UGA-Geosite-Biek-Brian-Head-Peak.indd Utah Geosites 2019 Utah Geological Association Publication 48 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors Robert F. Biek1 and Peter D. Rowley2 1Utah Geological Survey, P.O. Box 146100, Salt Lake City, UT 84114-6100, bobbiek@utah.gov 2Geologic Mapping Inc., P.O. Box 651, New Harmony, UT 84757 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. Brian Head Peak, Iron County 2 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 Utah Geosites 2019 Utah Geological Association Publication 48 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors Utah Geosites showcases some of Utah’s spectacular geology, both little-known localities and sites seen by visitors to Utah’s many national and state parks and monuments. Th e geosites refl ect the interests of the many volunteers who wrote to share some of their favorite geologic sites. Th e list is eclectic and far from complete, and we hope that additional geosites will be added in the coming years. Th e Utah Geological Survey also maintains a list of geosites https://geology.utah.gov/apps/geosights/index.htm. We thank the many authors for their geosite contributions, Utah Geological Association members who make annual UGA publications possible, and the American Association of Petroleum Geologists—Rocky Mountain Section Foundation for a generous grant for desktop publishing of these geosite papers. Design and desktop publishing by Jenny Erickson, Graphic Designer, dutchiedesign.com, Salt Lake City, Utah. Th is is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and fi gures that are not noted as copyrighted, provided the original author and source are credited. See the Utah Geological Association website, www.utahgeology.org, and Creative Commons https://creativecommons.org/licenses/by/4.0/ for details. Suggested citation for this geosite: Biek, R.F., and Rowley P.D., 2019, Brian Head Peak, Iron County, in Milligan, M., Biek, R.F., Inkenbrandt, P., and Nielsen, P., editors, Utah Geosites: Utah Geological Association Publication 48, 11 p., https://doi.org/10.31711/geosites. v1i1.47. 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 is at the junction of several diff erent geological provinces. We have the Basin and Range to the west and the Central Utah Hingeline and Th rust Belt down the middle. Th e Uinta Mountains have outcrops of some of the oldest sedimentary rock in Utah. Utah also has its share of young cinder cones and basaltic lava fl ows, and ancient laccoliths, stratovolcanoes, and plutonic rocks. Th e general public comes to Utah to experience our wonderful scenic geology throughout our state and national parks. Driving between our national and state parks is a breathtaking experience. Th e “Utah Geosites” has been a great undertaking by many people. I wanted to involve as many people as we could in preparing this guidebook. We have had great response from authors that visit or work here in the state. Several authors have more than one site that they consider unique and want to share with the rest of us. I wanted to make the guidebook usable by geologists wanting to see outcrops and to the informed general public. Th e articles are well written and the editorial work on this guidebook has been top quality. I would like to personally thank Mark Milligan, Bob Biek, and Paul Inkenbrandt for their editorial work on this guidebook. Th is guidebook could not have happened without their support. I would like to thank Jenny Erickson for doing the great desktop publishing and the many authors and reviewers that helped prepare the articles. Your work has been outstanding and will certainly showcase the many great places and geology of Utah. Last, but not least, Th ank you to the American Association of Petroleum Geologists, Rocky Mountain Section Foundation for their fi nancial support for this publication. Guidebook 48 will hopefully be a dynamic document with the potential to add additional “geosites” in the future. I hope more authors will volunteer articles on their favorite sites. I would like to fi ll the map with locations so that a person or family looking at the map or articles will see a great location to read about and visit. Enjoy Guidebook 48 and enjoy the geology of Utah. Peter J. Nielsen 2019 UGA President R.F. Biek and P.D. Rowley Brian Head Peak 3 INTRODUCTION Brian Head peak, the highest point on the west edge of the Mark- agunt Plateau at 11,307 feet (3447 m), provides stunning views westward into the Great Basin. The plateau is part of the High Plateaus, a subprovince of the Colorado Plateau. Few views in southern Utah so well demonstrate the huge difference between the badly broken Great Basin, where east-west crustal extension (pulling apart) produced north-trending faulted basins and inter- vening ranges, and the much less deformed and here higher Col- orado Plateau. The southwestern flank of the peak is the type sec- tion of the Brian Head Formation, an Eocene to Oligocene stream and lake deposit that is overlain by densely welded ash-flow tuff of the 27 to 26 Ma Isom Formation. The peak itself is capped by the moderately welded, 23.8 Ma ash-flow tuff of the Leach Canyon Formation (figure 1). These two regionally extensive, upper Oligo- cene ash-flow tuffs erupted suddenly and explosively from calde- ras near the Utah-Nevada border and made their way in minutes to their present position, devastating everything in between, millions of years before the episode of basin-range deformation formed the Great Basin and uplifted the High Plateaus. Both bear on the timing of basin-range deformation, and both can be visited on Brian Head. Furthermore, the Isom Formation itself is a key player in understanding Earth’s largest terrestrial landslide, the Markagunt gravity slide (see Sidney Peaks geosite). The south side of the peak offers the best exposures of the Leach Canyon Forma- tion, including its vitrophyre and basal surge deposits, which are seldom exposed glassy and sandy parts of typical ash-flow tuffs. LOCATION Brian Head peak, just off Utah Highway 143 north of Cedar Breaks National Monument, is accessible via Brian Head Peak Road (Forest Service Road 047) during the summer to early fall; it is closed by snow the remainder of the year. The first 1.9 miles (3 km), to a Forest Service trailhead parking area with pit toilet, is an improved gravel road typically accessible by cars. The last 0.75 miles (1.2 km) is rougher but usually still passable by high-clear- ance, two-wheel drive vehicles. An open shelter, built by the CCC (Civilian Conservation Corps) of local stone (the Leach Canyon Formation), caps the summit plateau. The best place to see the en- tire Leach Canyon Formation is at the south end of the peak at 37° 40' 48.2", 112° 49' 51", about 140 yards (125 m) south of the CCC shelter. Figures 2, 3, and 4 provide a geologic map, stratigraphic column, and cross section for the area. STRATIGRAPHY Regional Ash-flow Tuffs Utah’s middle Cenozoic landscape looked unimaginably different from that of today. Geologists refer to that former landscape as the Great Basin altiplano or Nevadaplano, a high-elevation region that stretched from the Sierra Nevada in eastern California eastward to what is now the Colorado Plateau (DeCelles, 2004; Best and oth- ers, 2009, 2013). The altiplano was studded with volcanic moun- tains and intervening basins, analogous perhaps to the modern Andean Altiplano of South America. It was onto this landscape 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. 4 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 L 3000 ft Brian Head PeakBrian Head PeakBrian Head Peak Cedar Breaks North View Overlook Cedar Breaks Cedar Breaks Cedar Breaks Cedar Breaks Tcp Qc Tcwt Ti Qms Tcwu Ti Tcwm Tbh Tql Ti Tcwm Qms Qacf Tbh Qac Tm(Ti) Tcwl Qc Tm Qmt Qc Tcp Qac Qac Qms Tm Qmt Qacf Qms Qacf Qms Ti Tcwl Ti Qmt Qmt Qacfo Qmsh Tbh Qms Tbh Tm(Ti) Qes Tm(Ti) Qes Tbh Qac TcwuTcwm Td Tm(Ti) Qal BLACK FAULT LE DGE BH071808-4, -5, -6 BH071808-7 BH071808-2, -3 BH071808-1 BH062310-1 2 6 42 53 37 14 71 16 Contact Normal fault Gravity slide fault Sinkhole Bedding Attitude15 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. The Brian Head ski area, and the southern end of the Yankee Meadows graben, is in the upper left part of the map. From Rowley and others (2013). R.F. Biek and P.D. Rowley Brian Head Peak 5 that dozens of widespread ash-flow tuffs accumulated—part of the middle Cenozoic “ignimbrite flare-up” of southwestern North America, which represents one of the largest episodes of subduc- tion-related, silicic volcanism known on Earth (figure 5). Ash-flow tuffs (ignimbrites in European parlance) are the deposits of pyroclastic flows, density currents of hot volcanic rock, ash, and gases derived from explosive volcanic eruptions. Pyroclastic flows can travel rapidly more than 100 miles (160 km) across the land- Miocene Markagunt Megabreccia Tm(Ti) Tm(Tnw) Tm(Tbh) Tm 450 (140) 300 (90) 40 (12) 150 (45) Mount Dutton Fm. Td 10 (3) Tm(Tqcb) Leach Canyon Formation Tql 55–100 (17–30) Isom Formation Ti 350 (107) Oligocene Wah Wah Springs Fm. Tnw 3–8 (1–3) Brian Head Formation Tbh 500 (150) Eocene uppermost unit middle unit upper limestone unit Tcw lower limestone unit Tcwm Tcwl 47 (14) Tcwu Tcwt w hi te m em be r 440 (135) 310 (94) TE R TI A R Y Paleocene pink memberC la ro n Fo rm at io n Tcp 1000 (305) 109 (33) 45–60(14–18) 23.0 33.9 ?55.8 50+ (15+) about 20–22 Ma 23.8 Ma 26–27 Ma 29.5 Ma 35 Ma 36 Ma Ma System and Stage LITHOLOGY AGE MAP UNIT SYMBOL MAP THICKNESS feet (meters)Series STRATIGRAPHIC COLUMN Figure 3. Stratigraphic column showing rock units of the Brian Head peak area. The Markagunt Megabreccia is the deposit of the Marka- gunt gravity slide. From Rowley and others (2013). Figure 4. Cross section through Black Ledge at Sydney Peaks. Note that line of cross section lies just north of and covers a wider area than that shown in figure 2. The Yankee Meadows graben, down-dropped blocks between the Black ledge and Rattlesnake Canyon faults, is covered with modern landslide deposits derived from the clay-rich Brian Head Formation. From Rowley and others (2013). Black Ledge Thin surficial deposits not shown WEST A 11,000 10,000 9,000 8,000 7,000 6,000 Navajo Ridge Parowan Canyon Utah State Route 143 Brian Head Resort well #2 (projected to section) Bend in section Dark Hollow Sidney Peaks EAST A' 11,000 10,000 9,000 8,000 7,000 6,000 Tm Ksj Kw Tcp Kgc & Kwcs Km Tcw Tbh Ti Ksj Kw Tcp Kgc & Kwcs Km Tcw Tbh Ti Ksj Kw Tcp Kgc & Kwcs Km Qms Qms Tm(Tbh) Ksd Navajo Ridge fault Ksd faultY a n k e e M e a d o w s g r a b e n Brian Head Resort well #1 (projected to section) Rattlesnake Canyon fault Tm Cretaceous, undivided Cretaceous, undivided E le va tio n in fe et M A R K A G U N T P L A T E A U scape, filling valleys that radiate away from volcanic highlands. One interesting and very useful characteristic of ash-flow tuffs, noted by Mackin (1960), is that ash-flow tuffs are emplaced in a geological instant over broad areas, and thus serve as important time horizons for correlating rock formations and understand- ing structural development of the region. Best and others (2013) summarized how our understanding of these ash-flow tuffs has evolved, beginning in the 1950s with J. Hoover Mackin who first realized that they were indeed the products of enormous cata- strophic eruptions of volcanic ash, not simply lava flows. Today, the calderas themselves are recognizable only through mapping of stratigraphic and structural relations between caldera in-fill and outflow deposits, inasmuch as 20 million years of subsequent ba- sin-range extension, erosion, and burial under intervening basins makes the calderas all but invisible in the modern landscape. The eruption of Oligocene to Miocene ash-flow tuffs in Nevada and Utah is part of a broad pattern of volcanism that migrated southward through time across northwestern North America from about 55 to 20 million years ago (Mackin, 1960; Cook, 1965; Armstrong and others, 1969; Stewart and Carlson, 1976; Stewart and others, 1977; Rowley, 1998; Rowley and Dixon, 2001). The southward migration resulted from complex plate tectonic interac- tions along the western margin of North America as outlined by Dickinson (2006) and Humphreys (2009). Ultimately, these and other researchers hypothesize a tear in the relatively cold and dense oceanic crust of a subduction zone. The tear allowed the subducting slab to peel away from the less dense continental crust above, opening a window through which relatively hot upper mantle rock ascended, feeding the magmatic flare-up. Best and others (2013) provided a comprehensive summary of the Great Basin ash-flow tuff province of Nevada and western Utah, where, from about 36 to 18 million years ago, more than 200 large eruptions from 42 calderas resulted in more than 16,500 cubic miles (70,000 km3) of tuff deposited over the landscape (figure 5). In the Indian Peak and Caliente caldera complexes in the eastern part of the tuff province, more than 50 large eruptions produced 6 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 an estimated 7600 cubic miles (32,000 km3) of ash-fl ow tuff s now spread over an area of 15,000 square miles (63,000 km2) in east-central Nevada and southwestern Utah (this volume is rough- ly enough to fi ll the Grand Canyon nearly eight times over). Nine of those eruptions are popularly known as “super eruptions,” each having ejected more than 240 cubic miles (1000 km3) of rock, in- cluding from older to younger the Wah Wah Springs, Lund, Isom, and Leach Canyon Formations, the latter two of which are found at Brian Head peak. In addition to ash-fl ow tuff s, these eruptions produced voluminous ash falls—fi ne-grained ash fall from the Wah Wah Springs eruption, for example, is recognized in western Nebraska (Best and others, 2013). Over time, repeated eruptions of the Indian Peak and Caliente caldera complexes produced ash-fl ow tuff s that fi lled topographic low areas in the Great Basin altiplano. Because ash-fl ow tuff s tend to be confi ned to former river val- leys far from their source area, the distal ends of many ash-fl ow tuff s behave geomorphically like basaltic lava fl ows (see the St. George inverted valleys geosite). Because it is relatively fl uid when it erupts, basaltic lava fl ows downhill until it becomes entrained in old stream valleys. Th is buries and displaces the stream, and because the basalt is typically harder than enclosing rocks and sediments of the valley walls, the stream shift s to the side of the fl ow and preferentially erodes adjacent, less-resistant rock. Ulti- mately, through a process called topographic inversion, this leaves the lava fl ow as a sinuous high ridge above the surrounding land- scape—the ridge marks the location of the former stream channel. Topographic inversion of the distal ends of many ash-fl ow tuff s naturally ensues as it does for basaltic lava fl ows. Th e distribution of regional ash-fl ow tuff s on the Markagunt Plateau and Sevier Plateau (the next range to the east) suggests that they may have been controlled by broad, east-fl owing stream valleys (Biek and others, 2015). When an ash-fl ow tuff blocked a stream valley, displaced streams stepped southward to create another valley later to be fi lled by a subsequent, younger ash-fl ow tuff . In this way, older ash-fl ow tuff s are typically found farther north in this part of southwestern Utah. Leach Canyon Formation Th e Leach Canyon Formation is a pinkish-brown, moderately welded (welding refers to the compaction of ash shards at the time of emplacement, resulting in a resistant rock even when welding is poor), moderately crystal-rich rhyolite ash-fl ow tuff that erupted from a caldera on the Utah-Nevada border nearly 24 million years ago. Th e formation has a volume of at least 430 cubic miles (1800 km3) and a distribution of at least 6000 square miles (15,000 km2) (Best and others, 1989b). For comparison, the volume of erupted products from the 1980 Mount St. Helens eruptions was about 1 cubic mile (4 km3). Th e south side of Brian Head peak reveals the most complete section of the Leach Canyon Formation on the Markagunt Plateau (Biek and others, 2015). Th ere, the classic three-part section of an ash-fl ow tuff is exposed, including an un- welded basal surge deposit, a thick vitrophyre (black glassy layer), and the moderately welded ash-fl ow tuff that caps Brian Head peak (fi gure 6). Th e seldom exposed basal surge deposit is crys- tal-rich, tuff aceous sandstone with wavy and low-angle cross-bed- ding, resembling sand dunes. It records deposition by relatively low-concentration, turbulent density currents as the ash-fl ow tuff moved rapidly across the landscape. Th e Leach Canyon Formation contains abundant white or light-pink, collapsed pumice fragments and several percent rock fragments, many of which are reddish brown; phenocrysts of plagioclase, slightly less but subequal amounts of quartz and san- idine, and minor biotite, hornblende, Fe-Ti oxides, and a trace of pyroxene make up 25% to 35% of the rock. Th e formation is about Figure 5. Southern Great Basin ash-fl ow tuff province that resulted from the middle Cenozoic (36 to 18 Ma) “ignimbrite fl are-up.” Several ash-fl ow tuff s (ignimbrites) from the Indian Peak and Caliente calde- ra complexes (blue lines) on the Utah-Nevada border spread eastward into southwest Utah and as far east as the Marysvale volcanic fi eld. Th e coeval Marysvale fi eld (purple, in southwest Utah) and calde- ras (black) are shown to emphasize the contrasting dominance of andesitic lavas and mudfl ows (most of the Marysvale fi eld) over more silica-rich ash-fl ow tuff s (most of the Great Basin). Th is fi gure does not show pre-36 Ma ash-fl ow tuff s and calderas from more northern igneous belts, nor does it show post-18 Ma tuff s and calderas from more southern igneous belts. Th e thick yellow line marks the western edge of the Great Basin altiplano; tuff s erupted from western Neva- da calderas fl owed mostly west down the west fl ank of the altiplano and are now preserved in exhumed paleovalleys across today’s Sierra Nevada Mountains. Th e western edge of Precambrian continental basement is defi ned geochemically by the dashed 87Sr/86Sr = 0.706 line. From Best and others (2013). 112°114°116°118°120° 42° 40° 38° 36° Arizona St. George Reno Elko Topographic barrier Salt Lake City Austin Tonopah Ely 0 10050 Kilometers 0 10050 Miles �eld Marysvale G R E A T N E VA D A S IE R R A B A S I N P L AT E A U C O L O R A D O Las Vegas Fig_8.02_NevUtah Nevada Utah Cal i fornia 87 Sr/ 86 Sri = 0.706 Andesitic lavaSilicic ignimbrite Western NV caldera Central NV caldera Indian Peak- Caliente caldera R.F. Biek and P.D. Rowley Brian Head Peak 7 100 feet (30 m) thick where it forms the resistant caprock of Brian Head peak. Interestingly, the Leach Canyon Formation is petro- graphically and chemically similar to the Haycock Mountain Tuff, a small, locally derived ash-flow tuff exposed to the east. This sim- ilarity led to confusion about the age of landslide deposits even- tually called the Markagunt gravity slide (Biek and others, 2015). The 22.8 Ma Haycock Mountain Tuff unconformably overlies the Markagunt gravity slide a dozen miles (20 km) east of Brian Head. The older, 23.8 Ma Leach Canyon Formation underlies and is locally involved in the slide. Isom Formation The 26 to 27 Ma, crystal-poor, densely welded, trachydacitic ash- flow tuff of the Isom Formation is present as far south as Brian Head peak and is spectacularly exposed for many miles along Black Ledge where it is about 350 feet (110 m) thick (figure 7). The Isom Formation is unusual in that it was so hot when erupted that it flowed like lava during its final stages (few tens of feet) of em- placement. For that reason it is commonly referred to as a tufflava or a rheomorphic ash-flow tuff—see, for example, Anderson and Rowley (1975, 2002), Andrews and Branney (2005), and Geissman and others (2010). Many Isom outcrops reveal secondary flow characteristics, including flow breccias, contorted flow layering, and linear vesicles, such that the unit was considered a lava flow until Mackin (1960) mapped its widespread distribution (300 cubic miles [1300 km3] today spread over an area of 9500 square miles [25,000 km2] [Best and others, 1989a]) and found evidence of glass shards, thus showing its true ash-flow tuff nature. The Isom is exposed at Brian Head peak where the lower part of the formation is classic tufflava about 80 feet (24 m) thick and the upper part is a flow breccia 60 to 90 feet (18-27 m) thick (Biek and others, 2015). Brian Head Formation The Brian Head Formation is characterized by white volcaniclastic mudstone, siltstone, sandstone, volcanic ash, muddy limestone, and minor conglomerate and multi-hued chalcedony, and it is known for its propensity for landsliding. These strata, rich in volcanic ash, were deposited in low-relief fluvial, floodplain, and lacustrine environments; they record the inception of volcanism in southwest Utah beginning about 37 million years ago (Sable and Maldonado, 1997) (figure 8). The base of the section is well exposed near the North Rim Overlook in Cedar Breaks National Monument; there, a thin rhyolitic ash bed overlies a thin pebbly conglomerate likely equivalent to the conglomerate at Boat Mesa in Bryce Canyon National Park. This ash bed yielded a U-Pb age on zircon of 35.77 ± 0.28 Ma, and several additional radiometric ages from Brian Head strata throughout the region show that it was deposited from about 37 to 33 million years ago (Biek and others, 2015). Thus, it is most- ly late Eocene in age, barely reaching into the Oligocene. The Brian Head Formation contains abundant trace fossils, includ- ing possible crayfish burrows and root traces (Golder and Wizev- ich, 2009; Golder and others, 2009), but aside from its basal varie- gated interval it is surprisingly unfossiliferous (Eaton and others, 1999). It also has colorful beds of chalcedony in various shades of white, gray, yellow, red, black, and brown, all typically with a white weathering rind. The chalcedony forms resistant beds as much as 10 feet (3 m) thick and is thought to have resulted from silicifica- tion of limestone beds (Maldonado, 1995; Sable and Maldonado, Figure 6. Leach Canyon Formation on the south side of Brian Head peak. Here, the classic three-part section of an ash-flow tuff is exposed, including an unweld- ed basal surge deposit, a thick vitrophyre, and moderately welded ash-flow tuff that caps Brian Head peak. A flow breccia of the Isom Formation (Ti) is present in the lower left corner of the photograph. Figure 7. A. The resistant Isom Formation at Black Ledge, looking south towards Brian Head peak. B. Contorted flow layering of this densely welded ash-flow tuff, an example of its rheomorphic nature. A B 8 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 1997; Schinkel, 2012). Being resistant, the chalcedony commonly litters slopes developed on Brian Head strata; it was commonly used for tools and arrowheads by Native Americans. Above all, however, the Brian Head Formation is known for its swelling soils and for its susceptibility to landsliding. Nearly all ex- posures on steep hillsides form large landslide complexes, includ- ing one that fi lls the 15-mile (24 km) length of Yankee Meadows graben, home to the resort town of Brian Head. Th e formation is susceptible to landslides because of its abundant clay derived from weathered volcanic ash. A more complete section of the Brian Head Formation is well exposed on the southwest fl ank of the Sevier Plateau (fi gure 9). Th ere, a transitional interval as much as 160 feet (50 m) thick, not present elsewhere, of fi ne-grained, slope-forming sandstone, siltstone, and mudstone of red, pink, yellowish-brown, and pur- plish-gray hues forms the base of the formation. Th is variegated interval yielded fossil turtles, charophytes (freshwater green algae), and fi sh suggestive of lacustrine environments of the Duchesnean North American Land Mammal Age (end of middle Eocene) (Feist and others, 1997; Eaton and others, 1999; Korth and Eaton, 2004). STRUCTURE Th e Markagunt Plateau is uplift ed with respect to the Great Basin by high-angle normal faults, some of which create a series of horsts and grabens west and north of Brian Head peak. Horsts are blocks that are upthrown by faults on each side, whereas grabens are blocks that are downthrown by faults on each side. Th ese horsts and grabens step down from the plateau to the adjacent Great Basin (fi gure 10) (Maldonado and others, 1997; Biek and others, 2015). Th e most prominent of these faults is the active Hurricane fault at the base of the plateau, which has a down- to-the-west vertical displacement of at least 6000 feet (1800 m) 113°00´ 112°30´ 38°00´ 37 °3 7´ 30 ˝ 0 0 5 Miles 5 Kilometers 143 143 148 14 R E D EN O C H G R AB EN H I L L S H O R S T NORTHERN CEDAR VALLEY GRABEN Hur ric an e fa u l t zo ne Iro n Sp rin gs th ru st f a ul t P A R O W A N V A L L E Y G R A B E N R ed H ills fault zo ne FI D D LE R S C AN YO N G R AB EN M id dl e R id ge fa u l t M AP LE R ID G E H O R ST Jo ne s H ill fa ul t SU M M IT M O U N TA IN G R AB EN Su m m it M ou nt ai n fa ul t PAROWAN GRABEN Su ga rlo af M ou nt ai n fa ul t SU GA RL OA F G RA BE N Na va jo R id ge fa ul t NA VA JO RI DG E HO RS T Ratt les na ke Can yo n fa ult YANKEE MEAD OW S G RA BE N Blac k Le dg e fau lt M A R K A G U N T P L A T E A UParagonah Parowan Summit Pa ra go na h fa u l t PA RO W AN CAN YON H O R ST Iro n Pe ak we st fa u lt IR ON PEAK GRA BE N Iro n Peak eas t faul t w es t Be ar Va lle y fa ult ea st Be ar Va lle y fa ult BE AR VA LL EY GRABEN Location Map Normal fault, bar and ball on down-dropped side Thrust fault, teeth on upper plate Parowan fault 15 Salt Lake City Brian Head Peak CBNM North View Overlook ¯ Figure 10. Major faults of the western Markagunt Plateau and Red Hills, and named grabens (shaded) and horsts. CBNM = Cedar Breaks National Monu- ment. From Biek and others (2015). Figure 8. Air-fall ash bed (white unit) in the Brian Head Formation just south of Haycock Mountain, which yielded a U-Pb age on zircon of 34.95 ± 0.83 Ma. Figure 9. Exceptional exposures of the Brian Head Formation on the southwest fl ank of the Sevier Plateau. Here, Brian Head strata are divisible into four parts: (1) a basal variegated unit (below the hikers and so not visible), (2) a lower, light-gray, fi ne-grained volcaniclastic unit, which includes a thick, bluish-gray bentonitic mudstone at its base (Tbh1), (3) a distinctive red-green-gray banded, fi ne-grained volcaniclastic unit (Tbh2), and (4) a thick, upper volcaniclastic unit (Tbh3). Brian Head strata are capped by Mount Dutton Formation volcanic mudfl ow deposits (Td), which here mark the base of the Sevier gravity slide.. R.F. Biek and P.D. Rowley Brian Head Peak 9 (Hurlow, 2002; Lund and others, 2007). Overall, the collection of horsts and grabens forms a highly faulted relay ramp between the Paragonah fault and the Hurricane fault (fi gure 11). Th e question of when this faulting began can be answered in part by the distribution of ash-fl ow tuff s. Whereas modern basin-range extension that led to the topography we see today likely began about 10 to 12 million years ago in the Cedar City area (Rowley and others, 1981; Hurlow, 2002), the initiation of such extension is much older. Because ash-fl ow tuff s like the Isom are widespread and were emplaced “in the blink of an eye,” they now serve as im- portant timelines that help constrain structural interpretations of southwestern Utah. Th e eruption of the Isom Formation was fol- lowed by the eruption of three regionally distinctive ash-fl ow tuff s: the 23.8 Ma Leach Canyon Formation, the 22.8 Ma Bauers Tuff Member of the Condor Canyon Formation, and the 22.0 Ma Har- mony Hills Tuff , all part of the Quichapa Group. Only the Leach Canyon is preserved as far east as Panguitch Lake; the Bauers and Harmony Hills tuff s are restricted to the western part of the Mark- agunt Plateau. Th e distribution of these ash-fl ow tuff s suggests that a west-facing topographic escarpment associated with early basin-range extension may have impeded eastward distribution of the latter two tuff s, as fi rst noted by Rowley and Barker (1978). Furthermore, a graben north of nearby Parowan juxtaposes Bear Valley and Brian Head strata that were later intruded by the 20 Ma Iron Peak laccolith, showing that early extension was active prior to about 20 million years ago (Biek and others, 2015). ACKNOWLEDGMENTS Our knowledge of southwestern Utah geology comes from com- bined decades of geologic mapping supported largely by the Utah Geological Survey and U.S. Geological Survey. As the acknowledg- ments of our many published geologic maps attest, we are indebt- ed to a great many people for their help over the years. Th anks to Grant Willis, Stephanie Carney, and Mike Hylland (all with the Utah Geological Survey) for their insightful reviews, and to Jenny Erickson (UGS) for draft ing the fi gures. REFERENCES Anderson, J.J., and Rowley, P.D., 1975, Cenozoic stratigraphy of southwestern high plateaus of Utah, in Anderson, J.J., Rowley, P.D., Fleck, R.J., and Nairn, A.E.M., editors, Cenozoic geology of southwestern high plateaus of Utah: Geological Society of America Special Paper 160, p. 1–51. 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