QUANTIFYING ACTIVE SALT DEFORMATION USING ALLUVIAL MARKERS AROUND SPANISH VALLEY AND MOAB, UTAH

The salt-tectonic features of the ancestral Paradox Basin in the eastern Colorado Plateau are well known to geologists, and have been deforming episodically since the Permian. Past workers have suggested that recent and ongoing deformation is driven by localized erosional unloading from the incising Colorado River system. Yet, the patterns and rates of Quaternary deformation are largely unexplored, and only recently have modern subsidence hazards been recognized. We are using stream terraces and alluvial-fill deposits as markers to quantify Quaternary salt deformation along the flanks of Spanish Valley, a salt-dissolution graben in eastern Utah encompassing the town of Moab. The study area includes two Colorado River tributaries, Mill Creek and Pack Creek, which drain the La Sal Mountains and cross Spanish Valley transversely and axially, respectively. Mapping of surficial deposits and structural relations, optically stimulated luminescence (OSL) dating, and surveying of alluvial deposits allow for correlation of terraces and calculation of deformation rates.

Field relations indicate graben subsidence around Moab is more rapid than the ~500 m/my rate of regional incision by the Colorado River, because lower reaches of tributaries are not incised as they cross the aggrading wetlands of NW Spanish Valley. Pilot OSL results along Mill Creek indicate that strath terraces recording rapid incision upstream are separated by graben-bounding faults from progressive deposition of alluvial gravels at the edge of the subsiding graben. Alternatively, initial chronostratigraphy along Pack Creek supports previous work suggesting a gradual transition from upstream incision to downstream subsidence and deposition, with gently dipping terrace treads in upper reaches and buried soils exposed downstream in the graben. Building upon these initial results will contribute to our broader goals of distinguishing multiple sources and scales of baselevel control in complex tectonic regimes and assessing active salt deformation hazards in the rapidly growing community of Moab.