EASTERN SNAKE RIVER PLAIN (IDAHO) and SOUTHWESTERN UTAH Geologic Excursions in Volcanology: Eastern Snake River Plain (Idaho) and Southwestern Utah

Geologic Excursions in Volcanology: Eastern Snake River Plain {Idaho} and Southwestern Utah

GUIDEBOOK - PART III

The Geological Society of America Rocky Mountain and Cordilleran Sections Meeting Salt Lake City, Utah May 2 - 4, 1983

WILLIAM P. NASH GREGORY D. HARPER General Chairman Field Trip Coordinator

KLAUS D. GURGEL Editor

~ UTAH GEOLOGICAL AND MINERAL SURVEY ;. a division of 'Il ~ Utah Department of Natural Resources and Energy ... \C) ~ Special Studies 61 May 1983 STATE OF UTAH Scott M. Matheson, Governor DEPARTMENT OF NATURAL RESOURCES AND ENERGY Temple A. Reynolds, Executive Director

UTAH GEOLOGICAL AND MINERAL SURVEY Genevieve Atwood, Director

BOARD Kenneth R. Poulson, Chairman ...... Brush Wellman, Incorporated Laurence H. Lattman, Vice Chairman ...... University of Utah lames H. Gardner ...... University of Utah Robert P. Blanc ...... Getty Oil 10 Brandt ...... Public-at-Large Elliot Rich ...... Utah State University E. Peter Matthies...... Sharon Steel Corporation

Ralph A. Miles, Director, Division of State Lands ...... exofficiomember

UGMS EDITORIAL AND ILLUSTRATIONS STAFF Klaus D. Gurgel ...... Editor Trena L. Foster, Nancy A. Close...... Editorial Staff Brent R. 10nes ...... Senior Illustrator Donald Powers...... Illustrator James W. Parker ...... Cartographer GEOLOGIC EXCURSIONS IN VOLCANOLOGY: EASTERN SNAKE RIVER PLAIN (IDAHO) AND SOUTHWESTERN UTAH Geologic Excursions in Volcanology: Eastern Snake River Plain (Idaho) and Southwestern Utah

GUIDEBOOK - PART III

The Geological Society of America Rocky Mountain and Cordilleran Sections Meeting Salt Lake City, Utah May 2 - 4, 1983

WILLIAM P. NASH GREGORY D. HARPER General Chairman Field Trip Coordinator

KLAUS D. GURGEL Editor

UTAH GEOLOGICAL AND MINERAL SURVEY a division of Utah Department of Natural Resources and Energy

Special Studies 61 May 1983 Editor's Note: The papers contained in this Guidebook were solicited by the organizers of the GSA Rocky Mountain and Cordilleran Sections and have been edited and given a common format; however, their style and content have not been formally reviewed by the Utah Geological and Mineral Survey. Contents Page Field Trip 4 - Holocene Basaltic Volcanism along the Great Rift, central and eastern Snake River Plain, Idaho ...... By Mel A. Kuntz, Richard H. Lefebvre, Duane E. Champion, John S. King, Harry R. Covington

Field Trip 9 - Mid-Tertiary History of the central Pioche-Marysvale Igneous Belt, southwestern Utah ...... 35 By Myron G. Best and Jeffrey D. Keith

v Utah Geological and Mineral Survey Special Studies 61, 1983 Guidebook Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting Salt Lake City, Utah - May 2 - 4, 1983

HOLOCENE BASALTIC VOLCANISM ALONG THE GREAT RIFT, CENTRAL AND EASTERN SNAKE RIVER PLAIN, IDAHO

Mel A. Kuntz u.s. Geological Survey, Federal Center, Denver, CO 80225 Richard H. Lefebvre Department of Geology, Grand Valley State College, Allendale, MI 49401 Duane E. Champion U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 John S. King Department of Geological Sciences, SUNY-Buffalo, Amherst, NY 14226 Harry R. Covington U.S. Geological Survey, Federal Center, Denver CO 80225

Part I: Craters of the Moon lava field and the northern part of the Great Rift

Part II: Kings Bowl and Wapi lava fields and the southern part of the Great Rift

INTRODUCTION The central and eastern parts of the Snake River The Snake River Plain is an arcuate topographic Plain constitute a broad, flat, lava plain consisting, depression, 50 to 100 km wide, that extends from at the surface, of basalt lava flows of the Snake near Payette, Idaho, on the west, for about 250 km River Group and thin discontinuous interbedded southeast to Twin Falls, and then for about 300 km loess, eolian sand, and alluvial fan deposits that northeast to near Ashton, Idaho (Figure 1). It is together have a total thickness of about 1 to 2 km bounded on the north by Mesozoic and early Terti­ near the area described in the present report (Zohdy ary granitic rocks of the Idaho batholith and by and Stanley, 1973; Stanley and others, 1977; Doher­ basin-range, block-faulted mountains that were up­ ty and others, 1979). Flows of the central and eas­ lifted in Tertiary and Quaternary time. It is bounded tern Snake River Plain were erupted from low vol­ on the southeast by basin-range, block-faulted canic vents that generally are aligned with, and mountains that also were uplifted in Tertiary and parallel to, volcanic rift zones that trend mainly at Quaternary time. It is bounded on the southwest by right angles to the axis of the eastern Snake River Tertiary rhyolitic and basaltic rocks of the Owyhee Plain. A volcanic rift zone is a narrow belt of faults, Plateau. Upper Tertiary and Quaternary rhyolitic grabens, noneruptive fissures, eruptive fissures, and basaltic rocks of the Yellowstone Plateau are spatter cones, spatter ramparts, cinder cones, lava present at the northeast end of the plain. Geologists cones, pit craters, and shield volcanoes. Most vents and geophysicists traditionally have divided the in volcanic rift zones are elongated and overlie erup­ Snake River Plain into eastern, central, and western tive fissures. Well defined volcanic rift zones of the parts based on different geological histories and Snake River Plain are as much as 10 km wide and geophysical features in each part (Mabey, 1976, 120 km long. The Great Rift is the best example of a 1978) . volcanic rift zone in the Snake River Plain. 2 Utah Geological and Mineral Survey Special Studies 61, 1983

115°W 113°W 112°W 111 0 W / I r I I I I 0 100 200 KM I ~-- I I I I / MONTANA I I (

TETON BLOCK

WYOMING NEVADA UTAH

Figure 1. Index map showing generalized geology of southern Idaho and localities referred to in the text.

In the central and eastern Snake River Plain, the en and McKee (I 978), and Mabey and others dominantly basaltic volcanism represents the latest (I 978). phase of a complex tectonic and volcanic history At least eight basaltic lava fields in the central and characterized by an earlier phase of dominantly eastern Snake River Plain are known, or believed, rhyolitic pyroclastic volcanism. Available to be less than 20,000 yrs old; they are the geological, geophysical, radiometric, and drilling Shoshone, Craters of the Moon, Kings Bowl, Wapi, data suggest that the rhyolitic rocks were erupted North Robbers, South Robbers, Cerro Grande, and from successive northeast-trending calderas. The Hells Half Acre lava fields (Figure 1). This two-day earliest calderas formed about 15 m.y. ago near field trip will visit remarkable examples of Holocene Twin Falls and were succeeded by progressively to latest Pleistocene basaltic lava flows and vents in younger calderas northeastward to Yellowstone Na­ the Craters of the Moon (COM), Kings Bowl, and tional Park. Models describing the formation of the Wapi lava fields. belt of calderas are discussed by Armstrong and The COM lava field, located on the northern part others (1975), Eaton and others (1975), Christians- of the Great Rift, includes more than 60 lava flows Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 3 and flow units, more than 25 cinder cones, and at COMNM. To be examined are vents and related least eight eruptive fissures and fissure systems. lava flows of the youngest (2,100 ± yrs old) eruptive 2 The field covers an area of 1,600 km , contains period of the COM lava field. To be emphasized are more than 30 km3 of lava, and is the largest basaltic dating and correlation methods used in establishing lava field of dominantly Holocene age in the conter­ the volcanic history of the COM lava field, the vol­ minous United States. COM lava flows were erupted canic and structural history of the northern part of during eight eruptive periods that began about the Great Rift, and the chemical diversity and evo­ 15,000 and ended about 2,100 yrs ago. lution of the COM lavas. A traverse from COMNM, 2 The Kings Bowl lava field is small 0.3 km , 0.01 southeastward across the ESRP to Pocatello, Idaho, km3) and consists of flows erupted during a single will end the first day. The second day will be spent fissure eruption on the southern part of the Great examining volcanic features at the Kings Bowl and Rift about 2,250 yrs ago. Wapi lava fields on the southern part of the Great The Wapi lava field is a low shield volcano that Rift. covers an area of about 330 km2 and contains about 14 he ideas and data to be presented on the field trip 6 km3 of lava. It also erupted about 2,250 yrs ago on have been formulated and collected by our col­ the southern part of the Great Rift. leagues and ourselves during the last ten years. The The first day begins at Idaho Falls, Idaho, in the work was supported by the USGS, DOE, and northeastern part of the plain, and proceeds NASA, and is still in preliminary form and subject westward, across the entire width of the eastern to further interpretation. We especially thank Elliott Snake River Plain (ESRP), to Craters of the Moon Spiker and Meyer Rubin of the USGS for their aid National Monument (COMNM) near Arco, Idaho. in radiocarbon dating, and Ron Greeley, Arizona Most of the volcanic vents in the Craters of the State University, for his counsel in helping us recon­ Moon (COM) lava field are aligned along the north­ struct the volcanic history of the Kings Bowl and ern part of the Great Rift and are located within the Wapi lava fields.

PART 1: CRATERS OF THE MOON LAVA FIELD AND THE NORTHERN PART OF THE GREAT RIFT By Mel A. Kuntz, Richard H. Lefebvre, and Duane E. Champion

RADIOCARBON DATING AND Only one or a few flows in each eruptive period EVOLUTION have been dated. Thus, the duration of each erup­ OF THE C01\1 LAVA FIELD tive period is not established. Figure 5 shows the Early studies of the COM lava field (Stearns, evolution of the COM lava field through the eight 1928; Murtaugh, 1961) outlined the general strati­ eruptive periods. Individual lava flows, their source graphic relations among lava flows. Later studies vents, and the approximate ages of each eruptive (Prinz, 1970; Valastro and others, 1972; Lefebvre, period are listed in Table 1. 1975; Kuntz and others, 1980; 1982a; 1982b) V olcanic features to be viewed on the first day of refined previously established stratigraphy, pro­ the trip are chiefly those of the youngest eruptive duced detailed geologic maps of the lava field, and period (A) of the COM lava field. attempted to date key flows by the radiocarbon method. Samples for radiocarbon studies include PALEOMAGNETIC STUDIES charcoal from tree molds and organic material in Paleomagnetic measurements have been used to sediment buried by flows. The radiocarbon and stra­ correlate and approximately date lava flows and tigraphic data indicate that the COM lava field groups of flows to aid in deciphering the volcanic formed during at least eight eruptive periods (H, history of the lava fields along the Great Rift. Lava the oldest, through A, the youngest) that began flows record the local geomagnetic field at the time about 15,000 and ended about 2,100 yrs ago. Each of eruption and cooling. Geomagnetic field changes eruptive period was probably only a few tens of due to secular variation occur at a geologically rapid years or possibly a few hundred years long. Eruptive rate (4°/century, Champion and Shoemaker, 1977) periods were separated by intervals of quiesence and thus permit assignment of lava flows to groups that lasted a few hundred to a few thousand years. that have similar paleomagnetic direction. 4 Utah Geological and Mineral Survey Special Studies 61,1983

Table 1. Eruptive periods, approximate age of eruptive periods, informal names of lava flows of upper part of Snake River Group, and source vents for lava flows in the Craters of the Moon, Wapi, and Kings Bowl lava fields. Eruptive Approx. Informal name and lava type period age of major flows* Source vents (queried where uncertain) A Broken Top (p) Eruptive fissures, east and south flanks of Broken Top cinder cone Blue Dragon (p) Eruptive fissures south of Big Craters cinder cone complex Trench Mortar Flat (p) Eruptive fissures between Big Cinder Butte and The Watchman cinder cones 2,100 to North Crater (p) North Crater cinder cone 2,300 Big Craters (Green Dragon) (p) Eruptive fissure at north end of Big Craters cinder cone yrs B.P. Kings Bowl (p) Fissure vents north and south of Kings Bowl Wapi (p) Vents at Pillar Butte Serrate (a-b) North Crater cinder cone (?) Devil's Orchard (a-b) North Crater cinder cone (?) Highway (a-b) North Crater cinder cone (?) or tholoid vent (?) between Grassy Cone and Sunset Cone cinder cones B Vermillion Chasm (p) Eruptive fissures at Vermillion Chasm Deadhorse (p) Eruptive fissures north and south of Black Top 3,500 to Butte cinder cone 4,500 Devil's Cauldron (p) Devil's Cauldron yrs B.P. Minidoka (p) Obscure vents about 5 km northeast of New Butte cinder cone Larkspur Park (p) Rangefire (p) Black Top Butte cinder cone (?) C Indian Wells north (a) Big Cinder Butte cinder cone (?) Indian Wells south (a) Big Cinder Butte cinder cone (?) 5,800 to Sawtooth (a) Big Cinder Butte cinder cone (?) 6,200 South Echo (p) Eruptive fissures south of Echo Butte cinder cone yrs B.P. Sheep Trail Butte (p-a) Sheep Trail Butte cinder cone Fissure Butte (p-a) Fissure Butte cinder cone Sentinel (p) The Sentinel cinder cone

D About Silent Cone (a) Silent Cone cinder cone 6,600 Carey Kipuka (a) Silent Cone cinder cone (?) yrs B.P. Little Park (a) Silent Cone cinder cone (?) Little Laidlaw Park (a) Silent Cone cinder cone (?) E 7,300 to Grassy Cone (p) Grassy Cone cinder cone 7,900 Laidlaw Lake (p) Grassy Cone cinder cone yrs B.P. Lava Point (a) Great Rift northwest of Echo Crater cinder cone (?) F 10,000 to Pronghorn (p) Great Rift near Sheep Trail Butte cinder cone 11,000 Bottleneck Lake (p) Great Rift near Sheep Trail Butte cinder cone yrs B.P. Heifer Reservoir (p) Great Rift near Crescent Butte cinder cone G 12,000 to Sunset (p) Sunset Cone cinder cone 12,900 Carey (p) Sunset Cone cinder cone yrs B.P. Lava Creek (p-a) Vents near Lava Creek H Kimama (p) unknown Bear Den Lake (p) unknown About Baseline (p) unknown 15,000 Little Prairie (p) unknown Echo Crater and (or) yrs B.P. Lost Kipuka (p) unknown Crescent Butte cinder cones(?) No Name (p) unknown Brown flow (p) unknown

* (p) Chiefly pahoehoe flows (a-b) aa' and block flows (a) chiefly aa' flows (p-a) pahoehoe and aa' flows Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 5

Conversely, dissimilar paleomagnetic directions MgOandCaO. suggest that two lava flows do not belong to the Evolved COM flows first were erupted about same group. 6,600 yrs ago in eruptive period D (Figure 6) and Paleomagnetic studies have helped to solve diffi­ continued to be erupted, along with more primitive cult stratigraphic problems for the COM lava field. flows, in eruptive periods C, B, and A. For example, we thought initially that all thick aa flows in the southern part of the COMNM were VOLUMES OF LAVA erupted in the same eruptive period from a source Stratigraphic relations, areal extent, thicknesses, vent at or near Big Cinder Butte. Paleomagnetic and radiocarbon ages of COM lava flows, give the inclination data indicate two distinct eruptions of aa data plotted in Figure 7. The age and volume data lava flows from at least two source vents. Thus, we reveal that volcanism along the northern part of the recognized groups of aa flows in eruptive periods C Great Rift is volume-predictable, i.e., the volume and D from source vents at Big Cinder Butte and of each eruption is a function of the elapsed time be­ Silent Cone (Table 1). Petrochemical data and radi­ tween it and the preceding eruption. This relation­ ocarbon ages led us to split one of these two aa ship has implications to the regional stress field and groups into two additional groups; initially we the magma plumbing system. placed the Lava Point aa flows in eruptive period D, In Figure 7, the break in slope of line 2-2' from now we place it in period E (Table 1). Because they line 1-1' coincides with the marked change in have identical paleomagnetic field directions, we composition of COM lavas beginning at eruptive correlate the Sunset and Carey flows that moved period D. We attribute the increased rate of volca­ northeast and southwest, respectively, from the nism since 6,600 yrs ago to the addition of supplies northern part of the Great Rift. Without the paleo­ of evolved (Si02 less than 52 percent) lava to the magnetic data, we had assumed originally that the nearly constant rate supply of non-evolved (Si02 > Carey and Sunset flows represented separate erup­ 52 percent) lava over the last 15,000 yrs. tions from separate source vents. PETROGRAPHY AND CHEMICAL FIELD TRIP ROAD LOG: FIRST DAY COMPOSITION OF COM LAVAS Snake River Plain basalts are texturally, Idaho Falls to Craters of the Moon mineralogically, and chemically uniform (Stone, National Monument to Pocatello 1967). The basalts are olivine tholeiites that consist STOP 1 is for an overview of the regional geogra­ typically of olivine (F080-S~' plagioclase (An70-S0)' phy and geology of the ESRP. Older lava flows ferroaugite, titanomagnetite, ilmenite, and brown (about 12,000 yrs old) of the COM lava field, that glass. An average chemical composition from 37 are not well exposed within COMNM, will be analyses is given in Table 2. viewed at STOP 2. The remaining three stops will COM flows differ significantly from typical Snake be in COMNM. These stops are selected to study River Plain basalt (Table 2). COM lavas contain features produced during the latest (2,100 yrs old) phenocrysts of olivine (FoSO -lO) , plagioclase eruptions along a 10-km segment of the Great Rift. (An60-40), brown clinopyroxene, titanomagnetite, STOPS 3, 4, and 5 require short (a few kilometers) ilmenite, and brown glass. Some contain orthopy­ walks over terrain that has a relief of 50 to 75 m. roxene and apatite. Evolved basalts (i.e., greater Despite trails, sturdy shoes or boots are than about 52 percent Si02) contain xenoliths and recommended. A small canteen of water is recom­ xenocrysts of corroded anorthoclase, plagioclase mended also but is not absolutely necessary. Ham­ (AnSS-lS), green clinopyroxene, olivine (F02S-1~' mers are unnecessary in the Monument; rock col­ and zircon. Average chemical analyses of several lecting is prohibited. different types of COM flows are listed in Table 2 to illustrate the degree of chemical variability. These Mileage Description flows range from alkali basalt to latite in the Incre- Cumu­ nomenclature of Cox and others (1979). Note in mental lative Table 2 that the most primitive COM flows 0.0 0.0 Start of road log. Junction U.S. (columns 2, 3, 9, 12) differ from the average Snake Highway 20 and 1-15 in Idaho River Plain olivine tholeiite in that they have more Falls, Idaho. Proceed west on Ti02, total iron, Na20, K20, and P20 S, and less U.S. 20. Road crosses loess- 6 Utah Geological and Mineral Survey Special Studies 61, 1983

Table 2. Average chemical analyses of Snake River Plain basalts and other selected, informally named, lava flows from the Craters of the Moon lava field, Idaho (data for COM, Kings Bowl, and Wapi lava flows from Leeman and others (1976] and unpublished analyses by the U.S. Geological Survey). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Si02 46.59 45.32 44.27 46.87 46.02 46.20 46.32 55.84 44.55 56.29 48.76 45.84 63.25 58.22 51.00 50.37 49.08 45.85 45.98 46.77 Ti02 2.77 3.24 3.86 3.12 3.19 3.48 3.14 1.72 3.65 1.51 2.89 3.22 0.56 1.21 2.53 2.49 2.71 2.27 2.54 3.02 Ab03 14.80 14.22 13.80 13.78 13.66 13.46 12.98 13.95 13.56 14.67 13.81 14.14 14.57 14.17 13.93 13.99 13.83 14.94 15.59 14.99 Fe203 2.14 1.79 2.23 2.57 1.29 1.61 2.30 1.65 1.90 1.59 1.19 1.77 1.04 1.20 1.60 1.40 2.04 1.74 1.55 1.06 FeO 11.58 14.66 14.49 14.05 14.74 14.34 14.81 10.54 14.35 10.51 14.25 13.11 7.24 9.92 13.34 14.11 13.61 11.10 11.06 12.66 MgO 7.71 4.26 5.07 3.84 4.09 4.30 4.12 1.59 5.12 1.65 3.61 5.99 0.26 1.10 2.88 3.12 3.29 9.95 8.01 6.54 CaO 9.69 7.65 8.34 7.35 7.70 7.96 7.86 4.96 8.85 4.61 7.05 9.91 2.96 4.08 6.62 6.77 6.89 10.20 10.49 9.12 MnO 0.20 0.26 0.27 0.29 0.26. 0.24 0.27 0.20 0.25 0.21 0.24 0.24 0.19 0.19 0.28 0.29 0.26 0.19 0.18 0.20 Na20 2.44 3.35 3.33 3.70 3.61 3.23 3.33 3.70 3.21 4.07 3.54 2.89 4.13 4.02 3.50 3.50 3.59 2.53 2.27 2.57 K20 0.63 1.73 1.58 2.02 1.89 1.68 1.85 2.93 1.53 3.19 2.02 1.23 4.80 3.66 2.24 2.08 2.04 0.45 0.64 0.83 P205 0.58 2.62 2.45 2.13 2.33 2.40 2.26 0.94 2.52 0.77 2.03 2.06 0.11 0.55 1.61 1.53 1.68 0.57 0.69 0.68 CO2 0.03 0.09 0.09 0.03 0.07 0.10 0.03 0.03 0.10 0.06 0.08 0.03 0.02 0.06 0.04 0.01 0.07 0.02 0.01 0.32 H20+ 0.40 0.88 0.19 0.23 0.28 0.57 0.43 0.35 0.29 0.17 0.49 0.25 0.24 0.30 0.29 0.23 0.34 0.16 0.15 0.17 Total 99.56 100.07 99.97 99.98 99.13 99.57 99.70 98.40 99.88 99.30 99.96 100.68 99.37 98.68 99.86 99.89 99.43 99.97 99.61 98.93

I. Average of 37 analyses of Snake River Plain basalts (Stone, 1967). 11. Minidoka flow, eruptive period B (average 01'5 analyses) 2. Kimama now, eruptive period H (average of 2 analyses). 12. Vermillion Chasm flow, eruptive period B (average of 2 analyses) 3. Lava Creek now, eruptive period G (average of 15 analyses) 13. Highway flow, eruptive period A (average on analyses) 4. Sunset flow, eruptive eriod G (average of 5 analyses) 14. Serrate flow, eruptive period A (average of 6 analyses) 5. Pronghorn flow, eruptive period F (average 01'3 analyses) 15. Big Craters now, eruptive period A (average of 12 analyses) 6. Lava Point flow, eruptive period E (average of5 analyses) 16. North Crater now, eruptive period A (average of 3 analyses) 7. Grassy Cone now. eruptive period E (average of 4 analyses) 17. Blue Dragon flow, eruptive period A (average of 9 analyses) 8. Silent Cone now, eruptive period D (average of 3 analyses) 18. Kings Bowl lava field (average of 3 analyses) 9. Sheep Trail Butte now, eruptive period C (average of 4 analyses) 19. Wapi lava field (average of3 analyses) 10. Sawtooth now, eruptive period C (average of4 analyses) 20. Hells Half Acre lava field (Karla, 1977)

and alluvium-covered Snake thicker loess rather than for River Plain basalt with only younger Rifle Range Butte local exposures of the flows. flows.

2.5 2.5 Osgood Road intersection; con­ 3.2 13.1 Brunt Road intersection; con­ tinue on U.S. 20. tinue on U.S. 20. At 4:00 is North Butte, a low shield 1.0 3.5 Shelley-New Sweden Road volcano. intersection. Continue on U.S. 20 over irregular topography of 2.0 15.1 Intersection with unnamed hummocky flow surfaces road just over crest of hill. Fea­ mantled by loess. tures to be viewed at this locality, in a counterclockwise 2.7 6.2 Skyline Gun Club on right. direction, include: 2:00 Kettle Road begins ascent of Rifle Butte, a low shield volcano; Range Butte flows. 1 :30 south end of the Lemhi Range in far distance; 12:30 2.0 8.2 One of several vents in the Lost River Range in far Rifle Range Butte vent com­ distance; 11:00 East Butte, a plex on right. rhyolite dome; 10:30 Big Southern Butte, a rhyolite 1.7 9.9 Highway leaves surface of dome; 10:00 highest point Rifle Range Butte flows and marks an extremely broad, low continues west onto the older shield volcano that constitutes Butterfly Butte flows (drainage the Hells Half Acre lava field. follows contact) . More sub­ The lava field is the dark sage­ dued topography of Butterfly and cedar-covered area on the Butte flows is due, in part, to horizon south of the highway. Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 7

The knobs to the south of the Butte is at the intersection of apex of the shield are spatter the axis of the ESRP and the cones along the Hells Half Lava Ridge-Hells Half Acre Acre eruptive fissure system. volcanic rift zone (see Figure This fissure system is repre­ 2). At 10:00 to 11:00 are East sented by a set of open cracks Butte, Middle Butte, and Big that extends northwest from Southern Butte rhyolite domes the northwest edge of the lava that project above surrounding field (see Figure 3) . basalt lava flows. The three rhyolite domes are discussed in 0.6 15.7 Road to left leads to Seventeen­ more detail at STOP 1. mile Cave, an opening (sky­ light) in a lava tube in Butterfly 2.4 30.2 Road to left leads to television Butte flow. Just beyond road to transmitters atop East Butte. cave, highway crosses contact onto flows from vent at Kettle 0.1 30.3 Boundary of Idaho National Butte. Engineering Laboratory (INEL). INEL, formerly called 3.5 19.2 Mile marker 288. Road to right the National Reactor Testing leads to a skylight entrance, Station, was established in about 4 mi north of highway, 1949, so that the U.S. Atomic in a lava tube in Kettle Butte Energy Commission (AEC) , flow. The area is known as later the Energy Research and "Owl Cave," or the Wasden ar­ Development Administration cheological site. Faunal mate­ (ERD A) and now the Depart­ rials include bison, mammoth, ment of Energy (DOE), could camel, pronghorn, grizzly build, operate, and test various bear, wolf, and other animals. types of nuclear reactors. More Fluted points and bone tools than 50 reactors have been suggest that the rock shelter built to date. INEL occupies 2 was used by bison and mam­ 894 mi2 (2320 km ). moth hunters as long ago as 12,500 yrs B.P. (S. J. Miller, 0.7 31.0 Road to right leads to Argonne written communication, 1975). National Laboratory reactor and related facilites (see Figure 1.1 20.3 Twentymile Rock is 1/8 mi off 2, "ANL"). road to left. This northernmost exposure of the Hells Half 0.5 31.5 Small tree-covered hill on Acre field is now a National horizon, to right of East Butte Historic Site and was a land­ (9:00), marks top of unnamed, mark to early settlers as they low rhyolite dome that is crossed the eastern Snake mostly covered by basalt lava River Plain. flows. Basalt flows that dip south cap Middle Butte. 0.9 21.2 Highest point on skyline to left (sou th) is the vent area for the 5.8 37.3 Small unnamed lava cone at Hells Half Acre lava field (see 3:00. Figure 2). 0.4 37.7 Cedar Butte at 10:30, left of 1.6 22.8 Small lava cone at 3:00. Big Southern Butte.

5.0 27.8 Road to right leads to crest of 2.3 40.0 Road to right leads to Auxiliary Microwave Butte. Microwave Reactor area. 8 Utah Geological and Mineral Survey Special Studies 61, 1983

LEMHI RANGE

\ / \ / • I I I 1 LOST RIVER RANGE

~' BIG SOUTHERN BUTTE ...... ~, • CERRO GRANDE VENT "­•

o 5 10 15 KILOMETERS EXPLANA TION I I I I ...... Volcanic vent showing orientation of feeder fissure WJJr;:;0~ :~;::iSri:f Z:~;R ,\, Open fissures o Rhyolite dome

Figure 2. Map showing volcanic and structural features in the central part of the eastern Snake River Plain, Idaho (modified from Kuntz and Dalrymple, 1979). Q c 0.: (1) 0- 0 0 l' "'ti Pl ~ w I Q CIl >-- DOME OF MIDOLE BUTTE UNNAMED DOME DOME OF EAST BUTTE :::0 EAST 0 WEST n 8600' :;>:;" '< 6000' ~ 0 5000' C ;::::s £;; 4000' S· Pl ;::::s 0- 3000' n 0 '""l 2000 ' § CD ASALT '""l Pl ;::::s {,:',;' ;~.~':~ ~ :':::.:~~ CIl ~ (1) ~ O· ~ ~ ;::::s CFl KM ~ (1) ~ S· go

~ '-D Figure 3. East-west cross section through rhyolite domes at East and Middle Buttes, Idaho (from Kuntz and Dalrymple, 1979) 00 W

'-D 10 Utah Geological and Mineral Survey Special Studies 61,1983

2.1 42.1 Highway descends onto Rye K-Ar ages for the rhyolite Grass Flat. domes are given in Table 3. The internal structure of 1.2 43.3 STOP 1. Turn left and park in East Butte is known from study lot next to piles of gravel. This of the orientation of the flow stop is designed to review the layering in its rhyolite lava origin and volcanic history of flows. The flow banding gener­ the ESRP, as outlined in the ally defines inward- dipping, Introduction. The local geolo­ concentric layers similar to gy, including East, Middle, and those of a short, stubby carrot Big Southern Buttes, will be or the lower half of an elongat­ pointed out and related to the ed onion. Flow layering parallel regional geologic framework. to an original upper surface Climb to the top of the pile of occurs in many rhyolite domes road metal, face south, and but only locally at East Butte. note the following landmarks: The orientation of flow layering 9:00 East and Middle Buttes suggests that East Butte is a are the two steep-sided hills protrusion of lava that was too that rise as much as 350 m viscous to flow, and that the above the surrounding magma rose as inclined concen­ basalt-lava-covered terrane. tric sheets (K un tz and An unnamed hill, about 1 km Dalrymple, 1979). southwest of East Butte, repre­ 12:00 Cedar Butte (Spear, sents the apex of another rhyo­ 1979) is a unique volcanic com­ lite dome (see Figures 2 and plex on the ESRP. The vent 3). Rhyolite lava flows and area consists of a circular py­ breccias are exposed at East roclastic cone more than 100 m Butte and at the unnamed hill, high and 1.2 km in diameter but not at Middle Butte. The and an elongated arcuate vent, steep sides of Middle Butte are 1 km long and 30 to 60 m high. covered by thick accumulations The arcuate vent extends of talus composed of blocks north-northwest from the base that have been dislodged from of the pyroclastic cone. Field the approximately 75-m-thick and petrographic data show layer of basalt lava flows that that the two vents were active cap the butte. However, at different times; the pyroclas­ magnetic and gravity data sug­ tic cone is younger. Lava flows gest that the core of Middle at Cedar Butte have a range in Si0 content from 54 to 67 Butte consists of rock that is 2 less dense and less magnetic percent; thus they are composi­ than basalt, and is probably tionally similar to flows of the rhyolite (Don Mabey, USGS, COM lava field. A K-Ar age on oral communication, 1978). a flow from Cedar Butte is These factors and the structural 400,000 yrs (Kuntz, 1978). data described below all indi­ 1:30 Big Southern Butte cate that the three hills are the (Spear, 1979) is the most upper parts of rhyolite domes. prominent landmark in the Figure 2 is an east-west cross ESRP; it rises 760 m above the section through Middle and surrounding lava plain. The East Buttes that is based on butte is oval in plan, about 6.5 mapping, a drill core (see km in longest dimension, and Figure 3, well no. 1), and inter­ elongated in a northwest­ pretation of geophysical data. southeast direction. Big South- Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 11

Table 3. Potassium-argon ages of rhyolite domes in eastern Snake River Plain, Idaho (from Kuntz and Dalrymple, 1979). Rhyolite dome Age Reference East Butte 0.58 ± 0.09 m.y. Armstrong and others (1975)

Middle Butte Rhyolite not exposed; basalt flow capping Armstrong and others (1975) butte is 1.9 + 1.2 m.y.

Unnamed dome between 1.42 ± 0.02 m.y. Kuntz and Dalrymple, (1979) Middle Butte and East Butte

ern Butte is a volcanic dome (1979) concluded that the well complex that consists of two penetrated caldera fill material. coalesced cumulo-domes. A The geographic relations of the 350-m-thick section of basaltic Arco volcanic rift zone, the lava flows is exposed on the axis of the ESRP, the Lava north flank of the butte. The Ridge-Hells Half Acre volcanic basalt section was uplifted and rift zone, and other localities tilted by the rise of the volcanic near this stop, that are referred dome. Three K-Ar ages and to in the text, are shown in one fission-track age show that Figure 3. Big Southern Butte is about 300,000 yrs old (Kuntz, 1978). 0.1 43.4 Return to highway and head Faults, grabens, Big South­ west (left). Junction U.S. High­ ern Butte, Cedar Butte, and ways 20 and 26. Continue west the vent for the Cerro Grande on combined Highways 20-26 lava field are structural and vol­ toward Arco. Highway 26 to canic features that define the left leads to Blackfoot. Arco volcanic rift zone (Figure 2). A electrical sounding profile across the ESRP from 4.3 47.7 Highway now on alluvium of Blackfoot to Arco (Zohdy and Big Lost River. Stanley, 1973) suggests an upper crustal structure in this 1.1 48.8 Road to left leads to EBR-1 area that consists of an upper (Experimental Breeder Reac­ layer, 1.5 to 5 km thick, of tor-I), the first reactor built at basaltic rocks, and a lower INEL. Construction of EBR-1 layer of sedimentary and was begun in 1949 to test a rhyolitic rocks of unknown plutonium-fueled reactor for thickness. A 3,160-m-deep ex­ electrical power generation. ploratory geothermal test well The reactor was decommission­ CINEL-1, see Figure 2) was ed in 1964, and in 1966 the drilled at INEL in 1978 facility was designated a Regis­ (Doherty and others, 1979). tered National Historic Land­ More than 2,400 m of rhyolite mark open to the public. ash flow tuffs, rhyolite lava flows, and interbedded air-fall ash material are present in the 0.5 49.3 Borrow pit to left in alluvial gra­ lower part of the well. Less vels of Big Lost River. than 1 km of basaltic lava flows and interbedded continental sediments overlie the rhyolite 1.0 50.3 Big Lost River Rest Area on section. Doherty and others left. Highway leaves Big Lost 12 Utah Geological and Mineral Survey Special Studies 61,1983

River flood plain west of rest Turn left on main street of area. Arco.

3.3 53.6 Crater Butte at 11:00. l.7 68.6 Highway crosses the Big Lost River. 4.5 58.1 Road to left leads to spectacular crater vent in Crater Butte. 0.6 69.2 Highway rises up onto higher alluvial terrace along Big Lost River. 0.6 58.7 Tumuli in flows from Crater Butte to right. 0.4 69.6 Arco Airport. Vent area for the Lost River Butte flows is at 0.5 59.2 Leaving INEL. 2:30. The flow margin is north of the highway and continues 0.2 59.4 J unction with Idaho State for about 1 mi (I.6 km) past Highways 22 and 23. Continue the airport. The Lost River on Highways 20-26. Butte flows rests on gravel-size alluvium of the Big Lost River. l.7 61.1 Six Mile Butte, a broad shield volcano, at 9:00. Note profile 5.3 74.9 The low rounded flow front on of cinder cones along the Great the left is that of the Sunset Rift in Craters of the Moon Na­ flow of the COM lava field. tional Monument at 10:00 (the The 12,000-yr-old Sunset largest cone is Big Cinder pahoehoe flows traveled more Butte). Pioneer Mountains, than 17.5 km from a source north of the COMNM, at vent at Sunset Cone in 11:00-1:00. The Pioneer COMNM to this site. The Mountains immediately north Sunset flow margin is subparal­ of COMNM consist chiefly of lel to the highway all the way to Eocene extrusive and intrusive STOP 2. rocks and Paleozoic, including Carboniferous sedimentary l.2 76.1 Tongue of Sunset flow crosses rocks (Skipp and Hait, 1977). highway. For next several miles on right, note large, eastward verging folds in Carboniferous l.5 77.6 Another tongue of Sunset flow and Permian carbonate rocks crosses highway. in south-facing slopes of the Arco Hills (Skipp and Hait, l.6 79.2 Outliers of Ordovician-Kin­ 1977) . nikinic Quartzite at top of hill at 3:00. l.5 62.6 Highway now on alluvial fan formed by streams emerging 0.4 79.6 Sign for "Rough Road." from Arco Hills to right. 0.2 79.8 Highway rises onto Lava Creek 0.7 63.3 Butte City. flow of the COM lava field.

0.3 80.1 Highway rises onto overlying l.0 64.3 Railroad tracks. Sunset flow, where it is in con­ tact with the Lava Creek flow. 2.1 66.4 Enter Arco. 0.1 80.2 STOP 2. Turn off onto wide 0.5 66.9 Junction U.S. Highway 93. shoulder on right. Walk east Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 13

back to the contact of the The Lava Creek alkali basalt Sunset and Lava Creek flows. flow is hypo crystalline and At this, the first of four stops ranges from microporphyritic in the COM lava field, we will to porphyritic. Phenocrysts are see the two oldest flows (of mainly olivine and plagioclase; eruptive period G; see Table 1 the latter are common near the and Figure 5) at the north end south vent. The Sunset flow is of the field and their source hypocrystalline and micropor­ vents. Flows about 15,000 yrs phyritic with microphenocrysts old lie at the nearly inaccessible of olivine and plagioclase. Both south end of the COM lava the Sunset and Lava Creek field. The Lava Creek flow was flows represent the erupted about 12,600 yrs ago "primitive" eruptive products from the southernmost Lava that are common in the early Creek vent along the Great history of the COM lava field Rift wi thin the Pioneer (Table 2). Mountains. The flow cascaded about 500 m in about one mile to the level of the Snake River 1.4 81.6 Mile Post 234. Plain at the site of the old town of Martin. It then traveled as 2.0 83.6 Mile Post 232. Junction with an aa flow past the site of STOP road to Blizzard Mountain to 2 until it reached a point about right. Flow with serrated 19 km from its source. profile on left is the Serrate The Sunset flow is approxi­ flow. The jagged profile is due mately 12,010 yrs old. It erupt­ to cinder cone monoliths that ed from Sunset Cone at the the Serrate flow transported to north end of the Craters of the the east when North Crater (in Moon National Monument. COMNM) was partially de­ The S unset flow consists of stroyed (to be discussed at pahoehoe and its surface is STOP 3). hummocky with pressure ridges and plateaus (look south of highway), collapse 0.7 84.3 Boundary of Craters of the depressions, ropes, and Moon National Monument. squeeze-ups. The Sunset flow is progressively more ash cov­ 0.7 85.0 On right and left sides of road ered toward COMNM because are downwind cinder accum­ dominant westerly winds blew ulations from Sunset Cone. tephra onto it during succeed­ ing eruptions from Sunset 1.1 86.1 Entrance to Craters of the Cone. Moon National Monument, Walk on the S unset flow Visitors Center on left. Stop about 50 ft in from highway to for snack and rest stop at Visi­ observe the surface of this tors Center, then follow the 12,000-yr-old pahoehoe flow. loop road into the Monument. Blue glassy crust that is common on younger pahoehoe flows of the COM lava field is 0.4 86.5 Entrance to campground. The present here, but most has cinder-covered latite block been obliterated by weather­ flow in the campground is one ing, vegetation, and sediment of the most felsic flows of the cover. COM lava field, and it is con- 14 Utah Geological and Mineral Survey Special Studies 61, 1983

sidered to be part of the undat­ drive; bear right. ed Highway flow (to be dis­ cussed at STOP 3). 0.2 88.2 Tongue of Big Crater flow on left in valley between Paisley 0.1 86.6 Tongue of blue-crusted North Cone on right and Inferno Crater flow on left. This flow is Cone on left. The vents for this one of several 2,100-yr-old flow will be seen at STOP 3. In­ basalt-hawaiite pahoehoe flows ferno Cone at 9:00, Big Craters common along the loop road. cinder cone complex at 12:00, We will visit the source of this North Crater at 2:00. flow in North Crater at STOP 3. 0.6 88.8 Turnout to Inferno Cone park­ 0.2 86.8 Turnout on right for North ing lot. Crater flow trail. Continue on loop road, which crosses North 0.2 89.0 Turn right on road to Spatter Crater flow. Cones parking lot.

0.1 86.9 Turnout on right is at the east 0.1 89.1 STOP 3. Park in lot (vans flank of North Crater. This is return to Visitors Center). A one of the trailheads to the 3.6-km walk to the Visitors North Crater-Spatter Cones Center will constitute STOP 3. trail. Continue on loop road. The hike is designed to view eruptive features and vent 0.4 87.3 Big Craters flow on right man­ areas along a part of the Great tles the western flank of Paisley Rift that was active about Cone. We will see the fissure 2,100 yrs ago. A segment of vents for this pahoehoe and aa the Great Rift 10 km long, that basalt-hawaiite flow at STOP 3. extends from North Crater For the next 1.3 mi, the loop southeast to the Watchman road follows the north, east, cinder cone (Figure 4), was and south flanks of Paisley active at various times during Cone. the latest eruptive period (A) of the COM lava field. The 0.2 87.5 Devils Orchard flow on left. hike begins at the Spatter This heavily cinder-covered Cones, traverses the west side latite block flow was thought of the Big Craters cinder cone by early workers to be quite complex, crosses into North old. We correlate it with the Crater, extends along the Highway and Serrate flows of North Crater flow, and ends at eruptive period A (Table 1). the highway fault near the Visi­ The Serrate, Devils Orchard, tors Center. The vents to be and Highway flows are consid­ seen on the walking tour pro­ ered to be quite young on the duced about 3.5 km3 of lava basis of stratigraphic and paleo­ that now cover about 20 per­ magnetic data, which will be cent of the COM lava field. At discussed in more detail at about the same time as this STOP 3. activity, a rift segment of com­ parable length was active also 0.4 87.9 Entrance to Devils Orchard and formed the Kings Bowl Nature Trail on left; continue and Wapi lava fields to the on loop road. south of the COM lava field (Figure 4). The Kings Bowl­ 0.1 88.0 Intersection with one-way loop Wapi area will be visited on the Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 15

EXPLANATION <:;'; CINDER CONE--Showing ;:./ crater rim * SMALL CONE OR VENT ~:~- THOLOID (?) LAVA DOME ERUPTIVE FISSURE 43°30'

~ SCARP o BURIED VENT ~?- OPEN CRACKS GREAT RIFT

11

Buried vent complex for Minidoka-Larkspur .. ~~~ Park flows

CRATERS OF THE MOON LAVA FIELD

43°00'

10 KILOMETERS I

Figure 4. Map showing volcanic and structural features along the Great Rift, and the Craters of the Moon, Kings Bowl, and Wapi lava fields, Idaho (from Kuntz and others, 1982b). 16 Utah Geological and Mineral Survey Special Studies 61, 1983

D c

113°30 / // t I I Serrate-Devils Orchard­ Highway flows f I -'

Blue Dragon flows

KILOMETERS 50 I

Figure 5. Evolution of the Craters of the Moon lava field in eight eruptive periods, H, the oldest, through A, the youngest (from A Kuntz and others, 1982b). Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 17

---7113~30' , 113°30' I ( I \ \ Lava Creek Vents _/"'__ , ,,="~), /: ~ Lava Creek Topographic .../-) r- flows boundary of ~- {_r ~ eastern Snake \ I ~//' Sunset flows River Plain ~) /Crescent Butte (/ suns~:..~~:. /-./ -/ ;r-----~...,:1Baseline / __ f"--" J (}~ --) --.~; Echo Crater,' ~." flows --; '\ ~." ~ --; ~ / ( Little Prairie \ \..) / ~ flow \ eM No Name \ \ flows

I \ / ; \1 \ Bear Den~ \ Lake flows \) ,./ "fJ I I ~Brown ) ",/ 43°15' / / flows I " / / / / // ,,/'/ / / " /////// r Klmama flows CC ...",.v-/ ~~>' I ~~J ~------.~\_) ~------'-~~~~--' G

second day of the field trip. Cones area. After viewing one The Spatter Cones formed in spatter cone, take trail to west the waning activity along a that ascends to south end of short (I-km-Iong) eruptive fis­ the Big Craters cinder cone sure that extends southeast of complex. the south end of Big Craters Big Craters is a cinder cone cinder cone. Most of the lava complex that contains at least that forms the extensive Blue nine nested cones. On the Dragon flow was erupted from southwest rim, agglutinated the Great Rift in the Spatter spatter mantles the inner wall >-' KEY TO SYMBOLS 00 FOR INFORMALLY NAMED LAVA FLOWS Period A Period 0 64~ Broken Top • Silent Cone • Blue Dragon o Carey Kipuka ... (~ Trench Mortar Flat ... Little Park • ~\ North Crater Little Laidlaw Park • \III 621- + I I Big Craters 6 10 I Period E I I Kings Bowl I 1 Grassy Cone • I I Wapi , I Laidlaw Lake A 601- Serrate o 10 : Lava Point • 1 I Devils Orchard • 1 1 Highway 1 I Period F , I :. \ Period B Pronghorn • re\ , I 58~ I 1 1+-' , I Vermillion Chasm • Bottleneck Lake A ,4-.\ I I Deadhorse Heifer ReserVOir • : ' , I I. A: + I.' , , 1 I Devils Cauldron 1, I ' , , , I , I 561- Minidoka A Period G , ' i ' Larkspur Park Sunset I I 1+1 18 \ COM • • " \ \ r Rangefire A , ' \t} Carey I ' \0 I lava flows Lava Creek ,I 'I I \ 54 • , I C , 1 Period C /i' I 1 Indian Wells N, Period H , ' I 1 I \ # • , ' 1 \ 1 \ Sawtooth + All analyzed flows • , .' I \ 1 \ South Echo , ' 1 6 \ ,I 1' :. \ 52 , 1 I + \ ~ Sheep Trail Butte A , 1 I \ : ~.I 0'" Fissure Butte • I.... : \. \ \ ~A \ C75 Sentinel r .t) I \ , 1 I XI \~,A \ s: I \ p,? 50 \ /i' til 1 ' I 1 I I' ~ \ I \ \ ...~J ,r-\ It X I (~ , 1 \ 2. 'A I , \ IA I \ ' a I I I A I , 1 I I CD I I I \ D , , \1 x \ 0 \ , , 1 ' I ' , , , I I 0- 48 I 1 I \ I I \ .1 (JQ , I I \ 1 , \ I I A I , I \ I \1 ,' (s' ,... '---., \ , I t \ :. . \ I I' e:. , A·' ",'" \ ,. ., '. I I 1 (Jj p,? I A I , , \ I (e\ " I, \ 1 , ::J 46 / t (, " I :. 1 , I~ ___.!-J \ II B I W ' 0- /,-'\ ',~j , 1 I,:i~K' Ings ow" apl , I \!/ 10/ ~ , • I I ' / / F 1 , ~ lava flows , I E 'l' S' \.!/ A CD \}1 >-j 44 l/G B H I_I e:. I C C/l c >-j <: COM lava flows CD 42 "<: C/l '0 CD n 40 e 16 14 12 10 8 6 4 2 0 C/l 8' YEARS BEFORE PRESENT X103 0- (p' Ul 0"- Figure 6. Si02 (weight percent) versus age of eruption for analysed rocks of the era ters of the Moon, Kings Bowl, and Wapi lava fields, Idaho. >-'

>-' \0 00 W 35 Cl 2' c ~ (!l oCT 1' o l' 30 "'"Ij ~ w I Cl A C/l >­ ;:tl o o 25 () ;:0\ >< B '< ~ M o c E ;:::s .::£. s­ s· ~ W 20 ;:::s 0- ~ n ::J o .....J o >-j 0 5 > o g E EXPLANATION ~ w 15 ;:::s C/l ~ (!l r- o·~ 52% Si0 2 ) lava \0 00 G w

/ Equation of lines Correlation coeficient 5~ ~ 1-1' Y = -1 .86X + 31 .82 r = 0.997 1 H 2-2' Y = -2.69X + 37.11 r = 0.999 ol 16 14 12 10 8 6 4 2 o

3 YEARS BEFORE PRESENT (X 10 )

Figure 7. Relation of cumulative volume of erupted lava to ages of lava for the Craters of the Moon lava field, Idaho. A-H are eruptive periods, youngest ...... to oldest. \0 20 Utah Geological and Mineral Survey Special Studies 61, 1983

of the south and southeast broken by slumping during a parts of the complex. The relatively explosive eruption of mantle drapes over the rim of the High way-Devils Or­ the complex and covers the chard-Serrate group of flows, outer wall (visible from STOP and crater-wall remnants were 4). About 100 m north along rafted away on flows from the trail, remnants of a lava vents in and near North Crater. lake lie along the north wall of Some of these crater-wall rem­ the inner crater. Just south of nants now appear as monoliths the lava lake remnant, the in the Serrate and Devils Or­ crest of a small cinder cone has chard flows and as kipukas sur­ a red streak aligned parallel to rounded by the North Crater the eruptive fissure. Late-stage flow. From the crest of the trail corrosive steam from the fis­ on the west side of North sure oxidized the black Crater, slumped blocks and cinders. Lava issued from slump scarps can be observed several satellite vents at the on the northwest and northeast base of Big Craters cinder cone flanks of North Crater cinder complex along its western cone. The trail descends into (left) flank and travelled to the the vent area at North Crater. southwest. Additional nested An entrance through a skylight craters are viewed in the Big into a lava tube is along the Craters complex along the trail near the contact of the trail. As the trail descends the North Crater pahoehoe flow west slope of Big Craters, it and the southwest part of the passes near small craters on the inner wall of North Crater. Far­ west flank of the complex. ther east along the trail, near Where the trail flattens out, it the vent for the North Crater passes near a few small eruptive flow, is a large block of agglu­ fissures to the left of the trail. tinated cinders that contains a The trail crosses eruptive granulitic xenolith. Such fissures, source vents for the xenoliths, as well as xenoliths Big Craters flows, in the area of pumiceous glass, are between the Big Craters cinder common in cinders in the walls cone complex and the south­ of North Crater. Leave the trail west flank of North Crater. Big and walk north-northwest Craters flows travelled both across the surface of the North east and northwest from this Crater pahoehoe flow toward area. This lava has an olive­ the highway. Ropes, hum­ green to greenish-brown crust, mocky surfaces, small which is useful in distinguish­ monoliths, and the blue crust ing Big Craters flows in areas color of the North Crater flow where they abut younger and are well displayed. Contacts be­ older flows. tween green-crusted Big Cra­ The trail continues north tc ters flows and blue-crusted the west rim of North Crate North Crater flows can be ob­ cinder cone, which has had a served near the steep scarp complex history. Clearly, (called the "highway fault") North Crater cinder cone was parallel to and several hundred larger than it is at present. We yards south of the highway. We believe that much of the believe this fault formed northwest, north, and north­ during collapse of the north east flanks of North Crater was flank of North Crater and the Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 21

related eruption of the facing southwest toward Big Highway-Devils Orchard­ Cinder Butte, note the Serrate group of flows. following: 11 :00 the two eas­ Climb the scarp and walk to ternmost cinder cones in this the Visitors Center for lunch. direction are Half Cone in the foreground and Crescent 3.8 92.9 Depart Visitors Center and Butte, with its distinctive cres­ then travel 2.7 mi along loop cent shape, in the background; road to parking lot at Inferno 11: 10 in this direction, the Cone. dark saddle shaped cone is Blacktop Butte, the most 2.7 95.6 STOP 4. Inferno Cone parking southerly cone along the north­ lot on left. Climb path up side ern part of the Great Rift, and of Inferno Cone. At top of 12 mi distant. Many of the cone, facing north, geographic more than 25 cinder cones in features to be noted, in a clock­ the COM lava field can be seen wise direction, are: 9:30 to from this vantage point, but 10:00 Big Craters cinder cone they are too numerous and complex; 10:30 Grassy Cone closely spaced to differentiate cinder cone; 11 :00 North them. Crater cinder cone; 12:00 From the southwest rim of Sunset Cone directly behind the crest of Inferno cone (no Visitors Center with Pioneer sign) , one may observe the Mountains in far background; "plumbing system" that was 1:00 Paisley Cone. The profile responsible for the eruption of against the mountains of the the vast eastern lobe of the vegetated surface of the Sunset Blue Dragon flow, the largest flow is visible beyond Paisley of all the 2,100-yr-old flows. Cone. On the near side of the The plumbing system consists Sunset flow is the Serrate flow of, from the northwest to that extends to the east (right) southeast, 1) an eruptive as far as Round Knoll (2:30). fissure, located in the southern Round Knoll is a grass-covered part of Big Craters cinder cone kipuka of older Snake River complex and beneath the area Plain flows and cinders. of the spatter cones; 2) pit At the "Snake River Pla­ craters, such as Crystal Pit, teau" display sign on the east that overlie the southern end rim of the summit of Inferno of the eruptive fissures; 3) Cone, note Big Southern Butte perched lava ponds, such as and the surrounding terrane. Big Sink waterhole, that are To the left of Big Southern located on the upper part of a Butte are East (left) and lava tube system that extends Middle (right) Buttes that east and south of the eruptive appear to be one butte. Low fissure, and 4) a lava tube shield volcanoes are clearly system that contains numerous visible to the left and right of skylight entrances into tubes Big Southern Butte. The eas­ (cave area along the Caves tern part of the vast Blue trail). Farther east are rootless Dragon flow is in the fore­ vents, where lava moving ground. through the tube system was At the "Great Rift" display extruded through openings in sign on the south rim of the tu be ceilings. Also visible, summit of Inferno Cone, directly south beyond Big Sink 22 Utah Geological and Mineral Survey Special Studies 61,1983

waterhole and the Lava above mechanisms. Cascades, is Broken Top cinder cone and the area to be visited 0.5 97.6 Road crosses Blue Dragon slab at STOP 5. Two 2,100-yr-old pahoehoe. fissures slice across the north­ east and southwest sides of 0.1 97.7 STOP 5. A short walk (about Broken Top cinder cone. 1.5 km) will constitute STOP Source vents for the youngest 5. Park in Tree Molds and Wil­ flow in the COM lava field, the derness Trails parking lot. Broken Top flow, are on the Walk back on road east to Buf­ eruptive fissure on the east and falo Caves trailhead on south­ northeast flanks of Broken Top. west side of Broken Top near Return to parking lot. Turn 2,100-yr-old fissure. Turn left ( one way) and proceed past right (southwest) onto trail; do entrance to Spatter Cones park­ not take trail to left, which as­ ing lot. cends Broken Top. The trail to Buffalo Caves drops down into 0.2 95.8 Entrance to Spatter Cones a fissure on the west side of parking lot, bear left. Broken Top, which has been filled by a tongue of Blue 0.2 96.0 Big Cinder Butte on horizon. Dragon lava. Follow cairns on Continue straight ahead. Blue trail to southeast and walk on Dragon slab pahoehoe lies to the surface of the Blue Dragon the right between the road and flow. To the left, the the spatter cones. West flank southwest-facing wall of the fis­ ofInferno Cone to left. sure has been mantled with spatter and bombs erupted 0.5 96.5 Turn right on road to parking from the fissure. Many faults lot for Tree Molds and Wilder­ that trend parallel to the Great ness Trails. Rift cut the west side of Broken Top. Walk along trail to contact 0.3 96.8 Turnout on right for viewing of Broken Top flow with Blue Lava Cascades. Here Blue Dragon flow. Pahoehoe toes of Dragon lava flowed in a radial Broken Top flow lie on top of pattern from a lava lake in Big Blue Dragon flow. The Broken Sink waterhole, a perched lava Top flow, though not dated pond. The eastern rim of Big numerically, is stratigraphically Sink waterhole lies several younger than the 2,076-yr-old hundred feet west of the Lava Blue Dragon flow and therefore Cascades parking area. the youngest flow in the COM lava field. The Broken Top 0.3 97.1 On left is a slab of Blue Dragon flow is mainly a lava lake in flow that mantles the north this area; large slabs of the side of Broken Top. Either the crust of the lake occur on the flow was considerably inflated right side of the trail. The as it passed this site or the lava Broken Top flow is also blue­ "sloshed" up onto the side of crusted, especially in squeeze­ the cone as it changed direction ups that rise through cracks in from south to east. There are the crust of the lava lake. The several other "slosh slabs" pre­ squeeze-ups appear to be from sent in the Monument, some a younger flow. The surface of of which do not seem to be ex­ the Broken Top flow has a rela­ plained by either of the two tively light color due, in part, Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 23

to the weathering of its highly 0.6 99.6 Road to Caves parking lot on vesicular crust and to relatively right; continue straight on loop greater amounts of vegetative road. cover than nearby flows. Con­ tinue on trail (follow cairns) to 0.2 99.8 Devil's Orchard block flow on Buffalo Cave. right. Buffalo Cave, in the Broken Top flow, shows several inter­ esting features: lava stalactites, 0.3 100.1 Intersection; turn right. curbing (showing successive flow levels on the cave walls), 0.1 100.2 Entrance to Devil's Orchard ro pes (showing flow direc­ parking lot on right; continue tion), and floors formed by in­ straight ahead. complete crusts of several levels of the lava stream. What 1. 7 101. 9 Visitors Center. Park for quick was the direction of movement rest stop. Leave COMNM, of lava in the tube system? turn right (east) on Highway Where was the source vent(s)? 20-26, pass through Arco, turn Follow southwest flank of right on Highway 20-26 in Broken Top cinder cone east to Arco, and return to intersec­ cinder path (the old Wilderness tion of U.S. Highways 20 and Trail). Turn left and take path 26 at location of STOP 1. up southeast side of Broken Top. The Broken Top flows 42.7 144.6 Intersection of U.S. Highways came from vents to the right of 20 and 26. Turn right on High­ this path. At a very small way 26 toward Blackfoot. borrow pit on the right side of the trail (on the north flank of 0.3 144.9 Highway crosses Rye Grass Broken Top), turn left off path Flat, a grass-covered kipuka be­ and climb to the top of the tween Snake River Plain lava cinder cone. (Spread out and flows. walk amidst the sage and bitter­ brush when making this ascent 0.8 145.7 Distal flow fronts of Cerro so as not to form foot paths on Grande lava field on right. The the side of the cone). The top Cerro Grande lava field is of the cone is strewn with cin­ about 10,000 yrs old and was ders and large (1 m) bombs fed from a vent at the base of erupted from the fissure on the the southeast flank of Cedar west flank of Broken Top. Here Butte, the prominent butte im­ we will have a last overview of mediately southeast of Big the Monument and review Southern Butte (see Figure 2). o bserva tions, da ta, and concepts. Return northwest 3.6 149.3 Bingham County line. To right (toward Big Craters) to cinder is the town of Atomic City. path and descend to roadway and walk to parking lot. The 1.6 150.9 Sign: "Leaving INEL." bus will return to the Visitors Center for a short restroom 0.6 151.5 Intersection, Atomic City to stop. Leave Tree Molds­ right; continue straight ahead. Wilderness parking lot. 0.9 152.4 Table Legs Butte to right. This shield volcano has a high­ 1.3 99.0 Turn right on loop road. crowned vent due to lava lake 24 Utah Geological and Mineral Survey Special Studies 61,1983

activity. Middle and East chemical analysis is given in Buttes to left. Table 2.

1.1 153.5 Low hills on right represent 0.9 173.4 Highway drops off Hells Half rootless vents on tube system Acre flows. in flow that traveled east from Table Legs Butte. 0.1 173.5 Highway crosses Peoples Canal. Gravel-size alluvium 0.3 153.8 Borrow pit on right shows ac­ from former courses of the cumulation of at least 1 m of Snake River is common along loess on Snake River Plain the sides of irrigation canals. flow. This area was inundated by the 1976 flood caused by the failure 3.2 157.0 Kipuka filled with loess on left. of Teton Dam.

1.7 158.7 Ascend Taber Butte, a low 5.1 178.6 Highway crosses Snake River. shield volcano whose crest is at 11:30. 0.1 178.7 Intersection of U.S. 26 and 1-15. Turn right on entrance 2.2 160.9 Low shield vent area of Taber ramp to 1-15. Head south on Butte, at 9:00. Portneuf Range 1-15 toward Pocatello. Black­ at 1:00. foot on left after turning on in­ terstate highway. 11.6 172.5 Highway crosses distal tongues of the Hells Half Acre lava 4.5 183.2 Overpass, Willie Road. On left field. Pressure ridges and pres­ is Blueberry Hill. This broad, sure plateaus are common round hill (diameter of 9 km) morphologic features of this probably overlies a rhyolite 4,000- to 6,000-yr-old pahoe­ dome, but no rhyolite is hoe flow. The Hells Half Acre exposed. Basalt flows dip out­ lava field (Karlo, 1977) covers ward on its north, west and 2 an area of about 430 km , and south flanks. contains about 3 km3 of lava. The lava field probably formed 1.1 184.3 The broad, low shield volcano during a single eruptive event on right is Ferry Butte, a land­ that lasted perhaps several mark for early travelers on the months or a few years. The Oregon Trail. This was one of flow units that make up the the few safe places to ford the lava field all flowed from a cen­ Snake River. tral vent complex about 1,000 m long and 300 m wide. The 6.8 191.1 Fort Hall and Fort Hall Indian vent complex consists of an Reservation to right. Continue elongated crater that contains on 1-15. In the early 1800's, remnants of lava lakes. A few Fort Hall was a trading post in small cinder cones, aligned this part of Idaho. parallel to an eruptive fissure system lie in the crater and 3.9 195.0 Pocatello Range on left, Mi­ beyond its edges. An extensive chaud Flats on right. Pocatello lava tube system reaches south Range consists chiefly of a and east of the vent area. Lava basement complex ofPrecamb­ flows of the Hells Half Acre rian quartzite, sandstone, lava field consist of olivine argillite, and metavolcanic tholeiite; a representative rocks. Several thrust sheets Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 25

have been recognized. The Pre­ flood upon its emergence onto cambrian rocks are overlapped the Snake River Plain by Miocene rhyolite tuffs of (Trimble, 1976). the Starlight Formation. Mi­ chaud Flats is a fan-delta that 3.7 198.7 Junction ofl-15 and 1-86. consists of boulders, sand, gravel, deposited by Bonneville END OF FIRST DAY OF FIELD TRIP.

PART 2: KINGS BOWL AND WAPI LAVA FIELD AND THE SOUTHERN PART OF THE GREAT RIFT By Duane E. Champion, John S. King, and Harry R. Covington

The Kings Bowl and the Wapi lava fields are and Covington, 1977). Flanking cracks are older shown in generalized form in Figure 4. The volcanic than the central fissure, and typically are less than 1 features to be observed on the second day of the m wide. The central eruptive fissure consists of field trip contrast with the features observed on the discontinuous, linear, en echelon cracks, 2 to 3 m first day. The Kings Bowl and Wapi lava fields are wide, that locally are filled with breccia and feeder relatively simple, the products of single eruptive dikes. bursts. They are not compound lava fields that The Kings Bowl lava field is characterized by thin record mUltiple eruptions such as the COM lava (typically < 0.2 m), fissure-fed pahoehoe lava field. However, there are also marked contrasts be­ [lows, lava lakes, low natural levees, spatter tween the Kings Bowl and Wapi fields that reflect ramparts, spatter cones, and explosion pits. Large different styles of eruption for each field. The Kings spatter cones and spatter ramparts adjoin explosion Bowl lava field is small and accumulated adjacent to pits at South Grotto and Creons Cave, and light­ a 7-km-Iong eruptive fissure segment of the Great colored blankets of lapilli tephra spread eastward Rift. The Wapi lava field is larger and probably from the explosion pits (Figure 8). The largest began initially as a fissure eruption and, with more explosion pit on the Kings Bowl rift is Kings Bowl, prolonged activity, progressed to a sustained erup­ 85 m long, 30 m across, and 30 m deep. A lava lake tion from a central vent complex. surrounded Kings Bowl before the explosive The compositions of both Kings Bowl and Wapi eruption. Prominent basalt mounds, believed to be lava flows resemble the average olivine tholeiite of the remnants of levees, define the limits of the lava the Snake River Plain (Table 2), in contrast to the lake. Blocks as large as 10 cm in diameter were evolved COM lava flows from the northern part of hurled explosively westward as far as 245 m. The the Great Rift. lapilli-ash tephra resulting from the explosion was carried eastward by the prevailing winds; tephra KINGS BOWL LA V A FIELD about 1 mm in diameter occurs as far as 1.2 km east The Kings Bowl lava field consists of about 0.005 of Kings Bowl (see Figure 8). The tephra blankets km3 of pahoehoe lava flows that cover an area of an area of 0.15 km2 (King, 1977). As the larger about 3.3 km2 along the southern part of the Great blocks that resulted from the explosion fell on the Rift, 12 km southeast of the Craters of the Moon west side of Kings Bowl, many broke through the lava field (Figures 4 and 8). The lavas were erupted crust of the lava lake and through squeeze-ups. At about 2,220 ± 100 yrs ago from a 7-km-Iong central some localities, the impacting projectile can be fissure of the Kings Bowl rift set (King, 1977). The found in place beneath the crust. Calculations by set consists of a central eruptive fissure that trends King (1977) show that the volume of the ejected N. 10° W., flanked by two subparallel, noneruptive material falls far short of that needed to refill the sets of cracks that are from 600 to as much as 1,100 cavity of Kings Bowl, indicating collapse in the vent m from the main fissure (see Figure 8 of this report area subsequent to the explosive vent. 26 Utah Geological and Mineral Survey Special Studies 61,1983

EXPLANA TION

- - Contact

~'-.... Cracks

--++++++- Eruptive fissure CJ Explosion pit 03 Locations along walking tour Flow direction - '- Area of lava mound concentration Extent of phreatic material Spatter cone = Road, maintained

======Road. not maintained

o 4000 FEET I I I I I I o 500 1000 METERS

\ ~ \\ \ \~ \

MODIFIED AFTE R GREELEY. TH EILI G. AN D KING (1977) Figure 8. Generalized geologic map of the Kings Bowl lava field , Idaho, showing features discussed in text. Circled numbers refer to localities on walking tour for STOP 1, Day 2. Map modified from Greeley and others (1977) . Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 27

WAPI LAVA FIELD bon age on this sample was 2,270 ± 50 yrs B.P. (S. Unlike the COM lava field, the Wapi lava field is Robinson, USGS, personal communication 1978). a monogenetic volcano and typical of many of the Thus the Wapi and Kings Bowl lava fields formed low shield volcanoes that make up most of the pre­ simultaneously. sent surface of the central and eastern Snake River The contemporaneity of the Wapi and Kings Plain. Thus, study of the Wapi low shield is impor­ Bowl lava flows is supported by paleomagnetic data tant in understanding the processes involved in the (Figure 9). Precise directions of ramanent magneti­ formation of this type of volcano. zation obtained from two separate outcrops (17 km The Wapi field covers a 326-km2 area that is apart) within the Wapi field agree closely with each elongated north-south along the Great Rift. The other and with the direction of magnetization of margin of the field is smooth on the north and east lavas from the Kings Bowl field. In addition, the sides, where it ponded against the regional slope of paleomagnetic data suggest that the Wapi field took the Snake River Plain. On the south and west sides, a short time to form, probably several months to a the margin is formed of long, lingular flows where few years, despite the large volume (approximately 3 they filled drainages of small intermittent streams. 6.5 km ) of basalt erupted. The slope of the Wapi lava field over distances of Ramanent magnetization directions of lava flows 10-20 km is less than one degree. The flat slope is a near Pillar Butte differ from those from lava flows consequence of 1) the very fluid pahoehoe flows, 2) of the Wapi lava field. The difference can be at­ the relatively high rates of lava effusion, and 3) the tributed to rotation associated with summit deflation original flat slopes over which the flows moved. The near Pillar Butte at the close of the eruption, or to a only area of the lava field that has a steeper slope is younger age of lava flows near Pillar Butte than the near the vent area of Pillar Butte, where the slopes age of the main part of the Wapi field. The latter range from 5° to 7° (Champion and Greeley, 1977). possibility seems unlikely in that flows near Pillar The Wapi field is composed of numerous flow Butte both underlie and overlie flows of the rest of units of pahoehoe lava that are piled side by side the Wapi field. and atop one another; this forms a type of lava flow Surface morphologies of the Wapi lava field are described by Walker (1972) as "compound." Expo­ readily observable and are characteristic of low sures in many kipukas along the south and west shield volcanoes of the Snake River Plain. Near sides of the lava field suggest that the flows there Pillar Butte, thin flows were fed on the surface from are about 5-10 m thick. Thicknesses of the Wapi the vent complex; leveed-channel aa' flows, shelly flows are 15-25 m, except at Pillar Butte, where the pahoehoe rootless flows, and pahoehoe flows com­ total thickness may be 100 m (Champion, 1973). posed of toes are common. Other lava flows of the Near the margins of the field, the flow units are Wapi field are pahoehoe in surface texture and were larger and tend to have a greater local relief (as fed by tubes. Ubiquitous features are "pressure much as 10m) and are characterized by large pres­ ridges" and "collapse depressions." Unfortunately, sure plateaus, flow ridges, and "collapse both features are incorrectly named with respect to depressions." The transition in the size of the flows their mechanisms of formation. True pressure from the periphery to the interior of the field is ap­ ridges in lava flows are analogous to pressure ridges parently a function of proximity to the vent area; of sea ice that form from wind pressure pushing up thus, closer to the vent, many small pahoehoe flows ridges transverse to the direction of the wind. The have filled depressions in earlier flow units and lava ridges of the Wapi lava field are parallel to the generally leveled the local relief. direction of lava flow (Champion, 1973). Thus, the The numerical age of the Wapi lava field was un­ term "flow ridge" is used here in contrast to known until recently. Wapi lava flows overlie and "pressure ridge." Pressure ridges have been ob­ flowed into open cracks of the King's Bowl rift set, served to form on the surface of a lava lake in which in turn produced the Kings Bowl lava field. Mauna Loa (Champion, 1973). Nichols (1946), in a The weighted average of three radiocarbon ages on paper describing the ridges on the McCartys lava charred sagebrush found under the Kings Bowl field in New Mexico, recognized flow ridges and at­ lavas is 2,220 ± 100 yrs B.P. (King, 1977). In 1977, tributed them to collapse of the inflated crust over a a sample of charred root material was obtained, by lava flow. Ronald Greeley and Ronald Papson of Arizona Collapse depressions have been thought to form State University, from an excavation under the by collapse of the roofs of lava tubes. In a study of Wapi lavas at the east edge of Wapi Park. A radiocar- 100 collapse depressions (Champion, 1973), none N 00 0°

Wapi at Pleasant Valley Kings Bowl 2220 yr B.P.

Cave Basalt-Oo- 1925 yr B.P. / Wapi at Wapi Park 2270 yr B.P. c Craters of the Moon r; Pillar Butte ::r' 2100yr B.P. a (j) 0 0" (Jqn· ~ ;:s~ 0 0. 270 rl------+------~------~------r_------+_------~ 90° ~ 5· (j) '""I ~ Vl C '""I <: Figure 9. Equal-area diagram of directions of remanent magnetization, with circles of 95 percent confidence, obtained from lava flows of the Wapi and (j) '< Kings Bowl lava fields and other 14C dated lava flows of the Pacific Northwest. Arrow shows the trend in field direction due to secular variation of the geo­ Vl '0 magnetic field during the time interval between 2,300 and 1,900 yrs B.P. (j) n E e-Vl o. (D. CFl 0\...... - ...... - '-D 00 W Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 29 were found to be related to an uncollapsed lava tube 4.6 25.2 Kings Bowl lava field in view system. From their shapes and positions, it is appar­ ahead. Pillar Butte and part of ent that the collapse depressions of the Wapi field Wapi lava field in view to left. are instead related to an internal flow system. The gentle slopes of the Wapi field make it seem unlikely 0.6 25.8 Queens Bowl crater to the that a hydrostatic head could exist to drain tube right. This explosion crater is systems. Observations of the pattern of flow, pre­ located on a north-trending rift served in surface textures around collapse system that is older, but similar depressions, the morphology of the depressions, in appearance, to the Kings and the striations on inward-facing scarps of Bowl rift set. depressions, suggest that depressions form early in the cooling of the flow, and not from collapse; 1.1 26.9 Flows of the Kings Bowl lava rather, they can be thought of as localities where the field to the left. lava crust never was inflated. 0.3 27.4 STOP 1. Crystal Ice Cave park­ FIELD TRIP ROAD LOG: SECOND DAY ing area. At this stop we will examine the Kings Bowl lava American Falls to Kings Bowl and field (Figure 8) and we will dis­ Wapi lava fields and return cuss its eruptive history and Mileage Description relationship to the Great Rift. Incre- Cumu­ Features to be observed while mental lative walking include the main erup­ 0.0 0.0 Start log at traffic light at junc­ tive fissure, spatter cones spat­ tion of Business 1-86 and Idaho ter ramparts, lava lakes, lava Rt. 39 in American Falls. Pro­ levees, basalt mounds, the ceed west on Idaho Rt. 39 Kings Bowl explosion pit, and across the American Falls Dam effects of the explosive event toward Aberdeen, following at Kings Bowl. signs to Crystal Ice Cave. Walk to top of lookout directly west of Cyrstal Ice 6.3 6.3 Road junction. Turn left on Cave parking area (locality 1, North Pleasant Valley Road. Figure 8). The view to the west and southwest is of a former Road junction. Turn right; 8.4 14.7 lava lake surface. North and cemetery on right. west of the lava lake are several basalt mounds, believed to be 1.0 15.7 Road bears left. remnants of levees that con­ 1.9 17.6 Pillar Butte in view ahead on tained lava lakes. horizon. Pillar Butte marks Proceed northward from the summit of the lava cone of the lookout, along the main erup­ Wapi lava field. tive fissure, to an area where age relationships of Kings Bowl 0.4 18.0 Road junction. Turn right. lava flows and noneruptive fis­ sures of the Great Rift are ap­ 1.5 19.5 Pillar Butte and part of Wapi parent (locality 2, Figure 8). lava field in view to left. The Cracks cut the older flow, but rounded hill in the middle are overlapped by the younger ground is a kipuka vent sur­ flow. rounded by lava flows of the Proceed west across the Wapi field. main eruptive fissure to an area of levees that contained a 1.1 20.6 Cattle guard and road junction. lava lake (locality 3, Figure 8). Take left fork. Lava tubes and lava channels 30 Utah Geological and Mineral Survey Special Studies 61, 1983

that resulted in overflow of the serve the grooved walls re­ levee system also can be ob­ sulting from phreatic served here. eruptions. Retrace trail out of" Proceed southward along the the crater, turn north and main eruptive fissure to an return along trail to parking area with squeeze-ups, ejecta area. blocks, basalt mounds, and sur­ Leave Crystal Ice Cave park­ face of the lava lake (locality 4, ing lot on same gravel road Figure 8). "Flow-out" features traveled from American Falls, can be seen between the basalt traveling to the southeast. mounds. The lava lake is flat at Parts of the next segment of this locality and it is broken the field trip require a high­ into large plates as a result of centered or 4-wheel drive subsidence of the lake. Bulbous vehicle, particularly in wet squeeze-ups resulting from weather. lava oozing through cracks be­ tween plates on the lava lake 2.2 29.6 Turn sharply right onto dirt surface are abundant. Lithic road. Proceed first west, then blocks thrown out by the explo­ curve south. sive event at Kings Bowl litter the surface; impact craters in 0.3 29.9 Turn right onto dirt road and the lava lake surface from such proceed nearly due west. In blocks can be observed. near distance are spatter vents Proceed eastward across the and near-vent flows of the main eruptive fissure to locality southern part of the Kings at the north end of the Kings Bowl lava field. Bowl crater, near the tourist wall, where a feeder dike in­ 0.8 30.7 Continue west past road on trudes the main eruptive fis­ right. sure (locality 5, Figure 8). Follow the trail southward 0.1 30.8 STOP 2. This area constitutes around the east side of Kings an optional stop (time Bowl crater to the crater permitting) to observe fissures entrance. Here, the surface of the Kings Bowl segment of east of the main eruptive fis­ the Great Rift. Extensional sure has been blanketed by deformation produced fissures fine tephra erupted during the as much as 0.5 m across at this phreatic explosions at Kings locality. These fissures belong Bowl. At the crater entrance, to the eastern set of three paral­ the upper flows display a swirl­ lel fissure sets. The eastern set ing pattern described by Swan­ of fissures is separated from fis­ son (1973) as a flow variety sures of the central set by characteristic of near-vent about 0.7 km. Systems of activity. Farther down the trail, nearly parallel fissures also a soil horizon separates the characterize the area immedi­ young Kings Bowl lava flows ately southeast of the COM from older massive flows in lava field (Figure 4). Although the crater wall. Continue down the Kings Bowl fissure system the trail to the north end of the trends N. 10° W., the individu­ Kings Bowl crater where a al fissures trend N. 22° W. in feeder dike is exposed in the an en echelon manner central eruptive fissure. Walk (Champion, 1973). This trend north into the fissure to ob- suggests a component of left Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 31

shear in this area of the Great be carried on this traverse, par­ rift. ticularly in the hot summer months. Descriptions of locali­ 0.2 31.0 Proceed west past left turn. ties on this traverse were provided, in large part, by 0.2 31.2 Cross the Great Rift and erup­ Champion and Greeley (1977). tive fissures that produced the Walk about half way from Kings Bowl lava field. Spatter STOP 3 to the north end of the cones occur both north and Pillar Butte vent complex south of the road at this point. (locality 1, Figure 10). As a landmark during the traverse, 0.1 31.3 Turn left and proceed south on walk toward the left of Pillar dirt road. Road traverses the Butte. The hummocky relief of length of Wapi Park, a vent the lava surface is characterized complex that trends south­ by pressure ridges and local col­ westerly, and is older than the lapse depressions. Local relief nearby Wapi and Kings Bowl changes from low over the lava fields. pressure plateau areas to fairly high (10 m) near flow ridges. l.5 32.8 Turn right and proceed south­ This form of topography is southwest between the edge of characteristic of the margins of the Wapi lava field and the old the Wapi field, but quite dif­ vents of Wapi Park. ferent from that of the Pillar Butte vent complex. 0.1 32.9 Proceed south-southwest past Continue south-southeast left turn. for about one mile to the con­ tact between lavas of the main 0.4 33.9 A hand-dug tunnel here yield­ Wapi lava field and tube- and ed charred sagebrush beneath surface-fed flows of the Pillar lava flows of the Wapi field, Butte vent complex (locality 2, whose radiocarbon age is Figure 10). The Pillar Butte 2,270 ± 50 yrs B.P. flows at this locality emanated from the northwest side of the 0.8 34.1 One of the paleomagnetic sites Pillar Butte area. Observable for the Wapi lava field is locat­ here are tubes and channels in ed on the inflated pressure pla­ the flows and small pit craters. teau to the west of the road; Proceed southeast to the data are the same for the Pleas­ north flank of the summit area ant Valley arm of the Wapi of Pillar Butte (locality 3, field, which projects from the Figure 10). This area is a com­ eastern margin of the field, 20 plex of several pit craters and km to the south (see Figure 9). vents that fed flows to the northern part of the lava field. 0.3 34.4 STOP 3. End of the dirt road Lava flows of the summit area at the southern end of Wapi consist chiefly of shelly pahoe­ Park. A walk that does NOT hoe that contains numerous follow a trail to and from the channels that slope to the Pillar Butte vent complex con­ north. Shelly pahoehoe forms stitutes STOP 3 and begins and near vent areas from gas­ ends at this locality (STOP 3, charged fluid lavas (Swanson, Figure 10). The traverse is 1973). about 5 km long and requires Traverse south, along the about 4 hours. Water should eastern margin of the summit 32 Utah Geological and Mineral Survey Special Studies 61, 1983

To Crystal Ice Cove KINGS BOWL and Stop 1 LAVA FIELD

To American Falls \ ~

II \ 'I \ ~\ \ \ " \ " \ 42°55' \ \\ "\' " "" ,\ "~,, EXPLANA TION \ '/~ ~~

--- Contact ~'\ Cracks -+--t--t- Eruptive Fissure Vent, rim of 0 crater outlined Location along 03 walking tour Direction of \ walking tour Road, maintained

~ = = ~= = Road. not maintained ,

5000 FEET I I 1000 METERS

Figure 10. Generalized geologic map of the Pillar Butte area of the Wapi lava field , Idaho, showing features discussed in text. Circled numbers refer to localities on walking tour for STOP 3, Day 2. Map modified from Champion and Greeley (I 977). Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections [v'leeting, 1983 33

region, and note more shelly REFERENCES CITED pahoehoe and slab lava. Pro­ Armstrong, R. L., Leeman, W. P., and Malde, H. E., ceed to the eastern edge of the 1975, K-Ar dating of Quaternary and Neogene vol­ major pit crater complex of the canic rocks of the Snake River Plain, Idaho: Ameri­ Pillar Butte vent area (locality can Journal of Science, v. 275, p. 225-251. 4, Figure 10). The western wall Champion, D. E., 1973, The relationship of large-scale of the vent area displays layered surface morphology to lava flow direction, Wapi lava rootless flows of the summit field, southeast Idaho: MS thesis, State University of region in cross section. In the New York, Buffalo, 44 p. pit craters, observe ledges of Champion, D. E., and Greeley, Ronald, 1977, Geology of lava lakes that filled the pit cra­ the Wapi lava field, Snake River Plain, Idaho, in ters and then subsided. Thin Greeley, Ronald, and King, J. S., eds., Volcanism of the eastern Snake River Plain, Idaho; A comparative deposits of lapilli, generated by planetology guidebook: Washington, D. C., National vigorous fountaining, occur Aeronautics and Space Administration, p. 133-152. along the east margin of the Champion, D. E., and Shoemaker, E. M., 1977, Paleo­ vent area. magnetic evidence for episodic volcanism on the Snake River Plain (abs.): National Aeronautics and Continue south to Pillar Space Administration Technical Memorandum Butte, an excellent vantage 78,436, p. 7-9. point (locality 5, Figure 10) for Christiansen, R. L, and McKee, E. H., 1978, Late Ceno­ the general aspects of the Wapi zoic volcanic and tectonic evolution of the Great Basin and Columbia intermontane regions, in Smith, lava field as well as the topogra­ R. B., and Eaton, G. P., eds., Cenozoic tectonics and phy and features of the eastern regional geophysics of the western Cordillera: Snake River Plain. The origin Geological Society of America Memoir 152, p. of Pillar Butte is unclear; its 283-312. relief is due either to deflation Covington, H. R., 1977, Preliminary geologic map of the of the area around it or to infla­ Pillar Butte, Pillar Butte NE, Pillar Butte SE, and Rat­ tion directly beneath it. tlesnake Butte quadrangles, Bingham, Blaine, and Power counties, Idaho: U.S. Geological Survey Open­ File Report 77-779, scale 1:48,000. Proceed northwest to an area Cox, K. G., Bell, J. D., and Pankhurst, R. J., 1979, The containing well-exposed, east­ interpretation of igneous rocks: George Allen and facing, subsidence faults that Unwin, Ltd., London, 450 p. have truncated flow channels Doherty, D. J., McBroome, L. A., and Kuntz, M. A., and tubes (locality 6, Figure 1979, Preliminary geologic interpretation and litho­ 10). The orientation of these logic log of the exploratory geothermal test well faults is parallel to fissures of (INEL-1), Idaho National Engineering Laboratory, the Kings Bowl Rift system. eastern Snake River Plain, Idaho: U.S. Geological From this locality, continue Survey Open-File Report 79-1248, 10 p. northwest to locality 2, and Eaton, G. P., Christiansen, R. L, Iyer, H. M., Pitt, A. M., Mabey, D. R., Blank, H. R., Zietz, 1., and Gettings, then west-northwest back to M. E., 1975, Magma beneath Yellowstone National STOP 3, where the vehicles are Park: Science, v. 188, no. 4190, p. 787-796. parked (Figure 9). Karlo, J. F, 1977, The geology and Bouguer gravity of the Hells Half Acre area and their relation to volcano­ Retrace route north out of tectonic processes within the Snake River Plain rift Wapi Park, east across the zone, Idaho: Ph.D. dissertation, State University of southern edge of the Kings New York, Buffalo, 152 p. Bowl field, and along to the King, J. S., 1977, Crystal Ice Cave and Kings Bowl Crater, eastern Snake River Plain, Idaho, in Greeley, main gravel road back to Amer­ Ronald, and King, J. S., eds., Volcanism of the eas­ ican Falls. tern Snake River Plain, Idaho; A comparative pla­ netology guide book: Washington, D.C., National Aeronautics and Space Administration, p. 133-152. END OF ROAD LOG. Kuntz, M. A., 1978, Geology of the Arco-Big Southern 34 Utah Geological and Mineral Survey Special Studies 61,1983

Butte area, eastern Snake River Plain, and potential States, in Smith, R. B., and Eaton, G. P., eds., Ceno­ volcanic hazards to the Radioactive Waste Manage­ zoic tectonics and regional geophysics of the Western ment Complex and other waste storage and reactor Cordillera: Geological Society of America Memoir facilities at the Idaho National Engineering 152, p. 93-106. Laboratory, Idaho: U.S. Geological Survey Open-File Murtaugh, J. G., 1961, Geology of Craters of the Moon Report 78-691, 70 p. National Monument, Idaho: Unpublished M.S. Kuntz, M. A., and Dalrymple, G. B., 1979, Geology, geo­ thesis, University ofIdaho, Moscow, Idaho, 99 p. chronology and potential volcanic hazards in the Nichols, R. L., 1946, McCartys basalt flow, Valencia Lava Ridge-Hells Half Acre area, eastern Snake County, New Mexico: Geological Society of America River Plain, Idaho: U.S. Geological Survey Open-File Bulletin, v. 57, p. 1049-1086. Report 79-1657, 72 p. Prinz, M., 1970, Idaho rift system, Snake River Plain, Kuntz, M. A., Lefebvre, R. H., Champion, D. E., Idaho: Geological Society of America Bulletin, v. 81, McBroome, L. A., Mabey, D. R., Stanley, W. D., no. 3, p. 941-947. Covington, H. R., Ridenour, J., and Stotelmeyer, R. Skipp, Betty, and Hait, M. H., Jr., 1977, Allochthons B., 1980, Geological and geophysical investigations along the northwest margin of the Snake River Plain, and mineral resources potential of the proposed Idaho: Wyoming Geological Association Guidebook, Great Rift Wilderness area, Idaho: U.S. Geological Twenty-ninth Annual Field Conference, p. 499-515. Survey Open-File Report 80-475,48 p. Spear, D. B., 1979, The geology and volcanic history of Kuntz, M. A., Mabey, D. R., Champion, D. E., Stanley, the Big Southern Butte-East Butte area, eastern W. D., Lefebvre, R. H., Spiker, E. McBroome, L. c., Snake River Plain, Idaho: Ph.D. dissertation, State A., and Covington, H. R., 1982a, Geologic and University of New York, Buffalo, 136 p. geophysical maps of the Great Rift Wilderness Study Stanley, W. D, Boehl, J. E., Bostick, F. X., and Smith, H. area, Blaine, Butte, Minidoka, and Power counties, W., 1977, Geothermal significance of magnetotelluric Idaho: U.S. Geological Survey Miscellaneous Field soundings in the eastern Snake River Plain­ Studies Map MF-1462, scale 1:125,000, 2 sheets. Yellowstone region: Journal of Geophysical Kuntz, M. A., Champion, D. E., Spiker, E. C., Lefebvre, Research, v. 82, p. 2501-2514. R. H., McBroome, L. A., 1982b, The Great Rift and Stearns, H. T., 1928, Craters of the Moon National the evolution of the Craters of the Moon lava field, Monument, Idaho: Idaho Bureau of Mines and Geol­ Idaho: in Bonnichsen, Bill, and Breckinridge, R. M., ogy Bulletin 13, 57 p. eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26 (in press) . Stone, G. T., 1967, Petrology of Upper Cenozoic basalts Leeman, W. P., Vitaliano, C. J., and Prinz, M., 1976, of the Snake River Plain, Idaho; Ph.D. dissertation, Evolved lavas from the Snake River Plain; Craters of Boulder, University of Colorado, 392 p. the Moon National Monument, Idaho: Contributions Swanson, D. A., 1973, Pahoehoe flows from the to Mineralogy and Petrology, v. 56, p. 35-60. 1969-1971 Mauna Ulu eruption, Kilauea volcano, Lefebvre, R. H., 1975, Mapping in the Craters of the Hawaii: Geological Society of America Bulletin, v. Moon volcanic field, Idaho, with LANDSA T 84, p. 615-626. (ERTS) imagery: International Symposium on Trimble, D. E., 1976, Geology of the Michaud and Poca­ Remote Sensing of the Environment, 10th tello quadrangles, Bannock and Power counties, Proceedings, v. 2, no. 10, University of Michigan, Idaho: U.S. Geological Survey Bulletin 1400, 88 p. Ann Arbor, p. 951-957. Valastro, S., Jr., Davis, E. M., and Zarela, A. G., 1972, Mabey, D. R., 1976, Interpretation of a gravity profile University of Texas radiocarbon ages: Radiocarbon, across the western Snake River Plain, Idaho: v. 14, p. 468-470. Geology, v. 4, p. 53-55. Walker, G. P. L., 1972, Compound and simple lava flows ___, 1978, Regional gravity and magnetic anomalies in and flood basalts: Bulletin Volcanology, v. 36, p. the eastern Snake River Plain, Idaho: U.S. Geological 579-590. Survey Journal of Research, v. 6, p. 553-562. Zohdy, A. A. R., and Stanley, W. D., 1973, Preliminary Mabey, D. R., Zietz, Isidore, Eaton, G. P., and interpretation of electrical sounding curves obtained Kleinkopf, M. D., 1978, Regional magnetic patterns across the Snake River Plain from Blackfoot to Arco, in part of the Cordillera in the Western United Idaho: U.S. Geological Survey Open-File Report. Utah Geological and Mineral Survey Special Studies 61, 1983 Guidebook Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting Salt Lake City, Utah - May 2 - 4, 1983

MID-TERTIARY HISTORY OF THE CENTRAL PIOCHE-MARYSVALE IGNEOUS BELT, SOUTHWESTERN UTAH

Myron G. Best Department of Geology, Brigham Young University, Provo, UT 84602

Jeffrey D. Keith Queens University, Kingston, Ontario, Canada K7L 3M6

INTRODUCTION OLIGOCENE VOLCANISM This paper deals with the character of Cenozoic The lowest part of the volcanic sequence (Figure magmatism in the central part of the Pioche­ 2) consists of rhyolitic ash-flow tuffs, thick rhyolite Marysvale igneous belt in the Needle Range and and pyroxene andesite flows, and local volcanic southern Wah Wah Mountains of southwestern sandstones belonging to the Sawtooth Peak Forma­ Utah (Figure 1). This belt is one of several that tion and the overlying Escalante Desert Formation trend east-west across Nevada and western Utah (Grant, 1978; Campbell, 1978). The Lamerdorf (Stewart and others, 1977; Rowley and others, ash-flow tuff near the top of the latter has a K-Ar 1978). Following a long period of erosion that had age on biotite of 32.3 ± l.1 m.y. (Best and others, reduced the terrain to a subdued relief of generally 1983). These units are thickest in paleovalleys that less than 100 m, magmatic activity began in the mid­ commonly trend more or less east-west. It was not Oligocene and continued, with few interuptions, until deposition of the voluminous tuffs of the N ee­ into the Miocene until about 18 m.y. ago. After a dIes Range Group 30 to 28 m.y. ago that most of the hiatus of about 5 m.y., activity resumed again, but remaining hills of Paleozoic sedimentary rocks were only for a brief period of about one million years. covered. By the time of deposition of the densely There has been no magmatism manifest since. welded andesitic tuffs of the latest Oligocene Isom Oligocene magmatic rocks are generally of inter­ Formation 25 m.y. ago, topographic relief in south­ mediate calc-alkaline composition and include western Utah was nil because this formation is on voluminous, regionally extensive ash-flow tuffs and the order of a few tens of meters thick almost every­ minor local lava flows. Large caldera complexes where in the central part of the Pioche-Marysvale formed as the result of catastrophic explosive expul­ belt. The near uniformity of its thickness also im­ sion of thousands of cubic kilometers of ejecta. In plies absence of any tectonic disturbance causing contrast, the two episodes of Miocene magmatism, differential uplift during the late Oligocene. from 23 to 18 and 13 to 12 m.y. ago, produced bimo­ dal mafic-silicic assemblages, chiefly rhyolite lava Needles Range Group flows, tuffs, minor shallow intrusions, and local Since the pioneering efforts of J. H. Mackin and mafic lava flows that were erupted from many local his students in the 1950s and early 1960s (Mackin, centers. No calderas developed during the 1960; Cook, 1965), the crystal-rich dacitic tuffs of Miocene, although major extensional block faulting the Needles Range Group have been recognized as along a northeast trend occurred during the early a lithologically distinctive and remarkably wides­ Miocene magmatic pulse. These faults localized pread time marker in the succession of mid-Tertiary some of the hydrothermal activity that accompanied ash-flow tuffs in the eastern Great Basin and high emplacement of Miocene silicic magmas. plateaus of south-central Utah. Mapping at a scale

35 116° W 39°. 0'\ .:::.. )/ ...... :...... :.: .> :-': / .. :-::... 114° ??%/ 112° ..',j<~: ././ ~~:: // / %!~./ ;: '.'

. '. . / / ./ .' I /.;c:/ .' .. ' .' . . . . ." / /// ...... :>. /d".//./'.:. ....// //./ /./.// ...... : 01// I.// / ./// <'.. .: :.. . ~ .':/' "/ .... ;::;(v":~ '. '. ~ / W7:'MILFORD" :.... : ~/ ~// , ... / .... ~:.I.:~I .. ~~<. /~/// / ....,"j ...•...... j1..~:: ". ///~: ..... )% .. //// /. / :// / .;...... I ••. .z;. ~ ,l"" +.:-, ." .'/. /. 3 <·6 MARYSVALE 8~.:: ...... ::)' '/ /.~//.~;.I -: / .... ::-~ I ~'.'.''.':'. ."'". ..'.:. 0:······· .. ··· '.' . :.' . (/;/~~ / / /. .... /. ... /1--"/ / .... .1 '.-:ll'~' ..BEAVER:...·· ~/./

I: .. :..... :-./ ...... : < .:/; / .. : ...... :,' ..... t·;·.>·.// • . . . " . . . . F'./::~./. "/ ..... /~'./...... 0;~I ,SO .:. . . '.' '.' ::/ ./ .. ,./ .. : ". / .. ' './ ;.' . /.//~~~ ...... /.: /~. . >/ /jf'. :.' ';.":<:\.,0." .,''''' <.:...... ;1 c//>//////)/;/ '.. :' ..... ". '.' .': ~/~//~ W/w~///// . c ,.' ...... :..... 'i»./> ...•.•.•. :.!: .. .. .••. / p; j ;.~ /:~\ ;./C?t'~;.· ~/;/:/// ::r ,:'/: ..... / ...... «< ...... Cl . (i) ~. <1·7:::/;·~///~//// ~//// o 370". 0- ... //// I, /// 0 ,Nevada ----;---- (fQ ::M1:~./;:?/;.~ ,/:~;/J ... ~~./.:;;/~~/.~ //~~W/;~/;/ n' ~ __ _ _ _ km 50 / /. I I \ P" ~ ::; Arizona -- - -- I ---I ~L P- ~ PIOCHE-MARYSV ALE l " ·1 5' (i) '""I IGNEOUStz:3 BELT:.'."~ " ____U~~hJ i: ~ « CIl C ··' >-j ~.. : ". . . [Z] Sediment " "", Arizona <: (i) D D ary rocks ' '-0,,~. '< Quaternary Tertiary volcanic Tertiary granitic CIl rocks "0 alluvium rocks (i) (') §.: CIl 2 P­ Figure 1. Tertiary volcanic (stipled) and intrusive igneous (black) rocks of the Pioche-Marysvale igneous belt in southern Utah and eastern Nevada. (ii' [JJ Quaternary alluvium is blank. NR, Needle Range; WWM, Wah Wah Mountains; TU, Tushar Mountains; WRM, White Rock Mountains. 0'\ >-'

>-' '-D 00 W Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 37

The Cottonwood Wash Tuff is thickest in the northern Needle Range and in easternmost Nevada where its source caldera is perhaps largely, if not entirely, concealed beneath younger deposits, espe­ cially alluvial valley fill (Figure 3). The Wah Wah Springs Formation includes dense­ ly to weakly welded lapilli ash-flow tuffs composi­ tionally distinct from other Needles Range tuffs in that hornblende phenocrysts are more abundant E than biotite, and quartz phenocrysts are generally u.. silicic flows, intrusions, and tuffs c small ( 2 mm) and constitute no more than 2 to 3 3: ctI percent of the tuff. The unit is especially unique in .: 22 m terms of its reversed magnetic polarity (Gromme' Q) Ol and others, 1972; Shuey and others, 1976); other ,-_-~?~,~_?~~_2,'~< ,- of Paleozoic nous ash-flow sheets known anywhere (Figures 3 34 Sawtooth Pk. Fm. - and 4). It crops out over an area of almost 25,000 2 :... ~ - :-- - and local Mesozoic rocks km - chiefly west, north, and east of the source caldera (see below). To the south, the sheet is cov­ Figure 2. Stratigraphic relations of volcanic units in the Wah ered by a thick, widespread sequence of Miocene Wah Mountains and Needle Range. Regional ash-flow sheets rocks (Williams, 1967; Stewart and others, 1977) are stippled. but appears to be thin because of a regional highland that existed during deposition. The average thick­ of 1:24,000 by the U.S. Geological Survey combined ness of the tuff outside the caldera, from data with paleomagnetic investigations (Shuey and shown on Figure 4, is 0.2 km. The total volume of others, 1976) have defined the areal extent and re­ 5,000 km3 calculated from these area and thickness gional stratigraphic correlation of the tuffs in Utah values must surely be subject to unknown amounts (Figures 3 and 4). Less is known of the Needles of adjustment because of the effects of pre­ Range tuffs in eastern Nevada as little has been deposition topography, variable compaction and done there since the reconnaissance work of Cook porosity, uncertainties in the original extent of the (1965) and Gromme' and others (1972). deposit (particularly to the south of the caldera), in Although originally defined as a formation, with the area of the caldera, the thickness of caldera­ the individual tuff sheets comprising members, it is filling tuff (Iocally at least 2km), and the amount of now apparent that the tuff sheets constitute a strati­ east-west crustal extension in the Great Basin since graphic group of formations, each formation being a the Miocene. genetic entity manifesting a period of catastrophic The possibility that the enormous volume of the explosive eruption and caldera collapse. Four tuff Wah Wah Springs Formation originated from more formations have been recognized in southwestern than one magma-caldera source cannot totally be Utah; in ascending stratigraphic order they are the discounted. However, the unique and uniform Cottonwood Wash, Wah Wah Springs, Lund, and magnetic polarity of the tuff, the absence of any Wallaces Peak (Best and others, 1973). All are other candidate source, and the large size of the crystal-rich, lapilli ash-flow tuffs with plagioclase Indian Peak caldera (at least 60 km x 20 km, uncor­ comprising about half of the phenocrysts, and com­ rected for east-west post-Oligocene crustal binations of biotite, hornblende, and quartz the re­ extension) favor a single source. maining half. The Indian Peak caldera that collapsed during w 00

Wallaces Peak Tuff, thickness in meters 113°00' 112°00' W 30 Nevada Utah , I T L 60 Lund Tuff, thickness in meters

o DELTA

C 80 Cottonwood Wash Tuff, thickness in meters

115°00' 114°00' 39°00' BAKER I~- o C FILLMORE z C CD 20 < , c..... ill , ill C g-,::T 7S CRYSTAL PEAK t::. C C o CURRANT C RICHFIELD C C215 C135 CL c C I 105 i C460 g; '_+- C L12 ;:J C30 5 CI W Q L100+ ATLANTA I 300c C 67 o CD L21 SUNNYSIDE I MARYSVALE o L L W o I C CL 0- 30S L SSC30S MILFORD (JQ L70 LL 1 cLw o· L70++ C L W 2- 47 900l L L ~ 2S0L o :::l INDI~N PEAK t::. l Ji BEAVER 0- L l W L LC W60 L lsoo 20L L ~ l L SOO s· ~ L 2- l C/) 38°00' - L7S0 Ll L l l1 00 s:: 400 '""1 l- -< CD o '< o miles 40 PIOCHE C/) '0 !! I CD I iii n o kilometers 60 L o PANGUITCH E C/) 2" 0- (ii' Figure 3. Distribution and thickness of known exposures of the Cottonwood Wash, Lund, and Wallaces Peak tuffs in southwestern Utah and southeastern [JJ 0\ Nevada. Data in Nevada from Cook (1965) and in Utah from numerous published maps...... '-0 00 W fl exposures of Intercaldera Member Cl ~ • 0.: (1l exposures of Outflow Member, with thickness in meters : 113°00' 112°00' 0- • o 60 o l' '"t;j o exposures of Outflow Member, with paleomagnetic data .+30+ ~ o w o DELTA I caldera ring fault, dashed where inferred or approx. located Cl / CZI>­ o~ topographic wall of caldera WAH WAH SPRINGS TUFF (") ~ Q o 114°00' ~ o 39°00'+ BAKER c I ~ ;:l I o FILLMORE 21C,+ S 11s<' 00' S' 1 ; p; D>~ ::T .60+ 70+: 0. ;:l D> ,1 ·.0 0 0 0- o o •• RICHFIELD n 0.0. • :60+ o o CRYSTAL PK CO o .... C CURRANT • .430 • e: •••• ~ 0 • • • ~ ~ 90+ •• • p; .29 r"' • 120+ •• 350 \'II • ;:l I. • CZI o • 0 z (1l I ):> • +.41 :!. S I· )7~ I -+---I o· MARYSVALE ;:l I-t-.. o 83 I (FJ T- o ATLANTA c I, •••• ~. c SUNNYSIDE ::30~ ~ o MILFORD (1l ~ .:... z...... (1l Z 1%/0;1// "I '0~ ):> • \ • 116 / :!. • • 120 S" • BEAVER • C!Cl t ~'W~"~: ~ IND'c'~ PK • o • 0 ...... fffl, WI; , ·.10 itO0 \0 ~ ~ 00 ? ••• 180+ W () ~Z y ~ '1/;, \.J. • ." -t. (;;,# . •• • 0~ ~. o .." i& '" %'i%:' i! •• 250 '?? '0; "'.0. o :0 ... ,;1 /' • ~~0 ? _ o 38°00'+ • A _ • ,'S o --1- o PIOCHE ,1 o o o o miles 40 1(1) I ,", I I o kilometers 60 • o PANGUITCH

Figure 4. Distribution and thickness of known exposures of the Wah Wah Springs in southwestern Utah and southeastern Nevada. Data in Nevada from Cook (1965), and Gramme' and others (1972); in Utah fram Shuey and others (1976), and numerous published maps.

W \0 40 Utah Geological and Mineral Survey Special Studies 61,1983

~ --\ I' \ I eruption and deposition of Outflow Member of Wah ! Wah Springs Tuff CTwo) over Cottonwood Wash (~ Tuff CTc) s ~" N_·:':'_:T_~~··_!_W-==O=:'='~_----:r;-;:~-.::;..'.:.:::.:.:::... :=.:. Te·· ? ? 9 ~ \ -~ ?

\ possible source caldera of \ Tuff of Marsden Spring Member of Escalante Desert Formation 2 CTe)

episodic subsidence of Indian Peak Caldera along ring fault; landslide deposits CTb) that caved off caldera wall interfinger with deposits of Intracaldera Member of Wah Wah Springs Tuff CTwi) (

, Te

ring fault

continued subsidence along ring fault and a second fault about 7 km to the north caused large brecciated landslide or fault slices CTb) to cave away from receeding caldera wall: continued eruption of Wah Wah Springs Tuff (undivided. Tw)

topographic ',",,Twci:: ' wall Tc

Pz recurrent minimum \\ total offset 2 km

total subsidence at least 2.1 km

resurgent doming, erosion and deposition of Ryan Spring Tuff (Tr) and Lund Tuff (TI) within 2 m.y. future site of Indian Peak

: ~::., .-~ TI ? Tc ',~ Tr dacite T ban d T ,I I : ' I " f .... I ~ w~ " -,' dome J.,-~~==... ' -'_'-'Twi Te I~'?".~-:- . ','. '. ,',"

. ".'

Figure 5. Idealized and schematic north-south cross sections through the central Needle Range showing the evolution of the northeastern segment of the Indian Peak Caldera. Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 41 catastrophic eruption of the Wah Wah Springs For­ directly on the Intracaldera Wah Wah Springs Tuff mation is a gourd-shaped structural depression that Member. However, 10 km farther southeast the straddles the Utah-Nevada border. The northern Ryan Spring Tuff reappears, but is never more than and western extent of the caldera in Nevada is 500 m thick, and the Lund Tuff is still as much as known only imperfectly from reconnaissance 750 m. These relations suggest resurgent doming of investigations; the eastern segment appears to be the eastern part of the Indian Peak caldera before buried beneath Pine Valley between the Needle deposition of the Ryan Spring Tuff. Range and the Wah Wah Mountains and Miocene The source calderas of the Ryan Spring and Lund and younger deposits conceal its southern Tuffs have not been located but the distribution and extremities. However, its northeastern segment is great thickness of the tuffs suggest they are probably well exposed in the east-tilted Needle Range just nested within the topographically enlarged basin of north of Indian Peak. Subsidence began along the the Indian Peak caldera. ring fault, exposed 1.5 km north of the peak, after Only minor shows of Cu-Fe-skarn mineralization much of the Outflow Member of the Wah Wah S pr­ appear along the ring fault of the Indian Peak caldera ings Formation was extruded (Figure 5). Episodic about 6 km northwest of Indian Peak where a caving of older volcanic rocks from the unstable porphyry intrusion, compositionally similar to the caldera wall into the deepening depression created Wah Wah Springs tuffs, intruded Paleozoic carbo­ tongues of landslide breccia that interfinger with nate rocks that are now marbles. caldera-filling tuff. There are conspicuous lateral va­ Isom Formation riations in the appearance of the Intracaldera The last regionally extensive intermediate­ Member of the Wah Wah Springs Formation within composition ash-flow tuffs to be deposited in the the caldera that probably represent different batches Oligocene, about 25 m.y. ago (Fleck and others, of magma from an inhomogeneous chamber; these 1975), were those comprising the Isom Formation. variations in space and probably in time have yet to These lapilli tuffs with sparse phenocrysts of plagi­ be investigated. Recurrent subsidence along the oclase and pyroxene occur widely over much of northeast segment of the ring fault and along a southwestern Utah and adjacent Nevada and consti­ parallel fault to the north created such instability tute excellent stratigraphic markers, in part because that massive gravity-driven fault slides moved of their densely welded character and the thinness southward toward and into the deepening of the sheets (generally less than 10m). The loca­ depression; remnants of these slides are now found tion of the source caldera is unknown, but unusually as cataclastic (not mylonitic) and chiefly monolitho­ thick sections occur locally in the southern Needle logic volcanic breccias north of the ring fault. Range and central White Rock Mountains in Wedges of Wah Wah Springs tuff lie within this se­ Nevada. quence of breccias that is as much as 800 m thick. MIOCENE MAGMATISM Some of these wedges are internally sheared where­ Two episodes of Miocene magmatic activity, 23 as others several meters thick are only of densely to 18 and 13 to 12 m.y. ago, produced similar bimo­ welded vitrophyre. The topographic wall of the cald­ dal assemblages of mafic and silicic rocks in the cen­ era receded 10 km north of the ring fault as a conse­ tral part of the Pioche-Marysvale belt. Magmas quence of the caving and faulting (Figures 5, 8, and invaded the uppermost crust and erupted onto the 9). surface at numerous places without the formation Overlying the fault-landslide breccias in the of calderas. The earlier of these episodes more or Needle Range are the lithic rhyolite ash-flow tuffs less overlaps the period of extrusion of the ash-flow of the Ryan Spring Tuff that contain sparse phe­ sheets of the Quichipa Group 24 to 21 m.y. ago nocrysts of plagioclase and biotite. Although absent from caldera sources south of Pioche, Nevada beyond the topographic wall of the caldera, this unit (Williams, 1967). Only one of these sheets, the is as much as 560 m thick inside the depression. Bauers Tuff Member of the Condor Canyon Forma­ Additional caldera fill is represented in the overlying tion with an age of 22 m.y. (Fleck and others, 28 m.y. old Lund Tuff, a compound cooling unit 1975), is present in the Needle Range and Wah that is up to 600 m and possibly 1,000 m thick Wah Mountains; it is generally less than 20 m thick within the topographic depression of the caldera and densely welded. and less than 55 m outside it just to the north. Six kilometers southeast of Indian Peak the Ryan Blawn Formation Spring Tuff pinches out and 500 m of Lund Tuff lies Rocks of the early Miocene bimodal magmatic 42 Utah Geological and Mineral Survey Special Studies 61, 1983 association comprise the Blawn Formation (Best Mid-Miocene mafic flows are more restricted in and others, 1983). The base of the unit is a area extent than those of the Blawn Formation. sequence of rhyolitic pyroclastic rocks, chiefly They have less Si and K and, therefore, are of a crystal-poor, weakly welded lapilli ash-flow tuffs. more basaltic composition. Overlying these tuffs are K-rich mafic flows that are most extensively exposed on the east flank of Thus, the bimodal association of the mid­ the Wah Wah Mountains. Si02 (weight percent) Miocene Steamboat Mountain Formation comprises ranges from 54 to 62 and K20 from 2.2 to 4.0, so high-silica topaz-bearing rhyolite and basaltic lavas that they may be classed as trachyandesite to in contrast to the early Miocene Blawn Formation latite - not basalt. that comprises rhyolites generally not so enriched Overlying the mafic flows are more rhyolitic tuffs in Si and F and mafic lavas with intermediate similar to those beneath. These are in turn capped amounts of Si and high K. by, or locally interfinger with, rhyolite lava-flows that range from virtually aphyric to crystal-rich with Two younger episodes of bimodal rhyolite-basalt phenocrysts of quartz and sanidine several magmatism - 8 to 6 m.y. and less than 3 m.y. millimeters across in addition to plagioclase, biotite old - are manifest farther northeast in the and rare hornblende. Pioche-Marysvale igneous belt, chiefly in the Many crystal-rich rhyolite dikes and other Mineral Mountain area, and west of Fillmore in the shallow intrusions of early Miocene age occur in Black Rock Desert (Best and others, 1980; Rowley both the Wah Wah Mountains and Needle Range. and others, 1978). In the former, several intrusions are exposed from south of The Tetons northward to Pine Grove - where an intrusive-vent complex is host to EARLY MIOCENE TECTONISM disseminated Mo and W at depth (Keith, 1982). In The tectonic pattern of the central part of the the Needle Range, two swarms of dikes are exposed Pioche-Marysvale belt is a composite of north and in an upthrown horst that forms the backbone of northeasterly trending features (Figure 1). Fault­ the range. There is no recognizable difference in the bounded northerly-aligned mountain ranges are radiometric age between the northern swarm that truncated at their southern ends by the northeast­ trends east-northeast and the southern that trends trending northern margin of the Escalante Valley. northeast - both are 22 to 19 m.y. old. Rhyolite The southern termini of these ranges are linked by a lava flows that were undoubtedly fed from the same prominent zone of northeast-striking high-angle dike swarms occur in adjacent grabens. faults that coincides with belts of early and middle Miocene silicic magmatic activity (Figure 6). Data Steamboat Mountain Formation show that the northeast-striking faults were chiefly Mafic and silicic rocks extruded during the mid­ early Miocene and possibly related to a left-lateral, Miocene episode 13 to 12 m.y. ago comprise the almost east-west shear couple. Development of the Steamboat Mountain Formation. Many centers of northerly trending more conspicuous basins and activity developed, as in the early Miocene episode, ranges typical of this part of the Basin and Range but the silicic ones lie in a northeast trending band province apparently followed the early Miocene parallel to and just to the south of the Blawn centers event. (Figure 6). Steamboat Mountain silicic activity also The pattern of northeast-striking faults of early began with the explosive eruption of gas-charged Miocene age is displayed in Figure 6. The virtually magma, forming pyroclastic deposits around vents. unfaulted segment of the Wah Wah Mountains Mostly capping but locally interfingering with these north of Pine Grove Canyon where few Tertiary vol­ sequences are extrusions of flow-layered rhyolite. canic rocks are exposed contrasts sharply with the Rhyolite tuffs of the Steamboat Mountain Forma­ segment to the south where numerous subvertical, tion are petrographically indistinguishable from northeast-striking faults continue southwestward those of the Blawn Formation. However, lava flows into the southernmost Needle Range and into the tend to be less phenocrystic and are uniformly more southern White Rock Mountains in Nevada; enriched in Si and F and have less Ti, F~, Mg, and however, our mapping there is incomplete. Ca than Blawn rhyolites. Only some of the younger These northeast-striking faults, as well as the one Blawn rhyolite lava flows, and the 18 m.y. old Staats northwest-striking fault zone extending from Blue rhyolite intrusion, contain a little topaz whereas in Mountain to Blawn Mountain in the Wah Wah Steamboat Mountain flows topaz is a virtually ubi­ Mountains, have vertical displacements as great as quitous vapor-phase crystal in lithophysal cavities. 1 km. The northeast-striking faults are slightly ob- 0 0 0 113 30' 113 00' 38 30', a r- c i' ~ 't-r :E (II » cr' i o I 1\// ~ :::c o ~';l'\ r;:) ~tc ""C < i ..... 1· DlIDl :::.~ ~I:J" MILFORD VJ \ ~~ i \\ o I ~, ,",';.' a U'l::» PG //. o~ SHAUNTIE (') <~., '(~' ';l'\ -.' \J . ---- .. '< ~ \ ~c>( o ~ /r: c HILLS]' ::; IP. ~ p;- 38 0 15'1- -- ST~BM~<# _ ,:=', S· ~ ~ ::; 0- :t. n ~ ' "" '-D o 00 It..·./ Miocene rhyolite flows VJ 38°00'L~ ,\~ (age uncertain)

Miocene(?) granite, quartz monzonite ~-<..

Figure 6. Generalized distribution of faults and Miocene igneous rocks in the central part of the Pioche-Marysvale igneous belt. IP, Indian Peak; CS, Cougar Spar mine; SM, Steamboat Mountain; PG, Pine Grove; ST, Staats mine; BM, Blawn Mountain; BW, Blawn Wash; BLM, Blue Mountain; CM, Cima mine; BR, Broken Ridge; BSFZ, Bible Spring fault zone; TT, The Tetons. +>­ VJ 44 Utah Geological and Mineral Survey Special Studies 61,1983

lique to the overall trend of the Pioche-Marysvale 32.0 32.0 Junction of gravel road to belt because of a more northerly strike (Figure 1). Lund. STOP 1. Overview of A crude en echelon pattern is apparent. Although Needle Range (also referred to no unequivocal strike-slip displacement of map as the Mountain Home Range units has been found, several well-exposed sub­ at its northern end and the vertical fault surfaces have strongly developed sub­ Indian Peak Range farther horizontal slickensides. These are common along south). The rocks comprise a the Bible Spring fault zone in the low hills linking complexly faulted but generally the southern termini of the Needle Range and Wah east-tilted sequence of Oligo­ Wah Mountains. The emplacement of the cene ash-flow tuffs and Paleo­ northeast-striking early Miocene rhyolite dikes zoic sedimentary rocks. Con­ around the Cinna mine in the Wah Wah Mountains tinue south and west on gravel and the two swarms near the Cougar Spar mine in road toward Lund. the Needle Range indicates a northwest-southeast direction of extension. 4.6 40.2 Intersection. Continue south­ The age of the northeast faulting is well­ westerly toward Sawtooth constrained. Oligocene volcanic rocks and overlying Peak, not south down Pine older members of the Blawn Formation are cut and Valley. tilted whereas 13 to 12 m.y. old rhyolites of the Steamboat Mountain Formation are generally not 11.2 51.3 Intersection southeast of Saw­ faulted and lie unconformably on top of the older tooth Peak; continue west. rocks. At The Tetons the constraints are even tight­ er because unfaulted 18 m.y. old Blawn rhyolite 2.7 54.0 Vance Spring. Take second rests unconformably on tilted and eroded Oligocene road (the one just past corral) units; older Blawn units as well as the 22 m.y. old heading northwest and then Bauers Tuff are tilted in fault slices 4 km to the north. southeast (Figure 1). Additional evidence for early Miocene block 2.6 56.6 Intersection. STOP 2. Walk faulting are deposits of chaotic megabreccia and as­ about 230 m northeast along sociated coarse alluvial deposits that occur along the road and then view outcrops northeast striking Bible Spring fault zone and con­ along low hill just south of and tain abundant clasts of 22 m.y. old Bauers Tuff. paralleling road back toward These deposits, whose total area of exposure is intersection. First exposures 2 about 4 km , appear to represent landslide debris are in the upper part of the Cot­ that sloughed off uplifted fault blocks. The deposits tonwood Wash Tuff, the basal underlie virtually unfaulted, subhorizontal, 13 to 12 formation of the Needles m.y. old rhyolite lava flows of the Steamboat Moun­ Range Group (Figure 7). This tain Formation. Locally, as at the northeast end of lapilli ash-flow tuff is usually the Bible Spring fault zone, altered Steamboat red-brown to pink except for a Mountain rhyolite flows are cut by faults that might local gray vitrophyre near the be reactivated early Miocene fractures. base of the sheet. Here, however, the upper part of the FIELD TRIP ROAD LOG: FIRST DAY deposit is gray, probably be­ Stops will be made in the Needle Range to exam­ cause of slight hydrothermal ine Oligocene volcanic rocks, chiefly regional ash­ activity along nearby faults. flow tuffs. Log begins in Milford at the junction of Nonetheless, the crystal-rich State Highways 257 and 21. Proceed west on High­ character of the Needles Range way 21. tuffs is obvious; about one-half Mileage Description of the phenocrysts are plagio­ Incre- Cumu­ clase. In the Cottonwood Wash mental lative Tuff large euhedral books of 0.0 0.0 Milford. Head west on Highway biotite several millimeters 21. across are abundant. A few per- Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 45

tensity of welding diminishes until just south of the road in­ tersection the tuff is only moderately welded and rather Tw porous; light-colored devitri­ fied pumice lapilli and biotite phenocrysts are conspicuous. Turn around and drive south back towards Vance Spring.

0.5 57.1 STOP 3. Along roadside are o mile exposures of gray Lund Tuff, I I the upper-most of the three o kilometer 6 Wah Wah Springs Tuff principal sheets of the Needles Outflow Member Range Group (Figure 7). Here, outside the Indian Peak Isom Formation Cottonwood Wash Tuff caldera, the Lund is only about 30 m thick and is weakly o ~ welded. In addition to the dom­ Lund Tuff pre-Needles Range Tuffs inant plagioclase, other pheno­ crysts include pale amethyst Figure 7. Geologic map of area around STOPS 2 and 3, Sees. quartz and biotite. Hornblende 4 and 5, R. 18 W., T. 28 S. (Best, 1976; Best and Hintze, is less abundant. Brown sphene 1980) . occurs in trace amounts. A cent of large, embayed, and short distance to the west, broken quartz phenocrysts are dark-colored rubble is all that obvious. Much smaller horn­ remains of a thin ash-flow blendes are generally visible sheet of the 25 m.y. old Isom only in thin sections and green Formation. augite occurs in trace amounts. Continue south on road The first conspicuous exposure toward Vance Spring. of the overlying Wah Wah Spr­ ings Formation of the Needles 2.1 59.2 Intersection at Vance Spring. Range Group is a black to dark Depending upon road condi­ gray vitrophyre a few meters tions and snow cover, proceed thick produced by intense either to stops 4 and 5 or to al­ welding near the base of the ternate stop A4. ash-flow deposit. In this area, To reach stop 4, turn south which is about 12 km north of at intersection and proceed its source caldera, the Wah toward Ryan Spring. Wah Springs tuff is 300 to 500 m thick. In the overlying red­ 2.7 61.9 Intersection; continue straight brown and slightly devitrified (west) . tuff the characteristic phe­ nocryst assemblage is easily 1.9 63.8 Ryan Spring. STOP 4. Exposed seen. Plagioclase, hornblende, here is a thick section, totaling and biotite occur in order of de­ some 1200 m, of ash-flow tuffs creasing abundance. Augite that were deposited within the and quartz phenocrysts less Indian Peak caldera after its than a millimeter across are collapse during catastrophic minor. Upwards in the section expulsion of the Wah Wah toward the southwest the in- Springs Formation (Figure 8). 46 Utah Geological and Mineral Survey Special Studies 61, 1983

~ sandstone ~ Isom Formation

andesite flow Tc EJ Lund Tuff

o R;~ III Ryan Spring Tuff Q. m...., III G dacite flow : ..... :'·.Tw:: [3]...... Wah Wah Springs Tuff, in breccia sequence

25 ---L-. I~ Tb; ~I ' I - / Cambrian TI brecciated Cotton- I wood Wash Tuff j sed. roc k s

I".T~O. :.1 ------.------f Wah Wah Springs Tuff, Outflow Mbr. [EJ Cottonwood Wash Tuff 8 T28~18W Escalante Desert Fm. R 19WjT29S and minor Paleozoic rocks (mostly brecciated)

Tbe Qa /,.~j.. !:..:.= .. ~ o mile ...~-----.--.- - '. I Tbc I --~ (- Ordovician I \ o km '---- \ ..

Figure 8. Geologic map around Ryan Spring STOP 4 showing the topographic wall of the Indian Peak caldera and 1200 m thick caldera-fill sequence (Best, 1976; Best, Grant, and Holmes, 1979). Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 47

This section lies on a complex tuff is densely welded for hund­ sequence, as much as 800 m reds of meters. thick, of cataclastic and chiefly Continue down road mono lithologic breccias of pre­ (southwest) through the wet Wah Wah Springs volcanic spring area. rocks with intercalated wedges of Wah Wah Springs tuff. 1.1 64.9 Turn left (southeast) at During catastrophic eruption in tersection. of thousands of cubic kilome­ ters of Wah Wah Springs tuffs, 1.4 66.3 Fork in road; continue straight sUBsidence occurred along the (east) . ring fault of the caldera 4 km south of Ryan Spring. Ap­ 1.5 67.8 STOP 5. Cataclastic, variably parently, subsidence was so altered, monolithologic brec­ great (> 3 km) that gravity­ cias of the Cottonwood Wash driven fault slices (or large Tuff along the road are overlain landslides) caved into the by several meters of Wah Wah deepening depression from the Springs tuff vitrophyre, then unstable caldera wall, extend­ by more Cottonwood Wash ing the topographic rim of the breccia and finally by the basal caldera 10 km northward from member of the Ryan Spring the initial scarp of the ring fault Tuff (Figure 9). The thick (Figure 5). Discontinuous Wah Wah Springs vitrophyre wedges of Wah Wah Springs here is either 1) a fault slice de­ tuffs, some similar to that oc­ rived from a thick section to curring as the Outflow Mem­ the north and transported ber to the north, lying within here, or 2) a remnant of a the sequence of breccias sug­ much thicker in situ deposit gest continued eruption during that was welded, compacted, subsidence. and then decapitated by subse­ The Ryan Spring Tuff overly­ quent faulting/landsliding, or ing the sequence of breccias in­ 3) an in situ thin deposit that cludes crystal-poor and locally was welded and compacted by lithic, rhyolitic ash-flow tuffs immediate burial beneath the with minor volcanic sandstones overlying Cottonwood Wash and debris flow deposits. Plagi­ breccia deposit. In any case, oclase phenocrysts comprise 2 there is evidence here for trans­ to 15 percent and biotite 1 to-2 port of fault/landslide masses. percent of the tuffs. The lower To the southeast about 900 tuff member has conspicuous m is a spectacular exposure of light-colored haloes surround­ brecciated pre-Wah Wah ing voids in the matrix. Springs volcanic rocks with a The overlying 28 m.y. old subhorizontal slickensided sur­ Lund Tuff is a compound cool­ face above cataclastic layering. ing unit that is 600 m thick Continue on road. here, compared to only 30 m at Stop 3 just north of the 0.2 68.0 Intersection; turn right topographic rim of the caldera. (south) . Because of this much greater thickness there is a well­ developed basal vitrophyre 0.9 68.7 Intersection; turn left (east). several meters thick and the overlying red-brown divitrified 0.6 69.3 Road forks; continue up ridge. 48 Utah Geological and Mineral Survey Special Studies 61, 1983

o mile 1 I I I o km \ Tbc ~ / ...... ,.,,'. : ", '.. ~

'. ," Te '" . ~ ~----~II \\ II II \1 \1 "Ti::" \\ \\ \\ 1217 1"3T18 -?

-, ,- ..... ~ .' . .

",":',':. : -> .T/.'

~Wi •

k.\T!"_~:_:,] Lund Tuff EEJ Ryan Spring Tuff Wah Wah Springs Tuff.lntracaldera Member II, Tb-/i / ..... ~ breccias of pre-Wah Wah Springs ~ Wah Wah Springs Tuff, undivided volcanic rocks ~ Wah Wah Springs Tuff, Intrusive Escalante Desert Formation Porphyry Member

Figure 9. Geologic map of ring fault of the Indian Peak caldera STOPS 4A, 5 and 6 (Best, Grant, and Holmes, 1979). Northwest corner of map joins with southeast corner in Figure 8 map. Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 49

1.4 70.7 Intersection in saddle. STOP Return to Highway 21 and 6. Ring fault of Indian Peak Milford. caldera (Figure 9). Pre-Wah To reach alternate Stop A4 Wah Springs tuffs to the north from Vance Spring, continue are juxtaposed against caldera­ on road going east that passes fill rocks on the south. The just south of Sawtooth Peak. caldera-fill sequence exposed 2.7 73.4 Intersection southeast of Saw­ here comprises southward tooth Peak overlooking Pine thinning wedges of non­ Valley. Turn right (south). cataclastic volcanic breccias tliat interfinger with tuffs of 0.6 74.0 Road divides; continue south­ the Intracaldera Member of east. the Wah Wah Springs Forma­ tion. The wedges of breccia are 0.4 74.4 Intersection; continue straight essentially monolithologic ac­ (east) . cumulations of Cottonwood Wash Tuff and some other fa­ 0.3 74.7 Road divides; turn right miliar pre-Wah Wah Springs (south) . volcanic rocks that originated as landslide masses caving 0.5 75.2 Road comes in from right from the unstable wall of the (west); continue straight ahead caldera. These breccias are not (southeast) . cataclastic as are those north of the ring fault. The Intracaldera 4.5 79.7 Major intersection; turn right Member of the Wah Wah Spr­ (west) . ings Formation is a compound cooling unit in which zones of 4.0 83.7 Road divides; turn right (west). black vitrophyre alternate with 3.6 87.3 ALTERNATE STOP A4. orange-brown to gray-green View of Lund and Ryan Spring devitrified but still densely Tuffs that filled the topographi­ welded tuff. The Intracaldera cally enlarged caldera depres­ Member varies petrographical­ sion (Figure 9). The road for ly within the exposed area of the past several miles has the caldera. In places, it has a passed through the Lund Tuff. higher concentration of quartz The exact thickness of this phenocrysts than the Outflow compound cooling unit here is Member. Generally, it contains indeterminate because of fault­ lapilli-size lithic fragments and ing but is probably on the order locally these are cognate clasts of 700 m. Beneath the Lund of texturally different Wah Tuff are two members of the Wah Springs rock. Locally, Ryan Spring Tuff. near the ring fault some acci­ Continue on road to west. dental clasts of older rock are several meters across and com­ 0.6 87.9 Intersection; proceed right prise most of an outcrop. (west) through gate to top of Shows of Cu-mineralization ridge. occur along the ring fault 4 km to the west where dikes of an 0.7 88.6 Intersection. STOP 6. See intrusive porphyry composi­ mile 70.7 tionally similar to the Wah Wah Springs tuffs have invaded FIELD TRIP ROAD LOG: SECOND DAY Paleozoic carbonate rocks a­ long the caldera wall. Stops will be made in the southern Wah Wah 50 Utah Geological and Mineral Survey Special Studies 61,1983

~ turn left (south) across cattle Blawn Formation guard. Upper Tuff Member 0.9 38.3 Intersection; continue straight o (southwest) . Lund Tuff 0.9 39.2 Intersection; continue straight (south) . Escalante Desert Formation 0.2 39.4 Road divides; turn left o (southeast) . I Paleozoic rocks o km 0.9 41.5 Road divides; turn left (east).

Figure 10. Geologic map of area around the Cinna mine STOP 7, 0.2 Old building. STOP 7. Walk mostly in Sec. 5, R. 15 W., T. 31 S. (Morris and others, 1982). 41.7 down dry wash to south to view a section through the 23 Mountains to examine bimodal associations of sili­ to 18 m.y. old tuff member of cic and mafic igneous rocks of early and middle Mio­ the Blawn Formation (Figure cene age. Log begins just north of Minersville at the 10). The exact source of the py­ junction of Highways 21 and 130. roclastic material exposed here is uncertain as several vent Mileage Discription areas existed in the southern Incre­ Cumu­ Wah Wah Mountains during mental lative the early Miocene. At the top 0.0 0.0 Head south on Highway 130. of the section at the northeast end of the wash are ash-flow 0.2 0.2 Intersection with first road lapilli tuffs with intervening south of Beaver River. Turn thin layers of reversely graded right (west) . air-fall or surge material. The. pyroclastic material is chiefly 0.4 0.6 Intersection; turn right (north) vitric with only minor amounts and drive around corner past of quartz, biotite, and sanidine Minersville Cow Palace Associ­ phenocrysts; xenoliths of Lund ation heading west. Tuff and flow-layered rhyolite are rare. Lapilli of green perlite 1.3 1.9 Main road veers right (north­ in the uppermost part of the west) ; continue straight (west) . section suggest it may be as young as 12 m.y. because the 11.2 13.1 Union Pacific railroad tracks. only other green perlite occur­ rences in the region lie a few kilometers to the southwest in 3.2 16.3 Road joins in from right; pro­ Broken Ridge where such ceed left (southwest). K -Ar ages have been deter­ mined. Farther down the wash 1.5 17.8 Intersection. Turn right (north­ and down section, following an west) . interval of no exposure, is a meter-thick air-fall layer of 13.7 31.5 Road JOIns in from left pumice lapilli that has a subtle (southeast); head right normal gradation in clast size. (northwest) . At the bottom of the section is another vitric ash-flow tuff. 5.9 37.4 Intersection with minor road; Return to old building and Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 51

then proceed east along road past old mercury retorts to Cinna mine. Brown, iron­ stained jasperoid north of the road is silicified lapilli tuff of the Blawn Formation. A major high-angle fault separates [GJ these rocks from Paleozoic car­ Needles Range Group bonate rocks to the south and ~ Tn Escalante Desert east. Shallow-seated hydrother­ Formation mal activity probably manifest [iJ as hot springs at the surface Paleozoic rocks was localized along this fault. Veins of gypsum carry disse­ minated native sulfur, cinnabar -1- (gray-black on weathered Tn surfaces) and jarosite. The age ,~.~~'. Qa of high-angle northeast-strik­ ~iTn' ing block faulting is bracketed Qa Tbo between 22 and 12 m.y., and at o /mile ';'~'~'~~~,.:;' least locally between 22 and 18 35" I o km m.y. To the southwest of here, Qa the Bible Spring fault zone of Figure 11. Geologic map of The Tetons area STOP 8 (Morris tilted horst and graben slices and others, 1982). underlies undeformed 12 m.y. rhyolite of the Steamboat southeast at the top of this Mountain Formation. The faulted and tilted section is the faulted rocks include the 22 22 m.y. old Bauers Tuff. The m.y. old Bauers Tuff. North of age of faulting, therefore, lies here, The Tetons are remnants between 22 and 18 m.y. of 18 m.y. rhyolite lying un­ The vitric tuff in the saddle comformably on tilted Oligo­ between The Tetons contains cene rocks (see Stop 8 and minor feldspar crystals and Figure 11). The time of hy­ abundant clasts up to a meter drothermal activity here at the in diameter of the underlying Cinna mine could be about Lund Tuff. This is not a welded 20 ± m.y. with a 12 m.y. ash-flow or air-fall tuff as the overprint, but no radiometric vitric particles are neither data are available. pumice nore curved sliver-like shards of discintegrated 4.3 46.0 Retrace route back to main pumice. The nonvesicular char­ gravel road; turn left (west) . acter of the vitric clasts possibly indicates derivation by explo­ 3.0 49.0 Intersection with jeep trail; sive decrepitation of a glassy turn right (northeast) . dome that encountered water near the surface. 2.1 51.1 End of road between The Upper parts of the tuff, as on Tetons. STOP 8. Rhyolite tuff the flank of the hill to the west, and capping rhyolite lava flow are argillized. The overlying with a K-Ar age of 18.3 ± 0.7 flow-layered rhyolite is virtual­ unconformably overlie tilted ly aphyric with only sparse Oligocene volcanic rocks (Fig­ small phenocrysts of quartz ure 11). Four kilometers to the and sanidine. Topaz and fluor- 52 Utah Geological and Mineral Survey Special Studies 61, 1983

\. -/ \ 1/

;<13°36' 4 km a o mile I I Phase Five Smoky Quartz o km lIJ [8J Phase Four ~ Lost Revenue Pine Grove Porphyry Pine Grove GJ Sheet Silicate I' .. ' . • .. 1 i ,Tee I East End Porphyry !. ' .. ' ,'.: GfJ East End Figure 12. Geologic map and schematic east-west cross section of the Pine Grove volcanic center STOPS 9, 10, and 11 (Abbott and others, 1981; Keith, 1982). ite are very rare vapor-phase ite, montmorillonite, and car­ crystals in vugs. bonate minerals. Relict phe­ nocrysts of quartz, alkali 2.1 53.2 Return to main gravel road and feldspar, plagioclase, biotite, turn right (north) ; continue and garnet lie in a felsic matrix. into Pine Valley. Nine hundred to more than 6.0 59.2 Intersection with main valley 2,000 m below the surface lies road; turn right (north) . a disseminated Mo-Wore de­ posit explored and evaluated 12.2 71.4 Intersection on east side of by Pine Grove Associates, a wash; continue east toward joint venture of Phelps-Dodge Wah Wah Mountains. Corporation and Getty Oil l.5 72.9 Road joins in from left; turn Company. right (southeast). The Pine Grove Porphyry is but one of the seven silicic 5.0 77.9 STOP 9. A small porphyry sill porphyry bodies in the Pine about 1 m thick was intruded Grove volcanic center (Figure into the uppermost part of the 12). Though mostly concealed Cambrian Prospect Mountain and strongly altered at the Quartzite (Figure 12). surface, these bodies have been encountered in over 30 0.4 78.3 STOP 10. Principal exposures km of drill core and can be dis­ of the largely concealed Pine tinguished on the basis of con­ Grove Porphyry body (Figure trasting textures and modal 12). The rock here is moderate­ and bulk chemical composi­ ly to highly altered to an assem­ tions (Keith, 1982). The East blage of quartz and sericite End Porphyry is a vent-facies with locally abundant kaolin- deposit formed by early ash- Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 53

flow eruptions (see Stop 12). ~ i Tbm 1 Subsequently, two dome­ Blawn Formation, Mafic Flow Member (includes forming bodies, the Pine talus mantles) Grove and Smoky Quartz Por­ phyries, were intruded. Shortly E Blawn Formation, after formation of this compo­ Garnetiferous Tuff site dome, cobbles of rhyolite Member porphyry were eroded and are ~~ now found exposed in a thin 150m Formation conglomerate layer in Blawn Wash about 14 km to the EJ Needles Range Group southeast. Wallaces Peak Tuff Two intrusive bodies that produced Mo-mineralization, Phase Four and Phase Five Porphryies (Figure 12), differ Figure 13. Geologic map of a part of the east t1ank of the Wah in several important aspects. Wah Mountains around STOP 12 (Abbott and others, 1981). Phase Four Porphyry has a transitional upper contact with ment of Phase Five Porphyry Pine Grove Porphyry, euhedral and the dominant episode of quartz phenocrysts, a low alkali mineralization. feldspar/plagioclase ratio, a Post-mineralization events relatively coarse matrix, and include intrusion of a few nar­ weak mineralization. Phase row trachyandesite(?) dikes Five Porphyry exhibits broken and eruption of trachyandesite quartz phenocrysts, a high al­ (K-rich mafic) lavas of the kali feldspar/plagioclase ratio, Blawn Formation across the a fine-grained matrix, and region (unit Tbm, Figure 13). much stronger mineralization. A few post mineralization rhyo­ Contemporaneous emplace­ lite dikes cross-cut Pine Grove ment of rhyolite and dacite and are probably an unrelated porphyry occurred during erup­ younger event. tion of the ash-flow tuff se­ 0.1 78.4 Take right fork (southeast). quence and also between intru­ sion of Phase Four and Phase 0.2 78.6 STOP 11. East End Porphyry. Five Porphyries. The rhyolitic This rock contains phenocrysts phase of the mixed intrusitons of quartz, sanidine, plagioclase is designated Lost Revenue and very minor amounts of Porphyry and the dacitic phase biotite, hornblende, and garnet is Sheet Silicate Porphyry. Mix­ in an altered felsic matrix. tures of these two units are Lithic fragments include common. Both units exhibit quartzite, shale, and limestone. moderately broken quartz phe­ That this rock body is the py­ nocrysts and have similar roclastic filling of a volcanic matrix crystal sizes. Textural vent, rather than an intrusion, evidence suggests that the is indicated by the presence of second episode of emplace­ locally significant amounts of ment of Lost Revenue and moderately flattened pumice Sheet Silicate Porphyry proba­ lapilli that are now altered to bly occurred coincident with in­ clay minerals (Keith, 1982) ; creased convection and very rarely the body is still volatile exsolution rates im­ glassy and spherulitic. Drill­ mediately prior to emplace- hole information indicate the 54 Utah Geological and Mineral Survey SpeCial Studies 61, 1983

body is rootless, resting on the 7.4, MgO 3.9, CaO 6.0, Na 0 2 flared, funnel-shaped sides of 20S 2.9, K20 3.3, P 0.3. Other the vent and cut off by the samples have Si0 as low as 57 2 dome-forming porphyries (Fig­ and K 0 up to 4.0. 2 ure 12). Initial plinian eruptions from The East End body also con­ the Pine Grove magma system tains fragments of rhyolite and deposited up to 4 m of poorly dacite porphyry - probably sorted and non-welded air-fall pieces of the Lost Revenue ash, lapilli, and lithic blocks. In and Sheet Silicate units. A few addi tion to glass shards and fragments appear to have been pumice there are crystals of quite ductile when incor­ quartz, sandine, and plagioclase porated, suggesting they might with minor to trace amounts of represent bits of magma spatter biotite, garnet, titanomagne­ ejected along with the East End tite, zircon, monazite, and pyroclasts. xenotime. Overlying these 0.2 78.8 Return to main Pine Grove deposits are lapilli ash-flow road and turn right (east). Con­ tuffs of similar composition tinue over divide to east flank that are locally over 150 m of range. thick. Lithic fragments include Isom and Needles Range tuffs, 7.0 85.8 STOP 12. Garnet-bearing Tuff phyllitic shale, quartzite, and Member of Blawn Formation limestone. In these tuffs, the (Figure 13). Ascending the hill proportions of biotite and to the south of the road the sec­ garnet vary and near the mid­ tion begins in the Wallaces dle of the sequence there are Peak Tuff, a local ash-flow trace amounts of hornblende, sheet in the Needles Range augite, apatite, and ilmenite. Group that lies above the Lund Despite the fact that the Tuff. It is overlain by the 25 Pine Grove intrusions (Stops m.y. old Isom Formation 9-11) have eroded approxi­ which here comprises three mately 1.7 km from the early cooling units; this tuff is a Miocene paleo-surface (Figure crystal-poor vitric tuff with less 12) correlation with the py­ than 10 percent phenocyrsts of roclastic deposits exposed at plagioclase and minor augite. this stop is made possible by Overlying the Isom Formation the presence of unique acces­ is the garnet-bearing Blawn sory garnet in all rhyolitic in­ tuff upon which lies the K­ trusive and extrusive units rich-Mafic Flow Member of (Keith, 1982). Microprobe the Blawn Formation. In these analysis of garnets from ash­ lavas, phenocrysts of augite, flow and intrusive units dem­ hypersthene, plagioclase, and onstrates that both are yttrium­ minor altered olivine lie in a rich almandine-spessartine gar­ black glassy matrix. A sample nets of identical chemical from across the canyon north composition. In addition, of here gave a whole-rock included within garnets from K-Ar age of 23.2 ± 1 m.y. both units are titanomagnetite (Best and others, 1980). grains of identical composi­ Chemical analysis of the same tion. Contained within the sample yielded (in weight garnet and titanomagnetite are percent) Si0 60.5, Ti0 1.0, small crystals of U -Th -bearing 2 2 A1 0 15.9, Fe 0 (total Fe) monazite, zircon, and xeno- 2 3 2 3 Guidebook, Part 3 - GSA Rocky Mountain and Cordilleran Sections Meeting, 1983 55

time. This sequence of succes­ 1035-1050. sively included mineral phases Best, M. G., Mehnert, H. H., Keith, J. D., and Naeser, C. of identical composition seems W., 1983, Miocene magmatism and tectonism in and to be conclusive evidence of near the southern Wah Wah Mountains, southwes­ comagmatic origin for the ash­ tern Utah: U.S. Geological Survey Bulletin (in press). flow tuffs and Pine Grove in­ Campbell, D. R., 1978, Stratigraphy of pre-Needles Range Formation ash-flow tuffs in the northern trusive units. Microprobe ana­ Needle Range and southern Wah Wah Mountains, lyses of other minerals and Beaver County, Utah: Brigham Young University whole-rock major and trace ele­ Geology Studies, v. 25, p. 31-46. mept analyses of both intrusive Cook, E. F., 1965, Stratigraphy of Tertiary volcanic rocks and extrusive units also sub­ in eastern Nevada: Nevada Bureau of Mines Report stantiate this correlation. 1l,61p. In addition, the pyroclastic Fleck, R. J., Anderson, J. J., and Rowley, P. D., 1975, deposits have a distribution Chronology of mid-Tertiary volcanism in High Pla­ and thickness that is compati­ teaus region of Utah: Geological Society of America ble with venting of the Pine Special Paper 160, p. 53-6l. Grove magma system. The tuff Grant, S. K., 1978, Stratigraphic relations of the Escalante Desert Formation near Lund, Utah: Brigham Young rests on 25 m.y. old Isom For­ University Geology Studies, v. 25, p. 27-30. mation and is capped by Gromme', C. S., McKee, E. H., and Blake, M. c., Jr., trachyandesite flows with an 1972, Paleomagnetic correlations and potassium­ age of 23 m.y. (Best and argon dating of middle-Tertiary ash-flow sheets in others, 1980). The age of the the eastern Great Basin, Nevada and Utah: Geologi­ tuff bracketed by these two cal Society of America Bulletin, v. 83, p. 1619-1638. units agrees with the K-Ar age Keith, J. D., 1982, Magmatic evolution of the Pine Grove of 24 m.y. from Pine Grove porphyry molybdenum system, southwestern Utah: Porphyry (Abbott and others, Unpublished Ph.D. dissertation, University of 1981) . Wisconsin. Mackin, J. H., 1960,' Structural significance of Tertiary volcanic rocks in southwestern Utah: American Jour­ nal of Science, v. 258, p. 81-13l. REFERENCES CITED Morris, H. T., Best, M. G., Kopf, R. W., and Keith, J. D., Abbott, J. T., Best, M. G., and Morris, H. T., 1981, Pre­ 1982, Preliminary geologic map and cross sections of liminary geologic map and cross sections of the Pine The Tetons quadrangle and adjacent part of the Ob­ Grove-Blawn Mountain area, Beaver County, Utah: servation Knoll quadrangle, Beaver and Iron U.S. Geological Survey Open-File Report 81-525. counties, Utah: U.S. Geological Survey Open-File Best, M. G., 1976, Geologic map of the Lopers Spring Report 82-778. Quadrangle, Beaver County, Utah: U.S. Geological Rowley, P. D., Lipman, P. W., Mehnert, H. H., Lindsey, Survey Miscellaneous Field Studies Map MF-739. D. A., and Anderson, J. J., 1978, Blue ribbon Best, M. G., Shuey, R. T., Caskey, C. F., and Grant, S. lineament, an east trending strucutral zone within K., 1973, Stratigraphic relations of members of the the Pioche mineral belt of southwestern Utah and Needles Range Formation at the type localities in eastern Nevada: Journal of Research, U.S. Geological southwestern Utah: Geological Society of America Survey, v. 6, p. 175-192. Bulletin, v. 84, p. 3269-3278. Shuey, R. T., Caskey, C. F., and Best, M. G., 1976, Distri­ Best, M. G., Grant, S. K., and Holmes, R. D., 1979, Geo­ bution and paleomagnetism of the Needles Range logic map of the Miners Cabin Wash and Buckhorn Formation, Utah and Nevada: American Journal of Spring quadrangles, Beaver County, Utah: U.S. Science, v. 276, p. 954-968. Geological Survey Open-File Report 79-1612. Stewart, J. H., Moore, W. J., and Zietz, I., 1977, East-west Best, M. G., and Hintze, L. F., 1980, Geologic map of the patterns of Cenozoic igneous rocks, aeromagnetic Sawtooth Peak Quadrangle, Beaver County, Utah: anomalies, and mineral deposits, Nevada and Utah: U.S. Geological Survey Miscellaneous Field Studies Geological Society of America Bulletin, v. 88, p. Map MF-1l52. 67-77. Best, M. G., McKee, E. H., and Damon, P. E., 1980, Williams, P. L., 1967, Stratigraphy and petrography of the Space-time-composition patterns of late Cenozoic Quichipa Group, southwestern Utah and southeas­ mafic volcanism, southwestern Utah and adjoining tern Nevada: Unpublished Ph.D. dissertation, Uni­ areas: American Journal of Science, v. 280, p. versity of Washington. UTAH GEOLOGICAL AND MINERAL SURVEY

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