5FIRST DAY: MIDDLE TRIASSIC STRATIGRAPHY AND AMMONITE

BIOSTRATIGRAPHY IN WESTERN NEVADA: FOSSIL HILL TO FAVRETCANYON

SPENCER G. LUCAS, NORMAN J. SILBERLING, JAMES F. JENKS, HUGO BUCHER,JUSTIN A. SPIELMANN AND LARRY F. RINEHART

Assembly Point: Exit 119 on Interstate 80, east of Reno.Departure Time: 8 AMDistance: 141.7Stops: 3 (one optional)

SUMMARY

Today’s trip traverses an essentially west-to-east transect from Fossil Hill in the southern Humboldt Rangethrough the southern end of the Tobin Range to Favret Canyon in the Augusta Mountains (Fig. 1.3). All stops are inMiddle Triassic strata that yield ammonite and halobiid bivalve assemblages critical to Anisian-Ladinian biostratigra-phy. These are basinal strata of the Fossil Hill Member of the Prida Formation (to the west) and Favret Formation (tothe east) that record a paleogeographically complicated pattern of syndepositional subsidence (Nichols and Silberling,1977) attributable to back-arc extension. The strata are silty shales and dark lime mudstones deposited below wavebase in an euxinic paleoenvironment. Fossil content is mostly of planktonic and nektonic organisms, especially ichthyo-saurs, radiolarians, conodonts, halobiid bivalves and cephalopods. The stops focus on the megafossil biostratigraphy,particularly of the cephalopods.

The Prida and Favret formations are part of the Star Peak Group, which is composed mainly of carbonaterocks ranging from late Spathian through the Carnian in age (Fig. 1.4). Beneath the Star Peak Group are mid-Spathianand older Lower Triassic, and possibly uppermost Permian, volcanic rocks of the Koipato Group. Strata above theStar Peak group are mainly fine-grained, terrigenous-clastic rocks —sometimes referred to as the Auld Lang SyneGroup, but more commonly as the “mud pile”—of a large Norian to Lower Jurassic deltaic complex and its thickoffshore equivalents. All of the rocks of the Star Peak Group to be visited on Days 1 and 2, together with the underlyingand overlying lower Mesozoic rocks, characterize the lower Mesozoic cover of the so-called Golconda terrane.Although not among the scheduled field-trip stops, the basinal, offshore strata of the Auld Lang Syne Group occur justwest of the Golconda terrane, which is characterized by upper Paleozoic rocks of the Golconda allochthon. (Fig. 1.1).Because these strata everywhere are evidently structurally juxtaposed with the lower Mesozoic rocks of the Golcondaterrane, they are regarded as a distinct allochthonous tectonic block, the Jungo terrane (Silberling, 1991), which willenter into the discussion on Day 2. All of the lower Mesozoic rocks in northwest Nevada, except the easternmostexposures of those belonging to the cover on the Golconda terrane, lie seaward of the initial-strontium 0.706 isopleth,which is generally interpreted to be the oceanward limit of continental crust.

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Traveling east on U.S. Interstate 80, leave high-way on exit 119 (to Oreana). 0.1Stop sign; turn right. 0.4Road forks; continue straight (towards Roch-ester). 0.2Road forks; go straight. Humboldt Rangeahead. The Humboldt Range is named after Ger-man geologist (among other things) BaronFriedrich Heinrich Alexander von Humboldt

(1769-1859). John Charles Frémont conferredthe name during his explorations of 1845-1846.The higher parts of the southern Humboldt Rangeexpose Upper Permian(?) and Lower Triassicvolcanic rocks (principally andesite, rhyolite tuffsand flows) of the Koipato Group (Wallace etal., 1969). The overlying Triassic carbonaterocks of the Star Peak Group crop out primarilyalong the western and eastern margins of therange. The southern part of the Humboldt Rangethus describes a broad, south-plunging, antiform.

Lucas, S.G. and Spielmann, J.A., eds., 2007, Triassic of the American West. New Mexico Museum of Natural History and Science Bulletin 40.

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3.4We are now driving up Limerick Canyon on thewest limb of this antiform. 1.8Enter Spring Valley, named after its naturalsprings. 2.2Road curves left; Rochester Gold Mine at 2:00.A group of prospectors, in the 1860s, namedthe canyon in which they struck “pay dirt” Roch-ester, after their native city in New York state.The first major production from the Rochesterdistrict was during World War I and the yearsshortly thereafter. Development of the district byopen-pit mining during recent decades is prima-rily for gold, although production of silver as aby-product makes the district one of the majorproducers of silver in North America. 1.3Road forks; take unpaved road to left thatgoes to Unionville. 0.2Crest of hill; road curves to left. View to east isof Buena Vista Valley and the East Range be-yond that. Roadcuts here are in volcanic rocksof the Koipato Group. The Buena Vista Valley,between the Humboldt Range and the EastRange, was settled by men who fought in theMexican-American War at Buena Vista (Mexico)in 1847, who supposedly thus named the valleyto commemorate their victory. However, the val-ley is quite striking, and we suspect its name,which means “nice view” in Spanish, commemo-rates the scenery. 4.1To the right is the ghost town of Fitting, whichhas an interesting history of placer gold mining.First located by American prospectors, duringthe last part of the 19th Century, it came to beoperated by Chinese miners, thousands of whom

in the 1860’s had been imported into this coun-try as laborers on the Central Pacific Railroad,the western segment of the first transcontinentalrailroad. Early (in American terms) mining dis-tricts in the west were self regulated. Hence, thisplacer gold district was divided into individualclaims measuring only 20 x 20 ft in area. A shaftwas hand dug through the semi-consolidatedgravels to depths of 10 or more meters on eachclaim, and the higher grade gravel directly abovethe bed rock was then excavated and processed.Waste rock was dumped into an adjoining aban-doned shaft. All in all, a very dangerous busi-ness. Cave-ins were not uncommon, and the re-mains of a number of Chinese miners are nodoubt still buried here. Unionville, a silver mining camp founded in theearly 1860’s, and now known as a “ghost townthat refused to die,” is located in a beautiful, nar-row valley in the Humboldt Range about 15 milesto the north of here. 2.0Road forks; go straight. 0.7Road curves to right to proceed south. Note al-kali playa to left in valley floor and StillwaterMountains ahead on the skyline. The name ofthe mountains is from the Stillwater Slough, alarge and deep slough about 40 miles south ofhere named for its sluggish (“still”) water. 2.8Road forks; go right. Granite Mountain is highpeak at 9:30. 2.0Turn left on two-track road. 1.0Road forks; go right. 0.5Stop where road begins steep ascent.STOP 1. Fossil Hill to right. Fossil Hill is one ofthe first known and most extensively studied Tri-assic ammonite localities in Nevada (Figs. 1.5-1.6). The first descriptions of Triassic fossils fromthis area were published by Gabb (1864) andMeek (1877). Middle Triassic ammonites fromhere and other localities in the Humboldt Rangefigured in Hyatt and Smith’s (1905) classic mono-graph on the Triassic cephalopod genera ofNorth America. The name “Fossil Hill” subse-quently appeared in J.C. Merriam’s (1908)monograph on Triassic ichthyosaurs, many ofwhich came from nearby “Saurian Hill.” Ammo-nites from Fossil Hill became the basis of James

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FIGURE 1.3. Route of day 1 roadlog against relevant terranes ofwestern Nevada (after Silberling, 1991).

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Perrin Smith’s (1914) monographic treatment ofthe Middle Triassic ammonites (and other ma-rine invertebrates) from North America; thismonograph listed 110 species of ammonites from

Fossil Hill. Subsequent commercial fossil col-lecting at Fossil Hill brought ammonites to theBritish Museum that figured in the now classicmonographs of Spath (1934, 1951).

FIGURE 1.4. Distribution of Star Peak outcrops (below) and diagrammatic cross section (above) of the Star Peak Group from thenorthern Humboldt Range (NW) to the Augusta Mountaisn (SE) (from Nichols and Silberling, 1977). Shaded border shows the area ofthe “Sonoma Range” one-degree quadrangle.

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The 1905 expedition to Saurian and Fossil hills,on which both Smith and Merriam were partici-pants, was organized and financed by Miss AnnieAlexander, heiress to the C & H Sugar fortuneand an avid amateur naturalist. Her field notesfrom this expedition were published by Zullo(1969), and a full account of this remarkable ladyis given by Stein (2001). Smith (1914) was aware that the section atFossil Hill crossed the Anisian-Ladinian bound-ary, but most of his collections from Fossil Hillwere attributed to his all-inclusive “Daonelladubia zone.” Reevaluation of the fauna basedon closely spaced, bed-rock collections(Silberling, 1962; Silberling and Nichols 1982)resulted in the recognition of 12 successive, fau-nally distinct”beds” distributed through three up-per Anisian zones: the Rotelliformis, Meeki, andOccidentalis zones.

Bucher (1989, 1992; Monnet andBucher, 2005a, b) established the current bios-tratigraphic framework (Figs. 1.6-1.7). The old-

FIGURE 1.5. Photograph of Fossil Hill showing approximate position of Anisian-Ladinian boundary.

est assemblage here is the Gymonotocerasmimetus Zone of Monnet and Bucher (2005a).The base of the late Anisian, which is defined bythe G. weitschati Zone in the Augusta Moun-tains, has not been documented in the southernHumboldt Range. Documentation of new andmore detailed biostratigraphy across the Anisian-Ladinian boundary at Fossil Hill is now in prepa-ration. The section has been sampled formagnetostratigraphy but is remagnetized. Thereis no published 13C record. Currently, the Anisian of the Prida and Favretformations in Nevada can be resolved into 31biochronologic units based on ammonoids, themost highly resolved Triassic succession in theentire world (Fig. 1.7). U-Pb age calibration ofthe Anisian (Ovtcharova et al., 2006) indicates,by correlation with south China, a 246.9 ± 0.4Ma age for the early middle Anisian Hyatti Zoneand a 244.6 ± 0.5 Ma age for the late middleAnisian Shoshonensis Zone. These ages fit wellwithin interpolations for the isotopic ages of the

9during the Unionvillites hadleyi Subzone (lateHyatti Zone). Large fluctuations (up to 50%) inammonoid diversity took place during the latemiddle and late Anisian, without a first-ordertrend and with comparatively lower turnover. TheHadleyi peak resulted from limited exchangesbetween the low and high paleolatitude faunas(typical high-latitude genera such asAmphipopoanoceras, Arctohungarites andTetsaoceras occur in the Hadleyi subzone, butare represented by a few individuals only).After the stop, retrace route back to thenorth-south road at the valley edge. 1.4Road intersection; go straight. 0.1Turn right on main road. 2.0Road intersection; turn right to proceed south-ward. 2.2Road forks; continue straight. Note that Middleand Upper Norian as well as Lower Jurassicammonoids occur in the low hills of the Pershingdistrict at the extreme southern end of theHumboldt Range. 13.3Intersection with paved road; turn left. Ahead,to the south, before the turn are the Buena VistaHills, from which a large tonnage of magmaticiron ore was shipped during the 1940’s and 50’sfrom a gabbroic complex of Jurassic age. Roadnow heads across the Antelope Valley toMcKinney Pass. The Antelope Valley, of course,takes its name from the pronghorn antelope,denizens of the valley floors in this part of Ne-vada. 2.1Pavement ends. 19.0Crest of McKinney Pass, which divides the EastRange to the north from the Stillwater Range tothe south. The Augusta Mountains (Cain Moun-tain is the high peak) are ahead and expose a fullsection of the calcareous Star Peak Group.Favret Canyon is the v-shaped valley north ofCain Mountain. Pleasant Valley is in the fore-ground. Slope wash of limestone here atMcKinney Pass is of late Early Norian age, andfrom a relatively thin limestone unit within thethick section of dominantly fine-grained clasticrocks of the Auld Lang Syne Group, which over-lies the Star Peak Group. The name East Range is shortened from “East

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upper and lower limits of the Anisian of, respec-tively, ~240 Ma and ~247 Ma (Mundil et al.,1996; Lehrmann et al., 2006; Galfetti et al., inpress). Thus, the gross average duration of eachbiochronologic unit is ~0.23 my, without tak-ing the intervals of separation (gaps) intoaccount. Ammonoids show a protracted recovery dur-ing the early Anisian, having a peak in diversity

FIGURE 1.6. Distribution of ammonoid taxa in the lower part of theFossil Hill section (from Monnet and Bucher, 2005a).

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FIGURE 1.7. Correlation of Anisian ammonoid zonations of Nevada, British Columbia and the Sverdrup basin (from Monnet andBucher, 2005b).

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Humboldt Range,” used on some old maps butnow reserved for a mountain range in easternNevada. The Augusta Mountains were namedby one of the geologists of the Fortieth ParallelSurvey, Arnold Hague. Cain Mountain com-memorates Hiram Cain, an early settler in the1800s. Favret Canyon was named for Joe Favret,the Basque caretaker during the late 1930’s atthe “Jenkins” ranch about 12 miles northwest ofCain Mountain. McKinney Pass is in the southwestern part ofthe old “Sonoma Range” quadrangle, coveringone degree of latitude and longitude. The geo-logic framework of this huge area was estab-lished by Henry G. “Fergie” Ferguson (1882-1966), S.W. Muller (see above), and Ralph J.Roberts (1911-present!) immediately before andafter World War II. Ferguson and Roberts weregeologists with the U.S. Geological Survey(USGS) and were mainly responsible for thePaleozoic rocks, Cenozoic volcanics, structure,and ore deposits of the quadrangle. Muller, whoseresearch was supported by the USGS, concen-trated on the Triassic rocks and the structuresinvolving them. Their findings, published in theearly 1950’s as four 1:125,000 scale geologicmaps of the Sonoma Range quadrangle, are ofenduring utility despite having been accomplishedbefore the interpretation of sedimentary environ-ments or the analysis of structural strain werewell understood, and before isotope dating tech-niques and plate tectonics theory were estab-lished. Furthermore, Triassic ammonite zonationthen still mainly followed that of Mojsisovics, andTriassic conodonts had not yet been discovered(for better or worse). All the more remarkable isthat starting in the late 1920’s, Ferguson andMuller, at first working independently and thenas a team, also established the basic geology ofthe old “Hawthorne” and “Tonopah” one-degreequadrangles (Muller and Ferguson, 1939;Ferguson and Muller, 1949) farther south innorthwestern Nevada, where we will be visitingon Field Trip Day 3. 4.2Road forks; go right. Road to left leads toWinnemucca. The Tobin Range is ahead, andthe Pleasant Valley is in the foreground. A con-

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spicuous normal-fault scarp having as much as 5m of displacement along the western edge of theTobin Range resulted from the Pleasant Valleyearthquake of October 2, 1915. 2.7Road enters from left; curve hard right into Pleas-ant Valley. 4.2Road forks; take the left fork (go straight).2.0Crest of small burm; note Cain Mountain andFavret Canyon ahead. 0.6Road intersection; go left under power lines.2.1Crest of hill. STOP 2 (optional). Here, we canexamine Favret Formation (Fossil Hill Member)ammonoid-bearing strata in low hills to left. Af-ter stop, drive to south to McCoy Springs. 1.8Just before homestead and spring turn left atthe fork in the road. Dixie Valley to south.Numerous hot springs are present in Dixie Val-ley and the southern end of Jersey Valley. The60 megawatt Dixie Valley Geothermal powerplant, located about 20 miles to the southwestalong the Stillwater Range, is the state’s largestgeothermal plant. 0.5Hard left; note Cain Mountain (Fig. 1.8). DixieValley Formation red beds are below FavretFormation. 2.0Road intersects cross roads; turn right. 0.4Turn left on two-track road. 1.8End of road.STOP 3. Favret Canyon (Figs. 1.9-1.10). Dur-ing this stop we will take a rather steep hike upthe north side of the canyon to the more gentlysloped plateau at the top and once there, we’llbegin our traverse on top of the massive Lowermember of the Favret Formation and graduallywalk up through much of the Middle Anisianportion of the Fossil Hill member (Hyatti Zoneand type area of the Taylori Zone). TheShoshonensis Zone is also represented on thisside of the canyon, but not nearly as well as onthe south side. Looking across to the steep southside of the canyon, you will have an excellentview of the entire Fossil Hill member whichranges up to the Occidentalis Zone, as well asthe overlying Augusta Mountain Fm. Also obvi-ous will be a major fault that runs along the range

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FIGURE 1.8. Cain Mountain showing stratigraphic unit and location names used by Monnet and Bucher (2005a).

front. In the Favret Canyon area, the lower part ofthe Hyatti Zone is condensed and although fos-sils of the Intornites mctaggarti subzone arelocally abundant, they are strongly re-crystallized(Bucher, 1992). Interestingly, ammonoids areusually outnumbered by coiled nautiloids in thesebeds. The intermediate Constrictus andAmericanus subzones, with type areas in the PridaFormation of the northern Humboldt Range, havenot as yet been reported from the Favret For-mation (Bucher, 1992). Ammonoid faunas be-longing to the uppermost Unionvillites hadleyisubzone consistently occur in calcareous, nodu-lar sandstone a few meters above the base ofthe Fossil Hill member. These beds are laterallytransported sandy sediments with obvious sort-ing of ammonoids (Bucher, 1992). Four intervals of closely spaced limestone beds

provide excellent lithostratigraphic markerswithin the middle Anisian part of the Fossil Hillmember throughout the Augusta Range and thesouthern Tobin Range as well. The base of thelowest marker coincides with the base of theTaylori Zone (Nicholsi subzone) and its top withthe Spivaki subzone of the same zone. Am-monoid faunas of the Escheri subzone are foundin the next “main cliff”. Ascending, the first “up-per small cliff” coincides with the base of theShoshonensis Zone, whose type area is in theMcCoy Mine area, one of tomorrow’s stops.Its middle part yields the Rieberi subzone faunaand the Ransomei subzone fauna is found at itstop. The top of the second “upper small cliff”yields the Wallacei subzone fauna. Ammonoidsof the overlying Fergusoni subzone, whose typearea is also in the McCoy Mine area, have onlyrecently been reported from the Augusta Range,

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FIGURE 1.9. Outcrop of Anisian strata in Favret Canyon looking southward.

in the Oliver Gulch area, about 1.5 km south ofFavret Canyon. Faunas of the topmost Mojsvarisubzone, whose type area is also south of FavretCanyon, occurs a couple of meters below twoclosely spaced thin bedded sand sheets, whichin turn, are ~ 10 m above the top of the “uppersmall cliff”. This isolated pair of turbidites locallycontains accumulations of fragmentary reptilebones (e.g. thalattosaurids). The oldest lateAnisian fauna of the Gymnotoceras weitschatiZone, together with the oldest representative ofDaonella occurs 6 to 10 meters above the pairof thin sandstone layers. The remaining 100-120m of the upper part of the Fossil Hill Membercontains the same late Anisian succession as thatof the type locality of Fossil Hill . The type areas for the Gymnotoceras

weitschati and mimetus Zones are also locatedin the small canyons and gulches to the south ofFavret Canyon, where the faunas are better de-veloped and the outcrops for these particularintervals of the stratigraphic column are usuallybetter. Ammonoids representative of only theCordeyi subzone of the Weitschati zone havebeen found in Favret Canyon. For the most part, ammonoid preservation isbetter in some respects in this area than at FossilHill, where the beds have been subjected to con-siderable “heating” from nearby intrusive rocks.This generally holds true for the other sites (southend of the Tobin Range, McCoy Mine area) onthe eastern side of the Fossil Hill basin as well.The examination of suture lines on specimens fromFossil Hill can be difficult, whereas it is relatively

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FIGURE 1.10. Distribution of Anisian ammonoid taxa in the Oliver Gulch section (from Monnet and Bucher, 2005a).

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easy for ammonoids from this area After stop, retrace route back down the valleyto the main north-south road. 1.7Intersection with main north-south road; turnright. 0.5Road to left; go straight to continue north upthe Jersey Valley. The Jersey Valley took its namefrom the now defunct town of Jersey City (1876-1877). 6.6Intersection with main road. Go right (straight).Fish Creek Mountains to right (east), TobinRange to left (west). The Fish Creek Mountainstake their name from Fish Creek, a stream onthe east side of the range. The Tobin Range wasnamed for the family of that name, who lived inWinnemucca. The “Desert Hot Tub” located near some hotsprings along our route, is something of an engi-neering marvel that includes an elaborate systemof PVC pipes and “tennis ball valves” for fillingand draining. It was built from local rock andcement several years ago by an enterprising souland his wife who camped at the site in their mo-tor home for a couple of weeks. Apparently, theyhad nothing better to do. 10.6Cattleguard; conical hill to left is Squaw Tit. 0.7At Jersey Summit, road forks; bear right. EnterBuffalo Valley. 1.5Lander County line (leave Pershing County);Mount Tobin at 9:00. Created in 1862, LanderCounty honors Frederick W. Lander, chief en-gineer for the Central Overland Route (a wagonroute). In particular, Lander negotiated a trucewith the Paiute Indians, for which he was hon-ored. Lander died a Brigadier General in 1862from wounds received at a battle in Virginia dur-ing the American Civil War. Pershing County, established in 1919, wasnamed after General John Joseph Pershing ofWorld War I fame. 3.1Road enters from right; continue straight. 3.3Road to Buffalo Valley Ranch enters from left;continue straight. The high Tobin Range to the

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left is underlain by highly deformed upper Pa-leozoic deep-water sedimentary rocks (e.g., ra-diolarian cherts) forming the upper plate of theGolconda thrust fault, which is generally held tobe late Permian(?) or earliest Triassic in age.Unfortunately, an obvious direct overlap of thisfault by Triassic strata, such as those of theKoipato Group, is nowhere exposed, so its ageis inferred from various structural arguments. Tothe right of the road, Cenozoic volcanic rockscover most of the Fish Creek Mountains. 5.9Pass under power lines. China Mountain, cappedby nearly horizontal Triassic rocks of the StarPeak Group, at 9:30 on skyline. Battle Moun-tain straight ahead on skyline. Battle Mountainexposes rocks of the Golconda allochthonemplaced by the Golconda thrust on lower-plateupper Paleozoic sedimentary rocks, which in turnrest unconformably on Paleozoic strata of thestill older Roberts Mountains thrust allochthon.4.2Cattleguard; Copper Canyon Mine at 10:30 atbase of Battle Mountain. 2.9Road intersection at tailings pond; go straight.Reese River Valley to right; Shoshone Range oneastern skyline. The Reese River was named forJohn Reese, a guide and settler of the mid-1800s. The Shoshone Range takes its name from theShoshone Indians. 4.4Intersection with paved State Highway 305.Turn left and drive up Reese River Valley.Shoshone Range to right, Battle Mountain to left.Battle Mountain takes its name from a skirmishwith Shoshone Indians in 1857. 11.3Cross I-80 overpass and enter Battle Moun-tain. Wagon trains bound for California traversedthis valley in the 1840’s and 50’s. Battle Moun-tain was born in 1869 when the Central PacificRailroad pushed through here on its way to thehistoric meeting with the Union Pacific RR atPromentory Point, Utah. A post office was es-tablished in 1870. Today, this little town is affec-tionately known as the “armpit of America.”End of first-day road log.