Nature and Timing of Enigmatic Banded Fault Fabrics in the Navajo Sandstone, Salt Valley, Utah

Carolyn Tewksbury-Christle, Fort Lewis College
Robert Krantz, Fort Lewis College
Ethan Ribble, Fort Lewis College
Eleanor Fluckiger, Fort Lewis College
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Abstract

The NW-SE trending Salt Valley graben, located northwest of Moab, Utah, is bounded by conjugate sets of normal faults formed above a collapsed, salt-cored anticline. These faults offset subhorizontal Jurassic Navajo and Entrada Sandstone bedding and are commonly characterized by typical brittle deformation structures, including normal faults and cataclastic deformation bands. At the Tower Arch Trail in Arches National Park, the Navajo Sandstone has eye-catching and enigmatic deformation features accommodating normal-sense offset and consisting of alternating light and dark material in zones at high angles to bedding. To determine the significance of these banded fault fabrics (BFFs) in a regional framework, we characterized BFF morphology, orientations, and kinematics and determined the relative ages of features in the Tower Arch area.

Our twelve documented faults have normal sense separation (2-5 m throw) and are predominantly NE-dipping (~60°), consistent with broader graben orientations. BFFs are present along ten of these faults and exhibit mullion structures and/or slickenlines sub-parallel to dip. BFFs define 0.1-3.0 m thick shear zones characterized by alternating light and dark bands that encapsulate lozenges of intact Navajo Sandstone with well-developed primary bedding and older deformation bands. Bedding and deformation bands at the shear zone margins and within lozenges are truncated by the BFFs. BFF bands are isoclinally folded with thickened hinges and narrow limbs, and one deformation band is drag-folded into a BFF. Inside some BFFs, discreet cm-scale faults cross-cut the bands but do not extend beyond the BFF margins. Unlike classic deformation band microstructures, grain sizes in the BFFs and the undeformed Navajo Sandstone are comparable, and compaction with very minor grain crushing significantly reduces porosity in the BFFs.

BFFs are macroscopically ductile and exhibit minimal cataclastic processes associated with some deformation bands. Relative timing also suggests that (1) the host Navajo sandstone attained enough lithification and burial for deformation bands to form prior to BFFs, and (2) if truly "soft" shear occurred, then some unknown mechanism reduced sandstone stiffness. Interactions with reactive fluids potentially sourced from the salt at depth could have dissolved the cement, with BFFs subsequently accommodating strain during on-going normal sense slip until cement reprecipitation hardened the shear zone enough to return to brittle failure. These anomalous fault structures record unique deformation that has not been previously documented but is kinematically compatible with regional-scale salt tectonics. Resolving the mechanisms by which these BFFs form, however, requires continued research.

Session

Deformation in the upper crust