REACTIVE FLUID EXPULSION DURING PROGRESSIVE DEFORMATION IN THE FOLD LIMB OF THE SAN RAFAEL SWELL, UTAH, USA

A series of reaction fronts radiating from fractures are observed in the Navajo Formation in the eastern flank of the San Rafael Swell (monocline structure) in central Utah, USA. Mobilization of metals by reducing fluids and re-precipitation of oxides may be linked to the structural deformation history and the successive opening and closing of fractures and faults, facilitating pumping of fluids. In the field area the Navajo Formation comprises cross-bedded sandstone units interpreted as sand dunes deposited in an aeolian environment during early Jurassic. During the Laramide orogeny the sedimentary succession of Jurassic sandstones and mudstones were uplifted and folded over N-S striking underlying faults. Later uplift and erosion has exposed the section and the geology may be investigated in numerous cross cutting gullies. Relict plumes sourced from in between bedding surfaces and from fractures are observed at the field locality as successive reaction fronts where oxides have precipitated and formed solid rims, interpreted as resulting from pressure driven intrusion of reducing fluids. Dendritic growth of Mn-oxides is observed from solid precipitation fronts, indicating that diffusion-driven transport succeeds advection and carry large mass fractions. Numerous deformation bands in six directional populations are observed to both obstruct and allow fluid migration between generations of expulsion events. The latest generation of fractures is roughly perpendicular to the regional fold-axis, displaces reaction fronts and deformation bands, and do not appear to expel fluids before becoming completely sealed by oxide precipitates, quartz and carbonate. Thus it appears that the bulk part of advective and diffusive mass transfer took place during the earlier stages of deformation. Understanding these processes is of importance in reservoir-seal characterization for CO₂ storage, as leakage and reactive transport may be quantified in relation to deformation type and force.