Geology of the Picture Rock Hills quadrangle, Southwestern Keg Mountains, Juab County, Utah (Thesis and maps)

Geology of the Picture Rock Hills quadrangle, Southwestern Keg Mountains, Juab County, Utah

GEOLOGY OF THE PICTURE ROCK HILLS QUADRANGLE, SOUTHWESTERN KEG MOUNTAINS, JUAB COUNTY, UTAH by Ann Marie Staub A t h e s i s submitted to the f a c u l t y of the U n i v e r s i t y of Utah in p a r t i a l f u l f i l l m e n t of the requirements f o r the degree of Master of Science in Geology • Department of Geology and Geophysics U n i v e r s i t y of Utah Spring 1975 UNIVERSITY OF UTAH GRADUATE SCHOOL S U P E R V I S O R Y C O M M I T T E E A P P R O V AL of a thesis submitted by ANN MARIE STAUB I have read this thesis and have found it to be of satisfactory quality for a master's I have read this thesis and have found it to be of satisfactory quality for a master's Date / / WILLIAM P. NASH Member, Supervisory Committee UNIVERSITY OF UTAH GRADUATE SCHOOL F I N A L R E A D I N G A P P R O V AL To the Graduate Council of the University of Utah: I have read the thesis of _ A n n M j r i e S t a u b in its final form and have found that (1) its format, citations, and bibliographic style are consistent and acceptable; (2) its illustrative materials including figures, tables, and charts are in place; and (3) the final manuscript is satisfactory to the Supervisory Committee and is ready for submission to the Graduate School. MAX P. ERICKSON Member, Supervisory Committee Approved for the Major Department STANLEY H. WARD Chairman /Dean Approved for the Graduate Council STERLING M. MCMURRIN Dearr of the Graduate School C O N T E N T S Page I l l u s t r a t i o n s vi A b s t r a c t viii I n t r o d u c t i o n 1 L o c a t i o n , A c c e s s i b i l i t y , and C u l t u r e 1 C l i m a t e , V/ater Supply, and V e g e t a t i o n 1 P r e v i o u s Geologic I n v e s t i g a t i o n s k P r e s e n t I n v e s t i g a t i o n and Acknowledgements k General Geology 6 Geologic H i s t o r y 7 P r e - v o l c a n i c Rocks 12 P a l e o z o i c Sedimentary Rocks 12 B r e c c i a 13 Rocks of T e r t i a r y Age 15 I n t r u s i v e Rocks 16 Keg G r a n o d i o r i t e Porphyry 16 B r e c c i a Pipe 20 Volcanic Rocks 21 Older Volcanic Rocks 22 A n d e s i t e Porphyry 23 Keg Spring Andesite ( D i o r i t e Porphyry) 25 Red Mountain Crystal Tuffs 30 Younger Volcanic Rocks 37 Keg Mountains Tuff 38 Keg Mountains R h y o l i t e *tl Drum Mountains Rhyolite 52 P i c t u r e Rock Quartz L a t i t e Porphyry 53 S p h e r i c a l S t r u c t u r e s Occurring in the Younger R h y o l i t i c and Quartz-1 a t i t i c Rocks 59 Topaz and Other Minerals Occurring in C a v i t i es in the Keg Mountains R h y o l i t e 65 Topaz 65 P s e u d o b r o o k i t e 68 Hematite 69 Quartz 69 B i x b y i t e 69 Pa^e C h a r a c t e r and Nature of Emplacement of V o l c a n i c l a s t i c d e p o s i t s 71 P y r o c l a s t i c flow 72 Ground surge 73 Air f a l l 7^ Lahar 75 Age of the T e r t i a r y Volcanic Rocks 75 Rocks of Quaternary Age 76 B a s a l t 76 Q u a t e r n a r y Sediments 77 S t r u c t u r e 78 References Cited 86 v I L L U S T R A T I O N S F i g u re 1 Index map of Utah 2 Map showing l o c a t i o n of e x t r u s i v e c e n t e rs 3 Geologic c r o s s s e c t i o n through the Kane Spring-West Mountain area k Photograph of j o i n t i n g in Keg Spring Andesite 5 Photograph of bedding in the younger p a r t of the Red Mountain Crystal Tuffs 6 Photograph of the t y p i c a l c h i l l border of a r h y o l i te i n t r u s ive 7 Photograph of the southwestern part of the r h y o l i te plug south of the J e r i c h o Road 8 Oblique a e r i a l photograph showing the P i c t u r e Rock Quartz L a t i t e Porphyry in the extreme southwestern Keg Mountains 9 Geologic c r o s s s e c t i o n through the h i g h e s t h i l l in s e c . 36, T. 13 S . , R. 10 W., showing P i c t u r e Rock Quartz L a t i t e Porphyry o v e r l y i n g l i t h i c tuff 10 Photomicrograph of s p h e r u l i t i c r h y o l i t i c g l a ss 11 Photograph of l i t h o p h y s a l s t r u c t u re 12 Photomicrograph of l i t h o p h y s a l layers 13 Photograph of intergrown topaz c r y s t a l s in a l i c h e n - covered Keg Mountains R h y o l i t e matrix \k Photograph of p s e u d o b r o o k i t e c r y s t a l c l u s t e r on Keg Mounta i ns Rhyoli te 15 Vertical a e r i a l photograph showing the l i n e a r f e a t u re on the west s i d e of the Keg Mountains P l a t e Geologic map of the P i c t u r e Rock H i l l s q u a d r a n g l e, s o u t h w e s t e r n Keg Mountains, Juab County, Utah !n Pocket G e n e r a l i z e d g e o l o g i c map of c e n t r a l Juab County, Utah, showing l o c a t i o n of hypothesized c a l d e r a s In Pocket v i i A B S T R A C T The a r e a covered by t h i s report is the P i c t u r e Rock H i l ls q u a d r a n g l e and comprises about 60 square m i l e s . ft includes the s o u t h w e s t e r n Keg Mountains and a small part of the Drum Mountains, Juab County, Utah. The s o u t h w e s t e r n Keg Mountains is composed c h i e f l y of e x t r u s i ve and i n t r u s i v e igneous rocks of T e r t i a r y age. L a c u s t r i n e and a l l u v i al d e p o s i t s of Quaternary age cover the o l d e r rocks and f i l l in the v a l l e y s . The major i n t r u s i v e rock in the area is the Keg G r a n o d i o r i te Porphyry which is probably T e r t i a r y and pre-Miocene in age. The v o l c a n i c rocks, which comprise most of the. exposure in the a r e a , are d i v i d e d into two g r o u p s , s e p a r a t e d by an unconformity. The o l d e r v o l c a n i c rocks a r e poorly exposed. They include a n d e s i te porphyry, Keg Spring A n d e s i t e , and Red Mountain Crystal Tuffs. The Red Mountain Crystal Tuffs unit is the most widespread of the o l d er rocks and is predominately a s h - f l ow t u f f s . It is the youngest unit in the o l d e r group and is probably middle Miocene. The younger v o l c a n i c rocks include the Keg Mountains Tuff, Keg Mountains R h y o l i t e, and P i c t u r e Rock Quartz L a t i t e Porphyry. The Drum Mountains Rhyolite i s e q u i v a l e n t to the Keg Mountains R h y o l i t e . The younger v o l c a n ic rocks probably range in age from l a t e Miocene t o l a t e P l i o c e n e . The r h y o l i t e , tuff, and q u a r t z l a t i t e porphyry are interbedded one with t h e o t h e r and a r e about the same age. Topaz, p s e u d o b r o o k i t e , s p e c u l a r h e m a t i t e , and b i x b y i t e occur in c a v i t i e s in the Keg Mountains R h y o l i t e. Several d i f f e r e n t types of s p h e r i c a l s t r u c t u r e s occur in the younger e x t r u s i v e and i n t r u s i v e rocks: r a d i a l l y s t r u c t u r e d s p h e r u l i t e s, l i t h o p h y s a e , and "thunder e g g s " . The s p h e r u l i t e s and l i t h o p h y s a e were formed by t h e d e v i t r i f i c a t i o n of the rocks. P r e v i o u s i n v e s t i g a t o r s hypothesized the e x i s t e n c e of c a l d e r a s in t h e a r e a . Field i n v e s t i g a t i o n done for t h i s r e p o r t does not s u b s t a n t i a te t h e g e o l o g i c evidence used for t h a t i n t e r p r e t a t i o n. I N T R O D U C T I O N L o c a t i o n , A c c e s s i b ? 1 i t y , and Culture The mapped a r e a , the P i c t u r e Rock H i l l s q u a d r a n g l e , includes the s o u t h w e s t e r n Keg Mountains and a small p a r t of the n o r t h e a s t e r n Drum Mountains and is e n t i r e l y w i t h i n Juab County, Utah. The a r e a is bounded by p a r a l l e l s 3 9 ° 3 7 , 3 0 n and 39°45' and m e r i d i a n s 112°52,30,! and 113°00l ( f i g . 1) and comprises a p p r o x i m a t e l y 60 square m i l e s. The n e a r e s t town, D e l t a , is 36 m i l e s s o u t h e a s t. The a r e a is e a s i l y reached by the paved road which c o n n e c t s the Spor Mountain area and U.S. Highway 6 and 50. Access w i t h i n the area i s provided by s e v e r a l good d i r t roads and marginal t r a i l s p a s s i b le o n l y with four-wheel d r i v e v e h i c l e s. The study area has no permanent r e s i d e n t s . The only economic a c t i v i t y is sheep g r a z i n g from November to A p r i l . Numerous mining c l a i m s have been staked on much of the area s e v e r a l times in the p a s t; n o t h i n g of economic value has been proved, however. CIimate, Water Supply, and Vegetat ion The c l i m a t e of the area is s e m i a r i d , l i k e t h a t of most of the Basin and Range p r o v i n c e . Staatz and Carr (1964) compiled data from nearby weather s t a t i o n s . The local mean annual p r e c i p i t a t i o n is i n f e r r e d from t h e i r data to be between 5 and 10 i n c h e s , and the mean annual t e m p e r a t u r e to be around 50° F ( S t a a t z and Carr, 1964, p. 5 ). 2 113( 112° s lh2° \ > / \ o I I I % <-. \ O Proyo \ 111° 110° Sal Lake .City z r A Eureka \ _x {/ t.^. , i V O P r i ce | 41° i *-i i I I oDel ta -j TV" Ii • I i > -s ! 39< -I ,-J .A/ o Cedar City I I i 3 8 ° 37° F i g u r e 1.-Index map of Utah showing l o c a t i o n of P i c t u r e Rock H i l ls q u a d r a n g l e (l) and Keg Pass quadrangle ( 2 ) , Juab County, Utah. 3 Kane Spring is the only source of water in the study a r e a . The q u a l i t y of the water is not good, however, as it c o n t a i n s at l e a st 1,500 p a r t s per m i l l i o n d i s s o l v e d s u b s t a n c e s (Waddell, 1967, t a b l e 1). The e l e v a t i o n of the s p r i n g is about 5620 f e e t . The exact source of t h e water is not known, but the water a p p a r e n t l y emerges j u s t above t h e dark red a n d e s i t e b r e c c i a at the base of the Keg Mountains Tuff. The local v e g e t a t i o n is t y p i c a l of the North American high d e s e rt environment. The v e g e t a t i o n type and d i s t r i b u t i o n is dependent on a l t i t u d e and p r e c i p i t a t i o n ; two v e g e t a t i o n zones a r e a r e s u l t of that dependence. The p l a n t s in the lower, d r i e r a r e a s a r e g e n e r a l l y not more than 0 . 5 meters high. The predominant p l a n t s a r e v a r i o u s s p e c i e s of sage­brush (Artemesia s p . ) , Brigham's tea (Ephedra n e v a d e n s i s ) , s h a d s c a le (A11rip1 ex c o n f e r t i f o 1 i a ) , gray molly (Kochia v e s t i a ) , bunch g r a s s e s, b a r r e l c a c t i , - p r i c k l y pear c a c t i (Opuntia p o l y c a n t h a ) , and r a b b it brush (Chrysothamnus s p . ) a l s o grow in the lower a r e a s. All of the above p l a n t s , except r a b b i t brush, grow in the h i g h e r, mountain a r e a s . The v e g e t a t i o n in the higher a r e a s tends to be l a r g er than t h a t of the f l a t s . Juniper ( J u n i p e r u s u t a h e n s i s ) is common in t h e n o r t h c e n t r a l part of the mapped a r e a . Very few j u n i p e r s grow in the c e n t r a l a r e a . Mountain mahogany (Cercocarpus l e d i f o l i u s ) and c r e o s o t e bush (Larrea s p . ) a l s o grow in the higher a r e a s . In a d d i t i on a few pinon pines (Pinus e d u l i s ) grow on t h e n o r t h e r n slope of the mountain n o r t h e a s t of Kane Spring. 4 P r e v i o u s Geolog?c I nvest: i g a t ions The g e o l o g i c l i t e r a t u r e on the Keg Mountains, which is meager, c o n s i s t s of short d e s c r i p t i o n s of the v o l c a n i c rocks by Erickson ( 1 9 6 3 ) , a few s e n t e n c e s in S t a a t z and C a r r ' s (1964) paper on the geology of the Thomas Range, a few comments in a t h e s i s on the n o r t h e r n Drum Mountains (Newell, 1971), and a r e c o n n a i s s a n c e study of hypothesized c a l d e r a s in Juab County (Shawe, 1972). The geology of the area is g e n e r a l i z e d on the 1/250,000 g e o l o g ic maps of Stokes and Madsen, J r . ( I 9 6 I ) , Erickson (1963), and Shawe (1972). P r e s e n t Invest ? g a t ion and Acknowledgements The p r e s e n t work is a study of one 7i minute q u a d r a n g l e , the P i c t u r e Rock H i l l s q u a d r a n g l e , involving an area of a p p r o x i m a t e l y 60 s q u a r e m i l e s . Field work was done i n t e r m i t t e n t l y between June, 1971, and J u l y , 1974. During 1971, the geology was mapped on v e r t i c a l a e r i al photographs and was l a t e r t r a n s f e r r e d to t o p o g r a p h i c base maps when t h e y became a v a i l a b l e . After 1971, t h e geology was mapped on the t o p o g r a p h i c base maps. The a r e a was mapped at a s c a l e of 1/24,000 and is here reproduced at t h a t s c a l e ( p i . 1). During the course of t h e s t u d y , 76 t h i n s e c t i o n s and 103 p o t a s s i u m - s t a i n e d p o l i s h e d rock s l a b s were s t u d i e d . Electron microprobe and x - r a y d i f f r a c t i on d e t e r m i n a t i o n s of s e l e c t e d m i n e r a l s were made. P l a g i o c l a s e composi­t i o n s were determined using a f o u r - a x i s u n i v e r s a l s t a g e. The w r i t e r acknowledges the f i n a n c i a l support of the Utah Geological and Mineral Survey, the use of Brush Wellman's p it f a c i l i t i e s , the t e c h n i c a l a s s i s t a n c e and counsel provided by Kennecott E x p l o r a t i o n S e r v i c e s and American Smelting and Refining Company, and the counsel of my committee. I a l s o wish to thank Mr. and Mrs. Dwain Marshall and Alan B l a i r for t h e i r company in the f i e l d and t o extend s p e c i a l thanks t o my husband, Gordon B l a i r , for h i s f i e l d a s s i s t a n c e , t o l e r a n c e , and p a t i e n c e. G E N E R A L G E O L O GY The rocks in the P i c t u r e Rock H i l l s q u a d r a n g l e , Keg Mountains, range in age from P a l e o z o i c to Recent. A s i n g l e o u t c r o p of Paleozoic l i m e s t o n e which is too small to r e p r e s e n t on P l a t e 1 o c c u r s in the q u a d r a n g l e . The r e s t of the area is u n d e r l a i n by T e r t i a r y igneous rocks and P l e i s t o c e n e and Recent a l l u v i um and lake s e d i m e n t s. Approximately t h i r t y percent of the q u a d r a n g l e is u n d e r l a i n by T e r t i a r y igneous rocks. The f o l l o w i n g u n i t s are d i s t i n g u i s h e d in t h e T e r t i a r y igneous r o c k s : Keg G r a n o d i o r i t e Porphyry, Keg Spring A n d e s i t e ( d i o r i te p o r p h y r y ) , a n d e s i t e porphyry, Red Mountain Crystal Tuffs, Keg Mountains Tuff, Keg Mountains R h y o l i t e , Drum Mountains R h y o l i t e , and P i c t u r e Rock Quartz L a t i t e Porphyry. The p r e v i o u s l y named u n i t s a r e l i s t e d in t h e ir approximate c h r o n o l o g i c order with the Keg G r a n o d i o r i t e Porphyry being t h e o l d e s t . The Drum Mountains R h y o l i t e is i d e n t i c a l 1 i t h o l o g I c a 1 1y and s t r a t i g r a p h i c a l 1 y to the Keg Mountains R h y o l i t e and is assumed to be t h e same age. Some of the Keg Mountains Tuff is younger than part of the Keg Mountains R h y o l i t e. Q u a t e r n a r y sediments c o n s i s t p r i n c i p a l l y of Lake B o n n e v i l l e gravel sand, s i l t , and marl or t h e i r c o n s o l i d a t e d c o u n t e r p a r t s . Recent a l l u v i um is a l s o p r e s e n t l o c a l l y . Remnants of a l l u v i a l fans o l d e r than Lake B o n n e v i l l e a r e a l s o p r e s e n t . A s i n g l e o u t c r o p of b a s a l t of presumed Q u a r t e r n a r y age o c c u r s in the s o u t h e a s t e r n p a r t of the q u a d r a n g l e . Geologic History Although t h e r e is no exposed rock r e c o r d , the p r e - T e r t i a r y h i s t o ry of the area is i n f e r r e d to be the same as or s i m i l a r to the h i s t o r y of nearby a r e a s where a rock record is exposed. Staatz and Carr (1964), Thomas (1958), and Newell (1971) can be c o n s u l t e d for the p r e - T e r t i a ry • h i s t o r y of the r e g i o n. The age of the f i r s t igneous a c t i v i t y recorded in the area is u n c e r t a i n ; it is probably T e r t i a r y and pre-Miocene. The f i r s t igneous a c t i v i t y r e s u l t e d in the formation of a b r e c c i a . This b r e c c i a has a high p e r c e n t a g e of sedimentary c l a s t s in an igneous m a t r i x (p. 13-15). I t c l o s e l y resembles the b r e c c i a pipes in the West T i n t i c and Sheeprock Mountains (Morris and Kopf, 1967). B r e c c i a t i o n probably was t h e r e s u lt of e x p l o s i v e degassing of an ascending magma. The next igneous a c t i v i t y in the a r e a probably a l s o is T e r t i a ry and o c c u r r e d before Miocene time. The Keg G r a n o d i o r i t e Porphyry which i s now exposed in the w e s t e r n Keg Mountains was emplaced as a shallow, p o s s i b l y s u b v o l c a n i c , i n t r u s i v e . The uniformly m i c r o c r y s t a l 1 i ne groundmass of the Keg G r a n o d i o r i t e Porphyry c o n t r a s t s s h a r p l y with the d e v i t r i f i e d g l a s s shard m a t r i x of s i m i l a r rocks in n e i g h b o r i n g a r e a s. One of the s i m i l a r rocks, the p l a g i o c l a s e c r y s t a l tuff of the Thomas Range ( S t a a t z and Carr, 1964, p. 7 8 - 7 9 ) , is p o s s i b l y the e x t r u s i ve e q u i v a l e n t of t h i s i n t r u s i v e e v e n t . Whether the i n t r u s i v e rock is r e l a t e d to the b r e c c i a mentioned above is unknown. Erosion exposed t h e g r a n o d i o r i t e porphyry to near i t s level of exposure before l a t er v o l c a n i s m . Younger u n i t s uncomformably o v e r l i e the Keg G r a n o d i o r i te Fbrphyry. 8 Keg Spring A n d e s i t e flows and p y r o c l a s t i c s were d e p o s i t e d over most of the n o r t h e r n Keg Mountains. Andes?tic rocks on the western f l a n k of the Simpson Mountains a r e p o s s i b l y r e l a t e d to t h e Keg Spring A n d e s i t e (Erickson, 1963, P' 26). The g r e a t e r t h i c k n e s s of t h i s u n i t, d i k e s of i d e n t i c a l composition, and p o s s i b l e vent agglomerates in the Keg Pass quadrangle imply a source for the Keg Spring Andesite in t h at q u a d r a n g l e . The a n d e s i t e porphyry was d e f i n i t e l y emplaced before the Red Mountain Crystal Tuffs, but i t s exact age r e l a t i v e to the o t h e r o l d er u n i t s is u n c e r t a i n . The a n d e s i t e porphyry is probably e x t r u s i v e and might be p a r t of e i t h e r the Keg Spring Andesite or the p l a g i o c l a se c r y s t a l tuff of the Thomas Range. A s e r i e s of r h y o l i t i c to q u a r t z - l a t i t i c c r y s t a l t u f f s , the Red Mountain Crystal Tuffs, were d e p o s i t e d during middle Miocene time a f t er t h e emplacement of the i n t e r m e d i a t e rocks. Most of the e a r l i e r t u f fs a r e not welded and may a c t u a l l y be d i k e s in p a r t . The younger t u f fs a r e h i g h l y welded, massive, a s h - f l ow t u f f s. The o l d e r v o l c a n i c rocks were f a u l t e d , t i l t e d , and eroded p r i or t o the emplacement of the younger v o l c a n i c r o c k s . Dips of up t o 50° were recorded on some bedding and compaction p l a n e s . The d i f f e r e n c es of o r i e n t a t i o n s of bedding or compaction f e a t u r e s of a d j a c e n t outcrops can only be e x p l a i n e d by f a u l t i n g . The t u f f s and r h y o l i t e s of the younger v o l c a n i c u n i t s were emplaced on e r o s i o n a l s u r f a c e s that are in some p l a c e s p e r p e n d i c u l a r to t h e bedding or compaction planes of t h e o l d e r u n i t s. 9 The second period of volcanism probably began in the l a t e Miocene and ended in the l a t e P l i o c e n e (p. 76). During t h i s time, r h y o l i t ic t u f f s and lavas were e r u p t e d from e i g h t v o l c a n i c c e n t e r s in the q u a d r a n g l e ( f i g . 2 ) . A s i n g l e e r u p t i v e sequence at any of these c e n t e r s g e n e r a l l y c o n s i s t s of a bedded t u f f o v e r l a i n or i n t r u d e d by r h y o l i t e . The e r u p t i v e c e n t e r s in the a r e a were not sources of very e x t e n s i v e flows as were the c e n t e r s in t h e e a s t e r n Keg Mountains and t h e Thomas Range. The s m a l l e s t flow in the Thomas Range is about 11 s q u a r e km.; the l a r g e s t in the s o u t h w e s t e r n Keg Mountains is about 4 s q u a r e km. R h y o l i t i c green g l a s s was emplaced in or around vent or f r a c t u r e d a r e a s where i t l o c a l l y remelted p r e - e x i s t i n g tuff and r h y o l i t e ( S t a a t z and Carr, 1964, p. 100). Some v o l c a n i c c e n t e rs - show e v i d e n c e of repeated a c t i v i t y . East of Kane Spring at l e a s t two t u f f - r h y o l i t e e r u p t i v e sequences a r e p r e s e n t ( f i g . 3)- Minor e r o s i on o c c u r r e d between e r u p t i o n s . Removal of some of the upper c h i l l f a c i es of r h y o l i t e s and the t r u n c a t i o n of beds in the t u f f occurred before t h e emplacement of o v e r l y i n g u n i t s. A b r e c c i a pipe of u n c e r t a i n age intruded the younger Red Mountain C r y s t a l Tuff. This pipe may be r e l a t e d to the l a t e Miocene to l a te P l i o c e n e r h y o l i t i c volcanism or may be the record of a s e p a r a te e x p l o s i v e event. Basin and Range f a u l t i n g probably o c c u r r e d in the area at the same time t h a t the younger v o l c a n i c rocks were emplaced and may a c t u a l l y have begun before the c r y s t a l t u f f s were e r u p t e d . Faulting which broke up t h e Red Mountain Crystal Tuffs may have been Basin and Range t y p e. 10 ^ , i c R 10 W R 9 W i g u r e 2.-Map showing the l o c a t i o n of vent a r e a s for the younger v o l c a n i c rocks in the P i c t u r e Rock H i l l s q u a d r a n g l e . Topography reduced from P l a t e 1. Scale a p p r o x i m a t e l y 1/76,000. F i g u r e 3*-Geologic c r o s s s e c t i o n through West Mountain-Kane Spring Mountain a r e a . Line of s e c t i o n is N. 87° E. Trm^, o l d e r Red Mountain Crystal Tuffs; Tkmrw, p o r p h y r i t i c Keg Mountains R h y o l i te a t V/est Mountain; Tkmr, Keg Mountains R h y o l i t e ; Tkmt, Keg Mountains Tuff; Tks, Keg Spring A n d e s i t e . The numbers ^ and ^ i n d i c a t e local r e l a t i v e a g e s . Scale a p p r o x i m a t e l y 1/24,000. 12 The a r e a continued to be eroded; l a r g e a l l u v i a l fans developed in p a r t s of the Keg Mountains. In P l e i s t o c e n e time, the w a t e r s of Lake B o n n e v i l l e covered the area to an e l e v a t i o n of 5200 f e e t . Gravel, sand, and marl were d e p o s i t e d in the l a k e , and s h o r e l i n e f e a t u r e s such a s wave-cut t e r r a c e s and bars developed. The lake level f l u c t u a t ed s e v e r a l times during the P l e i s t o c e n e and f i n a l l y was lowered to the l e v e l of the Great S a l t Lake. In Recent time, a veneer of a l l u v i um has been d e p o s i t e d l o c a l l y on t h e o l d e r lake sediments and a l l u v i al f a n s . P r e - v o l c a n i c Rocks P r e - v o l c a n i c rocks in the Keg Mountains a r e P a l e o z o ic s e d i m e n t a r y rocks and the b r e c c i a d e s c r i b e d below. P a l e o z o i c Sedimentary Rocks A s i n g l e o u t c r o p of limestone which is probably P a l e o z o i c in age o c c u r s e a s t of the road j u s t n o r t h of the c a t t l e guard in s e c . 23, T. 13 S . , R. 10 W. ( p i . 1). The o u t c r o p is l e s s than 20 f e e t (about 6 m e t e r s ) in d i a m e t e r . The younger Red Mountain Crystal Tuff a p p a r e n t l y o v e r l i e s the l i m e s t o n e . The l i m e s t o n e is l i g h t grey, f r a c t u r e d , and p a r t i a l l y s i l i c i f i e d and c l o s e l y resembles Cambrian Limestones in the Drum and Simpson Mountains. The e x t e n t to which t h is l i m e s t o n e or o t h e r P a l e o z o i c rocks u n d e r l i e the v o l c a n i c rocks could not be d e t e r m i n e d . Rocks of P a l e o z o i c or Precambrian age probably u n d e r l i e much of the a r e a , however. 13 B r e c c ia Northwest of Kane S p r i n g , very small o u t c r o p s of a b r e c c i a of q u e s t i o n a b l e o r i g i n o c c u r . This u n i t is too small to r e p r e s e n t on P l a t e 1. Much l a r g e r o u t c r o p s of the same u n i t a r e p r e s e n t in the Keg Pass quadrangle to the n o r t h ( f i g . 1; p i . 2 ) . The b r e c c i a c o n s i s ts of numerous c l a s t s of many types in an a l t e r e d , probably igneous, m a t r i x . More than half of the c l a s t s a r e sedimentary. The b r e c c i a is p o s s i b l y e i t h e r a l a h a r , a p y r o c l a s t i c - f l ow d e p o s i t , or a b r e c c i a p i p e . Vague c l a s t alignment, c l a s t contact r e l a t i o n s h i p s , and the type and q u a n t i t y of c l a s t s p r e s e n t seem to s u p p o r t the b r e c c i a pipe i n t e r p r e t a t i o n . Intermediate v o l c a n i c rocks in some of the mountain ranges in Nevada have a basal b r e c c i a which g r o s s l y resembles the b r e c c i a . Not enough work has been done to d e t e r m i n e the exact n a t u r e of the b r e c c i a. L i t h o l o g y . The b r e c c i a c o n s i s t s of numerous c l a s t s of many types in an a l t e r e d , probably igneous, m a t r i x. Rock types p r e s e n t as c l a s t s a r e limestone pebble conglomerate, o o l i t i c l i m e s t o n e , a l g a l ball l i m e s t o n e , massive grey l i m e s t o n e, i n t e r b e d d e d tan s h a l e and grey l i m e s t o n e , massive d o l o m i t e , g l a u c o n i t ic s a n d s t o n e , p u r p l i s h crossbedded q u a r t z i t e , pink to red q u a r t z i te pebble conglomerate, white to s l i g h t l y pink q u a r t z i t e , fucoidal micaceous s h a l e s , dark grey g n e i s s , black amygdaloidal b a s a l t, c o m p l e t e l y d e v i t r i f i e d p e r l i t i c g l a s s , and s e v e r a l u n i d e n t i f i a b le Igneous rocks. Formations probably r e p r e s e n t e d a r e Mutual Q u a r t z i t e, P r o s p e c t Mountain Q u a r t z i t e , Cabin S h a l e , Busby Q u a r t z i t e , and T r a i l er Limestone. The b a s a l t c l a s t s a r e u n l i k e the Quaternary b a s a l ts exposed south of the Keg Mountains ( p i . 2 ) . Those b a s a l t s a r e the n e a r e s t b a s a l t s in p l a c e in the r e g i o n . Clast s i z e ranges from l e ss t h a n 1 mm. to about 1 m. in the study a r e a . North of the a r e a , c l a s ts g r e a t e r than 30 m. in diameter a r e p r e s e n t . Sedimentary rocks • c o n s t i t u t e more than 60 p e r c e n t of the c l a s t s. A b l a c k c a r b o n a t e " r i n d " e n c l o s e s some of the l a r g e r limestone b l o c k s . The rind is up t o 6 cm. t h i c k and includes some iron oxide and g a r n e t . The q u a r t z i t e c l a s t s have a h i g h l y f r i a b l e o u t e r zone where they a r e in c o n t a c t with the m a t r i x. The m a t r i x is an a l t e r e d , l i g h t o l i v e green m a t e r i a l whose o r i g i n a l c h a r a c t e r is no longer e v i d e n t . It is p o s s i b l y igneous and t u f f a c e o u s l i k e the m a t r i x of the b r e c c i a pipes in the West T i n t ic Mountains (Morris and Kopf, 1967). The m a t r i x has been c a l c i t i z ed w i t h local si 1 i c i f i c a t i o n and c h l o r i t i z a t i o n . Minor s e r i c i t e and clay a l t e r a t i o n i s a l s o p r e s e n t . Considerable s o l u t i o n a c t i o n occurred a f t e r the emplacement of the b r e c c i a . S t y l o l i t e s a r e very common in t h e margins of c a r b o n a t e c l a s t s . A few q u a r t z c r y s t a l fragments appear t o be h a l f p r o t r u d i n g from c a r b o n a t e c l a s t s . The c a r b o n a t e was p r e f e r e n t i a l l y removed by s o l u t i o n i n g where a p r e s s u r e point was c r e a t e d by t h e p r o x i m i t y of a q u a r t z g r a i n. Sortie c r y s t a l l i n e m a t e r i a l occurs in the m a t r i x . Small p l a g i o c l a se ( 0 . 3 mm.) l a t h s and c r y s t a l fragments occur in the m a t r i x . The p l a g i o c l a s e is g e n e r a l l y at l e a s t half a l t e r e d to c a l c i t e , clay m i n e r a l s , and c h l o r i t e . Fragments of myrmekitic i n t e r g r o w t h s (0.4 mm.) were noted in t h i n s e c t i o n . Quartz c r y s t a l fragments up t o 0.5 mm. a c r o s s a l s o occur in the m a t r i x . A few crushed b i o t i t e f l a k e s that 15 a r e a l t e r e d p a r t i a l l y to iron oxide a r e p r e s e n t . Minor p y r i t e a l so o c c u r s in the m a t r i x . It is a l t e r e d to g o e t h i t e . Magnetite and h e m a t i t e blebs a l s o occur in the m a t r i x. Rocks of T e r t i a r y Age All rocks of T e r t i a r y age exposed in the mapped a r e a a r e igneous. T e r t i a r y v o l c a n i c rocks comprise about 95 p e r c e n t of the bedrock; the r e s t a r e shallow p l u t o n i c or s u b v o l c a n i c rocks. C l a s s i f i c a t i o n All of the igneous rocks in the a r e a have a g l a s s y or micro-c r y s t a l l i n e groundmass. The most a c c u r a t e way t o c l a s s i f y rocks with g l a s s y groundmass is by t h e i r norm because the modal a n a l y s i s would o n l y be a phenocryst composition. No chemical a n a l y s i s were made of rocks from t h e study a r e a , so no norms were c a l c u l a t e d . The rock names used in t h i s r e p o r t a r e t h e r e f o r e based on the phenocryst composition u n l e s s o t h e r w i s e noted. Sodium c o l b a l t i n i t r i t e potassium t e s t s were made on p o l i s h e d rock s l a b s to determine the amount of potassium-b e a r i n g m i n e r a l s in the groundmass or m a t r i x. The rock c l a s s i f i c a t i o n system used in t h i s r e p o r t is the system proposed by T r a v i s (1956, p. 3-11). Universal s t a g e methods a r e d e s c r i b e d by Slemmons (1962) were used t o determine the a n o r t h i t e content of p l a g i o c l a s e p h e n o c r y s t s, P l a g i o c l a s e c r y s t a l s for which a n o r t h i t e content was determined g e n e r a l l y had c a r l s b a d - a l b i t e twinning. Determination of p l a g i o c l a se composition by o p t i c a l methods was o f t e n impossible because of zoning v a r i a b i l i t y and width, e x s o l u t i o n , and replacement. A range of a n o r t h i t e c o n t e n t s is recorded for each rock. 16 R e f l e c t e d l i g h t methods were used to determine the n a t u r e of the opaque m i n e r a l s. I n t r u s i v e Rocks The l a r g e s t exposure of i n t r u s i v e rocks in the a r e a is the g r a n o d i o r i t e porphyry which crops out n o r t h e a s t of Red Mountain ( p i . 1). In a d d i t i o n , a s i n g l e b r e c c i a pipe was i d e n t i f i e d , but it i s not l a r g e enough to be r e p r e s e n t e d at the s c a l e of P l a t e 1. R h y o l i t e d e s c r i b e d as the Keg Mountains Rhyolite is commonly p r e s e nt as d i k e s and p l u g s . These r h y o l i t e bodies a r e d e s c r i b e d with the e x t r u s i v e r h y o l i t e s , but a r e r e p r e s e n t e d s e p a r a t e l y on P l a t e 1. Keg Granodior? t e Porphyry The Keg G r a n o d i o r i t e Porphyry crops out in the c e n t r a l part of t h e mapped a r e a , n o r t h e a s t of Red Mountain ( p i . 1). The same u n it c r o p s out more e x t e n s i v e l y in the Keg Pass q u a d r a n g l e , north of the a r e a ( p i . 2 ). Rocks s i m i l a r in hand specimen appearance to the Keg G r a n o d i o r i te Porphyry a r e the p l a g i o c l a s e c r y s t a l t u f f s of the Thomas Range ( S t a a tz and Carr, 1964, p. 78-79) and the p l a g i o c l a s e a n d e s i t e of the Drum Mountains/(Newel 1, 1971, P- 2 7 - 2 8 ) . The Keg G r a n o d i o r i t e Porphyry d i f f e r s in t h a t it has f r e e q u a r t z and l e s s c a l c i c p l a g i o c l a s e . The groundmass of the Keg G r a n o d i o r i t e Porphyry is c r y s t a l l i n e ; that of t h e o t h e r two u n i t s is d e v i t r i f i e d g l a s s s h a r d s. The exact n a t u r e of the Keg G r a n o d i o r i t e Porphyry body exposed in the study area could not be determined because of poor o u t c r o p. 17 The w r i t e r and o t h e r s who have examined t h i s unit i d e n t i f i e d several p h y s i c a l c h a r a c t e r i s t i c s to support a s h a l l ow i n t r u s i v e o r i g i n: The uniform c r y s t a l 1?n?ty of the groundmass, high phenocryst p e r c e n t a g e, and the general weathering c h a r a c t e r . The uniform c r y s t a l 1 i n i t y is t h e most important c h a r a c t e r i s t i c of the rock. If the u n i t had been an e x t r u s i v e l i k e the p l a g i o c l a s e c r y s t a l tuff of the Thomas Range it had once been c o n s i d e r e d part of, r e c r y s t a l 1 i z a t i o n of a once g l a s sy m a t r i x would not have produced a uniformly c r y s t a l l i n e groundmass. No c l e a r c o n t a c t r e l a t i o n s h i p s between the g r a n o d i o r i t e porphyry and any o t h e r unit in the study area was found. Rock fragments derived from t h i s unit occur in b r e c c i a s interbedded with the Keg Mountains Tuff in the Keg Pass q u a d r a n g l e . Dikes which a r e p a r t of the Keg Spring A n d e s i t e i n t r u d e the Keg G r a n o d i o r i t e Porphyry in the Keg Pass q u a d r a n g l e . The K e g ' G r a n o d i o r i t e Porphyry had p r e v i o u s l y been included in the Keg Mountain I g n i m b r i t e (Erickson, 1963, p i • 0 a n d as p a r t of the q u a r t z - l a t i t i c welded ash flow t u f f (Shawe, 1972, p. B69). Shawe's welded a s h - f l ow tuff is the same u n i t as the p l a g i o c l a s e c r y s t a l tuff of S t a a t z and Carr (1964, p. 78-79). The g r a n o d i o r i t e porphyry is massive and has no a p p a r e n t alignment of m i n e r a l s . Although j o i n t i n g is g e n e r a l l y well developed in t h is u n i t in the Keg Pass q u a d r a n g l e , the o u t c r o p s in the study area a r e too v/eathered to a l l ow a c c u r a t e measurements of j o i n t o r i e n t a t i o n. The g r a n o d i o r i t e porphyry o u t c r o p s , p a r t i c u l a r l y in the Keg Pass q u a d r a n g l e , have a rounded c h a r a c t e r t y p i c a l of many g r a n i t e s . The porphyry tends to form s u r f a c e s of very low r e l i e f. 18 Li t h o l o g y . The following l i t h o l o g i c d e s c r i p t i o n of the Keg G r a n o d i o r i t e Porphyry is based on the study of rocks from the mapped a r e a and of rocks c o l l e c t e d from b e t t e r exposures in t h e Keg Pass q u a d r a n g l e . Differences where they e x i s t a r e noted. F r e s h l y broken s u r f a c e s of the g r a n o d i o r i t e porphyry are s p e c k l e d grey green. However, the mafic m i n e r a l s , upon which the c o l o r is very much dependent, are never u n a l t e r e d so t h a t completely u n a l t e r e d rock probably has an a p p r e c i a b l y d i f f e r e n t color than the specimens observed. The Keg G r a n o d i o r i t e Porphyry c o n t a i n s an average of 65 percent p h e n o c r y s t s in a m i c r o c r y s t a 1 1 i n e groundmass. The porphyry in the Keg Pass q u a d r a n g l e has s l i g h t l y more p h e n o c r y s t s. Highly a l t e r e d rock fragments a r e the only f o r e i g n m a t e r i al p r e s e n t in the Keg G r a n o d i o r i t e Porphyry. These fragments have been reduced to f i n e - g r a i n e d c h l o r i t i c c l o t s. P l a g i o c l a s e (An^Q t o An/jg) p h e n o c r y s t s c o n s t i t u t e 15 t o kS percent of the rock and average 27 p e r c e n t in the study a r e a . The c r y s t al s i z e in a l l o u t c r o p s v i s i t e d ranges from 1 t o 12 mm. and a v e r a g e s 3 mm. C r y s t a l s in the study area rocks a r e not over 5 mm. and average 3 mm. Clumping of p l a g i o c l a s e c r y s t a l s is not uncommon. Carlsbad, broad a l b i t e , and some p e r i c l i n e twinning is p r e s e n t . P l a g i o c l a s e in the s t u d y area is at l e a s t 60 p e r c e n t a l t e r e d to c h l o r i t e , c l a y , c a l c i t e, s e r i c i t e , h e m a t i t e , b i o t i t e , q u a r t z , and potassium f e l d s p a r. A l t e r a t i o n tends to be more i n t e n s e in c r y s t a l cores or along c e r t a in composition zones. The more i n t e n s e l y a l t e r e d a r e a s a r e more c a l c i c. 19 Quartz l o c a l l y comprises 1 t o 10 p e r c e n t of the rock and averages 7 p e r c e n t . Quartz p h e n o c r y s t s in the Keg G r a n o d i o r i t e Porphyry range in s i z e from 1 t o 5 mm. and average 2 mm. The p h e n o c r y s t s a r e very much embayed; the embayments a r e f i l l e d with groundmass m a t e r i a l. The embayments a r e probably due to a n a t e c t i c m e l t i n g and r e c r y s t a l 1 i z a - t i o n r a t h e r than c o r r o s i o n (Rittman, 1962, p. 202). B i o t i t e , hornblende, and a u g i t e once comprised 6 t o 23 percent of the rock. The degree of p r e s e n t a l t e r a t i o n makes it impossible to d e t e r m i n e how much of each mineral was once p r e s e n t . About 70 p e r c e nt of the mafics is b i o t i t e or i t s a l t e r a t i o n p r o d u c t s . The b i o t i te ranges in s i z e from 1 t o k mm. and averages 2 mm. The b i o t i t e is at l e a s t half to completely a l t e r e d to iron o x i d e s , c h l o r i t e , sphene, c a l c i t e , and e p i d o t e . The b i o t i t e books v/ere expanded by sphene, e p i d o t e , and c h l o r i t e pods. Some a p a t i t e i n c l u s i o n s were p r e s e n t. Bent b i o t i t e f l a k e s were noted in s e v e r a l t h i n s e c t i o n s made from rocks c o l l e c t e d north of the study a r e a . A few u n a l t e r e d cores of a u g i te and hornblende a r e p r e s e n t in s e v e r a l t h i n s e c t i o n s . Together the hornblende and a u g i t e comprised up to 7 p e r c e n t of the rock. The hornblende p h e n o c r y s t s range in s i z e from 1 t o 3 mm. The hornblende i s p r i m a r i l y a l t e r e d to c a l c i t e and c h l o r i t e with minor sphene, e p i d o t e, h e m a t i t e , and b i o t i t e . A l t e r a t i o n g e n e r a l l y follows c l e a v a g e p l a n e s. Augite is more than 95 p e r c e n t a l t e r e d to c a l c i t e , c h l o r i t e , h e m a t i t e, and f e l d s p a r. Opaque m i n e r a l s comprise up t o 7 p e r c e n t of the Keg G r a n o d i o r i te Porphyry. All of the opaque m a t e r i a l is e i t h e r h e m a t i t e or m a g n e t i t e. 20 A few g r a i n s of p y r i t e were noted in the porphyry to the n o r t h of the s t u d y a r e a. The groundmass of the porphyry averages 35 p e r c e n t of the rock. Less groundmass is p r e s e n t in t h i s unit in the Keg Pass q u a d r a n g l e. The groundmass is c r y s t a l l i n e with an average g r a i n s i z e s l i g h t l y less than 0.025 mm. Potassium s t a i n s were made on p o l i s h e d s l a b s to d e t e r m i n e c o m p o s i t i o n . About half of the groundmass is q u a r t z and about half is potassium f e l d s p a r . The groundmass f e l d s p a r is a l t e r ed t o c a l c i t e , c h l o r i t e , and v a r i o u s clay m i n e r a l s . A few 0.05 mm. p l a g i o c l a s e c r y s t a l s were p r e s e n t . The groundmass is pale p i n k i sh g r e y to grey green in hand specimen. • B r e c c i a Pipe A s i n g l e b r e c c i a pipe occurs n o r t h of the c a t t l e guard in s e c . 23, T. 13 S . , R. 10 W. It is an oval with a long a x i s trend of N. hS° W. and measures 33 by 25 f e e t (about 10 by 7 m e t e r s ) . It intruded the younger p a r t of the Red Mountain Crystal Tuffs. The p i p e is h i g h l y s i l i c i f i e d and has minor c a l c i t e vein f i l l i n g. The o r i g i n a l c h a r a c t e r of the groundmass is no longer e v i d e n t due to t h e si 1 i c i f i c a t i o n . The c l a s t s are predominately younger Red Mountain C r y s t a l Tuffs, and no r e a d i l y i d e n t i f i a b l e sedimentary c l a s t s o c c u r r e d. C l a s t s range in s i z e from sand s i z e to about 2 f e e t (about 2/3 m e t e r ). Other b r e c c i a pipes a r e probably p r e s e n t in the a r e a , but poor exposures prevented p o s i t i v e i d e n t i f i c a t i o n of p o s s i b l e b r e c c i a s in o t h e r p l a c e s. 21 Volcanic Rocks Volcanic rocks comprise n e a r l y a l l of the bedrock in the south­w e s t e r n Keg Mountains and a r e exposed in the r e s t of the Keg Mountains. They a l s o crop out in most of the nearby mountain r a n g e s : Dugway, Drum, Thomas, D e s e r t , and Simpson. A well developed unconformity d i v i d e s the v o l c a n i c rocks into two g r o u p s . The unconformity is commonly obscured by a l l u v i u m , as a r e many of the c o n t a c t s . The old e r o s i o n a l s u r f a c e has c o n s i d e r a b le r e l i e f in some p l a c e s : almost 400 f e e t (about 120 meters) near the mountain e a s t of Kane Spring. Volcanic rocks in the southwestern Keg Mountains a r e not as well exposed as t h o s e in the Thomas Range t o the w e s t . Division of the younger v o l c a n i c group i n t o d i s t i n g u i s h a b l e u n i t s and c o r r e l a t i o n of s i n g l e flows is r e l a t i v e l y simple in most of the Thomas Range. In the Keg Mountains, however, d i v i s i o n is in most p l a c e s impossible because of the lack of good exposure and t h e v a r y i n g n a t u r e of the u n i t s . The r h y o l i t e exposed e a s t of Kane Spring can be d i v i d e d i n t o two age g r o u p s : an upper younger r h y o l i t e and a lower o l d e r r h y o l i t e ( f i g . 3). This d i v i s i o n is v a l i d only at t h a t p l a c e because the u n i t s a r e not c o n t i n u o u s and the r h y o l i t e s cannot be c o r r e l a t e d 1 i t h o l o g i c a 1 1 y. Whether the o l d e r r h y o l i t e at West Mountain is the same as the o l d er o r younger r h y o l i t e e a s t of Kane Spring could not be d e t e r m i n e d . There a r e , however, two d i s t i n c t r h y o l i t e s at each p l a c e ( f i g , 3). C o r r e l a t i o n of u n i t s in the o l d e r v o l c a n i c group is more d i f f i c u lt t h a n c o r r e l a t i o n of the younger u n i t s because they a r e poorly exposed, 22 many of the exposures a r e badly weathered, the geometry of the b o d i e s is unknown, and f a u l t i n g , deformation, and e r o s i o n have c o m p l i c a t e d the geology. Because of t h e s e problems, the o l d e r v o l c a n ic rocks a r e p r i m a r i l y grouped by rock type. Flow s t r u c t u r e in lava and bedding in t u f f are common and d i s t i n ct in the younger v o l c a n i c rocks in the Keg Mountains, as they a r e in the younger v o l c a n i c rocks in the Thomas Range. The flow s t r u c t u re o r i e n t a t i o n is d i r e c t l y r e l a t e d to the shape of the r h y o l i t e body, the n a t u r e of the s u r f a c e the r h y o l i t e is o v e r r i d i n g , and the height above t h e flow base or c h i l l zone ( C h r i s t i a n s e n and Lipman, 1966 p. 676). D i s t i n c t flow s t r u c t u r e and bedding is g e n e r a l l y absent in the o l d e r v o l c a n i c rocks. These rocks tend to be very massive or have been a l t e r e d enough to o b l i t e r a t e any primary s t r u c t u r e s . In a few p l a c e s , however, planar s t r u c t u r e s i n t e r p r e t e d to be bedding, compaction, or flow s t r u c t u r e were well enough p r e s e r v e d to determine t h e i r a t t i t u d e , but not enough good exposures a r e p r e s e n t to determine t h e s t r u c t u r e. Most of the T e r t i a r y v o l c a n i c rocks in the mapped area a r e e i t h er r h y o l i t i c or q u a r t z - l a t i t i c ; none a r e more mafic than a n d e s i t e. 01der v o l c a n i c rocks Older v o l c a n i c rocks comprise s l i g h t l y l e s s than half of the bedrock exposed in the a r e a . Three u n i t s have been d i s t i n g u i s h e d: a n d e s i t e porphyry, Keg Spring Andesite ( d i o r i t e p o r p h y r y ) , and Red Mountain Crystal Tuffs. Diorite porphyry and c r y s t a l tuff are the most widely exposed rock types of the o l d e r v o l c a n i c rocks ( p i . 1). 23 Exposures of the d i o r i t e porphyry a r e l i m i t e d to a r e a s in the n o r t h e rn one t h i r d of the mapped a r e a ( p i . 1). The c r y s t a l t u f f s are exposed fn the n o r t h e r n two t h i r d s , with the major area of o u t c r o p being in t h e c e n t r a l part of the map. The a n d e s i t e porphyry crops out In the n o r t h e r n one half of the a r e a. R e l a t i v e ages between u n i t s of the o l d e r v o l c a n i c rocks could not be p o s i t i v e l y determined because of the lack of good exposure. A n d e s i t e Porphyry • A n d e s i t e porphyry is the name a s s i g n e d to s e v e r a l small i s o l a t ed o u t c r o p s . These rocks a r e s i m i l a r , but not i d e n t i c a l , 1 ? t h o l o g i c a l ly and more c l o s e l y resemble each o t h e r than o t h e r u n i t s . All of the rocks a r e a l s o p a r t of the o l d e r v o l c a n i c r o c k s . The a n d e s i t e porphyry may be p a r t of the Keg Spring Andesite or p a r t of the p l a g i o c l a se c r y s t a l tuff of the Thomas Range ( S t a a t z and Carr, 1964, p. 7 8 - 7 9 ). This u n i t is exposed in two p l a c e s in the a r e a : northwest of Kane Spring and j u s t north and south of the J e r i c h o Road ( p i . 1). The a n d e s i t e porphyry is o v e r l a i n by the younger p a r t of the Red Mountain Crystal Tuffs north of the J e r i c h o Road. Northwest of Kane S p r i n g , the Keg Mountains Tuff unconformably o v e r l i e s the a n d e s i te porphyry. Very crude bedding is p r e s e n t in t h i s unit south of the J e r i c ho Road and n o r t h of the r h y o l i t e plug. It s t r i k e s N, 07° E. and dips 42° E. L i t h o l o g y . The a n d e s i t e porphyry is a r e d d i s h to p u r p l i s h brown rock which c o n t a i n s 16 t o 30 p e r c e n t p h e n o c r y s t s . No f o r e i g n rock fragments a r e p r e s e n t , but the rock is l o c a l l y a u t o b r e c c i a t e d. 24 P l a g i o c l a s e (An/^g) p h e n o c r y s t s c o n s t i t u t e s 15 t o 30 p e r c e n t of t h e porphyry. The g r a i n s i z e v a r i e s from 1 t o 6 mm. Up t o 50 percent of the p l a g i o c l a s e is a l t e r e d to s e r i c i t e , c a l c i t e , and h e m a t i t e . Some zoning and broad a l b i t e twinning is p r e s e n t . The p l a g i o c l a s e is present as remnants of glomerophyric a g g r e g a t e s which were ripped a p a r t before or during e r u p t i o n . Some post emplacement f r a c t u r i n g of the c r y s t a ls has a l s o o c c u r r e d. Red-brown to greenish-brown b i o t i t e f l a k e s and s h o r t books c o n s t i t u t e 1 t o 10 p e r c e n t of the rock. The b i o t i t e ranges in s i ze from 0.6 to 4 mm. and a v e r a g e s 2 mm. Some bent f l a k e s a r e p r e s e nt in rocks from northwest of Kane S p r i n g . The b i o t i t e is n e a r ly c o m p l e t e l y a l t e r e d to h e m a t i t e. Hornblende c o n s t i t u t e d up t o 7 p e r c e n t of the rock near the J e r i c h o Road. Most of the hornblende has a l t e r e d to c a l c i t e and iron o x i d e s . Quartz in t h e form of broken bipyramids c o n s t i t u t e s l e s s than 2 p e r c e n t of the rock and is absent in some p l a c e s . It ranges in s i ze from 1 t o 3 mm. Opaque m a t e r i a l , most of which is rounded g r a i n s of m a g n e t i t e, c o n s t i t u t e s up t o 5 p e r c e n t of the rock. The m a g n e t i t e is p r e s e nt as g r a i n s averaging 0.4 mm. or as d u s t. Minor a p a t i t e euhedra averaging 0.25 mm. long a r e p r e s e n t. The groundmass is a p p a r e n t l y s t r u c t u r e l e s s . It ranges in color from a deep red brown to a dark pink to a p u r p l i s h brown. Very t i ny p l a g i o c l a s e l a t h s in the groundmass a r e s l i g h t l y o r i e n t e d , a p p a r e n t ly by flowage. Northwest of Kane S p r i n g , the groundmass is micro- 25 c r y s t a l l i n e and most of it is q u a r t z . Near the J e r i c h o Road, about 10 p e r c e n t of the groundmass is a l t e r e d to c a l c i t e . Secondary q u a r tz v e i n i n g is p r e s e n t near the J e r i c h o Road a l s o. Northwest of Kane S p r i n g , the a n d e s i t e porphyry is highly f r a c t u r e d , and the f r a c t u r e s are f i l l e d with pink and white c a l c i te and red j a s p e r . Veinlets a r e up t o 6 mm. a c r o s s and c o n s t i t u t e about 8 p e r c e n t of the rock volume. Keg Spring Andesite ( D i o r i t e Porphyry) The Keg Spring Andesite and L a t i t e u n i t was o r i g i n a l l y named and b r i e f l y d e s c r i b e d by Erickson (1963, p. 26). T e c h n i c a l l y , the part of the u n i t exposed in the map a r e a is a d i o r i t e porphyry ( T r a v i s, 1956). The name a n d e s i t e has been r e t a i n e d for t h i s u n i t because the major p a r t of the u n i t which l i e s o u t s i d e the map a r e a is an a n d e s i t e. The u n i t is composed of both flows and p y r o c l a s t i c d e p o s i t s. The u n i t is exposed in the n o r t h e r n one t h i r d of the a r e a ( p i . 1) where i t has an a p p a r e n t t h i c k n e s s of over 300 f e e t (about 100 m e t e r s ). I t forms very poor o u t c r o p and has a low r o l l i n g s u r f a c e where it is not capped by a more r e s i s t a n t rock. Much of i t s map a r e a on P l a t e 1 i s i n f e r r e d . The r e l i a b i l i t y of mapping for t h i s u n i t is good, however, because of i t s d i s t i n c t i v e w e a t h e r i n g . It weathers to form a t h i c k brown rock mantle with a high p r o p o r t i o n of f i n e powdery m a t e r i a l (Erickson, 1963, p. 26). The n e a r e s t a n d e s i t i c rocks o u t s i d e the Keg Mountains a r e the a n d e s i t e s on the west flank of the Simpson Range and in the Drum Mountains. The a n d e s i t e s on the western flank of the Simpson Range 26 a r e p o s s i b l y r e l a t e d to the Keg Spring A n d e s i t e ( E r i c k s o n , 1963, p . 26). The Keg Spring Andesite ( d i o r i t e porphyry) is b e l i e v e d to have a source in the Keg Pass q u a d r a n g l e . Greater t h i c k n e s s e s of t h i s unit and p o s s i b l e vent b r e c c i a s occur west of Keg S p r i n g . Dikes of c o m p o s i t i o n and appearance i d e n t i c a l to t h e Keg Spring A n d e s i t e occur in t h e Keg Pass quadrangle a l s o. The Keg Spring Andesite is a p p a r e n t l y one of the o l d e s t v o l c a n ic rocks in t h e a r e a . The a n d e s i t e porphyry u n i t (p. 23) is thought to be o l d e r than t h e Keg Spring A n d e s i t e , but no c o n t a c t r e l a t i o n s h i p was found. The Keg Spring A n d e s i t e is o v e r l a i n by Keg Mountains Tuff and i n t r u d e d and o v e r l a i n by Keg Mountains R h y o l i t e . Keg Spring Andesite c l a s t s a r e included in the younger Red Mountain Crystal Tuff, Keg Mountains Tuff, and Keg Mountains R h y o l i t e . The Keg Spring Andesite is a r b i t r a r i l y a s s i g n e d a middle T e r t i a r y age. Rough alignment of b i o t i t e in t h e u n i t northwest of Kane Spring p r o b a b l y r e p r e s e n t s flow banding and has a s t r i k e of N. 23° W. and a dip of 39° S. The base of the u n i t was not found In t h e a r e a and t h e top has been eroded away. The base of t h i s unit crops out n o r t h of Bed Spring in t h e Keg Pass q u a d r a n g l e . This u n i t and the Red Mountain C r y s t a l Tuffs had a c o n s i d e r a b l e e r o s i o n a l s u r f a c e before the Keg Mountains Tuff and the Keg Mountains R h y o l i t e were emplaced. The Keg Spring Andesite is l o c a l l y highly j o i n t e d ( f i g . 4 ). Spacing v a r i e s from several c e n t i m e t e r s to about two m i l l i m e t e r s. Several j o i n t s e t s a r e p r e s e n t . The dominant j o i n t s s t r i k e s l i g h t ly n o r t h of e a s t and a r e roughly v e r t i c a l ( f i g . 4 ) . Less n o t a b l e is the 27 F i g u r e k.-Photograph of j o i n t i n g in Keg Spring A n d e s i t e about 1.2 km s o u t h e a s t of Kane S p r i n g . S t r i k e of major j o i n t s ranges from N. 70° E. t o N. 75° E. Dip is w i t h i n h° of v e r t i c a l. 28 N. 30° W. - N. 60° W. set which has a clip of about 35° N. The o t h er j o i n t s s t r i k e p r i m a r i l y n o r t h - n o r t h w e s t and have m o d e r a t e l y s o u t h e r ly d i p s . The d i p of the j o i n t s is g e n e r a l l y not c o n s i s t e n t from one area t o a n o t h e r ( p i . 1). Some of the j o i n t i n g is undoubtedly the r e s u l t of c o o l i n g of the u n i t . The r e l a t i v e amounts of c o o l i n g and t e c t o n i c a l ly caused j o i n t i n g could not be determined. L i t h o l o g y . The Keg Spring A n d e s i t e is a dark to medium grey f a i r ly well indurated rock of i n t e r m e d i a t e composition. Yellowish-brown to reddish-brown c o l o r i n g of the rock is due p r i m a r i l y to thermal a l t e r a t i o n caused by p r o x i m i t y to younger r h y o l i t i c dikes and flows. L o c a l l y f o r e i g n rock fragments occur in t h i s u n i t : q u a r t z i t e, s h a l e , and several u n i d e n t i f i a b l e rocks. These fragments c o n s t i t u te l e s s than 1 p e r c e n t of the rock where p r e s e n t . None of the fragments in t h e study area are g r e a t e r than 3 cm. These f o r e i g n rock fragments a r e e i t h e r i n c o r p o r a t e d s u r f i c i a l m a t e r i a l or vent wall m a t e r i a l. The u n i t c o n t a i n s 50 to 70 p e r c e n t p h e n o c r y s t s in a dense a p h a n i t i c groundmass. P l a g i o c l a s e (Anj^-Anjg) c o n s t i t u t e s 25 t o kS percent of the rock. I t forms subhedral to euhedral c r y s t a l s . Two g e n e r a t i o n s of p l a g i o c l a s e p h e n o c r y s t s were noted in one t h i n s e c t i o n . Some of the l a r g e r c r y s t a l s are broken but the p i e c e s a r e s t i l l a d j a c e n t . Crystal range in s i z e from 0.3 to 3 mm. and average 1 mm. Some of the c r y s t al e x h i b i t o s c i l l a t o r y zoning. A s i n g l e c r y s t a l had a core of An^g, became more c a l c i c outwards to An^g, and had a rim of An/^, Some of t h e p l a g i o c l a s e c r y s t a l s a r e at l e a s t 30 p e r c e n t a l t e r e d to c a l c i te and minor s e r i c i t e. 29 Light to dark brown b i o t i t e f l a k e s c o n s t i t u t e 5 t o 15 p e r c e n t of t h e rock. They range from 0.4 to 4 mm. a c r o s s and average 1 mm. About 20 p e r c e n t of the b i o t i t e f l a k e s a r e b e n t . Most have r e a c t i o n rims of i r o n o x i d e s . A l t e r a t i o n to c h l o r i t e and iron oxides v a r i e s from s l i g ht t o complete. The b i o t i t e has i n c l u s i o n s of z i r c o n and a p a t i t e . In hand specimen, much of the b i o t i t e appears b l a c k . Oriented n e e d l e ­l i k e i n c l u s i o n s were p r e s e n t in a few f l a k e s. Hornblende was recognized in one t h i n s e c t i o n where i t c o n s t i t u t ed 7 p e r c e n t of the s e c t i o n . The hornblende ranges in s i z e from 0.4 to 0 . 6 mm. It was n e a r l y completely a l t e r e d to iron o x i d e , c a l c i t e , and p e n n i n i t e . Some hornblende may have been p r e s e n t in o t h e r s e c t i o n s, but a l t e r a t i o n is such t h a t i t is now u n r e c o g n i z a b l e . The hornblende a l s o had a r e a c t i o n rim of iron o x i d e s. Quartz c o n s t i t u t e s 5 t o 12 p e r c e n t of the rock. The c r y s t a l s are embayed and rounded and range in s i z e from 0.5 to 5 mm. Some s i x - s i d ed s e c t i o n s were observed. A few c r y s t a l s a r e broken but the fragments a r e s t i l l a d j a c e n t . These c r y s t a l fragments may have been bipyramids. Some secondary q u a r t z was noted r e p l a c i n g b i o t i t e , hornblende, f e l d s p a r , and groundmass. The Keg Spring A n d e s i t e c o n t a i n s up t o 5 p e r c e n t opaque m a t e r i a l - most of which is m a g n e t i t e and h e m a t i t e. The groundma'ss is a p h a n i t i c and well i n d u r a t e d . It is p o s s i b ly ashy and in p l a c e s looks l i k e it may have been g l a s s s h a r d s . A few c r y s t a l l i t e s were noted in the groundmass. The groundmass is 15 t o 25 p e r c e n t a l t e r e d to c a l c i t e and when t r e a t e d with the sodium c o l b a l t n i t r i t e potassium t e s t , took an i n t e n s e b r i g h t yellow s t a i n. 30 The groundmass was probably a high potassium g l a s s or had a p p r e c i a b le p o t a s s i um added to it a f t e r emplacement. Red Mountain Crystal Tuffs The Red Mountain Crystal Tuffs were named for t h e i r o c c u r r e n c e at Red Mountain, the prominant h i l l north of the J e r i c h o Road ( p i . 1). Red Mountain is u n d e r l a i n e n t i r e l y by welded a s h - f l ow t u f f s assigned t o t h e Red Mountain Crystal Tuffs u n i t . The c r y s t a l t u f f s crop out e x t e n s i v e l y in the n o r t h e r n two t h i r d s of the area ( p i . 1). Landforms developed on the t u f f s vary from low, s l i g h t l y r o l l i n g s u r f a c e s to very s t e e p h i l l s up to 500 f e e t (about 150 meters) high. Most of the landforms, however, have l e s s than 150 f e e t (about 50 m e t e r s ) of r e l i e f. The Red Mountain Crystal Tuffs occur throughout the r e s t of the Keg Mountains ( p i . 2 ) . The c o l o r , t e x t u r e , mineral composition, s t r a t i g r a p h i c p o s i t i o n , and o u t c r o p c h a r a c t e r i n d i c a t e t h a t the Red Mountain Crystal Tuffs a r e probably e q u i v a l e n t in p a r t or e n t i r e l y to t h e q u a r t z - s a n i d i n e and s a n i d i n e welded t u f f s of the Thomas Range and Drum Mountains. A s i m i l a r r h y o l i t i c welded tuff occurs in t h e Desert Mountain t o the e a s t of the Keg Mountains ( p i . 2 ). The Red Mountain Crystal Tuffs a r e a p p a r e n t l y the youngest unit in the o l d e r v o l c a n i c rocks. Northwest of Kane S p r i n g , r h y o l i t ic c r y s t a l tuff assigned to the Red Mountain u n i t has i n c l u s i o n s which were a p p a r e n t l y d e r i v e d from the Keg Spring A n d e s i t e . The younger p a r t of the Red Mountain Tuffs o v e r l i e s the a n d e s i t e porphyry unit n o r t h of the J e r i c h o Road and it o v e r l i e s limestone in s e c . 23, T. 13 S . , R. 10 W. (p. 12). Because i t Is probably e q u i v a l e n t to the 31 q u a r t z - s a n i d i n e and s a n i d i n e welded t u f f s in t h e Thomas Range, the Red Mountain Crystal Tuffs a r e probably middle Miocene in age (p. 7 5 ), Although in most o u t c r o p s , the t u f f s lack i n t e r n a l s t r u c t u r e , in a few p l a c e s d i s t i n c t bedding or flow banding is p r e s e n t ( p i . 1; f i g . 5 ). The t u f f s a r e a t l e a s t 500 f e e t (about 150 m e t e r s ) t h i c k and a re assumed to have been d e p o s i t e d n e a r l y h o r i z o n t a l l y . Considerable t i l t i n g and some f a u l t i n g a p p a r e n t l y occurred a f t e r emplacement of the c r y s t a l t u f f s and b e f o r e the emplacement of the younger v o l c a n i c rocks. Dips of up t o 50° a r e t h e r e s u l t of t h e t i l t i n g . Some o u t c r o p s of the younger Red Mountains Crystal Tuffs a r e h i g h l y j o i n t e d . Most of these j o i n t s a r e columnar and a r e the p r o b a b l e r e s u l t of c o o l i n g of an a s h - f l ow s h e e t. The Red Mountain Crystal Tuffs have been g r o s s l y s e p a r a t e d into two p a r t s . The o l d e r tuff has much l a r g e r , unbroken s a n i d i n e twins and q u a r t z c r y s t a l s . Small pieces of broken c r y s t a l s dominate the younger t u f f . The mineralogy of the two i s a p p r o x i m a t e l y the same. Clasts d e r i v e d from the o l d e r t u f f were found in t h e younger, broken c r y s t al t u f f at Red Mountain. Some p a r t s of what was mapped as t h e younger t u f f are g r a d a t i o n a l with the o l d e r tuff. T e x t u r e s of the o l d e r p a r t of t h e Red Mountain Crystal Tuffs a re p o o r l y p r e s e r v e d . The upper p a r t of the o l d e r t u f f is l o c a l l y an a l t e r n a t i n g bedded t u f f . Relative phenocryst r i c h beds a l t e r n a t e with t u f f n e a r l y devoid of p h e n o c r y s t s . This p a r t of the o l d e r tuff is probably a i r f a l l t u f f . Textures in the r e s t of the o l d e r p a r t of the c r y s t a l tuff a r e u n c l e a r such t h a t some of the rock included in the t u f f may a c t u a l l y be d i k e s . The g e n e s i s of the younger p a r t of the iure 5.-Photograph of bedding in t h e younger p a r t of the Red Mountain Crystal Tuffs. Photograph was taken on t h e south s i de of the h i g h e s t point in NW£ s e c . 23, T. 13 S . , R. 10 W. The bedding s t r i k e s N. 35° E, and dips 59° S, 33 Red Mountain Crystal Tuffs is more c l e a r . The younger t u f f s are very t h i c k , massive, h i g h l y welded t u f f s which a r e undoubtedly ash-flow t u f f s . Flattened l a p i l l i are not uncommon west of Red Mountain in t h i s u n i t . . The o l d e r tuff crops out in t h r e e p l a c e s in the P i c t u r e Rock H i l ls q u a d r a n g l e . The l a r g e s t o u t c r o p is a 4000 by 1000 foot (about 1300 by 400 meter) s t r i p on t h e e a s t e r n margin of the l a r g e o u t c r o p of Red Mountain Crystal Tuffs south of the J e r i c h o Road. Northwest of Kane Spring and west of West Mountain a r e two very much s m a l l e r outcrop a r e a s . Older t u f f l i t h o l o g y . The o l d e r c r y s t a l tuff is a brownish-pink to d a r k grey compact to welded tuff of r h y o l i t i c to q u a r t z l a t i t ic c o m p o s i t i o n . Local bleaching and baking with the c o r r e s p o n d i n g color v a r i a t i o n is not uncommon. No f o r e i g n rock fragments were noted. Most samples of the t u f f c o n t a i n from 25 t o 75 p e r c e n t phenocrysts in a groundmass of u n c e r t a i n composition. A few i n t e r b e d s of f i n e ­g r a i n e d t u f f with few to no p h e n o c r y s t s a r e p r e s e n t. Quartz g e n e r a l l y c o n s t i t u t e s 10 t o 25 p e r c e n t of the rock. It g e n e r a l l y forms embayed and f r a c t u r e d bipyramids and c r y s t a l fragments r a n g i n g in s i z e from 0.5 to 6 mm. F r a c t u r e s in the c r y s t a l s a r e not healed and include some groundmass m a t e r i a l . Many i n c l u s i o n s which look l i k e dust occur in the q u a r t z . Quartz bipyramids weather out of t h e rock in some p l a c e s. S a n i d i n e g e n e r a l l y c o n s i t u t e s 10 to 30 p e r c e n t of the rock. Euhedral to subhedral phenocrysts a r e up t o 1.6 mm. long. Crystal 34 fragments range in s i z e from 0.5 to 10 mm. Baveno twinning was observed in t h i n s e c t i o n . Sanidine twins commonly weather out of the rock. P l a g i o c l a s e (Anjj to An^y) g e n e r a l l y c o n s t i t u t e s 5 t o 30 p e r c e nt of the rock. Crystals and c r y s t a l fragments range in s i z e from 0.3 t o 6 mm. A few euhedra a r e p r e s e n t . Most of the p l a g i o c l a s e occurs as broken c r y s t a l s or fragments of c r y s t a l a g g r e g a t e s . The c r y s t a ls a r e twinned and have o s c i l l a t o r y zoning. The p l a g i o c l a s e is g e n e r a l ly a l t e r e d to c a r b o n a t e , c l a y , and b i o t i t e . Sphene is included in the p l a g i o c l a s e . B i o t i t e f l a k e s and s h o r t books c o n s t i t u t e d about 1 p e r c e n t of the rock. They range from 0.5 to 3 mm. a c r o s s . Inclusions of sphene and and a p a t i t e a r e p r e s e n t . Secondary hexagonal p l a t e s of a red mineral a r e p r e s e n t in t h e b i o t i t e ; they average l e s s than 0.01 mm. a c r o s s. The b i o t i t e is almost completely a l t e r e d to s e r i c i t e. Less than 1 p e r c e n t hornblende was p r e s e n t in the rock. The hornblende is completely replaced by h e m a t i t e , b i o t i t e , and a f i n e ­g r a i n e d , low b i r e f r i n g e n t m a t e r i a l . The c r y s t a l o u t l i n e and cleavage t r a c e s a r e s t i l l e v i d e n t . Tiny z i r c o n i n c l u s i o n s are p r e s e n t. If o t h e r mafic m i n e r a l s were o r i g i n a l l y p r e s e n t , they a r e now t o o a l t e r e d to be recognized. The groundmass is h i g h l y a l t e r e d to c a l c i t e , s e r i c i t e , c l a y s, b i o t i t e , and c h l o r i t e such t h a t the o r i g i n a l c h a r a c t e r is not d e f i n i t e. A few small p l a g i o c l a s e euhedra (0.5 mm.), b i o t i t e f l a k e fragments ( 0 . 2 mm.), secondary q u a r t z , z i r c o n euhedra (0.01 to 0.02 mm.), and h e m a t i t e g r a n u l e s a r e p r e s e n t in the groundmass. 35 Younger t u f f U t h o l o g y . The younger p a r t of the Red Mountain Crystal Tuffs is a brownish-pink to pink welded rock of r h y o l i t i c to q u a r t z - l a t i t i c composition. It is composed of c r y s t a l s , c r y s t a l fragments, and welded g l a s s shards and rock fragments. Locally the t u f f may be b r i c k red or white due t o thermal a l t e r a t i o n by l a t e r v o l c a n i c s, I n c l u s i o n s of s e v e r a l d i f f e r e n t foreign rock types occur in the younger t u f f : q u a r t z i t e , s h a l e , d i o r i t e porphyry (Keg Spring A n d e s i t e ), an i n t e r m e d i a t e p l a g i o c l a s e porphyry, and the o l d e r tuff of the Red Mountain Crystal Tuffs. Fragments of m a t e r i a l i d e n t i c a l to the younger t u f f i t s e l f a l s o o c c u r s . D i s t r i b u t i o n of t h e s e fragments is not uniform; c l a s t s a r e more abundant at Red Mountain than at o t h e r p l a c e s. Some of t h e t u f f near Red Mountain c o n s i s t s of more than 75 p e r c e n t of c l a s t s of m a t e r i a l i d e n t i c a l to the e n c l o s i n g younger tuff. C r y s t a l s and c r y s t a l fragments c o n s t i t u t e 35 t o 70 p e r c e n t of the rock and a r e included in what was a groundmass of g l a s s s h a r d s. S a n i d i n e c r y s t a l fragments c o n s t i t u t e 8 t o 25 p e r c e n t of t h e rock. They range in s i z e from 0.5 to 5 mm. and average 2 mm. P l a g i o c l a s e c r y s t a l fragments (Any t o An3g) c o n s t i t u t e 0 t o 25 p e r c e n t of the rock. They range in s i z e from 0.02 t o 1.5 mm. Some of the c r y s t a l fragments have been s h a t t e r e d in p l a c e , p o s s i b ly because of compaction or post-emplacement flowage of the t u f f. O s c i l l a t o r y zoning and very f i n e a l b i t e twinning is p r e s e n t. Myrmekitic i n t e r g r o w t h s a r e p r e s e n t on some c r y s t a l margins, The p l a g i o c l a s e in the t u f f in s e c . 30, T. 12 S . , R. 9 W., has an o r t h o c l a s e rim. The p l a g i o c l a s e v a r i e s in f r e s h n e s s from one p l a ce t o a n o t h e r . In some p l a c e s , it is n e a r l y u n a l t e r e d ; in o t h e r s , it is 36 a t l e a s t 80 p e r c e n t a l t e r e d to s e r i c i t e , c a l c i t e , e p i d o t e , iron o x i d e s, t h e c l i n o z o i s i t e. Quartz c r y s t a l fragments and bipyramids c o n s t i t u t e 12 t o 40 p e r c e n t of the rock. They range in s i z e from 0.02 to 5 mm. The q u a r tz as embayed and rounded and in some p l a c e s has f r a c t u r e s which a r e f i l l ed w i t h groundmass m a t e r i a l . Strain l a m e l l a e were noted in some fragments. Black b i o t i t e f l a k e s or s h o r t books c o n s t i t u t e d up t o 3 p e r c e n t of t h e rock. The b i o t i t e ranges from 0.2 to 1.5 mm. a c r o s s . Some bending o f the f l a k e s has o c c u r r e d . A l t e r a t i o n v a r i e s from one a r e a to a n o t h e r, but averages about 50 p e r c e n t . A l t e r a t i o n is e i t h e r e x t e n s i v e iron o x i d e replacement or iron oxide rims with q u a r t z expansion pods. Less than 1 p e r c e n t hornblende was p r e s e n t in the rock. It is ( g e n e r a l l y a l t e r e d to iron oxides with only the c r y s t a l o u t l i n e and c l e a v a g e t r a c e s being p r e s e r v e d. Less than 1 p e r c e n t a u g i t e was p r e s e n t in the rock. It always has an iron oxide rim. M a g n e t i t e euhedra c o n s t i t u t e l e s s than 1 p e r c e n t of the rock. It ranges in s i z e from 0.05 to 0.1 mm. Hematite is c l o s e l y a s s o c i a t ed w i t h the m a g n e t i t e. Trace amounts of o t h e r m i n e r a l s a r e p r e s e n t . Euhedral to subhedral sphene ranges in s i z e from 0.1 to 0.3 mm. and c o n s t i t u t e s much l e s s than 1 p e r c e n t of the rock. Zircon euhedra a l s o c o n s t i t u t e much l e s s than 1 p e r c e n t of the rock. They a r e p r e s e n t in the groundmass and q u a r tz and range in s i z e from 0.03 to 0.05 mm. Radiation-damaged haloes surround the z i r c o n . Apatite euhedra occur in the groundmass and f e l d s p a r and range in s i z e from 0.01 to 0.08 mm. The groundmass was a t one time fused g l a s s shards and is now t h o r o u g h l y d e v i t r i f i e d . The d e v i t r i f i c a t i o n types a r e p a t c h i ly d i s t r i b u t e d and i n c l u d e s p h e r u l i t i c growth, a x i o l i t i c growth of f e l d s p a r and s i l i c a in g l a s s s h a r d s , and replacement by g r a n o p h y r ic a g g r e g a t e s of f e l d s p a r and s i l i c a . A c o n c e n t r i c p a t t e r n caused by v a r i a t i o n of g r a i n s i z e and composition is superimposed on t h e r a d i al s t r u c t u r e of the s p h e r u l i t e s. Younger vol can ? c rocks About h a l f of the bedrock in t h e P i c t u r e Rock H i l l s quadrangle c o n s i s t s of t h e younger v o l c a n i c r o c k s . These rocks a r e s e p a r a t ed from t h e o l d e r v o l c a n i c rocks by an unconformity. An e r o s i o n al s u r f a c e which l o c a l l y had up t o hOO f e e t (about 120 m e t e r s ) of r e l i ef was p r e s e n t before the emplacement of the younger u n i t s . Units of the younger rocks- a r e of p o s s i b l e l a t e Miocene t o l a t e middle P l i o c e ne age (p. 76). Included in the younger v o l c a n i c rocks a r e t h e Keg Mountains R h y o l i t e , the Keg Mountains T u f f s , the Drum Mountains R h y o l i t e , and t h e P i c t u r e Rock Quartz L a t i t e Porphyry. The Keg r h y o l i t e s and t u f f s could not be s t r i c t l y divided into subgroups as t h e Topaz Mountain R h y o l i t e and v i t r i c t u f f had been in t h e Thomas Range t o the west ( S t a a t z and Carr, 1964, p. 8 6 - 8 7 ) . The subgroups in the Thomas Range c o n s i s t of, in ascending o r d e r , v i t r i c t u f f, v o l c a n i c b r e c c i a , and r h y o l i t e lava flows. The same type of e r u p t i ve sequence is p r e s e n t in the s o u t h w e s t e r n Keg Mountains. The e r u p t i on of r h y o l i t e lava flows was g e n e r a l l y preceded by d e p o s i t i o n of t u f f. Keg Mountains Tuff The name Keg Mountains Tuff has been given to the e q u i v a l e nt of the v i t r i c tuff of the Thomas Range. This t u f f occupies t h e same s t r a t i g r a p h i c p o s i t i o n as the Topaz Mountain Tuff and is the same approximate composition and age. The Keg Mountains Tuff crops out throughout the a r e a and forms l e s s than 10 p e r c e n t of the bedrock. Remnants of the Keg Mountains Tuff a r e found u n d e r l y i n g r h y o l i te f l o w s , around e r u p t i v e c e n t e r s , and f i l l i n g in old v a l l e y s and low a r e a s . The t u f f is highly s u s c e p t a b l e to e r o s i o n because of i ts g e n e r a l lack of i n d u r a t i o n . The n a t u r e of t u f f d e p o s i t i o n and the p r e s e n t o u t c r o p p a t t e r n of the Keg Mountains Tuff i n d i c a t e t h a t the t u f f unit was much more e x t e n s i v e than i t is today. Where t h e tuff is p r o t e c t e d by more r e s i s t a n t r h y o l i t e capping, i t g e n e r a l l y forms r a t h e r s t e e p s l o p e s . The t u f f is g e n e r a l l y well exposed a l t h o u g h in p l a c e s it is p a r t i a l l y covered with r h y o l i t e t a l u s. In s e e s . 2 and 3, T. 13 S . , R. 10 W., and s e e s . 26 and 35, T. 12 S . , R. 10W., the t u f f is u n p r o t e c t e d and was eroded to form a low r e l i e f s u r f a c e . The s u r f a c e was covered with a veneer of r h y o l i t e t a l u s and is now being exhumed. Where i t is exposed the base of the t u f f u n i t commonly o v e r l i es u n i t s of the o l d e r v o l c a n i c rocks or o l d e r Keg Mountains R h y o l i t e. The a g g r e g a t e t h i c k n e s s of the Keg Mountains Tuff is highly v a r i a b l e because of the n a t u r e of tuff d e p o s i t i o n (p. 71-75) and e r o s i o n . Tuffs are g e n e r a l l y t h i c k e r near t h e i r vent and d e c r e a se in t h i c k n e s s and c l a s t s i z e away from the v e n t . The t h i c k n e s s es a r e commonly h i g h l y modified by e r o s i o n because of the lack of i n d u r a t i o n of the t u f f . Where t h e base and the top a r e exposed, a g g r e g a t e t h i c k n e s s of the Keg Mountains Tuff ranges from about 30 f e e t (about 10 m e t e r s ) to n e a r l y 350 f e e t (about 100 m e t e r s ). The Keg Mountains Tuff in the P i c t u r e Rock H i l l s quadrangle is t h e record of tens of e x p l o s i v e e r u p t i o n s . Several d i f f e r e n t types of v o l c a n i c l a s t i c s a r e p r e s e n t . A i r - f a l l , surge, and p y r o c l a s t i c - flow types a r e well r e p r e s e n t e d (p. 7 1 ) . Welding of some of the p y r o c l a s t i c - f 1 o w beds is p a r t i c u l a r l y well developed south of Kane S p r i n g . Minor normal f a u l t i n g in t h e t u f f was noted in a few p l a c e s. Nowhere did t h e v e r t i c a l displacement exceed 2 f e e t (about two t h i r ds of a m e t e r ) , however. Li t h o l o g y . A b r e c c i a commonly occurs at the base of the t u f f . The n a t u r e of the basal b r e c c i a is d i r e c t l y dependent on t h e u n d e r l y i ng rock and t h e vent wall composition. Northeast of Kane S p r i n g , the basal b r e c c i a is up t o 1.2 meters (about k f e e t ) t h i c k and is composed of d i o r i t e porphyry (Keg Spring Andesite) c l a s t s in a dense d a r k - r ed ashy m a t r i x . Above t h i s bed is a s l i g h t l y d i f f e r e n t b r e c c i a - t u f f. The c h i e f c l a s t s p r e s e n t a r e d e r i v e d from the younger Red Mountain C r y s t a l Tuffs. The basal bed in s e e s . 23 and 26, T. 13 S . , R. 10 W., i s composed of l a r g e blocks of Red Mountain Crystal Tuffs in an ash m a t r i x . The ash m a t r i x is i d e n t i c a l to t h e o v e r l y i n g t u f f ; the basal b r e c c i a was a p p a r e n t l y formed by i n c o r p o r a t i o n of s u r f i c i a l material d u r i n g e r u p t i o n of the i n c o r p o r a t i o n of vent wall m a t e r i a l . The ho basal b r e c c i a in t h i s area is up t o 25 f e e t (about 8 m e t e r s ) t h i c k. The bulk of the t u f f is a f r i a b l e y e l l o w i s h to w h i t i s h rock c o n s i s t i n g c h i e f l y of pumice l a p i l l ? in an ash m a t r i x . Minor amounts of small c r y s t a l s or c r y s t a l fragments a r e l o c a l l y p r e s e n t . Clasts of rocks o t h e r than pumice a r e l o c a l l y dominant. Greenish-yellow, grey, o r a n g e , green, and b r i c k - r e d reds a r e a l s o p r e s e n t in t h e t u f f . The t u f f is baked to a medium t o dark pink where i t is o v e r l a i n or intruded by a younger r h y o l i t e . The t u f f has t h r e e g r o s s l y d i f f e r e n t p a r ts in some p l a c e s : a basal b r e c c i a , a middle bedded s e c t i o n of t u f f and l a p i l l i s t o n e , and, uncommonly, an upper v i t r i c t u f f . The basal b r e c c i a was b r i e f l y d e s c r i b e d above. The upper v i t r i c tuff is composed e s s e n t i a l l y of a few q u a r t z and s a n i d i n e c r y s t a l s in an ash m a t r i x. The l i t h o l o g i c d e s c r i p t i o n of the major p a r t of the t u f f follows. The rock fragments content of the u n i t is v a r i a b l e ; rock fragments c o n s t i t u t e from about 10 t o over 80 p e r c e n t of the rock volume. Nearly a l l of the rock fragments a r e of v o l c a n i c o r i g i n . Most of the rock fragments a r e l i g h t colored pumice in v a r i o u s s t a g e s of d e v i t r i f i c a t i on or a l t e r a t i o n . Other major types p r e s e n t a r e r h y o l i t i c g l a s s , grey r h y o l i t e , p o r p h y r i t i c r h y o l i t e , baked-red r h y o l i t i c and q u a r t z l a t i t ic c r y s t a l t u f f , and baked-red d i o r i t e porphyry (Keg Spring A n d e s i t e ). Less common a r e g r a n o d i o r i t e porphyry, q u a r t z i t e , l i m e s t o n e , and b i o t i t e g r a n i t e c l a s t s. The pumice, g l a s s , and r h y o l i t e fragments a r e l o c a l l y highly d e v i t r i f i e d or h i g h l y a l t e r e d . The pumice commonly has become a m i x t u r e of c l a y and c a r b o n a t e m i n e r a l s which can e a s i l y be removed w i t h a f i n g e r n a i l . Unaltered or v i t r i c m a t e r i a l is a l s o l o c a l l y p r e s e n t. Pore spaces and c a v i t i e s l o c a l l y c o n s t i t u t e up to 20 p e r c e nt of the rock volume. C a v i t i e s a r e up to 3 mm. a c r o s s , but average c l o s e r to 1 mm. Chalcedony p a r t i a l l y f i l l s some of the c a v i t i e s. C r y s t a l s or c r y s t a l fragments are g e n e r a l l y v i s i b l e in hand specimen. They c o n s t i t u t e from 2 t o 20 p e r c e n t of the rock. Most of the c r y s t a l s a r e q u a r t z and s a n i d i n e . The q u a r t z and s a n i d i ne form anhedral to euhedral c r y s t a l s as well as c r y s t a l fragments. Minor amounts of b i o t i t e , p l a g i o c l a s e , and m a g n e t i t e a r e a l s o p r e s e n t. P l a g i o c l a s e is anhedral to subhedral or f r a g m e n t a l . It has some o s c i l l a t o r y zoning and is twinned. The c r y s t a l l i n e m a t e r i a l is g e n e r a l l y l e s s than 2 mm. a c r o s s. The m a t r i x of the t u f f is a f i n e ash which c o n s i s t s of c l e a r tan t o brown g l a s s . Most of the g l a s s in t h i n s e c t i o n was s h a r d s . The m a t r i x of the t u f f exposed s o u t h e a s t of Red Mountain is c a l c i t e . The c l a s t s in t h i s t u f f are for the most p a r t f r e s h. • Keg Mountains Rhyolite • R h y o l i t e crops out throughout the a r e a and comprises about kO p e r c e n t of the e x p o s u r e . Outside the a r e a , s i m i l a r r h y o l i t e occurs in t h e e a s t e r n and n o r t h w e s t e r n Keg Mountains, n o r t h e r n Drum Mountains, Thomas Range, and Dugway Range ( p i . 2 ) . The r h y o l i t e s of t h e s e o t h er a r e a s and the Keg Mountains R h y o l i t e a r e a p p a r e n t l y i d e n t i c al 1 i t h o l o g i c a l l y . They a r e p o s s i b l y d e r i v e d from the same magma. Another s i m i l a r , but not i d e n t i c a l , r h y o l i t e occurs in the Honeycomb H i l l s , west of the Fish Springs Range. 42 The Keg Mountains R h y o l i t e is moderately r e s i s t a n t to e r o s i on and forms r a t h e r s t e e p s l o p e s . Soil cover is minimal so t h e r h y o l i te t e n d s not to support v e g e t a t i o n. R h y o l i t e in t h e Thomas Range t o t h e west forms l a r g e flows. Flows a r e p r e s e n t in the s o u t h w e s t e r n Keg Mountains, but t h e s e a re of small e x t e n t and a r e more l i k e domes than flows. Many r h y o l i te d i k e s and plugs a r e p r e s e n t . Why l a r g e r e x t r u s i v e r h y o l i t e bodies a r e p r e s e n t in one a r e a and not in a n o t h e r is not known. The r h y o l i t e flows g e n e r a l l y r e s t conformably on Keg Mountains Tuff. Where t h e Keg Mountains Tuff r e s t s on r h y o l i t e , the upper c h i ll or b r e c c i a f a c i e s of t h e r h y o l i t e was a p p a r e n t l y eroded p r i o r to or was scoured off during t u f f d e p o s i t i o n . Keg Mountains R h y o l i t e a l so unconformably o v e r l i e s Keg Spring A n d e s i t e and t h e younger Red Mountain C r y s t a l Tuffs. Keg Mountains R h y o l i t e i n t r u s i v e s i n t r u d e Keg Spring A n d e s i t e , the younger Red Mountain Crystal T u f f s , Keg Mountains Tuff, and o t h e r Keg Mountains R h y o l i t e b o d i e s . These i n t r u s i v e s have been given a s e p a r a t e d e s i g n a t i o n on P l a t e 1. All of the r h y o l i t e was not emplaced at the same time. Where p o s s i b l e , local age d i f f e r e n c e s were i d e n t i f i e d and i l l u s t r a t e d on P l a t e 1. Keg Mountains R h y o l i t e bodies of two d i s t i n c t ages occur a t several places in the q u a d r a n g l e : West Mountain, the mountain e a s t of Kane Spring ( f i g . 3 ) , the south s i d e of PB H i l l , and in s e e s. 13, 23, and 24, T. 13 S . , R. 10 W. C o r r e l a t i o n of e i t h e r of the two d i f f e r e n t age r h y o l i t e s from one l o c a l i t y ot another is impossible because of the noncontinuous n a t u r e of t h e r h y o l i t e s , lack of good e x p o s u r e , and the l i t h o l o g i c s i m i l a r i t y. A r h y o l i t e plug south of the J e r i c h o Road e x h i b i t s many of the c h i l l f e a t u r e s d e s c r i b e d on pages 46 t o 49. The c h i l l border of the plug is p a r t i c u l a r l y well developed. The black o b s i d i a n border is about 1 meter wide and is f a i r l y c o n t i n u o u s . Just i n s i d e the black g l a s s is the red s p h e r u l i t i c r h y o l i t e . A f a i r l y sharp c o n t a c t between t h e s e two f a c i e s is p r e s e n t in the southwestern part of the plug ( f i g . 6 ) . Flow banding is p r e s e n t in both f a c i e s ( f i g . 6 ) . The red s p h e r u l i t i c r h y o l i t e f a c i e s grades i n t o the grey f a c i e s . Figure 7 is a photograph showing the s p a c i a l r e l a t i o n s h i p s of the f a c i e s in the s o u t h w e s t e r n p a r t of the plug. The younger Keg Mountains R h y o l i t e exposed at V/est Mountain is more p o r p h y r i t i c than the r e s t of the Keg Mountains R h y o l i t e . It s t r o n g l y resembles the P i c t u r e Rock Quartz L a t i t e Porphyry, but it has c o n s i d e r a b l y l e s s p l a g i o c l a s e . The r h y o l i t e at West Mountain l o c a l ly has up t o 50 -percent p h e n o c r y s t s In a g l a s s y or d e v i t r i f i e d groundmass. The younger r h y o l i t e at West Mountain was e r u p t e d from the c e n t r al p a r t of West Mountain and formed a dome which probably did not have much more a r e a l e x t e n t than it has today. Flow b r e c c i a is p r e s e nt everywhere on the c h i l l margin of the younger r h y o l i t e at West Mountain. It grades downward i n t o the u n d e r l y i n g t u f f - b r e c c i a and has i n c o r p o r a t e d some t u f f - b r e c c i a. L i t h o l o g y . The Keg Mountains R h y o l i t e v a r i e s in c o l o r , t e x t u r e, banding, abundance and type of p h e n o c r y s t s , and the p r e s e n c e or absence of s p h e r u l i t e s or l i t h o p h y s a e . These f e a t u r e s vary such that p a r t s of the same r h y o l i t e body may look l i k e d i f f e r e n t r h y o l i t e s. F i g u r e 6.-Photograph of the t y p i c a l c h i l l border of a r h y o l i te . i n t r u s i v e . The c h i l l border is p a r t of the r h y o l i t e plug south of the J e r i c h o Road. The hammer is on t h e o b s i d i a n - r ed s p h e r u l i t i c r h y o l i t e c o n t a c t . The hammer is 18 inches long. F i g u r e 7 . - - P h o t o g r a p h of the s o u t h w e s t e r n p a r t of the r h y o l i t e plug s o u t h of the J e r i c h o Road. Locations of o b s i d i a n , red s p h e r u l i t ic r h y o l i t e and grey r h y o l i t e a r e i n d i c a t e d on t h e photograph. F i g u r e 6 was taken on the extreme r i g h t o u t c r o p of the o b s i d i a n. he A l s o , specimens from the same f a c i e s of d i f f e r e n t r h y o l i t e bodies cannot be d i s t i n g u i s h e d one from a n o t h e r. The younger r h y o l i t e s in the Keg Mountains, l i k e the younger r h y o l i t e s in t h e Thomas Range, can be roughly d i v i d e d i n t o t h r e e major f a c i e s : o b s i d i a n , red s p h e r u l i t i c r h y o l i t e , and grey r h y o l i t e . In a d d i t i o n r h y o l i t i c flow b r e c c i a and green g l a s s a r e included in the r h y o l i t e u n i t s . The boundaries between the f a c i e s vary from very sharp d i s t i n c t c o n t a c t s to g r a d a t i o n a l changes o c c u r r i n g over several m e t e r s. Some flow b r e c c i a is a l s o a s s o c i a t e d with each of the f a c i e s. A t h i n layer of o b s i d i a n is g e n e r a l l y p r e s e n t at the base of flows and a t the borders of dikes and p l u g s . The red s p h e r u l i t ic r h y o l i t e is g e n e r a l l y above the o b s i d i a n flow base or w i t h i n the o b s i d i a n c h i l l border of dikes and p l u g s . The t h i c k n e s s of the red s p h e r u l i t i c r h y o l i t e v a r i e s c o n s i d e r a b l y , even w i t h i n the same r h y o l i t e body. It v a r i e s from a few c e n t i m e t e r s to tens of m e t e r s. The red s p h e r u l i t i c r h y o l i t e g e n e r a l l y grades i n t o grey r h y o l i te which comprises the bulk of the Keg Mountains R h y o l i t e . A minor q u a n t i t y of v e s i c u l a r g l a s s y to a p h a n i t i c r h y o l i t e occurs at the tops of a few r h y o l i t e s. S t a a t z and Carr (1964, p. 99-102, p i . 1) d e s c r i b e d and mapped r h y o l i t i c "green g l a s s " as a u n i t s e p a r a t e from t h e i r Topaz Mountain R h y o l i t e . Because the green g l a s s bodies in the s o u t h w e s t e r n Keg Mountains a r e very small (p. 5 1 ) , it is included in the Keg Mountains R h y o l i t e unit on P l a t e 1. The green g l a s s bodies are a p p a r e n t l y the r e c o r d of s e p a r a t e r h y o l i t i c i n t r u s i o n s in vent and h i g h l y f r a c t u r ed a r e a s . hi The d i f f e r e n t r h y o l i t e f a c i e s were not mapped s e p a r a t e l y because t h e y could not be a d e q u a t e l y r e p r e s e n t e d at 1/2*4,000. R h y o l i t e flow b r e c c i a is g r a d a t i o n a l with u n b r e c c i a t e d r h y o l i te and is included in the Keg Mountains R h y o l i t e on P l a t e 1. The flow b r e c c i a appears to have been formed by the breaking up of f a i r l y s o l i d, a l r e a d y cooled r h y o l i t e and is more common near the base, edges, or t o p s of r h y o l i t e flows. O b s i d i a n . The o b s i d i a n is g e n e r a l l y black or b l a c k i s h grey, but dark brown, red-brown, g r e y - g r e e n , grey, and r e d d i s h - p i n k c o l o r i n g is not uncommon. Flow banding is very common in t h i s f a c i e s , although not as d i s t i n c t i v e as i t is in the o t h e r f a c i e s . in a few p l a c e s , the o b s i d i a n is composed of black and red a l t e r n a t i n g bands of g l a s s. P e r l i t i c cracks a r e a l s o common. All of the o b s i d i a n in the quadrangle is p o r p h y r i t i c . Phenocrysts c o n s t i t u t e 1 t o 20 p e r c e n t of the rock and average about 5 p e r c e n t. Subequal amounts of q u a r t z and s a n i d i n e a r e p r e s e n t as the major p h e n o c r y s t s . Less than 1 p e r c e n t each of p l a g i o c l a s e , b i o t i t e , sphene, z i r c o n , and m a g n e t i t e is p r e s e n t . Phenocrysts range in s i z e from 0 . 3 to 2 mm. and average 1 mm. Red, brown, grey, and black r a d i a l l y s t r u c t u r e d s p h e r u l i t es ( p . 61) occur in t h i s f a c i e s . They range from s t r u c t u r e s v i s i b le o n l y m i c r o s c o p i c a l l y to s t r u c t u r e s up to 2 cm. in d i a m e t e r. A second type r a d i a l l y s t r u c t u r e d s p h e r u l i t e was found in the o b s i d i a n f a c i e s northwest of Red Mountain. This type is d e s c r i b ed on page 6 1. 48 Hemispherical s t r u c t u r e s of s o l i d r h y o l i t e (p. 63) are g e n e r a l ly p r e s e n t in the upper p a r t of the o b s i d i a n f a c i e s . They a r e not common In d i k e s p o s s i b l y because the c h i l l f a c i e s a r e too narrow for the development of s p h e r u l i t e s . The c o n t a c t between the o b s i d i a n and red s p h e r u l i t i c r h y o l i t e is in p l a c e s g r a d a t i o n a l with l e n s e s of the red s p h e r u l i t i c r h y o l i t e enclosed in the o b s i d i a n . The s o l i d hemispherical s t r u c t u r e s d e s c r i b e d on page 63 a r e a c t u a l l y more c h a r a c t e r i s t i c of t h e red s p h e r u l i t i c r h y o l i t e than they a r e of the o b s i d i a n . These s t r u c t u r e s mark the beginning of the change from o b s i d i a n to red s p h e r u l i t i c r h y o l i t e. L o c a l l y the o b s i d i a n is a u t o b r e c c i a t e d and grades into a flow b r e c c i a . Two b a s i c types of flow b r e c c i a a r e p r e s e n t . One is composed of angular blocks of black or grey o b s i d i a n is a m a t r i x of red to b l a c k o b s i d i a n . The second type is e s s e n t i a l l y the f i r s t type which has undergone p a r t i a l welding. It is composed of l e n t i c u l a r o b s i d i an b l e b s ( g e n e r a l l y blacker than the s u r r o u n d i n g o b s i d i a n ) in an o b s i d i an m a t r i x . The m a t r i x has a well developed flow s t r u c t u r e and does not appear to have been composed of s h a r d s. Red s p h e r u l i t i c r h y o l i t e . The red s p h e r u l i t i c f a c i e s of the r h y o l i te i s composed of numerous small s p h e r u l i t e s in a g l a s s y or a p h a n i t ic groundmass. It is g e n e r a l l y r e d d i s h brown but is a l s o v a r i o u s shades of red or grey. Although the s p h e r u l i t e s range in s i z e from s t r u c t u r e s v i s i b le o n l y m i c r o s c o p i c a l l y to s t r u c t u r e s 5 mm. in d i a m e t e r . Most a r e e a s i ly v i s i b l e in hand specimen. S p h e r u l i t e s c o n s t i t u t e from 50 to 85 p e r c e n t of the rock and a r e g e n e r a l l y not p e r f e c t l y s p h e r i c a l . They 49 commonly c o a l e s c e along flow s t r u c t u r e to form r a t h e r continuous l a y e r s with u n d i s t u r b e d g l a s s in between. The s p h e r u l i t e s commonly i n c l u d e or p a r t l y include p h e n o c r y s t s and p r e - e x i s t i n g flow o r i e n t ed c r y s t a l l i t e s or m i c r o l i t e s. Flow s t r u c t u r e is very common in t h i s f a c i e s and is s l i g h t ly more i r r e g u l a r than the flow s t r u c t u r e of the o b s i d i a n f a c i e s. Some of the red s p h e r u l i t i c r h y o l i t e has up t o 5 p e r c e nt p h e n o c r y s t s . These p h e n o c r y s t s are c h i e f l y subhedral to euhedral q u a r t z and s a n i d i n e . Minlscule amounts of b i o t i t e , p l a g i o c l a s e, m a g n e t i t e , and z i r c o n a r e p r e s e n t in a few p l a c e s. The groundmass is g e n e r a l l y a red-brown g l a s s which c o n t a i ns some c r y s t a l l i t e s or m i c r o l i t e s . A mosaic of q u a r t z and s a n i d i ne d e v i t r i f i c a t i o n products are groundmass in a few p l a c e s. Grey rhyol? t e . The grey f a c i e s of the r h y o l i t e is g e n e r a l l y a l i g ht g r e y , but in some p l a c e s it is p i n k i s h or p u r p l i s h grey, l i g ht brownish grey, medium grey, or w h i t e . The grey r h y o l i t e is g e n e r a l ly a dense a p h a n i t i c rock c o n t a i n i n g a small q u a n t i t y of s p h e r u l i t e s and p h e n o c r y s t s of q u a r t z and s a n i d i n e in a g l a s s y or d e v i t r i f i ed groundmass. L o c a l l y the r h y o l i t e e x h i b i t s a r a t h e r p e c u l i a r weathering a p p e a r a n c e which has been c a l l e d "honeycomb" by S t a a t z and Carr (1964, p. 9 7 ) . This weathering is probably the r e s u l t of the m i g r a t i o n of s i l i c a and calcium in t h e s e r o c k s . Water c o n t a i n i ng d i s s o l v e d s i l i c a and calcium e v a p o r a t e s at the o u t e r s u r f a c e s of the r h y o l i t e o u t c r o p and d e p o s i t s the s i l i c a and calcium. Their d e p o s i t i on 50 c a u s e s the o u t e r few c e n t i m e t e r s to be more i n d u r a t e d than the i n t e r i o r of the o u t c r o p . The l e s s i n d u r a t e d r h y o l i t e is *-Sen p a r t i a l l y weathered out leaving p a r t of the more d u r a b l e o u t e r s u r f a c e. Flow banding is very common in the grey r h y o l i t e f a c i e s and is more c o n t o r t e d than the banding of the o b s i d i a n or red s p h e r u l i t ic r h y o l i t e . The flow banding near the base of the flows tends to be more t a b u l a r and becomes s t e e p e r , more i r r e g u l a r , and c o n t o r t ed h i g h e r in the flows. Above the r e g u l a r , t a b u l a r flow banding near t h e base is a zone where the flow banding is folded into broad f o l d s. Above t h i s , the banding is c o n t o r t e d into t i g h t , smaller and smaller a m p l i t u d e f o l d s . The flow laminae a r e c l o s e l y spaced and continuous near the base. Laminae become t h i c k e r and more obscure with height above the base. The flow banding is the s i t e for l a y e r s of s p h e r u l i t e s and l i t h o p h y s a e in many p l a c e s. Near what was the top of some r h y o l i t e flows, is a l i g h t g r e y i s h - brown v e s i c u l a r r h y o l i t e . This g l a s s y rock c o n t a i n s numerous small c a v i t i e s e l o n g a t e d p a r a l l e l to flow banding. S p h e r u l i t e s ( r a d i a l l y s t r u c t u r e d ) are p r e s e n t in a few p l a c es in the grey r h y o l i t e f a c i e s . They a r e g e n e r a l l y m i c r o s c o p i c and are very much l e s s abundant in the grey r h y o l i t e f a c i e s than they a r e in o t h e r f a c i e s. Lithophysae (p. 6l) a r e l o c a l l y abundant in the grey r h y o l i t e; t h e y a r e not found in the o b s i d i a n or red s p h e r u l i t i c r h y o l i t e. T h e i r diameter ranges from 0.8 to 20 cm. Topaz, h e m a t i t e , q u a r t z, and b i x b y i t e have been found a s s o c i a t e d with l i t h o p h y s a e. 51 Small i r r e g u l a r c a v i t i e s are a l s o p r e s e n t in the grey r h y o l i t e. These c a v i t i e s are up t o 5 cm. a c r o s s and in some p l a c e s c o n t a in c r y s t a l s of t o p a z , p s e u d o b r o o k i t e , h e m a t i t e , and q u a r t z (p. 6 5 - 7 0 ). The grey r h y o l i t e l o c a l l y has up to 30 p e r c e n t p h e n o c r y s t s. The average is c l o s e r to 10 p e r c e n t . Most of the p h e n o c r y s t s are m i c r o s c o p i c . Sanidine and q u a r t z a r e p r e s e n t in subequal amounts and a r e the c h i e f p h e n o c r y s t s . They range from 0.3 to 2 mm. in s i z e. P l a g i o c l a s e l o c a l l y c o n s t i t u t e s up to h p e r c e n t of the rock. It o c c u r s as subhedral to euhedral c r y s t a l s t h a t range from 0.1 to 1.3 mm. long. Very few p l a g i o c l a s e c r y s t a l s were used to determine c o m p o s i t i o n . The composition determined ranges from A n ^ to An-^y. More m a t e r i a l should be examined. B i o t i t e and m a g n e t i t e a r e the chief d a r k m i n e r a l s in the r h y o l i t e and never c o n s t i t u t e more than 1 p e r c e nt of the rock. P l a g i o c l a s e , b i o t i t e , and m a g n e t i t e a r e not p r e s e nt e v e r y w h e r e . - Zircon, sphene, t o p a z , h e m a t i t e , and hornblende each c o n s t i t u t e c o n s i d e r a b l y l e s s than 1 p e r c e n t of the rock. The groundmass is e i t h e r g l a s s or d e v i t r i f i c a t i o n p r o d u c t s. The g l a s s ranges in c o l o r from n e a r l y c o l o r l e s s to a medium brown. C r y s t a l l i t e s , m i c r o l i t e s , and p e r l i t i c cracks have been noted in some p l a c e s . D e v i t r i f i c a t i o n products of the g l a s s a r e g e n e r a l l y grano-p h y r i c a g g r e g a t e s of f e l d s p a r and s i l i c a. Green g l a s s . Included with the Keg Mountains R h y o l i t e on P l a t e 1 a re small a r e a s of what S t a a t z and Carr (1964, p. 99-102) termed "green g l a s s " . All but two of the green g l a s s o u t c r o p s p r e s e n t are too small to r e p r e s e n t on P l a t e 1. The l a r g e s t o u t c r o p is about 200 52 m e t e r s in diameter and is in the S£NE£ sec 25, T. 12 S . , R. 10 W. The green g l a s s forms i r r e g u l a r bodies of v a r i a b l e s i z e. The green g l a s s grades i n t o the Keg Mountains R h y o l i t e and Keg Mountains Tuff. The green g l a s s has melted and p a r t i a l l y a s s i m i l a t ed some of the t u f f and r h y o l i t e ( S t a a t z and Carr, 1964, p. 100). The t r a n s i t i o n zone between the green g l a s s and the t u f f or r h y o l i t e is g e n e r a l l y highly d e v i t r i f i e d or a l t e r e d. The green g l a s s is a green or g r e e n i s h - g r e y highly p e r l i t i c g l a s s. L o c a l l y the g l a s s c o n t a i n s numerous r a d i a l l y s t r u c t u r e d s p h e r u l i t e s. These s p h e r u l i t e s range in s i z e from s t r u c t u r e s v i s i b l e only micro­s c o p i c a l l y to s t r u c t u r e s 1.5 cm. in d i a m e t e r . Hollow s t r u c t u r es c a l l e d "thunder eggs" a r e a l s o p r e s e n t in the green g l a s s in a few p l a c e s (p. 63). Both s t r u c t u r e s weather out of the green g l a s s. Flow s t r u c t u r e is p a r t i c u l a r l y well developed l o c a l l y. C r y s t a l l i t e s and m i c r o l i t e s are a l i g n e d by flowage. Drum Mountains Rhyolite A g r e y r h y o l i t e flow occurs in the southwestern corner of the q u a d r a n g l e ( p i . l ) . This r h y o l i t e was mapped s e p a r a t e l y because it i s p a r t of the r h y o l i t e s which were e r u p t e d from sources in the Drum Mountains. It i s , however, i d e n t i c a l 1 i t h o l o g i c a 1 1 y to the grey r h y o l i t e f a c i e s of the Keg Mountains R h y o l i t e d e s c r i b e d e a r l i e r in t h i s t e x t . No o b s i d i a n or red s p h e r u l i t i c r h y o l i t e is exposed. Some h e m a t i t e occurs in or near f r a c t u r e s . These oxides and the a l t e r a t i on around the f r a c t u r e s were a p p a r e n t l y d e r i v e d from vapors escaping along the f r a c t u r e s. 53 P i c t u r e Rock Quartz L a t i t e Porphyry A body of q u a r t z l a t i t e porphyry was mapped at the southwest e x t r e m i t y of the Keg Mountains ( p i . 1). This t h r e e by one mile (about 5 by 2 km.) body is probably a dome (p. 54). A s i m i l ar r h y o l i t e occurs at West Mountain (p. 43). The P i c t u r e Rock Quartz L a t i t e Porphyry was named for the P i c t u r e Rock R e s e r v o i r j u st southwest of the u n i t . The u n i t had p r e v i o u s l y been included in the a l k a l i r h y o l i t e u n i t (Topaz Mountain R h y o l i t e e q u i v a l e n t ) by Shawe (1972, f i g . 2) and in the Keg Mountain I g n i m b r i t e by Erickson (19^3, p l . 1). The P i c t u r e Rock Quartz L a t i t e Porphyry is s i m i l a r to the p o r p h y r i t i c r h y o l i t e t h a t S t a a t z and Carr (19&4, p. 82-83) assigned tQ t h e o l d e r v o l c a n i c group in t h e Thomas Range. The porphyry forms low r i d g e s or h i l l s l e s s than 200 feet (about 60 meters) high. The long dimension of o u t c r o p s is commonly p a r a l l e l to the s t r i k e of the flow banding. The P i c t u r e Rock Quartz L a t i t e Porphyry is a p p a r e n t l y about the same age as the Keg Mountains R h y o l i t e and Keg Mountains Tuff. A c r e a m i s h - y e l l ow l i t h i c t u f f of the Keg Mountains Tuff o v e r l i e s the P i c t u r e Rock Quartz L a t i t e Porphyry at i t s northernmost exposure ( p l . 1). Keg Mountains R h y o l i t e immediately north of the P i c t u re Rock Quartz L a t i t e Porphyry is h i g h l y iron s t a i n e d and s i l i c a veined w h i l e the o u t c r o p s of the porphyry a r e n o t . This s u g g e s t s t h a t the P i c t u r e Rock Quartz L a t i t e Porphyry is younger than p a r t of the Keg Mountains R h y o l i t e. A small part of the q u a r t z l a t i t e porphyry is s h a t t e r e d and d e v i t r i f i e d near the Keg Mountains Tuff outcrop in SiSV/£ s e c . 25, T. 13 S . , R. 10 W. ( p l . 1). The l a r g e s t block is 1 meter in d i a m e t e r, and the average fragment s i z e Is k to 10 cm. About 8 f e e t (about 2 m e t e r s ) of Keg Mountains Tuff o v e r l i e s the q u a r t z l a t i t e porphyry a t t h i s l o c a l i t y. The P i c t u r e Rock Quartz L a t i t e Porphyry o v e r l i e s a l i t h i c tuff on i t s southern c h i l l border ( f i g . 8 ) . The l i t h i c t u f f does not have enough a r e a l e x t e n t to be r e p r e s e n t e d on P l a t e 1. The t u f f c l a s t s are such t h a t they suggest the t u f f was formed as a vent c l e a r i n g process p r i o r to the emplacement of the o v e r l y i n g q u a r t z l a t i t e porphyry. The t u f f is u n l i k e any of the o t h e r t u f f s in the a r e a. The border c h i l l zone of the P i c t u r e Rock Quartz L a t i t e Porphyry in s e c . 30, T. 13 S . , R. 9 W., and where it o v e r l i e s the l i t h i c tuff ( f i g . 8) c o n s i s t s of angular blocks of black to red g l a s s y q u a r tz l a t i t e porphyry in a s i m i l a r p e r l i t i c g l a s s m a t r i x . The p e r l i t ic g l a s s porphyry p e r s i s t s for about 35 f e e t (about 9 m e t e r s ) above the b r e c c i a t e d c h i l l border. An e l l i p t i c a l body of the P i c t u r e Rock Quartz L a t i t e Porphyry i s rimmed with o b s i d i a n c h i l l border. The flow banding on the margins of the body s t r i k e s s u b p a r a l l e l to the margin and dips toward the c e n t r a l part of the body. The southern c h i l l border is not the southernmost exposure of the rocks a s s i g n e d to t h i s u n i t . The p o r p h y r i t i c rocks south of the c h i l l border were a s s i g n e d to the P i c t u r e Rock Quartz L a t i t e Porphyry because t h e i r c h a r a c t e r and l i t h o l o g y more c l o s e l y resembled the porphyry than the Keg Mountains 55 F i g u r e 8 . - - O b l i q u e a e r i a l photograph showing the P i c t u r e Rock Quartz L a t i t e Porphyry in the extreme southwestern Keg Mountains. Trace of the roughly e l l i p t i c a l c h i l l border of the P i c t u r e Rock Quartz L a t i t e Porphyry is o u t l i n e d . Trm, Red Mountain Crystal Tuff; Tkmr, Keg Mountains R h y o l i t e ; Tkmt, Keg Mountains Tuff; Tpr, P i c t u r e Rock Quartz L a t i t e Porphyry; t» l i t h i c t u f f underlying t h e P i c t u r e Rock Quartz L a t i t e Porphyry. 56 R h y o l i t e . Multiple e r u p t i v e s u r g e s , f a u l t i n g , or the behavior of the porphyry during emplacement could a l l account for the r e l a t i o n s h i ps observed in the f i e l d . On t h e south s i d e of the h i g h e s t h i l l in s e c . 36, T. 13 S . , R. 10 W., the P i c t u r e Rock Quartz L a t i t e Porphyry o v e r l i e s a l i t h i c tuff ( f i g . 9 ) . The q u a r t z l a t i t e porphyry south of t h e t u f f is i n t e r p r e t e d to be a c o n t i n u a t i o n of the porphyry o v e r l y i ng t h e t u f f . There may be a f a u l t between the t u f f and the southern o u t c r o p of the porphyry or the porphyry may simply have o v e r r i d d en t h e edge of the t u f f and scoured it o u t . The exposure is not good enough to determine e x a c t l y what happened. Li t h o l o g y . The P i c t u r e Rock Quartz L a t i t e Porphyry is mostly l i g ht grey in c o l o r , but is black, dark grey, or r e d d i s h brown at c h i ll m a r g i n s . It c o n t a i n s 25 t o 70 p e r c e n t p h e n o c r y s t s in a g l a s s y or d e v i t r i f i e d groundmass. In some p a r t of the o b s i d i a n c h i l l border, the f e l d s p a r phenocrysts tend to be a l i g n e d by flowage. S a n i d i n e p h e n o c r y s t s c o n s t i t u t e 5 to 30 p e r c e n t of the rock. They range from 1 t o 5 mm. in length and form euhedral or subhedral c r y s t a l s . P l a g i o c l a s e p h e n o c r y s t s c o n s t i t u t e 7 t o 25 p e r c e n t of the rock. The p l a g i o c l a s e composition ranges from AnjQ to An/jQ and averages c l o s e r to An^Q . About a f i f t h of the p l a g i o c l a s e is p r e s e n t as 0.2 t o 0.6 mm. long l a t h s . The r e s t of the p l a g i o c l a s e occurs as fragmented c r y s t a l s or c r y s t a l a g g r e g a t e s t h a t range in s i z e from 0 . 8 to 7 mm. Remnants of o s c i l l a t o r y zoning a r e p r e s e n t in the now twinned p l a g i o c l a s e . Some of the p l a g i o c l a s e is s h a t t e r e d with minor d i s p l a c e m e n t of the r e s u l t i n g c r y s t a l fragments. Quartz f i l l s some 57 - 4 9 5 0 F i g u r e 9--Geologic c r o s s s e c t i o n s k e t c h through the h i g h e s t h i l l in s e c . 36, T. 13 S . , R. 10 W., showing P i c t u r e Rock Quartz L a t i te Porphyry o v e r l y i n g l i t h i c t u f f . Line of s e c t i o n is N. 07° E. . S c a l e 1/2400. of the f r a c t u r e s . Needle-lik e i n c l u s i o n s in one c r y s t a l aggregate were o r i e n t e d e i t h e r p a r a l l e l or p e r p e n d i c u l a r to the 010 twin p l a n e s. A p a t i t e is p r e s e n t as 0.02 to 0.1 mm. i n c l u s i o n s in the p l a g i o c l a se and c o n s t i t u e s about 1 p e r c e n t of the p l a g i o c l a s e volume. Zircon i n c l u s i o n s were a l s o p r e s e n t. Quartz c o n s t i t u t e s 10 t o 30 p e r c e n t of the rock. It forms embayed and rounded bipyramids and c r y s t a l fragments ranging in s i ze from 0.2 to 4 mm. Dusty m a t e r i a l is included in the q u a r t z . The q u a r t z as is the p l a g i o c l a s e , is f r a c t u r e d . Copper-colored b i o t i te f l a k e s or s h o r t books c o n s t i t u t e about 2 p e r c e n t of the rock. They range from 1.5 to 3 mm. a c r o s s . Some bent and broken f l a k e s are p r e s e n t . In most of the t h i n s e c t i o n s s t u d i e d , the b i o t i t e is c o m p l e t e l y a l t e r e d to iron o x i d e s . Apatite i n c l u s i o n s a r e p r e s e nt in the b i o t i t e. In one t h i n s e c t i o n , a few g r a i n s of what was hornblende s t i ll r e t a i n hornblende c r y s t a l o u t l i n e and c l e a v a g e t r a c e s. Opaque m a t e r i a l c o n s t i t u t e s up to 3 p e r c e n t of the rock. It has an average g r a i n s i z e of 0.5 mm., is rounded, and is probably magnet i t e . Some sphene being replaced by q u a r t z was noted in one t h in s e c t i o n . A few very small g r a i n s of a mineral t e n a t i v e l y i d e n t i f i ed a s a l l a n i t e are p r e s e n t in the same s e c t i o n. The groundmass of the q u a r t z l a t i t e prophyry appears to have been g l a s s but is now somewhat v a r i a b l e because of d e v i t r i f i c a t i o n. In the c h i l l border of the P i c t u r e Rock Quartz L a t i t e Porphyry, t h e groundmass is n e a r l y e n t i r e l y g l a s s . This g l a s s has p e r l i t ic 59 c r a c k s and appears cloudy because of the high content of a very s m a l l, a l i g n e d t r i c h i t e s . About 3 p e r c e n t of the rock is composed of s p h e r u l i t e s which were formed a f t e r emplacement of the porphyry. The s p h e r u l i t i c a r e a s a r e s l i g h t l y browner than the surrounding tan c o l o r e d g l a s s . S p h e r u l i t i c growth has not d i s t u r b e d the t r i c h i te alignment or c o n c e n t r a t i o n. Groundmass of the P i c t u r e Rock Quartz L a t i t e Porphyry in f a c i es o t h e r than in the c h i l l f a c i e s is up t o 75 p e r c e n t s p h e r u l i t i c. S p h e r u l i t e s are up t o 0.3 mm. in r a d i u s . The r e s t of the groundmass i s now c r y s t a l l i n e with an average g r a i n s i z e of 0.0k mm. The groundmass is 30 t o 50 p e r c e n t a l t e r e d to s e r i c i t e and c a l c i t e . Most of the now c r y s t a l l i n e m a t e r i a l is potassium f e l d s p a r or q u a r t z. S p h e r i c a l s t r u c t u r e s occurr? ng i n the younger rhyoli t ! c and q u a r t z - l a t i t i c rocks Several d i f f e r e n t types of s p h e r i c a l s t r u c t u r e s occur in flows and i n t r u s i v e s of the younger v o l c a n i c rocks. The two most widely commented on a r e s p h e r u l i t e s and l i t h o p h y s a e . Another t y p e , the g e o d e - l i k e " t h u n d e r eggs" have been r e f e r r e d to as both s p h e r u l i t es and l i t h o p h y s a e (Ross and Smith, 1961, p. 38; S t a a t z and Carr, 1964, p. 101). Solid s p h e r i c a l s t r u c t u r e s which s u p e r f i c i a l l y resemble c o n c r e t i o n s found in sedimentary rocks a l s o occur in the v o l c a n i c s. The s o l i d s t r u c t u r e s have been r e f e r r e d to as s p h e r u l i t e s ( S t a a tz and Carr, 1964, p. 94). S p h e r u l i t e s haye been s t r i c t l y d e s c r i b e d as r a d i a t i n g s t r u c t u r es composed of f i b e r s of f e l d s p a r and s i l i c a m i n e r a l s ( f i g . 10). Often 60 F i g u r e 10.-Photomicrograph of s p h e r u l i t i c r h y o l i t i c g l a s s. I n c o m p l e t e l y formed r a d i a l l y s t r u c t u r e d s p h e r u l i t e s a r e p r e s e nt in a f a i r l y u n d e v i t r i f i e d p e r l i t i c g l a s s of the Keg Mountains R h y o l i t e . Slight c o l o r changes in the s p h e r u l i t e s cause a c o n c e n t r i c zoning, (x 19) 61 a c o n c e n t r i c p a t t e r n is superimposed on the b a s i c r a d i a l s t r u c t u re when a v a r i a t i o n in c o l o r occurs ( f i g . 10)(Ewart, 1971, p. 425). The s p h e r u l i t e s were formed f a i r l y l a t e in the emplacement of the v o l c a n i c s (Ewart, 1971). The s p h e r u l i t e s surround p h e n o c r y s t s and a r e superimposed on a p r e v i o u s l y e x i s t i n g flow s t r u c t u r e r e p r e s e n t ed by the alignment of c r y s t a l l i t e s . The s p h e r u l i t e s range from about 0 . 5 to 11 mm. in d i a m e t e r . This type of s p h e r i c a l s t r u c t u r e weathers out of d e v i t r i f i e d green g l a s s , o b s i d i a n , and red s p h e r u l i t ic r h y o l i t e as s p h e r e s. A v a r i e t y of the r a d i a l l y s t r u c t u r e d sph e r u l i t e occurs in the o b s i d i a n f a c i e s of the r h y o l i t e flows about 1 km. west northwest of Red Mountain. Radially o r i e n t e d a c c i c u l a r bundles of p o t a s s ic f e l d s p a r and s i l i c a mineral f i b e r s form t h i s type. The bundles so not form complete spheres and a r e of nonuniform l e n g t h . The c r y s t al s i z e is the same as in Figure 10 which is a photomicrograph of the c o m p l e t e l y s p h e r i c a l r a d i a l l y s t r u c t u r e d s p h e r u l i t e d e s c r i b e d above. A l o n g i t u d i n a l s e c t i o n through the i n d i v i d u a l bundles of c r y s t al f i b e r s c l o s e l y resembles a s e c t i o n through the completely s p h e r i c al s p h e r u l i t e in Figure 10. Lithophysa are n e a r l y s p h e r i c a l s t r u c t u r e s c o n s i s t i n g of many t h i n c o n c e n t r i c l a y e r s of c r y s t a l l i n e m a t e r i a l s e p a r a t e d by t h in almost continuous spaces ( f i g . 11). The l a y e r s of c r y s t a l l i ne m a t e r i a l average 0.1 mm. t h i c k and a r e composed p r i n c i p a l l y of p o t a s s i c f e l d s p a r with some s i l i c a m i n e r a l s . The c e n t e r of a l i t h o p h y s a l layer has a c o n c e n t r a t i o n of h e m a t i t e and m a g n e t i t e. 62 F i g u r e 1 1 . - - P h o t o g r a p h of l i t h o p h y s a l s t r u c t u r e . Layers are composed p r i n c i p a l l y of p o t a s s i c f e l d s p a r with some s i l i ca m i n e r a l s . Later q u a r t z , chalcedony, and topaz engulf the l i t h o p h y s a l l a y e r s . Figure 12 is a photomicrograph of the l a y e r i n g . (x 3) 63 Stubby f e l d s p a r c r y s t a l s appear to have grown outward and inward from t h e c e n t r a l c o n c e n t r a t i o n of iron oxides ( f i g . 12). Some l i t h o p h y s ae have l a r g e open spaces due to local wide spacing of the l a y e r s or to incomplete formation of the l a y e r s . True l i t h o p h y s a e pass g r a d a t i o n a l ly i n t o i r r e g u l a r c a v i t i e s ( P a t t o n , 1908, p. I 8 9 ) . They range from 0.5 t o 20 cm. in d i a m e t e r. Somewhat s p h e r i c a l nodules weather out of some of the r h y o l i t e s. These nodules resemble s e p t a r i a n nodules on the e x t e r i o r ; both have t h e same type e x t e r i o r crack p a t t e r n . The c r a c k i n g a p p a r e n t l y occurred d u r i n g cooling of the surrounding r h y o l i t e . These s t r u c t u r e s are hollow and a r e commonly l i n e d or f i l l e d completely with chalcedony, banded a g a t e , o p a l , q u a r t z c r y s t a l s , or c a l c i t e c r y s t a l s . The s t r u c t u r e l e s s r h y o l i t e p o r t i o n of t h e s e nodules was not examined in t h i n s e c t i o n . These s t r u c t u r e s a r e commonly c a l l e d "thunder eggs". S o l i d hemispherical s t r u c t u r e s composed of a p h a n i t i c or p o r p h y r i t i c r h y o l i t e are p r e s e n t in the o b s i d i a n and red s p h e r u l i t ic r h y o l i t e of the Keg Mountains R h y o l i t e . These s t r u c t u r e s s u p e r f i c i a l ly resemble c o n c r e t i o n s found in sedimentary rocks. Their b a s e s , however, merge with the surrounding rock. They e x h i b i t planar s t r u c t u r e s which have the same o r i e n t a t i o n as nearby flow banding. A few of these h e m i s p h e r i c a l s t r u c t u r e s are p a r t i a l l y hollow and have l a t e r p a r t i al f i l l i n g s of b o t r y o i d a l chalcedony. O r i g i n . After the r h y o l i t i c to q u a r t z - 1 a t i t i c e x t r u s i v e s and i n t r u s i v e s became f a i r l y s o l i d , they began to d e v i t r i f y . The flow s t r u c t u r e s are u n d i s t u r b e d by the s p h e r u l i t e s which enclosed the flow o r i e n t e d c r y s t a l l i t e s . The o t h e r s p h e r i c a l forms were not examined 64 F i g u r e 1 2 . - - P h o t o m i c r o g r a p h of l i t h o p h y s a l l a y e r s . Layers are p r i n c i p a l l y composed of p o t a s s i c f e l d s p a r with some s i l i ca m i n e r a l s . The c r y s t a l s in the l i t h o p h y s a l layers a r e c o n s i d e r ­a b l y l a r g e r than the c r y s t a l f i b e r s in the r a d i a l l y s t r u c t u r ed s p h e r u l i t e s ( f i g . 10). Hematite and m a g n e t i t e g r a n u l e s occur in t h e c e n t e r s of the l a y e r s . The c r y s t a l s grew both outward and inward from the c e n t e r of the l a y e r s . The l a y e r s enclosed o l d e r p h e n o c r y s t s and were in t u r n e n c l o s e d by l a t e r topaz, q u a r t z , and chalcedony. The spaces between the l a y e r s in t h is f i g u r e a r e completely f i l l e d with chalcedony. (approximately x46) 65 t o determine if the flow s t r u c t u r e was u n d i s t u r b e d by t h e i r f o r m a t i o n. Both the r a d i a l l y s t r u c t u r e d s p h e r u l i t e s and the l i t h o p h y s a e are formed by t h e d e v i t r i f i c a t i o n of the g l a s s e s (Ewart, 1971, p. 425). Topaz and o t h e r m i n e r a l s o c c u r r i n g ?n cav? t i e s ?n the Keg Mountains Rhyoli te Topaz, p s e u d o b r o o k i t e , h e m a t i t e , q u a r t z , and b i x b y i t e c r y s t a ls occur in c a v i t i e s in the Keg Mountains R h y o l i t e . A few minute p u r p le f l u o r i t e c r y s t a l s a l s o occurred in c a v i t i e s . The o c c u r r e n c e of t h e se m i n e r a l s in v o l c a n i c s is not unique; they a l s o occur in the younger group r h y o l i t e s of the Thomas Range which l i e s to the west of the Keg Mountains. These two o c c u r r e n c e s appear to be i d e n t i c a l . A d i s c u s s i on of the Thomas Range o c c u r r e n c e and a l i s t of o t h e r , s i m i l a r o c c u r r e n c es can be found in S t a a t z and C a r r ' s paper on the Thomas Range (1964, p. 102-108). • D i s t r i b u t i o n of the m i n e r a l s is s p o r a d i c w i t h i n any s i n g l e r h y o l i t e body, and the m i n e r a l s do not occur in every body. All of the m i n e r a l s do not occur t o g e t h e r. Topaz. The w r i t e r found topaz c r y s t a l s in the Keg Mountains in 1971; topaz had not been r e p o r t e d p r e v i o u s l y from t h i s a r e a . D i s t r i b u t i on i s s p o r a d i c w i t h i n any one r h y o l i t e body, and topaz is not p r e s e nt in every r h y o l i t e body. Topaz is found in l i t h o p h y s a e , c a v i t i e s , and s o l i d r h y o l i t e . Pieces of topaz c r y s t a l s a r e found in many of the d r a i n a g e s which head in the t o p a z - b e a r i n g r h y o l i t e . Topaz in the Keg Mountains R h y o l i t e is about as abundant as i t is in t h e Topaz Mounta?n R h y o l i t e. 66 Topaz has been found in the r h y o l i t e s j u s t west of Kane Spring, l i miles (about 2^ km.) s o u t h e a s t of Kane S p r i n g , j u s t southwest of Red Mountain, and from j u s t south of to l i miles (about 2\ km.) n o r t h w e s t of PB H i l l , Figure 13 is a photograph of topaz from the Keg Mountains. Topaz is an alumtnum f 1 u o r o s i 1 i c a t e , A12CSi0^)(F,0H)2, that c r y s t a l l i z e s in the orthorhombic system. Its habit is a prism with dipyramid t e r m i n a t i o n . The topaz from the Keg Mountains has the same c r y s t a l form as the Thomas Range topaz d e s c r i b e d by A i l i n g (I887, p. 146-147). They both have two p i n a c o i d s , b(010), c ( 0 0 l ) ; two p r i s m s, m ( H 0 ) , 1(120); two brachydomes, y ( 0 4 l ) , f ( 0 2 l ) ; and four pyramids, I ( 2 2 3 ) , u ( H l ) , o ( 2 2 l ) , e ( 4 4 l ) . The pyramid, r ( 1 2 l ) , was recognized in a d d i t i o n to the above forms on topaz from the Keg Mountains. P a t t o n (1908, p. I83) s e p a r a t e s the topazes from the Thomas Range i n t o t h r e e groups: t r a n s p a r e n t topaz, rough opaque t o p a z , and smooth opaque t o p a z . All g r a d a t i o n s between P a t t o n ' s groups occur in the Keg Mountains t o p a z . The t r a n s p a r e n t topaz c r y s t a l s g e n e r a l l y have one good t e r m i n a t i o n and an a t t a c h e d opaque end. Some doubly t e r m i n a t ed c r y s t a l s t h a t a r e a t t a c h e d in the middle have been found. Crystal face development is e x c e l l e n t on t h e t r a n s p a r e n t c r y s t a l s while development on t h e opaque c r y s t a l s tends to be poor and s i m p l e . The opaque topaz has many t i n y q u a r t z i n c l u s i o n s ; the p r e s e n c e of t h e s e i n c l u s i o n s has p r o b a b l y i n h i b i t e d the formation of more complex forms. Most of the Keg Mountains topaz c r y s t a l s a r e l e s s than 1.5 cm. long; the l a r g e st founds was 2.5 cm. long. F i g u r e 1 3 . - - P h o t o g r a p h of intergrown topaz c r y s t a l s in a l i c h e n - covered Keg Mountains R h y o l i t e m a t r i x . Specimen is from the s o u t h s i d e of PB H i l l , (x 5-3) 68 Some of the c l e a r topaz c r y s t a l s a r e wine, r o s e , or amber when t h e y a r e f r e s h l y broken out of the r h y o l i t e . Most lose t h e i r color upon exposure to heat or s u n l i g h t . The cause of the c o l o r and the c o l o r change in the topaz is unknown. Permanent amber c o l o r can be given to o t h e r w i s e c o l o r l e s s topaz by exposing it to x - r a y f l o r e s c e n ce r a d i a t i o n for 20 minutes (Whelan, 1972). Two t o p a z e s from the Keg Mountains and one each from the Thomas Range, Honeycomb H i l l s , and Ouro P r e t o , Minas G e r a i s , Brazil were analyzed with t h e e l e c t r o n microprobe to find gross chemical s i m i l a r ­i t i e s . All four samples a r e e s s e n t i a l l y i d e n t i c a l in Cr, Mg, and Si c o n t e n t . The v o l c a n i c topazes are n e a r l y i d e n t i c a l in Fe, Ca, Mn, and Al c o n t e n t , but have more Mn, Al, and Fe and l e s s Ca than the Ouro P r e t o t o p a z . Ti content is s i m i l a r in both the Ouro P r e t o and Honeycomb H i l l s samples, but is half as much as the c o n t e n t in topaz from e i t h e r the Thomas Range or Keg Mountains. The v o l c a n i c topazes have at l e a s t a t h i r d more F than the Ouro P r e t o sample. Topaz form t h e Keg Mountains is the h i g h e s t in F c o n t e n t . More m a t e r i a l should be analyzed to determine if the samples used were t y p i c a l. Pseudobrook? t e . Pseudobrookite had p r e v i o u s l y not been r e p o r t e d from t h e Keg Mountains. This o c c u r r e n c e is the f i f t h known l o c a l i t y in the United S t a t e s . Staatz and Carr (1964, p. 106) enumerate t h e o t h e r four l o c a l i t i e s . Pseudobrookite has been found around PB H i l l and in NE^NEi s e c . 36, T. 12 S . , R. 10 W. P s e u d o b r o o k i t e is commonly a s s o c i a t e d with h e m a t i t e and pink q u a r t z . The r h y o l i t e near the s i t e s of p s e u d o b r o o k i t e c r y s t a l l i z a t i on i s more a l t e r e d than the r h y o l i t e f a r t h e r away. The p s e u d o b r o o k i te 69 and the a l t e r a t i o n are b e l i e v e d to have come from the i n t e r a c t i o n of vapors escaping along f r a c t u r e s from c o o l i n g lava and the r h y o l i t e. P s e u d o b r o o k i t e is a r a r e oxide of iron and t i t a n i u m ; i t s s t o i c h i o ­m e t r i c composition is Fe203.TiO . The c r y s t a l system of pseudobrookite i s t e t r a g o n a l ; the h a b i t is p r i s m a t i c . The b l a c k , s u b m e t a l l i c c r y s t a ls a r e found as a c c i c u l a r c r y s t a l s in r a d i a t i n g c l u s t e r s ( f i g . 14). I n d i v i d u a l c r y s t a l s a r e g e n e r a l l y l e s s than 3 mm. in l e n g t h. P s e u d o b r o o k i t e was p o s i t i v e l y i d e n t i f i e d using x - r a y d i f f r a c t i o n. Hemat?te. Hematite, Fe203, is very common in the Keg Mountains R h y o l i t e. I t is g e n e r a l l y found a t t a c h e d to and included in topaz and q u a r tz c r y s t a l s or a s s o c i a t e d with p s e u d o b r o o k i t e. The b l a c k , m e t a l l i c c r y s t a l s occur in two d i f f e r e n t forms. The f i r s t type is the t a b u l a r form with the p i n a c o i d , c ( 0 0 l ) , and the rhombohedron, r ( l 0 1 l ) . The c r y s t a l s with t h i s form range in diameter from 1 t o 3 rnm. The second form is t h i n , s k e l e t a l , hexagonal p l a t es which range in diameter from about 0.3 to 10 mm. Q u a r t z . Besides comprising p a r t of the l i t h o p h y s a l s t r u c t u r e and being an e s s e n t i a l mineral in the r h y o l i t e , q u a r t z a l s o occurs in c a v i t i es and l i t h o p h y s a e with topaz or p s e u d o b r o o k i t e. The q u a r t z c r y s t a l s are g e n e r a l l y c l e a r bipyramids up to 1 mm. a c r o s s . Some l i g h t blue or pink c r y s t a l s are p r e s e n t . Crystal faces a r e g e n e r a l l y smooth. B i x b