MAPPING OF MOAB AND SPANISH VALLEYS TO EXPLORE SUBSIDENCE PATTERNS IN A QUATERNARY SALT GRABEN

Moab and Spanish valleys in eastern Utah are depocenters in a salt-dissolution graben hypothesized to be actively deforming. Recognition of subsidence and sinkhole hazards in the city of Moab motivates our effort to map graben-bounding structures and deformed Quaternary deposits. Our broader goal is to combine mapping with stream-terrace surveys and geochronology to quantify rates and patterns of late-Pleistocene deformation. Mapping is in collaboration with the Utah Geological Survey and supported by the USGS EDMAP program, and covers the south half of the Rill Creek and north half of the Kane Springs 7.5’ quadrangles. We iterate between field mapping and digital VrTwo photogrammetry to produce the final product.

Mapping confirms previous workers’ hypotheses of post-150 ka deformation, as late-Pleistocene stream terraces of two regional drainages are deformed by graben subsidence in distinct patterns. Terraces along the axial-flowing Pack Creek thicken and down-warp in central Spanish Valley, whereas terraces along the cross-cutting Mill Creek are discontinuous and offset across faults at the graben margin in Moab Valley. A cosmogenic radionuclide burial age of a formerly continuous piedmont above Spanish Valley constrains graben initiation to post-1.5 Ma, since which there has been >400 m of subsidence. Optically stimulated luminescence dating of Mill Creek terraces suggests 100-200 m of subsidence since ~150 ka in Moab Valley, translating to a ~1 mm/yr displacement rate comparable to that of Utah’s Wasatch Fault.

Fault systems bounding the graben are comprised of one or two master structures marked by >50 m of displacement, deformation bands visible at the outcrop scale, and 1-5 m-wide zones of fault breccia. Along the SW margin, shallow and discontinuous subsidiary faults with synthetic and antithetic displacement accommodate strata rolling over into the graben. At the NE margin, bedrock is pervasively jointed and locally overlain by brecciated overburden hypothesized to have fallen into fissures during past episodes of extension. Mapping and geochronology have demonstrated that the Moab-Spanish Valley salt graben is Quaternary in origin, and that though displacement rates may be comparable, graben-bounding structures are more heterogeneous and discontinuous than typical normal faults.