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Triassic and Lower Jurassic Formations of West Central Nevada by Siemon W

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Triassic and Lower Jurassic Formations of West Central Nevada by Siemon W 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
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