The Grabens, Canyonlands National Park, Utah: Gravity Slide of the Paradox

English Abstract:

The Grabens, Canyonlands National Park, Utah is an area of arcuate horsts and grabens formed by extending and gliding Pennsylvanian and Permian brittle rocks down dip towards the Colorado River over the Pennsylvanian Paradox salt. Dissolution and deformation of salt enhanced the extensional fracture system, widening individual fractures. Slumped blocks that constrict river flow and create rapids are evidence that gravity gliding is still active.

Scaled analogue models were constructed with sand and silicon to recreate brittle layer extension as the underlying salt deforms on a slightly inclined sliding plane. During the early stages, deformations within the ductile layer resulted in unequal thickness, exhibiting areas of spreading and compression where deep-seated, normal or reverse faults bounding the horsts and grabens form. During spreading, shallow-penetrating faults developed within existing horsts and grabens in the distal and medial areas and propagated towards the source area. Extensional features in the analogue slide translation and source zones are similar to the Grabens area and its vicinity east of Shay Mountain.

We suggest that a large, multi-stage and slow-moving gravity driven slide affects the Canyonlands area west of the Shay Mountain up to the Colorado River. It is caused by a deforming salt layer underneath. Deformation factors including basement faults; sinking basin; and sediment overburden were present by the end of Middle Pennsylvanian. The slide was triggered by Shay Mountain intrusion during the Cretaceous as evidenced by uniform thicknesses of Late Triassic overburden in the Bridger Jack and Boundary Butte. The sliding translated the upper brittle layer initially towards the northwest and eventually to the west due to thicker and stronger sedimentary confinement to the north. When the Colorado River in the west started eroding the toe of the translating mass during the Middle Miocene, retrogressive failure started, forming The Grabens.