Ludlow and Pridoli (Upper Silurian) graptolite biostratigraphy of the central Arctic Islands: a preliminary report

ALFRED C. LENZ Department of Geology, The University of Western Ontario, London, Ont., Canada N6A 5B7

Received December 18, 1989

Revision accepted April 18, 1990

A fauna, comprising slightly more than 30 species of Upper Silurian graptolites, has been recognized from the Arctic Islands. The following preliminary biostratigraphic biozones are recognizable: Lobograptus progenitor, Saetograptus fritschi linearis, and Bohernograptus bohemicus tenuis zones of Ludlow age; and the Pseudomonoclimacis parultinus, Pseudomonoclimacis ultimus, Monograptus bouceki, "Pristiogratpus" transgrediens, and Monograptus cf. birchensis zones of Pridoli age. The last-named zone may range into the Devonian. The faunal composition and zones are relatively close to coeval faunas of northern Yukon. They are, however, distinctly depauperate in comparison with the faunas of Poland, Czechoslovakia, or Kazakhstan.

Une faune, incluant un peu plus de 30 espbces de graptolites du Silurien supCrieur, a kt6 reconnue dans les iles de 1'Arctique. Les biozones biostratigraphiques prkliminaires qui suivent ont CtC identifiCes : les zones d'bge du Ludlowien i Lobograptus progenitor, Saetograptus fritschi linearis et Bohemograptus bohemicus tenuis; et les zones d'bge du Pridoli i Pseudomonoclim- acis parultimus, Pseudomonoclimacis ultimus, Monograptus bouceki, << Pristiograptus D transgrediens et Monograptus cf. birchensis. La derniere zone nommCe peut s'Ctendre dans le DCvonien. La composition faunique et les zones sont relativement proches de d'autres faunes contemporaines du nord du Yukon. Elles sont, cependant, moins peuplCes comparativement aux faunes de Pologne, de TchCcoslovaquie ou du Kazakhstan

[Traduit par la revue]

Can. J. Earth Sci. 27,1074-1083 (1990)

Introduction Studies of Upper Silurian graptolites of the Arctic Islands are

few. Graptolites of this age were first reported by Thorsteinsson (1958), who recognized (from oldest to youngest) the Mono- graptus nilssoni, Monograptus bohemicus, Monograptus sp. N and sp. 0, Monograptus tumescens minor, Monograptus ultimus, and Monograptus sp. P and sp. T zones. As was typical for the time, all zones, including those now assignable to the Pridoli, were correlated with Ludlow strata. Evaluation of these zones is difficult, partly because the new zonal species of Thorsteinsson were never illustrated and partly because the morphological definition of the important early Pridoli species "M." ultimus and related forms was not well known; it is only since the important study of Jaeger (1986) that the diagnostic features of the ultimus group have been clarified.

Two other works may be noted: four species of Ludlow graptolites were illustrated in Jackson et al. (1978), and a single Ludlow retiolitid was illustrated in Lenz and Melchin (1987).

Regional reports noting the occurrences or listing the names of Upper Silurian graptolite species include Kerr (1974) and Thorsteinsson and Glenister (1963) on Bathurst Island, Trettin (1979) on Ellesmere Island, and Robson (1985) on western Melville Island.

This present preliminary study, based solely on flattened, carbonized graptolites, derives from three well-sampled sec- tions from Cape Sir John Franklin (Devon Island), Twilight Creek (Bathurst Island), and Irene Bay region (Ellesmere Island) and a cursory sampling from Cape Phillips and Snow- blind Creek (Cornwallis Island). Furthermore, a few scattered samples were collected from each of Abbott River (Cornwallis Island) and Middle Island (northeastern Melville Island) (see Fig. 1). The study sets up a preliminary biostratigraphic

FIG. 1. Index map of graptolite localities. 1-5 are sections sampled stratigraphically: 1, Snowblind Creek (75"111N, 93'47'W; 2, Cape Phillips (type section) (75"37'N, 94"301W); 3, Twilight Creek (76"10fN, 99"101W); 4, Cape Sir John Franklin (76"42.5'N, 96'53'W); 5, Irene Bay area (79"04'N, 82"14'W). A and B are "spot" sample localities: A, Abbott River (75"14'N, 95'36'W); B, Middle Island (Melville Island) (75"52'N, 111°54'W). The broken line is the approximate boundary between platformal carbonates (to east) and basinal shales and carbonates of the Cape Phillips Formation (to west).

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LENZ

TABLE 1. Stratigraphic distribution of graptolites of Twilight Creek (loc. 3)

Level (m)

zonation, compares the faunas and zones with coeval faunas of northern Yukon (Jackson and Lenz 1972; Jackson et al. 1978; Lenz 1988) and, to a lesser extent, of Europe and central USSR, and, finally, illustrates most of the species by line drawings.

Stratigraphy The Cape Phillips Formation is a basinal, graptolite-bearing,

stratigraphic unit occurring throughout much of the central regions of the Arctic Islands. At its type section on northeastern Cornwallis Island, Thorsteinsson (1958) recognized three infor-

mal members, A, B, and C. Only member C, which is of late (or latest) Llandovery to Late Silurian or Early Devonian age, is of relevance to this paper. On Cornwallis Island and, in geneal, western Devon Island and Irene Bay, that member comprises relatively soft, argillaceous lime mudstones and calcareous shales. Elsewhere, as at Twilight Creek, equivalent and younger strata become progressively more silty and (or) dolomitic and are moderately cliff forming. Resedimented carbonates, rang- ing from granules to multimetre-sized blocks, often occur through these sequences. In Pridoli rocks of Irene Bay, for

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CAN. 1. EARTH SCI. VOL. 27, 1990

TABLE 2. Stratigraphic distribution of graptolites of Cape Sir John Franklin (loc. 4)

Y +: .4 .- .y

Level (m)

.I.. . I . . . .

example, allochthonous carbonate blocks make up as much as 30% of the volume.

Upper Silurian rocks are about 400 m thick at Twilight Creek, more than 500 m at Cape Sir John Franklin, and in excess of 1000 m at Cape Phillips but only about 200 m at Irene Bay.

the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature 1983). Within the zones, recognition may be based on the association of several or more species, i.e., an assemblage. The stratigraphic range of each species of each section, arranged in order of first appearance, is shown in Tables 1-5. Line drawings illustrating all important species, which are given Geological Survey of Canada (GSC) numbers, are shown in Figs. 3 and 4.

Biostratigraphy Eight graptolite biozones are recognized in this study, the first

three of Ludlow age, the remainder Pridoli. Generally speaking, the base of each zone is defined by the first appearance of the name species and as such may be considered an interval zone of

Lobograptus progenitor Zone The L. progenitor zone, which directly overlies the latest

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TABLE 3. Stratigraphic distribution of graptolites of Snowblind Creek (loc. 1)

L1 3 .il U L1 - E

a 2 c? .-. E =; .s =

i; 2 z .r, k .r=, e 3 3 5 .z ; a .- <$& 4 .s .? g .y. u I-. - 3 Q, 4 - L $ &$ - 2 $ 8 ;.s::. .: c% b < - ? S @ ' * - - - g $

= . - E Z & L C $ 2 - 2 $ & % 2:E- 2 q $2 5:s g 7% 2 n $ z c F; Q Y 4 2 r l Q : rr:

- NOTE: Numbers in left column are in strati-

graphic order, oldest at the bottom, while num- bers in parentheses are field designations.

TABLE 4. Stratigraphic distribution of graptolites of Cape Phillips (loc. 2)

ij

.2 <

Level (m)

Wenlock Zone of Pristiograptus ludensis (Lenz and Melchin 1990), is defined by the first appearance of the name species (Figs. 3E, 3F), or Bohemograptus bohemicus bohemicus (Figs. 3A, 3B), the latter being very long ranging. Saetograptus roemeri (Figs. 35, 32) also ranges through the entire zone but not higher. Species occurring in lesser numbers, or ranging into higher zones, include Pseudomonoclimacis richardsonensis

TABLE 5. Stratigraphic distribution of graptolites of Irene Bay area (loc. 5)

Level (m)

(Figs. 3G-3I), Pseudomonoclimacis tetlitensis (Figs. 3K-3M), Pseudomonoclimacis bispinosus, Plectograptus macilentus, and Balticograptus cf. erraticus, the latter two being rare. Furthermore, retiolitids such as Spinograptus, Holoretiolites, and Agastograptus have been recovered from nodules in this zone. Surprisingly, species common in coeval strata elsewhere, such as Monograptus uncinatus, Saetograptus colonus, Saeto- graptus varians, and Pristiograptus turnescens, are absent or are not certainly recognized. Significantly also, Neodiversog- raptus nilssoni is totally absent from the study collections of flattened material, although that species is reported from nodules in Thorsteinsson (1958).

Thorsteinsson's (1958) M. nilssoni Zone from the region and the N. nilssoni Zone of Jackson et al. (1978) and L. progenitor Zone of Lenz (1988) from northern Yukon are more or less precise equivalents of the progenitor Zone of this study.

The zone is clearly longer ranging than most early Ludlow zones of Europe (see Fig. 2). It equates with the nilssoni and probably the scanicus zones of the British Isles (Warren 1971; Rickards 1976), the nilssoni, progrenitor, and parascanicus zones of Poland (Teller 1969), the nilssoni, progenitor, and scanicus zones of Czechoslovakia (Pfifbyl 1983), and the nilssoni and progenitor zones of the Baltic region (PashkeviEius 1979).

Saetograptus fritschi linearis Zone The base of the zone is defined primarily on the first appeance

of the name giver (Figs. 3s-3U). Equally characteristic, but less common, are the first appearances of Pseudomonoclimacis dalejensis (Figs. 3V-3Y, 3AA), which ranges into the overly- ing zone, and the morphologically very distinctive Monograp- tus ceratus (Figs. 3N-3P). "Pristiograptus" sp. A (Figs. 4C-

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1078 CAN. J . EARTH SCI. VOL. 27, 1990

I NORTHERN BRITISH CZECHOSLOVAKIA POLAND KAZAKHSTAN, ARCTIC ISLES USSR

SERl ES ISLANDS

Rlckards 1976; Holland ( piibyl 1983; ( Teller 1969) and Palmer 1974) Jaeger 1986) o or en 1983,1986)

b i r c h e n s i s t r a n s g r r d ions microdon - t r o n s g r e d i e n s t r o n s p r e d ~ e n s perbrev is aksa jens i r

J t r a n s g r r d i e n s praeci puus odmirob i l i r NO p r r n e r l p e r n e r i p r r n e r i

0 bouceki bouceki b o u c r k i boucek i boucrki -

I GR APTOLITES lochkov iens is band01 etovi - p r i d o l i e n s i s aamsonowiczi lochkovi ensis-

c h e l r n i e n s i a c h r l m i r n s i s b r a t u r u l t l m u a u l t i m u s u l t i r n u s b u g r n s ~ u s p r i d o l i e n s i s -

a u l t i m u a formosus-

porul t i m u r f o r m o r u r p a r u l t i mu8 f o r m o r u s bessobaensis

f r a g m e n t a l i s fecundus k o z l o w s k i i k o z l o w r k i i i n r i g n l t u r i n a x p e c t a t u r i n e x p e c t a t u s

bohrmicur bohemicus bohemicus a u r l c u l a t u s bohemicur 0 t r n u i s t r n u i r t e n u i s " bohemic,ys cornutus t rnu is -

badr butovicensis A bohemicus

Q longur

l r i n t w a r d i n e n s i s l e i n t w o r d i n e n s i s f r i t s c h i f r i t s c h i l i n r a r i r h e m i a v r r s u s

l i n r a r i s l i n e o r i s i n v e r t us J t u m r s c r n s

s c a n l c u r s c a n i c u s p a r o r c a n i c u s p r o g e n i t o r p r o g e n i t o r

p r o g r n l t o r p r o g e n i t o r n i l r r o n l n i l s s o n i n i l s s o n i

FIG. 2. Comparison and correlation of Cape Phillips Ludlow and F'ridoli graptolite zonal schemes.

4E) appears to be confined to the zone, and P. richardsonensis and P. tetlitensis continue through the zone.

The zone, as presently used, is identical with the zone of the same name of northern Yukon (Lenz 1988). On the other hand, that zone, as used earlier by Jackson et al . (1978) in northern Yukon, is now recognized to be much larger in scope and to have encompassed the linearis and the tenuis zones of this study. Based on the common occurrence of the zonal species and P. dalejensis, the zone appears closely correlative with the linearis Zone of Czechoslovakia (PTibyl 1983). In a similar vein, the Arctic zone is probably coeval with the combined invertus to leintwardinensis zones of Poland (Teller 1969), although the absence of cucullograptids in the Arctic sequence prevents detailed correlation.

Bohemograptus bohemicus tenuis Zone This low-diversity zone represents the highest Ludlow unit

and is defined solely by the first appearance of B. bohemicus tenuis (Figs. 4A, 4B). In the study region, as well as in northern Yukon, that subspecies consistently appears stratigraphically higher than B. bohemicus bohemicus (Jackson and Lenz 1972;

Jackson et al. 1978). The subspecies is also recognized as younger than the typical morph in the British Isles (Holland and Palmer 1974) and in Poland (Teller 1969; Urbanek 1970). This zonal period would appear to represent the flowering of bohemograptids: in addition to the common subspecies noted above, there are Bohemograptuspraecornutus in the study area, "Bohemograptus" helicoides and Bohemograptus cf. cornutus in northern Yukon (Jackson and Lenz 1972), and Bohemogratus cornutus and B. praecornutus in Poland (Teller 1969; Urbanek 1970).

Species extending into, and ending in, the zone comprise "Pristiograptus" sp. A, P . dalejensis, P. tetlitensis, S . fritschi linearis, and Linograptus posthumus.

Pseudomonoclimacis parultimus Zone This zone, of earliest Pridoli age, is defined by the appearance

of the name species (Figs. 4J-4L). The appearance of this and related and superseding species constitutes a striking change in morphology, one that is characterized by the more or less temporally progressive development of distinct proximal thecal lappets (see Jaeger 1986). The zone contains a few other, rarer

FIG. 3. (A, B) Bohemograptus bohemicus bohernicus (Barrande). Middle Island, 122.5 m; Twilight Creek, 160 m. GSC 98240 and 98241. (C, D) Lobograptus sp. Twilight Creek, I56 and 162 m. GSC 98242 and 98243. (E, F) Lobograptusprogenitor Urbanek. Twilight Creek, 160 m, and Middle Island, 122.5 m. GSC 98244 and 98245. (G-I) Pseudomonoclimacis richardsonensis Lenz. G, I , Twilight Creek, 160 m; H, Snowblind Creek, collection 2. GSC 98246,98247, and 98248. (J, Z) Saetograptus roemeri (Wood). Twilight Creek, 166 m; Cape Sir John Franklin, 170 m. GSC 98249 and 98250. (K-M) Pseudomonoclimacis tetlitensis Lenz. Twilight Creek, 176 m. GSC 9825 1,98252, and 98253. (N-P) Monograptus ceratus Lenz. Cape Sir John Franklin, 221.5 m; Abbott River, collection 5B; Cape Sir John Franklin, 205 m. GSC 98254,98255, and98256. (Q, R) Bohemograptus praecornutus Urbanek. Twilight Creek, 180 m. GSC 98257 and 98258. (S-U) Suetogratpus fritschi linearis (BouEek). S, Snowblind Creek, collection 6; T, U, Twilight Creek, 176 m. GSC 98259, 98260, and 98261. (V-Y, AA) Pseudomonclimacis dalejensis (BouEek). V, Y, Snowblind Creek, collection 3; W, X, AA, Cape Sir John Franklin, 191 m. GSC 98262,98263,98264,98265, and 98266. All figures approx. x 5.0, except P, which is X 5.5.

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1080 CAN. J. EARTH SCI. VOL. 27, 1990

TABLE 6. Comparison of the occurrences, by zone, of important species in northern Yukon and the Arctic Islands using the zonal scheme of the Arctic Islands

3 .- h .O -

Species

birchensis transgrediens bouceki ultimus parultimus tenuis linearis progenitor

NOTE: 1 , Arctic Islands occurrences; 2, northern Yukon occurrences; 3, occurrences in both Arctic Islands and northern Yukon.

species, particularly Monograptus formosus (Fig. 41) and Mono- the overlying broadly defined zone of "Pristiograptus" trans- graptus cf. beatus (Figs. 4T, 4U). grediens but is given separate status because the bouceki Zone is

The zone corresponds, more or less precisely, to the M . for- widely recognized in Europe (see Fig. 2) and is known from mosus Zone of Jackson et al. (1978) and Lenz (1988) of northern Yukon (Lenz 1988). The zone is one of the very low northern Yukon where, in addition, P . richardsonensis is diversity and appears to mark the highest (local) occurrence of present. L. posthumus.

Pseudomonoclimacis ultimus Zone Characteristic of the zone is the first appearance of the zonal

species (Figs. 4V-4Z), as well as the occurrence of Monograp- tus branikensis (Figs. 4F-4H). In some instances, however, the separation of P . parultimus and P . ultimus is difficult, particu- larly in stratigraphic intervals where there appear to be forms transitional between the two species. The zone is particularly well devloped at the Irene Bay section, where it ranges through at least 70 m of strata.

The zone clearly corresponds to the Pristiograptus bugensius zone (= P . ultimus: fide Jaeger 1986), as well as the overlying Pristiograptus chelmiensis Zone of Jackson et al . (1978) and Lenz (1988). Yukon equivalents of the zone contain, in addition, " Pristiograptus" chelmiensis and " Pristiograptus" separabilis (see Lenz 1988), which, according to Jaeger (1986), are both probable synonyms of P . ultimus.

"Pristiograptus" transgrediens Zone This zone is, in essence, defined by the post-bouceki

occurrence of the name species (Figs. 4P-4R) and as such is the least precisely defined and most long-ranging zone. It, further- more, is a "gap-filling" zone, rather more in the nature of the "interzone of Monograptus transgrediens" of Czechoslovakia (Jaeger 1986). The zone contains only one other species, "Pristiograptus" cf. transgrediens, which may, in fact, be conspecific. The Arctic zone probably correlates with what Jackson et al. (1978) and Lenz (1988) called the Pristiograptus transgrediens praecipuus Zone in northern Yukon.

Monograptus cf. birchensis Zone The zone is recognized solely on the occurrence of the zonal

species (Figs. 4M-40), which occurs in abundance at Twilight Creek and, especially, at Cape Sir John Franklin. Elsewhere, it may correspond, in part at least, to the occurrence of Mono-

Monograptus bouceki Zone graptus uniformis &rangustidens in northern Yukon (Lenz The presence of this zone is questionable in that the zonal 1988).

species (Figs. 4M-40) is recognized only in Twilight Creek, Monograptus cf. birchensis ranges through a considerable and even there it is rare. The occurrence might be lumped with thickness of strata, particularly at Cape Sir John Franklin, where

FIG. 4. (A, B) Bohemograptus bohemicus tenuis (BouEek). Snowblind Creek, collections 4 and 7. GSC 98267 and 98278. (C-E) "Pristiograptus" sp. A. C, D, Snowblind Creek, collection 6; E, Abbott River, collection 5B. GSC 98269, 98270, and 98271. (F-H) "Monograptus" branikensis Jaeger. Twilight Creek, 240 m. GSC 98272,98273, and 98274. (I) Monograptus formosus BouEek. Irene Bay, 6 m. GSC 98275. (J-L) Pseudomonoclimacis parultimus (Jaeger). Twilight Creek, 230, 230, and 195 m. GSC 98276, 98277, and 982728. (M-0) Monograptus cf. birchensis Berry and Murphy. M, N, Cape Sir John Franklin, 599 m and collection "2"; 0, Twilight Creek, 530 rn. GSC 98279, 98280, and 98281. (P-R) "Pristiograptus" transgrediens (Perner). P, R, Cape Phillips, 1351 rn; Q , Twilight Creek, 270 m. GSC 98282,98283, and 98284. (S) Monograptus bouceki Pribyl. Twilight Creek, 290 m. GSC 98285. (T, U) Monograptus cf. beatus Koren. Twilight Creek, 195 m. GSC 98286 and 98287. (V-Z) Pseudomonoclimacis ultimus (Pemer). V , Twilight Creek, 220 m; W-Z, Irene Bay, 70,58,75,75, and 58 m. GSC 98288, 98289, 98290, 98291, and 98292. All figures approx. X 5.0, except C and Y, which are X9.5.

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1082 CAN. J. EARTH SCI. VOL. 27, 1990

it is found intermittently through at least 200 m of strata. The NOTE ADDED IN PROOF: T. N. Koren in a personal communication absence of associated species, however, makes it impossible to received after the paper was in galley proof points out that the perneri determine whether it is restricted to the uppermost Pridoli or Zone of Kazakhstan shown in Fig. 2 should be the perneri kazakh-

ranges from pridoli to ~~l~ ~ ~ ~ ~ ~ i ~ ~ , as does the ~~~~d~ stanensis Zone. She further suggests that the species parultimus and

occurrence of M~~~~~~~~~~ birchensis ( B ~ ~ and Murphy ultimus should not be assigned to the genus Pseudomonoclimacis

1975). Mikhailova. The writer disagrees with that suggestion.

Comparison and correlation of the zones Ludlow and Priodoli graptolites of the Arctic Islands are more

readily compared with those of northern Yukon (Lenz and Jackson 1971; Jackson and Lenz 1972; Jackson et al . 1978; Lenz 1988) than of any other region. A comparison of the most common or important species of the two regions is shown in Table 6. The table, however, shows a number of important differences; for example, species such as S. colonus, Saeto- graptus compactus compactus, S. varians, Pristiograptus tumes- cens, and Monograptus cf. balticus do not co-occur in lower Ludlow strata of the two regions, although Thorsteinsson (1958) listed Pristiograptus tumescens minor from the Cape Phillips Formation. Furthermore, although B. bohemicus bohemicus and B . bohemicus tenuis are abundant and widespread in both re- gions, as they are elsewhere in the world, species such as B. cf. cornutus, B . praecornutus, and ~ o h e m o ~ r a ~ t u s heli- coides do not co-occur in upper Ludlow strata.

In Pridoli strata, " Monograptus" norjordi has, to date, been found only in northern Yukon (Lenz and Jackson 1971), whereas "M." branikensis and Monograptus cf. pridoliensis occur in the Arctic Islands. The apparent absence of P. parul- timus from nothern Yukon may, on the other hand, be a matter of nonrecognition to date. Prior to the critical study of Jaeger (1986), the morphology of that species was poorly known, as was its probable descendant P . ultimus. Monograptus cf. birchensis, although abundant in the study area, has not, as yet, been recovered from northern Yukon.

Comparison with the biostratigraphic zones of Europe and the USSR shows that in a very broad sense the same general faunal sequence prevails (see Fig. 2). In detail, however, and keeping in mind the preliminary nature of the present study, the differences are considerable. A comparison with the Ludlow faunas of the British Isles (Rickards 1976); Ludlow and Pridoli faunas of Poland (Teller 1964, 1969; Urbanek 1970) and Czechoslovakia (Piniyl 1983); and the Pridoli faunas of the USSR (Koren 1983, 1986) and Czechoslovakia (Jaeger 1986) shows the Arctic Islands zonal subdivisions and overall faunal diversity to be depauperate. Particularly striking are the absence of Cucullograptus and Neocucullograptus, the rarity of the mor- phologically unusual and more uncommon species of Bohemo- graptus in the Ludlow of the Arctic Islands, and the rarity or absence of Pridolian species such as Monograptus pridoliensis, Monograptus lochkoviensis, and Monograptus perneri.

Acknowledgments At various times individuals provided valuable field assis-

tance and companionship, donated small collections, or provided information on the localities of graptolites. They are B. D. E. Chatterton, T. de Freitas, J. M. Hill, J. Jin, M. J. Melchin, D. G. Peny, E. C. Prosh, C. C. Ryley, J. A. Shaw, and S. W. White. To them I extend my thanks. Financial support was through a Natural Sciences and Engineering Research Council of Canada operating grant, and logistical and field support has been continuously provided by the Polar Continental Shelf Project.

BERRY, W. B. N., and MURPHY, M. A. 1975. Silurian and Devonian graptolites of central Nevada. University of California Publications in Geological Sciences, No. 110.

HOLLAND, C. H., and PALMER, D. C. 1974. Bohemograptus, the youngest graptoloid known from the British Silurian sequence. In Graptolite studies in honour of 0. M. B. Bulman. Edited by R. B. Richards, D. E. Jackson and C. P. Hughes. Special Papers in Palaeontology, No. 13, pp. 215-236.

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