AN ANALYSIS OF FRACTURES AROUND THE SEVIER FAULT TRANSFER ZONE NEAR ORDERVILLE, UTAH

Structural discontinuities—such as opening mode joints, shear fractures, and faults— tend to occur in close geographic proximity to one another; however, the timing relationships between these structures is not always easy to discern in the field. In southwestern Utah, the Jurassic Navajo Sandstone is cut by large-scale normal faults associated with the Sevier fault zone, making it perfect for observing several fracture types. The aim of this study is to complete a dynamic and kinematic analyses of the fractures near a major fault and to determine the chronologic relationships between the fractures. Specifically, we observed a previously unnamed segment of the Sevier Fault Zone— herein referred to as the Mountain Lion Den Fault— previously interpreted as a west dipping normal fault striking 035°. The primary field area is the Red Hollow Canyon/Elkheart Cliffs region, located southeast of Orderville, Utah.

For this study, orientations (dip and dip direction) of fracture data within the Navajo Sandstone were measured and tracked on eight different scanlines. Scanline fractures were plotted on stereonets and averages determined. GPS locations were taken on a Trimble G7X at ends of each scanline for GIS mapping. Schmidt Hammer (L-type) data were taken to compare rock strengths near the Mountain Lion Den Fault.

Fracture analyses show a general NNE strike similar to the Mountain Lion Den fault strike. Despite a few outliers, scanline averages typically strike within 10°-15° of the 035° strike of the Mountain Lion Den fault. We interpret movement along the fault initiated around the same time some of the fractures formed. The fractures likely formed in front of the Mountain Lion Den fault at oblique angles to its strike as the fault propagated northward. These results suggest that an area of weakness formed in Red Hollow Canyon, allowing the fault to propagate easily at 035°. These results compare favorably to previous brittle fracture studies within propagating fault zones. Outliers in the data could be associated with NW rotation of σ3, similar to joints in Zion NP. Schmidt Hammer data show that oxidized beds have greater maximum compressive strengths than bleached zones in the Navajo Sandstone.