MAPPING THREE GENERATIONS OF FRACTURES IN BRYCE CANYON NATIONAL PARK: A GEOCORPSTM AMERICA INTERNSHIP

Bryce Canyon National Park is famous for its spectacular hoodoos, fins, and windows exposed in the Paleocene to Eocene Claron Formation. In a general sense, these features formed through frost wedging in vertical fractures and differential weathering of flat-lying carbonate-cemented stream deposits and paleosols. Less obvious conjugate fractures associated with the Late Oligocene to Early Miocene Rubys Inn thrust fault occur throughout the park and may provide zones of weakness that influence weathering and erosion. During a summer 2014 GeoCorps^TM America internship, we sought to better understand relationships between conjugate fractures, stratigraphy, and hoodoo formation.

In addition to the vertical fractures and the conjugate fractures, our detailed geologic mapping revealed a third generation of variously oriented fractures. Unlike the conjugate fractures that show contraction in the north-south direction, this generation exhibits extension likely related to the nearby Paunsaugunt normal fault. It likely post-dates the conjugate fractures as evidenced by spectacular striations that are better developed and preserved than those indicating reverse slip. This newly discovered generation likely utilizes zones of weakness that originated during contraction. We also discovered cross-cutting relationships that suggest the vertical fractures pre-date or are contemporaneous with contractile conjugate fractures.

We propose a three-stage fracture history for the area. The first stage produced vertical fractures before the Rubys Inn thrust fault. The second stage produced conjugate fractures during contraction of the Rubys Inn thrust fault. The third stage produced extensional fractures likely associated with more recent Basin and Range extension. Alternatively, the first and second stages may be contemporaneous as some conjugate fractures cut the vertical fractures while many others do not.

Vertical fractures seem to have had the greatest influence on hoodoo and fin formation. However, windows commonly occur in fins that exhibit conjugate fractures, large competency differences between beds, or both. Future research will investigate how competency differences control deformation, and how the relationship between competency and fracturing controls window development.