GEOPHYSICAL SURVEYS IN THE GRABENS OF CANYONLANDS NATIONAL PARK, UTAH

The grabens of Canyonlands National Park, Utah, are developed in a sequence of Pennsylvanian to Permian clastic sedimentary rocks, with movement of the faults and valley floors accommodated by flow in the underlying Paradox evaporites. Although the bounding faults are well exposed above the surface, sediments obscure most of the bedrock floors complicating the measurement of fault displacements. Previous estimates of sediment thickness, averaging between 10 and 15 m, were widely accepted, but poorly substantiated. Seismic and gravity data from field work in 1999 [Grosfils et al.,2003], showed that the maximum sediment thickness in Devil's Lane graben is at least 90 m and typical thicknesses along the graben are about 60 m.

In the summer of 2005, we conducted seismic and gravity surveys in Cyclone graben, a larger graben adjacent to Devil's Lane. Six P-wave seismic refraction lines were shot parallel to the graben axis using an accelerated weight-drop source. A shear wave refraction line was shot coincident with one of the P-wave lines. One reflection line was shot perpendicular to the graben axis. Forty-nine gravity stations, spaced at 100 m intervals along the graben axis were measured with a Lacoste-Romberg G meter with Aliod nulling. Relative locations of the gravity stations were optically surveyed, and the survey was georeferenced with WAAS corrected GPS.

Analysis of our seismic lines shows that sediment thickness in Cyclone graben is similar to that in Devil's Lane. Maximum sediment thickness is approximately 80 m. Cyclone graben is generally deeper in its southern half with sediment thickness there averaging about 60 m versus about 30 m in the north end of the graben. Parallel refraction lines in the center of the graben show that the sediments are about 40 m thinner near the master fault than along the axis suggesting “step-like” faulting in the graben floor as proposed in some structural models and suggested by some bedrock exposures. Our complete Bouguer gravity anomaly is over three times larger than that measured in Devil's Lane. This anomaly can only be reconciled with our seismic results, assuming any plausible density contrast between sediments and bedrock, by salt diapirism beneath the graben floor as proposed in some previous structural models.

Grosfils, Schultz and Kroeger, 2003, J Struct Geol, v. 25, p. 455-467.