INVESTIGATING THE ROLES OF SALT TECTONICS AND HILLSLOPE MASS MOVEMENTS IN FORMING THE CATARACT CANYON KNICKZONE, UTAH USING CHRONOSTRATIGRAPHY OF COLORADO RIVER TERRACES

Cataract Canyon is the steepest knickzone on the Colorado River and it includes the added complexity of salt tectonics where river incision, topography, canyon-slope processes, salt flow and related deformation are coupled. The general consensus is that differential unloading caused by recent river incision initiated flow of evaporites at depth which, together with the gentle dip of the surrounding strata, resulted in lateral spread and collapse of overlying rock units downslope into Cataract Canyon. What is not understood is the cause of this anomalously steep (2.1 m/km), ~65 km knickzone. Potential influences on river gradient include (1) rock uplift due to passive salt diapirsm in the canyon corridor, (2) increased sediment supply from mass-wasting of the deformed canyon walls, and/or (3) lithologic control as the river incised through the core of the Monument Upwarp exposing resistant limestones. To test these competing hypotheses, we date and map a series of Colorado River terraces along ~60 km of Meander Canyon, the reach upstream of Cataract Canyon.

Terraces are preserved up to 143 m above the river at the start of Meander Canyon and decrease in elevation and number downstream approaching Cataract Canyon. Chronology is provided by multiple luminescence and cosmogenic dating techniques. Twenty-four total samples have been collected and ages range from ~300 to 50 ka. Terrace correlation reveals that terraces converge downstream toward Cataract Canyon, suggesting that the Colorado River through Meander Canyon has changed gradient during the Quaternary. The convergence of Mid to Late Pleistocene reconstructed paleo-profiles may be tied to regional transient incision and/or potentially the maintenance of high baselevel at the head of Cataract Canyon due to diapiric salt tectonics. The modern low gradient (0.2m/km) of Meander Canyon is abrupt in comparison to the Pleistocene paleo-profiles suggesting that it is currently adjusting to a post Late Pleistocene deluge of sediment by hillslope mass-wasting in Cataract Canyon. These results reveal the dynamic influences of transient incision, sediment supply, and local deformation on channel gradient.