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BIOSTRATIGRAPHY, MIDWAY PROPERTY, NORTHERN BRITISH COLUMBIA" (1040/16) By M. J. Orchard Geological Survey of Canada and s. Irwin The University of British Columbia

KEYWORDS: Jennings River, conodont faunas,Earn Group, Sylvester Group (Gabrielst:, 1969). It has been reassignr:d to McDame Group, Sylvester allochthon. the Earn Group as named by Gordey et al. (1982a). The group consists of black slate, thin-bedd:ed (occassion;~lly) INTRODUCTION calcareous siltstone, thin to thick-bedded sandstone and chert-pebble conglomerate. There areeconomically siglifi- This is a preliminary report on the conodont faunas which cant baritic, siliceousand :sulphide-rich exhalites within this have been collected from the area of the Midway silver-lead- unit (Nelson and Bradford, 1986). Two generally coarsening- zinc manto depositin Jennings River map area (1040116), 80 upward sequences have been identified in the Earn arcund kilometres west of Watson Lake, Yukon and IO kilometres Midway by Cordilleran Engineering Ltd. The basal black south of theBritish Columbia - Yukon border. Approx- shales of the Earn were deposited unconformably over the imately 40 conodont collections have been recovered in the McDame carbonates. Thetupper contact i:s the basal thrust of area since 1982, as part of property and regional mapping the Sylvester allochthon (Nelson and Bradford, 1987). projects. During the summer of 19x7,78 samples were collectedfor conodont processing by S. Irwin.These samplesform part of abroader study of theDevonian- SYLVESTER GROUP Mississippian conodont faunas andbiostratigraphy of the metalliferous Earn Group within miogeoclinal areas of The Sylvester Group (used here in a restricted sense to northern British Columbia. exclude autochthonous strata) consists csf an allochthon of Upper Paleozoic to Tridsr,ic chert, limestone, greenstone, clasticand ultramafic rocks thrust over autochthonous or GEOLOGY parautochthonous strata along the conl.inental margin of Previous work around the Midway deposit area includes North America in Middle Iurassic to Early time l:250000-scale reconnaissance mapping by Gabrielse (Gordey et al., 1982b). It 'was originally mapped as autoch- (1969) and 1:25 OOO reconnaissance mapping by Nelson and thonous Sylvester Group by Gabrielse (1963). Nelson and Bradford (1987). Additional property mapping has been car- Bradford (this volume) hwe subdivided the allochthon inco ried out by Cordilleran Engineering Ltd. three lithotectonic units, the lowest of which contains all the conodont faunas reported here. The tect'onic signature was McDAME GROUP developed during two or mreindependant tectonic episodes The Middle McDame Group is composed of (Harms, 1986; Nelson and Bradford, 1987). shallow, warm-marine carbonates that were deposited onto a subsiding shelf or platform (Gabrielse, 1963). In this study area the McDame is made up of a lower, dark grey, tan- CONODONT FAUNAS weathering,massive to laminated, fetid dolostone and an upper, dark grey, fossiliferous platy limestone. Devonian At present, conodontfaunas are known from the McDme macrofauna accumulations suggest an abundant but low di- Group, Earn Group and Sylvester allochthon. This has versity fauna, indicative of Intertidal to subtidalenviron- provided broad constraints on the ageof these units. Within ments. The McDame has undergone extensive karsting, at the area shown in Figure L23-1, the concNdonts range in age least some of it related to processes that predate Earn Group fromMiddle Devonian through Late . Six deposition. In the Midway area, the McDameGroup is conodont faunas are identiFied and discused below: conformably (?) underlain by Lower Devonian Tapioca sand- 1. Palmatolepis. stone (Nelson and Bradford, 1987). 11. Fanlennian Palmatolepis. 111. . EARN GROUP IV. 'Hindeodella' segqbrmis. The Devono-Mississippian Earn Group comprises a tur- V. bilineat~cs. biditic sequence that was originally included in the Lower - VI. Idiognathoides. * This project is a contribution lo the CanaddBritish Columbia Mineral Development Agreement. Bitish Columbia Ministry of Energy, Mines and Petroleum Resources. Geological Fieldwork, 1987, hper 19118-I 249 Dsronisn?tO Sylreater lllocthon ohert, a~gillite. limestone, volcanics. m serpentinite, gabbro. De.o.i.n-Mis.iaaippian Earn Group slate, siltstone. gi-eywacke, chert pebble 161 conglomerate. Middle Devonian McDase Group El dolomite, limestone. A, a ConodontSample locations (see Table 1) contours in feet

L Figure 1-23-1. The collection sites of conodont-bearing samples (A to g), and simplified geology of the Midway area (aftel Nelson and Bradford, 1987). 250 FAUNA I (Table 1-23-1). The fauna IS characterized by abunddnt 1:on- odonts: the frequency is suggestive of either very slow rates This faunawas recovered from dolomitic siltstone occupy- of sedimentation and concentrationasconodont lagdeposits, ing hollows at the top of the McDame limestone at one or an extremely productive environmentthat has experienced locality.It comprises poorlypreserved specimens of Icr- a kill. iodus, Palmarolepis and . Although these have not been determined to species level, they compare closely Fauna I1 is dominated by representatives of Palmarolepis with early Late Devonian, Frasnian forms. Additional mate- ex gr. glabra Ulrich and Bassler, accompanied by P. minula rial should provide amore precise age forthisfaunule, which Branson and Mehl, P. sub,perlobatu Branson and Mehl, t'. is important because it provides the first clear evidence for cf. P. regularis Cooper, .P. quadranrinodosulobara ?;an- Late Devonian sedimentationprior to that of the Earn Group. nemann, andP. triangularis Sannemann. Collectively, these It should also provide information on the nature and magni- represent theearly to middle Fa.mennian conodont tude of the hiatus between the two groups. zones of Upper P. crepida or P. rhomboidea. The fma belongs to the offshore-basinal hiofacies of Palmatolepis and A representative suite of lithotypes from core material of lacks all indicatorsof shallow water depcIsltlon. ' ' the McDame limestone (provided by D. Mundy, Cordilleran Engineering Ltd.) was analysed for conodonts, but few were Thisfauna is widespread in theepicratonic region of recovered; they can only he broadly dated as Devonian. This western Canada. It is a typical Earn GrolJp association (for probably results from a generally inhospitable environment, example, Gordey era/., 1982a, Fauna VI) in Yukon and the but large samplestaken selectively might produce diagnostic Kechika trough. South of Midway, in the Driftpile-Galaga collections. area, approximately coeval faunas bracket mineralized hon- zons within theEarn Group (McClayand Insley. 15185; Elsewhere within the Cordilleran miogeocline, Frasnian McClay et al., 1986). conodont collections are commonly found in Earn Group strata. Many collections of this age are associated with barite deposits in the Selwyn Basin,Yukon Territory (Dawson and FAUNA Ill Orchard, 1982). This faunacharacterizes the 2AC unit of the EmGroup in two drill cores on the Midway property (Figure 1-23-1, K, FAUNA 11 M), and has also been recovered from two localities wil.hin This faunais known from the basal I AC unit of the Lower the outcrop of the Ewen and Perrybarites. Fauna 111 is Earn Group in two drill cores from the Midway property recognized by the occurrence of Siphonodella, the rang,: of

TABLE 1-23.1. DETAILS OF THE CONODONT DATABASE DISCUSSED IN THIS PAPER.

?lll-lv* N VI? ?11CIV+ VI I1 J K L Y N P R S T U V W X Y 7. a b c I O which approximates the early to middle Toumaisian (Kind- faunule (HI also includes primitive Neogondolellu. A second erhookian). Siphonodella, theoffshore successorto Late collection (F) contains , a genus that is com- Devonian Palmatolepis, is found alone in some collections, monly found with Idiognathoides, although the former but more commonly, species of Gnathodus, Polygnuthus, rangcs through the Upper Carboniferous and lowermost and are associated, as . for example at the Perry Barite property (c). High diversity faunules such as this are certainly conducive to improved CONCLUSIONS zonal resolution, which may become critical to the question Within the study area, the following conodont-based bio- of Em-Sylvester separation. stratigraphic conclusions are reached: The Occurrence of Fauna I11 also in strata assigned to the (I) Dolomitic siltstone occupying hollows at the top of the Sylvester Group (f) underlines this problem. This sample McDame Group are Late Devonian in age, but signifi- contains each of the genera listed as occurring in the Perry cantly older than basal Earn Croup. Barite collection, butin addition includesBispathodus ex gr. srabilis, arelatively long-ranging taxon that occurs more (2) The hiatus between the McDame and the Earn groups is commonly, possibly exclusively, in samples referred to the equivalent to an undetermined part of the Frasnian plus Sylvester Group (see below). about five conodont zones within the . Elsewhere, Siphonodella is known to occur both in the (3) Informal Unit IAC, which occurs at or near the base of Antler Formation of east-central British Columbia (Struik the Earn Group in the Midway area, is referred to the andOrchard, 1986), which belongs in the same tec- Upper Palmatolepis crepida Zone or P. rhomboidea tonostratigraphic terrane as the Sylvester Group. and in the Zone of the early to middle Famennian. Earn Group of the Selwyn basin. (4) Both the informalUnit 2AC of the Earn Group, andstrata associated with the Ewen and Perry barites, are Early to FAUNA IV Middle Tournaisian in age. This is also the age range assigned to theoldest conodonts identified within Thisfauna, known from four localitiesreferred to the Sylvester Group. Sylvester Group, is typified by 'Hindeodella' segaformis. In (5) In thestudy area, the SylvesterGroup, as presently each of these collections (E, U, V, Y), Bispathodus ex gr. conceived, includes limestone of four differentages: stabilis also occurs, as do fewer Polygnathus and Early to Middle Tournaisian (Fauna Ill), Late Tournai- Pseudopolygnathus. The lasttwo genera also occurin Fauna sian (Fauna IV), Early Namurian (Fauna V), and Late 111, but B. ex gr. stabilis is not known to occur for certain Namurian to Bashkirian (Fauna VI). below Fauna IV. Some Midway samples (D, G, T) contain questionable specimens of Bispathodus and are therefore assigned a range of age, including levels younger than Fauna ACKNOWLEDGMENTS 1V (IV + =Early Visean). One additional collection from the JoAnne Nelson and JohnBradford provided excellent sup- Sylvester Group (a) lacks the segaformis element, but con- port in the field. Access to the Midway property and drill tains Eotaphrus; both these taxa are of Early Carboniferous, cores, given to us by Henrik Thalenhorst of Strathcona Late Tournaisian(lower Osagean)age andare therefore Mineral Services Limited, and the hospitality shown to us at combined in Fauna IV. the Midway camp, were greatly appreciated. The following The segaformis element iswidespread in areas marginal to collectors of conodontsamples are thanked: 6. Hall, D. the craton, andhas been reportedfrom stratiformbarite Maclntyre, T.A. Harms, W. Jakubowski,D. Mundy, J. deposits in Yukon Territory (Dawson and Orchard, 1982). Nelson, J. Bradford and K. Green.

FAUNA V REFERENCES In the study area, Fauna Vis known from a single locality Dawson, K. and Orchard,M.J. (1982): Regional Metal- within the Sylvester Group. It consists of Gnathodus logeny of the Northern Cordillera: Biostratigraphy,Cor- bilineatus, G. girtyi,Lochriea commutata and Idio- relationand Metallogenic Significance of Bedded , an association indicative of the Late Visean to Barite Occurrences in Eastern Yukon and Western Dis- Early Namurian (Late Mississippian). Elsewhere in the Cor- trict of Mackenzie, in Current Research, Geological dillera. this fauna is one of the most widespread (Orchard, Sursey of Canada, Paper 82-1 C, pages 31 -38. 1987); it occurs in both the Antler and Fennel1 formations of Gabrielse, H. (1963): McDame Map-area, Cassiar District. the allochthonous Slide Mountain terrane. and in limestone- British Columbia, Geological Survey of Canada, quartzite units of autochthonous Yukon strata (for example, Memoir 319, 138 pages. Gordey et al., 1982, Nos. 1-4, 7). (1969):Geology of Jennings River MapArea, BritishColumbia (104O), Geological Survey of FAUNA VI Canada, Paper 68-55. This fauna is known from oneor two localities within the Gordey,S.P., Abbott, J.G. and Orchard, M.J. (1982a): Sylvester Group. It is characterized by Idiognathoides, Devono-Mississippian (Earn Group) and Younger which indicatesa Late Namurian to Bashkirian(Early to Strata in East-central Yukon, in Current Research, Part earlyMiddle Pennsylvanian) age. As in other collections 6, Geological Survey ofCanada, Paper 82-1B, pages fromthe Slide Mountain terrane (Orchard, 1986), one 92-100. 252 Gordey, S.P., Gabnelse, H. and Orchard, M.J.(1982b): Sediment-hosted Stratiform Lead-zinc Deposits, I).E Stratigraphy and Structure of Sylvester Allochthon, Sangster, Editor, Mineralogical Association of Cancdc!, SouthwestMcDame Map Area, Northern British Short Course Notes, Volume 8, pages 85.120. Columbia, in Current Research, Part B, Geological Nelson, J. and Bradford, I. (1987): Geology of theArea Survey ofCanada, Paper 82-lB, pages 101-106. Around the Midway Deposit, Northern British Cohm- Harms, T. (1986): Structural and Tectonic Analysis of the bia (1040/16), British Columbia hlini.srry of Energy Sylvester Allochthon, Northern British Columbia: Im- Mines and Petroltwz Resources, ICieological Field- plicationsfor Paleogeography and Accretion, Un- work, 1986, Paper 1987-1, pages 181-192. published Ph.D. Thesis, University ofArizona. Orchard, M.J. (1986): Conodonts from Western Canadian Chert: Their Nature, Distribution and Stratigraphic McClay, K.R. and Insley, M.W. (1985): Structure and Min- Application, in R.L. Austin, Editor, Conodonts, Inves- eralization of the Driftpile Creek Area, Northern British tigative Techniques and Applications, Proceeding:$ of Columbia (94E/16, 94Fi14, 94Ki4, 94LiI). British the Fourth European Conodont Symposium (ECOS ColumbiaMinistry of EnergyMines and Petroleum IV), Chapter 5, pages 96.121, Ellrs-Horwood, U.K. Resources, Geological Fieldwork, 1985, Paper 1986-1. pages 343-350. ~ (1987): and Correlaion of the Harper Ranclh Group (Devonian-Permian:I, McClay, K.R., Insley, M.W., Way, N.A. and Aderton, R. Ashcroft Map Area (Y2l), Southern British Colum~ia, (1986): Stratigraphy and Tectonics of the Gataga Area, in Current Research, Geological Survey of Candu, Northeastern British Columbia (94Ei16, 94Fi14, Paper 87-IA, pages 743.749. 94Ki4, 94Lil, 94L/7,94L/8),British Columbia Minis- Struik,L.C. and Orchard, M.J. (1985), Upper hleo;:oic try of Energy Mines and Petroleum Resources, Geo- Conodonts From Ribbon Chert Delineate Imbricate logical Fieldwork, 1986, Paper 1987-1. pages 193-200. Thrusts Within the Antler Formation of Slide Mountain Maclntyre,D.G. (1981): Geologyand Stratiform Barite Terrane, Central British Columbia, Geologj,, Volume Sulphide Deposits in Northeastern British Columbia, in 13, pages 794-798.

253 Figure 1-24-1. Geology of the northern HorseranchRange. Inset shows index map and regional geology. Line A-B is the location of the cross- section in Figure 1-24-2. 254