Wenlock (Silurian) graptolite biostratigraphy of the ...

Wenlock (Silurian) graptolite biostratigraphy of the Cape Phillips Formation, Canadian Arctic Islands A. C. LENZ Department of Geology, The University of Western Ontario, London, Ont., Canada N6A 5B7 AND

M. J. MELCHIN

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by China University of Petroleum on 06/05/13 For personal use only.

Department of Earth Sciences, University of Waterloo, Waterloo, Ont., Canada N2L 3G1

Received June 15, 1989 Revision accepted August 22, 1989 The Canadian Arctic Islands Wenlock is represented by a diverse assemblage of graptolites from the Cape Phillips Formation. Cyrtograptids are continuously present throughout all but the uppermost Wenlock. Species of Cyrtograptus constitute the main basis for biostratigraphic zonation, supported to a much lesser degree by Monograptus, Monoclimacis, and Pristiograptus through most of the series. The uppermost Wenlock is dominated by Pristiograptus in association with rare Gothograptus. The following biostratigraphic zones and their correlations are proposed: (1) Cyrtograptus centrifugus - Cyrtograptus insectus Zone (earliest Wenlock); (2) Cyrtograptus aff. rigidus - Monograptus aff. riccartonensis Zone (late early Wenlock); (3) Cyrtograptus perneri - Monograptus cf. munchi Zone, in which Cyrtograptus multiramis is a particularly common element (mid-Wenlock age); (4) Cyrtograptus lundgreni - Monograptus testis Zone, divisible into a lower Monograptus testis n.ssp. and an upper Monograptus testis testis Subzone (late Wenlock); and (5) Pristiograptus ludensis Zone (latest Wenlock). The second and third zones are also possibly divisible into lower and upper subzones, but with less certainty. The degree of zonal subdivision in the Cape Phillips is lower than in Europe. It was observed locally that a number of the key species showed a high degree of overlap and some inconsistencies in their ranges from section to section. Paleoecologically, a few species, particularly Cyrtograptus preclarus, Cyrtograptus cf. malgusaricus, and Pristiograptus meneghini, appear to have preferred basin-margin regions.

Le Wenlockien dans les iles de I'Arctique canadien est reprCsentC par divers assemblages de graptolites provenant de la Formation de Cape Phillips. Les cyrtograptides sont prksents dans tout le Wenlockien sauf ti la toute fin. La zonation biostratigraphique repose, en premier lieu, sur le espkces de Cyrtograptus et, en second lieu, sur Monograptus, Monoclimacis et Pristiograptus, genres prksents au travers la presque la totalit6 de la sbrie. La fin du Wenlockien est dominke par Pristiograptus en association avec Gothograptus; ce dernier genre est rare. Les zones biostratigraphiques suivantes sont proposCes : (1) la Zone h Cyrtograptus centrifugus - Cyrtograptus insectus (dCbut du Wenlockien prkcoce) ; (2) la Zone a Cyrtograptus aff. rigidus - Monograptus aff. riccartonensis (Wenlockien tardi-pricoce) ; (3) la Zone ? Cyrtograptus i perneri - Monograptus cf. munchi dans laquelle Cyrtograptus multiramis est un Clement particulikrement frkquent (Age du Wenlockien moyen) ; (4) la Zone 21 Cyrtograptus lundgreni - Monograptus testis, divisible en une sous-zone infkrieure Monograptus testis n.ssp. et une sous-zone supkrieure h Monograptus testis testis (Wenlockien tardif) ; et (5) la Zone 21 Pristiograptus ludensis (Wenlockien terminal). Les deuxitme et troisitme zones sont possiblement divisibles en sous-zones infkrieures et suptrieures, mais avec moins de certitude. Le degrk de subdivision zonale dans la Formation de Cape Phillips est moindre que celui applicable en Europe. I1 a CtC observe localement que certaines espkces-clCs montraient un chevauchement important et certaines divergences dans leurs distributions d'une coupe a l'autre. Du point de vue palCoCcologique, quelques esptces, particulikrernent Cyrtograptuspreclarus, Cyrtograptus cf. malgusaricus et Pristiograptus meneghini, semblent avoir prCfCrC les milieux de marge de bassin. [Traduit par la revue] Can. f . Earth Sci. 27, 1-13 (1990)

Introduction Wenlock graptolites were first reported from the Cape Phillips Formation by Thorsteinsson (1958). He defined the formation and listed the Wenlock graptolite faunas, particularly from the type section (Cape Phillips), in some detail, recognizing nine separate but informal zones, three of which were named for and based on the occurrence of new species of Cyrtograptus. However, Thorsteinsson's faunal lists are, with the exception of the uppermost Wenlock samples, based on uncompressed specimens extracted from calcite concretions that occur through most of the section on northern Cornwallis Island. The present report is based entirely on nonisolated specimens. With the exception of the papers by Thorsteinsson (1955), Lenz (1974a, 1974b), Lenz and Melchin (1987a, 1987b), and Melchin and Lenz (1986), which describe specific aspects of the uncompressed faunas, these exquisitely, three-dimensionally preserved graptolites remain to be described in detail. Since their initial report on Cornwallis Island, Wenlock grap-

tolites have been reported from Bathurst Island (Thorsteinsson and Glenister 1963; Kerr 1974), Melville Island (Tozer and Thorsteinsson 1964), and Ellesmere Island (Ken 1976; Trettin 1979). The most recent paper dealing specifically with Wenlock graptolites from the Cape Phillips was that of Lenz (1978), who systematically described the crytograptids, and some other Wenlock species, from both northwestern and Arctic Canada. The present report is based on more detailed collection from a total of 10 sections ranging geographically from Melville Island in the west to Ellesmere Island in the east (Fig. 1). Most of these were collected during the summers of 1983-1984. Ashgill and Llandovery graptolite faunas from most of these same sections have been described elsewhere by Melchin (1987, 1989). Stratigraphy The Cape Phillips Formation is divided into three informal members, A, B, and C (Thorsteinsson 1958; Melchin 1987,


2

CAN. J. EARTH SCI. VOL. 27, 1990

FIG. 1. Wenlock paleogeography (modified after Trettin and Balkwill 1979) and location of sampled sections. (I) Snowblind Creek (MSC, SBC), 75"111N,93'47'W; (2) Cape Phillips (MCP), 75"37'N, 94"301W;(3) Rookery Creek (MRC), 75"22'N, 95'46'W; (4) Cape Becher (CB), 76'17'N, 95'25'W; (5) Twilight Creek (MTC,MTC2), 76"101N,99"101W;(6)Middle Island (MMI),75"52'N, 111°54'W;(7) Huff Ridge (MHR), 78"34'N, 83'32'W; (8) Irene Bay (MIB), 79"04'N, 82'15'W. Section localities designated with two codes (e.g., MSC, SBC) are separate sections that are so closely spaced as to make them indistinguishable on the index map.

1989). Member A ranges in age from Ashgill through middle Llandovery, while member B, which is somewhat restricted in distribution, is entirely late Llandovery in age. These two members record a gradual change from open-shelf conditions in the basal Cape Phillips and underlying Irene Bay formations, through a gradual deepening and drowning, to more slope to basinal depositional conditions. The transition from shallow to deeper water sedimentation appears to have been very gradational both vertically and laterally. It is only in the upper Llandovery that any signifiant amounts of coarser grained resedimented carbonates, turbidites, and debris flows begin to appear in the basin-marginal sections, indicating development of some platform-margin relief (Melchin 1989). Member C, which ranges in age from late Llandovery (latest Llandovery where member B is present) through to Late Silurian to Early Devonian, is a monotonous sequence of argillaceous lime mudstones and calcareous shales. In the Ludlow strata, the siliciclastic content begins to increase and the lithology changes to predominantly calcareous shales, shales, and siltstones. In some areas, such as Rookery Creek and Irene

Bay, portions of the lower part of member C are partially or competely dolomitized. The Wenlock portion of the formation varies widely in thickness, from 215 m at Cape Phillips and 166 m at Cape Becher, to about 90 m at Rookery Creek, Twilight Creek, and Huff Ridge to a very condensed 27 m at Irene Bay and 25 m at Middle Island. At the sections close to the platform margin, such as Cape Phillips, Cape Becher, and Irene Bay, the lithology constitutes medium brownish grey, wavy- to microripple-laminated, argillaceous lime siltstones with thin, calcareous shales. Higher in the Wenlock, the wavy-laminated lime siltstones gradually give way to varying proportions of darker coloured, wavy- to planar-laminated, argillaceous lime mudstones. In the uppermost Wenlock, the lime mudstones change to calcareous shales. Calcite concretions occur in variable abundance throughout most of the Wenlock section, especially in the Cornwallis Island area. Two of these sections, Cape Phillips and Cape Becher, show relatively great thicknesses, of the order of 200 m, whereas Irene Bay, which shows similar lithologies, is a much more condensed section, about

LENZ AND MELCHIN

TABLE1. Comparision and correlation of the Cape Phillips Wenlock graptolite zonal scheme


UK-Poland (Rickards 1976; Teller 1969)

Arctic Canada (this paper)

Cornwallis Is. (Thomsteinsson 1958)

27 m. This variation in thickness between sections of approximately equal position with respect to the platform margin is most likely due either to differing rates of basinward sediment movement between the two regions or to the fact that Irene Bay may have been a margin where sediment bypass rather than deposition was predominant. In the more distal sections, such as Rookery Creek, Twilight Creek,and Huff Ridge, the lime siltstones are less common, and dark brownish grey, wavy- to planar-laminated, argillaceous lime mudstone and calcareous shales predominate. Section thicknesses are fairly consistent at about 90-100 m. In contrast with the underlying upper Llandovery strata, coarser grained resedimented carbonates are rare in the Wenluck, except for some thin turbidite beds at a few of the shelf-marginal sections. Higher, in the Ludlow strata, coarser, resedimented carbonates again become more common, including some allochthonous blocks up to more than 1 m in size. This may record a second episode of building of platformmarginal relief, as seen in the shelf-edge biostromes and shoals reported by Sodero and Hobson (1979) and de Freitas (1987). While the significant thicknesses of carbonate silts and muds indicate substantial offshore movement of carbonate material, the virtual absence of coarser, resedimented material indicates low relief and a period of quiescence at the platform margin. M m work on the actual platform-marginal sequences will provide a better explanation of their development.

Nevada (Berry and Murphy

S. Mackenzie Mtns. (Lenz 1980)

TABLE2. Stratigraphic distribution of Wenlock graptolites of Snowblind Creek (MSC) valley south, locality 1

.-,=, .- e

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Biostratigraphy

The following discussion of the zonal scheme, summarized and compared in Table 1, focusses on major faunal elements. The stratigraphicranges of all species present in each section are shown in Tables 2- 10, the composite biostratigraphic ranges of key species are shown in Fig. 2, and line drawings of key species, which are given Geological Survey of Canada (GSC) type numbers, are illustrated in Figs. 3-5. The zonal scheme is discussed in order of oldest to youngest.

Cyrtograptus centrifugus - Cyrtograptus insectus Zone The base of the zone is recognized by the first appearance of either of the zonal species; in addition, the zone is equally

marked by the occurrence of Cyrtograptus n. sp. aff. Cyrtograptus lundgreni (sensu Lenz 1978). Species of lesser importance and (or) greater stratigraphic range include Monograptus jlemingi, Monograptuspriodon, Monograptus aff. pexilis, and Monograptus aff. probosciformis, the last two species being rare. BouEek (1933) recognized a separate (lower) centrifugus Zone and a (higher) insectus Zone, separated by an intervening murchisoni Zone. In contrast, BouEek (1960) subsequently reversed the order. Other workers (Teller 1969; Bjerreskov 1975; Jaeger 1975; Rickards 1976) recognized only a single centrzfugus Zone or an insectus Zone, believing them to be


TABLE3. Stratigraphic distribution of Wenlock graptolites of Snowblind Creek (SBC) valley north, locality 1 5

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rigiduscentrifugus- riccartonensis insectus L U Cyrtograptus centrifugus Cyrtograptus insectus Cyrtograptus aff. lundgreni Monograptus flemingi Monograptus aff. jlexilis Cyrtograptus preclarus Cyrtograptus aff. rigidus Monograptus aff. riccartonensis Pristiograptus meneghini Cyrytograptus cf. malgusaricus Cyrtograptus perneri Cyrtograptus cf. mancki Cyrtograptus cf. ramosus Cyrtograptus? cf. gracilis Monograptus cf. munchi Monograptus ex gr. firmus Spinograptus nevadensis Cyrtograptus multiramis Cyrtograptus hamatus Cyrtograptus lundgreni Cyrtograptus radians Monograptus testis n.ssp. Monograptus testis testis Gothograptus nassa Pristiograptus ludensis Pristiograptus aff. deubeli Pristiograptus sherrardae Agastograptus clathrospinosus

pernerimunchi L U

lundgrenitestis L U

ludensis

-

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FIG.2. Composite range chart of common and biostratigraphically significant graptolite species; L, lower; U, upper.

LEN2 AND MELCHIN

TABLE4. Stratigraphic distribution of Wenlock graptolites of Cape Phillips (MCP), locality 2 h


'$

9 L

Level (m) 375 355 348 344 335 334 323 314 309.5 305.5 30 1 300 297 292 289 262 256 255 254.5 253.5 253 25 2 25 1 250.5 250 249 247.5-248.5 247.5 223.5 218 215.5 213.5 212 197.5 195.5 193 190-200 189.5 189 188 185.5 183 182.5 181.5 181 180.5 180 178.5 177 175 173.5 172 171.5 171 167.5 165 163 162 160

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. . . . x . x x

x x

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TABLE5. Stratigraphic distribution of Wenlock graptolites of Rookery Creek (MRC), locality 3


,3

h

3

Level Im) 29 1 285.5 284 283 280.5 274.5 273 272.5 272 260-278 271.5 27 1 263 259-260 255-265 251.5 255 25 1 240.5 221-222 217.5 208.5 206.5-207 200

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. . . . .

. . . . . . .

. . . . . . . . . . . . . . . X . . . . . . . . . . . . . . . . . . . . X . X . . . . . . . . . .X . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .X . .

. . . . . . .

. . . . . .

X

. . . . .

. . . . .

. . . . .

. . . . .

. . . . . . . .x . . . . . . . . . . . . . . . X . . . . . . . . . x . . . . . . . . . . . . . x . . . . . . x . . . . . .x . . . . .

. . . . .

. .

. . . . .

. . . . .

X

. . . . .

. X . . . .

X

. . . . .

. .

. .

X X X X . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . X . X X . x . . . X

.

X X

. . . . . . . .

X . . . . . . . . . .

. . . . . . . . . . .

. . . . . . . X . . . . . . . . . . . . . . . X . . . . . . . . . . . . . . . . . . . . . . . .X X X X X X . . . . . . . . . . . . . . . . . . .x . . .x x . . . . . . . . . . . . . . . . . . . . . . . . . . .

X . . .

. . . .

. . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . .

X . X . X X . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

X X X X . . X

inseparable; all recognized the presence of the overlying murchisoni Zone, a zone not recognized in this study. The present study clearly indicates that the stratigrapic relationship of C . cenhifugus and C . insectus is haphazard, and only a single combined zone can be recognized. Moreover, Cyrtograptus murchisoni has not to date been recognized in the Arctic Islands or in the North American Cordillera. It is possible that the large, multibranched, relatively common C . insectus might be the ecological equivalent of C . murchisoni.

Cyrtograptus aff. rigidus - Monograptus aff. riccartonensis

Zone The base of the zone is most easily recognized by the first appearance of what is here identified as Monograptus aff. riccartonensis. The other index species, Cyrtograptus aff. rigidus, is relatively uncommon, as is Cyrtograptus preclarus, which appears to be restricted to the zone. Long-ranging species include Pristiograptus dubius, Paraplectograptus eiseli, Paraplectograptus praemacilentus, Monoclimacis jlumendosae,

FIG. 3. (A-C) Cyrtograptus insectus BouEek: (A) locality 2 (MCP), 181 m, GSC 95390, x0.5; (B) locality 5 (MTC2), 63 m, GSC 95391, x2; (C) locality 4 (CB), 39 m, GSC 95392, x 2. (D, E, R, S) Monograptus sp. aff. M. riccartonensis Lapworth: (D, E , R) proximal, mesial, and distal portions of the rhabdosome, locality 2 (MCP), 172 m, GSC 95393, GSC 95394, and GSC 95395, ~ 4 . 4 (S) ; long specimen, locality 3 (MRC), 221-222 m, GSC 95396, x4.4. (F, G) Cyrtograptus sp. aff. C . lundgreni Tullberg (sensu Lenz, 1978): (F) locality 2 (MCP), 163 m, GSC 95397, X2.8; (G) locality 1 (MSC), 36 m, GSC 95398, x2.8. (H) Cyrtograptus centrijiugus BouEek: locality 3 (MRC), 208.5 m, GSC 95399, x2.8. (I, M) Pristiograptus meneghini Gortani: locality 2 (MCP), 177 m, GSC 95400 and GSC 95401, X 5. (J-L) Monograptus sp. aff. M.jZexilis Elles (s. 1.): (J, K) locality 5 (MTCl), 64 m, GSC 95402 and GSC 95403, X 2.3; (L) locality 2 (MCP), 163 m, GSC 95404, X2.3. (N) Cyrtograptus sp. cf. C . malgusaricus (Golikov): locality 2 (MCP), 185.5 m, GSC 95405, X 1.3. (0)Cyrtograptuspreclarus Lenz: locality 2 (MCP), 180.5 m, GSC 95406, x4.4. (P, Q) Cyrtograptus sp. aff. C . rigidus Tullberg (sensu Lenz, 1978): (P) locality 6 (MTCl), 64 m, GSC 95407, X4.4; (Q) locality 5 (MTC2), 91 m, GSC 95408, x4.4.


LEN2 AND MELCHIN



8

FIG. 4. (A-C) Monograptus sp. ex gr. M.firmm BouCek: locality 5 (MTCl), 124 m, GSC 95409, GSC 95410, and GSC 94511, X7. (D-F) Monograptus sp. cf. M . munchi Manck: locality 8 (MIB), 85 m, GSC 95412, GSC 95413, and GSC 95414, X7. (G, K) Cyrtograptuspernerl BouEek: (G) locality 1 (MSC), 2 m below base, GSC 95414, x5; (K) locality 8 (MIB), 98 m, GSC 95416, X5. (H-J) Cyrtograptus mudtiramis Tornquist (a series with few to many curved cladia): (H) locality 5 (MTC2), 128 m, GSC 95417, x4.4; (I) locality 1 (MSC), 2 m below base, GSC 95418, X4.4; (J) locality 1 (MSC), 5 m, GSC 95419, X4.4. (L) Cyrtograptus n.sp. cf. C . mancki BouEek (note hooked cladial thecae): locality 2 (MCP), 249 m, GSC 95420, x5.5.

9

LEN2 AND MELCHIN


TABLE6. Stratigraphic distribution of Wenlock graptolites of Cape Becher (CB), locality 4

Level

. . . . . . .

x

. . . . . . . . . . . . .

. . . . . . . . . . . . . . . . x . . . x

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .x . . . . . . . . . . . . . . . . . . . . . x x x . . . . . . . . . . . . . . . X X X . . x

. .

. . . .

X

. . x . . . . . . . . . . .x . . x . . . . . . . X . . . . . . X

x .

. .

x . . . x . . . . . . . . . . . . X . . X . . X . . . . . . . . . .

. . . . . . .

. . .

. x . .

. . . . . . . . . . . . . . . . . . . x x . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . X . . . . . . .

. . . X . . . . X . . . . . . . .

. . . . . . . . . . . . . . . . .x . . . . X X X . . . . X . . . . . . . . . . . . . . . . . . . . . x . . . . . . . X

. . . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

x . . . . . . . . . . . . .

. . . . x . x . . . . . . . . . . . . . .

. . . . .

x . . . . . . . . . . . . . . . . x . x x . . . . . . . . . . . . . . . . . . x . . . . . . . . . . . . . . . . . . X X

. . . . . . . . . . . . . . . . . . .

and Monograptus antennularius. Two other species, Pristiograptus meneghini and Cyrtograptus cf. malgusaricus, appear generally to be found only in the lower and upper parts of the zone, respectively. Two subzones based on these species are tentatively recognized. Whether the two tentative subzones are roughly coeval with the (lower) M . riccartonensis and (upper) C . rigidus zones of BouEek (1933), Teller (1969), or Rickards (1976) (see Table 1) is not clear. The stratigraphically long range and the abundance of M . aff. riccartonensis throughout the zone and the sporadic occurrences of C . aff. rigidus, however, clearly preclude the recognition of the equivalent of the separate riccartonensis and rigidus zones in the Canadian Arctic. Cyrtograptus perneri - Monograptus cf. munchi Zone The base of the zone is marked variously by the appearance of either or both of the two index species and (or) Monograptus ex gr. jirmus (a species somewhat similar to Monograptus cf. munchi). Furthermore, Cyrtograptus multiramis is essentially restricted to the zone, although characteristically that species is much more common and abundant in the approximately upper

TABLE7. Stratigraphic distribution of Wenlock graptolites of Twilight Creek cliff section (MTC), locality 5

155.5 154 150 134.5 128 124 120 102 90 68 64

. . . . . .

X . . . . . . . . . . . . . . .X . . . . . x x . . . . . . x x . . . . . . x . . . . . . . x . . .

. . . . . . . . . . . . . . . . . . . X

. . . . .

X . X X X .

. . . . . . .x . . . . . . . .x . x . . . . . . . x . . x . . . . . .

x . x x x . . . . . . . .x . . . . . . . . . . . . . . x x . x x x x . . . . . . . . . . . . X . x x x . . . . . . . . . . . . . . . . . ? . . . . . . . . . . . . . . . . . X X X X

. . . . . . . . . . . . . . . . .

half of the zone. As a consequence, the zone is tentatively divided into lower and upper subzones, the latter being recognized by the acme of C . multiramis. Other species present in the zone include Cyrtograptus cf. mancki, rare Cyrtograptus cf. ellesi and Cyrtograptus cf. ramosus, the upper ranges of M. aff. jlexilis and M . aff. riccartonensis, and the lowest appearances of Cyrtograptus? cf. gracilis and Spinograptus nevadensis. The zone is readily correlatable with the combined flexilisellesi zones of Teller (1969) and Rickards (1976) (see Table 1). The present data, however, do not warrant the recognition of these as separate zones in the Arctic Islands. Cyrtograptus lundgreni - Monograptus testis Zone This zone is the most widespread and widely recognized of those of the Wenlock, in part because of the greater thickness of its strata. The zone is very widely recognized in the North American Cordillera (Beny and Murphy 1975; Lenz 1978, 1988). The zone is most commonly marked by the incoming of Cyrtograptus lundgreni but is also characterized by the presence of Cyrtograptus hamatus n.ssp. and Cyrtograptus radians, as well as by Monograptus testis (sensu lato). Of particular note is the occurrence of a generally earlier Monograptus testis n.ssp. and a later typical Monograptus testis testis. The former, in comparison with the typical morph, is characteristically less hooked and only partially spinose, and its thecae are simpler and more readily discernible in flattened specimens. The subspecies, and therefore possibly the subzone, may be present in Taimyr, USSR (see Obut et al. 1965, P1. 6, fig. 7). The combined lower M . testis n. ssp. and upper M. testis testis subzones appear to be approximately coeval with the lundgreni Zone of Rickards (1976), the radians and lundgreni zones of BouEek (1960) and Teller (1969), and the radians and testis zones of PaSkeviEius (1979).

CAN, J . EARTH SCI. VOL. 27, 1990


TABLE8. Stratigraphic distribution of Wenlock graptolites of Twilight Creek (MTC2), locality 5

Level (m) -

155 144 140 132 128 123 118 115.5 110 104 101.5 98 91 69.5 68 66 64.5 63

--

. . . . . . . . . . . . . . . . . . . . . . X X X X X X X X . . . . . . . X . . . . . . X . . . . x . . . . . . . . . . . . .X . . . . . . . . .X . X . . X . . X . . . . . . . . . . . . . . . X . . . x . X x . . . . . X . . . . . . . . .

. . . . X . . . . . . . . .

. . . . . . .

X . . . . . . . X . . X X . . . . . . . . . .

. . . . . . . . . . . .

x . . . . . . x x x x x x . . . . . . . . .

. . . . . . . . . . . .

. . . .X

. .

. . . .

x . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X . . x x . . . . . . . . . . . . . . . . . . . . x . x . . . . . . . . . . . . . . . . . . .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

. . . . . . . x . . . . . . . . . X . . . . . x X

. . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .x . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x . . . . . . . . . . . . . . . . . . . . . . . .

. . . .

X . . . X x x x

X X X X

x x

. . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . .

Pristiograptus ludensis Zone

The international recognition of a post-lundgreni-testis, latest Wenlock interval received impetus from the study of Holland et al. (1969), who described Pristiograptus ludensis and associated species from the upper part of the classic Wenlock sequence. The zone is locally represented by a fauna markedly different from that of the underlying lundgreni-testis Zone; particularly, it is marked by the absence of Cyrtograptus and the dominance of Pristiograptus over Monograptus (sensu stricto). Characteristic species include Gothograptus nassa (rare), Gothograptus eisenacki (Lenz and Melchin 1987a), and various pristiograptids such as Pristiograptus ludensis, P . cf. ludensis, Pristiograptus aff. deubeli (this species shows only very slight similarity to true P . deubeli of Jaeger (1959), but H. Jaeger (personal communication, 1988) has proposed the name Pristiograptus praedeubeli for a species extremely similar to this),

and Pristiograptus sherrardae, along with Agastograptus? clathrospinosus. Pristiograptus sherrardae, found at only one locality (see Table 8), was earlier referred to Pristiograptus etheringtoni by Lenz (1980). This same species is also recognized in uppermost Wenlock strata in the Tian Shan region, USSR (Koren' and Rinenberg 1988). The interval has been widely recognized in North America, being known from the Gasp6 region of Quebec (Lenz 1975), southern Northwest Territories (Lenz 1980), and Nevada (Berry and Murphy 1975). The zone is readily correlated with the combined nassa and ludensis zones of Rickards (1976) and PaSkeviZius (1979). Typically, M . testis (s .1.) disappears before the appearance of this fauna; the occurrence of M . testis testis in the fauna of this zone at one locality (Table 8) may, therefore, be anomalous. In this instance, however, the sampling interval may have spanned the boundary between the two zones.

FIG. 5. (A-C) Monograptyus testis n.ssp.: (A, B) locality 1 (MSC), 115 m, GSC 95421 and GSC 95422, X 5; (C) locality 3 (MRC), 284 m, GSC 95423, X5. (D-G) Monograptus testis testis (Barrande): (D, F ) locality 4 (CB), 199 m, GSC 95424 and GSC 95425, X5; (E, G) locality 1 (MSC), 152 m, GSC 95426 and GSC 95427, x 5. (H) Cyrtograptus hamatus n.ssp.: locality 1 (MSC), 115 m, GSC 95428, x 5. (I) Cyrtograptus hamatus Baily: locality 6 (MMI), 92.5 m, GSC 95429, x 5. (J, K) Cyrtograptusradians Tiirnquist: (J) locality 6 (MMI), 95 m, GSC 95430, x4.4; (K) locality 3 (MRC), 273 m, GSC 95431, x8. (L, M) Cyrtograptus lundgreni Tullberg: (L)locality 2 (MCP), 309.5 m, GSC 95432, x4.4; (M) locality 1 (SBC), 138 m, GSC 95433, x4.4. (N-P)Pristiograptus aff. deubeli (Jaeger): locality 5 (MTC2), 155 m, GSC 95434, GSC 95435, and GSC 95436, X 8. (Q, R) Pristiograptus sherrardae (Sherwin): locality 1 (SBC), 150 m, GSC 95437 and GSC 95438, X5.5 and x 8. (S, T) Pristiograptus ludensis (Murchison) sensu Wood: locality 5 (MTC2), 155 m, GSC 95439 and GSC 95440, x5.


LENZ AND MELCHIN


12

Discussion of the fauna


TABLE9. Stratigraphic distribution of Wenlock graptolites of Middle Island (MMI), locality 6

(1) Biogeographically, the Canadian Arctic Wenlock faunas are generally pandemic and are typical of basin and basinmargin settings elsewhere. Although the data are too few at present, the occurrences of species such as C . preclarus, C . cf. malgusaricus, and P. meneghini in basin-margin sections only (Tables 2 , 4 , 6 , and 10) but not in the remaining basinal sections suggest they may have had a restricted and (or) shallower water distribution. This suggestion is somewhat supported by the occurrence of C . preclarus in a relatively shallow-water, small-basin environment in the southern Mackenzie Mountains (Lenz 1988) but its absence from open-basin Wenlock faunas to the southwest (see Lenz 1980). In contrast with this, species such as M. ex gr. firmus, P. ludensis, C.? cf. gracilis, and C . cf. ramosus are restricted to the more distal basinal sections. (2) Generally speaking, Cyrtograptus is fairly common in the early and late Wenlock of Europe but is absent from the riccartonensis Zone (see BouCek 1933; Teller 1969; Rickards 1976; PaSkeviEius 1979). Too little information is available on graptolites of this interval in the non-Baltic part of the USSR, i d Wenlock deposits are, in general, noticeably sparse in China (Mu et al. 1986). By contrast, cyrtograptids, like the retiolitids (see Lenz and Melchin 1987a, 1987b), are distributed rather uniformly and continuously through the Arctic Wenlock strata. This is in part accounted for by the long range of C . aff.

Level (m)

. . . .

. . . .

. . . .

. . . . x . X . . . . x . . . . . . . x

. x x . . . . . .

. . . X . . . . . X . . . . . . . . . . . . . . . . .

. . . . . . X X X X

. . . . . x x x x . . x x x . . . .

X X X X X

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

TABLE10. Stratigraphic distribution of Wenlock graptolites of Irene Bay (MIB), locality 8

2

9

Level (m) -

101 98 97

96 94.5 93 92.5 91 88.5 86.5 85 84.5 83 80.5 79.5 77 75 73.5

-

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

X

. . . . . . .

X . .

. . . . . . . . . . . . . . . . . . . . .x . . . . .

. . . . .

. .

X

X .

x . .

. . . . . . . . . . . . . . . . . . x . . . . . . . . . . . . . . . . . . . . . X . . . . . x

. . . . . . . . . . . . . . .x

x . x

. . . . . .

. . . . . . . . . . . . . . . . . . . .

. . . . . . . . . .

X

. . . . . . . . . . . .

x . . . . . . . . . . . . . X x . . . . . . . . x . . . . . . . . . . . . . x . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . X

. .

x

. . . . . .

X . X X x X . .

. . . . . . . . . . . . . . . . . . . .

X . . . . . . . . . . . . . . .

. . . . . . .x x . . . . . . . . . . . . . . . . . .X . . .. . . . . . . . . . . . . . . . . . . . . . .x x x . . . . . . . . . . . . . . . . . . . . . x . . . . . . . . . . . . . . . . . . . . . .

x x . . . . . . . . . . . . . . . . . . . . . . .

LENZ AND MELCHIN


rigidus and C. multiramis and the occurrences of C. preclarus, C. cf. malgusaricus, and C. cf. mancki. (3) In contrast with old-world Wenlock graptolite sequences, those of the Arctic Islands are divisible into fewer biostratigraphic units. This is attributable to ( i ) the long range of some of the local "index" species (e.g., M. aff. riccartonensis, M . aff. jlexilis, and C. insectus); (ii) the rare occurrences of some species (e.g., C. cf. ellesi, C. preclarus, and Monograptus minimus); and (iii) the presence of new species o r tentatively identified species (e.g., C. aff. rigidus, C. cf. malgusaricus, C. cf. mancki, and M. aff. riccartonensis). Paradoxically, the synthesis of data from all sections collected to date results in the recognition of fewer biozones than potentially recognizable in a single richly fossiliferous section, particularly one such as the type Cape Phillips Formation (Table 4). The implication of this is important: namely, some of the biostratigraphically important species may have greater biostratigraphic ranges than are commonly appreciated.

Acknowledgments Financial support for this project was provided by a Natural Sciences and Engineering Research Council of Canada operating grant to A. Lenz and by The University of Western Ontario Northern Research Group to M. Melchin; logistical support to both in the Arctic Islands was provided by the Polar Continental Shelf Project. Graptolites from Cape Becher were collected by B. D. E. Chatterton. Field assistance was provided at various times by B. D. E. Chatterton, D. G. Perry, J. M. Hill, E. C. Prosh, C. C. Ryley, and S . W. White. T o all of these individuals we express our thanks. BERRY, W. B. N., and MURPHY, M. A. 1975. Silurian and Devonian graptolites of central Nevada. University of California, Publications in Geological Sciences, Vol. 110, pp. 1-109. BJERRESKOV, M. '1975. Llandoverian and Wenlockian graptolites from Bornholm. Fossils and Strata, 8: 1-93. BOUEEK,B. 1933. Monographie des obersilurischen Graptoliten aus der Familie Cyrtograptidae. Prace geologicko-paleontologickkho dstavu Karlovy university v Praze, E. 1 , pp. 7-84. 1960. Einige Bemerkungen zur Entwicklung der Graptolithenfaunen in Mitteldeutschland und Bohmen. Geologie, 9: 556-564. DE FREITAS, T. A. 1987. The Silurian carbonate-platformmargin and contiguous sponge biostromes of east-central Cornwallis ~sland, Canadian Arctic. M.Sc. thesis, The University of Western Ontario, London. HOLLAND, C. H., RICKARDS, R. B., and WARREN, P. T. 1969. The Wenlock graptolites of the Ludlow District, Shropshire, and their stratigraphical significance. Palaeontology, 12: 663-683. JAEGER, H. 1959. Graptolithen und Stratigraphic der jungsten thuringer Silurs. Abhandlungen der Deutschen Akademie der Wissenschaften zu Berlin, Klasse fiir Chemie, Geologie und Biologie, 2: 1-197. Die Graptolithenfiihrung im SilurtDevon des Cellon-Profils (Karnische Alpen). Carinthia 11, 165: 111-126. KERR,J. W. 1974. Geology of Bathurst Island Group and Byam Martin Island, Arctic Canada (Operation Bathurst Island). Geological Survey of Canada, Memoir 378. 1976. Stratigraphy of central and eastern Ellesmere Island, Arctic Canada. III. Upper Ordovician (Richrnondian), Silurian and Devonian. Geological Survey of Canada, Bulletin 260. KOREN',T. N. and RINENBERG, R. E. 1988. Sobvite lundgreni v pelagicheskikh fatsiyakh pozdnego venloka yuzhnogo Tyan-Shan. In Graptolity v Istorii Zernli. Edited by J. PaGkeviEius. Ministerstvo Narodnogo Obrazovaniya, pp. 11- 14. LENZ,A. C. 1974a. Evolution in Monograptuspriodon. Lethaia, 7: 265-272.

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