DEFORMATION OF WALL ROCKS AND OVERBURDEN SEQUENCES PROXIMAL TO SALT DIAPIRS IN SALT VALLEY, UTAH: IMPLICATIONS FOR PREDICTING SUBSEISMIC DAMAGE IN SALT TECTONIC SYSTEMS

Although deformation related to salt tectonics is generally considered to be an ancillary field of structural geology, owing to its relatively limited occurrence in the geologic record, the preponderance of salt-involved systems in multiple hydrocarbon-rich basins around the world (e.g., Gulf of Mexico, Atlantic passive margins, Iran, etc.) creates a necessity for understanding salt-related deformation. Traditionally, salt was considered to be relatively weak and thus was mostly unable to drive deformation of adjacent wall rocks and cover sequences. However, a number of recent numerical modeling studies have shown that mobilized and pressurized salt may have the ability to actively pierce overburden sequences (active diapirism) and drive pervasive deformation. Field evidence of these “damage” zones has remained elusive, but if these active diapirs are the dominant mechanism of salt evolution in many contractional systems, field observations must confirm this. The Salt Valley, Utah, which is a salt-cored breached anticlinal valley, is a perfect natural laboratory to examine deformation and strain accommodation in high porosity sandstones proximal to mobile salt. In Salt Valley, the paleo-salt wall contacts and adjacent sedimentary wall rocks are now exposed at the surface. Deformation in these types of systems is often the result of compaction (pore-collapse) and deformation band localization in high-porosity wall rock and cover sequences proximal to salt diapirs. Mapping transects normal to the paleo-salt wall reveal the presence of steeply dipping (60-75°) normal sense deformation/shear bands that define two plastic strain gradients; a long wavelength strain gradient with deformation band accumulated thicknesses related either to distance from the paleo-salt wall or the valley anticlinal axis, and a short wavelength strain gradient with band accumulations that increase in intensity approaching brittle normal fault planes. Additionally, intense zones of compactional deformation banding are recognized immediately adjacent to the paleo-salt wall (within 10s of m) and in “roof-pendants” in the valley. SEM analysis of deformation band damage zones reveal that the deformation bands preserved in all 26 samples occurred prior to significant cementation, which is here interpreted to indicate that these “rocks” actually behaved as critical-state materials during deformation. In this framework, we interpret that the long wavelength plastic strain gradients likely formed during outer-arc extension and doming of the regional anticlinal fold. The shorter wavelength gradients that increase approaching brittle normal faults, are interpreted to result from increasing extensional plastic strain localization that acts to strain-harden the material, eventually allowing it to fail as a Mohr-Coulomb rock and produce normal faults. Finally, the compactional plastic strain zones immediately adjacent to salt are interpreted to result from “tunneling” of the salt during active diapirism. These findings increase our understanding of the mechanisms that drive salt-related deformation in high-porosity rocks, and may allow us to better predict subseismic deformation in petroleum systems at depth