of deformation bands formed in the Jurassic Entrada, Navajo, and Wingate Sandstones in a contractional tectonic setting in the San Rafael monocline, and in the Entrada Sandstone in an extensional tectonic setting in the San Rafael Desert, Utah. I combined field-scale observations including band orientation, mutual crosscutting relations, and kinematic interpretations with petrographic point-count analyses using images obtained by high-resolution scanning electron microscopy. Based on structural criteria, I observed as many as six band generations. Younger generations of bands generally experience more cementation and porosity loss than older bands. I have identified that cataclasis is the dominant control over the amount of quartz cementation in bands of a similar lithology and burial history, as opposed to the time between band formation and exhumation, or available pore space. Within the same generation of bands, bands with more slip have more quartz cement. These findings highlight the importance of mechanical deformation in chemical diagenetic processes in sandstone. I find that the degree of cataclasis and quartz cementation does not differ between contractional and extensional tectonic settings, and that the hostrock compositional differences between the sandstones in this study do not cause significant differences in band diagenetic evolution.