The Roberts Mountains Formation, a regional stratigraphic study with emphasis on rugose coral distribution

GEOLOGICAL SURVEY PROFESSIONAL PAPER 973

The Roberts Mountains Forn1ation, a regional stratigraphic study with emphasis on rugose coral distribution

B1' CHARLES W. MERRIAM and EowiN H. McKEE Tl 7ith a section on Conodonts, b_rv JOHN W. HUDDLE

GEOLOGICAL SURVEY PROFESSIONAL PAPER 973

A study of stratigraph_v, facies, and coral distribution in the 1niddle Paleozoic (Silurian and De·uonian) limestone belt of the central and southwestern Great Basin

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1976 UNITED STATES DEPARTMENT OF THE INTERIOR

THOMAS S. KLEPPE, Serretm)'

GEOLOGICAL SURVEY

V. E. :McKelvey, Director

Library of Congress Cataloging in Publication Data

Merriam, Charles Warren, 1905- The Roberts Mountains Formation.

(Geological Survey Professional Paper 973) Bibliography: p. 43-45. Includes index. Supt. of Docs. no.: I 19.16:973 1. Geology, Stratigraphic-Silurian. 2. Geology, Stratigraphic-Devonian. 3. Rugosa. 4. Geology-Nevada. 5. Geology­ California. I. McKee, Edwin H., joint author. II. Huddle, John Warfield, 1907- joint author. III. Title. IV. Series: United States Geological Survey Professional Paper 973. QE66l.M45 551. 7'3'0979 76-608307

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-02893-0 CONTENTS

Page Page i\bstract ------1 Roberts Mountains Formation of the northern Introduction ------1 Toquima Range-Continued Purpose and scope of investigation ______2 Ikes Canyon window i\cknowledgments ______2 Mill Canyon sequence------22 History of investigation ______3 August Canyon sequence ------23 Paleontologic zoning of the Great Basin Silurian and Lower Northumberland window ------23 Devonian ______4 Roberts Mountains Formation in the Toiyabe Range ______23 Scarcity of reefs or bioherms in the Roberts Mountains For- Callaghan window ------24 mation ______4 Dry Creek area------24 Distribution of Great Basin Silurian limestone and dolomite Point of Rocks and Straight Canyon ______24 belts and the regional dolomite problem ______6 Pablo Canyon area ------25 Geographic distribution of Silurian limestones ______7 Roberts Mountains Formation in the Shoshone Mountains ____ 25 Roberts Mountains Formation of the Roberts Mountains, Nev __ 7 Silurian and Lower Devonian limestones of the northern Inyo Type section ______9 Mountains, Calif ______25 Stratigraphic relations to underlying and overlying Mazourka Canyon ------25 formations ------9 Vaughn Gulch Limestone ------25 Facies changes from limestone to dolomite ______10 Physical stratigraphy ------25 Stratigraphic paleontology of the type section______10 Stratigraphic paleontology ______26 Fossils of units 1 and 2 ______10 i\ge ------27 Fossils of unit 3 ______10 Sunday Canyon Formation ------28 Age of the Roberts Mountains Formation of the type i\ge ______------____ 28 section------10 The Silurian-Devonian boundary in central Nevada ______28 Roberts Mountains Formation of the northern Simpson Park i\n appraisal of stratigraphic and age values of rugose corals and Mountains ------11 associated fossils, with description of previously unknown Geologic setting______11 key Rugosa ____ ------29 Coal Canyon area ______11 Rugose corals of special stratigraphic value ______29 Coal Canyon fault zone ______11 Silurian nondissepimented Rugosa------29 Physical stratigraphy of the Pine Hill section______12 Family Kodonophyllidae ______29 Stratigraphic paleontology and age of Roberts Moun- Subfamily Kodonophyllinae Merriam ______30 tains Formation at Pine HilL______13 Subfamily Mycophyllinae Hill ______30 Fossils collected within the Coal Canyon fault zone __ 14 Family Streptelasmatidae Nicholson ______30 Physical stratigraphy of the Pyramid Hill section ____ 14 Family Stauriidae Edwards and Haime ______31 Problem ofthe Silurian-Devonian boundary at Pyramid Family Tryplasmatidae Etheridge ______31 Hill ______15 Family Pycnostylidae Stumm ______31 Roberts Mountains Formation, Tuscarora Mountains ______15 Compact cerioid Pycnostylidae of Silurian limestone Physical stratigraphy ofthe Bootstrap Hill section ______16 facies ------32 Stratigraphic paleontology of the Bootstrap Hill section __ 16 Dissepimented Rugosa of the Great Basin Silurian i\ge of the Roberts Mountains Formation at Bootstrap limestone facies ______32 Hill ______17 Family Spongophyllidae Dybowski ______32 Roberts Mountains Formation of the northern Monitor Range 18 Family Endophyllidae Torley ______34 Physical stratigraphy ______18 Family Chonophyllidae Holmes ______36 Stratigraphic paleontology and age of the Roberts Moun- Family Kyphophyllidae Wedekind ______36 tains Formation at Copenhagen Canyon ______18 Family Calostylidae Roemer------37 Roberts Mountains Formation at Brock Canyon, Monitor Family Lykophyllidae ______37 Range ------19 Fossils associated with rugose corals in Great Basin Silu- Roberts Mountains Formation at Dobbin Summit, Monitor rian limestones ______37 Range ------19 Dasycladacean algae as indicators of the Silurian ____ 38 Roberts Mountains Formation at Bare Mountain ______20 Conodonts of the Roberts Mountains Formation by Roberts Mountains Formation of the northern Toquima John W. Huddle ------38 Range ------­ 20 Locality register of major fossil localities in the Roberts Moun­ Petes Canyon window------­ 22 tains Formation and correlative strata ------39 Ikes Canyon window ------22 Selected references ______------43 June Canyon sequence ------22 Index ---~------47

III IV CONTENTS ILLUSTRATIONS

[Plates follow index]

PLATE 1. Stylopleura and Pycnostylus. 2. Stylopleura and Pycnostylus. 3. Fletcheria and Tryplasma. 4. Mucophyllum and Kodonophyllum. 5. Tonkinaria, Kyphophyllum, Kodonophyllum, and Brachyelasma. 6. Spongophyllum, Carlinastraea, and Australophyllum. 7. Carlinastraea, Chonophyllum, and Calostylis? 8. Carlinastraea and A ustralophyllum. 9. Kyphophyllum and Chonophyllum. 10. Verticillopora, Cladopora, Favosites, Kyphophyllum, Ketophyllum, and Hercynella. 11. Verticillopora, Kozlowskiellina, H omoeospira, Dicoelosia, Plectatrypa?, and Gypidula. 12. Coelospira, Fardenia, Kozlowskiellina, Conchidium, Cyathophylloides, Brachyelasma, and lycophyllid. Page FIGURE 1. Index map of part of Great Basin showing distribution of the Roberts Mountains Formation and correlative formations in the intermediate limestone belt in Nevada and California ------8 2. Geologic map of the Coal Canyon area, northern Simpson Park Mountains ______12 3. Sketch map of the northern Toquima Range showing Paleozoic sequences that contain Silurian rock in separate thrust sheets __ 21

TABLES

Page TABLE 1. Characteristic fossils of the Great Basin Silurian coral zones ------5 2. Occurrence chart of conodont species collected from Silurian and Lower Devonian formations in central Nevada ______40 THE ROBERTS MOUNTAINS FORMATION, A REGIONAL STRATIGRAPHIC STUDY WITH EMPHASIS ON RUGOSE CORAL DISTRIBUTION

By CHARLES W. MERRIAM and EDWIN H. McKEE

ABSTRACT limestones, mostly argillaceous, and some dolomitic A large part ofthe Silurian and the base of the Devonian Systems marine strata occupying a northerly oriented linear are represented in the central and southwestern Great Basin by belt within the central and west-central Great Basin. limestones of the Roberts Mountains and the Vaughn Gulch Forma­ Diverse fossil assemblages from many localities indi­ tions. In Silurian time the Great Basin region was inundated by northward-trending shelf seaways of three wide facies belts: the cate that in the time-stratigraphic sense this formation eastern regional dolomite belt which bordered the ancient land mass probably makes up much of the Silurian and the bot­ on the east, the intermediate limestone belt here dealt with, and on tom of the Devonian Systems as represented in the the west, a chert-shale-graywacke belt with some volcanic rocks that Great Basin of the western United States. The extends into the Pacific Border province, best represented in south­ eastern Alaska. Studies of rugose corals show that each of the three Silurian-Devonian boundary lies· within the upper or belts has distinctive corals and associated fossils not yet found in the even upper middle part of the formation. A reexamina­ other belts; biofacies differences greatly complicate geologic correla­ tion of field relations of the fossil-bearing strata and a tion across the facies boundaries. review of fossil comparisons with those of standard The Roberts Mountains Formation, mainly limestone, calcareous columns for the Silurian and Devonian Systems indi­ shale, and some dolomitic limestone, is widely distributed in the central Great Basin, where it has been mapped from the Tuscarora cates that the standard time scale based on graptolites Mountains on the north to the Toquima and Toiyabe Ranges on the includes rocks in the Great Basin that are somewhat south and west. It is known to be present as far north as south­ older than rugose corals would suggest. central Idaho but has been little studied in this region. The Vaughn The Roberts Mountains Formation is bordered on Gulch Limestone and its lateral shaly graptolitic facies, the Sunday Canyon Formation, occur to the southwest in the Inyo Mountains. the east by dolomite that prevails throughout the Thick eastern regional dolomite facies rocks such as the Lone Moun­ east-central and eastern Great Basin. On the west, tain Dolomite and the Laketown Dolomite come into close proximity where the boundary is theoretical, the limestones are to, or intertongue with, limestone of the middle belt in a few areas inferred to grade into shale, graywacke, coarse argil­ such as the Roberts Mountains, Bare Mountain near Beatty,, Nev., laceous rocks, volcanic rocks, and subordinate lime­ and the Inyo Mountains. In a number of places such as in the Tusca­ rora Mountains, the Simpson Park Mountains, the Roberts Mowntains, stone of the Pacific border province (Merriam, 1973a). the Monitor Range, and the Toquima Range, the Roberts Mountains Paleogeographically, three subparallel northward­ limestone has been subdivided into additional lithologic units given trending depositional facies belts are definable where new formational names. These include the Windmill Limestone of the southwest Cordilleran geosynclinal seaways of Johnson, the McMonnigal Limestone, and the Wenban Limestone. The names Masket Shale and GatecliffFormation were used by Kay middle Paleozoic (Silurian and Devonian) time crossed for rocks in the Toquima and Toiyabe Ranges that closely resemble what is today the Great Basin. From east to west these parts of the Roberts Mountains Formation elsewhere. In most areas belts are: the eastern dolomite belt, the intermediate the basal part of the formation is a black cherty limestone or dolo­ limestone belt, and the western or Pacific Border mite followed upward by calcareous shaly laminated and platy lime­ graywacke belt. The transition from the dolomite to stone that bears graptolites. The upper part consists of thin-bedded limestone belt passes through the classical facies of laminated limestone and slabby thicker bedded allogenic limestone . and dolomitic limestone yielding corals, brachiopods, and calcareous platform margin carbonate deposition. Limestones of algae as well as graptolites. the intermediate limestone belt persist to the south­ The Silurian-Devonian boundary lies within this upper interval. west margin of the Great Basin, where they are repre­ This boundary, defined on the basis of graptolites in the standard sented by the Vaughn Gulch Limestone in the north­ time scale, is more than 100 m lower than that based on rugose corals and calcareous algae that uses as reference the well-known ern lnyo Mountains, Calif. section in Gotland, Sweden. As rugose corals are among the diagnostic fossils of the Roberts Mountains Formation, this study was in­ INTRODUCTION spired by the need for dependable stratigraphic se­ The name Roberts Mountains Formation applies to quences in deformed Great Basin Paleozoic rocks to

1 2 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY

be used in conjunction with Rugosa and their strati­ In recent years the Roberts Mountains Formation graphic ranges. Studies of Great Basin rugose corals has assumed economic importance as host rock for the and associated fossils have demonstrated marked bio­ Cortez and Carlin gold ores of central Nevada. As a facies differences between the three facies belts that result, the known extent of the formation has been make difficult interbelt paleontologic correlation. intensively explored and prospected. Some economic aspects of ore search, geochemistry, and geophysical PURPOSE AND SCOPE OF INVESTIGATION prospecting are treated in other reports (T. E. Mullens, unpub. data, 1975). In Silurian and Devonian limestone areas of the central and southwest Great Basin, geologic mapping ACKNOWLEDGMENTS within the past 10 years necessitates restudy and re­ Geologic mapping used in this report includes re­ definition of the Roberts Mountains Formation and sults of D. C. Ross in the northern Inyo Mountains, correlative units, including those of contiguous dolo­ Calif., Harold Masursky and Jonathan C. Matti in the mite areas. J\mo_ng- primary objectives, this effort en­ northern Simpson Park Mountains, and J. H. Stewart tails search for realistic vertical and lateral formation in the central Toiyabe Range and Shoshone Moun­ bq~ndaries which reflect depositional facies relations. tains, Nev. Efforts by these geologists toward clarify­ The physical stratigraphydepends upon refinement of ing geologic structure and stratigraphy have been geologic mapping and paleontologic correlation as most helpful. Detailed geologic mapping in the south­ background, and the biostratigraphy presented here ern Tuscarora Mountains, Nev., by J. G. Evans, sheds leans heavily upon zonation of the Great Basin Silu­ new light on the source beds of earlier fossil collections rian and Devonian by means of corals and associated made in the Bootstrap mine area before large-scale to­ megafossils (Merriam, 1973a, b, c; 197 4). In this coral pographic maps made accurate field location possible. work, the Gotland, Sweden, Silurian yardstick is the These especially well preserved silicified fossils col­ primary standard of comparison and geologic correla­ lected by R. J. Roberts and associates near the tion. In some sections a wide gap exists between the Bootstrap mine in the years 1954 to 1958 first demon­ accepted Silurian-Devonian boundary as determined strated the northward continuation of the Roberts by the graptolite and conodont specialists and the Mountains Formation beyond the Humboldt River placement of the boundary by the coral specialist. For Valley. Rugose corals in Coal Canyon, Simpson Park example, the Monograptus uniformis Zone at the base Mountains, were first collected by R. J. Roberts; at a of the Devonian falls as low as the middle part of the later time, Silurian and Devonian coral collections by Silurian delimited by means of corals. Explanations J. G. Johnson and A. J. Boucot were contributed for use are sought for these discrepancies. in this study. Collections of corals by Leland Cress at The systematic and descriptive paleontology of many Maggie Creek, Tuscarora Mountains, revealed the rugose corals listed in this report is treated in recent presence of Silurian and Early Devonian corals in this works (Merriam, 1973a, b). A paleontologic appendix area. A well-preserved colonial rugose coral from the covers stratigraphically relevant but undescribed Sunday Canyon Formation in Mazourka Canyon col­ megafossils and evaluates certain little-known rugose lected by C. H. Stevens of San Jose State University is coral groups, calcareous algae, and conodonts for age ackno-wledged with thanks. Jonathan C. Matti made determination, correlation, and biostratigraphy. As no many helpful suggestions on the manuscript and re­ megafossils were found by the geologists who mapped viewed the conodont section. many exposures of the Roberts Mountains Formation, Graptolite identifications accompanying this report age designations of these structurally and stratigraph­ are the work of W. B. N. Berry. ically isolated beds are based on conodont studies. Many detailed stratigraphic sections have been Twelve plates illustrate some of the distinctive measured across the Roberts Mountains Formation megafossils of this formation. throughout its known extent by T. E. Mullens. Litho­ A detailed report by T. E. Mullens (unpub. data, logic data from these sections have been employed in 1975) covers comparative lithology, sedimentology, the present report, especially at Bootstrap Hill in the and petrology of the Roberts Mountains Formation and Tuscarora Mountains. its economic potential as a gold-bearing formation. J. W. Miller ably assisted Merriam in detailed geo­ Briefly touched upon herein are the complex en­ logic mapping of the Coal Canyon area and other parts vironmental and geochemical aspects of the multi­ of the northern Simpson Park Mountains. In the To­ faceted dolomite problem bearing upon the complete quima Range all the detailed geologic mapping provid­ change from marine limestone to pervasive diagenetic ing the basis for stratigraphic conclusions in that area dolomite passing into the eastern dolomite belt. is the work of E. H. McKee. Earlier geologic mapping INTRODUCTION 3 of parts of that range was done by Kay and Crawford Reconnaissance geology of the northern Monitor (1964). Range by Merriam and Anderson (1942) made known a All fossil photographs are the work of Kenji southerly extension of the Roberts Mountains Forma­ Sakamoto. tion. In Copenhagen Canyon, for example, the Silurian and Lower Devonian were found to be represented by HISTORY OF INVESTIGATION thinly laminated calcareous shaly rocks carrying grap­ Silurian limestones were first studied in the Roberts tolite faunas and thicker graded beds of bioclastic Mountains by Merriam while he searched for the base limestone. Kay and Crawford (1964) recognized the oc­ of the Devonian System in that area (Merriam, 1940, currence of the Roberts Mountains Formation (called p. 11). On the northwest side of Roberts Creek Moun­ by them the :Masket Shale) farther southwest in the tain, the lowest beds in the Nevada Formation iden­ Toquima Range, where various limestone and argil­ tified in 1940 contain the Acrospirifer kobehana fauna laceous limestones occupy parts of several separately of Early Devonian age and appeared at that time to mappable thrust plates. More recent detailed mqppin_g, conformably overlie massive dolomites identified as in that area by McKee (1976) sheds ad~ittonal light the Lone Mountain Dolomite of supposedly, but as yet upon the facies, paleontology, and,, corr.elation of the unestablished, Silurian or perhaps Early Devonian Roberts Mountains Formation in the Toquima Range. age. Comparison with the Lone Mountain type section Northwest of the type area, the Roberts Mountains 18 miles (29 km) to the south revealed significant Formation was extended into the northern Simpson lithologic differences. Whereas no limestone is present P&rk Mountains by R. J. Roberts and associates during in the Upper Ordovician or Silurian at Lone Mountain, reconnaissance geologic mapping of northern Eureka beneath the Lone Mountain Dolomite at Roberts Creek County in 19t)4-58. Knowledge of these rocks was Mountain is a thick fossiliferous Silurian sequence of further advanced by Winterer and Murphy (1960) and limestones and some dolomite named at that time the Johnson (1965'1. R. J. Roberts and associates extended Roberts Mountains Formation by Merriam (1940). the distribution of the Roberts Mountains Formation This unit occupied the stratigraphic interval between northward into the Tuscarora Mountains and Elko the underlying limestone and dolomite of the Hanson County during the Eureka County mapping program. Creek Formation and the blocky light-gray dolomite Since 1967, parts of the southern Tuscarora Mountains interpreted by Merriam (1940) as the higher part of the near Carlin, Nev., have been mapped in detail by the Lone Mountain Dolomite. Comparison of the sections U.S. Geological Survey. Where the formation is locally at Lone Mountain and Roberts Creek Mountain led to gold bearing, 1nuch has been learned of the complex extension by Merriam (1940) of the name Roberts structure and stratigraphy; geochemical and geophysi­ Mountains Formation to the dark dolomite beneath cal investigations have been carried out by economic the type Lone Mountain Dolomite in accord with the geologists in connection with drilling exploration be­ view that this was a dolomite facies of the Roberts yond the currently productive belt. Mountains Formation. Later mapping in the Roberts Ge'ologic mapping of the Cortez quadrangle by Gil­ Mountains by Winterer and Murphy (1960) clearly luly and Masursky (1965) and the Mount Lewis area demonstrated the lateral equivalence by the inter­ by Gilluly and Gates (1965) extended the known occur­ tonguing of the Roberts Mountains Formation and the rence of the Roberts Mountains Formation into the Lone Mountain Dolomite. In a recent study of the stra­ Cortez Mountains and northern Shoshone Range. The tigraphy and paleontology of the Lone Mountain formation has subsequently become economically im­ Dolomite at Lone Mountain (Merriam, 1973b), the portant as a gold ore host in that vicinity. dark dolomite previously referred to as dolomitic In 1968 the economic potential of the Roberts Moun­ Roberts Mountains Formation has been redesignated tains Formation as a source of gold in north-central as part of the Lone Mountain Dolomite because it was Nevada became sufficiently evident to justify a special considered desirable to retain the formational term study by the U.S. Geological Survey. The lithologic and Roberts Mountains for a predominantly limestone ·unit stratigraphic aspects of this study were undertaken by only. T. E. Mullens, who worked with quadrangle m~pping Above the type section of the Roberts Mountains parties and geochemists in economically pf;'orpising Formation, at Roberts Creek Mountain, the saccharoi­ areas. Mulle~s ha? carried ol1t detailed sectiop'II)eas­ dal Lone Mountain dolomites have yielded no fossils; uremeht and studies' of sedimentary petrology in many the possibility remains that the uppermost part of this areas, including those where mapping:was in progress thick dolomite sequence may correlate with the finer (Mullens, unpub. data, 1975). textured Beacon Peak Dolomite Member of the Nevada Mapping of the Horse Creek Valley quadrangle in Formation. the northern Simpson Park Mountains by Har,Q.ld 4 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY

Masursky, begun about 1960, carried eastward the preserved and represent several genera known in the geology begun by Gilluly and Masursky in the adjoin­ Silurian of Gotland, Sweden; England; eastern Europ·e; ing Cortez quadrangle (1965). A progress map was and Australia. Graptolites, the primary basis for sub­ transmitted by Masursky in 1971 for the use of the dividing the world Silurian System, are abundant in writers in conjunction with the Paleozoic stratigraphy. rocks of the lower part of the Great Basin Silurian and As the Horse Creek Valley quadrangle includes the occur at many places well into the Devonian. Great Coal Canyon section, first fully described by Johnson Basin Silurian and Devonian brachiopods are well rep­ (1965) and probably the best reference section for the resented, and much has been done to tie their occur­ Roberts Mountains Formation, parts of it have been rences and stratigraphic ranges to mapped areas and remapped in more detail by Merriam and Miller, a measured sections. The large dasycladacean algae of work still in progress. the genus Verticillopora are especially useful in Great Interest in southwestern Great Basin Silurian lime­ Basin Silurian strata. Verticillopora ranges upward stones as correlatives of the Roberts Mountains For­ from Great Basin Silurian coral zone B through zone E, mation developed in 1946 with geologic mapping of the apparently reaching a peak of development in coral Cerro Gordo mining district, Inyo Mountains, Calif. zone D (see below). (Merriam, 1963). At that time it became evident that Conodonts are found through much of the Silurian westward and northward, the Hidden Valley Dolomite and Devonian limestone of this province and are cur­ (McAllister, 1952) changed to limestone. Section rently being coordinated with age determinations measurement, reconnaissance geologic mapping, and based on associated megafossils. fossil collecting were begun in 1947 at Mazourka Can­ Study of the stratigraphic distribution of Silurian yon, where the coral genera and calcareous algae of Rugosa in correlative and overlapping reference sec­ these Inyo Silurian beds were found to be closely allied tions throughout the Great Basin makes possible a to those of the central Great Basin. Subsequently, Ross hypothetical fivefold zonation based on the ranges of (1963, 1966) carried out the refinements of detailed species and genera. Provisional zones are designated geologic mapping of the northern Inyo areas. A north­ by capital letters A through E in ascending strati­ ward change of Silurian limestone into shale facies is graphic order. Gaps in the coral record within a single demonstrated by the Ross mapping. reference section are filled in correlative sections to The underlying paleontologic motivation for this complete the overlapping composite zonal scheme. No study of Silurian limestone dates from the discovery by more than three coral zones have been recognized Merriam of well-preserved Rugosa in the Roberts within any one reference column; further collecting is Mountains in 1933, and in the northern Inyo Moun­ expected to eliminate some of these local gaps. tains about 1946. As general worldwide knowledge of The five coral zones ranging in age from Early to Silurian Rugosa progressed rapidly, coral studies lead­ Late Silurian are: ing to a provisional Great Basin Silurian coral zona­ tion were initiated by Merriam (1963), providing, to­ Age Coral zone gether with brachiopod studies, much of the paleon­ Late Silurian1 E tologic support for this contribution. Further incentive D came with the brachiopod investigations of J. G. Middle Silurian c Johnson and A. J. Boucot, who introduced European B comparisons, and with investigations by W. B. N. Early Silurian A Berry of graptolites of Great Basin Silurian and Devo­ nian rocks. Conodont studies by Gilbert Klapper coor­ SCARCITY OF REEFS dinated with field studies of M. A. Murphy added a OR BIOHERMS IN THE wealth of information about the Silurian and Devonian ROBERTS MOUNTAINS FORMATION of central Nevada. The most modern discussion of the formation is the study by Matti, Murphy, and Finney Reef and biohermal structures occur in Silurian (1975). marine limestone and dolomite rich in calcareous al­ PALEONTOLOGIC ZONING OF THE GREAT gae, stromatoporoids, crinoids, and corals in many areas throughout the world (Lowenstam, 1949). These BASIN SILURIAN AND LOWER DEVONIAN structures are parts of the Niagaran Series of the Rugose corals provide a useful means of strati­ Great Lakes and the Gotland of Sweden (Manten, graphic zonation in the Great Basin Silurian and De­ 1 Age designation based on comparison with the Gotland, Sweden, Rugosa. Coral zone E vonian limestone facies. These fossils; common in contains rocks of Lower Devonian age based on the occurrence of Monograptus uniform is, nearly all the bioclastic limestones, are generally well the accepted index for lowermost Devonian in the Great Basin. SCARCITY OF REEFS OR BIOHERMS 5

TABLE !.-Characteristic fossils of Great Basin Silurian coral zones

Geologic Coral age zone Rugose corals Tabulate corals Brachiopods Dasycladacean algae

Late Silurian1 ______E ______Stylopleura nevadensis Merriam Orthostrophia sp. Verticillopora annulata Stylopleura berthiaumi Merriam Schellwienella? sp. Rezak M ucophyllum oliveri Merriam Barrandella? sp. V erticillopora cf. V. Kodonophyllum mulleri Merriam Sicorhyncha? sp. annulata Rezak Rhizophyllum cf. R. enorme Rhynchospirina sp. (Oliver) Plectatrypa? sp. Rhizophyllum sp. D Oliver Atrypa sp. Chonophyllum simpsoni Merriam Meristella sp. Australophyllum (Toquima-phyllum) Kozlowskiellina sp. f johnsoni Merriam Carlinastraea tuscaroraensis Merriam Kyphophyllum nevadensis Merriam Kyphophyllum sp. c Salairophyllum? sp.2 D ______Brachyelasma sp. Dicoelosia sp. r V erticillopora annulata Rezak Stylopleura berthiaumi Merriam Gypidula sp. r Verticillopora cf. V. annulata Rezak Tonkinaria simpsoni Merriam H omoeospira sp. r Tryplasma duncanae Merriam Atrypa sp. Carlinastraea tuscaroraensis Kozlowskiellina sp. f Merriam Middle Silurian ______C ______Tryplasma newfarmeri Merriam Denayphyllum denayensis Merriam Entelophylloides (Prohexagonaria) occidentalis Merriam B______Streptelasmid corals (small) Halysites (Cystihaly­ V erticillopora sp. Brachyelasma sp. B sites) magnitubus Ryderophyllum ubehebensis Buehler Merriam Pycnactis sp. K Petrozium mcallisteri Merriam Early Silurian ______A ______Rhegmaphyllum sp. h Cladopora sp. Dalmanophyllum sp. A Heliolites sp. Palaeocyclus porpita subsp. Halysites sp. mcallisteri Merriam Rhabdocyclus sp. d Dicoelosia sp. Cyathophylloides fergusoni Dalmanella sp. Merriam Atrypa sp. Arachnophyllum kayi Merriam N eomphyma crawfordi Merriam

1 Age designation based on comparison with the Gotland, Sweden Rugosa. Coral zone E contains rocks of Lower Devonian age based on the occurrence of Monograptus uniform is, ·the accepted index for lowermost Devonian in the Great Basin. 2Salairophy/lum? from uncertain stratigraphic horizon at Coal Canyon, Simpson Park Mountains, Nev. Probably from coral zone E.

1971). The Roberts Mountains Formation, however, tional breccias may mark the vicinity of small patch shows little evidence of reefal structure, or even of reefs or bioherrns, but no inclined reef-marginal beds minor bioherms having initial bottom relief. In most or reef core rock was found. Some limestone strata of places, the lower part of the formation is thin and con­ the upper Roberts Mountains Formation in the Roberts sists of evenly laminated fine-grained argillaceous Mountains rich in corals and other benthonic_ or­ limestone with no reeflike characteristics. The upper ganisms appear to represent beds and banks of very part of the formation contains thin- to medium-bedded low bottom relief, but many are thick allodapic beds bioclastic limestone as well as fine-grained thin lami­ deposited in basins or on a gentle slope at greater nated beds. These rock types represent a facies that depths than reef formation and below wave base. must have formed at a moderate depth of water. Cor­ Stromatoporoids contributed much of the material als, calcareous algae, and brachiopods, mostly frag­ toward the building of reefs in the Niagaran of Illinois mental, are fairly abundant in parts of the formation, (Lowenstam, 1949) and in the Silurian of Gotland, but they have not developed into reeflike structures; Sweden (Manten, 1971). In the coral-rich Roberts indeed, in most places the fossils have been trans­ Mountains Formation limestones, stromatoporoids, ported with other calcareous debris as debris flows. though present, apparently were not important as reef At Coal Canyon, Simpson Park Mountains, deposi- builders. 6 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY DISTRIBUTION OF GREAT BASIN rian, the regional dolomite belt lies between the old SILURIAN LIMESTONE AND subdued landmass on the east and the intermediate DOLOMITE BELTS AND THE REGIONAL limestone on the west. Farther west the Pacific border DOLOMITE PROBLEM graywacke belt, through its extent from northern Areal distribution of Great Basin Silurian carbonate California to Alaska, is not known to include dolomite rocks in contrasting lithologic belts such as the inter­ of the diagenetic type. Fossil and lithologic evidence mediate limestone and eastern dolomite belts leads to indicates that the Silurian eastern dolomite and the paleogeographic speculation and review of the multi­ intermediate limestone both represent shallow marine faceted unsolved problems of the origin of marine environments, possibly with some deepening where dolomite. It is not a purpose of this report to enter into limestones appear, especially the allogenic graded the complexities of geochemical and metasomatic types, and further deepening into graptolitic fine­ change. Dolomites of the eastern belt certainly fall in grained and laminated beds. The western limit of the category of regional dolomite of diagenetic origin dolomite, here the Lone Mountain Dolomite, was the as contrasted with localized hydrothermal or meta­ western limit of reef development that migrated west­ morphic dolomites. In several mining districts of this ward by normal landward carbonate accumulation typ­ province, the hydrothermal type locally forms very ical of all large carbonate-producing regions. The gen­ large bodies that may easily be confused with the erally regressive series of carbonate rocks that were diagenetic type (Hewett, 1931, p. 57-68). Nonetheless, deposited in the Great Basin during the lower and some interpretation is appropriate in explaining the middle Paleozoic are strong evidence for the landward discrete lithologic and paleogeographic boundary zone accumulation of carbonate sediment (and hence retreat separating the eastern dolomite from the intermediate of the sea) produced in the shallow shelf seas. Very limestone belt within which the Roberts Mountains extensive tidal flats and mud shoals behind or east of Formation is found. the Lone Mountain reef are the present site of the re­ In relation to regional dolomites of vast extent like gional dolomites we see today. the eastern dolomite belt, we may seek explanation of Studies of rugose corals suggest that the Lone Moun­ its lateral boundaries from various fields of study in­ tain and Laketown Dolomites contain marine faunas cluding paleogeography, paleobiology, geochemistry, differing in biofacies from those of limestones in the marine environment and the sources of magnesium in­ Roberts Mountains Formation (Merriam, 1973a). This troduced to produce the double carbonate mineral does not, however, imply that the dolomite facies was dolomite. We may ask: how or why does proximity to initially deposited as dolomite sediment by direct the subdued Silurian land mass on the east favor chemical precipitation. Other ecological factors must diagenetic dolomitization while offshore limestones be sought: for example, differences of water tempera­ remain normal marine limestone? ture, depth, salinity, and food supply. Fossils are lo­ The dolomite question has been reviewed by many cally abundant in the dolomites, but unless silicified workers, including Van Tuyl (1916a, b), Steidtmann early, that is, prior to dolomitization, they are de­ (1911, 1917), Skeats (1918), Fairbridge (1957), and, stroyed by dolomitic recrystallization. Among the fac­ more recently many authors represented in Pray and tors to be considered is the proximity to the shoreline, Murray (eds. 1965), Bathurst (1971), and Hsu and where extensive mudflats were doubtless very broadly Schneider (1973). Beyond doubt, the Laketown and exposed at low tide to bring about diurnal evaporative Lone Mountain Dolomites, like the Niagaran of the concentration to produce brines. Percolation of the continental interior-nearly continuous dolomites that magnesium-rich brine into the carbonate sediments cover thousands of square miles and are locally more may have taken place by reflux action seaward from than a thousand feet thick-fall into a regional cate­ the mudflats as suggested by Adams and Rhodes (1960) gory. or by capillary transfer through the sediments as It has long been known that the amount of regional suggested by Friedman and Sanders (1967). Another dolomite increases downward through the geologic sys­ theory suggests that drainage from the landmass on tems from Holocene to Precambrian (Fairbridge, 1957) the east introduced additional magnesium as well as and (Daly, 1907, 1909). The Silurian is one of the sys­ calcium to become available for the additive process of tems containing much continuous regional dolomite in dolomitization. Movement of the carbonate-bearing North America, as in the Great Basin and the conti­ ground water upward through the nearshore mud­ nental interior. banks probably was facilitated by evaporation across Ulrich and Schuchert (1902) were perhaps the first the banks and the subsequent pumping action created to note that certain extensive regional dolomites bor­ by this system (Hsii and Siegenthaler, 1969, and Hsii der upon ancient shorelines. In the Great Basin Silu- and Schneider, 1973). Westward toward the boundary THE ROBERTS MOUNTAINS, NEVADA 7

zone between the eastern dolomite belt and the inter­ are known over a distance of about 325 miles (525 km) mediate limestone belt, the magnesium calcium ratio in a south-southwesterly direction from the Tuscarora became too low for dolomitization because of the ab­ Mountains of Elko County, northern Nevada, to the sence of shallow water evaporative sites and less ex­ northern Inyo Mountains, southeastern California tensive tidal mudflats. (fig. 1 ). The width of the slightly sinuous limestone It is possible that the organic community that popu­ band can only be estimated, but in most places, it is lated the nearshore fiats (now the dolomitic belt) pro­ more than 50 miles (80 km) wide and may exceed 90 duced proportionally more high-magnesium calcite or miles (145 km) before passing entirely into the west­ even some dolomite by their biochemical processes, ern, predominantly shale-graywacke facies. Whereas thereby fixing a large amount of magnesium into the the west boundary is inferred at this time because of system. In slightly deeper water with greater circula­ lack of exposures and subsurface geologic data, the tion, a different assemblage of organisms produced east boundary can be delimited within much narrower mainly calcite, and limestone was the carbonate depos­ limits of error in many places, as, for example, at ited. Sediments in this region were not dolomitized be­ Roberts Creek Mountain, Bare Mountain near Beatty, cause magnesium was not available. Nev., and the Inyo Mountains, Calif., where the lime­ Concentration of brine magnesium carbonate by stones and dolomites either intertongue, are interbed­ evaporation at low tide on mudflats as a major factor in ded, or crop out within a few miles of each other. the dolomitization process has been studied by a The intermediate limestone belt has been mapped and number of investigators under a theory involving su­ studied in detail in the Roberts Creek Mountains, the . pratidal mudflats to which marine waters are added northern Simpson Park Mountains and Cortez Moun­ only occasionally during exceptionally high springtides tains, the Copenhagen Canyon area of the Monitor and heavy storms. Greater concentration of magne­ Range, the northern Toquima Range, and the northern sium in the brines comes about by evaporation and by Inyo Range. Outcrop areas where the geologic mapping the solutions percolating downward and seaward (re­ is of a reconnaissance nature, or where the strati­ flux) to bring about metasomatic dolomitization of un­ graphic work has been confined to section measure­ derlying carbonate muds and sands already containing ment, are the Bootstrap mine area of the Tuscarora magnesium in organically secreted calcite matrices. In Mountains, Swales Mountain in the Independence such evaporative environments, gypsum and other Range, the Carlin Mine and Maggie Creek area in the salines should also be represented. Within the past 10 Tuscarora Mountains, parts of the northern Monitor years, penecontemporaneous dolomites attributed to Range, and the Wall and Pablo Canyon area of the supratidal environments have been found in the Per­ Toiyabe Range. A number of widely separated outcrop sian Gulf (Illing and others, 1965, Hsii and Schneider, areas of these limestones have been identified and sec­ 1973, DeGroot, 1973), in the Netherlands Antilles tions measured, but additional mapping or strati­ (Deffeyes and others, 1965). These discoveries illus­ graphic paleontology is needed. These areas include trate the local hydraulics and geochemistry of contem­ the Mount Callaghan area of the Toiyabe Range; the porary dolomite formation on a fairly large scale, but Ravenswood area of the Shoshone Range; areas near they do not appear to provide the entire key to resolv­ Reeds and Dry Canyons, Toiyabe Range; Dobbin ing the problem of vast regional dolomites like those of Summit, Monitor Range; the Northumberland area of the Great Basin Silurian. the Toquima Range; Bare Mountain area near Beatty; In reality, dolomitization on a regional scale as in and occurrences north of Wells near Antelope Peak. A the eastern Great Basin is probably the product of all comprehensive documented stratigraphic summary of the mechanisms suggested. Addition of magnesium these localities and others is presented in T. E. Mullens concentrated by evaporative processes on the inshore (unpub. data, 1975). tidal fiats, magnesium-rich solute introduced from the land drainage as well as crystalline dolomite clastic ROBERTS MOUNTAINS FORMATION OF debris introduced from exposed and eroding dolomite THE ROBERTS MOUNTAINS, NEV. formations (for example, the Ordovician Bighorn In the Roberts Mountains, the Roberts Mountains Dolomite), and redistribution of magnesium taken Formation is exposed over a large area on the west and from sea water by organisms and deposited as high northwest side of Roberts Creek Mountain, the type magnesium calcite must all be significant factors. area of the formation. Other outcrops occur near the north edge of the range between Willow aand Birch GEOGRAPHIC DISTRIBUTION OF Creeks. SILURIAN LIMESTONES The Roberts Mountains Formation in its type area Strata of the Silurian intermediate limestone belt includes a variety of dark-gray to bluish-gray lime- 8 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY

EXPLANATION I IDAHO Roberts Mountains Formation and correlative formations 42" ----'------+ I Fossil localities of stratigraphic importan:e with regard to the Limestone / Roberts Mountains Formation 1 Western Belt +2J/ Graywacke 1. Roberts Mountains Formation, type section. Roberts Creek Mountain, Roberts Mountains. I Shale 2. Roberts Mountains Formation, reference section. Coal 1 Belt Canyon, northern Simpson Park Range. 3. Roberts Mountains Formation. North pediment slopes of northern Simpson Park Range. 4. Roberts Mountains Formation. Cortez Mountains. S. Roberts Mountains Formation. Goat Peak, Shoshone Mountains. 6. Roberts Mountains Formation. Dry Creek area, Toiyabe Range. 7. Roberts Mountains Formation. Petes Canyon window, Toquima Range. I l~ z c::: I 1 71• Lone Mountain 8. Roberts Mountains Formation and Tor Limestone. June ~ .-,! / 21 ~~ureka >I> Canyon sequence. lkes Canyon window, Toquima Range. .lAustin 14 +2~xyoke Canyon ~,::c 9. Roberts Mountains Formation and McMonnigal Limestone. I .t·6 .7 ~~I OEiy Ibex Mill Canyon sequence. Ikes Canyon window, Toquima Range. 28 I 11fes1o~ Rab~it Hill Hills 10. August Canyon sequence. Gatecliff Formation of Kay and I Canyon•• 1 Dobbin Summit I Crawford (1964). lkes Canyon window, Toquima Range. 11. Roberts Mountains Formation. Northumberland window, : •• 2 tf Toquima Range. j"anhattan 0 I Eastern 1 2. Roberts Mountains Formation. Pablo Canyon area, Toiyabe Range. I 1 Dolomite 1 3. Roberts Mountains Formation. Clear Creek and Dobbin Summit area, Monitor Range. 0 14. Roberts Mountains Formation. Brock Canyon area, Monitor Range. ~a ic'~ft~~~~~~~.t+-----~T~o~n~o~p=ah~~~--~--B~e~lt~~~~~~~j----~ 1 S. Roberts Mountains Formation. Bowman Creek area, Toiyabe Range. Goldfield 1 6. Roberts Mountains Formation. Point of Rocks and Straight 0 Canyons area, Toiyabe Range. Pahranagat 1 7. Roberts Mountains Formation. Ravenswood area, Shoshone Range Mountains. ; 1 8. Roberts Mountains Formation. Callaghan window, Toiyabe Range. iL c 19-20. Beacon Peak Dolomite Member of Nevada Formation. Southern part e of Sulphur Spring Range. ii 2 1. Lone Mountain Dolomite, type section. Lone Mountain. c 2 2. Lone Mountain Dolomite. Mahogany Hills. f 1£. 2 3. Lone Mountain Dolomite. Southern part of Fish Creek Range. 24. Beacon Peak Dolomite Member of Nevada Formation. Oxyoke Canyon, Mahogany Hills. 360 2 S. Late Silurian rugose corals in limestone. Antelope Peak, Snake Mountains. 2 6. Roberts Mountains Formation, reference section. Bootstrap Hill, southern part of Tuscarora Mountains. 2 7. Roberts Mountains Formation. Maggie Creek area, southern Bakersfield 0 part of Tuscarora Mountains. 2 8. Rabbit Hill Limestone, type section; Roberts Mountains 08sert !1110"11118 Formation. Copenhagen Canyon area, Monitor Range. 29. Vaughn Gulch Limestone, type area. Northern part of Inyo 12o" 118° 116" 114° Mountains. 3 0. Sunday Canyon Formation. Barrel Spring area, northern 0 25 50 75 100MILES lnyo Mountains. I I I 31. Lone Mountain Dolomite above dark-gray limestone and I I I I I 0 40 80 120 160 KILOMETRES dolomite probably correlative with Roberts Mountains Formation. Bare Mountain area.

FIGURE 1.-Part of Great Basin showing distribution of the Roberts Mountains Formation and correlative formations in the intermediate limestone belt and fossil localities of stratigraphic importance with regard to the Roberts Mountains Formation. Approximate limits of the Late Silurian-Early Devonian depositional facies belts are shown by heavy lines. stones that are mostly thin bedded and weather in a places, the platy and laminated fine-grained beds con­ platy to flaggy fashion. The basal part of the formation tain abundant silt-size quartz granules. Coarse deposi­ is thin-bedded blue-gray limestone with black chert tional limestone breccias or flat-cobble mud breccias interbeds and tabular chert nodules. Above this in the with limestone matrix are present as lenses in the lower part of the formation is fine-textured thinly upper thicker bedded sequences. laminated limestone and calcareous shale with a few Coarse-grained light-gray dolomitic limestone and thin interbeds of coarse limestone. In the middle and dolomite occur as tongues and interbeds in the upper upper parts of the formation, there is a change to pre­ part of the formation; these dolomitic rocks can be dominantly coarser slabby-weathering blocky beds, a traced laterally into a continuous dolomite, called Lone few centimetres to about 1 m thick, with partings of Mountain Dolomite. Silicified fossils are characteristic shale or laminated limestone. Crinoidal debris is throughout the formation, especially 1n dolomitic abundant in these coarser beds, some of which are limestones. crinoidal bioclastic lenses containing abundant corals At the type area, the Roberts Mountains Formation and brachiopods. As noted by Winterer and Murphy is overlain by the Lone Mountain Dolomite. At some (1960, p. 123-129), some of the thicker beds are fossil­ other places, it grades into sequences of laminated rich calcarenite locall:y revealing graded bedding. In limestone interbedded with thicker bedded clastic THE ROBERTS MOUNTAINS, NEVADA 9 limestones that have been named the McMonnigal shaly partings yield graptolites. The basal chert, rest­ Limestone (Kay, 1960, Kay and Crawford, 1964, and ing upon the Hanson Creek Formation, which also is McKee and others, 1972), the Windmill Limestone cherty in many places, is a widely recognized unit of (Johnson, 1965, Matti and others, 1975), and the Wen­ the Great Basin Silurian. In some other areas it has ban Limestone of Gilluly and Masursky (1965). yielded large pentameroid brachiopods. Unit 2.-Unit 2, thickest of the three units in the TYPE SECTION type section of the Roberts Mountains Formation, is The type section of the Roberts Mountains Forma­ made up of platy to flaggy dark bluish-gray fine to tion is on the northwest side of Roberts Creek Moun­ fairly coarse textured impure limestones with inter­ tain between the south and middle forks of upper Pete beds of bioclastic limestone as much as 1 m thick. Cal­ Hanson Creek. The formation is about 2,000 feet careous shaly partings and thin laminated layers sepa­ (600 m) thick in the type section, where it occupies the rate the thicker beds, many of which are highly fos­ stratigraphic interval between the Hanson Creek siliferous (bioclastic); much of this debris is crinoidal. Formation and overlying dolomites classified here as The introduction of the thicker, rather coarsely clastic the Lone Mountain Dolomite. It is here about 70 per­ crinoidal beds distinguishes unit 2 from unit 1. Scat­ cent limestone, with a prominent cherty member at the tered black chert lenses and nodules are present as base and dolomitic limestone and dolomite interbeds or high as about the middle of unit 2. In the upper part of tongues in the upper 200 m. the unit are a few light-gray recrystallized layers that The Roberts Mountains Formation consists of well­ are slightly dolomitic. Tabulate corals, pentameroids, bedded laminated dark-gray to slightly bluish-gray, and other brachiopods are abundant in the many bio­ platy, flaggy, and slabby limestone, dark-gray graded clastic beds; the rugose corals are subordinate. bioclastic limestone, calcareous shale, and subordinate Unit 3.-Unit 3 is gradational with unit 2 through black chert in the basal part of the formation. Most of an interval wherein coarser textured crinoidal layers the chert, shale, and laminated to shaly limestones containing abundant Conchidium grade upward into a occur in the lower part of the formation; the beds be­ more uniformly thicker bedded sequence of light and come on the whole thicker, coarser textured, and more dark-gray, somewhat blocky-weathering limestones bioclastic upward, with fewer of the laminated interca­ that include coralline beds. About 7 5 m above its base, lations. Fossils are abundant in many horizons above unit 3 becomes prevailingly lighter gray and less well the cherty interval, those in the laminated or shaly bedded as it passes upward into the interbedded lime­ rocks are mainly graptolites. stone and dolomite of the upper half. The lower coral­ In the type section, differences in lithology and line beds contain large heads of colonial Rugosa of paleontology make feasible a three-unit subdivision of Great Basin Silurian coral zone C. Light-gray-weath­ the Roberts Mountains Formation as unit 1, 100 m; ering limestones in the upper middle part of unit 3 unit 2, 300 m; and unit 3, 200 m, in ascending strati­ contain abundant silicified corals of Great Basin Silu­ graphic order. rian coral zone D. Through the upper 122m, bedding is Unit I.-Unit 1, a fine-textured cherty unit, is made poorly defined, the weathering more blocky, and fossils up of a basal chert and dolomite 1-3 m thick overlain become fewer as the mixed carbonate rocks pass up­ by fine-grained thin-bedded laminated dark-gray lime­ ward into the coarser saccharoidal dolomite of the stone weathering platy and flaggy with shaly partings. overlying Lone Mountain Dolomite. The limestones weather light gray. The laminae usu­ ally reveal an alternation of calcareous and argilla­ STRATIGRAPHIC RELATIONS TO UNDERLYING ceous or silty layers; in places, millimetre-thick limy AND OVERLYING FORMATIONS layers alternate with layers containing much dark­ Careful examination of the contact separating the colored matter probably composed of iron oxides with basal chert of unit 1 from the underlying Hanson organic substances (Winterer and Murphy, 1960, p. Creek Formation has disclosed equivocal evidence of ·123). The laminated strata include dark chert layers disconformi ty or angular discordance in the type sec­ and nodules elongate parallel to bedding. The distinc­ tion. Fossil evidence elsewhere, as in the Tuscarora tive bluish-black chert-bearing interval of varying Mountains (Berry and Roen, 1963), also suggests that thickness less than 3 m at the bottom of the formation, some of the earliest Silurian may not be represented at is about 75 percent chert that contains subordinate un­ or near the contact and hence a paraconformity exists chertified and undolomitized limestone lenses. Upward in this part of the section. the chert decreases in amount, occurring in discontinu­ The upper limit of the Roberts Mountains Formation ous 1-5 em layers separated by laminated limestone. in its type section is gradational with the Lone Moun­ Fossils are scarce in the laminated limestone, but tain Dolomite (Lone Mountain unit 2 at Lone Moun- 10 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY

tain as redefined by Merriam in 1973). About 500 m of Fossils of early Middle Silurian (Great Basin Silu­ blocky dolomite between Roberts Mountains Forma­ rian coral zone B) were expected in the middle to upper tion unit 3 and richly fossiliferous lower beds of the beds of unit 2 but have not been found here. Rugose Devonian Nevada Formation (or the McColley Canyon coral genera present are Tryplasma, Palaeophyllum, Formation of Murphy and Gronberg, 1970) yielded no Diplophyllum, and Microplasma. Tabulate coral gen­ fossils. Future study of the Lone Mountain Dolomite at era in these beds are Aulopora, Cladopora, Heliolites, Roberts Creek Mountain may show that the part of it and Halysites. In the uppermost beds of unit 2, correlates with part of the finer textured Beacon Peak brachiopods are abundant, especially medium-sized Dolomite Member of the Nevada Formation of the Conchidium-like pentameroids, some of which have Eureka district. fine radial costae, others radial costae of medi urn strength. Of the latter, one species (pl. 12, figs. 24--26) FACIES CHA~GES FROM LIMESTONE TO DOLOMITE resembles the Norwegian Conchidium milnsteri Kiaer A planimetric map of the upper Pete Hanson Creek as described and figured by St. Joseph (1938). This area by Winterer and Murphy (1960, p. 120, fig. 2) that species, reported in southern Norway Silurian zone 5b, includes the type section of the Roberts Mountains St. Joseph considered to be Early Silurian (Llandove­ Formation illustrates the intertonguing relation of rian). Other brachiopods abundant in higher beds of unit 3 limestone to dolomitic beds of the Lone Moun­ Roberts Mountains unit 2 are the small Coelospira sp. tain Dolomite. Dolomitic limestones of these unit-3 and Ptychopleurella sp. tongues contain a fa una more closely related to that of FOSSILS OF UNIT 3 the Roberts Mountains limestone facies than to that of the Lone Mountain dolomite facies as known in the Unit 3 of the Roberts Mountains Formation includes Eureka area (Merriam, 1973a, b). It is in Roberts two Great Basin Silurian coral zones, C and D, and is Mountains unit 3 of the type area that we find the most the reference section for both. Coral zone C lies about clearly defined marginal interfingering of the inter­ 10 m above the base of unit 3 and has yielded only mediate limestone belt to the eastern dolomite belt in corals, of which the following Rugosa are diagnostic: the Great Basin province. Entelophylloides (Prohexagonaria) occidentalis Merriam STRATIGRAPHIC PALEONTOLOGY OF THE TYPE SECTION Tryplasma newfarmeri Merriam Denayphyllum denayensis Merriam The reference section for two of the five proposed Coral zone D lies about 90 m above the base and con­ Great Basin Silurian coral zones (Merriam, 1973a) oc­ tains the following diagnostic Rugosa: curs in the type section of the Roberts Mountains For­ Stylopleura berthiaumi Merriam mation, Silurian coral zones C and D in lithologic unit Tonkinaria simpsoni Merriam 3 of the formation. The other Great Basin Silurian Tryplasma duncanae Merriam coral zones have not been recognized in this section. Tabulate corals, dasycladacean algae, and brachiopods were abundant here in a diverse biota. The following FOSSILS OF UNITS 1 AND 2 are among the common associated fossils of coral Poorly preserved Monograptus sp. with plain tubular zoneD: thecae occur in unit 1 and the lowermost part of unit 2 Verticillopora cf. V. annulata Rezak (W. B. N. Berry, written commun., 1964). Fossils pres­ Dicoelosia sp. r ent in these lower beds are: Gypidula sp. r Massive favositids Small Conchidium-like Homoeospira sp. r Cladopora sp. pentameroids Kozlowskiellina sp. f Heliolites sp. Dicoelosia sp. Atrypa sp. Halysites sp. Eatonia? sp. Orthophyllum sp. Merista? sp. A(:f: OF THE ROBERTS MOUNTAINS FORMATION Pycnostylid Rugosa Atrypa sp. OF THE TYPE SECTION Monograptus sp. In the time-stratigraphic sense, the greater part, but The characteristic Lower Silurian rugose corals of not all, of the Silurian System as well as the lower part Great Basin Silurian coral zone A which would be ex­ of the Devonian System, is believed to be recorded in pected in these lower beds have not been found here; the 600 m of Roberts Mountains Formation of its type those absent include the genera Palaeocyclus, Dal­ section. Fossil evidence indicating the early part of manophyllum, Arachnophyllum, and Cyathophyl­ Early Silurian (Llandoverian) is lacking; the rest of loides. the system is present and is represented by graptolites, THE NORTHERN SIMPSON PARK MOUNTAINS 11 conodonts, and shelly fossils of Silurian age. The upper Grouse Creek and Red Hill at the northeast tip is cov­ part of the formation is Lower Devonian in age (Klap­ ered in this report. per and Murphy, 1974; Berry and Murphy, 1975). Between Grouse Creek and Red Hill, a distance of The basal chert of the Roberts Mountains Formation about 11 km, the Paleozoic rocks are exposed in two (bottom of unit 1 ), present in the type section and most areas: the Coal Canyon area (fig. 2) on the west and the other Great Basin Silurian carbonate sections of both Red Hill area on the east. Between these areas is a limestone and dolomite, seems to be a dependable wide band of volcanic rock and gravel cover. Over­ marker unit. That it does transgress hypothetical time thrust chert and shale of the Ordovician Vinini Forma­ planes from place to place has been suggested by F. G. tion crop out extensively in the Fye Canyon area on the Poole (written commun., 1971), and his geologic evi­ southwest; Pennsylvanian or Permian conglomerate dence is convincing. It has also been suggested that and limestone is found within the area and discon­ there may be two or more chert sequences of different formably overlies the older Paleozoic strata. ages at this place in the stratigraphic column of the Between Grouse Creek and Red Hill, the Paleozoic Great Basin (Jonathan C. Matti, written commun., beds dip 20° to about 60° E.; the steeper dips are in the 1974). This possibility remains to be demonstrated. vicinity of faults. Most faults have a northerly strike; The Silurian age of the basal chert has been estab­ in some places, faulting has caused an apparent thick­ lished in several widely separated localities. In the ening of the stratigraphic section. In general, the beds northern Monitor Range at Copenhagen Canyon, the become progressively younger eastward; the oldest ex­ basal chert contains fairly large pentameroid brach­ posed rocks are the uppermost beds of the Hanson iopods of Silurian character. At Mill Canyon, Toquima Creek Formation on the west in a minor canyon half a Range, a coral assemblage beneath the chert contains mile east of the mouth of Grouse Creek Canyon. Cladopora? sp., three species of Favosites, Halysites (or Nearly all the Silurian and Lower Devonian section is Cystihalysites ), ~~cystiphyllum" sp., and Brachyelasma exposed at Pine Hill and Pyramid Hill on the west. sp., a fauna suggesting a Silurian age. Graptolites from the chert in the central Toquima Range were COAL CANYON AREA identified by W. B. N. Berry as Climacograptus scalaris Pine Hill on the west side of Coal Canyon is under­ and Climacograptus cf. C. med~us; these species suggest lain by a nearly continuous east-dipping section of an Early Silurian (Early Llandovery) age. As shown by Roberts Mountains Formation truncated on the east by Berry and Roen (1963), the beds in the southern Tus­ the Coal Canyon fault (fig. 2). On the opposite or east carora Mountains that are a few feet above the basal side of Coal Canyon, Pyramid Hill is largely Rabbit chert marker contain graptolites of the Monograptus Hill Limestone of Early Devonian age under lain along riccartonensis Zone of early Wenlockian age. the lower east slopes of Coal Canyon by the uppermost Roberts Mountains unit 2 and the lower part of unit beds of the Roberts Mountains Formation. The upper 3 up to and including Great Basin Silurian coral zone C part of the Roberts Mountains Formation here was are provisionally classified as Middle Silurian (Wen­ named the Windmill Limestone by Johnson (1965). lock). The lower and middle beds of unit 3, including This unit of finely laminated fine-grained limestone coral zone D, are considered to be :Date Silurian (Lud­ and thin- to medium-bedded clastic limestones is a rec­ lovian); the higher Ludlovian faunas of coral zone E ognizable formation at a number of places elsewhere in have not been found at the type section. The graptolite the region. succession on which the currently accepted Silurian­ At places along the west slope of Coal Canyon, the Devonian boundary is based indicates that the upper Coal Canyon fault is marked by a north-south zone of part of unit 3 is Lower Devonian. shearing and brecciation about 76 m wide. Within and adjacent to the fault zone, the east bedding dips are ROBERTS MOUNTAINS FORMATION OF THE .steep and locally reversed to the west. The fault strikes NORTHERN SIMPSON PARK MOUNTAINS about N. 5o W.; although its overall dip was not de­ termined by field observation, it is inferred to be a GEOLOGIC SETTING high-angle normal fault with a possible steep east dip. Silurian and Devonian strata are exposed along the To the south, the Coal Canyon fault passes beneath lower north slopes of the Simpson Park Mountains volcanic cover. (Horse Creek Valley quadrangle, Nevada), most of COAL CANYON .FAULT ZONE which are blanketed by andesite and basalt flows. In some parts of the foothill area, coarse gravel and Continuous measurement of the stratigraphic sec­ fanglomerate obscure the Paleozoic strata. Only that tion across Coal Canyon from Pine Hill through part of the lower north slope of the range between Pyramid Hill must take into consideration a north- 12 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY

116°38'40" 40°30' .------, EXPLANATION

Volcanic rocks COAL CANYON FAULT }~

Rabbit Hill Limestone }I

A' Robert•M~Fonnation} ~ ~ Sl > Hanson Creek Formation } 8 ~ 0

A Contact 40°2'19' Geology mapped by C. w. Merriam, 1970-74, N assisted by J. W. Miller, 1969-70 ~-- 0 Fault Dashed where uncertain, t U, upthrown side; D, downthrown side 0 1000 2000 3000 4000 5000FEET \22 0 500 1000 1500 METRES Strike and dip of beds M1334 CONTOUR INTERVAL 200 FEET ® A A' Fossil locality 7500' See locality register

7000' Approximate position of beds with Monograptus praehercynicus

6500' 2 Approximate position of beds with Carlinastraea and Stylopleura 6000' FIGURE 2.-The Coal Canyon area, northern Simpson Park Mountains, showing relations of the Roberts Mountains Formation in the vicinity of the Coal Canyon fault. south fault break of undetermined magnitude. The are fault drag features suggesting either downward Coal Canyon fault zone follows the steep west wall of movement of the east or Pyramid Hill block or west­ the canyon for a distance of 800 m. It is best shown on to-east displacement of strata in the west or Pine Hill the main northeast spur of Pine Hill, where its north­ block on a low-angle thrust fault. The stratigraphic south trace is marked by a coarse calcite-impregnated sections on the two sides of the canyon, the Pine Hill limestone cataclastic breccia some 76 m wide. Minor block on the west, the Pyramid Hill block on the east, surfaces disclosing dip and direction of displacement are dealt with separately. were not found at the margins of this coarse breccia. The general trend of the breccia mass suggests that it PHYSICAL STRATIGRAPHY OF THE PINE HILL SECTION continues southward, mostly beneath talus, and passes A nearly continuous section of Roberts Mountains beneath a volcanic inlier in upper Coal Canyon basin Formation is exposed in Pine Hill west of the Coal without observed displacement of the volcanic body. Canyon fault. Only the uppermost beds of this section The fault appears to have had a steep dip, but the di­ have been disturbed within the Coal Canyon fault rection of inclination has not been determined. It is zone. The Roberts Mountains Formation in the Pine probably a high-angle normal fault, although it might Hill block is about 450 m thick and is here considered conceivably be a thrust fault subsidiary to a fault in as having two lithologic units. upper Coal Canyon herein named the Rocky Hills The lower Roberts Mountains unit of Pine Hill is thrust. Mapping in the Pine Hill block reveals a about 330m thick; the base is black chert, 1-2m thick, steepening of bedding dip as the Coal Canyon fault resting with sharp contact upon thick-bedded light­ zone is approached from the west. The steepened dips gray-weathering Hanson Creek dolomite and lime- THE NORTHERN SIMPSON PARK MOUNTAINS 13 stone. The succeeding Roberts Mountains consists of Coiled graptolites in beds a few feet above the basal laminated thin-bedded platy and flaggy-weathering chert are reported as late Early Silurian (late Llan­ dark-gray impure limestone with intercalations of cal­ doverian). Graptolite assemblages near the top of the careous shale or calcareous siltstone. There are a few lower 330-m unit include Monograptus praehercynicus bioclastic limestone interbeds 5 or 6 em thick contain­ Jaeger. ing brachiopods and other fossils. Graptolites are fairly Some limonitic silty limestone and siltstone beds common in the laminated limestone and shaly units. In about 140 m stratigraphically above the base of the the lower 30 m above the basal chert, there are a few Roberts Mountains Formation contain a graptolite as­ thin black chert bands and chert nodules. Lamination semblage reported upon by W. B. N. Berry (written is a characteristic feature of some of the thin limy beds commun., Oct. 16, 1972) as: as in other areas of Roberts Mountains outcrop. Lobograptus '"scanicus" In the lower part of the lower 330-m interval, bio­ Monograptus bohemicus? clastic limestone beds are few. A l-inch, slightly Monograptus colon us bluish-gray fine-grained limestone is crowded with Monograptus sp. (of theM. dubius type) small silicified corals and brachiopods; another thin The age of this horizon is considered by Berry to be bed with unsilicified fossils yielded a smooth medium­ early Ludlovian. Such a low position for early Ludlo­ sized Pentamerus-like brachiopod. vian, some 310m below the top of this section, suggests About 140 m above the formation base is a limonite that the Wen lockian part of the formation is relatively brown or reddish-stained silty sandstone bed contain­ thin compared with the Ludlovian. ing abundant graptolites. The upper 120-m lithologic unit of the Roberts The upper 120-m unit of the Roberts Mountains Mountains Formation at Pine Hill (the type Windmill Formation in the Pine Hill block is an alternating Limestone of Johnson, 1965) is primarily thick-bedded thick and thinly bedded fairly coarse-grained dark­ dark-bluish-gray medium- to fairly coarse grained bluish-gray limestone with calcareous shaly and silty bioclastic limestone with thin-bedded limestone and intercalations. The lithologic change from the lower calcareous shaly intercalations. The coarse-grained unit to the upper unit is distinct, but not abrupt. Most beds are made up of clastic debris, and most are of the thicker limestone beds are bioclastic and contain graded. This unit forms prominent outcrops and the well-preserved corals and other fossils; the most abun­ change from the platy and laminated limestones of the dant are massiveFavosites colonies. About 15m of this lower division is gradual. As described by Berry and limestone in the upper half of the interval, which car­ Mullens (written communs., 1968), the Monograptus ries a varied fauna of rugose corals and brachiopods, is praehercynicus beds must lie near the base of this referred to as the Carlinastraea beds. Thick beds of upper unit. Corals and brachiopods, especially corals, depositional breccia occur in the upper part of the are abundant in the upper unit; the most abundant Roberts Mountains Formation on the east side of Coal coral is the large, massiveFavosites. A zone about 20m Canyon in the Toquimaphyllum beds of the Pyramid thick above the middle of this unit has yielded. the Hill section. following fossils: The upper 120-m unit of the formation in the Pine Favosites sp. (massive) Hill block is terminated on the east by the Coal Can­ C ladopora sp. yon fault. It is possible that the total thickness of this Alveolites sp. unit in unfaulted sections is much greater than 120m. Syringopora sp. Striatopora? sp. STRATIGRAPHIC PALEOl\TOLOGY AND AGE OF Rhegmaphyllum? sp. ROBERTS MOU~T AINS FORMATION AT PINE HILL Brachyelasma sp. c Fossils collected through the 450 m of Roberts Moun­ Tryplasma sp., cf. T. duncanae (fragmentary) tains Formation in the Pine Hill block show the age to Tonkinaria simpsoni Merriam range from Early Silurian (late Llandoverian) for the Stylopleura sp. c, cf. S. berthiaumi Merriam lower beds to the Lower Devonian (Gedinnian) for the Carlinastraea tuscaroraensis Merriam upper 100m or so of the formation. This range in age is Gypidula sp. assigned on the basis of graptolites identified by Isorthis sp. (ventral valve) W. B. N. Berry (written commun., Nov. 8, 1968). The Small dalmanellid brachiopod (dorsal valve) upper 120-m unit contains abundant corals in the more This assemblage containing Tryplasma, Tonkinaria, massive beds; Carlinastraea, which is especially abun­ and Stylopleura suggests that of Great Basin Silurian dant, is considered representative of the Great Basin coral zone D at the Roberts Mountains Formation type Silurian coral zone E. section, although its most distinctive coral, Carlina- 14 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY straea, was not found at the type section. The species of is covered by talus mantle that is the source of weath­ Carlinastraea found at Pine Hill occurs in the upper­ ered fossil material collected here. Some of the float most fossil bed at Bootstrap Hill in the Tuscarora fossils, like many large Rugosa, however, are traceable Mountains and at locality M1446 on the pediment to their source beds. slope north of Red Hill in the Simpson Park Moun­ The uppermost Roberts Mountains Formation of the tains where it is associated with Australophyllum (To­ Pyramid Hill is made up of fine-textured dark- to quimaphyllum) johnsoni Merriam and Stylopleura. medium-dark-gray laminated platy to fl.aggy lime­ Such an association, in particular with To­ stone, and thick-bedded (2 cm-2 m) coarse slightly quimaphyllum, suggests that there is no profound bluish-gray bioclastic limestones, most of which are stratigraphic separation between the Carlinastraea graded. Some of the more massive beds are lenticular, beds at Pine Hill and those of Great Basin Silurian and locally they form prominent cliff exposures. The coral zone E on the east side of Coal Canyon (Pyramid bioclastic beds, which are composed in considerable Hill block), in which Australophyllum (Toquimaphyl­ part of crinoidal debris, also contain abundant corals, lum) johnsoni is the most distinctive rugose coral. Lit­ brachiopods, and other fossils. In some places all fossils tle is known of the stratigraphic range of any of these are silicified and weather out a limonite brown color. seemingly distinctive Rugosa. The thicker bedded limestones above the middle of the FOSSILS COLLECTED WITHIN THE unit include numerous lenticular bodies of coarse­ COAL CANYON FAULT ZONE textured depositional limestone breccia in which some clasts are more than a foot in greatest diameter and Several important fossil occurrences are within dis­ range in shape from subangular to rounded. In some of turbed rocks of the Coal Canyon fault zone, and be­ the beds the corals present are surrounded by a cause of this, their true stratigraphic position remains coarse-grained and fine-grained limestone matrix; uncertain. Among the fossils are the compact aseptate some beds are graded and of uniform thickness. Above pycnostylid Stylopleura? sp. c (pl. 2, figs. 1 and 2) from the interval of thicker bedded coral-rich limestone and locality M1379, Australophyllum sp. c (pl. 8, figs. 3-7), depositional breccia, the Roberts Mountains Forma­ from locality M1318 and Verticillopora annulata (pl. tion becomes thinner bedded in about its uppermost 23 10, figs. 1-3) from locality M1411. From their positions m, approaching the transition interval into overlying in the fault zone, it appears probable that all these Rabbit Hill Limestone. In general, the lower part of the fossils came from horizons stratigraphically higher Rabbit Hill is much like the platy thin-bedded Roberts than the Carlinastraea beds. Mountains beds, but coarser textured bioclastic beds of the Rabbit Hill lack the bluish cast of such fossil-rich PHYSICAL STRATIGRAPHY OF THE limestone beds in the Roberts Mountains. Tentaculites PYRAMID HILL SECTION is common in the lower part of the Rabbit Hill Lime­ In Pyramid Hill, east of the Coal Canyon fault zone, stone. The characteristic Leptocoelia of the Rabbit Hill about 400 m of east-dipping limestone and thinly Limestone becomes abundant in Leptocoelia beds about bedded calcareous shaly and silty deposits include the 100 m stratigraphically above the base of the Rabbit uppermost part of the Roberts Mountains Formation Hill as mapped. The dark-gray platy limestones and (the Windmill Limestone of Johnson, 1965) and the calcareous shale and silty beds of the Rabbit Hill com­ overlying Rabbit Hill Limestone. About 150 m of the monly weather very light gray. Roberts Mountains Formation is present here. The In the upper part of the Roberts Mountains Forma­ Rabbit Hill underlying most of Pyramid Hill continues tion of the Pyramid Hill section, the coral-rich more eastward. As the depositional contact between the two massive beds with depositional limestone breccia are formations is gradational, placement is arbitrary as referred to as the ~~upper coral zone with Toquimaphyl­ understood at this time. The paleontologic change in lum." On the west side of Coal Canyon in the Pine Hill corals, however, is profound with complete disappear­ block, the coral-rich beds are distinguished as the ance upward of all Rugosa of Silurian affinities and '~lower coral zone with Carlinastraea." The upper coral Rugosa classified previously as Silurian types (Mer­ zone with Toquimaphyllum is the primary reference riam, 1973a). Only the small solitary variety Syringa­ section for Great Basin Silurian coral zone E of Mer­ xon is present in the Rabbit Hill Devonian of this sec­ riam (1973a). These rocks are Lower Devonian in age tion. based on graptolites. Within the Roberts Mountains Formation along the All Pyramid Hill fossils dealt with in this report steep lower east slopes of Coal Canyon are bold mas­ came from a stratigraphic interval of about 50 m in the sive outcrops of the more thickly bedded lenticular and middle and upper parts of the Roberts Mountains For­ locally fossil-rich limestones. Much of this lower slope mation on the east side of Coal Canyon. This interval TUSCARORA MOUNTAINS 15 includes the thicker bedded coral-rich limestone and A float specimen from Coal Canyon assigned to the allogenic depositional limestone breccia. Some of the genus Salairophyllum, first reported from the Ural fossil material came directly from the bold outcrops in Mountains of the U.S.S.R., was probably derived from place, but much of it was collected on the talus below the higher Roberts Mountains limestone. In southeast­ the bold exposures. The common fossils of this 50 m ern Alaska, Salairophyllum occurs in the Late Silurian interval are: limestones (Merriam, 1972) associated with Con­ Stromatoporoids chidium alaskensis, an index fossil of the Ludlovian Favosites sp. (massive) Stage in Alaska. Cladopora sp. Verticillopora, the large dasycladacean alga, is Alveolites sp. mostly confined to Silurian rocks of the Great Basin. In Stylopleura nevadensis Merriam the Vaughn Gulch Limestone of the In yo Mountains, it Stylopleura berthiaumi Merriam crosses the line into earliest Devonian at the base of Mucophyllum oliveri Merriam the uppermost Vaughn Gulch limestone unit. Kodonophyllum mulleri Merriam Rhizophyllum cf. R. enorme (Oliver) PROBL£~1 OF THE SILURIAN-DEVONIA~ BOCJ\'DARY Chonophyllum simpsoni Merriam AT PYRAMID HILL Kyphophyllum sp. c A systematic study has been made (Merriam, 1973a) A ustralophyllum (Toquimaphyllum) johnsoni of the rugose corals from thick-bedded Roberts Moun­ Merriam tains limestone of the upper coral zone with To­ Orthostrophia sp. quimaphyllu.m, the reference section of Great Basin Schellwienella? sp. Silurian coral zone E as defined by Merriam (1973a). Barrandella? sp. This coral zone contains distinctive genera and sub­ Sicorhyncha? sp. genera represented in the Silurian of Gotland, Sweden, Rhynchospirina sp. in the Klamath Mountains Silurian of northern Plectatrypa? sp. California (Merriam, 1972), and in the Silurian of Aus­ Atrypa sp. tralia. None of these genera range into the overlying Meristella sp. Rabbit Hill Limestone. Kozlowskiellina sp. f It is significant that no fossils have been collected Verticillopora cf. V. annulata Rezak through a stratigraphic interval of about 100 m from The rugose coral assemblages in the coral-rich lime­ the top of the uppermost Roberts Mountains thick­ stone interval of the Pyramid Hill exposures are of bedded coralline limestone to the lowest fossils of the Silurian character as compared with assemblages of Rabbit Hill Limestone. The contact between the two the Gotland, Sweden, Silurian standard and with those formations as mapped is arbitrary, and from the coral of Silurian strata of the Klamath Mountains, Calif., evidence, with reference to Gotland, Sweden, the and Australia. Silurian-Devonian boundary is discretionary and as­ M ucophyllum oliveri, the most distinctive fossil in sumed to lie somewhere within the 100m interval. this fauna, bears a fairly close resemblance to The rugose coral evidence bearing upon the M ucophyllum of the Silurian Gazelle Formation of the Silurian-Devonian boundary clearly does not agree Klamath Mountains, Calif., and to Silurian Mucophyl­ with that derived from study of the graptolites and lum crateroides (Etheridge) of Australia. Similar cor­ from conodont research. The base of the Monograptus als from Gotland, Sweden, have previously been re­ uniformis Zone, which is accepted as defining the base ferred to Schlotheimophyllum or erroneously to of the Devonian, places the boundary about 250 m Chonophyllum. Kodonophyllum is represented in the lower in the section in the Pine Hill block on the west Gotland section, as is the slipper coral Rhizophyllum, side of Coal Canyon. To clearly resolve this type of which does, however, pass upward into the Early De­ boundary problem will require the cooperative effort of vonian of eastern Europe. Kyphophyllum is a Silurian specialists dealing with all the fossil groups repre­ genus in Gotland but may range into earliest Devonian sented in these rocks, emphasizing graptolites, cono­ in the Klamath Mountains. Toquimaphyllum, pro­ donts, brachiopods, and rugose corals. posed by Merriam (1973a) as a subgenus of the long­ ranging Australophyllum in Silurian and Devonian ROBERTS MOUNTAINS FORMATION, rocks, is itself known in the Great Basin in later Silu­ TUSCARORA MOUNTAINS rian beds. The little-known pycnostylid Rugosa, which include Pycnostylus and Stylopleura, are characteristic As a host rock of gold, the Roberts Mountains For­ of the Silurian System. mation has been traced widely in the Tuscarora Moun- 16 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY tains and nearby areas of Elko and northern Eureka PHYSICAL STRATIGRAPHY OF Counties, Nev. Structural complexity is everywhere THE BOOTSTRAP HILL SECTION characteristic of the formation, where, as part of the Measurement of the Roberts Mountains Formation autochthonous group of rocks, it is overridden by grap­ at Bootstrap Hill by T. E. Mullens (unpub. data, 1975) tolitic shales, argillites, and cherts of the Vinini For­ shows two major lithologic divisions totaling about 4 70 mation (Roberts and others, 1958). Complete strati­ m. This section does not include the base of the forma­ graphic sections of the Roberts Mountains Formation tion, which presumably is the black chert unit. The are rare here, but the basal chert is exposed 4 miles lower division, 180m of which is exposed, is made up of (6.4 km) southwest of the Big Six Mine (Berry and platy splitting laminated limestone with thicker inter­ Roen, 1963) and at the gorge on Maggie Creek 8 miles beds of bioclastic and pelletal limestone. Of these (12.8 km) northwest of Carlin. Rocks of the unnamed strata, the fine-textured laminated limestones pre­ Devonian limestone unit of Mullens (Evans and Mul­ dominate. The upper 275 m unit consists of thicker lens, 1976; T. E. Mullens, unpub. data, 1975) are above bedded coarser bioclastic and pelletal limestone with a the Roberts Mountains Formation at Bootstrap Mine few intercalcalations of laminated limestone, espe­ and at Maggie Creek. The section at the Carlin mine, cially the lower 50 m. Corals and brachiopods are though much faulted, seems to be nearly complete. abundant in some beds of the upper unit. The bioclastic An excellent partial section occurs at the Bootstrap and pelletal beds commonly exhibit graded bedding; mine on Boulder Creek 22 miles (35.2 km) north­ many of these beds include breccia fragments and northeast of Dunphy, a short distance north of the scour surfaces at the base. Eureka-Elko County line. Although the basal chert is The Bootstrap Hill section is possibly truncated at not exposed here, about 4 70 m of strata with several the top by a thrust fault (T. E. Mullens, written com­ coral-bearing fossil zones has been measured by T. E. mun., 1972). The coral-bearing bed, that of Carlinas­ Mullens (unpub. data, 1975) at this locality. Evans and traea tuscaroraensis, lies about 400 m stratigraphically Mullens (1976) divide this section of rocks into two above the bottom of the section. units, the lower 180 m called Roberts Mountains For­ STRATIGRAPHIC PALEONTOLOGY OF THE mation and the upper 27 5 m termed unnamed Devo­ BOOTSTRAP HILL SECTION nian limestone. We favor including the lower part of Three faunal zones are recognizable at Bootstrap the unnamed Devonian limestone unit in the Roberts Hill on the basis of corals and brachiopods. These Mountains Formation because it is similar to the upper range in ascending age from possible Great Basin Silu­ part of that formation at many places in central N e­ rian coral zone A to coral zoneD or E. The uppermost vada, in particular, the reference section at Coal Can­ beds have yielded no coral fauna, but conodonts are yon in the Simpson Park Mountains. The upper part of varieties found in the Rabbit Hill Limestone. the unnamed Devonian limestone unit of Evans and Lowest coral-brachiopod zone.-Two fossil collec­ Mullens (1976) looks like the Rabbit Hill Limestone tions were made by R. J. Roberts, one from the south­ and is probably correlative with that formation. An west base of Bootstrap Hill (M1120) and the other from alternative nomenclature, in line with other studies of the west base of the next hill to the south (M1412). the Silurian and Lower Devonian in central Nevada, Both collections were taken from talus masking the would term the lower part of the unnamed unit of lowest exposed strata. Evans and Mullens (the top of our Roberts Mountains The collection from locality M1412 is diverse and Formation) the Windmill Limestone (Johnson, 1965) distinctive; it includes, among other fossils, the follow­ and the upper part the Rabbit Hill Limestone. The ing forms: lumping of rocks into a general unnamed unit obscures Cyathophylloides sp. f basic lithologic similarities and correlations and is a Cladopora sp. step backward in our understanding of the paleotec­ Alveolitid tabulate coral tonics of the region. Collections of unusually well­ Conchidium sp. b preserved silicified fossils made in this vicinity by R. J. Fardenia sp. b Roberts during reconnaissance mapping of northern Ptychopleurella sp. Eureka County convincingly demonstrated the Silu­ Coelospira sp. b rian age of some of the limestones. Fossil collections Kozlowskiellina sp. b along the Mullens section traversed by Mullens and Merista sp. Merriam make possible a significant correlation with The fossils from locality M1120 at the southwest base the Coal Canyon reference section and indirectly with of Bootstrap Hill are Cyathoph_ylloides sp. f, the Roberts Mountains Formation type section. Brachyelasma sp. b, Heliolites sp., and Favosites sp. TUSCARORA MOUNTAINS 17

The fossils at both localities are silicified and as­ AGE OF THE ROBERTS MOUNTAINS FORMATION sociated with chertified sedimentary material that is AT BOOTSTRAP HILL characteristic of lower beds of the formation near the Coral and brachiopod evidence points to a Silurian basal chert marker. The marker bed presumably is age for part of the Bootstrap Hill section with an age lower in the section but is not exposed. The primitive range from Early Silurian (Great Basin Silurian coral colonial rugose coral Cyathophylloides in each of these zone A) for the lower beds containing Cyathophylloides assemblages resembles C. fergusoni of the Great Basin to Late Silurian (Great Basin Silurian coral zone D or Silurian coral zone A described in collections from the E) for the higher beds containing Carlinastraea. Like Toquima Range. the the type and reference sections of the Roberts Middle coral-brachiopod zone.-A 60-m strati­ Mountains Formation, the upper part is Lower Devo­ graphic interval starting 180 m from the base of the nian. This age, as elsewhere, is based on the graptolite section and at the bottom of the more massive bioclas­ standard which in central Nevada places the tic sequence contains fossil assemblages that include Silurian-Devonian boundary at a lower stratigraphic the following forms.: position than the corals would suggest. Except for Coelospira sp. b, the brachiopod as­ Cladopora sp. semblage from the lowest faunal zone shows little simi­ Coenites sp. larity to known assemblages from other areas; the Favosites (massive) brachiopods of Great Basin Silurian coral zone A are in Stylopleura sp. b general poorly known. Calostylis? sp. The faunas of the middle coral-brachiopod zone gen­ Kodonophyllum sp. b erally fall in line with a Silurian age, in particular the Kyphophyllum sp. b Rugosa Stylopleura, Calostylis?, Kodonophyllum, and Resserella sp. Kyphophyllum. Several of the brachiopods range up­ Leptaena sp. ward into the Devonian; those provisionally assigned Salopina? sp. to Salopina?, Coelospira, and Glassia or Cryptatrypa Gypidula sp. are possibly more in harmony with a Silurian age. Coelospira sp. The remarkable colonial rugose coral Carlinastraea, Trematospira? sp. though resembling Devonian forms conventionally as­ Atrypa sp. signed to Spongophyllum, is generically quite distinct. H owellella sp. Carlinastraea tuscaroraensis, which is specifically Glassia or Cryptatrypa sp. identical to that from Bootstrap Hill, occurs in the Upper fossil zone with Carlinastraea.-Collections Simpson Park Mountains at Coal Canyon in beds with from what is here called the Carlinastraea bed, 70 m a coral fauna quite similar to that of Silurian coral stratigraphically below the top of the Bootstrap Hill zone D in the Roberts Mountains type section. Car­ section, were made by R. J. Roberts in 1958 and were linastraea of a similar kind occurs in association with supplemented by additional collections by Mullens and Australophyllum (Toquimaphyllum) johnsoni and Merriam in 1969. The complex rugose coral Carlina­ Stylopleura cf. S. nevadensis in beds assigned to Great straea occurs in very large colonies 1 m or more in Basin Silurian coral zone E at locality M1446 in the diameter, as it does at Coal Canyon in the Simpson northern foothills of the Simpson Park Mountains. Park Mountains. Fossils found in the Carlinastraea Carlinastraea at Bootstrap Hill is therefore suggestive bed at Bootstrap Hill are: of either Great Basin Silurian coral zone D or E. Studies of graptolites from the lower 180 m of the Cladopora sp. Mullens section (unpub. data, T. E. Mullens, 1975) (by Alveolites sp. W. B. N. Berry) give evidence for a higher position in Favosites (massive) the Silurian System than do the corals. Monograptus Rhegmaphyllum sp. in beds from about 45 m above the base to about 90 m Syringaxon? sp. include: Tryplasma sp. cf. T. duncanae Merriam Monograptus sp. (plain thecae) Kyphophyllum? sp., cf. K. sp. b Monograptus sp. (appears to have thecae with Carlinastraea tuscaroraensis n. gen., n. sp. spines similar to those in M. chimaera) Salopina? sp. (dorsal valve) Monograptus sp. (of theM. dubius type?) Large dalmanellid brachiopod (ventral valve) Monograptus sp. (thecae appear to be similar to Abundant crinoidal debris those of M. uncinatus) 18 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY Berry's conclusion (written commun., Nov. 8, 1968) is that is assumed to be correlative with, or to include, that these graptolites are possibly of Ludlovian Late the basal Roberts Mountains chert marker of other Silurian age. Other graptolites from horizons about Great Basin sections. It is, however, classified as the 175 m and 185 m above the base of the section are: top of the underlying Hanson Creek Formation by Monograptus angustidens Pribyl Matti, Murphy, and Finney (1975). This conspicuously Monograptus uniformis Pribyl banded member consists of interbedded dark-gray or Their age is given as Earliest Devonian (Monograptus slightly bluish-gray fine-grained limestone and dark­ uniform is Zone). gray chert that weathers limonite brown. Uneven lenses and pinch-and-swell beds of chert, 2-12 em ROBERTS MOUNTAINS FORMATION OF thick, make up one-third to one-half of this cherty unit. THE NORTHERN MONITOR RANGE Above the cherty member, for at least 140 m to the top of the hill, are thin-bedded fine-grained argilla­ At Copenhagen Canyon near Rabbit Hill in the ceous limestones that are dark gray on fresh breaks northern Monitor Range, the Roberts Mountains For­ but weather light gray with buff- and brownish-colored mation is represented by thin-bedded laminated patches. A few dark-gray chert nodules occur at places graptolite-bearing silty limestone and thin- to in this interval. medium-bedded clastic limestones. It is overlain by Rabbit Hill Limestone. These Silurian and Lower De­ vonian strata were studied by Merriam and Anderson PHYSICAL STRATIGRAPHY (1942, p. 1687) and later described in more detail after The contact at the bottom of the lowest chert bed reconnaissance geologic mapping and stratigraphic reveals no convincing physical evidence of erosion and measurement by Merriam (1963). A recent study by disconformity with the underlying Hanson Creek Matti, Murphy, and Finney (1975) focuses on the stra­ Formation, of Late Ordovician and Early Silurian age. tigraphy and environment of deposition of the Silurian Of possible significance regarding the system bound­ and Lower Devonian rocks of this area and revises the ary is a persistent calcareous sandstone bed that lies reconnaissance map. about 12m stratigraphically below the chert. This fine The most complete and measurable sequence of sandstone contains abundant chert granules. Accord­ Roberts Mountains Formation in Copenhagen Canyon ing to F. G. Poole (oral commun., 1968), a sand of this underlies an unnamed hill whose summit is about 1.6 kind occurs widely in the central Great Basin in about km north-northeast of the top of Rabbit Hill in the the same stratigraphic position. This bed provides a SWlft sec. 36, T. 16 N., R. 49 E. The formation to the top useful datum with reference to base of the Silurian of the unnamed hill measured about 180m (Merriam, System; it is especially valuable in sections lacking the 1963, p. 38); beds of several hundred metres more are chert marker. present west of the hill but poorly exposed. A thickness The apparent thinning of the Roberts Mountains of 480 m was measured by Matti, Murphy, and Finney Formation at Copenhagen Canyon from the relative (1975) on this traverse. The section is made up of platy thick sections to the north is part of the gradual thin­ to shaly weathering argillaceous limestone and cal­ ning of the unit to the west and south. In the Toquima careous shale grading upward into thin- to medium­ Range to the west, two thrust plates containing rocks bedded clastic limestones with fine-grained laminated that were originally deposited farther west (presuma­ limestone interbeds. On fresh surfaces these argilla­ bly in the vicinity of the Toiyabe Range) contain com­ ceous beds are dark gray and fine textured; on weath­ plete sections of Roberts Mountains Formation that ering they become light gray or buff colored. Bioclastic are 106m thick and less than 3m thick, respectively. and graded beds occur as sporadic interbeds in the Similarly, in the northern lnyo Mountains, Calif., a middle part of the section and as the main lithologic northward change of the Vaughn Gulch Limestone to type in the upper part; they are about 12 em thick and graptolite-bearing shaly deposits was accompanied by are composed largely of fine sandy material with a thinning of strata representing the Vaughn Gulch crinoidal debris. The platy silty limestones contain an interval (Ross, 1966, p. 32). abundance of graptolites best seen on weathered sur­ faces. No thick coarse bioclastic limestone lenses con­ taining a diverse colonial coral fauna were found in STRATIGRAPHIC PALEONTOLOGY AND AGE this section, but brachiopods, which were collected OF THE ROBERTS MOUNTAINS FORMATION AT COPENHAGEN CANYON from high in the section, are reported by Johnson, Boucot, and Murphy (1967). Studies of graptolites and conodonts provide most of The lower 30-40 m of the Roberts Mountains Forma­ the paleontologic evidence of age of this formation in tion at the unnamed hill is a very cherty limestone the Monitor Range; they indicate the age of the forma- DOBBIN SUMMIT, MONITOR RANGE 19 tion as Silurian and Lower Devonian. Generically un­ ROBERTS MOUNTAINS FORMATION AT determined pentameroid brachiopods about 3 em long BROCK CANYON, MONITOR RANGE observed in the field near the top of the cherty unit At Brock Canyon on the west side of the range, the suggest that this unit is best assigned to the Silurian Roberts Mountains Formation is exposed as parts of System, in agreement with the graptolites and cono­ two separate thrust plates. One plate contains all the donts. The sporadic crinoidallimestone beds may even­ Middle and Upper Ordovician formations (Antelope tually yield other shell megafossils of diagnostic age Valley Limestone, Copenhagen Formation, Eureka significance. Quartzite, and Hanson Creek Formation) recognized to Graptolites identified by Ruedemann (Merriam and the east around Antelope Valley as well as the Roberts Anderson, 1942, p. 1687) came from surface rubble a Mountains Formation; the second plate contains few feet above the chert. These were identified as Roberts Mountains Formation directly on the Antelope Monograptus acus Lapworth and Monograptus pandus Valley Limestone. The Roberts Mountains Formation Lapworth, suggesting to Ruedemann the approximate in both plates is finely laminated thin-bedded graptoli­ age of the lower and middle Gala beds of England, or of tic silty limestone, but the basal chert is present in the Clinton Group (Middle Silurian) and younger Silu­ only one of the two sequences-the plate containing rian beds in New York State. the complete series of Ordovician formations. The for­ Several graptolite collections made by Mullens and mation is at least 180 m thick, but the total thickness Merriam from higher in the section and identified by of the formation in either plate is unknown as it is the Berry include the following forms: youngest unit in the respective sequences. (float near top of basal chert, 33m above base of forma­ tion) ROBERTS MOUNTAINS FORMATION AT Cyrtograptus sp. DOBBIN SUMMIT, MONITOR RANGE Monograptus priodon (Bronn) In the middle part of the Monitor Range at Dobbin Age: In the span of late Llandovery to early Wen­ Summit and Clear Creek, the Roberts Mountains For­ lock. mation has been mapped by F. J. Kleinhampl (float 33--84 m above base of formation) (Kleinhampl and Ziony, 1967). In that area the forma­ Cyrtograptus sp. tion is underlain by the Hanson Creek Formation and Monograptus flemingii (Salter) overlain by Rabbit Hill Limestone. This occurrence of Age: Probably Wenlock. the Roberts Mountains is of special significance as it is partly dolomitic, suggesting proximity to the border (100 m above base of formation) zone between the eastern dolomite belt and the inter­ Monograptus dubius (Suess) mediate limestone belt as at Roberts Creek Mountain Monograptus uncinatus Tullberg and at Bare Mountain near Beatty, Nev. Rocks from Gothograptus spinosus Wood the August Canyon thrust sequence in the Toquima Age: Early Ludlow; Monograptus nilssoni-M. Range that are in part correlative with the Roberts scanicus Zone. Mountains Formation are also dolomitic. The basal (approximately 106 m above base of formation) chert member was not found. Kleinhampl states: Monograptus sp. cf. M. nudus type? The Roberts Mountains Formation consists of two main units. The Monograptus sp. cf. M. jaculum type? lower contains alternating pale brown-weathering slope-forming platy limestone and thinner but conspicuous medium-dark-gr~y Age: In the span of middle Llandovery to Ludlow. ledgy limestone and dolomite containing abundant macerated foss1ls (90--150 m above base of formation) (crinoid columnals and corals). This unit closely resembles the Mas­ Monograptus flemingii (Salter)? ket Shale of Kay and Crawford (1964, p. 439) in the Ikes Canyon area of the Toquima Range. An overlying gray massive cliffy dolomite Monograptus sp. (of theM. dubius type) and limestone unit is commonly present in the Monitor Range and is Age: Probably late Wenlock. tentatively considered to be the upper part of the Roberts Mountains. Graptolites reported by Matti, Murphy and Finney It could be correlative with part of the Lone Mountain Dolomite. Because of the questionable age and name assignment and thinness (1975) include Monograptus spiralis and Retiolites of units, the Late Ordovician through Silurian strata are shown as geinitzianus angustidens 1m above the top of the chert undifferentiated dolomite on the northern Nye County geologic map. (33 m above the base of the formation as used here) In the Dobbin Summit area, the Roberts Mountains is no more than these forms are late Llandoverian. Collections from about 60-120 m thick and consists mainly of the lower unit. In con­ high in the formation (in the Windmill Limestone as trast, near Clear Creek, the lower member is 90 m thick and the upper gray massive dolomite member is 100m thick (Greene, 1953, mapped by Matti and others, 1975) include Monograp­ p. 27 ). Here the top is faulted out according to Greene. tus birchensis, M. praehercynicus, and M. hercynicus; The Rabbit Hill Limestone overlies the Roberts Mountains Forma­ those species indicate an Early Devonian age. tion in the Dobbin Summit area. The Rabbit Hill consists of very 20 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY fossiliferous somewhat platy slope-forming limestone that is poorly and Fish Creek Range, Eureka County, Nev., identifi­ exposed and weathers to chips and plates of pale yellowish- and pale able fossils are present in Lone Mountain unit 2 (Mer­ reddish-gray and yellowish-brown colors. The formation may form riam, 1973b). At Bare Mountain, the upper formation, an incomplete section as little as less than 100 to about 245m thick and truncated at its top by a low-angle fault. The fault zone is or Lone Mountain Dolomite of Cornwall and Klein­ marked by a discontinuous very dark gray chert. hampl, has yielded no fossils. The underlying dark unit (Roberts Mountains Formation) contains abundant silicified fossils in its middle part, in both the lime­ ROBERTS MOUNTAINS FORMATION stone and the dolomite interbeds. AT BARE MOUNTAIN Correlation of the Bare Mountain strata with those Of special interest are those localities where the of central Nevada is based somewhat more on general Great Basin Silurian gives evidence of a position near lithologic resemblance and stratigraphic sequence a lateral transition from dolomite of the eastern dolo­ than on fossil evidence. The silicified fossil as­ mite belt to limestone of the intermediate limestone semblages from the middle part of the lower dark-gray belt, such as Bare Mountain near Beatty, Nev., and the unit (Roberts Mountains Formation of Cornwall and Roberts Mountains type section of central Nevada. Kleinhampl ), which come mainly from dolomite inter­ At Chuckwalla Canyon, near Bare Mountain, beds, are not closely related to known faunas of the Cornwall and Kleinhampl (1960) mapped 300 m of type Roberts Mountains Formation. Most abundant Silurian rock as two formations. The upper unit, being are pentameroid brachiopods which include a large, entirely dolomite about 114 m thick, was assigned to smooth Pentamerus-like genus, Conchidium of the Lone Mountain Dolomite; the lower unit, about medium to large size, and Virgiana? sp. A similar 190m thick, was referred to the Roberts Mountains Pentamerus-like genus occurs in the lower part of the Formation; this lower unit is made up of alternating Roberts Mountains Formation at Coal Canyon, Simp­ members of dark-gray limestone, dolomitic limestone, son Park Mountains, Nev. In the type section, Roberts and dolomite; one-third to one-half of this sequence is Mountains pentameroids, especially Conchidium, are dolomite. A Silurian age assignment is reasonable, for present in great abundance near the top of unit 2, that at Bare Mountain these beds occupy a stratigraphic is, near the middle of the formation. The corals at Bare interval between the Late Ordovician Ely Springs Mountain are Heliolites sp., large Streptelasma-like Dolomite and superjacent dolomite assigned to the De­ solitary forms, large solitary Rhabdocyclus sp. B, and vonian. Stylopleura sp. resembling S. berthiaumi of the Great The Bare Mountain sequence can be compared with Basin Silurian coral zone D. Near the base of the Bare the Lone Mountain Dolomite at its type section. As Mountain section, the colonial Palaeophyllum sp. b oc­ presently interpreted by Merriam (1973b), the type curs (Merriam, 1973a). Lone Mountain includes a lower dark-gray dolomite It is significant that a very cherty dolomite member called Lone Mountain Dolomite unit 1 and an upper lies at the base of the Roberts Mountains Formation at light-gray dolomite termed Lone Mountain unit 2. Bare Mountain. This member occupies the position of Lone Mountain unit 1 is lithologically comparable in the basal chert marker of many Great Basin sections of many respects to the Roberts Mountains Formation of the Roberts Mountains Formation. Cornwall and Kleinhampl at Bare Mountain but dif­ fers by containing no limestone. ROBERTS MOUNTAINS FORMATION OF No fossils of stratigraphic significance were iden­ THE NORTHERN TOQUIMA RANGE tified by Merriam (1973b) during the course of his work in the Lone Mountain Dolomite of the type section. The Good exposures of the Roberts Mountains Formation middle part of Lone Mountain unit 1 contains partly are found north of Petes Canyon, in the area between silicified brachiopods and corals, locally in abundance. Ikes and Mill Canyons, and in the area around East None of the fossils prepared by acid etching were ge­ and West Northumberland Canyons in the northern nerically determinable; they include small pentamer­ part of the Toquima Range. The formation in these aids, small rhynchonellid brachiopods, and tabulate localities occurs in different tectonic plates that have corals. Lone Mountain unit 2, the upper light-gray presumably been thrust from sites of deposition west of unit, has yielded a poorly preserved cerioid rugose the Toquimas. A simplified geologic map, figure 3, coral and in dark-gray lenses of this upper division, outlines these structural relations. A total of five colonial forms probably of the genus Entelophyllum. sequences of lower and middle Paleozoic rocks are There are unconfirmed reports that larger pentamerids present in the thrust slices; when unraveled palinspas­ were collected from an unknown part of the Lone tically, they represent rocks from a large region in Mountain in its type section. In the Mahogany Hills central Nevada. THE NORTHERN TOQUIMA RANGE 21

Petes Canyon window

EXPLANATION Upper plate of Roberts Mountains thrust Lower plate of Roberts Mountains thrust ~11111111111~ Cenozoic, Mesozoic, and upper Paleozoic rocks Western and transitional facies of lower Eastern and some transitional facies of lower and middle-- Paleozoic rocks and middle Paleozoic rocks ....1...... ----········· Fault Thrust fault Contact Bar and ball on down thrown side Sawteeth on upper plate, dotted where concealed

FIGURE 3.-The northern Toquima Range showing Paleozoic sequences that contain Silurian rock in separate thrust sheets. Based on geologic mapping by E. H. McKee in 196~ 70. 22 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY

PETES CANYON WINDOW The total thickness of Roberts Mountains Forma­ tion, here made up entirely of the thin-bedded graptoli­ The Roberts Mountains Formation in the vicinity of tic limestone, is about 3m. Fragments of the graptolite Petes Canyon (fig. 1, loc. 7; fig. 3) is exposed in the Monograptus aff. M. praehercynicus indicate that the Petes Canyon window in the Roberts Mountains thrust few feet of strata that make up the formation in this (McKee and Ross, 1969). An incomplete section of the thrust plate correlate with the upper part of the forma­ formation is present here as the uppermost unit in the tion elsewhere in central Nevada and are of Lower De­ autochthon. A number of partial sections including the vonian age. Corals from the bottom part of the Tor basal black chert unit are found; the thickest of these, above the graptolite horizon are typical of the Great in the central part of sec. 10, T. 16 N., R. 46 E., south­ Basin Silurian coral zone E (McKee and others, 1972), east corner of Lander County, contains more than 100 which is included in the Lower Devonian because of m of strata. The thickness of the order of 300 m esti­ mated by McKee and Ross (1969) should probably be the underlying Monograptus praehercynicus. revised to about 200-250 m. The basal contact with rocks mapped -as the Hanson MILL CANYON SEQCENCE Creek Formation seems conformable, though obscured by small faults or alluvium in most places. No angular The middle thrust plate, named the Mill Canyon se­ discordance was found between the Roberts Mountains quence by Kay and Crawford (1964) (fig. 1, loc. 9; fig. 3), and Hanson Creek, but fossil evidence suggests a contains a complete section of the Roberts Mountains hiatus between these formations. Formation with both graptolites and corals. These The Roberts Mountains above the basal black chert strata are well exposed on the ridges above the lower is mostly thin-bedded platy-splitting graptolite­ reaches of Ikes Canyon. bearing argillaceous limestone. A few medium-bedded The formation here is about 110m thick and consists limestones contain a rich coral fauna, but the strati­ mostly of thin-bedded platy argillaceous limestone graphic relation of these beds is not known because of with increasing amounts of medium (5--30 em) graded structural complications. Graptolites, which provide beds of clastic limestone toward the top. There is no the evidence for the age of lower parts of the formation, chert at its base, and it rests with no obvious discon­ include monograptids ranging from upper Lower Silu­ formity on massive limestone that contains Upper Or­ rian (Llandovery) into the upper part of the Middle dovician fossils. The lowest fossils (graptolites) in the Silurian (Wenlock and possibly lower Ludlow). The Roberts Mountains of the Mill Canyon sequence occur coral-bearing limestone beds contain forms typical of within a foot of its base and are Middle Silurian forms the Rabbit Hill Limestone of Early Devonian age. (McKee, 1975). The fossil evidence indicates a hiatus representing a time period of considerable duration at IKES CANYON WINDOW the base of the formation. The upper contact with the JUNE CANYOl'\ SEQUENCE McMonnigal Limestone, gradational across an interval In the area between Ikes and Mill Canyons, carbo­ of about 25 m, was arbitrarily drawn where medium­ nate strata of lower and middle Paleozoic age are ex­ bedded gray clastic limestone typical of the McMonni­ posed in a window in the Roberts Mountains thrust gal becomes the dominant lithology. called the Ikes Canyon window (fig. 1, loc. 8; fig. 3). The Graptolites that occur throughout the formation in­ Roberts Mountains Formation (or its correlatives) oc­ clude late Middle Silurian (late Wenlock) forms at the curs in three somewhat different sequences of rocks base, Upper Silurian Ludlow) types somewhat higher, that have been thrust together and are now exposed in and Monograptus praehercynicus, which is Lower De­ this window. The upper thrust plate, named the June vonian, in the upper part of the formation and in the Canyon sequence by Kay and Crawford (1964), con­ bottom of the McMonnigal Limestone. tains a metre or so of thin-bedded, platy-splitting grap­ The Roberts Mountains--McMonnigal sequence has tolitic limestone between massive beds of Ordovician yielded coral faunas in normal order representing limestone and the Tor Limestone of Late Silurian and three, A, D, and E, of the five Great Basin Silurian Early Devonian age. The upper contact with the Tor is coral zones. Coral zone A is represented by conformable; the lower contact with Ordovician lime­ Arachnophyllum kayi with which Cyathophylloides stone appears conformable but may be a fault. In all fergusoni and N eomphyma crawfordi are associated. places but one (approximately 1,500 m west of Ikes Above this horizon, coral zone D contains Stylopleura cabin site), the massive limestone units (Ordovician nevadensis, Tonkinaria sp., and large Verticillopora limestone and Tor) are in fault contact, the relatively annulata. Coral zones A and D are in the Roberts incompetent Roberts Mountains limestone having Mountains Formation; coral zone E lies in the Mc­ been sheared out between them. Monnigal Limestone and includes A ustralophyllum THE TOIYABE RANGE 23

(Toquimaphyllum) johnsoni and a species of Kypho­ most of Silurian time is represented by the underlying phyllum. dolomite or at the basal contact with the underlying Caesar Canyon Limestone of Kay and Crawford. The AUGUST CANYON SEQUENCE upper part of the Masket Shale of Kay and Crawford is Rocks in the lowest thrust plate in the Ikes Canyon thin-bedded laminated silty limestone, part of which window (fig. 1, loc. 10, fig. 3) were named the August may correlate with the upper part of the Roberts Canyon sequence by Kay and Crawford (1964). It was Mountains Formation and with the Rabbit Hill Lime­ assumed by these geologists that this sequence of stone. It is of Lower Devonian age on the basis of a strata is in situ, but it is possible that these rocks, along variety of Lower Devonian conodont forms (McKee, with the overlying two plates (Mill Canyon and June 1976). Canyon sequences), have moved from another region. NORTHUMBERLAND WINDOW As its base has not been found, there is no structural evidence bearing on the problem; stratigraphic consid­ About 8 km south of the Ikes Canyon window, east­ erations such as regional facies patterns shed little ern to transitional facies (mostly carbonate) rocks are light on the site of deposition of the rocks. These strata exposed in the Northumberland window in the Roberts contrast so markedly with the strata of equivalent age Mountains thrust (fig. 1, loc. 11; fig. 3). The rocks in in the overlying thrust plates that new formational this window constitute the Prospect and Striped Hill names proposed by Kay (1960) and Kay and Crawford sequences of Kay and Crawford (1964), presumed to (1964) seem appropriate. Rocks correlative with the have been thrust from the southwest to their present Roberts Mountains Formation include all or most of location in the north-central part of the Toquima the Gatecliff Formation of Kay (1960), the Bastille Range (Kay and Crawford, 1964, fig. 8). Limestone Member of the Masket Shale of Kay and Both the Prospect and Striped Hill sequences con­ Crawford (1964), and probably the lower part of the tain Roberts Mountains Formation (called Masket upper member of the Masket Shale of Kay and Craw­ Shale and Gatecliff Formation by Kay and Crawford, ford (1964). The name Masket Shale was used by these 1964) as their uppermost unit. The formation is typi­ workers (1964) for strata in the Mill Canyon and June cally thin-bedded platy-splitting argillaceous tan­ Canyon sequences; in this report and in others (McKee weathering limestone similar to rocks mapped as and others 1972; McKee, 1976), those strata are called Roberts Mountains Formation in most other places in the Roberts Mountains Formation. central Nevada. Black cherty limestone occurs at the The Gatecliff Formation of Kay and Crawford is base of the formation. Monograptids from the chert comprised of three distinctive units, all dolomitic. It zone at Water Canyon north of East Northumberland overlies with no apparent discordance dark-colored Canyon are indicative of the Lower Silurian (lower limestone containing a distinctive fauna considered to Llandovery). An Early and Middle Silurian age is as­ be Upper Ordovician (McKee, 1976). This Upper Or­ signed to the overlying platy unit of the Striped Hill dovician limestone was named the Caesar Canyon sequence by Kay and Crawford (1964, fig. 5). Limestone by Kay and Crawford (1964). Corals indica­ ROBERTS MOUNTAINS FORMATION tive of the Silurian have been collected from the basal IN THE TOIYABE RANGE unit of the Gatecliff (McKee, 1976), which is a light­ gray lithographic dolomite. The middle unit of the The Roberts Mountains Formation crops out in Gatecliff is dolomite distinctive by the presence of widely separated localities along most of the extent of rounded quartz sand grains; no fossils have been found the Toiyabe Range. Many of these occurrences mark in this rock. The upper unit of the Gatecliff is dolomite the westernmost exposures of the formation at a given and chert similar to the basal chert unit of the Roberts latitude; these outcrops lie approximately along long Mountains Formation in most of central Nevada. No 117°15'. It is assumed that these outcrops have not fossils have been collected from this chert in the Au­ been tectonically transported (thrust) from the west as gust Canyon sequence, but a few miles south, s similar have many thrust plates in the Toquima Range. chert unit in what is called the Prospect sequence by Localities where partial or complete sections of Roberts Kay and Crawford (1964) has yielded graptolites that Mountains Formation are exposed include: Callaghan indicate an Early Silurian age. The Bastille Limestone window (fig. 1, loc. 18), about 24 km northeast of Aus­ Member of the Masket Shale of Kay and Crawford is tin; Dry Creek area (fig. 1, loc. 6), 11 km south-south­ mostly unfossiliferous gray dolomite with medium­ west of Austin; Point of Rocks and Straight Canyon bedded bioclastic limestones at the top. These lime­ area (fig. 1, loc. 16), about 27 km southwest of stones contain a rich coral fauna equated with the Austin; and Pablo Canyon (fig. 1, loc. 12), about 16 km Great Basin Silurian coral zone E. Part or possibly west of Round Mountain. The formation crops out at a 24 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY few other localities in the Toiyabe Range where it has Favosites and Zelophyllum sp., suggest a Silurian age, not been studied in detail. probably in the upper part of the system. In overall aspect, this section resembles the Mill Canyon se­ CALLAGHAN WINDOW quence of the Toquima Range, a sequence of strata in a thrust plate assumed to have originated in the general Lower Paleozoic carbonate strata are exposed in the area of the Dry Creek section. Callaghan window (fig. 1, loc. 18) in the Roberts Moun­ tains thrust about 24 km north of Austin (Stewart and POINT OF ROCKS AND STRAIGHT CANYON Palmer, 1967; Stewart and McKee, 1968). The Roberts A faulted sequence of lower Paleozoic strata is ex­ Mountains Formation is the top unit in the autoch­ posed in the western and central part of the Toiyabe thonous sequence and occurs in scattered outcrops in range in an area that includes Straight Canyon and an the southern and northwestern parts of the window. area of about 10 km2 south of Reeds Canyon, includ­ Since it is the top unit, its thickness is not known. ing Point of Rocks Canyon about 27 km south of Austin About 60 m can be measured in partial sections, but (fig. 1, loc. 16). Here the rocks dip generally to the the formation is probably three to four times thicker in north, forming a homoclinal sequence. This seemingly this part of the Toiyabe Range. The more than 1,000 m simple structure is complicated by at least one thrust reported by T. E. Mullens (unpub. data, 1975) seems parallel to the strike of bedding and other faults that excessive on the basis of other measured sections in the cut out, or repeat, parts of the section. Stratigraphic region. The Roberts Mountains Formation lies on dif­ thicknesses are estimates based on a number of trav­ ferent rock units at localities only a few miles apart. In erses across the section. The best exposure of the lower the Boone Creek area in the northern part of the win­ part of the Roberts Mountains Formation and under­ dow, it lies on Antelope Valley Limestone. There is no lying units is in Straight Canyon (sec. 18, T. 16 N., R. obvious angular discordance between these forma­ 43 E.) about 1.6 km west of the Kingston ranger sta­ tions, but a hiatus spanning the Upper Ordovician and tion. Here about 3 m of dark chert and limestone that possibly part of the Lower Silurian must be repre­ make up the basal part of the Roberts Mountains lies sented at the contact. In the southern part of the win­ on a composite unit about 15 m thick of cherty lime­ dow, it rests on rocks originally considered to be equiv­ stone and dark shale containing cryptolithoid trilo­ alent to the Hanson Creek Formation (Stewart and bites. These rocks are probably best correlated with Palmer, 1967) but now considered likelier to be cor­ units in the Toquima Range named Hanson Creek relative with the Copenhagen Formation (Stewart and Formation (in Fetes Canyon window) or Caesar Can­ McKee, 1975). About 40 m of strata lithologically dif­ yon Limestone in the August Canyon sequence of the ferent from the underlying Antelope Valley Limestone Ikes Canyon window. There is no angular discordance and overlying Roberts Mountains separates these for­ between the unit and the Roberts Mountains chert, but mations. At all places in the Callaghan window, the they are probably separated by a hiatus as elsewhere Roberts Mountains consists of a basal black chert in central Nevada. Above the chert is about 100 m or about 10m thick overlain by light-gray platy-splitting more of gray-weathering platy argillaceous limestone silty limestone that contains monograptids. No coral­ typical of the graptolite facies. A few monograptids line limestone has been found in the incomplete sec­ were found a short distance above the basal chert in tions of the formation in this area. Straight Canyon and higher in the section on the west side of Point of Rocks Canyon. The section from Point DRY CREEK AREA of Rocks Canyon to the north side of the adjacent un­ On the west side of the Toiyabe Range about 11 km named canyon to the north (unsurveyed sec. 36, T. southwest of Austin, in the Dry Creek area, the 17 N., R. 42 E.) reveals about 300 m of thin-bedded Roberts Mountains Formation (fig. 1, loc. 6) is the up­ Roberts Mountains Formation. This thickness, how­ permost unit of a section that includes Ordovician, ever, is subject to speculation and is probably exces­ Cambrian, and Precambrian strata below. About sive, for at least half the section is poorly exposed on 120m of tan- or gray-weathering platy argillaceous the dip slope, and numerous faults mapped at other limestone makes up the formation in this area. There places in the region are probably present here. Scat­ is no basal chert unit, and the formation lies with ap­ tered graptolites collected in these rocks indicate that parent conformity on medium-bedded Antelope Valley they are forms considered to range from late Early Limestone. A few fragments of Monograptus have been Silurian (late Llandovery) to Devonian in age (Stewart found in these platy rocks, and a collection of poorly and McKee, 1976). Further evidence for unresolved preserved corals comes from a thicker bed of limestone structural complications is that the succession of grap­ near the top of the section. The corals, which include tolite zones does not correspond to a simple traverse LIMESTONES OF THE NORTHERN INYO MOUNTAINS 25 across the section. It is suggested here that the actual Mountains and the Toquima or Toiyabe Ranges, a dis­ thickness of the formation in this part of the Toiyabe tance of about 200 km. Named the Vaughn Gulch Range is of the order of150 m. Thin- to medium-bedded Limestone by Ross (1963, p. B81), these beds were for bioclastic limestones become more numerous toward many years after their first geologic examination in the top of the formation. Graptolites in this transition 1912 believed to be entirely Devonian (Kirk, 1918; zone are of the Monograptus hercynicus group. Higher Stauffer, 1930). Geologic mapping of the Cerro Gordo in the section, a massive limestone (about 4 m thick) mining district by the U.S. Geological Survey in 1946 contains the corals Billingsastraea? sp. T, a new stimulated renewed interest in these limestones when species, and a new species of a Hexagonaria-like genus. it was suspected that the Silurian and Lower Devonian Hidden Valley Dolomite (McAllister, 1952) of the PABLO CANYON AREA southern Inyo and Panamint Mountains changes A faulted slice of Roberts Mountains Formation oc­ northward and westward to limestone as represented curs in Pablo Canyon in the Toiyabe Range about 16 at Mazourka Canyon. Systematic collecting and study km west of the town ofRound Mountain (fig. 1, loc. 12). of the Vaughn Gulch and Hidden Valley Rugosa and The top of the formation here is everywhere faulted, dasycladacean algae began in 1947 with the Cerro and it is faulted at the base in most places. In the few Gordo work, during which the Vaughn Gulch section places where the basal chert unit is exposed, it lies on was measured and mapped. A study of these rocks by 3-6 m of shaly and cherty rock that contains Ordovi­ Waite (1953) pointed to the Silurian age of most of the cian graptolites. Most of the formation is gray- to tan­ formation and demonstrated northward facies changes weathering thin-bedded platy-splitting silty limestone into more argillaceous graptolite-bearing beds in the typical of the graptolitic facies seen elsewhere in cen­ Mazourka Canyon area. Detailed geologic mapping of tral Nevada. The locality in Pablo Canyon is the west­ the Independence quadrangle by Ross (1963, 1965, ernmost occurrence of recognizable Roberts Mountains 1966) further clarified the stratigraphic and structural Formation at this latitude. relations, and work by Stevens and Ridley (197 4) brought forth additional field evidence of northwest­ ROBERTS MOUNTAINS FORMATION IN THE ward change to graptolitic facies within the Vaughn SHOSHONE MOUNTAINS Gulch. Correlation of the Vaughn Gulch with the A partial section of Roberts Mountains Formation Roberts Mountains Formation was made possible by crops out in the Shoshone Mountains in the Ravens­ comparative studies of its rugose corals and dasyclada­ wood area about 24 km northwest of Austin (fig. 1, loc. cean algae in the 60's. Stratigraphic details and fossil 17); the westernmost outcrops of the formation at this zonation as here presented came mainly from Mer­ latitude are almost directly north of the Point of Rocks riam's field investigation during the years 1946--48 and Pablo Canyons sections along long 117°15'W. when most of the fossil collecting was done by the U.S. The formation here, as at the other places at this gen­ Geological Survey. eral latitude, is mostly graptolite-bearing thin-bedded PHYSICAL STRATIGRAPHY platy-splitting silty limestone. About 250m ofrocks of this lithologic type occurs above a thin black chert­ The Vaughn Gulch Limestone, in its type section, bearing unit that in turn lies on either Middle Ordovi­ the ridge northwest of Vaughn Gulch near the mouth cian Antelope Valley Limestone or shale, chert, and of Mazourka Canyon 4 km northeast of Kearsarge limestone of an unnamed unit tentatively correlated (Ross, 1963, 1966), is an unbroken sequence about with the Copenhagen Formation (Stewart and McKee, 460 m thick resting conformably upon the Late Or­ 1976). dovician Ely Springs Dolomite and overlain uncon­ formably by Mississippian conglomerate and quartzite SILURIAN AND LOWER DEVONIAN of the Perdido Formation (McAllister, 1952, p. 22). The LIMESTONES OF THE NORTHERN formation consists of well-bedded medium- to dark-gray INYO MOUNTAINS, CALIF. impure carbonaceous limestone, argillaceous lime­ MAZOURKA CANYON stone, and calcareous siltstone including many fossil­ rich beds. Some of the more argillaceous and silty VAUGHN GULCH LIMESTONE interbeds weather in subdued fashion, becoming light Within the intermediate limestone belt, the south­ gray, stained pink, or orange in places. Black chert ernmost Silurian and Lower Devonian rocks of the zones are stratigraphically significant, occurring at the Great Basin crop out at Mazourka Canyon, northern base and top of the formation; elsewhere minor chert is Inyo Mountains, Calif. (fig. 1, loc. 29). No exposures of present as scattered nodules or thin lenses. Platy and these limestones have been found between the Inyo fiaggy exposures predominate, with limestone ranging 26 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY from 2.5 to 15 em separated by calcareous shaly or Silurian coral zone E and the Mississippian disconfor­ siltstone partings. Thicker limestone beds, some ex­ mity. Bioclastic beds within the topmost 25m contain ceeding 1 m, are fairly common; many of them are an abundance of poorly preserved fossils. Contorted coarsely bioclastic. These thicker beds weather out layers of dark-gray chert in the upper 10 m are as­ prominently and are most numerous in the middle part sociated with massive dark-bluish-gray crinoidallime­ of the formation. stone with silicified partly macerated limonite-stained As noted by Ross (1966, p. 31), readily mappable brachiopods, favositids, and both solitary and colonial lithologic subdivisions were not found within the large rugose corals. Below the upper beds, the lime­ Vaughn Gulch Limestone of the type section. Its faunal stone is partly laminated and platy down to the lower distribution, however, is conveniently subdivided into strata adjoining coral zone E where thicker bioclastic a lower part 107 m thick, a middle part 220 m thick, and coralline beds become more numerous. Graptolite and an upper part 140m thick. and conodont evidence reported by Stevens and Ridley Lower division of the Vaughn Gulch Limestone.­ (1974) indicate that the upper part of the formation is This part of the formation, about 107 m thick, consists Lower Devonian and probably as young as Middle De­ largely of platy to flaggy partly laminated dark-gray to vonian. bluish-gray argillaceous limestone weathering light STRATIGRAPHIC PALEONTOLOGY gray in places. It is more uniformly bedded with fewer bioclastic and fossil beds than higher parts of this for­ Large collections of fossils made by Stauffer (1930) mation. A persistent dark-gray chert member at the from limestone in what is now the type section of the bottom, about 5 m thick, corresponds to that occurring Vaughn Gulch Limestone are the basis for his com­ near the base of the Silurian System elsewhere in the parison and correlation with the Kennett Formation of Great Basin. Weak partial dolomitization and nodular northern California and assignment at that time of the chert decrease upward to the lowest fossil bed of Great limestone to the Devonian System. The fossils were Basin Silurian coral zone A, which lies 40 m above the meticulously zoned by Stauffer within a 500-m meas­ basal cherty beds. The few fossil beds in this division ured section comprising 14 numbered stratal units. In contain abundant Heliolites and favositids; more dis­ general, these units can be alined fairly well with tinctive zone indicators like Dalmanophyllum are un­ those of the present measurement. As the Silurian and common. Devonian Rugosa of western North America were un­ Middle division of the Vaughn Gulch Limestone.­ known at the time of Stauffer's work, the listed iden­ The 220 m of limestone constituting the middle part of tifications of these forms in the Vaughn Gulch have the Vaughn Gulch Limestone includes prominent little definitive significance by present standards. medium- to dark-gray richly fossiliferous coralline and Nonetheless, the bed-by-bed tabulations by Stauffer bioclastic beds in its upper half, of which the upper­ clearly reflect the overall coralline nature of much of most 100 m falls within Great Basin Silurian coral the formation and reveal the diversity of these coral zone E. The dark-gray bioclastic beds range from 25 em assemblages. The lists show a relative abundance of to more than 1 m and are commonly sculptured into tabulate corals including massive Favosites, the slen­ prominent ribs separated by subdued intervals of der digitate Cladopora, and the less commonHeliolites. thin-bedded silty or argillaceous limestone weathering Stromatoporoids are listed throughout but do not ap­ a lighter gray. Much of the bioclastic material of these pear to have been especially important as limestone thick fossil beds is crinoidal debris. Some of the fossils builders here. The spherical sponge H india is reported that weather in relief are complete and partially in several beds. Brachiopod and trilobite identifica­ silicified and impregnated with limonite. Below the tions are largely of questionable value, no doubt partly middle of this 220 m division, the limestones are, in because the preservation is poor, and these fossil general, thin bedded and show fewer bioclastic mem­ groups are comparatively scarce in the coral-rich beds. bers. No fossil accumulations with true biohermal re­ The abundant dasycladacean alga Verticillopora was lief were found. Chert is a minor constituent. Some at that time unrecognized in these deposits. 100m of beds in the middle of this 220-m division con­ The biostratigraphy ofthe Vaughn Gulch Limestone tain an abundance of the large dasycladacean alga as here presented is dependent upon study of the Verticillopora annulata. Though long ranging, these rugose corals and dasycladacean algae. Great Basin calcareous algae appear to be most numerous in the Silurian rugose coral zonation of Merriam (1973a), Great Basin Silurian coral zone D. treated elsewhere herein, is based upon combined Upper division of the Vaughn Gulch Limestone .-A range and distribution data for these fossils in the 140-m upper interval of the Vaughn Gulch Limestone Roberts Mountains Formation, the Hidden Valley includes those beds between the top of Great Basin Dolomite, and the Vaughn Gulch Limestone. The LIMESTONES OF THE NORTHERN INYO MOUNTAINS 27 dasycladacean alga V erticillopora, which ranges up­ Stauffer's rather lengthy faunal lists from the inter­ wards from Great Basin Silurian coral zone B to E and val of coral zone E include brachiopods, among them above this horizon, is abundant and probably peaks in Camarotoechia, Atrypa, Gypidula, Athyris, Tremato­ coral zone D. Of the five Great Basin Silurian coral spira, Schizophoria, and Eatonia. zones A through E, only zones A and E have been iden­ Fossils of the upper Vaughn Gulch Limestone.­ tified with certainty in the Vaughn Gulch Limestone. Except for the lowermost 8 m, the upper part of the The Vaughn Gulch Limestone, judged by compara­ Vaughn Gulch Limestone has yielded few well-pre­ ble thicknesses and fauna, occupies most or all of the served fossils. The lowermost beds contain the highest stratigraphic interval of the Hidden Valley Dolomite known Verticillopora, Australophyllum sp. v, and to the south and east. Great Basin Silurian coral zones Cladopora and other favositids. Near the top of the A and B are typified by distinctive rugose coral as­ formation at locality M1090 are poorly preserved semblages in the Hidden Valley, whereas zones C, D, rugose and tabulate corals, large Atrypa, and indeter­ and E have their reference occurrences in the Roberts minate spiriferoid brachiopods. Among the Rugosa are Mountains Formation. a large possible member of the Halliidae that resem­ Coral zone A .-Great Basin Silurian coral zone A is bles Aulacophyllum and a colonial genus that has fea­ represented in the lower 107 m unit of the Vaughn tures of Acinophyllum or Diplophyllum. Other fossils Gulch by beds containing the columellate solitary present are cystiphylloids, digitate and massive favo­ rugose coral Dalmanophyllum sp. A, about 40 m above sitids, and stromatoporoids. Australophyllum sp. v re­ the top of the basal chert marker units; also occurring sembles A. landerensis of the central Nevada Rabbit with the genus is a small tryplasmid, a pycnostylid Hill Limestone of Early Devonian age. The diverse and coral, favositids, Heliolites, and Camerotoechia. Other highly distinctive Rabbit Hill fauna has not been found zone A indicators like Palaeocyclus, Arachnophyllum, in this unit where it would be expected. and Cyathophylloides have not been found in the AGE Vaughn Gulch Limestone. Verticillopora beds.-Strata in the middle part of the The age of the Vaughn Gulch Limestone, based on 220-m middle unit of the Vaughn Gulch contain an its coral fauna, ranges from Early Silurian (Llandove­ abundance of the large Verticillopora annulata. This rian) to possible Early Devonian. Dalmanophyllum sp. algal genus ranges in decreasing numbers on up A in the lower unit is an indicator of Great Basin Silu­ through the overlying Great Basin Silurian coral zone rian coral zone A. Beds with abundant Verticillopora E into the lowermost beds of the upper 140-m unit of annulata in the middle of the middle lithologic unit of the Vaughn Gulch, above which it has not been found. the Vaughn Gulch probably bracket Great Basin Silu­ Although the rugose coral fauna of Great Basin Silu­ rian coral zone D although the rugose coral fauna of rian coral zone D has not been found in the Vaughn zoneD was not found in these strata, and it is probable Gulch, it is not improbable that the beds in which Ver­ that Verticillopora reached its greatest development in ticillopora annulata is most abundant occupy that coral zone D. Great Basin Silurian coral zone E, at the interval in which Silurian Verticillopora appears to top of the middle unit, when compared with the Got­ have peaked. land, Sweden, standard, is Late Silurian (late Coral zone E.-Great Basin Silurian coral zone E Ludlovian). occupies the uppermost 100-m interval of the middle In the absence of conclusive evidence from the coral unit of the Vaughn Gulch Limestone. These upper beds faunas, the upper lithologic unit of the Vaughn Gulch include units 12, 13, and 14 of Stauffer (1930, p. 86- is viewed as Late Silurian or Early Devonian. Aus­ 89). The large and diverse coral fauna contains an tralophyllum sp. v at its base resembles, but is not abundance of tabulates including Favosites, Clado­ conspecific with, A. landerensis from the Lower Devo­ pora, Syringopora, and Heliolites, together with the nian Limestone Rabbit Hill. Near the top is a large distinctive Silurian rugose corals Australophyllum Aulacophyllum-like coral, possibly of Early Devonian (Toquimaphyllum) sp. similar to A. johnsoni, Kypho­ age; the cystiphylloids and the Acinophyllum-like phyllum sp., and Chonophyllum-like species. Rhizo­ coral might also be Devonian. phyllum sp. D Oliver is represented by float material, Graptolite and conodont evidence and correlation doubtless from coral zone E, and probably by ~~calceola with the Sunday Canyon Formation noted by Stevens sandalina Lamarck" of the Stauffer list from his unit and Ridley (197 4, p. 28) suggests that the formation 12. Verticillopora ranges upward into coral zone E. extends upward to the Middle Devonian. With this assemblage is the brachiopod Plectatrypa sp. Comparable thicknesses of the Vaughn Gulch Lime­ Crinoidal debris is abundant; the stromatoporoids, stone and the more easterly Hidden Valley Dolomite, though present, appear to be subordinate. as well as stratigraphic relations with bracketing 28 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY units strongly suggest that both occupy about the same These forms suggest an age range in the Silurian from stratigraphic interval. Because of facies differences, Wenlock to Ludlow. A locality 45 m above the base of only Great Basin Silurian coral zone A has been recog­ the formation (USGS-D179-SD) yielded Monograptus nized. As shown by McAllister (1952), the Hidden Val­ cf. M. uniformis Pribyl, which is considered to be Early ley is largely of Silurian age and includes the reference Devonian. occurrences of Great Basin Silurian coral zones A and The problem of dating the Sunday Canyon is the B. The uppermost beds of this dolomite are of Early same as that relating to the Roberts Mountains For­ Devonian age in the Siegenian-Emsian range. It is mation in that the standard time scale based on grap­ probable that the highest Vaughn Gulch strata may be tolites is different than that based on corals and other correlative with the fossil-bearing part of the Hidden benthonic organisms and usually suggests relatively Valley Dolomite. This correlation is made by Stevens younger ages for particular horizons. In the Mazourka and Ridley (1974, fig. 2). Canyon exposures, it is not unlikely that most of the SUNDAY CANYON FORl\fATION graptolitic deposits are relatively low in the Silurian column. The Silurian and Lower Devonian limestones of the The rugose coral A ustralophyllum stevens i Merriam, intermediate limestone belt change facies to shale lat­ from the upper 12 m of the Sunday Canyon, is of possi­ erally within a relatively short distance in the Ma­ ble Early Devonian age. zourka Canyon area of the northern Inyo Range, Calif. Stevens and Ridley (197 4), on the basis of graptolites Detailed geologic mapping of the Vaughn Gulch Lime­ and conodonts, indicate that the formation extends to stone northward for 19 km toward Badger Flat by Ross the Middle Devonian. (1966) shows a progressive thinning of the formation and the change to calcareous shale, siltstone, and ar­ THE SILURIAN-DEVONIAN BOUNDARY gillaceous limestone. Tongues of Vaughn Gulch bio­ IN CENTRAL NEVADA clastic limestone reappear north of the type section, We do not attempt here to delve into the philosophi­ but within 8 km, near Barrel Spring, the general cal aspects of stratigraphy with respect to time bound­ character of the deposits has become such as to justify aries, lithofacies, biofacies, and systemic definitions. the use of another name, Sunday Canyon Formation, Placement of the Silurian-Devonian boundary within for rocks correlative with the Vaughn Gulch Lime-. the carbonate sequence of the central Great Basin is a stone. The more shaly facies continues northward problem when attempts are made to compare rugose another 10 km to Badger Flat and becomes prog­ coral zonation based on similarities with the Gotland, ressively thinner. How much of the thinning is due to Sweden, sequence with graptolite zonation which is depositional processes such as initial deepening with the accepted guide for time boundaries in this part of less sedimentation of open marine waters toward the the geologic column. Any study of the Roberts Moun­ north and how much to erosional thinning at the un­ tains Formation should, however, point out the con­ conformity with the overlying Mississippian is not flict, where and how it arises, and what the current known. The calcareous shales yield mainly graptolites, geologic thinking is on this matter. This problem is a but occasional small Vaughn Gulch-type bioclastic classic one in stratigraphy involving lateral change of lenses as far north as Al Rose Canyon contain rugose facies, time-transgressive rock units, and definition of corals. In general, however, the proportion of carbo­ time boundaries by faunas from different reference nate material decreases toward the north (Ross, 1966, sections at different places in the world. Specifically, p. 32). the Silurian-Devonian boundary is a troublesome AGE boundary that has been the subject of much discussion The age of the Sunday Canyon Formation has been and a number of international symposia beginning in determined mainly by means of abundant monograp­ 1958 and continuing to this time. The Silurian­ tids, nearly all collected near the base of the formation. Devonian Boundary Committee of the International Among the species present are (identified by W. B. N. Stratigraphic Commission has established a generally Berry and R. J. Ross, Jr.): accepted boundary which proves to be viable in the Monograptus cf. M. dubius (Suess) Great Basin and in most parts of the world. This Monograptus dub ius (Suess) boundary is based on graptolites. Monograptus sp. (theM. tumescens type) Placement of the boundary in many sections of lower Monograptus sp. (slender rhabdosomes, plain Paleozoic rock in central Nevada is difficult because geo­ thecae) logic structure is extremely complex and easily inter­ Monograptus sp. (theM. vulgaris type) preted sequences of strata are rare. We point for exam­ Monograptus cf. M. scanius Tullberg ples to the section at Coal Canyon in the Simpson Park STRATIGRAPHIC AND AGE VALUES OF RUGOSE CORALS 29

Mountains (see p. 11), which appears superficially to stone belt are described in papers by Merriam (1972; be simple but contains at least one fault with uncertain 1973a) and a paper by Oliver, Merriam, and Churkin, offset in a critical part of the section. Detailed mapping (1975). is almost always a necessity before attempting strati­ With continued geologic mapping, new Silurian graphic studies in this region, and frequently even rugose corals have since been discovered in the central with careful mapping and repeated visits to the field, Great Basin, and other Rugosa previously unknown in the relation of rock units remains clouded. When the certain assemblages have been found in association at geologic difficulties are added to the subtleties of new localities. The stratigraphic ranges of some genera faunal interpretation in a narrow belt of rocks depos­ and species have been extended by further collecting ited at the transition between shallow and deeper and stratigraphic investigation. water, one realizes why placement of the Silurian­ Recent researches upon little-known taxonomic Devonian boundary has been disputed in this part of groups of Rugosa have resulted in a greatly improved the world. Part of the problem rests on what fossils are understanding of their internal structure, their value available and what forms are used to define the as indicators of the Silurian and Devonian Systems, boundary-what yardstick we are measuring with. and their geologic ranges. The two principal groups of When only one form is present, for example corals or corals are those lacking dissepiments (nondissep­ graptolites, the section can be matched with similar imented corals) and those having dissipiments (dis­ standards elsewhere in the world. But, where more sepimented corals). The nondissepimented Rugosa than one form is present, as in central Nevada, and the with which we are concerned are the Family Kodono­ rocks have been correlated on the basis of lithology, the phyllidae, the Family Pycnostylidae, and the Family faunal yardsticks do not agree. Several basic explana­ Tryplasmatidae. The dissepimented Rugosa of special tions can account for this difference: various forms interest here are the Family Spongophyllidae, includ­ have different time ranges at different places in the ing the new colonial genus Carlinastraea. Others of world, or the standard sections such as that at Gotland, interest are assigned to the Family Endophyllidae, like Sweden, or the Din ant Basin, Belgi urn, do not contain the subgenus Australophyllum (Toquimaphyllum) and rocks wholly correlative. In central Nevada, the grap­ other forms of A ustralophyllum. Among the Family tolite Monograptus praehercynicus is used by Berry Kyphophy llidae, there are new and undescribed (1970) to indicate the base of the Monograptus unifor­ species of Kyphophyllum. m is Zone. This zone is considered by most members of The dasycladacean algal group including Verticil­ the Silurian-Devonian Boundary Committee of the In­ lopora is discussed briefly; insofar as known, this large ternational Stratigraphic Commission to be the lowest and complex alga commonly associated with Silurian Devonian zone, and hence is accepted here as basal Rugosa ranges upward only into the lower part of the Devonian although no agreement exists on precisely Devonian System. what graptolite species or subspecies should be used to denote the base of the zone. Monograptus praeher­ RUGOSE CORALS OF SPECIAL STRATIGRAPHIC VALUE cynicus is found in the Roberts Mountains Formation SILURIAN NONDISSEPIMENTED RUGOSA at a number of places, and in a few places it occurs in a stratigraphic interval that also contains the coralline Family KODONOPHYLLIDAE fauna diagnostic of the Great Basin Silurian coral zone The Family Kodonophyllidae, which so far as known E as defined by Merriam (1973a). In the central is confined to the Silurian of the Great Basin, com­ Nevada limestone belt represented by the Roberts prises solitary and colonial nondissepimented Rugosa Mountains Formation, the Silurian-Devonian bound­ having long lamellar septa, a very wide stereo zone, ary based on graptolites and on conodonts is a hundred and flat tabulae or distally arched tabellae that may metres or more lower in the same sequence of strata combine with septal tips t0 form an axial structure and than the boundary based on rugose corals and cal­ calicular boss. careous algae. Two subfamilies of Kodonophyllidae are recognized: Kodonophy llinae and Mycophy llinae. These sub­ AN APPRAISAL OF STRATIGRAPHIC families and their taxonomy have been discussed in ANDAGEVALUESOFRUGOSECORALSAND some detail by Merriam (1973a). ASSOCIATED FOSSILS, WITH DESCRIPTION The genus Kodonophyllum Wedekind is present in OF PREVIOUSLY UNKNOWN KEY RUGOSA the Late Silurian (Great Basin Silurian coral zone E) .Most of the Cordilleran Rugosa listed and referred to on the east side of the Coal Canyon fault, where it is in this report as related to the age and correlation of represented by the large Kodonophyllum mulleri Mer­ Great Basin Silurian rocks of the intermediate lime- riam (pl. 4, figs. 5, 6). Kodonophyllum sp. b (pl. 5, 30 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY figs. 22--24), described below, occurs in the middle M1314. In middle coral-brachiopod zone near bottom of coral-brachiopod zone at Bootstrap Hill, Tuscarora upper 275 m lithologic division. Mountains. The range of Kodonophyllum in the classic Gotland Subfamily KODONOPHYLLINAE Merriam Silurian column is Wenlockian to Ludlovian; occur­ rences in Great Britain are reported as Wenlockian. In Members of the subfamily Kodonophyllinae Mer­ the Great Basin, the occurrence in coral zone E is late riam are very thick walled fasciculate colonial and Ludlovian, in the Tuscarora Mountains at Bootstrap platelike solitary Rugosa having the characteristics of Hill in the middle coral-brachiopod zone, perhaps as the family Kodonophyllidae. The tabulae and tabellae old as high Wenlock, but more likely Ludlovian. arch distally, combining with septal ends to form an Certain early Middle Devonian (Great Basin Devo­ axial structure and a calicular boss. nian coral zone D) occurrences of Kodonophyllum-like Kodonophyllum mulleri Merriam (1973a), of the corals of the Nevada Formation (Grays Canyon Lime­ upper part of the Roberts Mountains Formation, is a stone Member of the Eureka area) have the very wide medium to large solitary coral with deep calice and peripheral stereozone of Kodonophyllum; the longitud­ large calicular boss (pl. 4, figs. 5, 6). The major septal inal features are unknown. These forms are probably count is about 48; septa, somewhat thickened through­ aberrant developments from the Siphonophrentis out, are excessively thickened in the wide peripheral (Breviphrentis) group that is characteristic of Great stereozone and in the axial structure, where they Basin Devonian coral zone D (Merriam, 1973c). are obscured by merging with stereoplasm and septal terminations. Subfamily MYCOPHYLLINAE Hill Kodonophyllum mulleri Merriam occurs in the Genus Mucophyllum Etheridge northern Simpson Park Mountains in the upper part of Members of the subfamily Mycophyllinae, charac­ the Roberts Mountains Formation; locality M1107, terized by large discoid to turbinate corallites with a Great Basin Silurian coral zone E. broad calicular platform and a fiat-bottomed central Kodonophyllum sp. b pit, are especially characteristic of the Silurian. Septa Plate 5, figures 22-24 are numerous and thick and in contact laterally form­ Figured material.-USNM 166481. Roberts Moun­ ing a wide, thick stereozone. Tabulae are complete and tains Formation; Bootstrap Hill, Tuscarora Mountains, straight to undulant. Longitudinal sections show the Nev., locality M1314. fine incremental layering of a very thick stereoone Diagnosis.-Slender, subcylindrical Kodonophyllum transected by nearly vertical trabecular pillars. with axial arching of wide tabulae, moderate to low for Schlotheimophyllum of the Gotland Silurian has a the genus. similar growth habit but differs by having a more com­ Transverse thin sections .-Thick major septa plex axial structure, more like that of Kodonophyllum. number about 24, most of which extend to the axial Mucophyllum is known from Australia and from the structure. Minor septa buried in stereozone, which is Klamath Mountains, Calif. All occurrences are in Silu­ one-fourth of corallite diameter. Septal grooves are rian deposits of late Wenlockian to Ludlovian age. broad and shallow. No discrete epitheca. Mucophyllum oliveri (pl. 4, figs. 1-4) has near­ Longitudinal thin sections.-Tabulae are rather horizontal and undulant tabulae that are thickened closely spaced, very uneven; many are complete, peri­ where merging with the longer septa. M. oliveri comes axially depressed, and rise nonuniformly toward axis. from the east block of the Coal Canyon fault at Coal Axial rise is moderate to low for this genus. Trabeculae Canyon, northern Simpson Park Mountains, in the in wide stereozone is steeply inclined outward. upper part of the Roberts Mountains Formation; local­ Comparison with related forms .-Kodonophyllum ity 1108; Great Basin Silurian coral zone E. mulleri of the Roberts Mountains Formation is a larger, more robust species with more numerous septa Family STREPTELASMATIDAE Nicholson than Kodonophyllum sp. b. It also has a more pro­ Genus Brachyelasma Lang, Smith, and Thomas nounced axial rise of tabulae. K. truncatum (Linnaeus) Simple, solitary, nondissepimented Rugosa with of the Gotland Silurian, a colonial form having partly complete tabulae ranges from Late Ordovician ceratoid corallites, has a somewhat wider stereozone into the Silurian of the Great Basin. The tabulae bend than the subcylindrical K. sp. b; otherwise features in downward as they approach the wall. Major septa do detailed thin-section resemble those of the Nevada not reach the axis, though they commonly extend more species. than half that distance. Some species have a fossula. Occurrence.-Roberts Mountains Formation; south­ Brachyelasma resembles the Devonian Breviphren­ ern Tuscarora Mountains, Nev. Bootstrap Hill, locality tis, which has a wider stereozone and undergoes re- FAMILY PYCNOSTYLIDAE STUMM 31

peated rejuvenescence to form long segmented coralla Family PYCNOSTYLIDAE Stumm not found in Brachyelasma. Pycnostylid rugose corals are among the most abun­ Two species of Brachyelasma occur in the Roberts dant and distinctive nondissepimented genera in cer­ Mountains Formation. Brachyelasma sp. c (pl. 5, figs. tain facies of the Silurian System. It is possible these 25, 26) comes from locality M1383 in the lower coral corals have Devonian descendants like Cyathopaedium beds of the west fault block at Coal Canyon, Simpson and Fletcherina. None are known in rocks of Late Park Mountains; Brachyelasma sp. b (pl. 12, figs. Ordovician age. 29--33) is from locality M1120 at Bootstrap Hill, Tus­ The pycnostylids are colonial fasciculate Rugosa carora Mountains, in the lower coral-brachiopod zone having subcylindrical corallites. The septa are very with Cyathophylloides. short and vertically continuous, or lacking. The stereo­ zone is narrow. Most tabulae are complete and un­ Family STAURIIDAE Edwards and Haime arched medially. There are no dissepiments. Reproduc­ Genus Cyathophylloides Dybowski tion is by multiple offsets peripherally from the calice Cyathophylloides, a primitive colonial nondis­ interior. sepimented rugose coral, is related to Favistella (or The Silurian genera are Pycnostylus Whiteaves, Favistina Flower) and to the phaceloid Palaeophyllum. Stylopleura Merriam, and Fletcheria Edwards and These corals are common in the Upper Ordovician, but Haime. Cerioid colonial genera like Maikottia Lav­ Cyathophylloides and Palaeophyllum range upward rusevitch and similar large undescribed genera from into the Lower Silurian. the Roberts Mountains Formation are provisionally Cyathophylloides forms massive cerioid colonies assigned to this family. with narrow corallites having 12-14 smooth, simple The taxonomy of the Pycnostylidae has been treated lamellar major septa that meet axially where they may in some detail by Merriam (1973a). be twisted but do not form a discrete axial structure. Genus Pycnostylus Whiteaves Tabulae are complete; minor septa are short to fairly With the type species Pycnostylus guelphensis Whit­ long. Cyathophylloides is represented in the Roberts eaves of the Guelph Dolomite, Guelph, Ontario, (pl. 1, Mountains Formation by C. fergusoni Merriam of figs. 16-19), Pycnostylus is characterized by only four Great Basin Silurian coral zone A in the Mill Canyon internal calice offsets or peripheral offsets. Pycnostylus sequence at Ikes Canyon, Toquima Range. A similar elegans Whiteaves probably has a greater number of species C. sp. f (pl. 12, figs. 27, 28) comes from the offsets (pl. 1, fig. 20). Pycnostylus is not known to pos­ lowermost coral-brachiopod zone at Bootstrap Hill sess the lateral connecting elements of Stylopleura. (locs. M1120, M1412). Several poorly known Asiatic and Australian corals Family TRYPLASMATIDAE Etheridge referred to by Hill (1940) as ((ampleximorphs" are doubtfully related to Pycnostylus. These corals are re­ The family Tryplasmatidae, characterized by ported to have amplexoid lamellar septa. Several spinose or acanthine septa, is especially common in little-known Silurian corals of similar character have rocks of the Silurian System. Tryplasmids have been been classified as Pycnostylus by Hill (1940) as reported in strata of the Devonian (Hill, 1956, p. F312; Amplexus by Grabau (1930) and by Shimizu, Ozaki, Oliver, 1960, p. 15) and are known in Late Ordovician and Obata (1934), and asAmplexoides by Wang (1947). rocks of Sweden. Reviews of the classification have Pycnostylus Whiteaves is probably not a junior syn­ been published by Hill (1936) and by Merriam (1973a). onym of Fletcheria Edwards and Haime as interpreted They are readily distinguished from the other nondis­ by several workers. Fletcheria tubifera Edwards and sepimented Rugosa that have wide complete tabulae Haime from Gotland has no septa but it does have a by their acanthine septa, and they do not have the wide distinctive wall structure, and some of the tabulae (or stereozone of the Kodonophyllidae or the multiple large tabellae) are distally convex (pl. 3, figs. 1-7). interior calice offsets of the pycnostylids. The Forms assigned by Norford (1962, pl. 15) to Fletcheria Stauriidae-like Palaeophyllum of the Late Ordovician and Silurian has medi urn to long lamellar septa. are probably best placed in Pycnostylus. A similar Pyc­ nostylus, Pycnostylus sp. 1 (pl. 2, fig. 10), occurs in the The general characteristics of the tryplasmids are Lone Mountain Dolomite but has not been found in the well shown by the undescribed Gotland species Try­ Roberts Mountains Formation. plasma sp. g of the Hemse Group (Ludlovian) (pl. 3, figs. 16-19), T. newfarmeri Merriam (pl. 3, figs. 10-15) Genus Stylopleura Merriam of Great Basin Silurian coral zone C, and T. duncanae The genus Stylopleura with type species S. berthi­ Merriam (pl. 3, figs. 8, 9) of Great Basin Silurian coral aumi Merriam (1973a), which was first found in the zone D. upper part of the Roberts Mountains Formation unit 3, 32 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY is a phaceloid to cerioid coral. It differs from Pycno­ Corals provisionally assigned to this informal group stylus by having a large number of subequal peripheral are Maikottia sp., cf. M. turkestanica Lavrusevich of offsets of internal calice offsets and long tubular­ the Porcupine River, Alaska (Merriam, 1973a), Sty­ connecting elements between corallites. Open bushy lopleura? sp. T of the Ikes Canyon area, Toquima forms have trumpet-shaped flaring calices. Range, Nev., and the compact thin-walled aseptate Of the described species of Stylopleura, Stylopleura pycnostylid species Stylopleura? sp. c from Coal Can­ berthiaumi of Great Basin Silurian coral zones D and E yon, Simpson Park Mountains, here described. (pl. 1, figs. 1-9; pl. 2, figs. 8, 9) has a more open Stylopleura? sp. c phaceloid construction with flaring calices than the Plate 2, figures 1, 2 other species. S. nevadensis of Great Basin Silurian coral zone E (pl. 2, figs. 3-6) tends to be of more Stylopleura? sp. c is a large compact species. Coral­ crowded phaceloid to subcerioid compact construction lites are as much as 24 mm wide and have a very thin with fewer lateral elements. Also found is Stylopleura straight wall. Septa are absent. Tabulae are closely cf. S. nevadensis, n. gen., n. sp. (pl. 2, fig. 7.) spaced, uneven, and have a pronounced sag axially. Most tabulae are entire, but some near the periphery Stylopleura sp. b are short and terminate against a complete tabula. The Plate 1, figures 10-12 similarity of this form to the Australian Silurian Stylopleura sp. b, known from isolated fragmentary Yassia Jones in longitudinal section is deceiving, as it corallites at Bootstrap Hill, Tuscarora Mountains, re­ lacks the dissepiments of Y assia in the strict sense. sembles S. berthiaumi, but the specimens in the collec­ Occurrence.-A single incomplete corallum from tion do not include those with reproductive calices. The Coal Canyon, Simpson Park ~viountains, Nev., locality long tubular connecting processes are well shown. M1379, in sheared Roberts Mountains limestone at the Stylopleura sp. b occurs at localities M1314 and west edge of the Coal Canyon fault zone. M1315 in the middle coral-brachiopod zone of the Roberts Mountains Formation at Bootstrap Hill, where DISSEPIMENTED RUGOSA OF THE GREAT BASIN it is associated with Kodonophyllum sp. b, Kypho­ SILURIAN LIMESTONE FACIES sp. b, and sp. phyllum Calostylis? Rugose corals with dissepiments, uncommon in the Stylopleura sp. c, cf. S. berthiaumi Merriam Cordilleran Late Ordovician, had become abundant by Plate 1, figures 13-15 mid-Silurian time with the appearance of the lyko­ Stylopleura sp. c, cf. S. berthiaumi is a fairly compact phyllids, Entelophylloides, Petrozium, and other gen­ phaceloid Stylopleura from the lower coral zone of the era. Several Late Silurian colonial dissepimented west block of the Coal Canyon fault at Coal Canyon, groups disappeared by the end of the Silurian or the northern Simpson Park Mountains. It has the multiple earliest Devonian, and new ones appeared in early calice offsets but none of the lateral connecting ele­ Middle Devonian time. Families of special importance ments of S. berthiaumi. This form occurs here in the in the Late Silurian of the Great Basin intermediate same beds with Carlinastraea tuscaroraensis and Ton­ limestone belt are the Spongophyllidae and the En­ kinaria simpsoni Merriam and is accordingly regarded dophy llidae. as an occupant of the approximate interval of Great Basin Silurian coral zone D. Family SPONGOPHYLLIDAE Dybowski Genus Carlinastraea, n. gen. COMPACT CERIOID PYCNOSTYLIDAE OF SILURIAN Type species .-Carlinastraea tuscaroraensis n. gen., LIMESTONE FACIES n. sp., here designated; Late Silurian, upper beds of the Unusually large phaceloid to cerioid Rugosa lacking Roberts Mountains Formation, Nev. multiple peripheral offsets and resembling Stylopleura Diagnosis.-Cerioid spongophyllid having slender, nevadensis Merriam are known from Late Silurian fairly thick walled corallites, a strong marginarium of limestones of the Cordilleran belt. Being compact or large to very large, steeply inclined lonsdaleioid dis­ appressed, these specimens do not show lateral con­ sepiments, and a nonuniform pattern of partially necting elements. Some, like Stylopleura? sp. T (Mer­ aborted septa in the tabularium of some corallites. riam, 1973a), have very wide corallites, a very thick Remarks.-Direct comparison of Carlinastraea with wall, and closely spaced tabulae; others, like the large the most nearly related genus, Spongophyllum Ed­ aseptate pycnostylid Stylopleura? sp. c from Coal Can­ wards and Haime, is not possible as the Edwards and yon in the Simpson Park Mountains, have a very thin Haime (1853) type material of S. sedgwicki (the type wall without septa. The septate forms have short lon­ species) has been lost and only the original published gitudinally continuous septa. illustrations (pl. 6, figs. 1-4) remain as a basis for FAMILY SPONGOPHYLLIDAE DYBOWSKI 33 reappraisal. These illustrations (Edwards and Haime, are no records of strongly lonsdaleioid cerioid Devo­ 1853, pl. 56, figs. 2, 2a-2e) with little doubt figure two nian corals that closely resemble group 1 of S. different corals: (1) group 1 (pl. 56, figs. 2d, e), a thin­ sedgwicki or the new genus Carlinastraea. walled genus having many small peripheral Species from the Ural Mountains, USSR, assigned to lonsdaleioid dissepiments, (2) group 2 (pl. 56, figs. 2, Spongophyllum by Shurygina (1968) are herein as­ 2a-2c), a genus having narrower, moderately thick­ signed to the new genus Carlinastraea. Shurygina walled corallites, lacking or nearly lacking conspicu­ (1968, p. 121-135, pl. 59, figs. 1-3) reported these ques­ ous lonsdaleioid dissepiments. It is not likely that the tionable species as Lower Devonian. The coral fauna two forms fall within the range of variation of a single that is included with Spongophyllum [Carlinastraea] genus and species; so far this possibility is unknown. by Shurygina in these Ural beds is more suggestive of The usually accepted characterization of Spongophyl­ the forms in the Late Silurian of Gotland. Among the lum is that of lonsdaleioid group 1, and it is with this genera from the Urals so listed are Spongophylloides, group that Carlinastraea is compared. Comparison of Tryplasma, Neomphyma, and Rhizophyllum. Also in­ Spongophyllum sedgwicki to Lonsdaleia by Edwards cluded is Salairophyllum, which in southeastern and Haime (1851, p. 425; 1853, p. 242) is further evi­ Alaska occurs with the large Late Silurian Con­ dence that the original diagnosis is that of a cerioid chidium alaskense and, at Coal Canyon, Simpson Park rugose coral with well-developed lonsdaleioid dissepi­ Mountains, Nev., occurs as float, doubtless from high ments. beds of the Roberts Mountains Formation. It is unfortunate that Jones (1929, p. 89) selected as the ((neotype" of Spongophyllum sedgwicki a float Carlinastraea tuscaroraensis n. gen., n. sp. specimen in ((a dark coloured pebble" from ((South Plate 6, figures 5-7; plate 7, figures 1-4 Devonshire." Although not figured by Jones in 1929, Type and figured specimens .-Holotype USNM thin sections of this ((neotype" were illustrated by 166482, upper beds of the Roberts Mountains Forma­ black and white drawings by Birenheide (1962, p. tion, locality M1313, Bootstrap Hill, Nev. Figured 6S-7 4, pl. 9, fig. 8, pl. 10, fig. 10). The Birenheide specimens USNM 166483, locality M1384, Coal Can­ figures show a coral without conspicuous lonsdaleioid yon, Nev.; USNM 166484, locality M1409, Cortez, Nev. dissepiments, agreeing reasonably well with non­ Diagnosis .-Large massive Carlinastraea. The lonsdaleioid group 2 of the Edwards and Haime type coralla have several hundreds of very narrow corallites illustrations. whose septa are marked peripherally by strong wall Comparing Carlinastraea with group 1 lonsdaleioid crests; septa are mostly thin, nonuniform, and irreg­ Spongophyllum of Edwards and Haime, the new genus ularly wavy; the largest lonsdaleioid dissepiments sub­ Carlinastraea differs by having a much thicker wall tend one-fifth of a corallite circumference. with strong wall crests and all corallites have much Transverse thin sections.-Wall is stereoplasmically larger lonsdaleioid dissepiments. The septa of Car­ thickened, has about 26 prominent, mostly obtuse wall linastraea, being less continuous radially than those of crests uniformly spaced. Major septa are thin, smooth, Spongophyllum sedgwicki, are marked peripherally by unevenly wavy, nonuniform, and number 12-14-the septal crests. Aborting of septa, a feature of Carlina­ longest reach the axis; septal crests are developed, per­ straea, is unknown in S. sedgwicki. Mature corallites ipherally in some corallites. Lonsdaleioid dissepiments without lonsdaleioid dissepiments, as in group 2 of the number 6 to about 10 in a peripheral cycle and vary Edwards and HaimeS. sedgwicki, were not observed. greatly in size; the largest subtend one-fifth of the Carlinastraea tuscaroraensis n. gen., n. sp., occurs in corallite circumference. Minor septa, weakly de­ the upper part of the Roberts Mountains Formation. As veloped, show as wall crests and sporadic outer septal recorded by Edwards and Haime, the Spongophyllum crests. types came from Devonian strata of Torquay, Devon­ Longitudinal thin sections.-Tabularium is less than shire, England. The Jones-selected neotype, being a one-fourth to more than one-third of corallite width; float cobble, fails to confirm this occurrence. Recent delicate tabulae are only in part complete, sag irregu­ studies of southern England Devonian Rugosa by larly proximally, and for the greater part are rather Taylor (1950), Middleton (1959), and Webby (1964) closely spaced. Lonsdaleioid dissepiments, predomi­ make no reference to Spongophyllum. Two species of nantly large with very steep axial inclination, are ar­ Spongophyllum (sp. A and sp. B) have been reported ranged in one to three columns on each side. from the McCann Hill Chert of Alaska by Oliver (in Fine structure .-Stereoplasmic wall is without dis­ Oliver and others, 1975); he considered it to be Early(?) crete trabeculae and shows very fine calcitic fibers to Late Devonian (Emsian? to Frasnian). Elsewhere in transversely oriented; faint lamellar texture in some the Cordilleran belt of western North America, there places is developed toward inner wall surface. 34 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY

Comparison with related forms .-Carlinastraea the Gazelle Formation, Klamath Mountains, Calif. giganteum (Shurygina) of the Ural Mountains, USSR Klamathastraea differs from Toquimaphyllum by hav­ (Shurygina, 1968, p. 135, pl. 59, figs. 1, 2), differs from ing a much wider tabularium, tabulae with a marginal the new species by having less developed wall crests, depression, and a pronounced tendency to abort septa straighter septa, and less steeply inclined lonsdaleioid in mature growth stages. dissepiments. Carlinastraea originalis (Zhmaev), also Australophyllum (Australophyllum) sp. c of the Urals (Shurygina, 1968, p. 134, pl. 59, figs. 3a-b), Plate 8, figures 3-7 has less developed wall crests and more aborted septa Diagnosis.-Large Australophyllum with corallites than C. tuscaroraensis. of greatly varying width; lonsdaleioid dissepiments are Occurrence.-Upper part of the Roberts Mountains in restricted peripheral patches only; tabulae closely Formation, northern Simpson Park Mountains, Nev.: spaced, in places crowded; septa are minutely wavy locality M1384, Coal Canyon, west side in beds with a with spines and bumps in the periaxial band. fauna believed to represent Great Basin Silurian coral Transverse thin sections .-Corallites, highly var­ zone D. Tuscarora Mountains, Nev., locality M1313, iable in size, range in diameter from 9 to 28 mm at the Bootstrap Hill. Carlinastraea cf. C. tuscaroraensis, same distal corallum surface. Septal count is about 62 Cortez Mountains, Nev., locality M1409, pl. 8, figs. 1, 2. in large corallites. Major septa reach the axis and are twisted near it; minor septa range from one-half to Family ENDOPHYLLIDAE Torley three-fourths length of major septa. Septa, locally Genus Ketophyllum Wedekind thickened in dissepimentarium, may be slightly thick­ The solitary genus Ketophyllum is common in the ened in tabularium, wherein they become minutely Silurian of Gotland, Sweden, and in eastern Europe. wavy and ornamented laterally with spines and nodes. Only fragments of Ketophyllum sp. t (pl. 10, figs. 18, Narrower corallites show stereoplasmic wall thick­ 19) have been collected by McKee in the Toquima ening. Some larger lonsdaleioid patches quite irregu­ Range, Nev. Some species of Ketophyllum are virtually lar. indistinguishable from the Devonian Tabulophyllum Longitudinal thin sections.-Tabularium width is except for the presence of a weak fossula in some forms one-fourth to one-third of corallite diameter. Dissepi­ of Ketophyllum (Wedekind, 1927); this structure is ments are medium to large in as many as nine columns usually not visible in Tabulophyllum. Both have a very on each side of wider corallites. Outer dissepiments are wide tabularium and closely spaced straight to some­ much less steeply inclined than inner ones. Tabulae what undulant tabulae, many of which are complete. are mostly close spaced or crowded; many are complete Both genera have a peripheral band of large lonsdale­ with a wavy proximal sag. Inner septal extensions ioid dissepiments, and the thin septa break up periph­ have numerous lateral spines and bumps. erally as septal crests. Comparison with other forms.-Australophyllum Ketophyllum sp. t occurs in the Toquima Range at (Toquimaphyllum) johnsoni Merriam of Great Basin locality M1393 in the Mill Canyon sequence on the Silurian coral zone E has a much more continuous and south side of Ikes Canyon a mile northwest of the uniform marginarium of lonsdaleioid dissepiments canyon mouth. that are less steeply inclined; its tabularium is nar­ rower than that of A ustralophyllum sp. c. A ustralo­ Genus Australophyllum Stumm phyllum landerensis Merriam has fewer septa, a more Large colonial heads of Australophyllum are com­ fully developed pattern of lonsdaleioid dissepiments, mon in Great Basin Silurian coral zone E, where it is and fewer lateral spines and bumps on axial extensions represented by the subgenus A ustralophyllum (To­ of septa. Australophyllum sp. v of Late Silurian and quimaphyllum). Other forms of Australophyllum Early Devonian beds in the Vaughn Gulch Limestone sensu stricto occur in the same uppermost beds of the has a similarly restricted pattern of lonsdaleioid dis­ Silurian limestone facies and, though sparsely repre­ sepiments but it differs by having narrower, thicker sented, range upward into the Lower Devonian Rabbit walled corallites and fewer septa; the septa are un­ Hill Limestone. Some Middle Devonian species that thickened and lack the abundant lateral spines and were referred to A ustralophyllum in the past have but nodes of A ustralophyllum sp. c. A similar coral from few sporadic lonsdaleioid dissepiments and are locality M1445 near the Big Six mine, Tuscarora perhaps best assigned to other genera. The Mountains, has the spiny septal extensions of A us­ lonsdaleioid subgenus Toquimaphyllum has a tralophyllum sp. c but has very few small lonsdaleioid worldwide geographic distribution. A similar form oc­ dissepiments and a thicker wall. The Tuscarora Moun­ curs to which the generic name Klamathastraea has tains coral also differs by having localized crossbar been given (Merriam, 1972) in Late Silurian beds of carinae. FAMILY ENDOPHYLLIDAE TORLEY 35

Occurrence.-Coal Canyon, Simpson Park Moun­ as septal crests. Some mature corallites have much tains; locality M1318 in Coal Canyon fault zone on shortened or obsolete septa. west side of canyon. Although this coral head was not A ustralophyllum (Toquimaphyllum) differs from collected in place and the locality is in deformed strata Australophyllum sensu stricto by the greater develop­ within the fault zone, its field location suggests that it ment of large lonsdaleioid dissepiments forming a con­ may have come from beds stratigraphically above the tinuous marginarium, by the presence of septal crests lower Carlinastraea coral beds of the west fault block. peripherally, and by its tendency to lose septa in some Australophyllum (Australophyllum) sp. v mature corallites. The septa of Toquimaphyllum lack the carinae reported by Stumm (1949, p. 34) in Aus­ Plate 8, figures 8-11 tralophyllum sensu stricto. Endophyllum differs by Diagnosis.-Australophyllum has predominantly having a much wider tabularium; the tabulae being narrow, fairly thick-walled corallites and a restricted nearly fiat to slightly arched medially with a periph­ pattern of lonsdaleioid dissepiments; septa are mostly eral sag. The type species of Endophyllum shows a ten­ thin with very few nodes and spines in periaxial band. dency to lose the outer wall, thus becoming partially Transverse thin sections .-Septal count is about 42 aphroid. As known, Endophyllum does not, like To­ in average corallites. Some major septa reach axis; quimaphyllum, have a tendency to shorten and lose minor septa, mostly less than half the length of the septa. major septa, commonly appear only as septal crests Australophyllum (Toquimaphyllum) johnsoni Mer­ peripherally. Septa are slightly wavy. Lonsdaleioid riam (pl. 6, figs. ~10) is the most abundant and dis­ dissepiments are medium and small and are restricted tinctive rugose coral of Great Basin Silurian coral zone or absent in some corallites. E (Late Silurian). The relations of this subgenus and Longitudinal thin sections.-Tabularium is one­ its worldwide distribution are discussed in some detail third to less than one-fourth of corallite diameter; by Merriam (1972a). tabulae are partly complete, closely spaced, and sag­ The type species Australophyllum (Toquimaphyl­ ging. Dissepiments are steeply inclined, 4--7 columns lum) johnsoni Merriam comes from the Roberts Moun­ on each side. Periaxial extensions of septa are nearly tains Formation of the Mill Canyon sequence of the devoid of nodes and spines. Thickened wall shows no Ikes Canyon area, Toquima Range, Nev. Much of the discrete trabecular structure. study material was collected from the upper beds of Comparison to related forms.-Australophyllum sp. the Roberts Mountains Formation at Coal Canyon, v is most closely related to A. landerensis Merriam Simpson Park Mountains. from Lower Devonian beds of the northern Toquima Australophyllum (Toquimaphyllum) johnsoni Merriam Range, Nev. The Devonian form differs by having Plate 6, figures 8-10 somewhat thickened and more wavy septa in the Collections from several localities in Silurian lime­ tabularium; the periaxial extensions of septa show stones between the northern Simpson Park Mountains more lateral nodes and bumps than in A. sp. v. Aus­ and the northern Inyo Mountains of California contain tralophyllum sp. c has scattered, much wider coral­ many complete and fragmentary colonies of this lites with a septal count of 62, and its periaxial septal species. A ustralophyllum (Toquimaphyllum) johnsoni extensions show abundant lateral spines and nodes. has been found only in strata of Great Basin Silurian Occurrence and age .-Mazourka Canyon, northern coral zone E. A detailed description of this species and Inyo Mountains, Calif.; upper middle and upper beds of a review of its taxonomy have recently been published the Vaughn Gulch Limestone. Locality M1093, bottom by Merriam (1973a) of the upper unit of the Vaughn Gulch Limestone of This species is characterized by wide corallites, the Late Silurian and Early Devonian(?) age. Locality major septa normally reach the axis and break up M1410 about 50 feet stratigraphically below M1093. peripherally as septal crests. Most mature corallites Subgenus Toquimaphyllum Merriam have a narrow tabularium and a correspondingly wide (subgenus of Australophyllum Stumm) dissepimentarium in which the outer dissepiments are Toquimaphyllum, a cerioid compact subgenus of nearly flat. Partially aseptate corallites show only Australophyllum, has a wide continuous marginarium abbreviated tips of major septa. of predominantly large lonsdaleioid dissepiments; the Occurrence.-Upper beds of the Roberts Mountains peripheral dissepiments are nearly fiat or dip axially Formation in the Mill Canyon sequence at Ikes Can­ at low angles. The tabularium is narrow to moderately yon, Toquima Range; locality M1114 (type locality). wide; the tabulae are closely spaced, sagging, and are Upper beds of the Roberts Mountains Formation in without a marginal depression. The septa are thin, east block of Coal Canyon fault zone, Coal Canyon, smooth, fairly straight, and discontinuous peripherally northern Simpson Park Mountains; several localities 36 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY

including M1106 and M1108. Foothill exposures 2.4 scribed species. In the Great Basin, Kyphophyllum oc­ km north of Red Hill, SEV!NE¥1 sec. 5, T. 25 N., R. 50 curs in Great Basin Silurian coral zone E (Late Silu­ E., Simpson Park Mountains; locality M1446. Upper rian). part of middle unit of Vaughn Gulch Limestone (these Kyphophyllum nevadensis Merriam specimens differ slightly from typical A. johnsoni), Plate 9, figures 1-3 Mazourka Canyon, northern Inyo Mountains; locality Kyphophyllum nevadensis has medium-wide to nar­ M1115. Australophyllum, possibly A. johnsoni, occurs row corallites of variable diameter with a small in the Antelope Peak section 19 km north of wells, number of major septa (about 22) and sporadic groups Elko County, Nev. of relatively large lonsdaleioid dissepiments. Family CHONOPHYLLIDAE Holmes Occurrence.-Ikes Canyon (Mill Canyon sequence), Genus Chonophyllum Edwards and Haime Toquima Range, Nev. Great Basin Silurian coral zone Chonophyllum shares some characteristics of E. Locality M1114, northwest side of Copper Mountain Ketophyllum and other members of the Endophyllidae, in lower part of the McMonnigal Limestone. but it is solitary and usually has a narrower tabular­ Kyphophyllum sp. b ium than most endophyllids. The lonsdaleioid pattern Plate 5, figures 15--21 of Chonophyllum is much more irregular or less un­ Kyphophyllum sp. b has very narrow corallites for iform and localized in peripheral patches. the genus, a few wavy major septa (about 20), very few Chonophyllum simpsoni Merriam (pl. 7, figs. 5, 6; lonsdaleioid dissepiments, and lateral offsets that are pl. 9, figs. 9, 10) from the uppermost part of the Roberts attached at former calice rims. The dissepiments are Mountains Formation is a large trochoid to ceratoid all steeply inclined or vertical. Longitudinal exterior species with major septa reaching the axis and a nar­ ribbing is strongly developed. row tabularium. Its tabulae are closely spaced, undu­ Occurrence.-Tuscarora Mountains, Nev. Locality lant, and mostly incomplete with marginal tabellae. M1314, middle part of the Roberts Mountains For­ Inner dissepiments are commonly anguloconcentric in mation. transverse section. Chonophyllum simpsoni Merriam occurs at locality Kyphophyllum sp. c M1108 in the east block of the Coal Canyon fault, Coal Plate 9, figures 4-8 Canyon, Simpson Park Mountains, Nev., the upper Kyphophyllum sp. c has large corallites for the part of the Roberts Mountains Formation. genus and a large number (about 60) of long, smooth, somewhat wavy septa, many of which are greatly Family KYPHOPHYLLIDAE Wedekind lengthened minor septa. Longitudinal sections of K. This large and somewhat heterogeneous provisional sp. c are similar to those of K. nevadensis with a few group of Silurian Rugosa as revised by Merriam large localized lonsdaleioid dissepiments. Some coral­ (1973a) contains about eight gene;ra as follows: lites are joined by lateral processes not observed inK. Kyphophyllum Wedekind nevadensis. Strombodes Schweigger Occurrence.-Coal Canyon, Simpson Park Moun­ Petrozium Smith tains, Nev. Locality M1380, upper part of the Roberts Entelophyllum Wedekind Mountains Formation. Entelophylloides Rukhin (as a subgenus) Entelophylloides (Prohexagonaria) Merriam Kyphophyllum sp. t Neomphyma Soshkina Plate 10, figures 14-17 ?Tonkinaria Merriam. Kyphophyllum sp. t has narrow corallites of greatly Of these genera, Kyphophyllum and Tonkinaria are of varying diameter. The larger corallites have about 20 greatest importance to the stratigraphy of the inter­ long, somewhat wavy major septa and very short mediate limestone facies. minor septa. All dissepiments are steeply inclined. The Genus Kyphophyllum Wedekind stereozone is relatively wide for this genus. Kyphophyllum builds large bushy phaceloid col­ Occurrence.-Toquima Range, Nev. Late Silurian onies; its subcylindrical corallites have a narrow and Devonian Tor Limestone of the June Canyon se­ stereozone and medium-narrow closely spaced tabulae. quence; locality M1397. Restricted segments of the marginarium are occupied Genus Tonkinaria Merriam by large, irregular lonsdaleioid dissepiments. In the Tonkinaria, a solitary or loosely phaceloid genus, Roberts Mountains Formation and Vaughn Gulch commonly has a flaring trumpet-shaped calice, cera­ Limestone, this genus is represented by K. nevadensis toid to turbinate corallites, and a narrow tabularium. Merriam, K. sp. b, K. sp. c, K. sp. t, and other unde- The wall is a narrow stereozone, the septa are thin, and FOSSILS ASSOCIATED WITH RUGOSE CORALS 37

major septa reach or are slightly withdrawn from the FOSSILS ASSOCIATED WITH RUGOSE CORALS axis. Dissepiments are mostly large, elongate, and IN GREAT BASIN SILURIAN LIMESTONES steeply inclined. Multiplication is by peripheral calice Tabulate corals, crinoids, brachiopods, and dasycla­ offsets, of which there may be several on a single flar­ dacean algae are common members as well as a Her­ ing calice rim. cynella-like mollusk (pl. 10, fig. 20) of Great Basin Occurrence.-Tonkinaria simpsoni Merriam, the Silurian coral communities. In general, the most type species, comes from locality M1100 (pl. 5, figs. abundant fossil remains of the limestone coralline 1-9) in Roberts Mountains Formation unit 3 and Great facies are favositids, including entire massive heads of Basin Silurian coral zone D of the reference section; Favosites (pl. 10, figs. 9-13) and digitate forms (pl. 10, locality M1103 in the Mill Canyon sequence at Ikes fig. 8). With these branching corals are the more slen­ Canyon, Toquima Range, Nev., in Great Basin Silu­ der ramose genera Cladopora (pl. 10, figs. 5--7) and rian coral zoneD; and locality M1383 (pl. 5, figs. 10-14) Coenites. Halysites is rather uncommon and seems to on the west side of Coal Canyon, Simpson Park Moun­ be absent from the higher Silurian beds. Crinoids are tains, where it occurs in the west block of the Coal represented by large volumes of crinoidal debris filling Canyon fault together with Carlinastraea tuscaroraen­ interstices and making crinoidal limestone, but no ar­ sis and Stylopleura. ticulated or partial crinoid crowns were collected. Family CALOSTYLIDAE Roemer Limestone beds that contain silicified fossils yield Genus Calostylis Lindstrom abundant well-preserved brachiopods during extrac­ tion of the coral material by the acid technique. Among Fragmentary and rather poorly preserved solitary such occurrences are the Great Basin Silurian zone D rugose corals from Bootstrap Hill, Tuscarora Moun­ coralline beds in unit 3 of the Roberts Mountains For­ tains, are referred provisionally to Calostylis. This mation type section, the upper coral-rich beds of Great Silurian genus occupies a peculiar relation to all other Basin Silurian coral zone E at Coal Canyon, Simpson Rugosa (Smith, 1930, p. 294) by possessing perforate Park Mountains, and the lower beds of this formation septa and a spongy axial structure. at Bootstrap Hill, Tuscarora Mountains, Nev. Calostylis? sp. (pl. 7, figs. 7, 8) occurs at locality A few of the more distinctive brachiopods occurring M1314, Bootstrap Hill, Tuscarora Mountains, Nev., in with Silurian corals are illustrated herein. The follow­ the middle part of the Roberts Mountains Formation. ing brachiopods occur in unit 3 of the type section Family LYKOPHYLLIDAE Wedekind Roberts Mountains Formation: Solitary lykophyllid Rugosa have not been found to Dicoelosia sp. r (pl. 11, figs. 1~22) be among the more abundant corals of the Great Basin Gypidula sp. r (pl. 11, figs. 25, 26) Silurian limestone. They may have been overlooked in Homoeospira sp. r (pl. 11, figs. 15--17) collecting in limestone, however, as they are fairly Kozlowskiellina sp. f (pl. 11, figs. 9-12) numerous in the Hidden Valley Dolomite. This family Silicified brachiopods from Great Basin Silurian is well represented in the Silurian of Gotland and coral zone E on the east side of Coal Canyon are: elsewhere in Europe where the genera include Phau­ Kozlowskiellina sp. f (pl. 11, figs. 13, 14) lactis, Cyathactis, Pycnactis, and Ryderophyllum. Plectatrypa? sp. c (pl. 11, figs. 23, 24) The Lykophyllidae are medium and large solitary From the Roberts Mountains Formation of the Rugosa with many columns of small to medium, rather Bootstrap Hill area, Tuscarora Mountains, unusually steeply inclined dissepiments and a fairly wide tabu­ well preserved silicified brachiopods were collected by larium made up of closely spaced, uneven, mostly in­ R. J. Roberts from a large talus slab at the base of the complete tabulae and flat tabellae. Greatly thickened slope where the lowest beds of the formation are partly septa of early growth stages persist ontogenetically to exposed. Other rocks in the same collection contained at least early adult growth. A fossula is not generally Cyathophylloides sp. f, an indicator of coral zone A. present in mature transverse sections. The following associated brachiopods are figured in In the Great Basin Silurian, the Lykophyllidae are this report: represented by abundant Ryderophyllum in Great Conchidium sp. b (pl. 12, figs. 1~22) Basin Silurian coral zone B of the Hidden Valley Fardenia sp. b (pl. 12, figs. ~15) Dolomite (Merriam, 1973a) but are unknown in higher Coelospira sp. b (pl. 12, figs. 1-7) horizons of the dolomite facies and have not been iden­ Kozlowskiellina sp. b (pl. 12, figs. 16, 17, 23) tified from limestone facies of the better known sec­ Kozlowskiellina sp. f, which occurs in both Great tions. A lykophyllid resembling Ryderophyllum from Basin Silurian coral zones D and E, bears a rather an undetermined stratigraphic horizon in the Tusca­ close resemblance to Plicocyrtina sinuplicata Havlicek rora Mountains is figured here (pl. 12, figs. 34-36). as figured by Lenz (1972, pl. 1, figs. 1-20). According to 38 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY

Lenz, the Canadian species is of Early Devonian (late group seems to have attained its peak of development Siegenian) age. in limestone facies of Great Basin Silurian coral zone In the type section of the Roberts Mountains Forma­ D. In the Vaughn Gulch Limestone of the lnyo Moun­ tion, the brachiopod Conchidium is most abundant and tains, for example, the middle part of the formation diverse near the top of lithologic unit 2 and below below Great Basin Silurian coral zone E is heavily Great Basin Silurian coral zone C. These pentameroid populated by Verticillopora annulata Rezak. The Ver­ brachiopod faunas have not been studied in detail. The ticillopora beds have yielded much of the best study form resembling Conchidium miinsteri Kiaer is illus­ material of this interesting calcareous algal family. trated (pl. 12, figs. 24-26). CONODONTS OF THE ROBERTS MOUNTAINS FORMATION DASYCLADACEAN ALGAE AS INDICATORS OF THE SILURIAN By jOHN W. HUDDLE The family Dasycladaceae of green calcareous algae Conodonts are abundant in much of the Roberts has a very long geologic range, through the Paleozoic Mountains Formation. The presence of these microfos­ to the present day. One of its subgroups, the Verticil­ sils where no megafossils occur makes their use espe­ loporaea (Rezak, 1959), throve in Great Basin Silurian cially important for stratigraphic studies and geologic seas, occupying environments favorable for the corals. mapping. Many conodont identifications were made for In the Great Basin province, the family is well repre­ geologists mapping in the Carlin gold mine vicinity sented by the genus Verticillopora Rezak (pl. 10, figs. and other areas of the central Great Basin where the 1-4; pl. 11, figs. 1-8), some of whose individuals at­ Silurian and Devonian strata are intensely deformed tained a relatively large size for the family. Verticil­ and would otherwise remain undated paleontologi­ lopora is a fairly reliable Great Basin Silurian indi­ cally. In order to correlate the isolated conodont occur­ cator, as its remains have not yet been recognized in rences with more continuous sections of the Roberts Ordovician strata and it is rarely found in rocks of Mountains Formation, during this investigation cono­ early Devonian age. Its usefulness as a Silurian fossil dont-bearing material was collected from beds yielding was not recognized until recently; the cylindrical and distinctive rugose corals and other megafossils of age conical thalli were generally ignored; collectors re­ significance. Conodont control sections have been garded them as fragmentary sponges or crinoid colum­ measured in areas that have been mapped geologically nar segments, both of which they resemble in gross in the Toquima, Monitor, and Simpson Park Ranges. surficial features. Moreover, the dasycladaceans not Much more bed-by-bed conodont collecting remains to uncommonly occur in crinoidallimestone. be done in these more continuous Silurian and Devo­ The classification of living and fossil dasycladacean nian control sections. An important conodont study of algae has been reviewed by Pia (1926, p. 105). In 1959, the type section of the Roberts Mountains Formation Rezak described Great Basin Silurian algal forms col­ and nearby locations in the Roberts Mountains is that lected by U.S. Geological Survey mapping parties. of Klapper and Murphy (1974); this study serves as a Most of these specimens were assigned to the new reference for comparison of other collected strati­ genus Verticillopora Rezak. This genus is usually pre­ graphic intervals in the central Great Basin. served by silicification. Its thallus commonly has the A chart (table 2) of Roberts Mountains Formation externally jointed appearance of a large crinoid stem conodont assemblages collected from the Toquima with numerous rather uniformly perforated columnar Range and localities between the Toquima Range and segments. Internally the structure differs markedly the Tuscarora Mountains lists conodont stratigraphic from that of a crinoid column by having a wide, com­ ranges. In addition, conodonts from the overlying Rab­ monly five-sided or polygonal central chamber that bit Hill Limestone are included. housed the stipe (Rezak, 1959, p. 119), surrounded by The largest assemblages of conodonts listed here cycles of small radiating tubes or canals that contained come from the upper part of the Roberts Mountains For­ the lateral rays. Externally each segment reveals the mation at Coal Canyon, Simpson Park Mountains, on small-ray canal pores (pl. 11, figs. 3, 6). Some individ­ both the west and east sides of the Coal Canyon fault, uals (pl. 10, figs. 1-3) have long, stout, tubular rootlike and in the August Canyon sequence at Gatecliff in the processes. Toquima Range. Here the name Bastille Limestone Verticillopora ranges upward from Great Basin Silu­ Member of the Masket shale of Kay and Crawford rian coral zone B through coral zone E and into the (1964) is used for strata correlative with part of the Early Devonian. Unlike some rugose coral genera, Roberts Mountains Formation. V erticillopora is present in both limestone and dolo­ Conodont form taxa are used in this report. Some mite facies of the Great Basin Silurian. This algal multielement taxa are recognized, but until the mul- LOCALITY REGISTER OF MAJOR FOSSIL LOCALITIES 39 tielement taxonomy is generally used, the form taxa in the upper part of Roberts Mountains Formation. will serve for stratigraphic studies. Bed-by-bed collecting will be required to firmly es­ Comments on some of the collections of specific stra­ tablish conodont zonal distribution in these western tigraphic interest are: Locality KV-70--19 (8827-SD) Silurian and Devonian strata. Lowermost Devonian on (table 2, No. 6), from the Carlinastraea beds, Coal Can­ the conodont scale conforms closely to that of the grap­ yon in the Simpson Park Mountains. tolite scale, placing the base of the Devonian System The presence of Spathognathodus remscheidensis indicates that below or within Great Basin Silurian coral zone E. Al­ this collection is Early Devonian according to conodont zonation. though only a few conodonts have been identified from The faunal list gives form taxa because it is simpler to use for the Rabbit Hill Limestone, most of these are distinc­ stratigraphic purposes until the multielement taxonomy is more tive Early Devonian forms not recognized in the completely established. Spathognathodus inclinatus and several of the bar-type conodonts listed above (table 2) are part of a mul­ Roberts Mountains Formation. tielement taxon called 'Apparat-H' by Walliser (1964), Hin­ deodella excavata by Jeppsson (1969), and Ozarkodina inclinata LOCALITY REGISTER OF MAJOR FOSSIL by Walliser (1972). LOCALITIES IN THE ROBERTS MOUNTAINS Locality KV-70--24 (8829-SD) (table 2, No. 7), Car­ FORMATION AND CORRELATIVE STRATA linastraea beds, Coal Canyon, Simpson Park Mountains. Roberts Mountains, Nev. Spathognathodus remscheidensis indicates an Early Devonian MllOO.-Roberts Creek Mountain quadrangle, Nevada. Northwest age for this collection and Rotundacodina elegans has been side of Roberts Creek Mountain on measured section 550 m N. 79° reported only from Early Devonian rocks. The new species of W. of summit 9219 at altitude 8,680 feet. Upper part of Roberts Spathognathodus is similar to S. remscheidensis but has aborted Mountains Formation, unit 3. and small denticles anterior to the basal cavity. M1102 .-Roberts Creek Mountain quadrangle, Nevada. Northwest Two conodont assemblages from the upper part of side of Roberts Creek Mountain. Lower beds of Roberts Mountains the Roberts Mountains Formation on the east side of Formation unit 3 with Prohexagonaria. In measured reference the Coal Canyon fault in the Pyramid Hill block (table section. 2, Nos. 8 and 9) are from the interval of Great Basin Northern Simpson Park Mountains, Nev. M1032.-Horse Creek Valley quadrangle, Nevada. East side of Coal Silurian coral zone E (collection 8216-SD, table 2, Canyon in NW% sec. 21, T. 25 N., R. 49 E. Rabbit Hill Limestone. No. 8, made by T. E. Mullens): Collected by R. J. Roberts, 1954. The list includes only form taxa, and several of the bar elements M1075.-Horse Creek Valley quadrangle, Nevada. Near mouth of belonged to multielement species. The age of the fauna is Early Coal Canyon, east side along top of ridge of summit 6909. Rabbit Devonian according to the known conodont ranges. This age is Hill Limestone, Lower Devonian. indicated by Spathognathodus remscheidensis, Spathognathodus M1076.-Horse Creek Valley quadrangle, Nevada. Near mouth of transitans, and the form of Spathognathodus inclinatus, which is Coal Canyon, east side. Mostly float below top of ridge of summit the one common in Lower Devonian rocks. The Icriodus is only a 6909 and downslope to west. Rabbit Hill Limestone, Lower Devo­ small fragment and is not specifically identifiable, but it also nian. suggests a Devonian age. The Scolopodus devonicus is known only M1105.-Horse Creek Valley quadrangle, Nevada. Coal Canyon; from Devonian collections. Many of the other conodonts range float at canyon bottom 0.6 km south of mouth of Coal Canyon. from Silurian to Devonian. Although there are few Late Silurian Material probably from upper part of the section or upper coral­ faunas described now (1973), all the evidence available indicates a rich limestone breccia below Rabbit Hill Limestone. Devonian age for this collection. M1105a.-Horse Creek Valley quadrangle, Nevada. Coal Canyon; Collection 8828-SD (table 2, No. 9): float at canyon bottom 0.6 km south of mouth of Coal Canyon. The two species of Rotundacodina were described by Carls and Material probably from upper part of the section or upper coral­ Gandl (1969) from the Lower Devonian of northeastern Spain. rich limestone breccia below Rabbit Hill Limestone. These species and Spathognathodus remscheidensis indicate an M1106.-Horse Creek Valley quadrangle, Nevada. Coal Canyon Early Devonian age for this collection. The new species of near mouth in SE% sec. 17, T. 25 N., R. 49 E.; east side of canyon at Spathognathodus, S. sp., differs from S. remscheidensis by having altitude about 6,300 feet. Coral-rich limestone depositional breccia aborted anterior denticles instead of tall anterior denticles as inS. in upper part of the Roberts Mountains Formation and below Rab­ remscheidensis. Spathognathodus sp. occurs with Spathog­ bit Hill Limestone. nathodus boucoti Klapper, an Early Devonian species, in a collec­ M1107.-Horse Creek Valley quadrangle, Nevada. About 0.6 km tion made by T. E. Mullens near Carlin, Nev. (USGS loc. 8447- south of mouth of Coal Canyon on east side of canyon and about SD). This occurrence supports my opinion that S. n. sp. cf. 60 m above canyon bottom. Coral-rich limestone and depositional Spathognathodus remscheidensis is an Early Devonian species. limestone breccia. Roberts Mountains Formation. Klapper reports (oral commun., March 1971) the occurrence of M1108.-Horse Creek Valley quadrangle, Nevada. Coal Canyon, what is probably the same species with aborted anterior denticles near mouth. Float collections below coral-bearing depositional in the Gedinnian Windmill Limestone of Johnson and Murphy breccia on east side of canyon below locality M1107. Roberts (1969) at a stratigraphic horizon of 1,617 feet (493 m) in the west Mountains Formation. Coal Canyon section in the Simpson Park Mountains and on Pete MlllO.-Horse Creek Valley quadrangle, Nevada. Coal Canyon, Hanson Creek in the Roberts Mountains. It is unfortunate that near mouth. Coral-rich breccia limestone near top of Roberts species of Icriodus are not present in these collections of Silurian Mountains section on east side of canyon. Collection made by A. J. Coral Zone E to confirm the Early Devonian age of the Zone in Boucot, 1964. terms of conodont chronology. Klapper and Murphy (1974), re­ M1310.-Horse Creek Valley quadrangle, Nevada. SE% sec. 17, T. ported I. woschmidti from Birch Creek in the Roberts Mountains 25 N., R. 49 E. West side of summit 6909 (Pyramid Hill) near top. 40 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY

TABLE 2.-0ccurrence chart of conodont species collected from Silurian and Lower Devonian formations in central Nevada [Conodont species listed are form taxa. Identification by .J. W. Huddle. Multielement apparatus reconstructions in the sense of Jeppson ( 19691 and Klapper and Philip (19711 have not been attempted]

9 10 11 12 13 14 15 flcodina sp ------­ X Belodella devonica (Stauffer)------­ X X X Belodella resima (Philip) ------X Belodella sp ______x X X X X Eantiognathus lipperti Bischoff______x Hibbardella sp ______x Hindeodella sp ------x x X X X X X X X Icriodus latericrescens Branson and Mehl (subspecies not de- termined) ------X Icriodus latericrescens huddlei Klapper and Ziegler ______X Icriodus latericrescens subspecies B Klapper1 ______b. X Icriodus latericrescens aff. I. latericrescens subspecies B Klap- per______x X Icriodus sp ------­ X X X X Ligonodina sp ------­ X Lonchodina walliseri Ziegler------X Lonchodina sp ______X Neoprioniodus excavatus (Branson and Mehl) ______X X X X X X X Neoprioniodus multiformis? Walliser ______X Oneotodus? beckmanni Bischoff------x Ozarkodina denckmanni Ziegler ------x X X X X Ozarkodina gaertneri Walliser ------­ X Ozarkodina media Walliser ------X X X X Ozarkodina aff. 0. media Walliser ______X Ozarkodina typica Branson and MehL ______X Ozarkodina ziegleri Walliser ------X Ozarkodina aff. 0. ziegleri Walliser ______X Ozarkodina sp ------x X Paltodus aff. P. migratus Rexroad ______X X Paltodus sp ______X X X Panderodus sp ______X X X X X ~ X Pelekysgnathus furnishi Klapper------X Pelekysgnathus sp ______X X Plectospathodus extensus Rhodes ______X X X X Plectospathodus aff. P. alternatus Walliser ______X Plectospathodus sp ______x X X Po?ygr:at~us foveolatus Philip and Jackson ______X Prwnwdma sp ------x X X Rotundacodina elegans Carls and Gandl ______X X Rotundacodina noguerensis Carls and Gandl ______X Scolopodus devonicus Bischoff and Sannemann ______x X Spathognathodus inclinatus inclinatus Rhodes ______x X X X X Spathognathodus primus (Branson and Mehl)l ______Spathognathodus eosteinhornensis Walliser1 ______Spathognathodus remscheidensis Ziegler1 ______X X X b, b, Spathognathodus n. sp., cf. S. steinhornensis remscheidensis x X X X Ziegler ______X Spathognathodus transitans Bischoff and Sanneman1 ______X Spathognathodus cf. S. transitans Bischoff ______x X X ~~t!!:::~:?:~::u~~~P:::::::::::::::::::::::::::::::::::::: X X X Trichonodella blanda (Stauffer) ------­ X Trichonodella blanda? (Stauffer)------X Trichonodella inconstans Walliser ______x X Trichonodella sp ______X X X

Approximate stratigraphic position Geographic locality: Formation: of fossil collection:

1. 9165-SD ______Toquima Range, Mill Canyon in Bastille Limestone Member of Mas­ 31 m above top of chert member of August Canyon structural se~ ket Shale of Kay and Crawford Gatecliff Fm. of Kay (1960). quence. (1964). 2. 9166-SD ______do ______Upper member of Masket Shale of 91 m above top of chert member of Kay and Crawford (1964). Gatecliff Fm. of Kay (1960). 3. 8214-SD ______Monitor Range, Copenhagen Roberts Mountains Formation ___ _ 119 m above base of formation. Canyon. 4. 8304-SD ______do ______do ______187 m above base of formation. 5. 9167-SD ______do ______Rabbit Hill Limestone ______Upper 31 m of formation at type section. 6. 8827-SD ______Northern Simpson Park Mountains, Thick-bedded upper part of Roberts Carlinastraea beds; unmeasured lo­ Coal Canyon. Mountains Formation (mapped cality about 460 m above base of as Windmill Limestone by John­ formation of west side of Coal son (1965) and Johnson and Mur­ Canyon.2 phy (1969). LOCALITY REGISTER OF MAJOR FOSSIL LOCALITIES 41

TABLE 2.-0ccurrence chart of conodont species collected from Silurian and Lower Devonian formations in central Nevada-Continued

Approximate stratigraphic position Geographic locality: Formation: of fossil collection:

7. 8829-SD ______------do ______do ______Carlinastraea beds; unmeasured lo­ cality near 8827-SD. 8. 8216-SD ______------do------______do------Unmeasured locality; east side Coal Canyon about 520 m above base of formation. 2 9. 8828-SD ______------do ______do------Unmeasured locality; east side Coal Canyon about 550 m above base of formation. 2 10. 9168-SD ______------do ______Rabbit Hill Limestone ______Unmeasured locality; east side Coal Canyon about 70 m above base of formation.2 11. 8228-SD ______Tuscarora Mountains, Bootstrap Roberts Mountains Formation ___ _ Lower 15 m of exposed section. Hill. 12. 8219-SD ______------do ______do ______37 m above base of exposed section. 13. 8219A-SD ______------do ______do ______119 m above base of exposed section. 14. 8215-SD ______------do ______Roberts Mountains Formation ___ _ 244 m above base of exposed section. (thick-bedded facies). 15. 8826-SD ______Carlin area, Maggie Creek ______Rabbit Hill Limestone ______Unmeasured locality east side of Maggie Creek.

'Some occurrences of this taxon are anomalous with respect to its stratigraphic range as previously reported from central Nevada (Klapper, 1968; Klapper and others, 1971: Klapper and Murphy, 19751, the Canadian Yukon CKlapper, 1969; Fahr~Eus 1971 ), and Europe CZiegler, 19711, and relative to monograptlds of the Monograptus uniformu; and Monograptus hercynicus groups. In order to accommodate this discrepancy, the anomalous stratigraphic occurrence of the taxon is shown as a Ll.. 2 The cited stratigraphic footage above base of the formation is estimated.

Upper 60 m of Rabbit Hill Limestone on this slope platy limestone hill 6909, altitude 6,320 feet. Upper beds of Roberts Mountains with abundant trilobites. Formation. M1317.-Horse Creek Valley quadrangle, Nevada. West side of Coal M1383.-Horse Creek Valley quadrangle, Nevada. Coal Canyon, Canyon in sec. 20, T. 25 N., R. 49 E. near middle of north bound­ west side sec. 17 near middle of south edge, T. 25 N., R. 49 E. At ary, altitude about 6,600 feet. About 503 m stratigraphically above crest northeast spur 670 m S. 79° W. of summit of hill 6909, al­ base of the Roberts Mountains Formation. Colonial rugose corals titude 6,580 feet. Roberts Mountains Formation. in ·place. M1384.-Horse Creek Valley quadrangle, Nevada. Coal Canyon M1318.-Horse Creek Valley quadrangle, Nevada. West side of Coal west side, NE% sec. 20, T. 25 N., R. 49 E.; 760 m S. 53° W. of Canyon, about 152m west of canyon bottom at altitude 6,400 feet. summit 6909, altitude 6,520 feet. Roberts Mountains Formation. Float material oflargeAustralophyllum, probably from Coal Can­ Carlinastrae a tuscaroraensis locality. yon fault zone in the Roberts Mountains Formation. M1411.-Horse Creek Valley quadrangle, Nevada. West side of Coal M1331.-Horse Creek Valley quadrangle, Nevada. Coal Canyon. Canyon 0.8 km south of canyon mouth at altitude 6,360 feet on SW%,SW% sec. 16, T. 25 N., R. 49 E.; east side of summit 6909 northwest side of small limestone knob, altitude 6,400 feet. (Pyramid Hill), altitude 6,840 feet just east of section line, 150 m Roberts Mountains Formation with Verticillopora. southeast of flag station 6909. Rabbit Hill Limestone. M1446.-Horse Creek Valley quadrangle, Nevada. Foothills on M1332.-Horse Creek Valley quadrangle, Nevada. Coal Canyon north side of Simpson Park Mountains; 2.4 km north ofRed Hill in area, top of Pyramid Hill (summit 6909), northwest of M1331, at SE%NE% sec. 5, T. 25 N., R. 50 E. Low pediment exposures of flag station. Rabbit Hill Limestone. Roberts Mountains Formation. M1333.-Horse Creek Valley quadrangle, Nevada. Coal Canyon M1448.-Horse Creek Valley quadrangle, Nevada. East side of Pine area. West side of Coal Canyon, NE% sec. 20 near section line, Hill, 150m south ofmiddle ofnorth edge of sec. 20, T. 25 N., R. 49 305 m northwest of hill 6400 at altitude 6,560 feet. About same E., near north-south midline of section; altitude 6,500 feet. Roberts zone as M1317. Dark-gray well-bedded limestone with abundant Mountains Formation. In coral beds with Carlinastraea. Favosites and other corals including a pycnostylid. Brachiopods M1449.-Horse Creek Valley quadrangle, Nevada. Top of main present. Roberts Mountains Formation. northeast spur of Pine Hill, 213 m north of south boundary of sec. M1334.-Horse Creek Valley quadrangle, Nevada. Coal Canyon 17, T. 25 N., R. 49 E., at north-south midline of section. Roberts area. SE% sec. 17, T. 25 N., R. 49 E. on west side of Coal Canyon Mountains Formation. In coral beds with Carlinastraea. 460 m northwest ofhill6400 at altitude 6,560 feet and 200m north M1450.-Horse Creek Valley quadrangle, Nevada. Coal Canyon, of M1333, Roberts Mountains Formation. Dark-gray limestone east side about 335 m south of canyon mouth at altitude 6,300 feet. with corals and brachiopods. Coral collection mostly float material. Great Basin Silurian coral M1379.-Horse Creek Valley quadrangle, Nevada. West side of Coal zone E. Roberts Mountains Formation. Canyon near its mouth, SE% sec. 17, T. 25 N., R. 49 E.; 460 m N. M1451.-Horse Creek Valley quadrangle, Nevada. Coal Canyon, 78° W. of summit of hill 6909, altitude 6,400 feet. East-dipping east side about 100m south of canyon mouth at altitude 6,280 feet. limestone of Roberts Mountains Formation, fault sliver in Coal Coral collections mostly loose float material. Great Basin Silurian Canyon shear zone. On crest of northeast-extending spur. coral zone E. Roberts Mountains Formation. M1380.-Horse Creek Valley quadrangle, Nevada. Coal Canyon, M1452.-Horse Creek Valley quadrangle, Nevada. Coal Canyon, 850 m south of canyon mouth; NE% sec. 20, T. 25 N., R. 49 E.; west side about 460 m south of canyon mouth, altitude 6,300 feet. 2,200 feet S. 33° W. of summit of hill 6909 on east side near top Within the Coal Canyon fault zone. Roberts Mountains Formation. small knoll in canyon, altitude 6,400 feet. Upper beds of Roberts M1453.-Horse Creek Valley quadrangle, Nevada. Coal Canyon in Mountains Formation, Great Basin Silurian coral zone E. NE% sec. 20, T. 25 N., R. 49 E., on top of small knoll 0.8 km south M 1381 .-Horse Creek Valley quadrangle, Nevada. Coal Canyon east of canyon mouth, altitude 6,400 feet. Great Basin Silurian coral side, 760 m south of canyon mouth, 520 m S. 40° W. of summit of zone E. Roberts Mountains Formation. 42 ROBERTS MOUNTAINS FORMATION, REGIONAL STUDY

M1454.-Horse Creek Valley quadrangle, Nevada. Coal Canyon Canyon, altitude 6,960 feet. Roberts Mountains Formation in beds about 975 m south of canyon mouth, altitude 6,400 feet on top of with a fauna similar to that in the Rabbit Hill Limestone sur­ spur on which the Middle Devonian Woodpecker Limestone rounded by graptolitic beds of Silurian age. Member of the Nevada Formation is apparently overridden by an M1393 .-Dianas Punch Bowl quadrangle, Nevada. South side oflkes upper plate comprising parts of the Roberts Mountains Formation Canyon, a little less than 1. 7 km northwest of canyon mouth. and overlying Rabbit Hill Limestone. McMonnigal Limestone. M1455.-Horse Creek Valley quadrangle, Nevada. Coal Canyon M1394.-Southeast corner of Wildcat Peak quadrangle. On north about 975 m south of canyon mouth, altitude 6,400 feet, on top of side of Mill Canyon 1,525 m west of east border of quadrangle on spur about 60 m east of M1454. Mixed Devonian assemblage ap­ top of spur, altitude 8,560 feet and 730 m north of south boundary parently from both the Middle Devonian Woodpecker Limestone of quadrangle. Bastille Limestone Member of Masket Shale of Kay Member of the Nevada Formation and the Lower Devonian Rabbit and Crawford (1964). Hill Limestone tectonically involved in the thrust. M1395.-Southeast corner of Wildcat Peak quadrangle. On north M1456.-Horse Creek Valley quadrangle, Nevada. South middle side of Mill canyon, altitude 8,000 feet and 2,400 m west of east part of sec. 17, T. 25 N., R. 49 E., at crest of main northeast spur, border of quadrangle. Bastille Limestone Member of Masket Shale altitude 6,600 feet. In thick-bedded limestone with rugose corals. of Kay and Crawford (1964). Roberts Mountains Formation. M1396.-Southeast corner of Wildcat Peak quadrangle, 1,220 m Cortez Mountains, Nev. west of east border of quadrangle and 1,130 m north of south M1409.-Cortez quadrangle, Nevada. West base of Cortez Moun­ boundary of quadrangle; altitude 8,500 feet. Bastille Limestone tains about 1.6 km southeast of the town of Cortez. NE% sec. 17, T. Member of Masket Shale of Kay and Crawford (1964). 26 N., R. 48 E., 2.6 km N. 76° E. of BM 5748, altitude 6,400 feet. MJ397.-Wildcat Peak quadrangle, Nevada. South side of Stone­ Roberts Mountains Formation. Carlinastraea; probable float berger Basin, 2.4 km southeast of top of Whiterock Mountain at material. altitude 9,100 feet, just east of letter "n" in word "Basin" of Sulphur Spring Range, Nev. "Stoneberger Basin" (Wildcat Peak quadrangle). Tor Limestone. MJ148.-Garden Valley quadrangle, Nevada. Southern part of Sul­ Northern lnyo Mountains, Calif. phur Spring Range. East side of range in northeast corner sec. 35, MJ090.-Independence quadrangle, California. East of Kearsarge at T. 23 N., R. 52 E. Lone Mountain Dolomite with corals 0.8 km mouth of Mazourka Canyon. In NE14 sec. 8, T. 13 S., R. 36 E. north-northwest ofBM 5867 on east-west spur, altitude 6,100 feet. Within 6 m of top Vaughn Gulch Limestone. Poorly preserved Southern Tuscarora Mountains, Nev. fossils of probable Early Devonian age. MJJ20.-Tuscarora Mountains near north end of Eureka County, MJ09J.-Independence quadrangle, California. At mouth of Nev.; "Round Mountain," bottom of hill at southwest end. Col­ Mazourka Canyon. Near east line of sec. 8, T. 13 S., R. 36 E. at lected by R. J. Roberts, June 1958. Roberts Mountains Formation middle of NE%, 366m north of Vaughn Gulch creek bottom, al­ limestone. titude 4,840 feet. About 46 m stratigraphically above chert unit at M1313.-Bootstrap mine area, Nevada. Bootstrap Hill section. base of Vaughn Gulch Limestone. Roberts Mountains Formation about 400 m stratigraphically MJ092.-Independence quadrangle, California. Same measured sec­ above base of section on top of knob. Carlinastraea bed. tion as M1091, west of east section line of sec. 8 in NE%, altitude M1314.-Bootstrap mine area. Bootstrap Hill stratigraphic section 4,880 feet. Middle part of Vaughn Gulch Limestone about 305 m 200-250 m stratigraphically above base. Roberts Mountains For­ stratigraphically above the Eureka Quartzite equivalent. mation. M1093 .-Independence quadrangle, California. East of Kearsarge at MJ3J5.-Bootstrap mine area. Bootstrap Hill stratigraphic section mouth ofMazourka Canyon. In NE14 sec. 8, T. 13 S., R. 36 E. Base about 230 m stratigraphically above base. Roberts Mountains of upper unit of Vaughn Gulch Limestone about 412 m strati­ Formation. graphically above Eureka Quartzite equivalent and 137 m below MJ4J2.-Tuscarora Mountains, Nev. Advance quadrangle sheet top of Vaughn Gulch Limestone. USGS Tuscarora 3SW (7-7-68); Elko County, Nev., near north MJ115.-Independence quadrangle, California. East ofKearsarge at edge of Eureka County. Bootstrap Hill area southwest ofBootstrap mouth of Mazourka Canyon. Measured section of Vaughn Gulch Hill. Just north of Eureka County-Elko County line at bottom of Limestone about 34m stratigraphically below horizon of M1093. ridge about 300m east of Dunphy-Tuscarora road and about 1,830 Rugose coral bed. m S. 17o W. oftop of Bootstrap Hill. Probably a large float boulder; M 1116 .-Independence quadrangle, California. At mouth of contains silicified brachiopods in abundance. Mazourka Canyon in measured section in which are localities MJ4J3.-Tuscarora Mountains, Nev. Northeast Eureka County. M1092 and M1093. Coral bed about 9-15 m stratigraphically Northeast of the Lynn window of R. J. Roberts. Collected by R. J. below M1093, near top middle unit of Vaughn Gulch Limestone. Roberts, 1958. Corals. Possibly in Jack's Creek drainage northeast MJJ28.-Independence quadrangle, California. East ofKearsarge at of Big Six mine. Roberts Mountains Formation. mouth of Mazourka Canyon. In measured section about 52 m Toquima Range, Nev. stratigraphically below M1093 in higher middle part of the M1103 .-Dianas Punch Bowl quadrangle, Nevada. Ikes Canyon. Vaughn Gulch Limestone. Beds with Verticillopora. About 1.6 km up canyon from mouth on north side and about MJ385.-Independence quadrangle, California. Sec. 8, T. 13 S., 100m in altitude above creek bottom, altitude 7,900 feet. South­ R. 36 E. Vaughn Gulch Limestone, Silurian. Probably same zone east of Ikes cabin. Roberts Mountains Formation or Masket Shale as locality M1128. Verticillopora. Collected by Craig D. of Kay and Crawford (1964). Coral-rich limestone with Verticil­ Ross, 1961. lopora. MJ4JO.-Independence quadrangle, California. Mazourka Canyon M1JJ4.-Dianas Punch Bowl quadrangle, Nevada. Ikes Canyon east of Kearsarge in measured section. At top of middle unit of area; 200 m northwest of summit 8474 (Copper Mountain), al­ Vaughn Gulch Limestone and about 15 m stratigraphically below titude 8,300 feet. In McMonnigal Limestone. Coral fauna with locality M1093. Toquimaphyllum. Great Basin Silurian coral zone E. Gotland, Sweden MJ147.-Northeast corner of Wildcat Peak quadrangle. SW%SW% M1382.-Gotland, Sweden. Hemse Group. Stop 40 of Internat. Geol. sec. 16, T. 16 N., R. 46 E., on top of divide west of Henry Meyer Cong. guidebook, 1960. Boardman collection, Sept. 10, 1960. SELECTED REFERENCES 43 SELECTED REFERENCES and Breeding, J. G., eds., Regional aspects of carbonate deposi­ tion-a symposium: Soc. Econ. Paleontologists and Mineral­ Adams, J. E., and Rhodes, M. L., 1960, Dolomitization by seepage ogists Spec. Pub. 5, p. 126-178. refluxion: Am. Assoc. Petroleum Geologists Bull., v. 44, no. 12 Friedman, G. M., and Sanders, J. E., 1967, Origin and occurrence of p. 1912-1920. dolostones, in Chilingar, G. V., ed., Carbonate rocks, Part A: Bathurst, R. G. C., 1971, Developments in Sedimentology 12- New York, Elsevier Publishing Co., p. 267-348. 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Paleontologists and raphy and correlation of McMonnigal and Tor Limestones, To­ Mineralogists Spec. Pub. 13, 180 p. quima Range, Nevada: Am. Assoc. Petroleum Geologists Bull., Rezak, Richard, 1959, New Silurian Dasycladaceae from the south­ v. 56, no. 8,p. 1563-1570. western United States, Pt. 2 of Johnson, J. H., Konishi, Kenji, McKee, E. H., and Ross, R. J., Jr., 1969, Stratigraphy of eastern and Rezak, Richard, Studies of Silurian (Gotlandian) algae: Col­ assemblage rocks in a window in Roberts Mountains thrust, orado School Mines Quart., v. 54, no. 1, p. 115--129. northern Toquima Range, central Nevada: Am. Assoc. Petro­ Roberts, R. J., Hotz, P. E., Gilluly, James, and Ferguson, H. G., 1958, leum Geologists Bull., v. 53, p. 421-429. Paleozoic rocks of north-central Nevada: Am. Assoc. Petroleum McLaren, D. J., 1970, Presidential address; time, life and bound­ Geologists Bull., v. 42, no. 12, p. 2813-2857. aries: Jour. Paleontology, v. 44, no. 5, p. 801-815. Ross, D. C., 1963, New Cambrian, Ordovician and Silurian forma­ Manten, A. A., 1971, Silurian reefs of Gotland-Developments in tions in the Independence quadrangle, lnyo County, California, Sedimentology 13: New York, Elsevier Pub. Co., 539 p. in Geological Survey research 1963: U.S. Geol. Survey Prof. Matti, J. C., Murphy, M. A., and Finney, S. C., 1975, Silurian and Paper 475--B, p. B74-B85. Lower Devonian basin and basin-slope limestones, Copenhagen ---1965, Geology of the Independence quadrangle, Inyo County, Canyon, Nevada: Geol. Soc. America Spec. Paper 159, 48 p. California: U.S. Geol. Survey Bull. 1181-0, 64 p. Merriam, C. W., 1940, Devonian stratigraphy and paleontology of ---1966, Stratigraphy of some Paleozoic formations in the Inde­ the Roberts Mountains region, Nevada: Geol. Soc. America Spec. pendence quadrangle, lnyo County, California: U.S. Geol. Sur­ Paper 25, 114 p., 16 pls. vey Prof. Paper 396, 64 p. ---1963, Paleozoic rocks of Antelope Valley, Eureka and Nye St. Joseph, J. K. S., 1938, The Pentameracea of the Oslo region, Counties, Nevada: U.S. Geol. Survey Prof. Paper 423, 67 p. being a description of the Kiaer collection ofpentamerids: Norsk ---1972, Silurian rugose corals of the Klamath Mountains re­ Geol. Tidsskr., v. 17, no. 4, p. 225--236, 8 pls., 23 text-figs. gion, California: U.S. Geol. Survey Prof. Paper 738, 50 p., 8 pls. Shimizu, Saburo, Ozaki, Kin-emon, and Obata, Tadahiro, 1934, Got- SELECTED REFERENCES 45

landian deposits of northwest Korea: Shanghai Sci. Inst. Jour., ---1952, Species of the Silurian rugose coral genus Tryplasma sec. 2, v. 1, p. 59-88, pls. 9-18. from North America: Jour. Paleontology, v. 26, no. 5, p. 841-843, Shinn, E. A., Ginsburg, R.N., and Lloyd, R. M., 1965, Recent supra­ pl. 125. tidal dolomite from Andros Island, Bahamas, in Dolomitization ---1964, Silurian and Devonian corals of the Falls of the Ohio: and limestone diagenesis-a symposium: Soc. Econ. Paleon­ Geol. Soc. America Mem. 93, 184 p., 2 figs., 80 pls. tologists and Mineralogists Spec. Pub. 13, p. 112-123. Taylor, P. W., 1950, The Plymouth Limestone, and the Devonian Shurygina, M. R., 1968, Poznesiluriyskiye rannedevonskiye rugozy Tetracorals of the Plymouth Limestone: Royal Geol. Soc. vostochnogo sklona severnogo i srednogo Urala [Late Silurian Cornwall (England) Trasn., v. 18, pt. 2, p. 146--160, 161-214, and Early Devonian rugose corals from the eastern slope of the 5 pls. northern and central Urals], in Ivanovsky, A. B., Korally pogra­ Ulrich, E. 0., and Schuchert, Charles, 1902, Paleozoic seas and bar­ nichnykh sloeb Silura i Devona Altae-Saianskoi gornoi oblasti i riers in eastern North America: New York State Mus. Bull., Urala [Corals from the Silurian-Devonian boundary beds of the v. 52, p. 633--663. Altay-Sayan Mountains and the Urals]: Moscow, "Nauka" [Pub. VanTuyl, F. M., 1916a, The origin of dolomite: Iowa Geol. Survey House], p. 117-150, pls. 49-65. (Akad. Nauk SSSR Sibirsk. Ot­ Ann. Rept. 1914, v. 25, p. 251-422. deleniye Inst. Geologiya i Geofizika). ---1916b, New Points on the origin of dolomite: Am. Jour. Sci., Skeats, E. W., 1918, The formation of dolomite and its bearing on the ser. 4, v. 42, p. 249-260. coral reef problem: Am. Jour. Sci., ser. 4, v. 45, art. 13, p. 18&-- Waite, R. H., 1953, Age of the "Devonian" of the Kearsarge area, 200. California [abs.]: Geol. Soc. America Bull., v. 64, no. 12, pt. 2, p. Smith, Stanley, 1930, Some Valentian corals from Shropshire and 1521. Montgomeryshire, with a note on a new stromatoporoid: Geol. ---1956, Upper Silurian Brachiopoda from the Great Basin Soc. London Quart. Jour., v. 86, p. 291-330, pls. 26--29. [Nevada, Utah]: Jour. Paleontology, v. 30, no. 1, p. 1&--18, pl. 4. ---1933, On Xylodes rugosus sp. nov., a Niagaran coral: Am. Walcott, C. D., 1884, Paleontology of the Eureka district [Nevada]: Jour. Sci., ser. 5, v. 26, no. 155, p. 512-522, 1 pl. U.S. Geol. Survey Mon. 8, 298 p., 24 pls. ---1945, Upper Devonian corals of the Mackenzie River region, Walliser, 0. H., 1964, Conodonten des Silus: Abhandlungen des Hes­ Canada: Geol. Soc. America Spec. Paper 59, 126 p., 35 pls. sischen Landesamtes fiir Bodenforschung, Heft 41, 106 p., 32 Stauffer, C. R., 1930, The Devonian of California: California Univ. pls. Pubs., Dept. Geology, v. 19, no. 4, p. 81-118, 5 pls. ---1972, Conodont apparatuses in the Silurian, in Lindstrom, M., Steidtmann, Edward, 1911, The evolution of limestone and dolomite, and Ziegler, W., eds., Symposium on conodont taxonomy: Geol. pt. 1: Jour. Geology, v. 19, nos. 4--5, p. 323--428. et Palaeontol., Sonderb. 1, p. 7&--79. ---1917, Origin of dolomite as disclosed by stains and other Wang, H. C., 1947, New material of Silurian rugose corals from methods: Geol. Soc. America Bull., v. 28, p. 431-450, pls. 22-28. Yunnan: Geol. Soc. China Bull., v. 27, p. 171-192, 2 pls. [Nan­ Stevens, C. H., and Ridley, A. P., 1974, Middle Paleozoic off-shelf king, China]. deposits in southeastern California: evidence for proximity of Webby, B. D., 1964, Devonian corals and brachiopods from the Bren­ the Antler orogenic belt?: Geol. Soc. America Bull., v. 85 dan Hills, West Somerset (England): Palaeontology, v. 7, pt. 1, p. 27-32. 22 p., 1 pl., 6 text figs. Stewart, J. H. and McKee, E. H., 1968, Geologic map of the Mount Wedekind, Rudolf, 1927, Die Zoantharia von Gotland (bes. Nordgot­ Callaghan quadrangle, Lander County, Nevada: U.S. Geol. Sur­ land): Sveriges Geol. Undersokning, ser. Ca, no. 19,94 p., 30 pls. vey Geol. Quad. Map GQ--730. Whiteaves, J. F., 1884, On some new, imperfectly characterized or ---1976, Geology and mineral deposits of Lander County, previously unrecorded species of fossils from the Guelph Forma­ Nevada, with a section on Mineral deposits by Harold K. Stager: tion of Ontario: Canada Geol. Survey, Palaeozoic Fossils, v. 3, Nevada Bur. Mines Bull. (in press). no. 4, p. 1-43. Stewart, J. H., and Palmer, A. R., 1967, Callaghan window-a newly Winterer, E. L., and Murphy, M.A., 1960, Silurian reef complex and discovered part of the Roberts thrust, Toiyabe Range, Lander associated facies, central Nevada: Jour. Geology, v. 68, no. 2, County, Nevada, in Geological Survey research 1967: U.S. Geol. p. 117-139, 7 pls. Survey Prof. Paper 57&--D, p. D56--D63. Ziegler, Willi, 1971, Conodont stratigraphy of the European Devo­ Stumm, E. C., 1949, Revision ofthe families and genera of the Devo­ nian, in Sweet, W. C., and Bergstrom, S. M., eds., symposium on nian tetracorals: Geol. Soc. America Mem. 40, 92 p., 25 pls. conodont biostratigraphy: Geol. Soc. Am. Mem. 127, p. 227-284.

INDEX

[Italic page numbers indicate major references]

Page Page Page A Big Six Mine ______16. 34 Clinton Group, New York State ______19 Acinophyllum 27 Bighorn Dolomite, Ordovician ______Coal Canyon, fault zone ______11, 12,13. 35 Acknowledgments ------2 Billingsastraea sp. T 25 fault zone, fossils______14 Acodina sp _ 40 Bioherms, scarcity ______4 Simpson Park Mountains 2, 5, 11, 13, Acrospirifer kobehana fauna ___ _ 3 Birch Creek ______7 14, 15, 16, 17' 20, 28. acus. Monograptus ___ _ 19 birchensis, Monograptus __ 19 29. 30, 31, 32. 33, 35, 40, 41 AI Rose Canyon ______28 bohemicus, Monograptus ______13 Coal Canyon area ______11, 15, 40.41 Alaska. southeastern __ _ 15 Boone Creek area ______24 Coal Canyon basin _ 12 alaskense, Conchidium ______33 Bootstrap Hill, Tuscarora Mountains 2. 14, 16, Coal Canyon fault______29, 30,32 alaskensis. Conchidium ______15 30, 31, 32, 33, 34. 41 Coelospira ______------17 Algae, calcareous ______2, 4, 5, 26,29 Bootstrap Hill section, age ______17 sp. b ______16, 17; pl. 12 dasycladacean 4, 5. 10. 15, 25, 26, 27. 29 lowest coral-brachiopod zone 16 sp ______10, 17 alternatus, Plectospathodus ----- 40 mi-ddle coral-brachiopod zone 17 Coenites sp ______17 Ah•eolites sp ____ _ ----13,15.17 physical stratigraphy _ 16 colonus, Monograptus ______13 Amplexoides ______---- 31 stratigraphic paleontology 16 Conchidlllm ______9, 20 Amplexus __ 31 upper fossil zone with Carlinastraea ______17 alaskense 33 angustidens, Monograptus ------18 Bootstrap mine area ______2, 7, 16 alaskenszs 15 Retiolltes geillltzianus _ 19 Boucot, A. J .. cited ______miinsteri ______10; pl. 12 annulata, Vertzczllopora 5, 10, 14, 15, 22, Boulder. Creek 16 sp. b 16; pl. 12 26, 27; pls.10. 11 Brachiopods __ 4, 5, 8, 10, 14, 15, 16, 17.-18, 20, 26, 27 sp ______pl. 12 Antelope Peak 7 dalmanellid_ 13. 17 Conodonts ______2, 4, 11, 15, 16. 18. 19, 26, 28,29 Antelope Valley ------19 pentameroid ______9, 10, 11, 13, 19,20 Copenhagen Canyon______3, 7. 11, 18,40 Antelope Valley Limestone_ 19, 24, 25 rhynchonellid ------20 Roberts Mountains Formation, age ______18 Antilles. Netherlands ______silicified ______13 stratigraphic paleontology ______18 Arachnophyllum 10, 27 spiriferoid 27 Copenhagen Formation ____ 19, 24, 25 kayi 5, 22 Brachyelasma ------30 Coral zone E. age ______4 Athyris 27 sp. b _ 16, 31; pl. 12 Coral zones. reference section ______10 Atrypa ______------27 sp. c __ 13, 31; pl. 5 Corals______2, 4, 5, 8, 14, 16, 20. 22,24 sp 5, 10, 15, 17 sp ------5, 11 dissepimented ______29 August Canyon sequence ______23. 24 Breccias, depositionaL 5, 13, 14, 15 nondisspimented ______29 August Canyon thrust sequence 19,40 Breviphrentis ______------30 pycnostylid ______27 Aulacophyllum _ 27 IBret•iphrentisl, Siphonophrentis ___ _ 30 rugose_ 1, 2, 5, 9. 10, 13. 14, 15, 20, 26, 27, 29 Aulopora ______10 age value ______29 Australia______4, 15,30 c cerioid _ 32 Australophyllum 15, 29, 34 colonial 2, 9, 17. 20. 26, 29, 32 landerenszs ______27, 34. 35 Caesar Canyon Limestone ______23.24 key ______29 steuensi ------28 Calcarenite 8 lykophyllid 32; pl. 12 !Australophylluml sp. c ______34; pl. 8 Calceola sandalina ------27 special stratigraphic value ______29 sp. v ______35; pl. 8 Callaghan window 23,24 stratigraphic value ______29 !Toquimaphylluml_ 29, 34, 35 Calostylis ------17 zonation______28 johnsoni 5, 14, 15, 17, 22, 23, sp ______17, 32; pl. 7 silicified ______13 27, 34, 35; pl. 6 Camerotoechia ------27 streptelasmids 5 sp ------­ 27 Carlin, Nev ------3, 41 tabulate______9. 10, 20,27 sp. c __ 14, 34, 35 Carlin gold ores. Nev _ 2 alveolitid ------16 sp. v ------27, 34, 35 Carlin Mine ______7, 16 Cordilleran belt ______32,33 IAustralophyllum I sp. c, Australophyllum ____ 34; pl. 8 Carlinastraea ______13, 14, 17,29,32 Cordilleran geosynclinal seaway ______1 sp. v, Australophyllum______35; pl. 8 beds 13, 14, 17, 35. 40, 41 Cortez gold or.es, Nev ______2 giganteum ------34 Cortez Mountains 3, 7, 34 B originalis __ _ 34 Cortez quadrangle 3.4 Badger Flat ------28 tuscaroraensis 5, 13. 16. 17, 32, 33: pis. 6. 7. 8 crateroides, Mucophyllum ______15 Bare Mountain ______7, 19 sp - pl. 8 Crawford, J.P., cited ______Roberts Mountains Formation ______20 [Carlinastraea], Spongophyllum 33 crau{ordi, Neomphyma 5, 22 Barrandella sp ______5, 15 Cerro Gordo mining district 4, 25 Cress, Leland, cited_ 2 Barrel Spring 28 Chert granules ______18 Crinoids_ 4, 8, 14. 17, 26,27 Bastille Limestone Member 23, 40 nodules 26 Cryptatrypa ____ _ ------17 Bathurst, R. G. C., cited __ chimaera, Monograptus ______17 sp ------­ ------17 Beacon Peak Dolomite Member. Chonophyllum 15, 27 Cyathopaedium _ 31 Nevada Formation __ 3, 10 simpsoni_ 5, 15; pis. 7, 9 Cyathophylloides ______--10,17,27,31 beckmannt, Oneotodus 40 Chuckwalla Canyon 20 ferguson~ ______------5, 17. 22,31 Belodella det•onica ------40 Cladopora 10, 26. 27; pl. 1 sp. f ______16. 31; pl. 12 resima _ 40 sp. v ______pl. 10 Cyrtograptus sp ______19 sp ______40 sp____ 5, 10, 11.13,15,16, 17 Cystihalysites______11 Berry, W. B. N., cited_ 2, 11, 13, 19, 28 Clear Creek ------19 ICystihalysitesl magnitubus, Halysites 5 berthiaumi, Stylopleura _ 5, 10, 13, 15, 20, Climacograptus medius ______11 Cystiphyllum sp ------11 31, 32; pis. 1, 2 scalaris 11 Cystiphylloids ______27

47 48 INDEX

Page Page Page D fergusoni, Cyathophylloides __ 5. 17, 22, 31 H Dalmanella sp __ ------5 Fish Creek Range ______20 Halliidae ______------27 Dalmanophyllum ______10, 26 flemingii. Monograptus 19 Halysites ______10, 11 sp. A ______5. 27 Fletcheria ___ ------· ______31 (Cystihalysitesi magnitubus ______5 Daly, R. A., cited______6 tubifera ______31: pl. 3 sp ------5, 10 denayensis, Denayphyllum ______5, 10 Fletcherina ______31 Hanson Creek dolomite ______12 Denayphyllum denayensis 5, 10 Fossils ______9 Hanson Creek Formation__ 3, 9, 11, 18, 19, 22, 24 denckmanni. Ozarkodina ------40 Coal Canyon fault zone _ 14 Hanson Creek limestone______12, 13 Devonian System _ 10 Mullens section ------16 Heliolites ___ 10, 26, 27 deuonica, Belodella ------40 Pyramid Hill ______14 sp ______5, 10, 16,20 det•onicus, Scolopodus ------40 silicified __ ------8, 14, 16, 17, 20 Hemse Group 31 Dicoelosia sp. r _ 5, 10: pl. 11 unit 1 10 Hercynella pl. 10 sp- 5, 10 unit 2 ___ ------10 hercynicus, Monograptus 19, 25 Dinant Basin, Belgium, standard ______29 unit 3 10 Hexagonaria 25 Diplophyllum ______10,27 foueolatus, Polygnathus ______40 Hibbardella sp ------40 Dobbin Summit, Monitor Range______7 furnishi. Pelehysgnathus 40 Hidden Valley Dolomite ______4, 25, 26, 27,28 Monitor Range, Fye Canyon area ______11 Hindeodella sp ------40 Roberts Mountains Formation -- 19 Hindia 26 Dolomite _ 1, 3, 8, 9 G Homoeospira sp. r ______5, 10: pl. 11 diagenetic origin ___ _ 6 gaertneri, Ozarhodina ______40 Horse Creek Valley quadrangle_ 3, 4, 11 eastern belt 6 Gala beds, England______19 Howellella sp______17 facies ______3,10 Gatecliff Formation __ 23,40 Hsu, K. J., cited ______6 origin ______6 Gazelle Formation 15, 34 huddlei, Icriodus latericrescens 40 Great Basin, Silurian ______geinitzianus angustidens, Retiolites 19 Humboldt River Valley ______Guelph, Ontario ______31 giganteum, Carlinastraea 34 hydrothermal type ______6 Glassia ______17 lithographic ______23 sp ------17 Icriodus latericrescens ______40 marine, origin 6 Gold __ _ 3 latericrescens huddlei ______40 penecontemporaneous______Gothograptus spinosus ______19 subsp. B ------40 problem ______Gotland, Sweden ______34 sp ______40 reefs_ 4 corals 2, 30 Ikes Canyon ______20, 22, 32, 34, 35 regional_ 6,7 reefs ______4. 5 Ikes Canyon window _ ------22, 23. 24 regional problem __ _ 6 rugosa______4, 15 August Canyon sequence ______23, 24 saccharoidal ____ _ standard ___ 27, 29 June Canyon sequence ______22 Dolomite belts, Great Basin, distribution 6 Graptolite facies ______24,25 Mill Canyon sequence _____ 22, 23, 24, 31, 34, 35 Dolomitization 7. 26 Graptolite standard, age ______17 inclinatus inclinatus, Spathognathodus_ 40 metasomatic Graptolite zonation______28 inconstans, Trichonodella ______40 regional __ Graptolites ______1, 4, 9, 10. 11, 13, 14, 15. Independence quadrangle ______25 Dry Canyon, Toiyabe Range _ 17' 18, 19, 22, 23, 24, 25. 26, 28, 29 Independence Range Dry Creek area ------23,24 Great Basin 4, 28 Intermediate limestone be It ______1, 7. 10, 19, 20, dubius, Monograptus ____ _ 13. 17, 19, 28 Grays Canyon Limestone Member, 25, 28, 29, 32 duncanae, Tryplasma 5, 10, 13, 17,31: pl. 3 Nevada Formation 30 International Stratigraphic Commission 28, 29 Graywacke belt, Pacific belt_ Introduction ______1 E Great Basin. brachiopods 4 Investigation, history------3 Eantiognathus lipperti ------40 chert sequences __ ------11 purpose ______2 East Northumberland Canyon ------20, 23 conodonts 4 &0~ 2 Eastern dolomite belt ____ _ 1, 7, 10, 19, 20 coral zonation, Silurian ______4, 26 Inyo Mountains, Calif_____ 1, 2, 4, 7. 15, 18, 28,35 Eatonia ------27 Silurian, reference section ___ _ 10, 14 limestones------25 sp ------10 deposi tiona! facies belts ______1 Inyo Range______7 elegans, Pycnostylus ------31: pl. 1 dolomite belts, distribution ______6 Isorthis sp _ ------13 Rotundacodina __ _ ------40 eastern dolomite belt 1, 7, 10, 19, 20 Elko County _ 3, 7, 16 graptolites 4, 28 J Ely Springs Dolomite __ _ 20.25 intermediate limestone belt___ 1, 7. 10, 19, jaculum, Monograptus _____ 19 Endophyllidae _ 29, 32,34 20, 25, 28, 29, 32 Johnson, J. G., cited ______2 Endophyllum _ 35 limestone facies ______4 johnsoni, Australophyllum (Toquimaphyllumi __ 5, 14, enorme, Rhizophyllum __ _ 5, 15 Silurian ______32 15, 17, 22, 23,27, 34,35: pl. 6 Entelophylloides 32 Lower Devonian, paleontologic zoning 4 June Canyon sequence ______22, 23 IProhexagonaria I occidentalis 5. 10 reference sections ______4 Entelophyllum ------20 Rugosa, dissepimented ______32 K eosteinhornensis, Spathognathodus_ 40 Silurian, coral zone A __ 4, 10, 16, 17, Kay, Marshall, cited 3 Eureka County 3, 16, 20 22, 26, 27, 28, 31 hayi, Arachnophyllum 5, 22 Eureka district __ _ 10, 30 coral zone B ______4, 10, 27, 28 Kennett Formation------26 Eureka Quartzite 19 coral zone C 4. 9, 10, 11, 31 Ketophyl/um ______34 Evans, J. G., cited coral zoneD ______4, 9, 10, 11, 13, 16, 17, sp. t ______34: pl. 10 Evans and Mullens, unnamed Devonian 20, 22, 26, 27, 30, 31, 32, 34 Klamath Mountains, Calif ___ 15, 30, 34 limestone unit _ 16 coral zone E ______4, 11, 13, 14, 15, 16, 17, Klamathastraea ______34 excauatus, Neoprioniodus __ ------40 22, 23, 26, 27, 29, 32, 34, 35 Kleinhampl, F. J., quoted ______19 extensus, Plectospathodus 40 limestones_ ------4 Kodonophyllidae ______29, 31 distribution 6 Kodonophyllinae ______29,30 F paleontologic zoning 4 Kodonophyllum ______15, 17, 29,30 Fairbridge, R. W., cited 6 Great Britain ______30 mu/leri ______5, 15, 29,30: pl. 4 Fardenia sp. b _ 16; pl. 12 Great Lakes, reefs ______4 truncatum ______30 Fault, thrust 16 Grouse Creek 11 sp. b______29, 30, 32: pl. 5 Favistella __ _ 31 Grouse Creek Canyon ______------11 Kozlowshiellina sp. b ______16, 17: pl. 12 Fat•istina_ 31 Guelph Dolomite, Guelph, Ontario 31 sp. f ______5, 10, 15: pl. 11 Fal'Osites 11, 13, 17, 24, 26, 27 guelphensis, Pycnostylus ______31; pl. 1 Kyphophyllidae ______29 sp. c _ pl. 10 Gypidula ______27 Kyphophyllum ______15, 17, 23,29 sp. d ------pl. 10 sp. r _ ------5, 10: pl. 11 nevadensis ______5: pl. 9 sp --- 13, 15, 16 sp ------13,17 sp. b ______17.32: pl. 5 Favositids 10, 26, 27 Gypsum __ _ sp. c ______5, 15: pl. 9 INDEX 49

Page Page Page Ky phophyllu m -Continued Monitor Range ______3, 7, 11, 40 Pacific Border graywacke belt 1, 6 sp. t ______pl. 10 physical stratigraphy______18 Pacific border province 1 sp 17, 27 Roberts Mountains Formation ______18 Palaeocyc/us ------10, 27 Dobbin Summit ______19 porpzta mcallisteri __ ------5 L Monograptids 22. 23. 24, 28 Palaeophyllum ______10, 31 Laketown Dolomite_ Monograptus 17, 24 sp. b ______------20 Lander County------22 acus ______19 Paleontologic zoning, Great Basin 4 landerensis, Australophyllum ______27, 34,35 angustidens ______18 Paleozoic rocks ------11 laterierescens, Icriodus ______40 birchensis 19 Paltodus migratus 40 huddlei. Icriodus ______40 bohemicus __ 13 sp ------40 subsp. B,Icriodus______40 chimaera ______------17 Panamint Mountains______25 Panderodus sp ______40 Leptaena sp ______17 co/onus ------13 Leptocoelia ______14 dubius______13, 17, 19,28 pandus, Monograptus _____ ------··------19 beds ___ ------14 flemingii ______19 Pelekysgnathus furnishi _ ------40 Ligonodina sp ______40 hercynicus __ 19, 25 sp ------40 Limestone, argillaceous __ 24, 26 jacu/um ______19 Pentamerus ______20 bioclastic ______13, 14, 16, 23, 25, 26,28 nilssoni-Monograptus scanicus Zone 19 Perdido Formation ______25 breccia 12, 14, 15 nudus ______19 Persian Gulf ______------7 cherty ______24 pandus 19 Pete Hanson Creek ______9 crinoidal ______19, 26 praehercynicus ______13. 19, 22, 29 Pete Hanson Creek area ------10 dolomitic ______8, 10 priodon ______19 Fetes Canyon ______20 Petes Canyon window ______22, 24 facies, Silurian _ ------32 riccartonensis Zone ______11 graptolitic ______------22,25 scanius ______28 Petrozium __ _ 32 intermediate, Silurian ______6 spiralis ------19 mcallisteri Inyo Mountains, Calif______25 tumescens 28 Pine Hill ____ _ ------11, 12 marine, reefs______4 uncinatus ------17, 19 age ------13 stratigraphic paleontology ______13 Ordovician------22 uniformis ------4, 18,28 Roberts Mountains ______22 wziformis Zone 2. 15, 18, 29 Pine Hill section, physical stratigraphy ____ 12, 14, 15 Limestones, lnyo Mountains, Calif ______25 uulgaris ------28 Plectatrypa sp. c ______pl. 11 Lower Devonian ______25 sp ------10, 13, 17, 19, 28 sp ______5, 15,27 Silurian ------25 Mount Callaghan, Toiyabe Range ______7 Plectospathodus alternatus ______40 geographic distribution ______Mount Lewis area ______3 extensus 40 lipperti, Eantiognathus ______40 Mucophyllum ______15,30 sp ------­ ------40 Lobograptus scanicus ______13 crateroides ______------15 Point of Rocks area 23, 24, 25 Lonchodina wa1liseri ______40 olh•eri 5, 15, 30; pl. 4 Polygnathus {ot•eolatus 40 sp ------40 Mudflats ______7 Porcupine River, Alaska ______------32 Lone Mountain ______3, 9, 10 Mullens, T. E., cited ------2, 16, 17 porpita mcallisteri, Palaeocyclus saccharoidal dolomites ______3 Mullens section ______16, 17 praehercynicus, Monograptus 13, 19, 22, 29 Lone Mountain Dolomite __ 3, 6, 8, 9, 19, 20, 31 mullen·, Kodonophyllum ______5, 15, 29, 30; pl. 4 Pray, L. C., cited ______6 dolomite facies ______10 mu/tiformis, Neoprioniodus 40 primus, Spathognathodus______40 lateral equivalence______3 miinsteri, Conchidium ______10; pl. 12 priodon, Monograptus ______19 Roberts Creek Mountain ______3, 10 Murray, R. C., cited______6 Prioniodina sp ______40 saccharoidal ______9 Mycophyllinae ______29, 30 (Prohexagonaria I occidentalis, Entelophylloides __ 5, 10 type section ---- 3, 20 Prospect sequence, chert unit ______23 unit 1 20 N Ptychopleurella sp ______10, 16 unit 2 ______9, 20 Neophyma ______33 Pycnactis sp. K ______5 Lone Mountain reeL___ 6 crawfordi ______------__ ---- 5, 22 Pycnostylid ______10, 14. 15, 31 Lonsdaleia ______33 Neoprioniodus excapatus ______40 Pycnostylidae______29.31 Ludlovian faunas __ _ 11 mu/tiformis ------40 cerioid ______------32 Ludlovian Stage, Alaska 15 Nevada Formation ______3, 10, 30 Pycnostylus ______15,31 Lykophyllids ______32; pl. 12 Beacon Peak Dolomite Member ___ _ 3, 10 elegans ______31; pl. 1 Grays Canyon Limestone Member______30 guelphensis ______31; pl. 1 M nemdensis, Kyphophyllum ______5; pl. 9 sp. 1 ______31; pl. 2 mcal/isteri, Palaeocyclus porpita _ Stylopleura 5, 15, 17, 22, 32; pl. 2 Pyramid Hill ______11, 12, 14 Petrozium ______newfarmeri, Tryplasma ______5, 10, 31; pl. 3 Silurian-Devonian Boundary ______15 McCann Hill Chert, Alaska______33 Niagaran Series, reefs ______4, 5 Pyramid Hill section ______13, 14 McColley Canyon Formation ______10 noguerensis, Rotundacodina ______40 physical stratigraphy ______14 McKee, E. H., cited______2 Northumberland area, Toquima Range 7 McMonnigal Limestone ______9, 22 Northumberland window ______23 R Maggie Creek, Tuscarora Mountains_ 2, 7, 16, 41 Prospect sequence ______23 Rabbit HilL ______18 magnitubus. Halysites !Cystihalysites) ______5 Striped Hill sequence ______23 Rabbit Hill Limestone 11, 14, 15, 16, 18, 19, 22, Mahogany Hills ------20 nudus, Monograptus 19 23, 27, 34, 40, 41 Maikottia 31 Ravenswood area, Shoshone Range _ 7, 25 turkestanica ------32 0 Red Hill ______------11,14 sp- 32 occidentalis, Entelophylloides (Prohexagonaria I __ 5, 10 Reeds Canyon, Toiyabe Range ______7, 24 Masket Shale __ _ ---- 3, 19, 23 olh•eri, Mucophyllum ______5, 15, 30; pl. 4 Reef, Lone Mountain ______Bastille Limestone Member ______23,40 Oneotodus beckmanni _ 40 Reefs, building ______Masursky, Harold, cited_ originalis, Carlinastraea ______34 core rock______Matti, Jonathan C., cited ______2 Orthophyl/um sp ______10 dolomite______4 Mazourka Canyon ______2. 4,25, 28,35 Orthostrophia sp ------5, 15 Gotland, Sweden______4, 5 Mazourka Canyon area _ 28 Ozarkodina denckmanni 40 Great Lakes 4 media, Ozarkodina ------40 gaertneri ______40 marginal beds ------5 medius, Climacograptus ______11 media ______40 marine limestone ______4 Merista sp ______10, 16 typica 40 Niagaran Series ______4, 5 patch ______5 Meristella sp ------5, 15 ziegleri ______40 Microplasma ------10 sp ------40 Roberts Mountains Formation, scarcity ______4 scarcity ______4 migratus, Paltodus _ ------40 Mill Canyon, Toquima Range ______11, 20, 22, 40 p remscheidensis, Spathognathodus ______40 Mill Canyon sequence ______22, 23, 24, 31, 34,35 Pablo Canyon, Toiyabe Range______7 Spathognathodus steinhornensis 40 Miller, J. W., cited ______2 Pablo Canyon area ______23,25 resima, Belodella ______40 50 INDEX

Page Page Page Resserella sp 17 thickness ____ 9, 12, 13, 14, 16, 18, 19, 20, 22, 25 simpsoni, Chonophy/lum-Continued Retiolites geinitzianus angustidens _ 19 Toiyabe Range ______23, 25 Tonkinaria ______5, 10, 13, 32; pl. 5 R habdocyclus sp. B 20 Toquima Range______3,20, 24 Siphonophrentis !Breviphrentis) ------_____ 30 sp. d Tuscarora Mountains _ ------15 Skeats, E. W., cited______6 Rhegmaphyllum sp. h_ type area ______----- 7, 8, 10 Spathognathodus eosteinhornensis ______40 sp - 13, 17 type section 3, 9, 10, 16, 17, 20 inclinatus inclinatus ____ 40 Rhizophyllum __ 15, 33 age ______10 primus ______40 enorme 5, 15 unit 1 ______9, 11 remscheidensis ______40 sp. D 5, 27 unit 2 ______9, 10, 11, 20 steinhornensis remscheidensis ______40 R hynchospirina sp _ 5, 15 unit 3 ______9, 10, 11, 31 transitans ___ _ 40 Roberts Creek Mountain __ _ 3, 7, 9, 10, 19 upper coral zone with Toquimaphyl/um ______14 sp -- 40 Roberts Mountains, Nev ---- 7, 14 Roberts Mountains thrust______22 spinosus, Gothograptus ______19 rugosa_ ---- 4 August Canyon sequence ------23 spiralis, Monograptus ______19 Silurian limestones __ 3 Ikes Canyon window ------22, 23 Sponge______26 Roberts Mountains Formation, age 1, 2, 11, 20 June Canyon sequence ______22, 23 Spongophyllidae ______29, 32 August Canyon sequence --- 23, 24 Mill Canyon sequence ______22, 23 Spongophylloides ______------33 Bare Mountain -- 20 Northumberland window ______23 Spongophyllum ______17, 32,33 black chert unit ___ _ -- 22, 23, 24, 25 Petes Canyon window______22 sedgwicki ______32, 33; pl. 6 Bootstrap Hill section, age ---- 17 Roberts, R. J., cited ______2, 16 [Carlinastraea] ______------33 physical stratigraphy _ 16 Rocky Hills thrust ______12 sp. A ______33 sp. B ______33 stratigraphic paleontology ___ _ -- 16, 32 Ross, D. C., cited______Brock Canyon, Monitor Range __ 19 Ross. R. J., Jr., cited ___ _ ------28 Stauriidae ______31 Callaghan window ___ _ 23,24 Rotundacodina elegans -- 40 Steidtmann, Edward, cited ______6 chert marker 18, 19, 20 noguerensis ______------40 steinhornensis remscheidensis, Spathognathodus __ 40 cherty dolomite member ------20 Round Mountain ______25 Stevens, C. H., cited ______2 Coal Canyon area _ 11' 14, 15, 35 Rugosa______2, 4, 5, 10, 13, 14, 15, 17, 25, 26, 27,29 stevensi, Australophyllum ______------28 Coal Canyon fault zone __ _ 11 age value ______29 Stewart, J. H., cited ______------Coal Canyon section __ _ ---- 4, 16, 20 assemblages ------15, 27 Straight Canyon area ______23,24 conodonts 2 Australia ------4, 15 Strepte/asma ______------20 Copenhagen Canyon ______3, 18 cerioid ______------32 Streptelasmatidae ______------30 age 18 colonial 2, 9, 17, 20, 26, 29, 32 Striatopora sp ______------13 stratigraphic paleontology 18 dissepimented ______------32 Striped Hill sequence ______------23 Cortez Mountains ____ _ 3 eastern Europe ___ _ ------4 Stromatoporoids ______4, 5, 15, 26, 27 Dobbin Summit, Monitor Range 19 England ___ _ ------4 Stylopleura ______13, 14, 15, 17,31 dolomite facies ---- 3,10 Gotland, Sweden _ ------4, 15 berthiaumi ______5, 10, 13, 15, 20, 31, 32; pis, 1, 2 origin key ------29 nevadensis ______5, 15, 17, 22, 32; pl. 2 dolomite problem Klamath Mountains, Calif ______15 sp. b ______17, 32; pl. 1 Dry Creek area ______23,24 lykophyllid ______32: pl. 12 sp. c ______13, 14, 32; pis. 1, 2 sp. T ______32 facies changes ______------10 nondissepimented, Silurian ------29 geologic mapping __ --- 2 pycnostylid ______10, 14, 15 sp ------20 geologic setting 11 Silurian ______4, 15 Sunday Canyon Formation _____ 2, 27,28 age ______28 gold __ _ 3 Silurian, stratigraphic distribution______4 graptolite faunas __ ---- 3 special stratigraphic value ______29 Swales Mountain ______7 Great Basin ______1, 4 stratigraphic value ______----- 29 Synprioniodina sp ______40 Ikes Canyon window __ _ ---- 22, 23, 24 zonation _ 28 Syringaxon ------14 June Canyon sequence ------22. 23 Ryderophyllum ubehebensis _ ------5 sp------17 lateral equivalence ___ _ 3 Syringopora ______27 limestone ______------1, 5, 14 s sp 13 limestone facies ------10 Salairophyllum __ ------15, 33 lithology ____ _ 5, 8, 9, 12, 13, 16, sp - 5 T 18, 19, 20, 23, 24 Salopina ______17 Tabulophyllum ______------34 lower coral zone with Carlinastraea ______14 sp ------17 Tentaculites ------14 Mill Canyon sequence ______22, 23, 24, 31, 34, 35 sandalina, Ca/ceola ______27 Thrust fault ------16 Monitor Range ______18 scalaris, Climacograptus 11 Thrust plates 18, 19, 22, 23, 24 physical stratigraphy______18 scanicus, Lobograptus _ 13 Toiyabe Range 2, 18, 23, 25 Pablo Canyon area ______23,25 scanius, Monograptus ______28 Tonkinaria ______------13 Petes Canyon window ______22, 24 Schellwienella sp __ 5, 15 simpsoni ------5, 10, 13, 32; pl. 5 Pine Hill, age ______13 Schizophoria __ ---- 27 sp ------22 stratigraphic paleontology ______13 Schlotheimophyllum 15, 30 Toquima Range_____ 2, 3, 7, 11, 17, 18, 19, Pine Hill section______12, 14, 15 Schneider, J., cited ______6 23, 24, 25, 34, 35, 40 Point of Rocks area __ 23, 24, 25 Schuchert, Charles. cited ------6 Ikes Canyon area ______32, 35 Prospect sequence ___ 23 Sco/opodus devonicus __ _ ------40 Mill Canyon sequence ______22, 23, 24, 31, 34, 35 Pyramid Hill, Silurian-Devonian sedgwicki, Spongophy/lum __ 32, 33: pl. 6 Roberts Mountains Formation ______3, 20, 24 Boundary, problem _ 15 Shales, graptolitic ______16 Toquimaphyllum ______14, 15, 34, 35 Pyramid Hill section ______13, 14 Shale-graywacke facies beds ______13 physical stratigraphy ______14 Shoshone Mountains, Nev ______2 (Toquimaphyllum!, Australophyllum ______29, 34, 35 redefinition ______2 Shoshone Range ______johnsoni, Australophyllum ______5, 14, 15, 17, reefs, scarcity ______4 Sicorhyncha sp ______------5, 15 22, 23,27, 34, 35; pl. 6 reference section______4, 16, 17 Silurian-Devonian boundary ______1, 2, 11, 15, 17 sp., Australophyllum ______27 Roberts Creek Mountain ______3, 5, 7 central Nevada__ 28 Tor Limestone _ ------22 Roberts Mountains, Nev ______7 problem ______15 Torquay, Devonshire, England ______33 Shoshone Mountains ______25 Pyramid Hill __ ------15 transitans, Spathognathodus ______40 Shoshone Range ______3 Silurian-Devonian Boundary Committee ______28, 29 Trematospira ____ ------27 Silurian limestones______4 Silurian limestone, Great Basin, distribution______6 sp ______17 Simpson Park Mountains ______3, 11 Silurian limestones, geographic distribution ______Trichonodella blanda ______40 Straight Canyon area ______23,24 Silurian System ___ 4, 10, 15, 19, 26, 31 inconstans ______40 stratigraphy______2, 9, 10, 12, 13 Silurian zone 5b, Norway ______10 sp ______40 stratigraphic paleontology______10 Simpson Park Mountains ______2, 3, 7, 14, 16, 17, Trilobite ______26 stratigraphic relations _____ 9 32, 35, 40 cryptolithoid ___ ------24 Striped Hill sequence______23 simpsoni, Chonophyllum ______5, 15; pis. 7, 9 truncatum, Kodonophyllum ______30 INDEX 51

Page Page Page Tryp/asma ------10, 13, 33 v Fertici//opora-Continued duncanae ______5, 10, 13, 17, 31; pl. 3 VanTuyl, F. M .. cited ______6 sp ------5; pl. 10 newfarmeri 5, 10, 31; pl. 3 Vaughn Gulch ______25 Vinini Formation ______11, 16 sp. g ______31; pl. 3 Vaughn Gulch Limestone __ 1, 15, 18, 25, 34, 35 Virgiana sp 20 sp ______13,17 age ______27 uulgaris, Monograptus _ 28 Tryplasmatidae _ __ _ 29, 31 black chert zones______25 Tryplasmid ______27, 31 chert member ------26 w tubifera. F/etcheria ______31; pl. 3 coral zone A ------27 tumescens, Monograptus ______28 coral zone E _ ------27 Wall Canyon, Toiyabe Range turkestanica, Maikottia ______32 lower division ______26 wallisen, Lonchodina ______40 Tuscarora Mountains, Nev __ _ 2, 3, 7, 9, 11, middle division ______26 Water Canyon ------____ 23 15, 16. 30, 34, 41 physical stratigraphy______25 Wenban Limestone------9 tuscaroraensis, Car/inastraea 5, 13, 16, 17, stratigraphic paleontology ______26 West Northumberland Canyon ___ 20 32, 33; pis. 6. 7, 8 thickness ______26 Western graywacke belL ______typica, Ozarkodina 40 type section_ 25,26 Willow Creek ______upper, fossils ______-- 27 Windmill Limestone 9, 11, 13, 14, 16, 19, 40 u upper division ______26 ubehebensis, Ryderophy/lum ______5 Verticil/opora beds ______27 Y, Z Ulrich, E. 0., cited ______Vaughn Gulch limestone unit ______15 uncinatus, Monograptus ______17, 19 Verticz/lopora 4, 15, 26, 27, 29 Yassia ______----- 32 uniformis, Monograptus ____ _ 4, 18, 28 ranges _ ------4 Zelophyllum sp ______24 Ural Mountains, U.S.S.R _ 15, 33, 34 annulata 5, 10, 14, 15, 22, 26, 27; pis. 10, 11 ziegleri, Ozarkodina ______40

PLATES 1-12 [Contact photographs of the plates in this report are available, at cost, from the U.S. Geological Survey Photographic Library, Federal Center, Denver, Colorado 80225] PLATE 1

FIGURES 1-9. Stylopleura berthiaumi Merriam 1-6. Great Basin Silurian coral zone D. Roberts Mountains Formation, unit 3. Locality MllOO, Roberts Creek Mountain, Nev. 1. View of broken calice interior showing three offsets. Paratype, USNM 159384a. x 2. 2. Lateral view of two corallites from a large colony (paratype USNM 165359) showing connecting elements. x 1. 3. Calice view showing multiple calice budding. Paratype USNM 159383. x 2. 4. Partial lateral view of holotype. USNM 159382. x 1 Y2. 5. Partial lateral view of paratype showing lateral element attached to Cladopora branch. USNM 159384. x 1. 6. Longitudinal thin section. USNM 165358a. x 2. 7-9. Great Basin Silurian coral zone E. Roberts Mountains Formation. Locality M1106, Coal Canyon, Simpson Park Mountains, Nev. USNM 159385. x 1. 7. Lateral view. 8. Calice view. 9. Partial interior view showing calice offsets. 10-12. Stylopleura sp. b 10-11. Silurian Roberts Mountains Formation. Locality M1314, Bootstrap Hill, Tuscarora Mountains, Nev. 10. Lateral view of broken corallite. x 1%. 11. Lateral view of piece of corallite showing base of hollow lateral element. x2. 12. Longitudinal thin section. x2. Silurian. Roberts Mountains Formation. Locality M1315, Bootstrap Hill, Tuscarora Mountains, Nev. 13--15. Stylopleura sp. c, cf. S. berthiaumi Great Basin Silurian coral zone D. Roberts Mountains Formation. Locality M1317, Coal Canyon, Simpson Park Mountains, Nev. x2. 13. Longitudinal thin section. 14, 15. Transverse thin sections. 16, 17. Pycnostylus guelphensis Whiteaves? Longitudinal and transverse sections. x2. Unenlarged copies of plate X, figures 4, 4a in Lambe (1901 ). Silurian, Guelph Dolomite; Ontario, Canada. 18, 19. Pycnostylus guelphensis Whiteaves. Unenlarged copies of two ofWhiteaves' type illustrations (Whiteaves, 1884, pl. 1, figs. 1a, b). Silurian, Guelph Dolomite; Ontario, Canada. x 1. 18. Transverse section of corallum. 19. Lateral view of broken corallite with two remaining offsets; note lack of exterior longitudinal ribbing. 20. Pycnostylus elegans Whiteaves. Anterior end of a mature corallite with several offsets. Note external ribbing. x 1. Unen­ larged copy of one of Whiteaves' (1884) type illustrations. Silurian, Guelph Dolomite; Ontario, Canada. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 1

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STYLOPLEURA AND PYCNOSTYLUS PLATE 2

FIGURES 1, 2. Stylopleura? sp. c Transverse and longitudinal thin sections. x2. Silurian, Roberts Mountains Formation. Locality M1379, Coal Canyon, Simpson Park Mountains, Nev. 3-6. Stylopleura neuadensis Merriam Great Basin Silurian coral zone E. Roberts Mountains Formation. Coal Canyon, Simpson Park Mountains, Nev. 3, 4. Longitudinal and transverse thin sections of same colony. Paratype, USNM 159431a. x 2. Locality M1380. 5. Upper surface of holotype, USNM 159431. x '12 . Locality M1107. 6. Longitudinal thin section of holotype showing a lateral connecting element. USNM 159431. x2. Locality M1107. 7. Stylopleura cf. S. neuadensis n. gen., n. sp. Longitudinal thin section. x2. Great Basin Silurian coral zone E. Roberts Mountains For­ mation. Locality M1381, Coal Canyon, Simpson Park Mo untains, Nev. 8, 9. Sty lopleura berthiaumi Merriam Great Basin Silurian coral zone D. Locality M1103, Ikes Canyon, Toquima Range, Nev. 8. Lateral view of flaring calice. x 1. 9. Lateral view of same individual showing connecting element. x 1. 10. Pycnostylus sp. 1 Longitudinal thin section. x3. Silurian, Lone Mountain Dolomite. Locality M1148, south­ ern Sulphur Spring Mountains, Nev.; 4.8 km south of Romano Ranch. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 2

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STYLOPLEURA ANDPYCNOSTYLUS PLATE 3

FIGURES 1-7. Fletcheria tubifera Edwards and Haime. 1. Lateral view of cora!lum. Copy of original type figure of Edwards and Haime (1851, pl. 16, fig. 5). Silurian, Gotland, Sweden. x 1 Yz . 2-7. Silurian, Slite Group (Wenlockian); Slite, Liinnaberget, Gotland, Sweden. 2. Lateral view of corallum showing corallite surface ornamentation. x2. 3. Transverse thin section. x 2. 4, 5. Longitudinal thin sections. x2. 6, 7. Transverse sections showing multiple calice offsets and wall structure. Note ab­ sence of septa. x6. 8, 9. Tryplasma duncanae Merriam Great Basin Silurian coral zone D. Roberts Mountains, Formation, unit 3. Locality MllOO, Roberts Creek Mountain, Roberts Mountains, Nev. 8. Lateral view of holotype, USNM 159374. x2. 9. Oblique cal ice view of para type, USNM 159375. x5. 10-15. Tryplasma newfarmeri Merriam Great Basin Silurian coral zone C. Roberts Mountains Formation, unit 3. Locality M1102, Roberts Creek Mountain, Nev. 10, 11. Longitudinal thin sections of holotype, USNM 159377. x3. 12, 13. Longitudinal thin sections of holotype, USNM 159377. x8. 14, 15. Transverse thin sections ofholotype, USNM 159377. x8. 16-19. Tryplasma sp. g These figures are all of the same corallum. Silurian, Hemse Group (Ludlovian); locality M1382, Gotland, Sweden. 16. Transverse thin section. x2. 17. Transverse thin section. x4. Enlargement of part of figure 16. 18, 19. Longitudinal thin sections. x2. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 3

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18

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13 12 10

FLETCHER/A AND TR YPLASMA PLATE 4

FIGURES 1-4. Mucophyllum oliueri Merriam Great Basin Silurian coral zone E. Locality Mll08, Coal Canyon, Simpson Park Moun­ tains, Nev. 1. Longitudinal section of para type, USNM 159391. Photographed in water. x 1. 2. Transverse section of holotype, USNM 159390. Smoothed surface photographed in water. x 1. 3. Longitudinal thin section of holotype, USNM 159390. x4. 4. Part of transverse thin section of holotype, USNM 159390, showing pattern of trabeculae. x 2. 5, 6. Kodonophyllum mulleri Merriam Transverse and longitudinal thin sections of holotype, USNM 159386. x4. Great Basin Silurian coral zone E. Locality Mll07, Coal Canyon, Simpson Park Mountains, Nev. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 4

MUCOPHYLLUM AND KODONOPHYLLUM PLATE 5

FIGURES 1-14. Tonkinaria simpsoni Merriam 1- 9. Great Basin Silurian coral zone D. Roberts Mountains Formation, unit 3. Locality M1100, Roberts Creek Mountain, Roberts Mountains, Nev. 1, 2. Lateral (x1V2) and calice (X2) views ofparatype, USNM 159402. 3, 4. Transverse (X4) and longitudinal (x3V2) thin sections ofparatype, USNM 159404. 5. Transverse thin section. USNM 159405. x2. 6. Calice view. x 2. 7. Longitudinal thin section of same individual. x3. 8, 9. Lateral exterior and calice views of holotype, USNM 159403. x 1 V2 . 10--14. Great Basin Silurian coral zone D. Roberts Mountains Formation. Locality M1383; Coal Canyon, Simpson Park Mountains, Nev. 10. Oblique lateral view of corallum showing bases of three offsets at cal ice rim. x2. 11-13. Calice rim of three coralla showing offsets. x2. 14. Calice view. x2. 15--21. Kyphophyllum sp. b Silurian, Roberts Mountains Formation. Locality M1314; Bootstrap Hill, Tuscarora Moun- tains, Nev. 15. Lateral view of coral lite showing a single offset. x 2. 16. Lateral view of partial corallum x2. 17. Lateral view ofcalice interior. x2. 18, 19. Transverse and longitudinal thin sections of corallite. x 3. 20, 21. Transverse and longitudinal thin sections of corallite. x3. 22--24. Kodonophyllum sp. b. Transverse and longitudinal thin sections of three corallites. x 3. Silurian, Roberts Moun­ tains Formation. Locality M1314, Bootstrap Hill, Tuscarora Mountains, Nev. 25, 26. Brachyelasma sp. c. Transverse and longitudinal thin sections of same individual. x 1 V2. Great Basin Silurian coral zone D. Roberts Mountains. Formation. Locality M1383; Coal Canyon, Simpson Park Mountains, Nev. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 5

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TONKINARIA, KYPHOPHYLLUM, KODONOPHYLLUM, AND BRACHYELASMA PLATE 6

FIGURES 1-4. Spongophyllum sedgwicki Edwards and Haime. Copies of original figures of the type material of Edwards and Haime (1850--54, pl. 56, figs. 2a, c, d, e). Torquay, England; according to Edwards and Haime from the Devonian strata. 1. Transverse section enlarged (Edwards and Haime, fig. 2d, x 1 'h). 2. Longitudinal section enlarged (Edwards and Haime, fig. 2e, x 1 'h). This section is conceivably from same corallum as figure 1. 3, 4. Transverse sections enlarged (Edwards and Haime, figs. 2c and 2a, x 1 Y2). These figures probably do not represent the same species and are possibly not congeneric with corals illustrated by figures 1 and 2 of this plate. 5--7 . Carlinastraea tuscaroraensis n. gen., n. sp. 5. Transverse thin section. x3. Great Basin Silurian coral zone D. Roberts Mountains Formation. Locality M1384, Coal Canyon, Simpson Park Mountains, Nev. 6, 7. Transverse and longitudinal thin sections of holotype, USNM 166482. x21/2. Roberts Mountains Formation. Locality M1313, Bootstrap Hill. 8--10. Australophyllum (Toquimaphyllum)johnsoni Merriam Transverse and longitudinal thin sections of holotype, USNM 150420. x2. Great Basin Silurian coral zone E. Locality M1114, Ikes Canyon, Toquima Range, Nev. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 6

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SPONGOPHYLL UM, CARLINASTRAEA, AND A USTRALOPHYLL UM PLATE 7

FIGURES 1-4. Carlinastraea tuscaroraensis n. gen., n. sp. 1-3. Transverse and longitudinal thin sections of holotype, USNM 166482. x6. Roberts Mountains Formation. Locality M1313, Bootstrap Hill, Tuscarora Mountains, Nev. 4. Longitudinal thin section. x 6. Great Basin Silurian coral zone D. Roberts Mountains Formation. Locality M1384, Coal Canyon, Simpson Park Mountains, Nev. 5, 6. Chonophyllum simpsoni Merriam Transverse a nd longitudinal thin sections of holotype, USNM 159408. x 1 Vz. Great Basin Silurian coral zone E. Roberts Mountains Formation. Locality Mll08, Coal Canyon, Simpson Park Mountains, Nev. 7, 8. Calostylis? sp. Transverse thin sections. x 3. Roberts Mountains Formation. Locality M1314, Bootstrap Hill, Tuscarora Mountains, Nev. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 7

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CARLINASTRAEA, CHONOPHYLLUM, AND CALOSTYLIS? PLATE 8

FIGURES 1, 2. Carlinastraea sp., cf. C. tuscaroraensis, n. gen., n. sp. Transverse and longitudinal thin sections of same cor all urn. X 2. Roberts Mountains Forma­ tion. Locality Ml409, Cortez Mountains, Eureka County, Nev. 3-7. Australophyllum (Australophyllum ) sp. c Locality Ml318, west side of Coal Canyon in fault zone, Northern Simpson Park Moun­ tains, Nev. All the thin sections are from same corallum. 3-5. Transverse thin sections. x 2. 6, 7. Longitudinal thin sections. x 2. 8-11. Australophyllum (Australophy llum) sp. v Vaughn Gulch Limestone. Mazourka Canyon, northern Inyo Mountains, Calif. 8, 9. Transverse and longitudinal thin sections of same corallum. x 2. Locality Ml410. 10, 11. Transverse and longitudinal thin sections of another corallum. x2. Locality Ml093. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 8

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CARLINASTRAEA AND AUSTRALOPHYLLUM PLATE 9

FIGURES 1-3. Kyphophyllum neuadensis Merriam Great Basin Silurian coral zone E. Locality M1114, lkes Canyon, Toquima Range, Nev. Holotype, USNM 159425. 1. Transverse thin section. x4. 2. Transverse thin section. x2. 3. Longitudinal thin section. x2. 4-8. Kyphophyllum sp. c Transverse and longitudinal thin sections. Great Basin Silurian coral zone E. Locality M1380, Coal Canyon, Simpson Park Mountains, Nev. 4, 6, 8. Transverse thin sections. x 2. 5, 7. Longitudinal thin sections. x2. 9, 10. Chonophyllum simpsoni Merriam Transverse and longitudinal thin sections of paratype, USNM 159409. x2. Great Basin Silurian coral zone E. Roberts Mountains Formation. Locality M1108, Coal Canyon, Simpson Park Mountains, Nev. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 9

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KYPHOPHYLLUM AND CHONOPHYLLUM PLATE 10

FIGURES 1- 3. Verticillopora annulata Rezak. Roberts Mountains Formation. Locality M1411, west side of Coal Canyon, northern Simpson Park Mountains, Nev. All figures are of the same individual. 1. Latex impression of exterior. x 1. 2. Chert filling of internal cavity. x 1. 3. Latex impression of interior. x 1. 4. Verticillopora sp., cf. V. annulata Rezak. End view showing central cavity and small radial canals to exterior. x Y2. Roberts Moun­ tains Formation, upper part. Great Basin Silurian coral zone E. Locality Mll06, east side of Coal Canyon near mouth, northern Simpson Park Mountains, Nev. 5-7. Cladopora sp. v. Lateral views of pieces of branching colonies. x 2. 8. Digitate favositid. Lateral view of piece of branching colony. x l'h . Vaughn Gulch Limestone, basal beds of upper unit. Locality M1093, Mazourka Canyon, northern Inyo Mountains, Calif. 9-11. Favosites sp. c. Transverse and two longitudinal thin sections. x 2. Roberts Mountains Formation. Locality M1411, west side of Coal Canyon, northern Simpson Park Mountains, Nev. 12, 13. Favosites sp. d. Transverse and longitudinal thin sections of form with very narrow corallites. x2. Roberts Mountains Formation, upper part. Great Basin Silurian coral zone E. Locality Mll06, east side of Coal Canyon near mouth, northern Simpson Park Mountains, Nev. 14-17. Kyphophyllum sp. t. Lower part of Tor Limestone. Locality M1397, Toquima Range, Nev. All fi gures are of the same corallum. 14. Transverse thin section. x2. 15-17. Longitudinal thin sections. x2. 18, 19. Ketophyllum sp. t. Transverse and longitudinal thin sections of same corallum. x2. McMonnigal Limestone. Locality M1393, Toquima Range, Nev. 20. Hercynella-like mollusk. Lateral view. x 1 Y2. Near locality Mll06, Coal Canyon, east side near mouth. Northern Simpson Park Mountains, Nev. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 10

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VERTICILLOPORA, CLADOPORA, FA VOSITES, KYPHOPHYLLUM, KETOPHYLLUM, AND HERCYNELLA PLATE 11

FIGURES 1-6. Verticillopora annulata Rezak. 1- 4. Great Basin Silurian coral zones D-E. Upper middle part of Vaughn Gulch Limestone. Locality Mll28, Mazourka Canyon, lnyo Mountains, Calif. 1. Transverse view. x 1 Yz . 2, 3. Transverse and lateral views of same individual. x 11/z. 4. Lateral view. x 1 'h . 5. Lateral view. x 1. Great Basin Silurian coral zones D-E. Locality M1385, Mazourka Canyon, Inyo Mountains, Calif. 6. Transverse view. x 11/z. Great Basin Silurian coral zone D. Locality Mll03, Ikes Canyon, Toquima Range, Nev. Roberts Mountains Formation. 7, 8. Verticillopora cf. V. annulata Rezak. Great Basin Silurian coral zone D. Roberts Mountains Formation, unit 3 near top. Locality MllOO, Roberts Creek Mountain, Nev. 7. Weathered specimen showing wall of internal cavity. x 3. 8. Weathered interior showing radial canals. x 2 Vz . 9- 14. Kozlowskiellina sp. f 9--12. Great Basin Silurian coral zone D. Roberts Mountains Formation, unit 3 near top. Locality MllOO, Roberts Creek Mountain, Nev. 9, 10. Exterior and interior of same ventral valve. x2. 11 , 12. Interior and exterior of same dorsal valve. x3. 13, 14. Exterior and interior of same ventral valve. x 2. Great Basin silurian coral zone E. Upper part of Roberts Mountains Formation. Locality Mll06, Coal Canyon, Simpson Park Mountains, Nev. 15-17. Homoeospira sp. r Dorsal, ventral, and anterior views of same individual. x 2. Great Basin Silurian coral zone D. Roberts Mountains Formation, unit 3 near top. Locality MllOO, Roberts Creek Moun­ tain, Nev. 18, 21, 22. Dicoelosia sp. r Great Basin Silurian coral zone D. Roberts Mountains Formation, unit 3 near top. Locality MllOO, Roberts Creek Mountain, Nev. 18. Interior of dorsal valve. x4. 21 , 22. Dorsal and ventral exterior views of same individual. x4. 19, 20. Dicoelosia sp. r. Interior and exterior views of same ventral valve. x2Vz . Great Basin Silurian coral zone D. Roberts Mountains Formation, unit 3 near top. Locality MllOO, Roberts Creek Mountain, Nev. 23, 24. Plectatrypa? sp. c Dorsal and ventral exterior of same individual. x2. Great Basin Silurian coral zone E. Locality Mll06, Coal Canyon, Simpson Park Mountains, Nev. 25, 25. Gypidula sp. r. Dorsal and ventral interior of same individua ls. x 2. Great Basin Silurian coral zone D. Roberts Mountains Formation, unit 3 near top. Locality MllOO, Roberts Creek Mountain, Nev. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 11

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VERTICJLLOPORA, KOZLOWSKIELLINA, HOMEOSPIRA, DICOELOSIA, PLECTATRYPA?, AND GYPIDULA PLATE 12

FIGURES 1-7. Coelospira sp. b Roberts Mountains Formation. Locality M1412, Bootstrap Hill, southern Tuscarora Mountains. 1. Exterior view of ventral valve. x4. 2, 3. Dorsal valve and ventral valve exterior views of same shell. x4. 4. Exterior view of dorsal valve. x 4. 5. Interior view of dorsal valve. x4. 6. Interior view of ventral valve. x4. 7. Interior view of dorsal valve. x4. 8--15. Fardenia sp. b Roberts Mountains Formation. Locality M1412, Bootstrap Hill, southern Tuscarora Mountains. 8. Interior view of ventral valve. x2. 9, 10. Exterior and interior views of same dorsal valve. x2. 11, 12. Exterior and interior views of same dorsal valve. x 2. 13, 14. Exterior and interior views of same dorsal valve. x2. 15. Interior view of ventral valve. x 2. 16, 17, 23. Kozlowskiellina sp. b Roberts Mountains Formation. Locality M1412, Bootstrap Hill, southern Tuscarora Mountains. 16, 17. Exterior and interior views of same ventral valve. x2. 23. Exterior view of dorsal valve. x 2. 18--22. Conchidium sp. b. Roberts Mountains Formation. Locali ty M1412, Bootstrap Hill area, southern Tuscarora Mountains, Nev. 18, 19. Exterior and interior views of same ventral valve. x 1. 20. Exterior view of ventral valve . . x2. 21 , 22. Exterior and interior views of same dorsal valve. x2. 24-26. Conchidium sp., cf. C. munsteri Kiaer. Type section of Roberts Mountains Formation on Pete Hanson Creek. Upper beds of unit 2. Roberts Creek Moun­ tain, Nev. 24, 25 . Exterior ventral and lateral views of same ventral valve. x 1. 26. Lateral view of same shell as in figures 24 and 25 split longitudinally to show medium septum and spondylium. x 1 V2. 27, 28. Cyathophylloides sp. f. Transverse and longitudinal thin sections. x4. Locality M1412, Bootstrap Hill area, southern T uscarora Mountains, Nev. 29--33. Brachyelasma sp. b. Locality M1120, Bootstrap Hill southwest side, southern Tuscarora Mountains, Nev. 29, 30. Transverse and longitudinal thin sections of same corallum. x 2. 31-32. Transverse thin sections of same corall ite. x 2. 33. Longitudinal thin section of corallite in figures 31 and 32. x 2. 34-36. Lykophyllid rugose coral. Locality M1413, northeast of Lynn window of R. J. Roberts, Tuscarora Mountains, northeast Eureka County. 34, 35. Transverse and longitudinal thin sections of same corallum. x 2. 36. Transverse thin section. x 2. GEOLOGICAL SURVEY PROFESSIONAL PAPER 973 PLATE 12

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30 33 35 36 COELOSPIRA, FARDENJA, KOZLOWSKIELLJNA, CONCHIDIUM, CYATHOPHYLLOIDES, BRACHYELASMA, AND LYCOPHYLLID