BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 47 PP. 241-252. FEBRUARY 29. 1936

TRIASSIC AND LOWER JURASSIC FORMATIONS OF WEST CENTRAL NEVADA BY SIEMON W. MULLER AND HENRY G. FERGUSON CONTENTS Page Introduction...... 241 Stratigraphy...... 242 Triassic system...... 243 Candelaria formation...... 243 Excelsior formation...... 244 Luning formation...... 245 Gabbs formation...... 248 Lower Jurassic series...... 249 Sunrise formation...... 249 Dunlap formation...... 250

INTRODUCTION During the past few years, the writers have been studying the stratig­ raphy and structural geology of the Tonopah and Hawthorne quad­ rangles, in western Nevada. An unusually complete section of Triassic and Lower Jurassic rocks merits brief preliminary description prior to the publication of more complete papers, now in preparation, dealing with the stratigraphy and structure of the region. The field work has been carried on at various times under the auspices of the United States Geological Survey, the Geological Department of Stanford University, and the Geological Society of America. The writers gratefully acknowl­ edge much assistance received from Dr. T. W. Stanton. The study has covered an area of about 7,700 square miles, lying in parts of Mineral, Esmeralda, and Nye counties, Nevada. Much of the area is covered with Tertiary sediments and lavas, and there are large bodies of granitic intrusives, satellites of the Sierra Nevada batholith. Paleozoic rocks, ranging in age from Cambrian to Permian, crop out in the southeastern part of the Hawthorne quadrangle and the southern and eastern parts of the Tonopah quadrangle. Unmetamorphosed Triassic and Lower Jurassic sediments are confined to the central and eastern parts of the Hawthorne quadrangle, particu­ larly the Candelaria Hills, Excelsior Mountains, Garfield Hills, Gillis (241)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/2/241/3430594/BUL47_2-0241.pdf by guest on 23 September 2021 242 MULLER AND FERGUSON----TRIASSIC AND LOWER JURASSIC OF NEVADA Range, Gabbs Valley Range, and Pilot Mountains, and in the north­ western part of the adjoining Tonopah quadrangle, including the Pilot Mountains, Cedar Mountains, Shoshone Range, Paradise Range, and Toyabe Range. This area of Triassic rocks is coincident with a belt of complex folding and thrusting of Jurassic age. STRATIGRAPHY The bedded Mesozoic rocks of the Hawthorne and Tonopah quad­ rangles have a total thickness of about 30,000 feet. They are predom­ inantly marine clastic sediments, cherts, and limestones, with a consider­ able proportion of more or less altered pyroclastic rocks and lavas in the lower and upper parts of the section. In age these rocks range from Early Triassic to Early Jurassic (Lias). Recent paleontologic studies by Muller have added much information regarding the succession of marine invertebrate faunas in North America in Triassic and Early Jurassic time. Particularly significant are new assemblages discovered in the Lower Triassic, Upper Triassic, and Lower Jurassic. The new Early Triassic fauna appears to be older than any of the Early Triassic marine faunas heretofore recorded from North America. The new faunal assemblages from the Upper Triassic show a striking resemblance to, if not identity with, the Noric fauna from the Mediterranean region. Marine beds containing an Upper Triassic (Rhae- tic) fauna, including such characteristic forms as Pteria contorta (Port- lock) and Choristoceras marshi Hauer, are recognized for the first time in western North America. Psiloceras-bearing beds, marking the begin­ ning of the Jurassic system, were found resting conformably and without any noticeable change in lithology upon the marine strata containing the Rhaetic assemblage. These newly discovered facts demonstrate an uninterrupted intermin­ gling of Mediterranean and western American marine faunas near the end of the Triassic period and at the beginning of the Jurassic period. They also compel modification of the generally accepted view that “in the time represented by the Pseudomonotis zone the European connection was cut off, and intermigration took place through the. Bering portal, which opened into the Arctic Sea. This fauna came down on both sides of the north Pacific, and spread southward even below California, probably under the influence of a Boreal current.”1 Likewise, the con­ clusions afforded by the new data are quite contrary to the view, ex­ pressed not long ago by other writers, that “everywhere, from Alaska to California, the oldest Jurassic beds lie unconformably upon the up­ 1J. P. Smith: Upper Triassic marine invertebrate faunas of North America, U. S. Geol. Surv., Prof. Pap. 141 (1927) p. 3.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/2/241/3430594/BUL47_2-0241.pdf by guest on 23 September 2021 STRATIGRAPHY 243 turned edges of the Triassic,”2 or that “nowhere on the (North American) continent is there any latest Triassic or earliest Jurassic marine sedi­ ment.” 8 The following formations comprise the Mesozoic section in this area: Feet Erosion surface capped by Tertiary volcanic rocks. Intrusion of granitic and subordinate basic rocks allied to the Sierra Nevada batholith. Dunlap formation ...... 5,000± Unconformity (apparent conformity locally) 1,200 Sunrise formation ...... 420 TT . (Gabbs formation ...... 10,000+ Upper Tnassic...... -!T!JLuning . formation, ...... Unconformity. 12,000+ Middle Triassic...... Excelsior formation ...... Contact not exposed. 3,000+ Lower Triassic...... Candelaria formation ...... Unconformity. Permian and older strata. The Permian rests unconformably on the Ordovician. Devonian lime­ stone and Mississippian (?) shale occupy a small area in the San Antonio Range, north of Tonopah. TRIASSIC SYSTEM CANDELARIA FORMATION The Candelaria formation, the lowermost Mesozoic unit in the area, derives its name from the Candelaria mining camp, which is situated in the low hills south of the Excelsior Mountains, about 16 miles south of Mina. The name was first used in 1921 by J. A. Burgess in an unpublished report on the Candelaria mining district. The type locality of this formation is about 2 miles southeast of the camp. The Candelaria formation is more than 3,000 feet thick and consists essentially of shales, sandy shales, and sandstones, some of which have a tuffaceous aspect, and scattered thin layers and lenticular bodies of limestone. Marine invertebrates are found only in a belt about 150 to 225 feet above the base of the formation. The Candelaria rests with marked erosional unconformity on the thin Permian sandstones and grits (equivalent to the Phosphoria formation, according to G. H. Girty) and, where these have been eroded, on the folded and beveled Ordovician strata. The lower assemblage of marine invertebrates from the Candelaria formation, besides several fragments of indeterminate ammonites, con­ 2 Ibid. ®C. H. Crickmay: Jurassic history of North America, Am. Philos. Soc., Pr., vol. 70, no. 1 (1931) p. 18.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/2/241/3430594/BUL47_2-0241.pdf by guest on 23 September 2021 244 MULLER AND FERGUSON----TRIASSIC AND LOWER JURASSIC OF NEVADA sists of a large number of specimens of the genus Pseudomonotis (Cla- raia), among which are recognized Pseudomonotis (Claraia) clarai Emm- rich, P. (C.) aurita Hauer, and P. (C.) cf. P. (C.) stachei Bittner. The importance of this group of pelecypods, recognized in this con­ tinent for the first time, is readily seen when it is realized that in other parts of the world they are restricted to the lower part of the Lower Triassic. The occurrence of these three mollusks in the Lower Triassic of North America makes them world-wide in distribution, and with their restricted stratigraphic range they become very valuable for correlation. Thus, it follows that the lower Candelaria fauna may be regarded with reasonable certainty as the time equivalent of the assemblage from the lower part of the Seis member of the Werfen beds of Tyrol. It is, as well, probably an equivalent, in part, of faunas from beds of late Otoceratan age in Greenland and India. The following fossils were obtained from the Candelaria formation, about 50 feet above the Pseudomonotis-bearing beds: Hedenstroemia (Clypites) cf. H. evolvens M. cf. M. tenuistriatum Krafft Waagen Proptychites cf. P. ammonoides Waagen Meekoceras gracUitatis White P. cf. P. tiilobatus Waagen M. cf. M. lilangense Krafft Most of these species appear to be new to this continent. Meekoceras lilangense and M. tenuistriatum heretofore have been found only in the so-called “Meekoceras beds” of the Himalayas, whereas Proptychites ammonoides, P. trilobatus, and Hedenstroemia (Clypites) evolvens were recorded only from the Ceratite marl of the Salt Range of India. It is therefore safe to correlate the ammonite-bearing beds of the Candelaria formation with a part of the Ceratite marl, on one hand, and with a part of the “Meekoceras beds” of the Himalayas, on the other. In Spath’s classification4 of the Triassic, this new fossil assemblage from Nevada may be assigned to Gyronitan “age”; according to J. P. Smith’s terminology,5 it belongs to the Genodiscus zone. EXCELSIOR FORMATION The Excelsior formation derives its name from the Excelsior Moun­ tains, where it is well exposed in the Gold Range mining district, about 6 miles southwest of Mina. It is the most widespread of the Mesozoic formations and is present in all the ranges of the two quadrangles except in the extreme south and east. As no complete section was obtained and faulting is widespread, the actual thickness is undetermined, but, unless 4 L. F. Spath: Catalogue of the fossil cephalopoda, pt. 4, The Ammonoidea of the Trias (1934) p. 27. 5J. P. Smith: Lower Triassic ammonoids of North America, U. S. Geol. Surv., Prof. Pap. 167 (1932) pi. facing p. 14.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/2/241/3430594/BUL47_2-0241.pdf by guest on 23 September 2021 TRIASSIC SYSTEM 245 the formation is repeated by undiscovered faults, more than 12,000 feet is present in the southern part of the Pilot Mountains, and there are compa­ rable amounts in other ranges. The Excelsior formation probably overlies the Candelaria formation, but the two were not found in contact. It is overlain unconformably by the Luning formation. The Excelsior consists dominantly of effu­ sive and pyroclastic rocks, with subordinate sediments. The lavas range in composition from andesite through quartz latite to rhyolite. Alteration, principally epidotization and chloritization, has affected the formation over wide areas. Keratophyres were identified only in specimens collected from mining districts, and it seems probable that albite, where present, is the result of later hydrothermal alteration. Cer­ tain intrusive rocks that cut the lavas and tuffs of the Excelsior forma­ tion are probably in large part contemporaneous with the lavas. These include much altered basic rocks of gabbroic and diabasic types and siliceous porphyritic intrusives similar in character to the rhyolitic flows. Volcanic breccias, especially those containing altered andesite frag­ ments, are abundant and in some sections exceed the effusive rocks in amount. In the Pilot and Excelsior ranges a considerable thickness, esti­ mated to exceed 8,000 feet, consists of massively bedded chert. Examina­ tion under the microscope shows this rock to be an extremely fine­ grained water-laid tuff, cemented and largely replaced by cryptocrystal­ line quartz. Interbedded with the chert are dark tuffaceous slate and a little impure sandstone, with some lava and breccia. The chert is also present in other ranges but is subordinate in amount to the lava and breccia. In places, lenticular beds of limestone occur in the Excelsior forma­ tion. These are sparingly fossiliferous, and at only one locality, near Wildhorse (Gillis) Canyon, in the Gillis Range, were identifiable fossils obtained. The following species were identified from this locality: Spirijerina cf. S. jragilis (Schlotheim) Hoemesia aff. H. socialis (Schlotheim) Spirijerina stracheyi Salter var. funicularis Schmidt. Lima (Plagiostoma) striata (Schlotheim) Gervilleia, three species Pleuromya mactroides (Schlotheim) of Goldfuss This assemblage is of considerable biostratigraphic importance, for, besides establishing the Middle Triassic age of the Excelsior formation, it also proves the presence in western North America of a typical German Muschelkalk fauna. LUNING FORMATION The Luning formation, of Upper Triassic age, derives its name from the little settlement of Luning, in Soda Springs Valley. The best expo­

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/2/241/3430594/BUL47_2-0241.pdf by guest on 23 September 2021 246 MULLER AND FERGUSON----TRIASSIC AND LOWER JURASSIC OF NEVADA sures of the formation are in the mountains bordering this valley—the Garfield Hills, on the west, and the Gabbs Valley Range and Pilot Moun­ tains, on the east. The Luning formation is present also in the Gillis Range, in the north-central part of the Hawthorne quadrangle, and in the Cedar Mountains, Paradise Range, Shoshone Range, and the south­ west end of the Toyabe Range, in the Tonopah quadrangle. The total thickness is estimated at more than 10,000 feet, though in most localities less is present. The area on the north slope of the Pilot Mountains, about 12 miles southeast of Luning, is selected as the type locality of this formation. The Luning formation rests unconformably on the Excelsior formation, but the contact, though indicating a considerable erosion interval, does not show marked angular unconformity. The lithology is varied. Lime­ stone and dolomite predominate, but where the formation is in contact with, or close to, the underlying Excelsior, as in the Pilot Mountains, there are one or more members consisting of siliceous argillite with conglomer­ ate whose pebbles consist almost exclusively of chert. Elsewhere, as in the Gabbs Valley Range, the formation consists dominantly of dark limestone and dolomite with subordinate argillite and shale. An inter­ bedded lava flow was found in the Garfield Hills, southeast of Pamlico. The limestone and particularly the calcareous shale forming the grada­ tional zones between the limestone and the argillite members are in most places abundantly fossiliferous; the dolomite and argillite are generally barren. Paleontologic evidence, together with the field relations between the Luning and the underlying Excelsior and overlying Gabbs formation, indicate that the Liming formation should be correlated with the Karnic stage of the standard Triassic section of Europe. In Cedar Mountain and the Shoshone Range, however, there is apparently no change in lithology near the upper limit of beds that are correlative with the Karnic stage, and it is therefore possible that in these localities deposits of Noric, and even of Rhaetic, time are included in the Luning formation. The most diagnostic material for the correlation of the Luning forma­ tion consists of marine invertebrates of at least three distinct faunal facies—an offshore ammonite facies, a near-shore pelecypod facies, and a coral-reef facies. In the lowermost ammonite assemblage the common species are Juva- vites (Anatomites) böhmi Gemmellaro, J. (A.) formosus Gemmellaro, J. (A.) elegans Gemmellaro, and KlamMhites cf. K. kellyi Smith. Asso­ ciated with these cephalopods are the pelecypods Myophoria cf. M. decus- sata Münster, “Cardium” arcaejormis Gabb, and Nucula sp. This fauna is distinctive of the Karnic stage of the European Upper Triassic. It also shows some resemblance to the fauna of Karnic age from Cali­

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/2/241/3430594/BUL47_2-0241.pdf by guest on 23 September 2021 TRIASSIC SYSTEM 247 fornia, as is indicated by the presence of Klamathites cf. K. kellyi and Juvavites (Anatomites) böhmi. The latter is almost conspecific with Juvavites (Anatomites) subintenruptus Hyatt and Smith, of California. In the ammonite assemblage found about 250 feet higher in the section the following species were recognized: Tropites cf. T. subbuUatus Mojsi- sovics, T. cf. T. welleri Smith, T. cf. T. ursensis Smith, Hoplotropites cf. H. omatissima (Gemmellaro), Juvavites (Anatomites) inflatus Gem- mellaro, and Arcestes sp. From this list it is clear that the assemblage is closely related to the fauna from the Juvavites subzone of the Tropites subbuUatus zone of California, which, in turn, is correlated with the upper Kamic of the Triassic section in the Alps and elsewhere. The uppermost ammonite fauna, occurring about 350 feet above the Tro-pites-bearing beds, comprised the following species: Gümbelites phi- lostrati Diener, G. jandianus Mojsisovics, Griesbachites (several spe­ cies), Styrites vermetus (Dittmar), S. signatus (Dittmar), S. heberti Mojsisovics, S. cf. S. malayicus Welter, Palicites mojsisovicsi Gem­ mellaro, Mojsisovicsites crassecostatus Gemmellaro, M. obsoletus Gem­ mellaro, M. tenuicostatus Gemmellaro, and Arcestes sp. This fauna, besides being new to North America, is of more than usual interest be­ cause, though occurring in a thickness of only a few feet (the bulk of the material was obtained from a bed 2 feet thick), it contains elements which in other parts of the world are known to occur in the Karnic, in the so-called “Kamisch-Norisch Mischfauna” of Diener, and in the Noric. This peculiar association of “heterochronous” elements in the Nevada assemblage presents a rather puzzling problem, the ultimate solution of which involves a re-study of the stratigraphic relations and ranges of the genera and species enumerated and a more critical study of their morphologic characters. Pending further investigation of this interesting assemblage, its age cannot be established with any degree of accuracy, but, because of its affinities with the “Karnisch-Norisch Mischfauna” of Diener, it is tentatively assigned to the Kamic-Noric transition. In the pelecypod assemblages the most distinctive and easily recog­ nizable species are Cardita beneckei Bittner, Myophoria kefersteini Mün­ ster, M. whateleyae (Buch), and Alectryonia montis-caprilis (Klipstein). Cardita beneckei and the two species of Myophoria are in the Alpine Triassic restricted to the middle Karnic, whereas Alectryonia montis- caprilis is usually found in the upper, but not the uppermost, Kamic. These bivalves are therefore sufficiently diagnostic to date the rocks as corresponding to the middle division, and possibly, in part, the upper division, of the Kamic stage of the standard European section. The coral reefs that are particularly well developed at a number of places in Pilot Mountains have yielded a large number of species of

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/2/241/3430594/BUL47_2-0241.pdf by guest on 23 September 2021 248 MULLER AND FERGUSON----TRIASSIC AND LOWER JURASSIC OF NEVADA corals and other invertebrates. Of the corals the following species have been recognized in the fauna: Montlivaultia norica Freeh L. bronni Volz M. marmorea Freeh Confusastrea borealis Smith Thecosmilia cf. T. jenestrata Reuss Thamnastraea rectilamellosa Winkler T. delicatula (Freeh) T. norica Freeh Isastraea parva Smith T. borealis Smith I. profunda Reuss Stephanocoenia juvavica Freeh I. vancouverensis Clapp and Shimer Stromatomorpha californica Smith Latimaeandra norica (Freeh) Among the brachiopods found in association with the corals are Spiri- ferina gregaria Suess, S. peneckei Bittner, Terebratula debilis Bittner, T. julica Bittner, and T. suborbicularis Münster var. typica Bittner. These brachiopods, without a single exception, are known to occur in Europe only in strata of middle and early Kamic age. Fragments of ammonites, found in the strata immediately overlying the coral reefs and identified as belonging to the genera Camites and Klamathites, further substantiate the Kamic age of the coral reefs. This is rather significant, because in the past a rather meager fauna of corals from the Pilot Mountains was regarded as a part of the so-called interregional coral zone of early Noric age. More complete Stratigraphie and paleon- tologic evidence in support of the Kamic age of the corals has been recently submitted for publication.6 GABBS FORMATION The name of the Gabbs formation is derived from the Gabbs Valley Range, where, in the New York Canyon area, the formation is best exposed. The formation at this locality is 420 feet thick and consists of purple to black shale and dark-brown to black limestone. The con­ tacts with the underlying Luning formation and overlying Sunrise forma­ tion are conformable. The Gabbs formation has been recognized only in the Gabbs Valley Range in the neighborhood of New York Canyon, in the Garfield Hills east and south of Pamlico, and in the Paradise Range. The richly fossiliferous section in New York Canyon has been selected as the type locality. The lower boundary of the Gabbs formation is defined by a marked lithologic change from massive dolomite and limestone to thin-bedded shale with a minor proportion of limestone. The shales and limestones of the Gabbs formation are dark-gray to jet-black in fresh exposure but weather to a peculiar grayish-purple color that contrasts with that of the adjacent Stratigraphie units. There is, however, no lithologic change between the uppermost Triassic Gabbs formation and the lower part 6S. W. Muller: Triassic coral reefs in Nevada, Am. Jour. Sci. (in press).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/2/241/3430594/BUL47_2-0241.pdf by guest on 23 September 2021 TRIASSIC SYSTEM 249 of the lowermost Jurassic Sunrise formation; the boundary between the two systems was therefore arbitrarily drawn in the 30 feet of nonfossilif- erous shale which lies between the uppermost occurrence of the Upper Triassic fossils and the lowermost Lower Jurassic. Three faunal assemblages are recognized in the Gabbs formation— one at the base of the formation, one in the middle, and one near the top. The fauna from the lower part of the formation is characterized by the ammonite species Sagenites giebeli (Hauer), S. princeps Mojsiso- vics, Rhabdoceras suessi Hauer, and Cochloceras fischeri Hauer. These species, recognized in North America for the first time, are diagnostic of the lower Noric of the European Upper Triassic. The fauna from the middle of the Gabbs formation is likewise new to North America and is characterized by the following species: Pinacoceras metternichi Hauer Placites symmetricm Mojsisovics Arcestes gigantogaleatus Mojsisovics Celtites arduird Mojsisovics "Arcestes nevadanw” Hyatt and Smith Rhacophyllites debilis (Hauer) (= A. intuslabiatus Mojsisovics) Proelydonautilm spirolobus (Dittmar) Cladiscites tomatus Bronn Gonionautilus securis (Dittmar) Paracladiscites multilobatus (Bronn) In Europe this fauna indicates the Pinacoceras metternichi zone in the Sevatic substage of the upper Noric stage. The fossils from horizons near the top of the Gabbs formation, al­ though not as abundant nor as well preserved as those from the lower horizons, are, nevertheless, sufficiently distinctive to permit correlation with the Pteria, contorta (“Avicula” contorta) zone of the marine Rhaetic of Europe. In this collection from Nevada the species associated with Pteria contorta are Choristoceras marshi Hauer, C. cf. C. ammoniti- forme Giimbel, Arcestes sp., and Nucula sp. It is noteworthy that the marine Rhaetic fauna has not been previously recognized in North America. LOWER JURASSIC SERIES SUNRISE FORMATION The Sunrise formation derives its name from Sunrise Flat, in the Gabbs Valley Range. The rocks included in this formation are shales, limestones, and sandstones. It begins with a gradational contact above the Gabbs formation and is separated by a more or less strongly marked unconformity from the conglomerate of the next younger Dunlap forma­ tion. The total thickness of the formation is about 1,200 feet. The selected type section of the formation is in the upper part of New York Canyon, west of Sunrise Flat. Here the rocks are well ex­ posed and, at least locally, are comparatively little disturbed. The paleontologic material collected from the type locality is also remarkably complete.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/2/241/3430594/BUL47_2-0241.pdf by guest on 23 September 2021 250 MULLER AND FERGUSON----TRIASSIC AND LOWER JURASSIC OF NEVADA Incomplete sections of the Sunrise formation were found in the Gar­ field Hills, southeast and south of Pamlico, and in the northwestern part of the Pilot Mountains, in the Hawthorne quadrangle; it is probably present in the Paradise Range, in the Tonopah quadrangle. The fossils obtained from the Sunrise formation permit correlation with the Hettangian and the Sinemurian substages and possibly with the lower part of the Charmouthian substage of the Lias of Europe. The lowermost faunal assemblage includes several species of the genus Psilo- ceas, a number of species of Caloceras and Pecten valoniensis Defrance, none of which has been heretofore recognized in North America. These species indicate correlation with the base of the Hettangian substage of the Lias, the lowest division of the Jurassic in Europe. Considerable importance is attached to this fauna from the Sunrise formation, as it fills a long-existing gap in the marine early Jurassic record of North America. The stratigraphically higher assemblage obtained from an oolitic mem­ ber, about 15 feet thick, contains, among other forms: Coroniceras of. C. bucklandi (Sowerby) Pecten acutiplicatus Meek Megarietites meridionalis (Reynes) Stylophyllopsis sp. Vermiceras rotator (Sowerby) Pholadomya nevadana Gabb Tmaegoceras (several species) According to the occurrence of similar and identical species in Europe and elsewhere, this part of the Sunrise formation is correlative with the base of the Sinemurian substage of the European Lias, corresponding to the Coroniceratan and in part Vermiceratan “ages” of Buckman. The third faunal assemblage, collected at horizons near the top of the Sunrise formation, contains: Deroceras armatum (Sowerby) Pholadomya nevadana Gabb D. cf. D. dendnodum (Quenstedt) Ph. multUineata Gabb This fauna is most nearly related to that from the lower part of the Charmouthian substage of Europe, corresponding to the Deroceratan “age” of Buckman, Spath, and others. DUNLAP FORMATION The Dunlap formation derives its name from Dunlap Canyon, in the Pilot Mountains, in the upper part of which a considerable thickness of the lower part of the formation is exposed. The formation has a maxi­ mum observed thickness exceeding 5,000 feet and is of widespread dis­ tribution. It is present in Excelsior Mountain, the Garfield Hills, the Gabbs Valley Range, and the Pilot Mountains, in the Hawthorne quad­ rangle ; and in Cedar Mountain, the Shoshone Range, the Paradise Range, and the south end of the Toyabe Range, in the Tonopah quadrangle.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/47/2/241/3430594/BUL47_2-0241.pdf by guest on 23 September 2021 LOWER JURASSIC SERIES 251 Except in New York Canyon, where it locally rests on the Sunrise forma­ tion with apparent conformity, there is a marked angular unconformity with all the older formations of the section, especially well marked in the Pilot Mountains at the heads of Dunlap and Cinnabar canyons, where the Dunlap formation rests on upturned cherts of the Excelsior forma­ tion, with an angular discordance of about 90 degrees. At the southeast end of Sunrise Flat and in the Garfield Hills, conglomerates of the Dunlap formation truncate folds in Luning limestones. Commonly, the lower part of the Dunlap is conglomeratic, and a char­ acteristic feature is the presence of enormous fan cones of unsorted angu­ lar material derived from the formation against which it rests. In places these fanglomerates contain lenses of limestone, shale, sandstone, and bedded conglomerate, implying that the fan-cone deposition took place close to sea level and gave place at times to deltaic conditions. Where the fanglomerates of the Dunlap formation were deposited in a basin bor­ dered on one side by the Excelsior formation and on the other by the limestones of the Luning formation, as in the Pilot Mountains and Gar­ field Hills, it is noticeable that most of the material is derived from the limestones and that only immediately adjacent to the contact do the con­ glomerates consist solely of pebbles of the more resistant cherts and vol­ canic rocks of the Excelsior formation. The upper part of the formation consists dominantly of red to brown sandstone, containing much volcanic material and probably including a considerable proportion of water-laid tuffs, conglomerates, principally with limestone pebbles and varying amounts of volcanic rocks. In the Gabbs Valley Range and the central part of the Excelsior Mountains, volcanic rocks predominate in the upper part of the section; elsewhere, though nearly everywhere present, they are subordinate. They include andesite, quartz latite and rhyolite flows, and breccias, and are generally much altered. They are not dis­ tinguishable petrographically from the lavas of the Excelsior formation. The Dunlap formation is nearly barren of fossils, and such as were collected are poorly preserved. The paleontologic evidence obtainable, together with the fact that in the New York Canyon section there is no angular discordance with the underlying Sunrise formation, suggest that the Dunlap is to be correlated with the Charmouthian stage of Lias of Europe. Nothing definite can be said as to the time represented by the whole formation, except that the great fan cones of limestone frag­ ments, the abundant conglomerates, and the volcanic rocks suggest rela­ tively rapid accumulation during a period of marked crustal instability.

Stanford Univebsity, California, inn U. S. Geological Suhvey, Washington, D. C. Manuscript received by the Secretary of the Society, December 20, 1935. Accepted by the Commitiee on Publications, 193S. Published by permission of the Dikeciob, TJ. S. Geological Survey.

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