Brian Head Peak, Iron County Robert F. Biek1 and Peter D. Rowley2 1Utah Geological Survey, P.O. Box 146100, Salt Lake City, UT 84114-6100, [email protected] 2Geologic Mapping Inc., P.O. Box 651, New Harmony, UT 84757 Utah Geosites 2019 Utah Geological Association Publication 48 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors Cover Image: Th e type section of the Brian Head Formation (Tbh) is on the ridge just right of center (by the Tbh label). Brian Head peak is capped by the Leach Canyon Formation (Tql), which overlies the Isom Formation (Ti). A modern landslide (Qms) is west and south of the peak. M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 Presidents Message I have had the pleasure of working with many diff erent geologists from all around the world. As I have traveled around Utah for work and pleasure, many times I have observed vehicles parked alongside the road with many people climbing around an outcrop or walking up a trail in a canyon. Whether these people are from Utah or from another state or country, they all are quick to mention to me how wonderful our geology is here in Utah. Utah Geosites 2019 Utah is at the junction of several diff erent geological provinces. We have the Basin and Range to the west and the Central Utah Utah Geological Association Publication 48 Hingeline and Th rust Belt down the middle. Th e Uinta Mountains M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors have outcrops of some of the oldest sedimentary rock in Utah. Utah also has its share of young cinder cones and basaltic lava Utah Geosites showcases some of Utah’s spectacular geology, both fl ows, and ancient laccoliths, stratovolcanoes, and plutonic rocks. little-known localities and sites seen by visitors to Utah’s many Th e general public comes to Utah to experience our wonderful national and state parks and monuments. Th e geosites refl ect the scenic geology throughout our state and national parks. Driving interests of the many volunteers who wrote to share some of their between our national and state parks is a breathtaking experience. favorite geologic sites. Th e list is eclectic and far from complete, and we hope that additional geosites will be added in the coming Th e “Utah Geosites” has been a great undertaking by many people. years. Th e Utah Geological Survey also maintains a list of geosites I wanted to involve as many people as we could in preparing this https://geology.utah.gov/apps/geosights/index.htm. guidebook. We have had great response from authors that visit or work here in the state. Several authors have more than one site that We thank the many authors for their geosite contributions, they consider unique and want to share with the rest of us. I wanted Utah Geological Association members who make annual UGA to make the guidebook usable by geologists wanting to see outcrops publications possible, and the American Association of Petroleum and to the informed general public. Th e articles are well written Geologists—Rocky Mountain Section Foundation for a generous and the editorial work on this guidebook has been top quality. grant for desktop publishing of these geosite papers. I would like to personally thank Mark Milligan, Bob Biek, and Design and desktop publishing by Jenny Erickson, Graphic Paul Inkenbrandt for their editorial work on this guidebook. Designer, dutchiedesign.com, Salt Lake City, Utah. Th is guidebook could not have happened without their support. I would like to thank Jenny Erickson for doing the great desktop Th is is an open-access article in which the Utah Geological publishing and the many authors and reviewers that helped Association permits unrestricted use, distribution, and reproduction prepare the articles. Your work has been outstanding and will of text and fi gures that are not noted as copyrighted, provided the certainly showcase the many great places and geology of Utah. original author and source are credited. See the Utah Geological Last, but not least, Th ank you to the American Association of Association website, www.utahgeology.org, and Creative Commons Petroleum Geologists, Rocky Mountain Section Foundation for https://creativecommons.org/licenses/by/4.0/ for details. their fi nancial support for this publication. Suggested citation for this geosite: Guidebook 48 will hopefully be a dynamic document with the potential to add additional “geosites” in the future. I hope more Biek, R.F., and Rowley P.D., 2019, Brian Head Peak, Iron County, authors will volunteer articles on their favorite sites. I would like in Milligan, M., Biek, R.F., Inkenbrandt, P., and Nielsen, to fi ll the map with locations so that a person or family looking at P., editors, Utah Geosites: Utah Geological Association the map or articles will see a great location to read about and visit. Publication 48, 11 p., https://doi.org/10.31711/geosites. Enjoy Guidebook 48 and enjoy the geology of Utah. v1i1.47. Peter J. Nielsen 2019 UGA President 2 R.F. Biek and P.D. Rowley Brian Head Peak Figure 1. The type section of the Brian Head Formation (Tbh) is on the ridge just right of center (by the Tbh label). Brian Head peak is capped by the Leach Canyon Formation (Tql), which overlies the Isom Formation (Ti). A modern landslide (Qms) is west and south of the peak. INTRODUCTION LOCATION Brian Head peak, the highest point on the west edge of the Mark- Brian Head peak, just off Utah Highway 143 north of Cedar agunt Plateau at 11,307 feet (3447 m), provides stunning views Breaks National Monument, is accessible via Brian Head Peak westward into the Great Basin. The plateau is part of the High Road (Forest Service Road 047) during the summer to early fall; Plateaus, a subprovince of the Colorado Plateau. Few views in it is closed by snow the remainder of the year. The first 1.9 miles southern Utah so well demonstrate the huge difference between (3 km), to a Forest Service trailhead parking area with pit toilet, is the badly broken Great Basin, where east-west crustal extension an improved gravel road typically accessible by cars. The last 0.75 (pulling apart) produced north-trending faulted basins and inter- miles (1.2 km) is rougher but usually still passable by high-clear- vening ranges, and the much less deformed and here higher Col- ance, two-wheel drive vehicles. An open shelter, built by the CCC orado Plateau. The southwestern flank of the peak is the type sec- (Civilian Conservation Corps) of local stone (the Leach Canyon tion of the Brian Head Formation, an Eocene to Oligocene stream Formation), caps the summit plateau. The best place to see the en- and lake deposit that is overlain by densely welded ash-flow tuff tire Leach Canyon Formation is at the south end of the peak at 37° of the 27 to 26 Ma Isom Formation. The peak itself is capped by 40' 48.2", 112° 49' 51", about 140 yards (125 m) south of the CCC the moderately welded, 23.8 Ma ash-flow tuff of the Leach Canyon shelter. Figures 2, 3, and 4 provide a geologic map, stratigraphic Formation (figure 1). These two regionally extensive, upper Oligo- column, and cross section for the area. cene ash-flow tuffs erupted suddenly and explosively from calde- ras near the Utah-Nevada border and made their way in minutes STRATIGRAPHY to their present position, devastating everything in between, Regional Ash-flow Tuffs millions of years before the episode of basin-range deformation formed the Great Basin and uplifted the High Plateaus. Both bear Utah’s middle Cenozoic landscape looked unimaginably different on the timing of basin-range deformation, and both can be visited from that of today. Geologists refer to that former landscape as the on Brian Head. Furthermore, the Isom Formation itself is a key Great Basin altiplano or Nevadaplano, a high-elevation region that player in understanding Earth’s largest terrestrial landslide, the stretched from the Sierra Nevada in eastern California eastward to Markagunt gravity slide (see Sidney Peaks geosite). The south side what is now the Colorado Plateau (DeCelles, 2004; Best and oth- of the peak offers the best exposures of the Leach Canyon Forma- ers, 2009, 2013). The altiplano was studded with volcanic moun- tion, including its vitrophyre and basal surge deposits, which are tains and intervening basins, analogous perhaps to the modern seldom exposed glassy and sandy parts of typical ash-flow tuffs. Andean Altiplano of South America. It was onto this landscape 3 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 Td Ti Tm(Ti) Tbh Qmt 6 T Ti UL FA Qac Qmt Tql Tbh E G ED Qms L Brian Head Peak 2 Qms BH071808-7 BH071808 Ti K -4, -5, -6 AC BL BH071808-2, -3 Ti BH071808-1 Qc Qmt Qmsh Qms 71 Ti Tcwm Tcwu Qmt 16 Tbh Qms 14 37 Tbh Qacf Qacfo Qc Tcp Qms Qacf Tcwl Qal Qacf Tcwm Tcwt Tm(Ti) 53 Tcwu Tm Tbh Qes Qac Qc Cedar Breaks North View Tm(Ti) L Tm Overlook Qes Tcwm BH062310-1 Tcp Tcwl Contact 42 Qac Normal fault Qms Qac Gravity slide fault Tm(Ti) Sinkhole 15 Bedding Attitude 3000 ft Figure 2. Geologic map of the Brian Head peak area. Map units are shown on figure 3 (surficial deposits are various shades of light yel- low). Note the large area of modern landslides (yellow with red triangle pattern) that resulted from failure of the Brian Head Formation.