[U+S 7 DAY (Geology of the Cerro Gordo Mining District Inyo County, California fiGEOLOGICAL SURVEY PROFESSIONAL PAPER 408 Prepared in cooperation with the State of California, Department of Natural Resources, Division of Mines IBqu/Redl a & iE 78 246 Set 1. V‘4O£~407c & 7M Fu‘Ltb | - uwivErsuy of L.- camas (Geology of the Cerro Gordo Mining District Inyo County, California By C. W. MERRIAM CEOLOGICAL sURVEY PROFESSIONAL PAPER 408 Prepared in cooperation with the State of California, Department of Natural Resources, Division of Mines UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:: 1963 Toes UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director EARTH SCIENCES LIBRARY For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 € Grp iz v2 08-403 ~ 2 CONTENTS sticky LIBRARY Page Page ilu tite tetsu 1 | Metal mines of the Cerro Gordo area___._.____________ 37 Introduction ~. _. 0 .. _ 10100 C000 [C 2 Cerro Gordo mine. . 0 y C. 37 Previous geological studies in the Cerro Gordo area. _ __.. 6 History of the Cerro Gordo mine. 37 Present investigation.. __. .._... _._ ___ _._} 0. 6 Mme development and ore search._______. 39 'l ..... } _} n. ncn" s History of the Cerro Gordo smelters_ _ _ ___ 41 € y & Character and grade of ores.._.__.._..-_._____ 42 Geolgglc setftmg """""""""""""""""""""" 7 Production of the Cerro Gordo mine. 43 Stratiqruphic 8 Geology of the Cerro Gordo mine..__________. 44 ..... .. ___ 8 Rocks of the Cerro Gordo mine.__________ 44 Pogonip stoup... ...... _._ _. __l l _} _-_ 9 Redimentary 44 (ureka quartzite. '.}. ._ .... -__ 9 Igneous rocks _...:.r.l 0 .c lu-" 45 (Piy Sprine« dolomite: <... . > 10 Union dike-:z--._ .-. _ l. 45 Cilunian system-... coins .. L lu coro c ._ 11 Jefferson dike:= ._ ___ 45 Hidden Yalley dolomite-.........;... /_ ;- 11 Dacite porphyry and andesite Mevonlan system... ...... __ _} _n ~ _". 13 ? pom?” dikes__________‘___ 46 Upper part of Hidden Valley dolomite (Devo- Magne§1an alteration in Cerro Gordo mfne_ 46 5 Geologic structure of the Cerro Gordo mine. 46 flan Dark OnI¥) c. co- co occ rer i i Cerro Gordo anticline..._..-_._ _._ 47 fost Burro formation.. ......_.._..._;- . ___ 14 Cerro Gordo fault...... _.._________ 47 Missle«ipplan system...... _.. ..... ___. __. 17 Normal faults with northwest strike... 49 in Mountain 17 f Ore occurrence in the Cerro Gordo mine... 53 erdido formation..s-_..._ l _ _____:_._~_.__ 18 Union cucu c. lol 0. 54 bainman shale __ ..} [ln c_ __ 20 Jefferson chimney..= [' 57 Pennsylvanian and Permian rocks._______________ 24 Despreciada footwall ore bodies...... ___. 57 eeler Canyon formation. 24 |! Jeffersogdiabasic dike ore bodies 57 Permian systém ________________________________ 25 Buena Vista he'mglng-wall ore bodies.... 59 A Zoro fissure vein.. .c unl .nl {oon {08} 59 , wens Valley formation.... -_ .._:°~__'~- 25 sf f 7 lst. Siliceous veins of the Cerro Gordo mine . . 60 Aniesple system . ... onlt. 28 Cerro Gordo zine ore bodies.... 61 rlassic marine rocks...._........._._:__;__- 28 Morning Star mine .:.. ___. 00 I~ / 63 Lithology and stratigraphy.______________ 28 Estelle funnel _ clu clin [0 "an 64 Age of Triassic marine rocks.-____________ 30 Charles Lease tunnel. __. On till 68 volcanic rocks......___:. }_____._:_ 31 Ignacio mine' cn n sli ll l 9 69 Igneous rocks of the Cerro Gordo area_________________ 31 Hari mine. nove c ul stela . 72 intrusive igneous 32 Crosscut [-l. loons ctl an _ 72 Cranitoid:rocks.-......_..._ ._: /_ ~' _/ __. 32 Sunset mme""'-j""“""“"-----»- --------- 23 Later andesitic and dacitic dikes..___.__._.__.__ 32 (pan Newhow n 0 col s- cre! gad c ooh. J, Fock alteration... .:... _. nn 0 _ . [01 000.000 33 Newtoyvn Hf er f reste. #. i am flamine ..... _u - . 76 ~...... ___ ___ 38 Rerseverance mine...... -_ a_ 76 Cerro Gordo anticline ..o... ..... __... 34 Silver mines in Belmont ___... TT Subsidiary _-_. ___. _/}~ 35 Newsboy mine...... ___ __" {O0 30" 77 Faulting in the Cerro Gordo area__._.___________. 35 belmont _. _u [00 78 ___ .l __ . /_ _ 35 | Register of important fossil 79 Cleavage or sheeting _. ._.. -_ 36 | References cited-______________________________-_____ 79 Acc ol deformation......._._...._:.. __. ___". 201 Index: .-. ._... cnl acct t tto, 81 III 719 Prats TABLE CONTENTS ILLUSTRATIONS [Plates are in pocket] . Geologic map of the Cerro Gordo mining district. . Generalized geologic map of the southern half of New York Butte quadrangle. . Isometric block diagram of the Cerro Gordo mine. 1 2 3. Composite map of workings, Cerro Gordo mine. 4 5 . Map of workings of the Morning Star mine. bo i- tw co 1 o orn ih to 16. 17, 18. 19. 20. 21. 22. 283. 24. 25. 26. 27. . Map of Charles Lease tunnel workings -.ls/ on sas 29. 30. ' 81. 82. 33. 34. . Map of main tunnel, Fils mine.." _. [nie signer ia ste tao noo coo aaa aan 36. or gm G9 bP i- . Triassic rocks of the Cerro Gordo mining . Index map showing the location of the Cerro Gordo mining district . View from a point near Hart Camp looking west across the west slope of the Inyo Mountains and Owens Lake basin to the Slorta .._ cocco oo ane ; Index map of the Inyo-Panamint c ose Oblique aerial photograph facing east slope of the Inyo Mountains just north of Cerro Gordo._______-___---.-- . View looking northwest at the top of the Inyo . View looking north along higher east slope of the Inyo Mountains near Cerro . View looking northwest along the west Inyo Mountains . View from a point near Hart Camp looking northwest along the west Inyo Mountains lala. . Generalized southwest-northeast structure section (C-C") at Crosscut tunnel north of Cerro Gordo.._______... 10. . View of Cerro Gordo looking east 12. 13. 14. View looking northeast toward Ubehebe through saddle at Cerro Generalized structure section (A-4') through the higher Inyo Range at Cerro Map of workings of Omego tunnel, Cerro Gordo mine. Looking northeast across San Lucas Canyon from a point north of Cerro Structure section (G@-G') running northeast-southwest (observer facing southeast) through Cerro Gordo mine.. Enlarged geologic map of the area surfounding Gerro l .c seas e Map of workings of Safeguard funnel __.... - -o- - Lenses Semidiagrammatic northwest-southeast longitudinal projected section (H-H') (observer facing northeast) (brough Coro. Gordo mine. ...-... nn cols ct lensing ooo it tos Map of workings of Bullion funnel .._ ._.. l LL sett preda- okie, econ sac Surface pits in the marble of the Lost Burro onn Map of workings, Union tunnel (Union mine)__________________________________________~_-‘_ ____________ Northeast-southwest section (F-F") (observer facing southeast) through Cerro Gordo mine at Jefferson chimney. Map of workings of Buena Vista, Santa Maria (86), and Hero tunhniel§L Horizontal projection showing relation of supergene Union zinc carbonate ores to Union chimney lead ores_-_. - Map of inner workings of Estelle tunnel.___..._.... i -- Southwest-northeast structure section (B-B') through higher Inyo Range along line of Estelle tunnel:::...._-.. Map of Castle Rock workings, Estelle omen Map of Ienacio mime workings.! _ coir a t (Ooo View looking southwest along west Triyo Mountains Slopes... nonce Map of Crosscut tunnel to tap Map of Sunset mine cron to aalto ooo. play of Upper Newtown mifc workings. t nc oop cop lone Map of lower tunnel of the Newtown mine Map of workings of Perseverance ming. Lance alt . 22 ua a a hale -s as one oe mie bo ae Sie beck TABLES . Stratigraphic column of the Cerro Gordo mining nono e Members of the Hidden Valley dolomite, Salt Mill . Pennsylvanian and Permian sequence in the southern Inyo Mountains, district. Les rend ece c, 5k <4 ~- Soe . Production of the Cerro Gordo mine, Inyo County, ~~ Page 12 24 44 GEOLOGY OF THE CERRO GORDO MINING DISTRICT, INYO COUNTY, CALIFORNIA ABSTRACT The Inyo Mountains near Cerro Gordo comprise strongly folded and faulted sedimentary rocks ranging in age from Ordovician to Middle Triassic. These were intruded by granitic bodies, aplite dikes, and by innumerable andesitic and dacitic dikes of later age. Though largely nonfoliated, the sedimentary rocks have undergone varying degrees of contact and hydrothermal metamorphism productive of horn- fels, calc-hornfels, phyllite, and quartzite. Tertiary basaltic rocks and tuffs cover older rocks at the southern tip of the range, but do not enter the area of the present map. Paleozoic rocks of the Cerro Gordo area are more than 11,000 feet thick and include all systems from Ordovician through | Permian. Mapped units which have wide distribu- tion in the Great Basin are the Pogonip group, Eureka quartzite, and Ely Springs dolomite of the Ordovician and the Chainman shale of Mississippian age. Silurian and De- vonian rocks are represented by 'the Hidden Valley dolomite and the Lost Burro formation, the former being largely Si- lurian, but embracing Lower Devonian strata at the top. The Lost Burro includes the Atringocephalus zone near the base and is of late Middle and Upper Devonian age. This unit is largely a nondolomitic marble in this area and is es- pecially important as host rock of the principal ore bodies. The Mississippian system is‘refiresented by two principal formations: Tin Mountain limestone below and Chainman shale above. A third unit, the Perdido formation, wedges in to the east between Tin Mountain and Chainman. Being less than 100 feet thick near Cerro Gordo, it has not been differentiated from the Chainman in mapping. Pennsylvanian and Permian strata are divided into two formations: Keeler Canyon formation of Pennsylvanian to Early Pimian age and Owens Valley formation of Permian age. These strata are predominantly impure carbonates, with subordinate shale, siltstone, sandstone, conglomerate and chert. Stratigraphic division of the Pennsylvanian and Permian was accomplished mainly by study of the abundant fusulinids.~ Some 4,000 feet of Lower and Middle Triassic rocks are ex- posed on the west side of the Inyo Mountains. (Of marine origin is the lower 1,800 feet which comprises shale, thinly- bedded limestone and thick-bedded lenticular reefy limestone. The upper part of the Triassic section comprises volcanic rocks and land-laid deposits in which reddish coloration is characteristic. The Triassic succession is incomplete, for on the west side of the Triassic belt the volcanic rocks are in fault contact with fusulinid-bearing Permian beds of the Owens Valley formation. Intrusive rocks of the Cerro Gordo area include the older granitic and aplitic rocks of possible Cretaceous age and younger andesitic and dacitic porphyry dikes. The younger By C. W. Mrrrran porphyry dikes occur in large numbers and for the greater part strike northwest. In the Cerro Gordo mine such dikes, in fractured condition, seem to have served as avenues of ascent for mineralizing solutions. Rocks of the Cerro Gordo area are extensively folded and faulted. Most significant structural feature is the large asymmetrical south-plunging Cerro Gordo anticline which forms a sort of backbone to the Inyo Range. On its flanks and crest are irregular subsidiary flexures. Bordering the major anticline are many smaller folds with northwest axial trend. These range greatly in magnitude and tightness, partly in response to varying competency of strata involved. Some of the folds are related to reverse faults or thrusts. The Cerro Gordo mine is situated in the axial zone of the anticline which carries its name. Faults having a northerly trend are characteristic of the region Among these is the important Cerro Gordo fault, master fault of the Cerro Gordo mine. Northwestward- trending normal faults greatly complicate geologic structure in. the Cerro Gordo mine, where certain of these offset ore bodies. Silver, gold, lead, zinc, and in minor amounts copper are the metallic commodities of the Cerro Gordo area. The Cerro Gordo mine formed by consolidation of the Union mine, San Felipe, and the Santa Maria far exceeds in production all others of the ares combined. Estimates of total Cerro Gordo mine production show about 4,400,000 ounces of silver, 37,000 tons of lead, and 12,000 tons of zinc from zinc carbonate ore. Year of peak production was 1874. More than half the lead and three-fourths of the silver were produced in the years 1869 through 1876. Ores of the Cerro Gordo mine occur in Devonian marble of the Lost Burro formation on the east or footwall side of the northward-trending Cerro Gordo fault. This fault is seem- ingly normal and carries Chainman shale down on the west against marble of the Lost Burro formation. Largest ore bodies were found in two channels which rake steeply to the south, which is the plunge direction of the Cerro Gordo anti- cline. The two principal ore channels known as the Union chimney on the north and Jefferson chimney on the south occur in fractured marble close to the master. Cerro Gordo fault. They were fed by fissures which formed in sympathy to movement on the master fault. Major ore bodies occurred also in the sheared Jefferson diabasic dike. Quartz veins with northwest strike yielded siliceous ores of silver, lead, and copper. Carbonate-zine ores are secondary, derived by leach- ing of sulfide ores in the Union chimney vicinity. Supergene zinc-carbonate ores replaced unmineralized Lost Burro mar- ble along bedding. In the lower part of the Union chimney, primary sulfide replacement was also controlled in part by bedding. 2 GEOLOGY OF THE CERRO GORDO MINING DISTRICT The Union ore channel was bottomed near the northwest- ward-trending San Felipe siliccous vein where the vein lies against a dacite porphyry dike. The very steep Jefferson chimney extended to a much greater depth, but was cut off below the 900 level by northwest-trending normal faults. Ore in the Despreciada section of the mine may represent faulted deeper parts of the Jefferson chimney. South of Cerro Gordo, the Morning Star mine, the Charles Lease tunnel, and the 8,100-foot low-level Estelle tunnel were opened to explore the Castle Rock siliceous vein and ground beneath gossans in the Tin Mountain limestone. The Estelle also provided means of searching for inferred deep continua- tions of the rich Cerro Gordo ore channels. Morning Star and Estelle production was small. That of the Morning Star came principally from the Gold stope in Lost Burro marble. Estelle ore was mined near the tunnel level from upper Hidden Val- ley dolomite east of the Cerro Gordo anticline axis. . Among lesser mines the Ella, the Perseverance, and mines in Belmont Canyon yielded siliceous silver-bearing ores used as fluxing material in the Cerro Gordo furnaces. These mines are in a wide zone of northwest shearing which includes northwestward-trending quartz veins of the tetrahedrite- galena-barite type characteristics of the region. West of Cerro Gordo the now inaccessible Ignacio mine lies in altered and intruded Chainman shale near the boundary 124° 42" -of NEW YORK BUTTE |a QUADRANG‘LE to ~ s\ - VPLake 4 ¢ f." s Santa Barb? i ~ ml OS ANGELES 114° | C 3 I I « a f nd -C 6 A| § with overlying silicated limestone of the Keeler Canyon for- ¢ ( & Rogea mation. Principal Ignacio silver production seems to have & come from a fissure zone along the northeastward-trending Ignacio fault. Westernmost mine of the Cerro Gordo area is the Sunset, which lies in partly silicated limestone of the Keeler Canyon. A small amount of lead and silver came from two narrow intersecting veins. INTRODUCTION The Cerro Gordo mining district derives its name from a limestone peak (alt, 9,184 ft) near the south end of the Inyo Mountains (fig. 1). Together with a lofty northern prolongation known as the White Mountains, the Inyos occupy a position near the west margin of the Great Basin. Across Owens Valley rises the impressive Sierran scarp (fig. 2) culminating in Mount Whitney. On the east lies the rugged Panamint terrane and be- yond it Death Valley. Roughly parallel mountain ranges and basins having a northwesterly trend charac- terize the Inyo-Death Valley region. Northward- trending geomorphic features, though evident, are less obvious than in the more typical Great Basin territory to the north and east. P : At the foot of the southern Inyo Mountains is Owens Lake (alt, 3,570 ft), now practically dry (Gale, 1915) because of diversion of Owens River. Saline Valley, a smaller dry lake basin (Gale, 1914), flanks the range on the east, its lowest point (alt, 1,059 ft) some 2,500 feet below the Owens Valley floor (fig. 3). Difference of alti- tude from the Sierran crest near Mount Whitney to the Owens Valley floor is about 10,000 feet, and therefore commensurate with that which separates the higher Inyo summits from the bottom of Saline Valley. 120° \ F ya: Q 100 MILES FIGURE 1.-Index map showing the location of the Cerro Gordo mining district (shaded) in New York Butte quadrangle. Desert climate of the Inyos resembles that of other mountain ranges in the southern Great Basin. At Keeler, on Owens Lake, precipitation averages about 3.15 inches per year (Lee, W. T., 1906, p. 18; Lee, C.H., 1912, p. 23) but is considerably greater on the higher mountain slopes. During winter the range is some- times snow covered above 6,000 feet; in fact, work stop- page at the Cerro Gordo mine by reason of heavy drifting snow is a not infrequent entry in the mine rec- ords. Cloudbursts in Keeler Canyon during the summer have many times partly destroyed the mine road, threatening vulnerable Keeler itself. Persistent streams are few and small in the arid Inyo Mountains, unlike the Sierra Nevada which borders Owens Valley on the west. Normal runoff appears to be greater on the east slopes of the Inyos than on the west, where the disparity in moisture between this range and the opposing Sierra is patently manifested by differences in vegetation and geomorphic develop- ment. - Except during storms the east flank of the Inyo Range is drained by a few minor spring-fed streams like that in Hunter Canyon 11 miles northwest of Cerro Gordo. Normally these do not exhibit surface flowage INTRODUCTION 3 FIGURE 2.-View from a the Sierra Nevada. sic shaly beds, Silurian rocks. point near Hart Cam Photograph by L. G. Hen far beyond the fan heads near the canyon emergence. Vegetation of the lower Inyo slopes includes such des- ert types as sagebrush, greasewood, rabbit brush, des- ert holly, and salt grass. Near habitations the in- troduced phreatophyte salt cedar has taken hold and continues to spread. Up to altitudes of about 7 500 feet are scattered stands of Joshua tree. Above this altitude the juniper and pifion pine are dominant, with thickets of mountain mahogany in favored situations. Much of the pine was cut for smelter charcoal during boom days of the Cerro Gordo mine. A famed and once spectacular producer of silver and lead, the Cerro Gordo mine is near the summit of the range at Cerro Gordo Peak. The terrane is extremely precipitous in this vicinity, especially along the east slope; north of Cerro Gordo it rivals the bold east face of the Sierra itself (fig. 4). This factor of ruggedness, coupled with scarcity of readily obtainable water, con- tributes immeasurably in this area to the difficulties of access and mine operation. Springs are few, small, and unreliable in the higher parts of the Inyo Mountains. Close to the range sum- mit, 314 miles north of Cerro Gordo are several small springs which emerge from the Chainman shale. These Cronn, and Mexican springs are named Belshaw, p looking west across the west slope of the Inyo Mountains and Owens Lake basin to In the middleground are westward-dipping resistant Triassic volcanic rocks and underlying marine Trias- Near the middle of the photograph at the lake margin is Smelter Hill, which is underlain by Ordovician and best. spring (pl. 1) ; water from them has in the past been piped to Cerro Gordo, providing an inadequate but wel- come supply. The three springs are known collectively as the Cerro Gordo springs. Water from better springs in granite areas of upper Hunter Canyon and Craig Canyon once reached Cerro Gordo briefly through an expensive and elaborate pumping system and a 12-mile pipeline. Nearest important center of population is Lone Pine, 15 miles northwest of Cerro Gordo beyond the north end of Owens Lake. At the foot of the range on the former lakeshore is the small village of Keeler, southern termi- nus of the Southern Pacific narrow-gage railroad. Kee- ler Canyon, with its mouth 114 miles northeast of the village, leads upward to Cerro Gordo Peak. When the narrow-gage railroad was completed, about 1883, Keeler became a shipping point for the Cerro Gor- do mine and other lead-silver mines at Darwin. Of late years the town has been kept alive by a fairly active Inyo County tale industr -_ Owens Valley is served by a broad-gage branch of the Southern Pacific Railway from Mojave, which connects with the narrow-gage line at Owenyo northeast of Lone Pine. The narrow-gage line today operates only from Keeler to Laws Station near Bishop, Calif. It continued formerly over Mont- e 4 GEOLOGY OF THE CERRO GORDO MINING DISTRICT gomery Pass to Nevada. Owens Valley is connected | four-wheel-drive vehicles made it possible to reach the with Mojave by an excellent paved highway which | range summit at Burgess mine following the old salt passes through Lone Pine to Bishop and beyond. From | company wagon road up Swansea Wash. (pl. 2). A a junction south of Lone Pine the paved highway to road along the mountain crest from Burgess mine to Death Valley now bypasses Keeler. Mexican Spring was likewise usable. - In days of active A very steep and not infrequently washed-out road | mining and prospecting many pack trails existed ; most follows part way up Keeler Canyon, rising 4,600 feet in | of these have fallen into a state of disrepair, though used a distance of about 744 miles to reach Cerro Gordo. occasionally by hunters and cattlemen. Elaborate From the mine it descends along San Lucas Canyon on aerial tramways have twice been constructed from the east slope to connect with a road from Lee Flat to | Keeler to Cerro Gordo. That now in existence operated the Bonham tale mines. The Cerro Gordo area is other- successfully from about 1915 to fairly recent times. A wise largely inaccessible to conventional wheeled ve- remarkable 13-mile tram (Gale, 1914, p. 416), now long hicles. During the course of this work, jeeps and other abandoned, was constructed about 1912 by a salt com- 18°00 17°00 Independence | Waucoba Wash ‘ Dry Mountain Tin MByuntain | \\ Grapevine Peak‘ quadrangle quadrangle | quadrangle quadrdagle 04\4/<° quadrangle o 0 (42:6? w C e" ‘V ‘o 3 S L 7 £ A Cp: 34 < & L Tin Mountain C '9,l,\ 3 G fa) [ /g" =% G 2 3. ' 1" I, , 7, | o rsarge P €_ 2, Ay T f O, < Quartz Spring C Lost Burro mine 4’ " RReward m 7 4 & Ix Nid re\rannceirk Bane qui%%9‘ g $ I Marble Canyon | Stovepipe Wellsv/¢ ; - aKeynote Peak quadrangle a y quadrangle uadrangle y 2 \ mor 3 4a (HHS << Ubehebeal 3 > s g\ pOwenyo $. 3 Peal on L Stovepipe P a New York Butte 10668 7° peplls 009 m Buygessfié < E %, % ming ,/ is ; *o, *» S A ¢ a § . f Stovepipe Wells k :Dotomite? 48 i Hotel o *e / © 4 A 12 XQ /Swanseq a |= ° 362 j 36°30 [(el -f tert -A ‘ Olancha E § -/ Keeler v Darwin Panamint Butte quadrangle is | f Owens. quadrangle 9 quadrang|e\ quadrangle . } ® m ine . Panamint Y, 2A Lake 52 y Lee mine culls FA *; $ Rosa { mine # J es Tale City Hills | : . y Tal ‘gi‘ly 00mm? Springs (Trio nz eo10 / OPHIR a ~~. Grancha (al MOUNTAIN [Zinc Hill Emigrant Canyon quadrangle \3 O Garwin mines \\ ® Darwin 4 N i O / : Monache Mountain _ | ‘S Haiwee Coso Peak Telescope Peak quadrangle & Reservoir P quadrangle quadrangle $ s quadrangle '% aTelescopé | Haiwee Reservoir 0 Peak )‘ < Mdturangoy, Pedk y le CC Oquodrcnge A ~ & 10 o 10 20 MILES < L a a 1 N paved % [.s | 36°00 FicurE 3.-Index map of the Inyo-Panamint region showing localities to which reference is made. 1. Fossill Hill, 2. Union Wash, 3. Beveridge Canyon, 4. Hunter Canyon, 5. Craig Canyon, 6. Daisy Canyon, 7. Bonham Tale mines, 8. Lee Flat, 9. Santa Rosa Flat. Nex *H 'f 4q 'uofue;y 4steq 'C ! wea; res uo Uone}s " f y ; . . s f T M . j1umg (p 'Sundg usotxo “Mn “awn—um wwwdmpuauwwuafl rflMMMdW: HHMMMNcM NAM-“M “WNW obmm ouf 'g cuisieum 4oi¥A outeg ;e punouSoio} U; (¥) qInt} pudo 'puno12910} Ut mwwnfiw as“ I ing 3807 00} J0 arquetm urfuoaog popusq Supm1op-gijp Sut{[19a0 ouo;sou( ure;unopt UL, ort r . ? j I I I I 1J°1 18 18010 uo syooI y1tq 'doj afuei uo jo; aworxo 1j8 (g) uofut; stan usg {opion old;) go yji0u sute;unop; oy} 30 38%o Supe; uydeasojoud fiutwa— szwSOI.v WHODLI 2 g 8 a & 8 6 GEOLOGY OF THE CERRO GORDO MINING DISTRICT pany. - It led from a plant northwest of Swansea across the Inyo Mountains to Saline Valley. The Cerro Gordo mining district is known above all for its yield of silver and lead, which reached a peak in 1874. From 1911 to 1919 carbonate zinc was likewise an important product. Gold and copper, recovered espe- cially from certain of the siliceous ores in this district, were actually minor commodities and byproducts of lead and silver extraction. Nonmetallic products of the region include salines and tale. For 60 years the salines were produced in various evaporating works near Keeler (Goodyear, 1888, p. 227 ; Gale, 1915, p. 253-264). Until about 1950 the Natural Soda Products Co. on the lakeshore south of Keeler pro- duced soda ash and other byproduct salines. Salt ship- ments from Saline Valley via the 13-mile aerial tram were discontinued 20 or more years ago, and the tram and salt mills allowed to decay. Tale has been exten- sively prospected for in the southern Inyo Mountains. In recent years this commodity became the principal export (Page, 1951). The mill of the Sierra Tale Co. is at Keeler. Other commodities for which prospecting has been done with indifferent success near Cerro Gordo are tre- molite asbestos, beryllium, and tungsten. Whereas, like Darwin, the geologic environment of the southern Inyos appears favorable for tungsten, no such minerals have with certainty been recognized at Cerro Gordo. There is an unconfirmed report of scheelite in the Union tunnel. ; Worthy of mention, though not in connection with gold, is the so-called Keeler gold mine and mill southeast of Keeler. - During World War II this mill was recon- ditioned for concentration of tungsten ore from the Darwin district. Since construction of the narrow-gage railroad in 1882-83, building-stone quarries have from time to time been worked in dolomite and marble along the west foot of the range. - According to Knopf (1918, p. 123), stone from these quarries was used in construction of the Mills Building in San Francisco. Several Los Angeles buildings are said to have been faced with it (Merrill, 1903 p7206-207; Hill, 1912). On the whole, the rock is too strongly fractured to provide good dimension blocks. Silurian dolomite at selected localities near Dolomite Station yields an attractive snowy-white product. After pulverizing, this material is currently shipped for use in terrazzo. Devonian limestone quarried at the Cerro Gordo imine has in recent years been transported by the tramline for commercial uses. The Union tunnel was used in the quarrying operation. In past years a poor quality of red and green Triassic "slate" was quarried in Slate Canyon. possible value for roofing granules. It may have PREVIOUS GEOLOGICAL STUDIES IN THE CERRO GORDO AREA Study of Cerro Gordo geology may be said to have begun with a visit by Goodyear in 1870. Published in 1888 his results are accompanied by a crudely sketched geological cross section of the Inyo Range north of Cer- ro Gordo. The ore deposits and their geologic occur- rence are first dealt with by Raymond (1873, p. 17-21), after an examination of the mine by him in 1872. Geol- ogy and ore deposits of the Inyo Mountains region are discussed in general terms by Fairbanks (1894, p. 473- 475; 1896a, p. 150), and Gale (1914, 1915) described the occurrence of salines in neighboring Owens Valley and Saline Valley. Geological work by Knopf at Cerro Gordo for the Geological Survey led to publication in 1914 of a prelim- . . inary paper on geologic structure and relations of the ore deposits, especially the zinc carbonate at that time undergoing active development. A second and more comprehensive report (Knopf, 1918) constitutes a geo- logic reconnaissance of the Inyo Range. Included is a review of the geology and ore deposits at Cerro Gordo. The work is amplified by very general reconnaissance geologic mapping and a treatment of the stratigraphy by Kirk (1918). Occurrence and production of ores at Cerro Gordo and nearby mines are discussed from the engineer's viewpoint by Tucker and Sampson (1938, p. 431-4833) and later by Norman and Stewart (1951, p. 58) of the California State Division of Mines. In 1951 an economic geology report on the tale deposits on the east side of the Inyo Range near Cerro Gordo was contributed by Page (1951, p. 23). PRESENT INVESTIGATION This study began in December 1942, as a U.S. Geo- logical Survey lead-zinc strategic-minerals project. Geological mapping was at that time initiated by the writer in several parts of western Inyo County as a preliminary step in appraisal of lead and zinc resources of the region. Among areas in which work was started are the Ubehebe district, the Lee mine area, the Santa Rosa mine area, and the Cerro Gordo area. At Cerro Gordo the surface near the mine was mapped by plane- table, and accessible underground openings were mapped. During 1944, when the mine was temporarily reopened, several visits were made by the writer for purposes of underground mapping. Beginning in July 1946, a long-term quadrangle- mapping project was undertaken by the U.S. Geological PRESENT INVESTIGATION T Survey in cooperation with the State of California. Coverage of a scheduled 15-minute quadrangle (New York Butte quadrangle) which includes the Cerro Gordo area was anticipated. Detailed geologic map- ping on aerial photographs and measurement of strati- graphic sections through the southern Inyo Range were begun by the writer, who was joined in this undertaking during the fall of 1946 by W. C. Smith. Owing to numerous interruptions during which both authors were engaged in other work, these field studies were continued through the fall of 1948. Aerial photographs were used as field sheets until 1949, when the New York Butte quadrangle topographic sheet became available. Geologic maps were first compiled on a planimetric base made from airphotos by the radial-line-strip tech- nique. However, the general geologic map here in- cluded (pl. 2) has for its base the New York Butte quad- rangle; except for the Cerro Gordo mining district (pl. 1), all geological features were transferred by inspec- tion from the aerial photographs on which the mapping was done. During July, August, and September 1950, June 1951, and June 1952, detailed geologic mapping was carried out by Merriam on the new topographic base, within the area covered by the accompanying Cerro Gordo mining district geologic map (pl. 1). Scope and Umitations.-The present contribution documents only part of the work carried out by the U.S. Geological Survey in the southern Inyo Mountains. It relates specifically to the geology of that part covered by the accompanying detailed geologic map (pl. 1), and especially to the immediate vicinity of the Cerro Gordo mine itself. Separate studies have been made by the writer on the stratigraphy and paleontology of the Pale- ozoiec rocks (Merriam, 1954). Detailed studies of Triassic rocks and of the areal and economic geology of the entire New York Butte quadrangle were par- ticipated in extensively by other members of the U.S. Geological Survey parties, and are subjects of separate reports in preparation. With reference to the details of petrology and metamorphism little has been accom- plished beyond the work of Knopf (1918). Emphasis of this work was placed upon stratigraphy (Merriam, 1947) in relation to structure of a fairly size- able area surrounding the Cerro Gordo mine in order that these data might be translated into problems of economic geology in the mine itself. ACKNOWLEDGMENTS The writer was assisted in the field during. the course of this investigation by R. D. Nininger, R. L. Griggs, R. C. Douglass, L. S. McGirk, Jr., and EM. MacKevett. In later phases of the fieldwork, W. C. Smith also took part in underground mapping at the Cerro Gordo mine and in surface mapping of adjacent parts of the New York Butte quadrangle. Laboratory assistance and consultation in connection with petro- graphic problems has been provided by E. M. Mac- Kevett, J. G. Moore, and R. K. Hose. During the course of this investigation several persons interested in Cerro Gordo mining and geological prob- lems were most helpful in providing underground maps of partly inaccessible mine workings. Among these contributors are H. E. Olund, formerly of Imperial Met- als Co. at Darwin, Calif., the late J. Percy Hart, J. J. Beeson, of Salt Lake City, Utah and J. B. Stone, of the Golden Queen mine, Mojave, Calif. The operators of the Golden Queen opened parts of the Cerro Gordo mine for examination in 1943 and 1944. Underground map- ping of previously inaccessible workings on the lower mine levels was begun by the Geological Survey at that time. From 1946 until 1949 the mine was kept open by W. C. Rigg and associates. Access to the important "China stope" parts of which were reopened by Rigg, provided a rare opportunity to study and map in this section of the mine. Acknowledgment is made espe- cially to W. C. Rigg and to J. B. Stone for use of camp facilities and for making available not only mine maps but earlier reports and pertinent information on history and production. Thanks are due them for permission to publish some of the data in connection with under- ground geology of the mines. ; Among illustrations accompanying this report are photographs taken by L. G. Henbest and one oblique aerial photograph of the Inyo Range by J. H. Maxson. GEOLOGIC SETTING The southern Inyo Range comprises strongly folded and faulted sedimentary rocks which range in age from Ordovician to Middle Triassic (table 1; pl. 2). Except for the upper part of the Triassic sequence, these rocks are largely of marine origin and include limestone, dolomite, quartzite, and shale. The upper part of the Triassic sequence is predominantly volcanic with inter- calations of terrestrial sediments. Paleozoic and Tri- assic rocks are penetrated by small- to intermediate- sized granitic bodies, by aplites, and by large numbers of andesitic dikes. As might be anticipated from the number of plutonic bodies and dikes, the sedimentary rocks of this area have undergone varying degrees of contact and hydro- thermal metamorphism. Foliation is, however, not pronounced though developed locally. - Black shale like the Chainman shale is here and there phyllitic; carbon- ate rocks are not uncommonly changed to marble or cale-hornfels. 8 GEOLOGY OF THE CERRO GORDO MINING DISTRICT In terms of paleogeography the Paleozoic section . ranging from Ordovician to Permian is quite represent- ative of the Great Basin province. Considered jointly with Cambrian rocks exposed farther north in the range the Cerro Gordo Paleozoic column is one of the more inclusive of the southwestern Cordilleran belt, and as such has in recent years received a considerable amount of detailed stratigraphic and paleontologic study. At the south end of the Inyo Mountains, the older rocks are largely covered by Tertiary volcanic rocks which are seemingly coextensive with those of the Coso Mountains region to the southwest. The volcanic rocks have been considerably faulted, but have not otherwise been appreciably deformed. STRATIGRAPHIC GEOLOGY The idea that Paleozoic sedimentary rocks of the southern Inyo Mountains embraced units quite similar to those in distant parts of the Great Basin has been current for many years and is expressed in an earlier publication (Kirk, 1918). The present study serves to support and to amplify this conception. Work by Mc- Allister (1952, 1955, 1956) in the neighboring Ubehebe district demonstrates intimate stratigraphic and pale- ontologic relationships with central Nevada, especially with the Eureka district (Nolan, Merriam and Wil- liams, 1956). Whereas in general these studies point up the provincial similarities from place to place in certain parts of the Great Basin column, they also bring out great local facies differences and add support to the existence of major lineal trends or axes on oppo- site sides of which the conditions of deposition and the faunas were in Paleozoic time quite different. For ex- ample, in the Ordovician of the west-central Great Basin, a western graptolitic shale and an eastern car- bonate facies may be distinguished (Merriam and Anderson, 1942). In these terms the southern Inyo Mountains lie in the eastern or carbonate facies. Significant local facies changes are well illustrated in the Silurian system by the nearly complete change in a northerly direction from dolomite at Cerro Gordo to limestone at Mazourka Canyon (fig. 3). Likewise between Cerro Gordo and the Ubehebe district the De- vonian strata change from limestone to dolomite in an easterly direction. Only the Paleozoic rocks of the area have been studied in detail by the writer. Work on the Triassic sedimen- tary and volcanic rocks was largely undertaken by other members of the Geological Survey parties and will be reported on separately. Especially important at Cerro Gordo are the Devo- nian strata which are host rocks for the major ore bodies. During the course of this work, attention has been given the possibility of stratigraphic control of ore deposition, widely recognized in other lead-zinc dis- tricts where the ores occur as replacement of limestone. The Paleozoic column at Cerro Gordo has an approxi- mate total thickness of 11,100 feet. Individual system thicknesses are of the following orders of magnitude. Feet Permian.. . 202 242 0 LL Sele a o d En EH Ha mea he ai a acs ie ned ame 2,000 pPennsylyaniafn- -.... c.. ns 2,000 MissISSIpDIANLL _L LL _.. _ _ coe l ol ean an neenee ee nee a i alue ae 1,550 Devonian- .... 1 22.200 202 000. . LE Acela oe reran te we me an he i ual ot ce ie 1,800 _._ 0 1 M L. cE L ges 1,500 Ordovician. c. _s lean edu LIMA 2,250 TABLE 1.-Stratigraphic column of the Cerro Gordo mining district ean Thick Age Formation ness Lithology (feet) Triassic Unnamed rocks 4,000+ | Andesite flows and pyro- clastic rocks with inter- calated red sandstone and shale. Marine shale and limestone. ----|-- Unconformity Permian Owens Valley 1,800+ | Silty and sandy limestone, formation fusulinid limestone, silic- eous conglomerate, lime- stone conglomerate, shale, siltstone, sandstone, and hornfels. -Local unconformity Sandy and pebbly fusulinid ------ Keeler Canyon 2, 200+ limestone, shale, siltstone, Pennsylvan- formation and marble. ian 8 Chainman shale 1,000+ | Dark-gray silty shale and 2 phyllite. Limestone inter- S beds. 8 3 Mississip- Perdido formation 0-200+ | Limestone, chert, siltstone, _ i pian and quartzite. O Tin Mountain lime- 350 | Dark-gray limestone, chert stone nodules. Devonian Lost Burro formation 1,600+] Light- and dark-gray marble, dolomite, quartzite. Early Devonian Hidden Valley dolo- 1,700+ | Massive light- and dark-gray and Silurian mite (Lower bound- dolomite, quartzite. ary difficult to establish in this area.) Ely Springs dolomite |240-550+ | Light- and dark-gray cherty dolomite. Eureka quartzite 400+ | Light-gray vitreous quartz Ordovician ite. Pogonip group (Basal | 1,350+ | Saccharoidat dolomite and part not exposed in limestone. this area.) ORDOVICIAN SYSTEM Strata of the Ordovician system comprise three units : Pogonip group, Eureka quartzite, and rocks provision- ally referred to the Ely Springs dolomite. These rocks as exposed in the area under discussion do not include a full representation of the Ordovician system as it is known in this part of the Great Basin, for the lower part of the Pogonip group does not crop out. The bound- ary with the Silurian is uncertain here, as at many i fi STRATIGRAPHIC GEOLOGY 0 places in the Great Basin, and cannot be given a spe- cific map delineation. For mapping purposes the Upper Ordovician beds assigned provisionally to Ely Springs dolomite are not differentiated from the Si- lurian and Lower Devonian Hidden Valley dolomite. POGONIP GROUP General features.-Rocks of the Pogonip group rang- ing in age from Early to Middle Ordovician have been redefined in the Eureka district of central Nevada (Nolan, Merriam and Williams, 1956, p. 23-29). In the type region the group embraces three formations which occupy the interval between the Upper Cambrian Wind- fall formation and the Eureka quartzite or its equiva- lent. No formational differentiation has been attempted in the southern Inyos, where it is well exposed in strati- graphic continuity only along the western foothills but is quite generally altered and not very fossiliferous. In the Ubehebe area (McAllister, 1952, p. 10) the group is better represented and would probably lend itself to stratigraphic subdivision. Here in the Inyos the Pogo- nip group is treated for preliminary mapping purposes as a single formation. Areal distribution.-Only a small exposure of the Po- gonip was recognized in the Cerro Gordo mining district (pl. 1) ; it lies on the south side of Bonham Canyon near the boundary of the older rocks with the heavy fanglomerate. At this place the Pogonip is overlain by Eureka quartzite on the east and is in fault contact with the Hidden Valley dolomite on the west. Along the western foothills of the range, extensive outcrops of the Pogonip are found at Smelter Hill, Front Ridge, the mouth of Brooklyn Canyon, and east of Granite Hill (pl. 2). Strata of this unit are well exposed to the north at Mazourka Canyon (fig. 3) and in the Tale City Hills at the south end of the Inyo Mountains (Hall and MacKevett, 1958, pl. 2). Thickness and lithology.-Only a small part of the Pogonip is measurable in the fault block at Bonham Canyon. On the west side of the Inyo Range at Front Ridge, a thickness of about 1,350 feet is present, al- though the base is not exposed. In the Front Ridge belt the Pogonip consists of thick- bedded blocky-weathering saccharoidal medium-gray dolomite and medium- to light-bluish-gray fine-grained marble and limestone. Interbedded with the limestone and marble is fine-textured dense quartzite. The blocky dolomite underlies the limestone and marble, the two units being separated by an interval of platy crinkly bedded brown-weathering siliceous shale or hornfels. Before alteration the hornfelsic beds were probably a cherty calcareous shale with fine-grained cherty lime- stone intercalations. | At Front Ridge where the Pogonip section is more continuous, it has been subdivided stratigraphically on an informal basis. The lower blocky dolomite is re- ferred to as Pogonip A, the intermediate shaly horn felsic beds as Pogonip B, and the upper limestone and marble unit as Pogonip C. Locally the dolomitizatio has affected Pogonip C as well as Pogonip A. 2 At Bonham Canyon, Pogonip C is an iron-stained me- dium- to light-gray sugary dolomite with patches which still retain a slightly bluish color. Receptaculites and large gastropods of the genera Maclurites and Pallis- eria are present. Age and correlation.-Where the Pogonip group is best shown in the central Nevada region, it includes at least three formations (Nolan, Merriam and Williams, 1956, p. 23-29). The lowest unit, known as the Good- win limestone, is of Early Ordovician age, the middle or Ninemile formation is also Early Orodovician, and the upper or Antelope Valley limestone is of Early and Middle Ordovician age. In the southern Inyo Moun- tains all fossils obtained form the Pogonip are from Pogonip C, the upper unit, which correlates with the Antelope Valley limestone of central Nevada. The large gastropods Maclurites and Palliseria, together with Receptaculites, are indicative of the Palliseria zone, which is the middle of three faunal zones in the Antelope Valley limestone. A. brachiopod fauna, prob- ably representing the lower or Orthidiella zone, was found on the west side of the Inyos east of Granite Hill (loc. 40). In the Ubehebe area (McAllister, 1952, p. 11), faunas representing the intervals of the Goodwin limestone and Ninemile formation have been collected. On lithologic grounds it appears likely that Pogonip B of the Inyos is roughly correlative with the Ninemile. EUREKA QUARTZITE General features.-The Eureka quartzite of late Mid- dle to Late (?) Ordovician age is one of the more useful stratigraphic keys in the Inyo Mountains and the Great Basin generally. Named for the Eureka district, Ne- vada (Nolan, Merriam and Williams, 1956, p. 29), this resistant light-colored vitreous quartzite overlies the Pogonip and is overlain by usually dark-gray dolomite or limestone of Late Ordovician (Richmond) age. Areal distribution.-The Eureka quartzite has a lim- ited distribution in the area under consideration. It rests upon Pogonip on the south side of Bonham Can- yon and occupies a larger area between Bonham Can- yon and San Lucas Canyon (pl. 1). Good exposures may be seen on the road along San Lucas Canyon at a point 2 miles northeast of Cerro Gordo. The Eureka crops out much more extensively in the west- ern foothills of the Inyo Range between Smelter Hill E 10 GEOLOGY OF THE CERRO GORDO MINING DISTRICT and Granite Hill (pl. 2), reappearing to the north at Mazourka Canyon (fig. 3). Thickness and lithology -On the west side of the Inyo Mountains, the Eureka quartzite has an average thickness of about 400 feet. It is characteristically a clean sugary dense vitreous quartzite in which clear rounded grains are easily recognized. Normally the texture is medium to fine with the grains cemented by siliceous matter. Color ranges from nearly white to light brown and dark grayish brown; weathered sur- faces show varying degrees of limonitic staining, de- pending upon the amount of iron impurity. Bedding ranges from massive to platy, but with heavier bedding predominant. Crossbedding is fairly common, espe- cially in the upper part of the formation. - Recerystalli- zation and additive silicic alteration have not uncom- monly obliterated original sand-grain borders, giving the rock a finely granular, almost homogeneous appearance. The Eureka quartzite is usually noncalcareous, unlike the otherwise quite similar Silurian and Devonian quartzite of this region ; these contain varying amounts of calcite or dolomite as cement or as actual interbeds. At Mazourka Canyon (fig. 3), however, there are dolo- mite interbeds in the Eureka, which is exceptional. Throughout its extent in the Great Basin, the Eureka quartzite rests on rocks ranging in age from Late Cam- brian to Middle Ordovician. The basal contact is at some points a disconformity, although no evidence of such break was noted in the Inyos. The Eureka quartzite of the southern Inyos lends it- self to stratigraphic zonation based on lithology and color differences. As in many sections of this forma- tion, the lower part, ranging in thickness from 125 to about 200 feet, is darker, more deeply iron stained, and tends to be less heavily bedded (Kirk, 1933, p. 28, 30), whereas the upper part is more massive, predomi- nantly white or lighter gray and more commonly cross- bedded. In San Lucas Canyon and the west Inyo foothills, the Eureka includes a basal dark-gray angillaceous member varying from 40 to about 75 feet in thickness. It comprises shale, silty shale, and fine sandstone. At Mazourka Canyon (fig. 3) these beds are fossiliferous and have been assigned to the "Barrel Spring for- mation" (Phleger, 1983, p. 5). As elsewhere in the Great Basin the lower shaly beds are discontinuous, being wholly absent at some Eureka outcrops of the Inyo Mountains and the Ubehebe area. In central Nevada (Nolan, Merriam and Williams, 1956, p. 30) the Eureka quartzite interval is locally more than half occupied by such deposits, which will be designated as a new formation in a forthcoming report. Age and correlation.-The Eureka quartzite lies be- tween Pogonip beds of Middle Ordovician age and strata which carry a Late Ordovician (Richmond) fauna. Where the lower shaly facies of the Eureka is present, it carries faunas of about middle Trenton age, thus indicating that the Eureka interval repre- sents some part of late Middle and possibly early Late Ordovician time. ELY SPRINGS DOLOMITE General features.-Dark-gray, often quite cherty, dolomite overlying the Eureka quartzite in the southern Inyo Mountains is provisionally referred to as Ely Springs dolomite, a name originally given to similar rocks in the distant Pioche region of southeastern Ne- vada (Westgate and Knopf, 1932, p. 15). The dark- gray, at some places almost black, Late Ordovician Ely Springs band contrasts sharply with the light-gray Eureka where seen from a distance. Unlike the base, where the lithologic change is abrupt, the top of the Ely Springs is not readily de- finable in this area. - Very similar types of dark cherty dolomite are in fact repeated within the overlying Hid- den Valley, and the Ordovician-Silurian boundary ap- pears to be definable only on a paleontologic basis. This factor together with strong deformation and mild alteration of the rocks in question leads the writer to the conclusion that separation of the Ely Springs from the Hidden Valley for mapping purposes is at present impracticable. Areal distribution. -The Ely Springs dolomite is ex- posed 114 miles southeast of Bonham's tale mines (pl. 1). It is best shown, however, in the rugged western foothills of the range between Granite Hill on the north and Smelter Hill on the south (pl. 2). Equivalent strata given local names from one region to another are represented throughout much of the southern Cordil- leran belt. Although generally associated with the Eureka quartzite, these strata extend far beyond the limits of even that widespread formation. Thickness and lithology.-Thickness of the Ely Springs cannot be given accurately because of uncer- tain position of its upper boundary in this region. At Mazourka Canyon (fig. 3), where the top is placed just beneath a massive 20-foot chert member, it is about 270 feet thick. - At Smelter Hill the formation is at least 240 feet thick, but the massive chert bed was not recognized. At Smelter Hill the lower 150 feet of the Ely Springs is a dark-gray very cherty saccharoidal dolomite, chert constituting 1% to 14 of the entire rock. The chert is usualy gray and occurs as very irregular nodules or as lenses elongated with bedding. Recrystallization of the chert has commonly led to development of felted aggre- STRATIGRAPHIC GEOLOGY 11 gates and rosettes of lime-silicate minerals. The nodu- lar cherty dolomite is inclined to be rather thinly bedded. Toward the top of the lower 150-foot member, the amount of chert decreases with passage upward into a less dark gray almost noncherty dolomite member about 90 feet thick. This higher member is heavier bedded, rather coarse textured or sugary granular, and blocky weathering. Above this member the blocky dolo- mite becomes light gray or nearly white in the vicinity of Smelter Hill and is classified for the greater part with the Silurian part of the Hidden Valley. Age and correlation.-Strata to which the name Ely Springs dolomite is applied at Cerro Gordo are similar lithologically to those which rest upon the Eureka quartzite at many points throughout the central and southern Great Basin. These rocks generally contain Richmond Late Ordovician faunas. To the northeast in Utah the name Fish Haven dolomite is used for virtu- ally the same unit; in central Nevada, where limestone locally takes the place of dolomite, it is the Han- son Creek formation. The Montoya limestone of New Mexico is likewise similar both lithologically and faunally. Loose streptelasmid horn corals, probably derived from this unit, were collected in the area between San Lucas Canyon and Bonham Canyon (pl. 1). These corals came from the vicinity of tale prospects in the Ely Springs dolomite adjacent to the large Eureka quartzite exposure. Scarcity of good fossils is ex- plained in part by rock alteration. Convincing evidence of Richmond age is found in Ely Springs dolomite of the Tale City Hills (fig. 3) at the south end of the Inyo Mountains. About 214 miles northwest of the Tale City mine, the writer collected silicified corals and brachi- opods at locality 39; representative Ely Springs fossils from this locality are listed as follows : Halysites (Catenipora) sp. Columnmaria cf. C. alveolata (Goldfuss) Streptelasmid corals, several types Heterorthis sp. Glyptorthis cf. G. insculpta (Hall) Thaerodonta sp. Lepidocyclus (at least two species) Platystrophia sp. Onniella cf. 0. quadrata Wang Zygospira n. sp. Atrophomena sp. Plaesiomys sp. SILURIAN SYSTEM A dolomite-quartzite sequence about 1,500 feet thick is assigned to the Silurian system. These strata occur between Late Ordovician Ely Springs dolomite and rocks of established Devonian age. In the southern Inyo Mountains no objectively mappable contact with the Ely Springs was recognized, and the Silurian-De- vonian relation appears to be one of transition. Deter- rents to fixing of these systemic limits are such factors as scarcity of fossils, shearing, fracturing, and rock al- teration; especially is this true along the Silurian-De- vonian boundary zone at Cerro Gordo. Hence the sys- tem boundaries both below and above remain indefinite. Complications noted in connection with the differenti- ating of Silurian rocks from those of the Ordovician and Devonian are by no means confined to this region ; such difficulties are encountered generally in the Great Basin, where the middle Paleozoic is normally dolomite containing few good fossils. § Silurian rocks of the adjoining Ubehebe district are described by McAllister (1952, p. 15) as the Hidden Valley dolomite. However, the typical Hidden Valley as originally defined is not entirely Silurian, but in- cludes Lower Devonian (Oriskany) dolomite at the top. For purposes of this report the term Hidden Valley do- lomite is applied, in the original sense, to Silurian and Lower Devonian strata of the southern Inyo Mountains. Later work may well point up the desirability of sepa- rating the Devonian part, either as an independent formation, or perhaps by considering it a member of the overlying Devonian Lost Burro formation. Facies changes complicate the Silurian stratigraphy of this region, as well illustrated by nearly complete change from dolomite at Cerro Gordo to limestone on the north at Mazourka Canyon (fig. 3). Also note- worthy is the introduction of thick local quartzite units west of the Ubehebe district. , For purposes of map representation the Hidden Val- ley dolomite and the Ely Springs dolomite are not differentiated at Cerro Gordo. HIDDEN VALLEY DOLOMITE General features.-The Hidden Valley dolomite, com- prising dolomite and quartzite, lies between the Ely Springs dolomite and the Devonian Lost Burro forma- tion. This discussion applies mainly to the Silurian part, which constitutes most of the formation. Details of the Early Devonian part of the Hidden Valley are dealt with more fully under rocks of the Devonian system. Type section of the Hidden Valley dolomite is in the Ubehebe district, 20 miles northeast of Cerro Gordo (McAllister, 1952, p. 15). At Cerro Gordo the forma- tion differs from the type section by including con- siderable amounts of quartzite and chert (Merriam, 1951) ; it has furthermore been subjected to rather wide- spread hydrothermal alteration, with local development of commercial tale deposits. Areal distribution.-The Hidden Valley dolomite crops out over a large area on the east limb of the Cerro 12 GEOLOGY OF THE CERRO GORDO MINING DISTRICT Gordo anticline, extending from the north side of Bonham Canyon southward to the Belmont mine area on the east side of the range (pl. 2). The formation is well exposed along the western Inyo foothills from Smelter Hill through Front Ridge. A separate belt ex- tends from the Dolomite Hills along the east side of Brooklyn Canyon to the mouth of Long John Canyon. Lack of outcrop prevents tracing of this dolomite into the seemingly equivalent Silurian limestone facies at Mazourka Canyon, 12 miles to the northwest. Thickness and lithology.-The Hidden Valley dolo- mite in this area is roughly 1,750 feet thick. Of this, roughly 1,500 feet is Silurian, the remainder of Early Devonian age. The formation consists of light- to medium-gray blocky dolomite, dark-gray cherty dolomite, arenaceous dolomite, and quartzite. Heavy-bedded blocky jointed saccharoidal dolomite makes up at least half the unit and ranges from medium and light gray to white on fresh fracture. A fine banding or lamination is noted here and there. Brown iron-stained dolomite lenses at- tain a thickness of 25 feet and a lateral extent of sev- eral hundred feet. Iron-stained fractures and joints are common. - In the upper part of the formation, light- gray dolomite contains abundant crinoidal debris. When pulverized some of the purer Hidden Valley dolo- mite is snowy white and has long been quarried for terrazzo and other commercial purposes. Dolomite of the Hidden Valley resembles that of the Pogonip group, but on the whole is inclined to be lighter gray. f As exposed in the Salt Mill Hills (pl. 2), the dark- gray cherty dolomite tends to exhibit thinner and more distinct bedding than noncherty phases. The dark-gray to black chert occurs in thin irregular nodules or dis- continuous interbeds a few inches thick. Locally the siliceous dolomite has been altered to talcy or tremo- litic material, in fact some of the minable tale deposits are an alteration product of this Silurian facies. The dark cherty Silurian dolomite is lithologically indistin- guishable from that of the Ely Springs dolomite with- out benefit of stratigraphy. Quartzitic rocks of the Hidden Valley formation range from dense vitreous white or light-gray types re- sembling Eureka quartzite to arenaceous dolomite con- taining varying proportions of rounded quartz grains. Bedding ranges from heavy in the vitreous quartzite low in carbonate cement to relatively thin, as where limy quartzite and dark gray cherty dolomite are inter- bedded. The quartz grains are well rounded and me- dium to fine. Quite characteristic is the non vitreous limy quartzite with white aphanitic carbonate cement. All gradations are found from this type to white dolo- mite with scattered quartz grains. - On the west side of the Inyo Range at Smelter Hill and Salt Mill Hills four lithologic units can be defined in the Hidden Valley dolomite as follows: Pasts 2.-Members of the Hidden Valley dolomite, Salt Mill Hills Thick Age Formation Mem- Lithology ness ber (feet) Middle and Late Lost Burro Devonian formation (Fault rela <--- D Light- to brownish- 150 Early Devonian gray blocky sac- charoidal dolomite. pomme ee Pean | '- C Light-gray and white 540 quartzite, limy quartzite, and arenaceous dolo- Hidden Valley mite. dolomite 1,740 ft. 5 ore 5 cust edium- to dark- 250 Silurian gray well-bedded cherty dolomite. A Light-gray to white 800 thick-bedded blocky saccharoidal dolomite. aon (Transitional relation) aHe Late Ordovician Egolorglirggs Between Bonham Canyon and the Belmont mine on the east side of the Inyo Range (pl. 1), total thickness of the Hidden Valley is roughly commensurate with that tabulated above for the western foothills. Details of the stratigraphy - differ somewhat. Rock types which characterize members B and C to the west appear to be mixed here in a sequence of interbedded vitreous quartz- ite, limy quartzite, and dark-gray dolomite, part of which is cherty. Quartzite zones range from a few inches to several feet in thickness but reach a maximum of about 45 feet in a few places. This thickness is less than the maximum on the west side at Salt Mill Hills, where the quartzite zones are on the whole more con- tinuous laterally. In the eastern sequence dark-gray dolomite and well-bedded cherty dolomite appear to predominate over quartzite, individual cherty dolomite lenses reaching a maximum thickness of about 60 feet. Blocky light-gray saccharoidal dolomite corresponding to member A underlies the quartzite-cherty dolomite interval, while above is the upper blocky dolomite of member D with its contained crinoidal debris. Details of the stratigraphy are interpreted with difficulty be- cause of the sheared and locally altered condition of these strata in Bonham Canyon. It is quite evident, however, that individual quartzite and cherty dolomite bodies pinch out rapidly in the depositional sense. To the east the quartzites apparently disappear before reaching the Ubehebe area (fig. 3), while to the west toward Owens Valley, these highly siliceous deposits are even more extensively developed. Northwestward in the direction of Mazourka Canyon, they again disap- STRATIGRAPHIC GEOLOGY 13 pear from the section, as the Hidden Valley dolomite changes to limestone. Age and correlation.-In Bonham Canyon the cherty dolomite associated with quartzite at locality 32 has yielded an excellent fauna of silicified material includ- ing corals and brachiopods which indicate a Middle Silurian age. The highly disturbed beds from which the material came are believed to represent either member B or member C, probably above the middle of the formation. Among the common forms are species of Halysites, Heliolites, and Favosites, together with abun- dant Atrypa, a large smooth pentameroid (possibly Pentamerus) and Schizoramma sp. The Schizoramma is significant for it resembles a species from the Browns- port formation of Tennessee (Amsden, 1949, p. 45), which is believed to be of Niagaran age. Elsewhere in the southern Inyo Mountains, fossils are extremely rare in the Hidden Valley dolomite. They are locally abundant and are silicified in dolomite of the type area. (McAllister, 1952, p. 16). The faunas include Porpites, Diplophyllum of. D. caespitosum, 2Catazyga sp., and large conical dasycladacean algae. These al- gae, to which the name Verticillopora annulata has been given by Rezak (1959), resemble closely those found at several other localities in Great Basin Silurian rocks. Where the Hidden Valley changes northward to lime- stone at Mazourka Canyon, the rocks are abundantly ”fossiliferous, but the material is poorly preserved. The faunas consist very largely of corals, only Atrypa and rhynchonellids (Fatomia bicostata Stauffer) being at all common among the brachiopods. The large da- sycladacean algae (Verticilloporea annulata Rezak) are most prolific here and provide a tie with the Hidden Valley of the type areas as well as with the Roberts Mountains formation of central Nevada and the Lake- town dolomite of western Utah. Among corals of the limestone facies at Mazourka are many conforming to the general features of & trombodes. Others are assigned to Chonophyllum, Rhizophyllum, Heliolites, Alveolites, and Cladopora. Also present are large cyathophyllids and bushy forms of the Phacelo- phyllum and Disphyllum types. The Silurian part of the Hidden Valley dolomite of the Inyo Mountains probably ranges in age from Early to Late Silurian and seemingly correlates with the com- bined Roberts Mountains formation and Lone Mountain dolomite of central Nevada (Nolan, Merriam, and Wil- liams, 1956, p. 36). As mentioned above, the Hidden Valley of the original definition (McAllister, 1952. p. 17) also includes Early Devonian. The age of these uppermost beds is dealt with under the section "Devon- ian system." 678-865 O-63--2 DEVONIAN SYSTEM Banded limestone and marble of Devonian age make the impressive east-facing cliffs and very rugged upper east slopes of the Inyo Range stretching northward from Cerro Gordo. Rocks of this system underlie roughly one-fifth of the area under consideration and are especially important as host rock of the major ore bodies. The Devonian system is represented by about 1,800 feet of strata, a thickness that exceeds the Silurian by a possible 300 feet (fig. 5). Unlike the Silurian of the Cerro Gordo area only a small part of the Devonian is dolomitized. T wo formations are involved, the Lost Burro forma- tion, which includes most of the system, and an under- lying Lower Devonian zone, which occupies the upper part of the Hidden Valley dolomite. Strata previously assigned to the Devonian in this region (Kirk, 1918, p. 36; Stauffer, 1930, p. 8-91) are largely, if not entirely, Silurian (Nolan, 1943, p. 153) and are considered under treatment of the Silurian part of the Hidden Valley dolomite. Certain other strata at Cerro Gordo previously included with the Carbonifer- ous (Knopf, 1918, p. 110) are actually Devonian. The Devonian section of the southern Inyo Mountains is about half the thickness of that in the central Great ,, Basin. Fossil evidence as well as lithology suggests | that most of the attenuation may well be in the middle part of the system at Cerro Gordo. Boundary of the Devonian with the Mississippian is marked by an abrupt and readily mappable lithologic FiGurB 5.-View looking northwest at the top of the Inyo Mountains 2 miles north of Cerro Gordo. Highest dark-gray strata are Tin Mountain limestone resting conformably on cliff-forming Devonian marble of the Lost Burro formation, host rock of the principal ore bodies at Cerro Gordo. Foreground underlain by Hidden Valley dolomite. — 14 GEOLOGY OF THE CERRO GORDO MINING DISTRICT change and by introduction of wholly new faunas. On the other hand the boundary with the Silurian remains uncertain, may be transitional, and falls somewhere within the upper 400 feet or so of the Hidden Valley dolomite wherein the Early Devonian Oriskany fauna is to be expected. Except for a few clean, washed quartz sands or quartzite the Devonian rocks of this region are almost entirely carbonate. UPPER PART OF HIDDEN VALLEY DOLOMITE (DEVONIAN PART oNLY) General features.-Fossil evidence shows conclu- sively in the type area (McAllister, 1952, p. 15, 17) that at least part of the upper 400 feet of the Hidden Valley is Early Devonian. Fossils are few in these rocks at Cerro Gordo, but largely on the basis of lithology the upper 250 to 350 feet of this formation (member D of the Hidden Valley) is likewise provisionally classified as Early Devonian. Areal distribution.-Rocks classified as member DD.of the Hidden Valley are shown along Bonham Canyon in the vicinity of the tale mines, and on the east side of San Lucas Canyon near the Perseverance mine. On the west side of the Inyo Range, these strata may be ob- served in the Salt Mill Hills and in the rugged terrane east of Brooklyn Canyon. Lithology and stratigraphy.-Upper beds of the Hid- den Valley dolomite (member D) may be observed near the trail from San Lucas Canyon to the Perseverance mine. About 800 feet northwest of the mine (loc. 25) these beds with a thickness of about 350 feet appear to be transitional between the main part of the saccha- roidal Hidden Valley dolomite below and Stringoce- phalus beds of the overlying Lost Burro formation, above. The quartzite of member C has lensed out in this section, but appears again within a mile along the strike to the south, toward the Belmont mine. The member D or transition beds comprise blocky medium- to light- gray dolomite, partly dolomitized bluish-gray limestone, and near the bottom thinner bedded light-gray-weath- ering very fine grained dolomite which contains abun- dant poorly preserved high-spired gastropods of the Loxzonema type (loc. 25). Upper beds of the Hidden Valley (member D) are well represented near the tale mines on the northeast side of Bonham Canyon (pl. 1). In this belt they have been strongly sheared and locally involved in the tale mineralization, but give the impression of a tran- sitional interval between the middle quartzite beds of the Hidden Valley and the Lost Burro formation. On the west side of the range at Brooklyn Canyon, the strata of member D are similarly transitional. Age and correlation.-The upper beds of member D of the Hidden Valley lie between the 8 tringocephalus beds of the lower part of the Lost Burro, which are of late Middle Devonian age, and the quartzite beds of the Hidden Valley (member C), which are of Niagaran (Si- lurian) age. In the Ubehebe district (McAllister, 1952, p. 17) the upper beds of the Hidden Valley in question have yielded an excellent silicified fauna of Oriskany (Early Devonian) age identical with that from the lower part of the Nevada formation in central Nevada (Merriam, 1940, p. 50). The Oriskany fauna includes "Spirifer" kobehana, "Spirifer" cf. "48." arenosus, and Papiliophyllum elegantulum Stumm. Whereas mem- ber D of the Hidden Valley is thus directly correlated with the lower part of the Nevada formation, there ap- pear to be neither beds nor faunas in the area under discussion which correspond to the overlying middle part of the Nevada. The gastropod-bearing very fine grained dolomite near the bottom of member D on the Perseverance mine trail agrees lithologically with the Beacon Peak dolo- mite member of the Nevada formation in the Eureka district (Nolan, Merriam and Williams, 1956, p. 42) and with the Sevy dolomite of western Utah. Apha- nitic well-bedded dolomite of this type appears in fact to be rather characteristic of the Lower Devonian in the Great Basin. At Eureka it rests disconformably upon the blocky saccharoidal Lone Mountain dolomite which correlates with at least the middle part of the Hidden Valley. Large Lozonema-like gastropods in member D are indeterminate but resemble forms which occur in the lower and middle parts of the Nevada formation. LOST BURRO FORMATION General features.-The name Lost Burro has been applied to Devonian strata in the Ubehebe district (McAllister, 1952, p. 18), which occupy the interval be- tween Hidden Valley dolomite and the Tin Mountain limestone of Mississippian age. Type section of the formation is at Lost Burro Gap, Ubehebe Peak quad- rangle. In the type area these rocks are largely dolo- mite, differing in this respect from approximately equivalent beds of the southern Inyos, which are in the main rather pure limestone or marble. The Lost Burro includes most of the Devonian rocks in this area, resting apparently without break upon the lower Devonian uppermost part of the Hidden Valley dolomite. This important ore host was long and appropriately known to the mining profession as "Cerro Gordo - marble", a term now formally eliminated by published adoption of Lost Burro; moreover the name "Cerro STRATIGRAPHIC GEOLOGY Gordo" has nomenclatorial priority elsewhere, having been published much earlier in application to other formations. Areal distribution.-The Lost Burro formation has been mapped from the Belmont Mine area on the east side of the range, along the higher more cliffy slopes to Bonham Canyon (pl. 1). Northward from Bonham Canyon the main marble belt of the Lost Burro departs eastward of the Inyo crest to occupy terrane of inter- mediate and lower altitude through the Daisy Canyon drainage to Hunter Canyon (fig 3). North of Hunter Canyon, near the east foot of the Inyo Range, marble of the Lost Burro formation extends into a marble- dolomite-calc-hornfels complex and is with difficulty distinguishable from the Silurian and older rocks. From Cerro Gordo north for 6 miles, the strikingly banded white, gray, and bluish-gray marble of this divi- sion is sculptured in rugged cliffy exposures which face east and rise abruptly above less steep terrane formed by the blocky Silurian and Lower Devonian dolomite. Marble of the Lost Burro reappears at the Lee mine on Lee Flat, east of the Inyo Mountains (fig. 3). In the west Inyo foothills a narrow belt of thoroughly marbleized and locally more strongly altered Lost Burro has been mapped for 6 miles along the east side of the Salt Mill Hills through the very steep terrane | east of Brooklyn Canyon (pl. 2). At Mazourka Can- 1, yon (fig. 3) to the northwest, the formation appears to } be entirely absent, having been removed by erosion at | the unconformity which separates Chainman shale and 7 Perdido from the Silurian limestone. | Thiekness.-The Lost Burro formation, measured in detail on the northwest side of Cerro Gordo Peak, is about 1,600 feet thick. North of Cerro Gordo and in the west Inyo foothills where these beds are highly deformed, an assumed average thickness of 1,500 feet accords with outcrop width. McAllister (1952, p. 18) has measured 1,525 feet of this unit south of Lost Burro Gap in the Ubehebe district. Sections measured else- where in that area by McAllister exceed 2,000 feet. Lithology.--The Lost Burro formation near Cerro Gordo is prevailingly massive craggy cliff-forming marble and limestone of white to bluish-gray color. Lo- cally the color ranges from white to light bluish-gray to dark gray. Viewed from the east the finely seulp- tured cliffs and crags of Lost Burro north of Cerro Gordo present a striking pattern, with intricate bands and patches of contrasting gray and white, an aspect peculiar to the formation in this area. In the duller gray blocky Silurian dolomite below, the outcrops are more uniform, making on the whole less precipitous staircaselike, slopes. Although this formation is predominantly thick- 15 bedded, there are subordinate flaggy and platy lime- stone units. At some points thick-bedded marble shows a varvelike millimeter lamination. The marble or limestone of the Lost Burro formation tends to be of fairly uniform finely crystalline to sub- porcellaneous texture and weathers smooth. Small bodies of more coarsely recrystallized marble are present. Except along fissures and fractures, where ad- ditive metasomatic activity has taken place, the rock consists mainly of calcite and has been used commer- cially where pure calcium carbonate is required.. With its normally low content of clay, silica, iron, and mag- nesium, the Lost Burro contains the purest limestone of the region. In the southern Inyo Mountains, unlike the Ubehebe district (McAllister, 1952, p. 18), the Lost Burro is mainly nondolomitic. Dolomitization was noted only near the base in a transition interval be- tween this formation and the normal upper dolomite of the Hidden Valley. Here and there in the transi- tion zone, the bluish-gray smooth-weathering limestons exhibits irregular patches and tongues of probably dia- genetic dolomite wherein the bluish gray is lost and the rock becomes saccharoidal, weathering with a rough sandy surface. The Lost Burro includes several siliceous zones in which quartz sand grains constitute the siliceous ma- terial. Chert is extremely rare in this formation at Cerro Gordo. On the contrary in the Ubehebe area McAllister (1952, p. 18) finds abundant chert in a lower 155-foot sandy dolomite not recognized in the Inyo Mountains. At Cerro Gordo, hydrothermal jasperiza- tion has taken place locally along fractures. Absence of chert is significant, for the overlying Tin Mountain limestone with abundant chert may on this basis be distinguished. Quartzite and sandy limestone characterize the lower part of the Lost Burro but actually constitute an in- significant part of the unit in terms of volume. The quartzite or sandstone beds range in thickness from a few inches to a maximum of 25 feet. Thin quartzite beds in places can pass laterally into inconspicuous limy quartz sandstone and limestone with scattered quartz grains. The standstone and quartzite are well sorted, showing medium to coarse rounded quartz grains. Northeast of the Cerro Gordo mine the thicker quartzite beds crop out as prominent ribs but vary greatly in density and thickness from point to point. Where dense and vitreous, these deposits are light gray or white, re- sembling vitreous Eureka quartzite or the denser quartz- ite of the Hidden Valley formation. Sporadic appear- | ance of these clean quartz sands as lenses or beds in an | otherwise rather pure limestone is somewhat enig- | matic, suggesting aeolian introduction. 16 GEOLOGY OF THE CERRO GORDO MINING DISTRICT At Cerro Gordo the lower part of the Lost Burro is abundantly fossiliferous. A great part of the limestone is of organic origin as attested by the biohermal masses of stromatoporoids and "spaghetti coral." Not uncom- monly these coral-rich lenses are dark gray, owing to their high carbon content and contrast sharply with light-gray barren limestone which surrounds them. Color banding and streaking previously referred to seems to be a matter of carbon distribution related to factors of sedimentation. At many points in the marble of the Lost Burro, in- tense compressional deformation is revealed by intri- cate minor flexures. In the vicinity of Cerro Gordo a steeply dipping fracture cleavage or sheeting shows prominently in the Lost Burro formation. These fea- tures, together with jointing, have influenced sculpture | considerably, especially as noted in the higher suni- mits and crags north of Cerro Gordo. The sheeting may easily be mistaken for bedding in the massive mar- bles. True bedding is revealed where these superim- posed structural features intersect quartzite interbeds. Stratigraphy.-The Lost Burro formation is under- lain with seemingly transitional relation by the Lower Devonian upper part of the Hidden Valley and is over- lain with apparent conformity but abrupt lithologic change by the Mississippian Tin Mountain limestone. The Tin Mountain differs from the Lost Burro by being uniformly dark gray and containing much chert. Whereas no evidence of disconformity was found at this boundary, the sharp lithologic change and the abrupt introduction of a Madison fauna suggests a hiatus. Stratigraphic control in the Lost Burro is especially important as an aid to underground structural interpre- tation at the Cerro Gordo mine, largely developed in this formation. Coral beds and quartzite exposed in San Lucas Canyon just north of the mine would be ex- pected to have value as underground structural keys. A lower lithologic zone (zone A) and an upper lith- ologic zone (zone B) are recognized in the Lost Burro formation of the Cerro Gordo mine vicinity. Zone A is 575 feet thick, as measured on the east side of San Lucas Canyon ; zone B is 1,025 feet thick and embraces the re- mainder of the formation. Darker bluish gray marbles and limestone are con- spicuous in zone A, predominating over the light gray and white phases. In the lower 200 feet spotty incipient dolomitization causes bleached patches in the normally bluish-gray marble. An important paleontological da- tum is the Stringocephalus bed which immediately over- lie the transitional interval showing patchy dolomitization. Quartz sand and quartzite members characterize the upper 250 feet of zone A, the most conspicuous being a 23-foot member and a higher 6-foot bed at the top of the zone. Sandstone layers ranging in thickness from 1 inch to about a foot are fairly common.. Scattered quartz sand grains occur in many of the limestone layers. Zone B, the upper of the two, exhibits a predominance of the light-gray to white heavy-bedded cliffy marble streaked and banded with bluish gray. Siliceous sand- stone layers are rare, though one a few inches thick oc- curs 640 feet below the top of the formation. "Spa- ghetti coral" and stromatoporoid beds are less numerous than in zone A, but dark-gray "spaghetti" beds occur sparingly up to an horizon 500 feet above the bottor1 of zone B. Possible value of these stratigraphic zones for pur- poses of structural geology is considered helow under geology of the Cerro Gordo mine. Age and correlation.-Stringocephalus, a diagnostic late Middle Devonian brachiopod, occurs near the base of the Lost Burro north of Cerro Gordo. Excellent faunas of Late Devonian age were collected from the upper part of this unit in the Ubehebe district. Thus the Lost Burro formation ranges in age from late Mid- dle to Late Devonian. Coral and stromatoporoid remains form much of the limestone in zone A, the lower part of the formation. The "spaghetti coral" limestone of this division is made up in part of poorly preserved slender CZadopora, but probably includes Amphipore, a digitate branching stromatoporoid common in beds of equivalent age throughout the Great Basin. Stringocephalus serves to correlate the basal Lost Burro with the upper part of the Nevada formation in central Nevada (Merriam, 1940, p. 24), where this genus ranges through about 400 feet of strata. Some 1,300 feet of the Lost Burro above Stringo- cephalus, roughly three-fourths of the formation, cor- relates with the Devils Gate limestone of central Nevada, and agrees with it remarkedly well in thick- ness. In the Eureka district, Nevada, the Devils Gate limestone averages about 1,300 feet in thickness and rests upon the uppermost member of the Nevada for- mation containing S¢ringocephalus. As at Cerro Gordo, "spaghetti coral" and stromatoporoids are builders of limestone in the lower part of the Devils Gate. Poorly preserved gastropods collected 2,000 feet northwest of the Newsboy mine (loc. 26) are pos- sibly Qrecopia mecoyi (Walcott), a characteristic species of the Spirifer argentarius zone in the upper part of the Devils Gate limestone. . Late Devonian spe- cies which characterize the Cyrtospirifer zone of the uppermost Devils Gate have not been found in the south- ern Inyos. Fossils of this zone, to be expected near the top of the Lost Burro, have been found by McAllister STRATIGRAPHIC GEOLOGY 17 (1952, p. 18) in the Ubehebe area, where they occur in an upper 35-foot sandy dolomite and sandstone bed (bed 5). The typical Lost Burro of the Ubehebe area differs by being to a considerable extent dolomite. Its thick- ness is, however, comparable to that of the limestone in the Lost Burro formation near Cerro Gordo, and like the Cerro Gordo section the late Middle Devonian Stringocephalus zone occurs near its base. Unrecognized in the southern Inyos and at Ubehebe are the distinctive Martinia kirki and Apirifer pinyo- nensis faunas; in central Nevada these occupy zones within the middle and lower parts of the Nevada forma- tion. Comparative thicknesses suggest absence of beds representing these zones at Cerro Gordo, where the entire Devonian is only about 1,800 feet thick, about half that of the central Nevada region. Oriskany Lower Devonian fossils in the upper Hidden Valley dolomite at Ubehebe are nonetheless clear evidence that strata equivalent in age to the lowermost Nevada are represented. Further details relating to paleontology and correlation of the Lost Burro and the Hidden Valley are being dealt with by the writer in a separate paper in preparation. MISSISSIPPIAN SYSTEM The Mississippian system is represented in the south- ern Inyo Mountains by two principal formations : these are the Chainman shale above and the Tin Mountain limestone below. A minor unit, the Perdido formation, lies between the Tin Mountain and the Chainman ; it has not been mapped separately. The Perdido is much thicker to the east in the Ube- hebe district where McAllister (1952, p. 22-25) has dis- tinguished it as a separate formation. West to east thickening of the Perdido between Cerro Gordo and the Ubehebe area is believed by the writer to take place at the expense of the lower part of the Chainman shale, of which it is accordingly viewed as a facies. TIN MOUNTAIN. LIMESTONE General features-The dark-gray Tin Mountain limestone of Early Mississippian age rests upon the Lost Burro formation and is overlain by the Perdido forma- tion, also of Mississippian age. Named for a peak in the northernmost Panamint Range (McAllister, 1952, p. 20) the Tin Mountain is a prominent cliff maker in its type area. In the Cerro Gordo area it forms a dark- gray band at the top of the cliff-making lighter colored Lost Burro (figs. 5 and 6). Areal distribution.-Tin Mountain limestone forms the top of Cerro Gordo Peak and composes most of the higher west slope between that point and the Morning Star mine to the south. West of San Lucas Canyon the intermittent dark band of Tin Mountain may be fol- FiGUrRE 6.-View looking north along higher east slope of the Inyo Mountains near Cerro Gordo. Left distance shows Chainman shale at crest. Middle distance, crags in sheared Tin Mountain limestone against light-gray Lost Burro formation on east. In foreground, fusulinid-bearing limestone of the Keeler Canyon formation down- faulted on Ignacio fault, lowed northward for some 5 miles near the top of the east-facing cliff slopes. North of Daisy Canyon on the very rugged east slopes and on the lower west side of the range, it is differentiated with difficulty from the squeezed and altered Lost Burro. The Tin Mountain limestone crops out near the Lee mine on Lee Flat (fig. 3) and at several points between the Lee mine and the Belmont mine. At Mazourka Canyon it has not been recognized beneath the unconformity which sepa- rates Perdido from the Silurian. limestone. Thickness.-The Tin Mountain is about 350 feet thick at Cerro Gordo Peak, thinning appreciably as it is fol- lowed northward along the crest of the Inyo Mountains. At Mexican Spring it has thinned to about 75 feet. In the type area the Tin Mountain is 475 feet thick. Lithology.-At Cerro Gordo the Tin Mountain lime- stone is a medium-bluish to dark-bluish-gray fine- grained limestone and its beds range in thickness from less than 1 inch to 2 or more feet. Weathering platy and flaggy to massive, this formation appears more massive from a distance than it actually is. Dark-gray and black chert is common as blobs and irregular lenses which are either iron stained or bleached light gray and white. Much organic material is present in the form of crinoid and coralline debris. White calcite of the crinoid fragments contrasts with the dark limestone matrix. Content of argillaceous matter is low, chief impurities being silica of the chert and the dark finely divided carbonaceous material. North of Cerro Gordo, where the Tin Mountain is affected by fracture cleavage, it forms jagged cliffy slopes. Distribution of chert nodules makes possible the distinction between bedding and cleavage, other- wise difficult. f Near Cerro Gordo the Tin Mountain limestone is thin- ner and more uniform lithologically than in the Ube- 18 GEOLOGY OF THE CERRO GORDO MINING DISTRICT hebe district, where, according to McAllister (195%, p. 20, 21), it shows pale-red partings, and in the lower part brownish-gray to pale-red calcareous shale. Stratigraphy.-Although the Lost Burro-Tin Moun- tain contact is an important system boundary, no physi- cal evidence of erosion or angular discordance was noted. It is nonetheless well defined lithologically and readily mappable, unlike most system boundaries in the Paleozoic of the Great Basin. The fairly uniform dark- gray band of the cherty Tin Mountain contrasts rather sharply with the streaked and patchy light-gray and darker gray Lost Burro beneath. In the type Tin Mountain, McAllister (1952, p. 21) recognized two members: (a) a lower unit 275 feet thick with beds 2 to 6 inches thick and including brownish- gray to pale-red calcareous shale; (b) an upper cliff- forming member 200 feet thick with beds a few inches to 2 feet thick and with faint pale-red partings. Except for pale-red partings the Tin Mountain at Cerro Gordo more nearly resembles the upper member. Seeming absence of the lower beds, which include red and brown calcareous shale, suggests erosion or nondeposition, | possibly at the Devonian-Mississippian boundary in the fiLsouthern Inyo Mountains. At Cerro Gordo the Tin Mountain limestone is over- lain with sharp contact by the fine-grained quartzite member which constitutes a westerly tongue of the Per- \ dido formation. Followed northward along the range crest the quartzite of the Perdido disappears at some points, as the Tin Mountain itself becomes thinner. Such behavior suggests disconformity at the Perdido- Tin Mountain contact. In keeping with this reasoning | is the observation that in Mazourka Canyon the Perdido | is spotty and rests with profound unconformity upon Si- lurian limestone. In the Cerro Gordo area the deposi- tional change at the Tin Mountain-Perdido contact is one of the more significant of the Paleozoic column. The normal marine carbonate environment gives way here to one of land-derived siliceous clastic materials which include silt, sand, and highly argillaceous shale. Such facies characterize the Chainman-Diamond Peak inter- val over very large areas in the Great Basin. Land- plant remains in the Chainman bespeak emergence and presence of exposed land at no great distance. To the east, conditions were somewhat different, for in the type Tin Mountain area McAllister (1952, p. 21) finds the relation of this carbonate unit to the overlying Per- dido to be one of gradation. Age and correlation.-Although the Tin Mountain limestone is loaded with crinoid remains and other fossil debris in the Cerro Gordo area, it has yielded few de- terminable fossils. The crinoid columnals are often of large size reaching a diameter of more than half an inch. Near the Cerro Gordo springs, 344 miles north- west of Cerro Gordo, the Tin Mountain contains Syringopora and horn corals of the Caninia type. Near the Lee mine (fig. 3) and in the long ridge to the north this limestone contains Syringopora, Triplophyllites?, and a distinctive brachiopod referred to here as Brachy- thyris sp. A. Spirifers found here resemble Spirifer rowleyi Weller or 8. grimesi Hall. In the Ubehebe district the formation has yielded abundant fossil mate- rial (McAllister, 1952, p. 21). On the southeast side of Perdido Canyon (loc. 36), the writer made collections which include the following : Aulopora sp. Syringopora sp. Ekvasophyllum n. sp. (Ekvasophyllum Parks, 1951) Canimia sp. Triplophyllites? sp. Chonetes cf. C. loganensis Hall and Whitfield Schuchertella cf. 8. chemungensis (Conrad) Orthothetes inflatus (White and Whitfield) Productus sp. (small form) Spirifer cf. 8. centronatus Winchell Spirifer cf. S. missouriensis Swallow Brachythyris sp. A (finely ribbed) Euomphalus cf. E. utahensis Hall and Whitfield Straparollus? cf. S. ophirensis Hall and Whitfield Platyceras sp. (possibly two small species) Crinoidal material The Tin Mountain faunas are of Early Mississippian age, indicating correlation with the Madison limestone. Likewise correlative is the Joana limestone of central Nevada (Nolan, Merriam and Williams, 1956, p. 54) which also contains faunas of Madison type. In central Nevada the conodont-bearing Pilot shale underlies the Joana. Conodonts from the lower part of the Pilot indicate a Late Devonian age (Hass, in Nolan, Merriam, and Williams, 1956, p. 53) ; the upper part of the Pilot is believed to be Early Mississippian in age. Although the Pilot shale is not recognized in the region under discussion, it is not unlikely that the lower part of the Tin Mountain with red calcareous shale in the Ubehebe district (McAllister, 1952, p. 2‘) may be of the same age as the lower part of the Pilot shale. Near Eureka, Nev., the lower conodont-bearing shale of the Pilot is reddish at some points. Further details relating to this part of the strati- graphic column are dealt with by the writer in a paper in preparation. PERDIDO FORMATION General featwres.-The name Perdido formation was given by McAllister (1952, p. 22-25) to a heterogenous and facies-variable sequence of strata with type section near Perdido Canyon, Ubehebe district. - Including silt- stone, sandstone, shale, conglomerate, chert, and lime- stone, the Perdido of the type area conformably overlies STRATIGRAPHIC GEOLOGY 19 the Lower Mississippian Tin Mountain limestone and is overlain with conformity by the upper part of the Chainman shale to which McAllister has given the local name "Rest Spring shale." Comparison of the Inyo Mountains and Ubehebe sec- tions indicates that the Perdido is possibly an eastern complex of lithologic facies which takes the place of lower and middle parts of the Chainman shale. To the east the Perdido of McAllister reaches a thickness of at least 600 feet ; to the west near Cerro Gordo it thins to | 50 feet and locally pinches out completely at points where black shale of the Chainman rests directly upon - Tin Mountain limestone. Because of its thinness in this | area the Perdido is included with the Chainman for , mapping purposes. Thickness and areal distribution.-The Perdido for- mation of the Inyo Mountains ranges in thickness from about 200 feet at Mazourka Canyon (fig. 3) to less than 50 feet at Cerro Gordo. It is spotty and variable in thickness at Mazourka Canyon. North of Cerro Gordo it is unrecognized at some points but reappears as the contact is followed along the strike. Along the Inyo crest (pl. 1) this unit has been traced intermittently from a point south of Belshaw Spring to the area south of the Morning Star mine. Strata assigned to this unit also occur east of the Belmont mine. Lithology.-With the type Perdido as noted by Mc- Allister (1952, p. 22-23) "heterogeneity is an outstand- ing characteristic." Siliceous clastic rocks are the most distinctive, ranging "from shale through siltstone and sandstone to conglomerate. Of these, siltstone is the most abundant. It is commonly light gray or pale red and weathers from yellowish to reddish browns." The limestone varies from dark gray with interbedded black and gray chert to medium gray and platy. It is largely fine grained and in part silty to sandy. Certain of the limestones become coarsely clastic, made up of crinoidal debris, shell fragments, and pebbles. Conglomerate in the type area is lenticular and coarse, consisting of re- worked rocks of the Perdido. The greatly thinned Perdido of the Inyo Mountains shows its characteristic diversity in certain sections; in others it is represented solely by a fine sandstone or quartzite. At Cerro Gordo the unit is a fairly uniform dense or flinty fine-grained nearly white or cream-col- ored tan-weathering quartzite. Appearance of this rock is virtually that of a novaculite. The unit ranges in thickness here from less than 50 feet to about 75 feet. It is rather massive, north of Cerro Gordo forming a conspicuous white cliffy exposure, which contrasts strongly in color with the underlying Tin Mountain limestone. Southeast of Cerro Gordo the resistant quartzite of the Perdido forms a caprock at several points on the range crest. The light-colored rock may easily be confused with Pennsylvanian silicated lime- stone or "tactite" of the Cerro Gordo area. Northeast of the Belmont mine fine-grained sands of the Perdido vary from light and medium gray to green- ish or pale olive gray, are slightly calcareous at some points, and show faintly a rather fine banding. Pinkish crinoidal limestone with rounded black chert pebbles and brachiopod fragments rests locally upon the sand- stone. Fucoidal markings characterize the calcareous sandstone or siltstone. East of Conglomerate Mesa (pl. 1) the Perdido at one exposure is partly calcareous and includes dark-olive-gray impure fine-grained calcar- eous sandstone of an aspect quite different from the light-colored novaculitelike sandstone of the Cerro Gor- do vicinity. An excellent exposure of highly diverse Perdido about 100 feet thick is found at the Crosscut tunnel, 1%, miles northwest of Cerro Gordo on the Pipeline trail (pl. 1). Resting with apparent conformity on platy Tin Moun- tain limestone, the Perdido here comprises calcareous silty shale with lumpy bedding and fucoidal markings, dark-gray siltstone and fine quartzite, silty limestone, and a few dark-gray crinoidal limestone beds. These deposits are partly hornfelsic, showing in places almost a slaty cleavage. Dark-gray Chainman shale con- formably overlies the Perdido. At Mazourka Canyon (fig. 3) 25 miles northwest of Cerro Gordo, the Perdido is lenticular, ranging in thick- ness from 40 to 200 feet within a few hundred feet along strike. It rests unconformably on limestone of Silurian age. Where the formation is thickest at Mazourka Can- yon, it includes three members. At the base a very local 90-foot member consists of medium- to rather coarse grained, heavy-bedded gray quartzite. The basal quartzite is overlain by 100 feet of extremely heteroge- neous coarse limestone breccia-conglomerate within which are lenses of coarse gray quartzite; medium- to light-bluish gray medium-grained sandy limestone oc- curs in lenses or as a general matrix surrounding the conglomerate pieces. Conglomerate cobbles are coarse and angular to subrounded and reach a diameter of 3% feet. Numerous limestone cobbles have the fine textured light-gray to white appearance of Devonian Lost Burro from which they are believed to have been derived. Other pieces are fine-grained cherty limestone and light-gray saccharoidal dolomite which may have been derived from rocks ranging in age from Ordovician to Devonian. An upper 20-foot member of the Perdido formation. consists of well-bedded silty medium- to dark-gray quartzite with interbeds of medium- to fine-grained light-gray limestone. This zone is seemingly tran- 20 sitional to the overlying plant-bearing black shale of the Chainman. Stratigraphy. - observations _ (1952, p. 22-23) in the type area of the Perdido with relation to the Perdido-Tin Mountain boundary indicate that "the two formations are somewhat gradational." Where ex- posures are poor, there is difficulty in mapping the con- tact. On the other hand the upper contact with "Rest Spring shale" in the type area is sharp. The "Rest Spring," unlike Perdido, normally has no reddish col- oration and contains no limestone in that area. ___ In the Inyo Range, where strata assignable to the | Perdido facies constitute much less of the Mississippian column, change from Tin Mountain limestone to sand- stone or quartzite of the Perdido is very abrupt and probably a disconformity. These relations may be seen to advantage north of Cerro Gordo. Local absence of the Perdido in this vicinity may be related to an hiatus. Evidence of pre-Perdido disconformity is convincing at Mazourka Canyon, where thickness differs greatly within short distances. This suggests that coarse Per- dido clastic rocks overlie a surface of considerable re- lief cut in limestone of Silurian age. The Lost Burro formation of Middle and Late Devonian age and the Tin Mountain limestone of Mississippian age are absent, but large marble and limestone cobbles in basal Perdido breccia-conglomerate appear to have been derived from the otherwise missing Devonian unit. To judge from the Cerro Gordo section, as much as 2,000 feet of combined Tin Mountain and Lost Burro may have been stripped away by pre-Perdido erosion. At Mazourka Canyon the relation of the uppermost 20-foot member of the Perdido to the black platy shale in the Chainman is apparently gradational. This change is in fact less abrupt than that which separates the 20-foot member from the underlying breccia-con- glomerate of the Perdido formation. Disconformity is suggested by westerly stratigraphic thinning and presence of conglomerate within the Per- dido of the Ubehebe district (McAllister, 1952, p. 24). Very great overall westerly thinning in the 12 miles which separate Cerro Gordo from the Ubehebe area is best explained as a function of lateral replacing facies relation between the easterly Perdido and the westerly Chainman shale, as outlined more fully below. Age and correlation.-Direct fossil evidence support- ing Late Mississippian age of uppermost type Perdido is the Cravenoceras fauna discussed by McAllister (1952, p. 24). Below the Cravemoceras fauna in the type Perdido column (loc. 37), the writer has collected a 7'7¢- plophyllites fauna in medium-gray sandy limestone; certain of the species are as follows : GEOLOGY OF THE CERRO GORDO MINING DISTRICT Triplophyllites sp. a Spirifer cf. 8. brazerianus Girty, or cf. 8. grimesi Hall Spirifer cf. 8. pellaensis Weller Spirifer missouriensis Swallow Echinochonchus sp. Dictyoclostus sp. Composita cf. C. sulcata Weller Kaskia cf. K. chesterensis Weller and Weller Deltopecten sp. The horizon of this assemblage is possibly that of Mc- Allister's (1952, p. 23) unit 10, which lies near the top of a 610-foot Perdido section. This limestone fossil as- semblage is similar to that collected from the lower limestone of the Chainman on the Pipeline trail (loc. 22) and resembles the fauna collected at locality 8 south of the Morning Star mine. McAllister's unit 10 is believed to be higher stratigraphically than either locality 22 or locality 8 of the Inyo Mountains section. CHAINMAN SHALE General features.-The name Chainman shale is used for strata of Mississippian age which lie between the Perdido formation (or the Tin Mountain where Perdido is absent) and the Keeler Canyon formation (fig. 7). Type section of the Chainman shale is at the Chain- man mine near Lane, Ely mining district, Nevada,. where black shale rests upon typical Joana limestone of Early Mississippian (Madison) age (Spencer, 1917, p. 24-26). Edwin Kirk (1918, p. 88), recognizing the similarity between Mississippian black shale of the Inyo Mountains and that of the Eureka district, Nevada, ap- propriately used the name "White Pine shale," at that time in general use. In recent years the name Chainman shale has been adopted at Eureka in preference to "White Pine Shale" and applies there (Nolan, Mer- riam, and Williams, 1956, p. 59) to a thick sequence of shale, siltstone, and conglomerate between the Joana FIGURE 7.-View looking northwest along the west Inyo Mountains slopes. Shows wide belt of dark-gray Chainman shale on right, over- lain on left by west-dipping Keeler Canyon formation. Triassic ma- rine beds on extreme left rest unconformably on Keeler Canyon beds where the Owens Valley formation was unrecognized. limestone below and the Diamond Peak formation above. Among the varied objections to use of "White Pine shale" as defined by Hague at Eureka (Hague, 1883, 1892) is lack of a clearly established type section and inclusion by Hague of Devonian as well as Mississippian strata. Joana limestone and Pilot shale (partly Devonian), now separate formations, were included in the "White Pine shale" of Hague. As dis- cussed below under age and correlation, the upper part of the sequence called Chainman at Cerro Gordo may well include a time-stratigraphic equivalent of some parts of the Diamond Peak formation but does not in- clude conglomerate and arenaceous limestone facies characteristic of the true Diamond Peak. Areal distribution.-Three principal belts of Chain- man shale have been mapped in the southern Inyo Mountains. On the east side of the range this unit crops out at a few points between Black Basin and the first canyon north of Bonham Canyon (pl. 1), but in this belt it is largely beneath fanglomerate. Along the range crest from a point southeast of the Morning Star mine to Cerro Gordo and northward past the Cerro Gor- do springs to Daisy Canyon, the Chainman is broadly exposed. A western belt extends along the east side of Salt Mill Hills to the Long John mine vicinity (pl. 2), and a fourth belt in the western foothills may be fol- lowed southward from Mazourka Canyon (fig. 3). From Daisy Canyon northward on the precipitous east side of the range occur highly irregular infolded areas of variously altered Chainman, which have been penetrated by granitoid rocks. Together with the un- derlying Tin Mountain the altered shale appears as nearly black bands contrasting with the associated light- gray and white marbles of the Lost Burro. Lithology.-The Chainman shale comprises dark-gray to black carbonaceous clay shale, silty shale, fine sand- stone, and limestone. Black noncalcareous clay shale predominates; this ranges from smooth fissile very fine locally almost papery shale to nonfissile dense platy and blockly-weathering argillite in which the shaly parting is less evident. Fine sands and silty sands occur as interbeds. Limestone is subordinate, occurring in lenses from a few inches to about 70 feet thick. The argillite in places has limonite coatings following the numerous joints and fracture surfaces. Sandstone beds commonly weather brown. The limestone ranges from dense fine-grained dark-gray carbonaceous types to those of a purer variety which are silty or sandy, me- dium grained, and of medium-gray to bluish-gray color. Coarse-grained crinoidal limestone is also present. *The name "Diamond Peak" was erroneously employed by Kirk (1918, p. 40) for various altered siliceous rocks, some of which are Permian. STRATIGRAPHIC GEOLOGY 21 Plant remains are locally abundant in the black shale. These are of two principal types:: (a) possible algae of vermiform fucoidal configuration and (b) branching stems and flat straight straplike or rushlike impressions of fragmentary nature, probably derived from a coal swamp environment. At some localities marine shells are associated with land-plant remains. The limestone lenses contain marine fossils. Certain of the cleaner, less carbonaceous limestone lenses carry faunas which differ in facies from those of the black shale. In the southern Inyo Mountains the Chainman shale has been affected widely by low-grade metamorphism. On the whole a structurally weak rock, this formation was subjected to igneous intrusion at many points, and its alteration is related in the main to contact or hydro- thermal action. The extent of alteration becomes greater toward the north, where granitoid rocks are much more in evidence at the surface than at Cerro Gordo. Degree of alteration varies from weak silicifi- cation or silication through mild slaty or phyllitic change to complete recrystallization, producing in the extreme a fine-grained black rock of almost lampro- phyric appearance. Northwest of Cerro Gordo, bor- dering an intrusive stock, the dense argillite is mildly silicified, assuming a flaggy and blocky-weathering ap- pearance. Pyrite oxidized to limonite is common. More strongly altered phases exhibit secondary silicate crystal growths ranging from minute specks to large knots. In the Mazourka Canyon area chiastolite is re- ported (Kirk, 1918, p. 38). On the east side of the range, where the road down San Lucas Canyon meets the Lee Flat-Bonham tale mine road (fig. 3), the silty Chainman is partly recrystallized with development of silicate needles. In spite of the metamorphism, im- pressions of productid brachiopods may still be seen. East of Brooklyn Canyon (pl. 2), strongly altered Chainman ranges from phyllitic to almost schistose lo- cally, showing needlelike crystal growths in a silvery gray sericitic groundmass. Slaty development with cleavage angle differing perceptibly from that of bed- ding seems rather uncommon, and in general foliation is not well developed in these altered phases. Thickness.-Exposures of Chainman shale cannot or- dinarily be expected to give true stratigraphic thick- ness because of the faulting, shearing, and drag fold- ing undergone by these incompetent beds. Along the Dolomite Canyon thrust-fault zone in the western belt, apparent thickness ranges from less than 50 feet in fault slivers to more than 1,000 feet in continuous near- vertical sections at Brooklyn Canyon (pl. 2). Near the Crosscut tunnel, 13% miles northwest of Cerro Gordo (pl. 1), about 1,100 feet of Chainman was measured, but allowance should be made for possible thickening by 22 GEOLOGY OF THE CERRO GORDO MINING DISTRICT drag folds. East of the Inyo Mountains the "Rest Spring shale" of McAllister (1952, p. 26), believed to represent only the upper part of the Chainman, attains a thickness of about 400 feet. The Chainman shale in the Diamond Mountains near Eureka, Nev., reaches a thickness of 3,000 feet, which exceeds the maximum at Cerro Gordo by some 2,000 feet. That the black shale facies of the Chainman thicken appreciably from east to west is evident between the Ubehebe area and Cerro Gordo. Depositional changes involved are considered below under stratigraphy. Stratigraphy.-The Chainman shale rests conform- ably on the Perdido formation and is overlain conform- ably by the Keeler Canyon formation. In discussing these units it is desirable to consider the Chainman and Perdido jointly, for together they occupy an inter- val whose top and bottom are fixed paleontologically, by the early Mississippian (Madison) Tin Mountain below and the basal Keeler Canyon of Atoka age above. At Cerro Gordo the Chainman and Perdido together are 1,000 feet thick, but only about 100 feet or less of this is Perdido. In the Ubehebe area on the other hand the Perdido thickens to some 600 feet, whereas the Chainman ("Rest Spring") above it thins corre- spondingly to 400 feet or less. Combined thickness of Perdido and Chainman is nonetheless roughly the same in the two areas, or about 1,000 feet. The litho- logic differences involved are explainable by facies interpretation, black Chainman shale occupying the time-stratigraphic interval of most of the Perdido to the west. Subjectively, the Chainman-Perdido boundary may be said to rise in the column eastward with inter- tonguing pattern, thus transecting imaginary time- stratigraphic planes. Facies change in a southeasterly direction from black shale of the Chainman at Cerro Gordo to a wholly car- bonate Mississippian section is even more notable than northeasterly change into the complex facies of the Per- dido. To the southeast in the direction of Darwin (fig. 3), the shale disappears (Hall and MacKevett, 1958, p. and the thick Argus Range Carboniferous col- umn (Hopper, 1947, p. 409) appears to be entirely lime- stone, lacking the Chainman shale facies. Relation of Chainman to the overlying Keeler Can- yon formation of Pennsylvanian and Permian age is seemingly gradational. An intermediate zone with al- ternating limestone and black shale beds is well shown in upper Soda Canyon and on the east side of the Inyo Range in the first canyon north of Bonham Canyon (pl. 1). Detailed stratigraphic zonation of the Chainman shale at Cerro Gordo on the basis of lithology and fossils is not yet feasible. Limestone bodies are thickest and § most numerous in the lower 250 feet of the formation, being well shown along the Pipeline trail north of Cerro Gordo, where the 3-foot fossiliferous limestone bed is roughly 200 feet above the base of the Chainman. In Mazourka Canyon (fig. 3) the lenticular 70-foot lime- stone lies with its base 150 feet above the bottom of the Chainman. South of the Morning Star mine, coarse- grained bluish-gray crinoidal limestone rests upon, and is partly lenticular within, sandstones representing a Perdido facies. In the same vicinity partly crinoidal limestone lenses occur in what appears to be lower black shale of the Chainman, but at a higher strati- graphic horizon than that discussed above. Near the Crosscut tunnel on the Cerro Gordo Pipeline trail, smooth, fissile to almost papery black clay shale occurs in the lower 400 feet of the Chainman, which in- terval also includes coarser silty interbeds as well as limestone lenses. Marine fossils of the black shale fa- cies are found in these lower beds, where they are associated with land-plant remains. West of the Pipe- line trail the upper 600 feet of the formation is poorly exposed. It apparently consists in the main of monoto- nous dark-gray and black silty shales, passing at the top into a 20-foot zone of platy calcareous shale, overlying which is the basal limestone of the Keeler Canyon with fusulinids. Age and correlation.-The Chainman shale is older than early Middle Pennsylvanian beds of the Keeler Canyon which overlie it and younger than the Tin Mountain limestone with Early Mississippian (Madi- son) faunas. Its age is considered to be Late Mississippian. Several fossil collections have been made in the lower part of the Chainman near Cerro Gordo, but thus far none have come from the 600 feet of strata in the middle and upper parts of the formation in this area. Material collected by Kirk (1918, p. 38-39) on the Pipeline trail north of Cerro Gordo was identified by Girty as follows: Solenomya? sp. Caneyella wapanuckensis Caneyella n. sp., aff. C. richardsoni Orthoceras, several species Gastrioceras aff. G. richardsonianum Goniatites sp. Eumorphoceras bisulcatum ? Plant remains Fish remains Of this association Girty makes the following com- ments: "The latter is an interesting and peculiar fauna of the Caney shale of Oklahoma and the related but less well-known fauna of the White Pine shale of Nevada. These faunas I refer to the Upper Mississippian." STRATIGRAPHIC GEOLOGY 23 During the present investigation, collections were made at locality 22 on the Pipeline trail 114 miles north- west of Cerro Gordo, where black shale and interca- lated limestone are abundantly fossiliferous. Black shale from 200 to 250 feet above the base of the Chain- man yielded the following : Orthoceras sp. (small form) Nautilus sp. (large form. possibly a Liroceras) Cravenoceras cf. C. nevadense Miller and Furnish Cravenoceras cf. C. richardsonianum (Girty) Caneyella wapanuckensis Girty Caneyella n. sp., cf. C. richardson Girty Shark teeth (coarsely serrated edge, % in. long) Land plants (abundant, fragmentary; one well-preserved fernlike type) This assemblage is virtually the same as that studied by Girty. Similar fossil associations were collected at locality 38 on the east side of the Inyo Range. A 3-foot silty limestone bed only a few feet above the black shale at locality 22 on the Pipeline trail contains the following fauna, and is pro- visionally correlated with the Brazer limestone : Triplophyllites sp. A Dictyoclostus cf. D. burlingtonensis (Hall) or cf. D. ferngle- nensis (Weller) & Productus n. sp., cf. P. semistriatus Meek ake Unlike Meek's type, this form does not have obsolete ribbing on the anterior half of the pedicle valve. Spirifer cf. 8. pellaensis Weller Spirifer cf. 8. brazerianus Girty or cf. 8. grimesi Hall Small goniatite with narrow venter; genus indeterminate A similar fauna of Brazer aspect is found at locality 8, which lies 3,000 feet southeast of the Morning Star mine, in coarse crinoidal limestone at the top of the sand in the Perdido and near the bottom of the black- shale facies of the Chainman.. The following fossils have been determined : Triplophyllites sp., ef. T. sp. A (abundant) Chaetetes sp. Spirifer cf. 8. brazerianus Girty Spirifer cf. haydenianus Girty Composita cf. C. lewisensis Weller Productus ef. P. richardsi Girty Productus cf. P. ovatus Hall Diaphragmus cf. D. elegans (Norwood and Pratten) Dictyoclostus sp. (large, coarsely ribbed) Rhynchopora? sp. Proetus cf. P. missouriensis Shumard fenestellid bryozoans (common) Pleurotomaria cf. P. brazeriana Girty In connection with lower limestone faunas in the Chainman of the Inyo Mountains, attention is called to a similar assemblage with Triplophyllites which occurs below the Cravenoceras beds in the Perdido formation of the Ubehebe area (loc. 37). This fauna, elsewhere discussed under the Perdido, probably occupies a higher stratigraphic position than the comparable limestone fauna with Triplophyllites sp. A on the Pipeline trail (loc. 22). Correlation of the Chainman shale is based to a large extent on cephalopods, which occur abundantly in thin highly carbonaceous limestone interbeds of the black shale facies. These cephalopods are the subject of de- tailed paleontological studies, especially those from the White Pine or Hamilton region of central Nevada (Youngquist, 1949, p. 276, 283). Diverse generically, the cephalopod assemblages are characterized by C'7c- venoceras and Eumorphoceras and are regarded as of Late Mississippian age. Little is known of their vertical ranges or stratigraphic zonation. Some of the central Nevada occurrences are believed to be in the upper part of the Chainman. The thick Diamond Range strati- graphic column of Chainman shale near Eureka, Nev., otherwise well suited for stratigraphic zonation, has not as yet yielded these cephalopods. In the Ubehebe district McAllister (1952, p. 23-24) finds a prolific cephalopod fauna with Cravemoceras about 600 feet above the base of the type Perdido, an horizon probably falling, in the time-stratigraphic sense well within the upper half of the Chainman column at Cerro Gordo. Thus while the only Cravemoceras oc- currence recognized at Cerro Gordo is in the lower part of the Chainman, that at Ubehebe seems to be in a stra- tigraphic position equivalent to the upper part of the Chainman, conforming perhaps to an upper position in the Chainman of the central Nevada Cravenoceras fau- nas discussed above. Studies of zoned collections with good stratigraphic control are needed to determine evo- lutionary trends and zonation of these interesting forms within the Chainman sequence. Failure to obtain fossils in the uppermost part of the Chainman at Cerro Gordo leaves open the possibility that it may conceivably be Early Pennsylvanian rather than Late Mississippian ; for the relation of the Chain- man to the overlying Keeler Canyon seems to be transi- tional and the lowermost Keeler Canyon fusulinids indicate an early Middle Pennsylvangfi‘ (Atoka) age. The combined Chainman and Perdido of the Inyo Mountains region occupy a time-stratigraphic interval seemingly coinciding with that filled by combined Chainman shale and overlying Diamond Peak forma- tion in the Diamond Mountains near Eureka, Nev. This conclusion follows if Tin Mountain and Joana Lime- stone are correlative and the Fusulinella zone (or Atoka zone) at the base of the Keeler Canyon correlates with the same zone at the base of the Ely limestone in the Diamond Mountains. The thickness discrepancy is great, however, for that of combined Chainman and Diamond Peak is of the order of 6,200 feet compared 24 with 1,000 feet for the Chainman and Perdido strati- graphic column of the southern Inyo Mountains. Ex- cept for shale the upper part of the Chainman at Cerro Gordo lacks characteristic lithologic types of the true Diamond Peak. PENNSYLVANIAN AND PERMIAN ROCKS In combination the Pennsylvanian and Permian rocks are about 4,000 feet thick; this thickness constitutes about one-third of the exposed 12,000-foot Paleozoic see- tion in the Cerro Gordo area. These strata are predomi- nantly impure carbonates with subordinate shale, cal- careous shale, siltstone, sandstone, conglomerate, and chert. Most of these are mildly metamorphosed; car- bonate rocks are in part recrystallized to marble and argillaceous rocks to argillite and hornfels. Commonly within a mile of granitic intrusions the limestone is al- tered to calc-hornfels and tactite. GEOLOGY OF THE CERRO GORDO MINING DISTRICT Pennsylvanian and Permian strata of the Cerro Gor- do mine area are divided in two formations : the Keeler Canyon formation of Pennsylvanian to early Permian ago inclusive, and the Owens Valley formation of Per- mian age (Merriam and Hall, 1957). Previously named units, the Reward conglomerate and Owenyo limestone (Kirk, 1918, p. 42-45) , are localized lenticular members of the Owens Valley formation ; being local facies these have not been utilized as map units. In previous discussions of undifferentiated Pennsyl- vanian and Permian rocks of this region, they have been referred to provisionally as Bird Spring (?) formation (McAllister, 1956). In the Ubehebe Peak quadrangle these strata increase in thickness to more than 5,000 feet (McAllister, 1955). The Bird Spring formation, defined in southern Nevada (Hewett, 1931, p. 21 ; 1956, p. 42; Longwell and Dunbar, 1936, p. 1202), is believed to embrace strata ranging in age from Late Mississippian to Permian. TaBu® 3.-Pennsylvanian and Permian sequence in the southern Inyo Mountains, Calif. Age Formation T11(ifckgess Lithology Characteristic fossils ce Sandy and silty limestonedwith chelrt p f tihe» (Mack), & ebbles, calcareous sandstone, sili- unctospirifer pulcher eek), Sp:- Upper part 180-500 geous conglomerate, limestone - cob- rifer pseudocameratus (Girty). e ble conglomerate, and quartzite. o Disconformity E Calcareous shaéesl with argillaceolus silty and sandy limestone intercala- £ Cn ya 8 Middle part 400-700 tions; silty clay shale; fine sandstone; 13“;sz Zsufigzhtgggzag erina; Herit o hornfels. Fusulinids abundant in " t Permian g limestone. Silty fusulinid limestone, lenticular fairly pure organic limestone, lime- | Pseudoschwagerina, Schwagerina, Lower part 1, 000 + stone mud-breccias, platy argilla- subordinate Parafusulina, Triti- ceous limestone, blocky limestone cites; Heritschia, Omphalotrochus. Angular unconform- conglomerate, and hornfels. ity on west side --Inyo Mountains Upper part Schwagerina, Triticites. Arenaceous silty and pebbly limestone § with shale intercalations; calcareous a, 2, 200+ sandstone and siltstone. Shale inter- 8 Middle part calations commonly pink or maroon. | Triticites. O Marble. ® Pennsylvanian e © y Fine-textured fusulinid-crinoidal lime- f stone with round black chert nodules | Fusulinella, Millerella. Lower part in "golf-ball beds." Tactite and marble. KEELER CANYON FORMATION Name and occurrence.-The Keeler Canyon forma- tion of Pennsylvanian to Early Permian age was named by Merriam and Hall (1957, p. 4-7) for exposures in upper Keeler Canyon, where the type section lies east of the Estelle tunnel portal and 2 miles south west of Cerro Gordo Peak (pl. 1). The formation is widely exposed in the southern Inyo Mountains and also underlies most of the Darwin Hills and the northern Argus Range. It generally forms smooth slopes, but re- sistant beds protrude locally to emphasize the incom- petent folded nature of these strata. In earlier stratigraphic studies by Kirk (1918, p. 40-41), these beds were in part referred to as "basal Pennsylvanian limestones" and in part as "later Penn- sylvanian limestone and shale"; siliceous and silicated STRATIGRAPHIC GEOLOGY 25 limestone southwest of the Cerro Gordo mine previously regarded by Kirk as "Diamond Peak quartzite" are mapped by the writer as Keeler Canyon formation. Lithology.-The Keeler Canyon formation comprises thin-bedded medium- to dark-gray impure silty and arenaceous to pebbly limestone and limy siltstone, with intercalations of pink or maroon fissile shale. Pebbly limestone in a few places grades into chert pebble con- glomerate with limestone matrix. Silicified fusulinids are in some places an important constituent of the peb- bly limestone. Clastic texture is shown by all these rocks, including the limestone. Bedding is inclined to be platy or flaggy, with few layers as much as 3 feet thick. The basal 150 to 200 feet differs lithologically from the overlying beds. «These lower strata are purer thin- bedded dark-gray crinoidal and fusulinid-bearing lime- stone containing near-spheroidal black chert nodules % to 2 inches in diameter. The cherty Fusulinello-bearing strata which were given the field designation "golf ball beds" constitute a reliable stratigraphic marker, hav- ing been recognized widely in the southern Inyo Moun- tains, the Darwin Hills, and the Argus Range. Thickness and stratigraphic relations.-Because of its incompetent, highly folded nature the Keeler Canyon formation does not lend itself to accurate thickness appraisal. In the Cerro Gordo area the formation is on the average about 2,200 feet thick, as judged by meas- urement of several sections which ranged from 1,300 to 2,500 feet. Where the section is greatly thinned, there is evidence that segments have been cut out by faulting. The Keeler Canyon formation rests conformably upon the Upper Mississippian Chainman shale. In the Dar- win quadrangle (Hall and MacKevett, 1958, p. 9) it is underlain by thinly bedded Lee Flat limestone which apparently occupies at least part of the normal interval of the Chainman shale.