NEW EVIDENCE FOR THE NATURE OF THE SADDLE MOUNTAINS FAULT ZONE

The western portion of the Columbia River Flood-Basalt Province in Washington and northeast Oregon is characterized by the Yakima fold belt. The fold belt consists of a series of generally west trending anticlines that are narrow (approx 5 km) and over 100 km long. The anticlines have a frontal fault zone that, in surface exposures, can dip from 30 degrees to as much as 75 degrees. Resolving the nature of the fault at depth is difficult due to the inability of most geophysical techniques to have the necessary resolution. XRF chemical analyses of basalt were used in the 1980s to develop a regional stratigraphy, which then could be used to look for stratigraphic repeats in boreholes drilled into these folds. The younger flows are compositionally diverse and could be easily recognized by drill cuttings, but the Grande Ronde Basalt (GRB), however, proved to be more difficult because of the slight but significant variations in compositions. In the 1990’s, new technology provided high precision XRF analyses making the GRB easier to recognize and providing a more-powerful tool for fault repeat recognition.

In 2006, EnCana drilled an exploratory hydrocarbon borehole about 5 miles from the Shell-ARCO BN 1-9 that was drilled in the Saddle Mountains anticline. The EnCana borehole was drilled south of the Saddle Mountains providing an unfaulted stratigraphic reference. An earlier study using older XRF basalt compositions suggested the Saddle Mountains fault did not penetrate the borehole constraining the dip angle to greater than 65 degrees. Comparing the new EnCana XRF borehole stratigraphy to the BN 1-9 reanalyzed basalt, we were able to resolve a 600 m repeat in the BN 1-9 GRB stratigraphy. This analysis suggests that the Saddle Mountains fault zone is dipping approximately 30 degrees to the south. This is the first conclusive data defining the structural dip of a frontal fault zone in a Yakima fold.