REPEATED CO2 FLUX ASSOCIATED WITH BASIN AND RANGE FAULTING, CENTRAL UTAH

The Gunnison Fault (GF) is an active N-S trending, normal fault in the eastern Basin and Range Province in central Utah. Deposits of surficial tufa occur along the GF and exhumed travertine veins and fault rocks occur along transverse faults intersecting the GF. Field mapping shows that the GF system has the geometry of a relay ramp structure, with overlapping fault segments intersected by transverse faults from the west. Fault kinematic data from the transverse Rock Canyon Fault (RCF) documents normal dip and normal-oblique slip. Repeated phases of fault rupture (brecciation) and seal (travertine precipitation) are observed in the RCF from field and hand sample mapping of fault rocks. Fault rock textures suggest that breccias formed by implosion and explosion, with the latter related to CO2 degassing. U-Th data from 2 multiphase breccia samples yield ages of ~500 Ka with large errors; further dating of a larger sample suite is needed to resolve the ages of discrete faulting episodes. Surficial tufa outcrops along the GF include perched spring tufa and cascade deposits along a potential fault scarp. Tufa growth textures suggest both biologically- and structurally-mediated control on precipitation of CO2 from water emanating from the GF. The deposits along the GF system are natural examples of fault-related CO2 leakage from a subsurface reservoir over long time scales. The integrated structural, geochronological, and isotopic data being acquired by this study will allow us to test our postulate that the GF marks normal-fault-controlled CO2 leakage from a subsurface Navajo Sandstone gas and oil reservoir, with leakage localized at fault intersections in a breached relay-ramp structure.