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S. Irwin British Columbia Geological Survey Geological Fieldwork 1987

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S. Irwin British Columbia Geological Survey Geological Fieldwork 1987 CONODONT 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 and biostratigraphy 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 Cretaceous 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 Devonian 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 Carboniferous. Six deposition. In the Midway area, the McDameGroup is conodont faunas are identiFied and discused below: conformably (?) underlain by Lower Devonian Tapioca sand- 1. Frasnian Palmatolepis. stone (Nelson and Bradford, 1987). 11. Fanlennian Palmatolepis. 111. Siphonodella. EARN GROUP IV. 'Hindeodella' segqbrmis. The Devono-Mississippian Earn Group comprises a tur- V. Gnathodus 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 Triassic 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 Polygnathus. 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- conodonts 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 Idiognathodus, 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 Protognathodus and Pseudopolygnathus are associated, as Permian. 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 Famennian. 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 and Orchard, 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.
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