CHARACTERIZATION AND DEVELOPMENT OF FRACTURE PATTERNS FORMED IN DOMED MASSIVE SANDSTONE, HENRY MOUNTAINS, UTAH

We present in-progress work studying fractures developed during doming of massive aeolian sandstone in the upper crust due to emplacement of an underlying igneous intrusion. We studied the Pulpit Arch, located on the eastern side of the Mt. Hillers laccolithic intrusive center in the Henry Mountains of southern Utah. The Pulpit Arch is an asymmetrical dome formed above an underlying, unexposed Oligocene tongue-shaped igneous intrusion that radiates outward from the larger Mt. Hillers laccolith not studied for the work presented here. To study fracture development during doming, we compare fracture patterns in the domed region to those in the same stratigraphic unit outside the domed region. The strata studied are aeolian Jurassic Navajo Sandstone.

The first order geometry of the Pulpit Arch dome is apparent from deflection of stratigraphy from the regional sub-horizontal bedding orientation. In map view, the Pulpit Arch structure has a tongue-shaped geometry that closes to the east. The distal portion of the structure has a diameter of ~2.5 km. Along a north-south profile, strata are deflected upward a maximum of ~200 m at the center of the structure. Dip reaches a maximum of ~12 degrees, and varies smoothly from the regional subhorizontal orientation; no clear hinges or limbs are apparent.

Across the Pulpit Arch, 24 stations were distributed as evenly as possible, with an average spacing of ~300 m. At each station, a representative 16 m² area was chosen within which we collected data on fracture type, orientation, planarity, length, and style of termination. In addition, the contact between the Navajo Sandstone and the overlying Page Sandstone was mapped. An unfolded area outside of the Pulpit Arch dome was analyzed as a control area using the same methods.

The data collected within the Pulpit Arch and control area are being analyzed to examine the characteristics of fractures formed during doming. Preliminary results indicate that the entire Pulpit Arch has a higher fracture intensity than the control area. The most distal portions of the dome, where strata are deflected upward, has a high fracture intensity. Moving towards the center of the dome, fracture intensity first decreases, then increases again near the crest of the structure. Further analysis will include correlation between fracture intensity and fold curvature.