A GRAVITY SURVEY IN WESTERN UNAWEEP CANYON: LOOKING FOR THE BEDROCK BOUNDARY AND ITS SHAPE

Changes in gravity along a profile are caused by variations in the density structure within the subsurface. The solution to a given gravity response is non-unique; density measurements help to constrain the number of possible models. Gravity data is then used to provide constraints in seismic interpretation. In this study, we are integrating results from a gravity survey conducted in western Unaweep Canyon with results from a seismic survey (Suarez et al.) conducted in the same region. Our main goal is to define the bedrock profile of Unaweep Canyon in this region, and estimate its depth. Additionally, gravity data (especially integrated with the seismic) might provide information on the character of the valley fill. This is of interest because the origin of Unaweep Canyon is enigmatic; hypotheses proposed include genesis by fluvial action, as well as by glaciation.

The survey consisted of a 38-station profile conducted along the transverse seismic line acquired in the summer of 2004. The survey was made with a Lacoste-Romberg gravity meter type D. Each measurement has been corrected for latitude, Free Air and Drift, and the Bouguer correction is underway. Fortunately, petrophysical and core data from a well drilled along the profile will provide density and velocity data that will help to constrain the Bouguer correction. The program used for the modeling (GM-Sys), assumes a 2 1/2 D model, in which the transverse profile is invariant along the axis of the valley. This assumes the valley shape is constant in cross section. The results from the gravity and seismic analyses will be compared and integrated to produce an interpretation consistent with both datasets. Our results should contribute to resolving the controversy surrounding the origin of Unaweep Canyon, owing to the contrast in transverse profiles produced by fluvial (“V”) versus glacial (”U”) processes.