Abstract

Pleistocene/Recent canyon cutting by the Colorado River has exposed evaporites of the Paradox Member of the Hermosa Formation (Pennsylvanian) in Canyonlands National Park, Utah. Subsurface lateral flow of these evaporites into the Colorado River Canyon (Cataract Canyon) has occurred due to stresses generated by unloading of the Paradox Member under the river. Thicknesses of rock overlying the Paradox Member range from 0 in the river canyon to about 470 m on valley divides adjacent to the canyon. Flow has been particularly active on the east side of the river because of the gentle regional dip to the west, toward Cataract Canyon. This lateral flow in a subsurface layer has superposed essentially horizontal tensile stresses on the regional compressive stresses due to gravity. The following series of distinctive structural features has developed in the brittle rocks overlying the Paradox Member in response to the evaporite flow and superposed stresses:

1. A pervasive, nearly rectilinear system of unusually wide open vertical joints. The fracture anisotropy controlling the orientations of these joints is probably inherited from an earlier, unknown regional stress system.

2. A system of curved graben, concave to the west towards the Colorado River. The faults bounding these graben are vertical in the open-jointed rocks within 100 m of the surface, but dip steeply inward toward graben axes below 100 m. Many complex fault features that might be considered evidence for multiple deformation may be readily understood as due to interaction between graben faults and pre-existing joints.

3. Broad, gentle anticlines trending parallel to the Colorado River and to the lower reaches of the major tributaries of the Colorado. These are due to upward flow of evaporites toward the erosion-created free surfaces. The trends of these anticlines follow the meanders of the valleys, and are thus genetically tied to drainage development. Domical plugs of evaporites tend to occur where major tributaries enter the Colorado River.

Scale-model simulations of the geometrical and mechanical features involved in the origin of the Canyonlands graben show similar structures, including graben faults that have interacted with early-formed joints. Mathematical models developed by Nye (1951, 1952, 1957, 1965) for valley glaciers are applicable to the scale models and, by extrapolation, to the Canyonlands graben complex. Fractures predicted for the brittle surface layers of valley glaciers are similar in pattern to those found in the scale models and in the field.