GEOLOGIC MAPPING OF THE JESSEN BUTTE 7.5' QUADRANGLE

The Jessen Butte quadrangle is located 6 miles west of Manila, Utah along the north flank of the Uinta Mountains. The north flank of the Uinta Mountains is bounded by two major thrust faults along which the majority of the Uinta Mountains were uplifted. The Henry's Fork thrust has maximum displacement along it in the western part of the Uinta Mountains. Displacement decreases along this fault as it is followed to the east where the Uinta Thrust picks up the displacement and increases to the east. The Jessen Butte quadrangle lies in the zone of transition of deformation between the Uinta Thrust fault in the southern portion of the quadrangle and the Henry's Fork Thrust fault in the north. Deformation between these two thrusts has produced a zone of complex folding and faulting throughout the quadrangle. Kinematic indicators along these faults indicate left-lateral oblique slip thrust movement.

The Nugget Sandstone is a 250 m (820 ft) thick poorly cemented eolian sandstone which makes up the north side of Jessen Butte. At the top of Jessen Butte, intense silicification of the Nugget Sandstone has altered it from an easily erodible tan sandstone to a purplish blue dense quartzite. This quartzite makes up the steep cliffs seen on Jessen Butte and provides the quartzite talus material which blankets most of Jessen Butte. Some of the quartzite boulders which have fallen from the top of Jessen Butte are up 4.5 m (15 ft) in diameter.

Two large landslides are also within the Jessen Butte quadrangle. One lies in the northwest corner of the quadrangle and is the result of collapse within the Bridger Formation off the eastern side of Phil Pico Mountain, which is in the quadrangle to the west. While this landslide covers 6.59 km2 (2.54 mi2) it sits on an area of low relief and shows no major signs of reactivation. The second landslide found in Sheep Creek Canyon in the southeast corner of the quadrangle. This landslide is made up of Uinta Mountain Group and is currently being undercut by Sheep Creek. It has seen recent activation, with the last major event being recorded in 1965 when a swollen Sheep Creek undercut the slide enough to cause a large debris flow through Sheep Creek Canyon. This event caused the loss of 7 lives.

This project is funded through the EDMAP program. Information from previous mapping was provided by Doug Sprinkel and Paul Jensen from the Utah Geological Survey.