Moab Fault

The Moab Fault was active during a period between the Triassic and early Tertiary, with a break from mid-Jurassic until at least mid-Cretaceous. It is associated with two salt anticlines formed within the Fold and Fault Belt of the Paradox Basin in east-central Utah. The Paradox Basin is part of the Colorado Plateau that formed during the Late Paleozoic. Movement on the basement faults began in the Proterozoic and was greatest during mid-Pennsylvanian Rocky Mountain tectonism. The fault was reactivated at ~60 Ma, likely due to reinitiated salt movement during the Laramide orogeny. Following its deposition, the salt was deformed to form a series of salt anticlines that were ultimately buried by 1–2 km of Jurassic to Tertiary sediments. == Fault geometry ==

The architecture of the Moab fault zone is highly variable, and has been studied by several authors. The first systematic study was conducted by Foxford et al. in 1998, which classified architectural elements into slip band zones, shaley gouge zones and sandstone cataclastics and breccias. Thirty-seven transect exposures within the Moab fault zone were described. These exposures provide excellent data on lateral variation and structure features within the fault zone. The most studied transects include the Moab Canyon, R191 Canyon, Corral Canyon, Courthouse Mine, Bartlett Wash, and Waterfall Canyon. The Moab Fault is a sharply defined brittle shear zone (1–10 m wide). The overall geometry of the southern fault segment is that of a faulted anticline, modified by a minor component of normal drag adjacent to the fault. The fault is composed of three main components: a poorly exposed southern section, a central section (where the greatest throws are found), and a complex branching northern section that tips out to the northwest. At the north end of the Moab valley, there is a fault transfer zone, where the fault steps east. This zone transfers the displacement along the fault from one segment to another. Within this zone there is very dense faulting. Along the southern segment, footwall bed dips define a structural high symmetrically disposed about the point of maximum throw. A prominent hanging wall feature of the southern segment is the Moab Anticline, with a crestal collapse graben accommodated by an array of normal faults. The Moab Anticline is an asymmetric fold with a wavelength of approximately 1 km, an amplitude of 350 m and a length of over 10 km. The internal geometry of the Moab fault zone is complex in terms of the numbers of slip zones, the partitioning of throw between them and the distribution of fault rocks, all of which vary over the fault surface. One study by Berg and Skar5 analyzes the arrangement of fractures in damage zones of the Bartlett segment of the Moab Fault. The Bartlett fault consist of a fault core surrounded by damage zones in the footwall and hanging wall. The fault core consists of a variety of fault rocks and entrained bodies of clastic host rocks, which indicate variation in strain intensity and deformation style. Berg and Skar suggest that the most important cause for asymmetric strain distribution is the development of the hanging wall syncline and the resulting asymmetric stress pattern expected to exist during fault propagation. == Sedimentation and stratigraphy ==

The sedimentation of the Moab area was primarily influenced by marine or lacustrine incursions into the margins of major Jurassic ergs. This area consists of a heterogeneous series of dominantly clastic sedimentary rocks. The basal Navajo Sandstone to top Entrada Sandstone interval is separated into six sedimentologically distinct stratigraphic units: the Navajo Sandstone, the Page Sandstone, the Dewey Bridge and Slick Rock members of the Entrada Sandstone, the Moab Tongue, and the Curtis Formation. The stratigraphy of the outcropping fault zone can be divided into three lithological groups: mudstone-dominated, mixed mudstone-sandstone, and sandstone-rich sequences. The mixed mudstone-sandstone sequences include interbedded fluvial and aeolian sandstones and floodplain/lacustrine mudstones and siltstones. Sandstone-rich intervals are primarily aeolian in origin. == Distribution of clays and cements ==

Distinctive types of veining, calcite cementation and iron oxide reduction exist adjacent to the fault, especially in Jurassic Navajo and Entrada sandstones. use the distribution of calcite cement as an indicator of paleofluid migration. They infer that fault-parallel fluid flow was focused along fault segments overprinted by joints and sheared joints. These conclusions support Chan et al.’s hydrogeologic model, which proposes that hydrocarbon and basin brines from Pennsylvanian source rocks migrated along the Moab fault, moving into the porous sandstone units where they interacted with oxygenated meteoric water. Chan et al. Iron oxide reduction is concentrated within high permeability, well-connected sandstones and spatially associated with cemented veins – indicating that the reduction event coincided with vein formation, and thus with the final stages of faulting. Permeability along the fault may have been promoted by the highly anisotropic shaley gouge fabrics or by fault zone fractures. After analyzing geochemical data of carbonate cements and iron-oxide reduced sandstones from the Moab Anticline, Garden et al. Clay smear processes are applied to instances of cross-fault flow when porous and permeable rocks, specifically sandstones and shales, are cut by normal faults. According to Foxford et al., show that a shale gouge ratio of ~20% defines the boundary between sealing and non-sealing faults, with faults sealing at shale gouge ratios above this ‘cut-off’ value. Shale gouge is present in the Moab Fault at values >c. 20%, but varies depending on the location of the transect. ==References==