Integrated analysis of the growth history of the Moab-Spanish Valley salt wall and the Moab fault system, An

Abstract

The Paradox Basin is a structurally unique and important petroleum basin in southeastern Utah and southwestern Colorado that provides pristine examples of diapiric salt structures and supra-salt faulting. The structural and stratigraphic architecture of the Paradox Basin was strongly influenced by the complex, dynamic evolution of the evaporite-rich Paradox Formation and development of NW trending salt wall structures. The Moab Fault System is one of the largest normal fault structures within the Paradox Basin and provides an ideal opportunity to study the linked evolution of salt structures and supra-salt faulting. The fault system trends parallel to the Moab-Spanish Valley salt wall, extending approximately 30km NW of the entrance to Arches National Park. Although the fault system has been extensively studied, the timing and mechanisms controlling faulting are not well understood. The literature presents a range of inconsistent dates for fault initiation, ranging from the Permian through the Quaternary, while extensional mechanisms include regional extension, salt evacuation, and salt dissolution. This study reveals new information and conclusions regarding the development of the Moab Fault System and the underlying Moab-Spanish Valley salt wall, answering questions that were previously in debate. Detailed mapping along the Moab Segment and Mill Canyon Linkage Zone of the Moab Fault System characterize important hanging wall structures: collapse v-shaped synclines on the limb of a rollover anticline along the southern portion of the Moab Segment and extensional fault tip monoclines along the northern portion of the fault system. The synclines and monoclines indicate brittle, post-depositional faulting. Kinematic data indicate primarily dip-slip fault motion with oblique slip concentrated along fault branch points and curved fault segments. Extension is towards the NW along the NE-striking Moab Segment, transitioning to mainly northward extension along the E-W striking segments of the Mill Canyon Linkage Zone. A series of closely spaced 2D cross sections and 3D surfaces were constructed by integrating detailed surface data with all available subsurface data including wells, one 2D seismic profile, published 2D cross sections, and published structure contours. The interpreted 3D geometries reveal a lack of evidence for growth faulting and a genetic relationship between the shape of the top of the underlying Paradox Formation and localized fault initiation. Throw distributions indicate that the highest point of the underlying asymmetric salt high, or pillow, corresponds to the maximum throw and the location of fault initiation of the Moab Segment. Additionally, the faults along the Mill Canyon Linkage Zone are located where the top of the Paradox Formation increases in slope, dipping to the north. These data suggest that the fault system developed after the formation of the diapiric salt structures and deposition of the strata exposed in the field, at a minimum (early Cretaceous). This study also suggests salt movement from beneath the hanging wall as a stronger candidate for fault initiation than regional extension due to the lack of additional large-scale normal faults (graben system) in the area surrounding the Moab Fault System, indicating more localized strain than expected during regional extension. Faulting may have initiated due to uplift of the CO plateau during the late Neogene, which substantially increased erosion and initiated salt dissolution along the Moab-Spanish Valley salt wall. This process would have lowered the differential pressure on the Paradox Formation evaporites along the salt wall, forcing salt to migrate away from the thick overburden surrounding the salt structure (high differential pressure) towards the main salt wall and thinner overburden (low differential pressure). The presence of cataclastic deformation bands across the fault system provides evidence for deformation at a substantial burial depth and, therefore, before complete uplift and exhumation. This proposed deformation mechanism suggests that the brittle hanging wall accommodated the evacuation of the underlying Paradox Formation through the initiation of the Moab Fault system.