ANALYSIS OF SYNDEPOSITIONAL DEFORMATION STRUCTURES IN PERMIAN-JURASSIC DUNE FIELDS OF THE COLORADO PLATEAU

Permian and Jurassic eolian strata across the Colorado Plateau show a remarkable display of intraformational, syndepositional deformation structures. These structures range from: (1) millimeter-to-centimeter-scale crinkly laminae, to (2) meter-scale contorted cross-strata and liquefied zones, to (3) decameter-scale rotated blocks, slumps and mass-flow deposits, to (4) formation-scale cylindrical clastic pipes. The clastic pipes, concentrated in the Entrada Sandstone near Lake Powell, can measure over 100 m in height and 70 m in diameter, and are the largest recorded such features in the continental stratigraphic record. Syndepositional deformation structures such as those observed in these units are not typically associated with a desert setting. However, some of the observed structures can be attributed to conditions within these Permian and Jurassic desert environments that include (1) the deposition/dissolution of evaporites in interdune areas, on widespread sabkhas, and in adjacent shallow marine environments, (2) a high water table, especially in response to adjacent marine transgressions, (3) dune loading of saturated, poorly consolidated substrates, and (4) progradation of dune systems over muddy marine sediments. Other features argue for external triggers to produce geologically rare events. The Jurassic Todilto Formation has been interpreted as a catastrophic marine flooding event, whereas Upheaval Dome may be an impact site. The Jurassic back-arc setting of the region can be readily associated with earthquakes. The effects of strong ground motion by earthquakes or an impact in units such as the Entrada and the underlying Carmel were likely considerably amplified because of the susceptibility of these units to deformation.