UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

Geologic Map of the Pinto Spring and

Part of the Atchison Creek Quadrangles,

Beaver and Iron Counties, Utah

By 

S. Kerry Grant and Myron G. Best

Open-File Report 79-1656 

1979

This report is preliminary and has not been

edited or reviewed for conformity with U.S.

Geological Survey standards.
DESCRIPTION OF MAP UNITS

ALLTTVIUM AND COLLUVIUM (OUATEPNARV) Unconsolidated, poorly 

sorted stream, fan, and slope-wash deposits of gravel, 

sand, and silt; includes moderately resistant older 

gravels, some of which are composed of boulders 10 m in 

diameter along Cottonwood Creek; also includes hummocky 

patches of rhyolite debris of possible mass-wastage origin 

on the flanks of hills capped by rhyolite; presumably less 

than 100 m thick on the flanks of the Indian Peak range, 

thickening toward the valleys on either side

RHYOLITE OF COTTONWOOD CREEK (MIOCENE)

Thick flow-folded lava flow Consists of gray, strongly 

porphyritic rock whose phenocrysts include large sanidine 

(10 mm) and quartz with less abundant smaller hornblende 

and plagioclase. Dark vitrophyre and lithophysal rock with 

fewer phenocrysts occurs along the faulted southern margin 

Feeder dike

RHYOLITE OF PINTO CREEK (MIOCENE) Resistant lava flows and 

dikes. Flows are pink with phenocrysts of smokey quartz, 

sanidine, plagioclase, and minor biotite, constituting 

15-25 percent of the rock. Groundmass is commonly blotchy 

due to late-stage differential crystallization. Secondary 

fluorite locally present. Vitrophyre and flow-breccia as 

much as 50 m thick form the base of the flows. A lithicrich, orange-pink ash-flow tuff, as thick as 100 m, 

underlies the lava flows in the southern and western parts 

of the quadrangle; this tuff contains 20 percent crystals 

of quartz, sanidine, plagioclase, and biotite. Thickness 

of flows is several hundred meters. Dikes are white, pale 

gray, tan and pink, flow foliated, aphanitic to coarsely 

porphyritic with 25 percent phenocrysts as large as 10 mm 

of quartz, sanidine, and plagioclase, and smaller altered 

mica and ubiquitous limonite pseudomorphs after pyrite. 

Gray to greenish-gray quartz latite, containing quartz and 

feldspar xenocrysts, occurs locally along dike margins. 

Dikes are commonly ^-10 m thick

PLUG COMPLEX (MIOCENE OR OLIGOCENE?) Resistant intrusions of 

black to greenish-gray quartz latite with steep flowfoliation and subhorizontal columnar jointing. Most of the 

rock has 10-15 percent phenocrysts of plagioclase, biotite, 

hornblende, and pyroxene. The margins of the eastern plug 

are brecciated and filled with light-greenish-gray quartz 

latite having larger and more abundant hornblende. The 

latite of the western plug contains fragments of quartz, 

feldspar, granite, and banded aplite as large as 2 m across 

concentrated along its margin. Also present in the western 

complex is a holocrystalline, nearly aphanitic rhyodacite 

containing xenocrysts of quartz and feldspar
VOLCANIC ROCKS OF COMMISSARY CPEEK (MIOCENE) A sequence of 

ash-flow tuffs, lava flows, and volcaniclastic sediments. 

The tuffs occur throughout the sequence and are generally 

pale pink and lithic bearing, with as much as 20 percent 

phenocrysts of quartz, sanidine, and biotite; one of the 

tuffs contains garnet. Most of the tuffs are loosely 

consolidated, but one near the middle of the sequence is 

firmly welded and grades from pale purple-brown tuff in the 

upper part downward through orange brown to dark olive gray 

in the basal vitrophyre. Phenocrysts comprise 10 percent 

of this welded tuff with sanidine most abundant in the 

upper part, and plagioclase most abundant in the lower 

part. Biotite is a minor constituent throughout. This 

welded tuff probably correlates with the tuff of Leigh Well 

in the Lund Ouadrangle in the west. Lava flows near the 

top of the sequence consist of a widespread red-brown 

rhyodacite containing phenocrysts of plagioclase and 

pyroxene and xenocrysts of quartz and hornblende, underlain 

by dark-brown-red foliated flow with 10-15 percent 

phenocrysts of plagioclase, biotite, and hornblende; pods 

and stringers of secondary chalcedony follow the flow 

foliation. Volcaniclastic beds of chalky volcanic 

sandstone, locally containing alunite south of the Cougar 

Spar mine, lie at the top of the unit, and rust-colored 

breccia lies at the base. The basal breccia has clasts, as 

much as 20 cm in diameter, of the flows in the unit Tva. 

The entire unit is several hundred meters thick

VOLCANIC ROCKS OF ATCHISON CREEK (OLICOCENE) Light- to mediumgray, strongly foliated flows or flow domes of quartz 

latite containing 5-15 percent phenocrysts of plagioclase, 

biotite, and conspicuous hornblende. Locally present are 

zones of autobreccia, black vitrophyre and, near the top of 

the unit, purple-red to brown ash-flow tuff which resembles 

tuffs in the Isom Formation. A dark purple-red to brown, 

finer-grained flow with strong foliation parting and sparse 

quartz xenocrysts occurs in the northwest corner of sec. 

17, R.18 W., T. 30 S., and a light-colored ash-flow tuff 

containing 5-10 percent quartz, sanidine, and biotite lies 

about 3 km farther northwest. Thickness of the entire unit 

is at least 500 m

ISOM FORMATION (OLIROCENE) Densely welded, massive to strongly 

foliated vitric tuffs in a variety of colors, including 

red, purple, orange, and gray. Phenocrysts of plagioclase 

and minor altered pyroxene comprise 5-12 percent of the 

rock. Compressed pumice lapilli are locally filled with 

vapor-phase minerals. Unit includes a lava flow containing 

larger and more abundant phenocrysts of plagioclase and 

pyroxene. Both tuffs and flow have tubular alined vesicles 

and both weather to plates or grus. As thick as 300 m. 

Age, 25 m.y. (Fleck and others, 1975)
NEEDLES RANGE FORMATION (OLIGOCENE) Sheets of densely to

moderately welded crystal-rich ash-flow tuff. The Needles 

Range can be divided into mappable parts over much of its 

extent, and should be assigned to higher stratigraphic 

rank. Age, 29 m.y. (Fleck and others, 1975) 

Lund Tuff Member Ranges from a pink-gray slightly welded 

top, through a light-red, moderately welded central part, 

to a dark purple-brown near the densely welded base of 

thick sections. Phenocrysts comprise 35-45 percent of the 

tuff and include mainly plagioclase, with prominent quartz, 

biotite, hornblende (conspicuous only near the base of 

thick sections) and traces of sphene and sanidine. Thick 

sections have brown volcanic fragments as much as 10 cm in 

diameter near the top. About 500 m thick in the southeast 

part of the map, but 100-200 m in the northwest

TUFF OF RYAN SPRING (OLIGOCFNE) A sequence of crystal-poor, 

lithic-rich ash-flow tuffs with minor volcanic sandstone 

described by Rauch (1975). Tuffs are pale brown to pink 

gray, moderately welded, with about 10 percent phenocrysts 

of mainly plagioclase with lesser biotite and traces of 

quartz and hornblende. Lithic fragments range from cobble 

size in the north to less than a centimeter elsewhere and 

consist of dark volcanic flow-rock of intermediate 

composition. East of the New Arrowhead mine, tuffs are 

possibly as much as 500 m thick and are overlain by a thin 

volcanic sandstone bed composed of 0.2 mm grains of quartz, 

plagioclase, and glassy rock fragments. Local traces of 

the unit occur at the contact of the Lund and Wah Wah 

Springs Tuff Members where none is shown on the map

WAH WAH SPRINGS TUFF MEMBER OF NEEDLES RANGE FORMATION

(OLIGOCENE) Crystal-rich ash-flow tuffs and a related 

plutonic porphyry containing 25-40 percent phenocrysts of 

mainly plagioclase with lesser hornblende, biotite, quartz, 

and a trace of pyroxene

Nonlithic intracauldron unit Light-gray to moderately welded 

tuff similar to that found between the inner and outer ring 

faults of the Indian Peak cauldron in the Miners Cabin Wash 

and Buckhorn Spring Ouadrangles (Best and others, 1979). 

Lithic fragments are scarce but are more vividly colored 

than in the lithic intracauldron unit; over 100 m thick
Lithic intracauldron unit Olive to brown-gray, densely 

welded ash-flow tuffs filling the Indian Peak cauldron. 

Lithic fragments of dark volcanic rocks, quartzite, and 

lesser argillite constitute as much as 15 percent of some 

intervals; lithic fragments increase in abundance toward 

the intercalated landslide breccias (Tbv) but some parts of 

the unit are nearly lithic free. Most lithics are 1-2 cm ' 

in diameter, but some exceed 2 m near the breccias. Pumice 

lapilli contain larger but fewer crystals and tend to blend 

into the surrounding matrix where welding is extreme. The 

upper part of the unit is purple and tends to fracture 

along the foliation in contrast to the lower portion which 

more commonly breaks across the compaction foliation. 

Hornblende in the upper part of the unit is commonly 

altered to an opaque oxide on the rim and a pale carbonate 

in the core. Some of the tuff near the lenses of breccia 

may have abnormally abundant quartz. High in the section 

near the breccias in the north part of Atchison Creek 

Ouadrangle the tuff is orange with black vitrophyre. This 

member may be as much as 2 km thick

Intrusive porphyry unit Olive-gray quartz monzonite porphyry 

intrusive into the base of the lithic intracauldron unit. 

Phenocrysts are somewhat less abundant, show better crystal 

outlines, and are larger (to 8 mm) in the porphyry than in 

the tuff host rock. The large central intrusive body has 

relatively more crystals than the peripheral bodies which 

have more nearly euhedral phenocrysts. Xenoliths of finegrained granodiorite and of much larger, but less abundant, 

quartzite are distributed throughout the porphyry. The 

intrusions are weakly resistant to erosion because 

widespread propylitic alteration destroyed most of the 

hornblende in the central body
INTRACAULDRON BRECCIA OF VOLCANIC ROCKS, UNDIVIDED

(OLIGOCENE) Breccia derived by caving of unstable cauldron 

walls. Clast components change from north to south in an 

irregular fashion. In the north, clasts of Cottonwood Wash 

Tuff Member of Needles Range Formation (Best and others, 

1973) are dominant. In the central area, lava flows or 

megabreccia blocks (Lipman, 1976) include gray to pink 

flow-foliated rhyolite with 5-10 percent phenocrysts of 

plagioclase, biotite, and hornblende, a slate-gray to darkbrown andesite, and dark-gray porphyry with 30-40 percent 

phenocrysts of plagioclase and pyroxene. In the south a 

light-gray quartz-rich ash-flow tuff comprises all clasts 

in one part of a breccia body and another part consists 

entirely of clasts of a darker-toned tuff with less quartz; 

the quartz-rich tuff may be of the Sawtooth Peak Formation 

(Conrad, 1969) and the darker tuff may correlate with a 

tuff underlying the Escalante Desert Formation in the Lund 

Quadrangle (Grant and Best, 1979). North and east of the 

New Arrowhead Mine, breccia clasts are exclusively of 

quartzite in one body and a megabreccia block of bleached 

carbonate occurs in another. Breccia bodies are as much as 

100 m thick

PALEOZOIC ROCKS, UNDIVIDED Silicified and bleached carbonate 

rocks, quartzite, and minor shale
EXPLANATION OF SYMBOLS 

CONTACT Dashed where inferred or approximately located

CONTACT OF LANDSLIDE MASS WITHIN INDIAN PEAK CAULDRON Tick 

marks on sole of body; dashed where approximately or 

schematically located

D?,

 [j * FAULT Dashed where inferred or schematically located; dotted

where concealed. Ball and bar, or D, on downthrown side; 

barb shows dip

. 14 STRIKE AND DIP OF BEDDING 

X

S*^ STRIKE AND DIP OF COMPACTION FOLIATION IN ASH-FLOW TUFFS

STRIKE AND DIP OF FLOW LAYERING IN LAVAS, FLOW-DOMES, AND 

REMOBILIZED TUFFS

^ Inclined

^ Vertical 

60

- -**- RHYOLITE DIKE Barb shows dip

- »-..OUARTZ VEIN Multiple large veins shown by one representative; 

areas of many small veins indicated by stippling. Veins 

range from a few millimeters to 5 m in width and locally 

coalesce to form wide zones. Comprised of prismatic and 

massive quartz, white crystalline and brown massive 

calcite, and shattered fluorite in lenses oblique to the 

vein. Veins cut or follow rhyolite dikes with which they 

are associated. Sparse pyrite occurs in a few veins

-m- m * OUARTZ VEIN ALONG FAULT Ball and bar on downthrown side
REFERENCES

Best, M. H., Grant, S. K., and Holmes, R. D., 197^, Geology of the

Miners Cabin Wash and Buckhorn Spring Quadrangles, western Beaver

County, Utah: U.S. Geological Survey Open-File Report 79-1612. 

Best, M. G., Shuey, R. T., Caskey, C. F., and Grant, S. K., 1P73,

Stratigraphic relations of members of the Needles P,ange Formation

at type localities in southwestern Utah: Geological Society of

America Bulletin, v. 84, p. 3269-3278. 

Conrad, 0. G., 1969, Tertiary volcanic rocks of Needles Range, western

Utah: Utah Geological and Mineralogical Survey Special Studies 29,

27 p. 

Everett, F. D., and Wilson, S. P., 1951, Investigation of the Cougar

Spar fluorspar deposit, Beaver County, Utah: U.S. Bureau of Mines

Report of Investigations 4820, 12 p. 

Fleck, R. J., Anderson, J. J., and Rowley, P. D., 1975, Chronology of

mid-Tertiary volcanism in High Plateaus region of Utah: Geological

Society of America Special Paper 160, p. 53-61. 

Grant, S. K., 1979, Intrusive rocks of the Indian Peak range, Utah:

Rocky Mountain Association of Geologists and Utah Geological

Association Guidebook for Basin and Range Conference, in press. 

Grant, S. K., and Best, M. G., 1979, Geology of the Lund Quadrangle,

Iron County, Utah: U.S. Geological Survey Open-File Report 79- 

Kreider, J. E., 1970, The Lund ash-flow tuff: Rolla, University of

Missouri, unpublished M.S. thesis, 73 p. 

Lipman, P. W., 1976, Caldera-collapse breccias in the western San Juan

Mountains, Colorado: Geological Society of America Bulletin, v«

87, p. 1397-1410. 

Rauch, P. C., 1975, Tertiary welded tuffs of the Ryan Spring area,

Needle Range, Beaver County, Utah: Rolla, University of Missouri,

unpublished M.S. thesis, 58 p. 

Thurston, W. R., Staatz, M. H., Cox, P. C., and others, 1954, Fluorspar

deposits of Utah: U.S. Geological Survey Bulletin 1005, 53 p.
INDIAN PEAK CAULDRON

Most of the Atchison Creek and Pinto Springs Quadrangles lie within 

the Indian Peak cauldron which formed as thousands of cubic kilometers 

of Wah Wah Springs Tuff Member was erupted and spread over southwestern 

Utah and southeastern Nevada (Best and others, 1979). The eastern 

segment of the ring fault controlling its subsidence is now buried 

beneath younger deposits but may not be far from the eastern limit of 

outcrop of the lithic intracauldron unit of the Wah Wah Springs (Twl). 

Subsequent resurgence of the floor of the cauldron, possibly during 

emplacement of an intrusive porphyry (Twi) whose composition is the same 

as the Wah Wah Springs tuffs, tilted the tuffs and landslide breccias 

filling the cauldron. Some of this resurgent uplift may have occurred 

along the existing ring fault (see east end of cross section A-A') 

before deposition of the tuff of Ryan Springs which is about a kilometer 

thick north of the map, pinches out in the north part of the Pinto 

Spring Quadrangle and then reappears and thickens to the southeast in 

the southeast corner of the map. The tuff of Ryan Spring and overlying 

Lund Tuff Member filled the depression surrounding the area of 

resurgence.