EROSIONAL FORCING OF ACTIVE SALT STRUCTURES, CANYONLANDS NATIONAL PARK, UTAH

Salt structures in the Paradox Basin of Utah and Colorado form in response to erosional unroofing and solute uptake and transport in ground and river water. This is expressed in the Needles District of Canyonlands National Park as an array of actively creeping extensional faults, a gentle flexural rollover, isolated diapirs and a sinuous anticline that collectively accommodate plastic flow and dissolution of an unconfined sequence of Paradox salt deposits. These structures have formed in response to incision of the Colorado River into evaporite deposits by removal of strata that previously confined them. Sampling of spring and groundwater suggests that solute uptake and transport is low across the extensional fault systems. Salt uptake increases dramatically however along the river canyon where evaporite caprock is in direct contact with the ground surface. Mass balancing of salt flow from InSAR derived measurements of surface strain suggest that diapiric uplift along the river is an order of magnitude lower than subsidence driven by flexure and extension in the Needles. This implies a correspondingly large increase in dissolution at the river and points toward direct coupling of halite in the subsurface with ground water lying beneath the canyon. Active strain due to diapirism along the river and its tributaries as defined from InSAR occurs in isolated structures that do not necessarily correspond to a pattern of uplift as shown by the continuous Meander anticline. In addition, the area north of the highly extended Needles District contains a well-developed anticline along the river, yet shows no InSAR signal that can be identified with broad flexure or extensional faulting. The anticline can thus be interpreted to form independently of flexure and extension above a plastically flowing salt substrate, perhaps related to insitu uplift driven by unloading from erosion of the river canyon without large contributions from lateral salt flow further east. Such a model appears to be supported by the presence of similar short wavelength, high amplitude anticlines situated along side tributaries in the river canyon adjacent to the Needles. This implies that the flexural rigidity of strata directly above the salt is low.