The paradox of the global standard Late Ordovician–Early Silurian sea level curve: Evidence from conodont community analysis from both Canadian Arctic and Appalachian margins

Palaeogeography, Palaeoclimatology, Palaeoecology 236 (2006) 246 – 271 www.elsevier.com/locate/palaeo

The paradox of the global standard Late Ordovician–Early Silurian sea level curve: Evidence from conodont community analysis from both Canadian Arctic and Appalachian margins Shunxin Zhang a,*, Christopher R. Barnes a, David M.S. Jowett b a

School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada V8W 3P6 b Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada K1S 5B6

Received 7 July 2004; received in revised form 13 September 2005; accepted 4 November 2005

Abstract Relative sea level fluctuations in the Late Ordovician–Early Silurian were largely driven by Gondwana deglaciation, three possible short-lived ice readvances, and the closure of the Iapetus Ocean. This study establishes and compares sea level events on the Canadian Arctic and Appalachian margins of Laurentia based on the pattern of conodont communities, and demonstrates that sea level events were not synchronous on the two margins. Conodont data were compared for a) the Arctic margin from the Cape Phillips Formation (Richmondian, Upper Ordovician through lower Sheinwoodian, Lower Silurian), Cornwallis Island, Canadian Arctic Islands, and b) the Appalachian margin from the upper Ellis Bay, Becscie, Merrimack, Gun River, Jupiter and Chicotte formations (Richmondian, Upper Ordovician, Rhuddanian, Aeronian, and most of Telychian, Lower Silurian), Anticosti Island, Quebec. The pattern of Late Ordovician–Early Silurian conodont communities is established based on three cluster analyses on conodonts from Cornwallis Island (4967 specimens representing 54 species from 77 samples, Cape Phillips Formation) and Anticosti Island (1980 specimens representing 21 species from 25 samples, upper Ellis Bay Formation; 24,839 specimens representing 42 species from 123 samples, Becscie, Merrimack, Gun River, Jupiter and Chicotte formations). Overall, the sea level curves inferred from the distribution of the conodont communities from Cornwallis and Anticosti exhibit gradual and rapid oscillating patterns, respectively. Specifically, during the time interval of the latest Ordovician and the Early Silurian (Rhuddanian), the sea level behaved differently in these two regions: 1) during the latest Ordovician, conodont community changes show the different patterns in the two regions, which reflects a transgression on Cornwallis, but a regression on Anticosti; 2) during the earliest Rhuddanian, similar conodont communities arose in both regions, but they indicate a regression on Cornwallis and a transgression on Anticosti; 3) in the early Rhuddanian, these communities were replaced by a deeper water community on Cornwallis and a shallower water community on Anticosti, which suggest a transgression on Cornwallis and a regression on Anticosti; 4) a highstand drove out almost all conodonts from Anticosti by the end of Rhuddanian, which is not seen on Cornwallis; 5) a deep-water conodont community remained almost unchanged on Cornwallis from the late Rhuddanian to the early Aeronian, whereas a shallow water conodont community returned to Anticosti in the early Aeronian. However, the sea level dropped on both Cornwallis and Anticosti during the late Telychian, and the sea level curves show a similar pattern. The difference in the sea level pattern during the late Richmondian, Rhuddanian and Aeronian on both the northern and southern margins of Laurentia reflected by conodont communities is supported by the studies on Quaternary glaciation/deglaciation events, where the

* Corresponding author. Tel.: +1 250 472 5378; fax: +1 250 472 5370. E-mail addresses: [email protected] (S. Zhang), [email protected] (C.R. Barnes), [email protected] (D.M.S. Jowett). 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.11.002

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melting of an ice sheet was accompanied by sea level change but with variable regional isostatic effects that do not generate a uniform global eustatic signal. D 2005 Elsevier B.V. All rights reserved. Keywords: Late Ordovician–Early Silurian; Sea level changes; Arctic margin; Appalachian margin; Conodonts; Cluster analysis

1. Introduction Since the early studies of seismic stratigraphy (Vail, 1975; Vail et al., 1977) and sequence stratigraphy (Posamentier et al., 1988; Posamentier and Vail, 1988), much effort has attempted to establish a global eustatic model through geological time. This assumed that eustatic control produced synchronous sequence boundaries worldwide. However, the global eustatic model has been questioned and opposed by several authors whose key papers are reviewed by Miall and Miall (2001, table 3). The Early Silurian spanned about 10 million years (approximately 435–425 Ma), and experienced the effects of melting of a massive terminal Ordovician ice cap in the southern hemisphere that covered much of western Gondwana. It has been estimated that the ice covered between 6–8  106 km2 (Hambrey, 1985) and 11.8  106 km2 (Crowley and Baum, 1991). This was probably as extensive as the Laurentide ice sheet during the Pleistocene glacial maximum (11.6  106 km2; Paterson, 1972) and the present area of the East Antarctica ice sheet (10.2  106 km2; Williams and Ferrigno, 1993). Three brief glacial readvances appear to have occurred in the Early Silurian based on stratigraphic evidence on Gondwana (e.g. Caputo, 1998) and oxygen isotope data (Azmy et al., 1998). For the last two decades, a vigorous attempt has been made to define an Early Silurian global eustatic sea level curve. Most authors have assumed that if the North American changes in sea level were related to the growth and decay of the Gondwanan ice sheet, then similar evidence should be found in the Early Silurian sequences on the other continents (e.g. Johnson, 1984; Johnson et al., 1991). The global sea level curves have been constructed by Johnson (1996), Johnson et al. (1998), and Johnson and McKerrow (1991), which emphasize five major highstands in sea level during the Llandovery and early Wenlock. These events were reported to be relatively comparable among Laurentia, Avalonia, Baltica, Bohemia, Cathaysia and Gondwana based largely on the analysis of correlated changes in benthic assemblages from different paleocontinents. Relying on Laurentian

sequences, Ross and Ross (1996) produced a sea level curve that is significantly different from those noted above. Based on facies changes on a number of paleocontinents, Loydell (1998) established a new sea level curve that differs markedly from those previously published, in terms of both the number and timing of fluctuations. Studies on the Early Silurian glaciations in Brazil (Grahn and Caputo, 1992; Caputo, 1998) generated a sea level curve contrary to that of Johnson and McKerrow (1991). The different curves were explained as being caused by both the paucity of graptolites from most sections studied (hence, reduced biostratigraphic control) and the admitted inadequacy of existing attempts to correlate between graptolite biozones and the biostratigraphic divisions based on evolving brachiopod lineages (Loydell, 1998). This present study analyzes the Late Ordovician and Early Silurian conodont community changes from both the Arctic and Appalachian margins of Laurentia, and demonstrates that sea level events were not synchronous on the two margins, especially during the time interval from the latest Ordovician to early Aeronian. It is proposed that the non-synchronicity was caused by the melting and readvances of the ice sheet and variable regional isostatic rebound that produced complex sea level changes that departed significantly from a uniform eustatic distribution, which is comparable to those associated with Quaternary glaciation and deglaciation (Farrell and Clark, 1976; Clark et al., 2002). 2. Geological background During the Late Ordovician and Early Silurian, the northern and southern margins of Laurentia were located at a paleolatitude of about 5–108N and 15–208S, respectively, referred to herein as the Arctic and Appalachian margins (Fig. 1). Cornwallis Island, Canadian Arctic Islands and Anticosti Island, Quebec were located on the Arctic and Appalachian margins during the Late Ordovician and Early Silurian, respectively. Both areas preserve some of the most exceptional stratigraphic records in the world for this time interval in terms of

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Fig. 1. Schematic paleogeography of Canada during the Silurian showing the relative positions of Anticosti and Cornwallis islands (modified from Norford, 1997).

completeness limited structural/thermal alteration and exceptional fossil record. The Upper Ordovician and Lower Silurian strata on Cornwallis Island were deposited in the Cape Phillips Basin, where the shelf-to-basin transitional facies occurred along the shelf region of the Fanklinian Orogen within a passive-to-convergent cratonic margin during Late Ordovician and Early Silurian (de Freitas et al., 1999) (Fig. 2). The Cape Phillips Formation extends from the Richmondian, Upper Ordovician through Pridoli, Upper Silurian or Lower Devonian. Graptolite-rich calcareous shale, argillaceous limestone and calcareous siltstone are the dominant lithofacies within this interval in which most of the conodonts are slender pectiniform, ramiform and coniform elements

(Jowett, 2000; Jowett and Barnes, 2000, submitted for publication). In contrast, Upper Ordovician and Lower Silurian strata on Anticosti Island were deposited in a shallow, storm-influenced, open-marine sublittoral environment (Petryk, 1981; Sami and Desrochers, 1992). The Anticosti Basin was relatively unaffected by postTaconic orogenic clastic influx that affected the New York–Ontario region to the southwest, so that it maintained a virtually complete and stable record of carbonate sedimentation. The Ellis Bay, Becscie, Merrimack, Gun River, Jupiter and Chicotte formations cover the interval of upper Richmondian (Upper Ordovician), Rhuddanian, Aeronian, and lower Telychian (Lower Silurian) (Fig. 3). Most of the Anticosti conodonts are characterized by robust ramiform and

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Fig. 2. Generalized paleogeographic map of the Llandovery, Early Silurian for northern Canada and Greenland (modified from de Freitas et al., 1999). Paleolatitudes adopted from http://www.scotese.com/mlordcli.htm.

pectiniform elements (McCracken and Barnes, 1981; Zhang and Barnes, 2002a). 3. Methodology and database The principal methodology employed in this study uses the changing pattern of conodont communities to interpret sea level changes. The conodont community changes recognized from a particular stratigraphic section reflect local sea level changes; comparison of the different local sea level changes establishes whether sea level behaved synchronously or variably in different localities. Cluster analysis is employed in this study to identify conodont communities as used recently by Zhang and Barnes (2002b,c, 2004b,c) and Zhang et al. (2005) who provided details of the data processing, the program running the cluster analysis, the cluster method, and the coefficient for similarity measurement. Three databases are involved in this present study: 1) Part of the authors’ previous study dealing with Late Ordovician–Early Silurian (Ashgillian–Llandovery)

sea level history on Anticosti Island, Quebec (Zhang and Barnes, 2002b). It includes 24,839 specimens representing 42 species from 123 conodont-bearing samples from approximately 540 m of strata divided into the Becscie, Merrimack, Gun River, Jupiter and Chicotte formations, Lower Silurian (Fig. 3). Depending on this database, one cluster analysis was performed, 11 conodont communities were recognized, and the Early Silurian sea level was inferred (Figs. 4 and 5) (Zhang and Barnes, 2002b, figs. 7–11). 2) Part of the database established by McCracken and Barnes (1981) contains 1980 specimens representing 21 species from 25 samples about 50 m of strata of upper Ellis Bay Formation crossing the boundary between Upper Ordovician and Lower Silurian, Anticosti Island (Fig. 3). Based on this database, one cluster analysis was performed, 4 conodont communities were recognized, and the sea level of Late Ordovician–Early Silurian was inferred (Figs. 6 and 7). 3) Another is part of the database produced by Jowett and Barnes (submitted for publication) that provides

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Fig. 3. Geological map of Anticosti Island (after Jin and Copper, 1999).

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Fig. 4. Q- and R-mode cluster analysis of 108 conodont-bearing samples and 42 species from Becscie, Merrimack, Gun River, Jupiter and Chicotte formations, Anticosti. Samples in Q-mode clustering order, taxa in R-mode clustering order and relative abundance of taxa as a graded series of dots. Intersections of Q- and R-clusters define conodont communities: P.u., Panderodus unicostatus; I.–O.–A., Icriodella inconstans–Ozarkodina gulletensis–Aulacognathus bullatus; Oz.s.–R.n., Ozarkodina strena–Rexroadus nathani; A.–P.–P.–C.–O., Apsidognathus tuberculatus–Pterospathodus celloni–Pterospathodus pennatus procerus–Carniodus carnulus–Ozarkodina polinclinata; Oz.a., Ozarkodina aldridgei; Oz.o.–R.k., Ozarkodina oldhamensis–Rexroadus kentuckyensis; P. sp., Panderodus sp.; I.d.–Ou.j.–Oz.p., Icriodella deflecta–Oulodus jeannae–Ozarkodina pirata; P.r., Panderodus recurvatus; O.–P.–P.–D., Oulodus? cf. Ou.? fluegeli–Pterospathodus siluricus–P. posteritenuis–Decoriconus fragilis; Oz.p., Oz. pirata (modified from Zhang and Barnes, 2002b).

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Fig. 5. Inferred paleodepth and sea level changes based on the distribution of the eleven conodont communities (see Fig. 4 for abbreviations) identified by cluster analysis through Becscie, Merrimack, Gun River, Jupiter and Chicotte formations, Anticosti (M=Merrimack Fm.; Ch=Chicotte Fm.) (modified from Zhang and Barnes, 2002b).

a detailed study on the conodont taxonomy and biostratigraphy from the Cape Phillips Formation. It consists of 4967 specimens representing 54 species from 77 conodont-bearing samples from three sections (Cape Phillips South, Cape Manning, and Cape Phillips North) of the Cape Phillips Formation on Cornwallis Island (Fig. 2). The lower sampled part of the Cape Phillips Formation is approximately 750 m thick, which comprises the upper Cincinnatian (upper Richmondian and Gamachian), Upper Ordovician, the Llandovery (Rhuddanian, Aeronian and Telychian) and the lower Wenlock (Sheinwoodian and lower Homerian), Lower Silurian. This present study only covers the interval from upper Richmondian through Sheinwoodian. One analysis is performed for the database of the Cape Phillips Formation on Cornwallis in this study. Twelve conodont communities are recognized, and they are well differentiated from each other from the interval of upper Richmondian to Sheinwoodian (Fig. 8).

The twelve conodont communities are named as, in ascending order of their first appearance, Amorphognathus ordovicicus (A.o.), Decoriconus fragilis (D.f.), Rexroadus kentuckyensis–Ozarkodina hassi– D. fragilis (R.k.–Oz.h.–D.f.), Walliserodus curvatus–Aspelundia petila I (W.c.–A.p. I), Dapsilodus obliquicostatus (D.o.), A. petila (A.p.), W. curvatus–A. petila II (W.c.–A.p. II), Aspelundia fluegeli– Aspelundia borenorensis–Walliserodus sancticlairi– Pterospathodus celloni (A.f.–A.b.–W.s.–P.c.), W. sancticlairi (W.s.) A. fluegeli–A. borenorensis–W. sancticlairi–Pterospathodus pennatus procerus (A.f.– A.b.–W.s.–P.p.p.), P. pennatus procerus (P.p.p.) and D. obliquicostatus–N. Gen. n. sp. (D.o.–N. Gen. n. sp.) communities (see Appendix for details about cluster samples, defining taxa, biostratigraphic range, lithofacies, diversity and abundance for each community; in order to make the community name clear, some unconfirmed species names in the original database, such as Aspelundia cf. A. borenoren-

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Fig. 6. Q- and R-mode cluster analysis of 25 conodont-bearing samples and 21 species from the upper Ellis Bay Formation crossing the boundary between Upper Ordovician and Lower Silurian, Anticosti. Samples in Q-mode clustering order, taxa in R-mode clustering order and relative abundance of taxa as a graded series of dots. Intersections of Q- and R-clusters define conodont communities: Ph., Phragmodus undatus; G.e.–G.h.–P.p., Gamachignathus ensifer–G. hastatus–Panderodus panderi; G.e., G. ensifer; Oz.o.–W.c., Ozarkodina oldhamensis–Walliserodus curvatus.

sis, are used as a formal species name herein). The Late Ordovician and Early Silurian sea level curve for the Arctic margin is deduced by the distribution of these communities along the sections of Cape Phillips Formation (Fig. 9).

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A.b.–W.s.–P.c. and A.f.–A.b.–W.s.–P.p.p. communities, and it is related to a higher species diversity range of 1– 17 with an average of 6.8. However, group II yields communities almost without Panderodus species and is dominated by slender coniform and ramiform conodonts, such as the A.p., W.c.–A.p., D.o.–N. Gen. n. sp. and D.o. communities (Fig. 10). This group has a lower species diversity range of 1–9 with an average of 3.3. The Panderodus species help to define a boundary between groups I and II, in which the fundamental changes among the Cornwallis conodont communities are reflected by the alternation of Panderodus-rich and Panderodus-poor communities. Panderodus is a common genus in high-energy, near-shore environments (Le Fe`vre et al., 1976; Aldridge, 1976, Aldridge and Jeppsson, 1984). P. unicostatus usually comprises over 50% of the fauna, locally exceeding 90%, from the Becscie, Gun River, Merrimack, Jupiter, and Chicotte formations on Anticosti Island (Zhang and Barnes, 2002a), and it dominates in near-shore environments (Aldridge and Mabillard, 1981). Conodont communities that only contain P. unicostatus as a single species appear to have tolerated both deep and shallow water environments represented in the platform sequence that other conodonts avoided, but it was probably limited in water depths of 30–120 m (Zhang and Barnes, 2002b). Three conodont communities are recognized from the Cape Phillips Formation being P. unicostatus-rich: the A.o. community is recognized from the upper Richmondian, and the A.f.–A.b.–W.s.–P.c. and A.f.–A.b.–W.s.–P.p.p. communities from the upper Telychian. These communities probably are good indicators of shallowing events on Cornwallis where otherwise the deep-water environment predominated during the Early Silurian. 5. Interpretation of sea level changes on Cornwallis Island, Canadian Arctic margin

4. The role of Panderodus species 5.1. Lowstand in late Richmondian, Late Ordovician Panderodus Ethington is a common and long-ranging genus from Middle Ordovician to Middle Devonian (Clark et al., 1981), and because of its limited biostratigraphic importance it has received limited attention. Overall, the conodonts from Cape Phillips Formation are divided into two major groups at the similarity level of 0.25 (Fig. 8). Group I contains abundant Panderodus unicostatus (Branson and Mehl) and P. sp., which make up about 30–60% of the fauna in most of the samples (Fig. 10). This group involves communities with robust ramiform and pectiniform conodonts, such as the A.f.–

The A. ordovicicus (A.o.) community is within the Panderodus-rich group, and is characterized by the nominate species. Amorphognathus species show a much higher abundance in inner sublittoral than outer sublittoral environments, and are typically restricted to the shallow water environment in the North Atlantic Realm in the Late Ordovician (Barnes and Fa˚hraeus (1975, figs. 2, 3). A. ordovicicus is the second most abundant among all ramiform and pectiniform species in the lower Vaure´al Formation, Richmondian, on

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Fig. 7. Inferred paleodepth and sea level changes based on the distribution of the four conodont communities (see Fig. 6 for abbreviations) identified by cluster analysis through the upper Ellis Bay Formation, Anticosti. The numbers on the left represent the members of Ellis Bay Formation.

Anticosti Island, where the paleobathymetry ranged from 20 to 70 m, being deepest in the lower Vaure´al (Nowlan and Barnes, 1981; Copper and Long, 1998). Considering that this community occurs immediately below a disconformity in the Upper Ordovician (Jowett and Barnes, submitted for publication) and is associated with rich P. unicostatus and occurs in dolostones the community is inferred to have developed during the lowstand and cooling period associated with the Late Ordovician glaciation noted by Melchin et al. (1991). This lowstand probably persisted from fastigatus Zone through the lowest persculptus Zone (Fig. 9). 5.2. A transgression in the latest Richmondian, the latest Ordovician D. fragilis (Branson and Mehl) is common in more offshore areas of the shelf (Aldridge and Jeppsson, 1984), and it is also the characteristic species in the Dapsilodus Association that commonly occurs in the deepest conodont-bearing environment (Aldridge and Jeppsson, 1999). The D. fragilis community recognized in the Cornwallis basin with very low diversity and abundance (see Appendix) represents a survival cono-

dont fauna during the extinction at the end of the Ordovician. Considering the lithofacies change from dolostone with a possible paleokarst surface hosting the A. ordovicicus community in upper fastigatus Zone–pacificus Zone, to the black laminated calcareous mudstone interbedded with black fissile calcareous shales hosting the D. fragilis community in persculptus Zone, the changes in both conodont communities and lithofacies fit perfectly the model proposed by Brenchley et al. (2001) and Brenchley (2004), in which the survival fauna in the persculptus Zone was related to a global warming, change in carbon cycling, sea level rise and widespread anoxia after the Late Ordovician glaciation. Therefore, the D.f. community indicates a transgression in the latest Ordovician. 5.3. Regressive–transgressive cycles in the Rhuddanian, Early Silurian Three communities, R. kentuckyensis–O. hassi–D. fragilis (R.k.–Oz.h.–D.f.), W. curvatus–A. petila I (W.c.–A.p. I) and D. obliquicostatus (D.o.), exhibit a consecutive replacement of each other from lower acuminatus Zone through acinaces Zone, Rhuddanian.

S. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 236 (2006) 246–271 Fig. 8. Q- and R-mode cluster analysis of 77 conodont-bearing samples and 54 species from the Cape Phillips Formation, Cornwallis. Samples in Q-mode clustering order, taxa in R-mode clustering order and relative abundance of taxa as a graded series of dots. Intersections of Q- and R-clusters define conodont communities: D.f., Decoriconus fragilis; R.k.–Oz.h.–D.f., Rexroadus kentuckyensis–Ozarkodina hassi–D. fragilis; A.p., Aspelundia petila; W.c.–A.p. II, Walliserodus curvatus–A. petila II; W.c.–A.p. I, W. curvatus–A. petila I; D.o.–N. Gen. n. sp., Dapsilodus obliquicostatus–N. Gen. n. sp.; D.o., D. obliquicostatus; P.p.p., Pterospathodus pennatus procerus; A.o., Amorphognathus ordovicicus; W.s., Walliserodus sancticlairi; A.f.–A.b.–W.s.–P.p.p., Aspelundia fluegeli–Aspelundia borenorensis–W. sancticlairi–P. pennatus procerus; A.f.–A.b.–W.s.–P.c., A. fluegeli–A. borenorensis–W. sancticlairi–P. celloni. 255

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Fig. 9. Inferred sea level curve based on the distribution of the twelve conodont communities (see Fig. 8 for abbreviations) identified by cluster analysis through the Cape Phillips Formation, Cornwallis Island. i: insectus Zone; c: centrifugus Zone; m: murchisoni Zone. Legend: 1, mudstone; 2, platy-shaly lime mudstone; 3, dolostone; 4, shaly/laminated dolostone; 5, interbedded shale and lime mudstone; 6, interbedded calcareous shale and lime mudstone; 7, interbedded dolomitic mudstone and shale; 8, lime mudstone and shale; 9, vuggy, nodular, bituminous dolostone; 10, dolomitic shale; 11, concretion. Numbers with gray color represent conodont barren samples.

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Fig. 10. The relative abundance (percentage of the total fauna) of Panderodus species among different communities in the Cape Phillips Formation. Sample numbers are shown on the left axis; the order of the samples is same as in Fig. 8.

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The R. kentuckyensis–O. hassi–D. fragilis (R.k.– Oz.h.–D.f.) community first occurs in the lower accuminatus Zone, lowest Silurian, and maintains a stable occurrence until the middle atavus Zone. The three nominate species represent different depositional conditions revealed by previous studies. Although D. fragilis (Branson and Mehl) is widely accepted as a quieter, more offshore to slope specialist (Aldridge and Jeppsson, 1984), R. kentuckyensis (Branson and Branson) is considered to be restricted to the highenergy channel deposits in the shallow water; O. hassi (Pollok, Rexroad and Nicoll) has a wide geographic distribution, but tends to be restricted to the low-energy depositional conditions in near-shore environments (Zhang and Barnes, 2002c). The combination of the three species, with the black shale becoming progressively more calcareous in acuminatus Zone than in persculptus Zone (Melchin et al., 1991), may indicate environment changes that improved the anoxic condition and the shallowed water depth compared to that represented by the D.f. community in the latest Ordovician. The W. curvatus–A. petila I (W.c.–A.p. I) community is only recognized from one sample (cm2b-1) in the middle of the atavus Zone, and is different from the W.c.–A.p. II community in that it contains the nominate species of the R.k.–Oz.h.–D.f. community, and has a high diversity (Fig. 8). However, the appearance of W. curvatus (Branson and Branson) and A. petila (Nicoll and Rexroad) reveals a significant change among the conodont communities, and may indicate a deepening phase, as a) W. curvatus is much more common in distal offshore environments than near-shore and favoured by the deeper water brachiopod Clorinda community of Ziegler et al. (1968) (Idris, 1984, figs. 4, 5), and it is most common at or below storm-wave base (Zhang and Barnes, 2002b,c); b) the delicate Aspelundia species are dominant in the Lower Silurian slope facies of North Greenland (Armstrong, 1990), and are the main component of Aspelundia–Dapsilodus fauna that was suggested as favouring an open oceanic environment in the Lower Silurian of Canadian Cordillera, northern Yukon Territory (McCracken, 1991); and c) the sample is at the transition from carbonate to black shale. Thus, both the change of the conodont fauna from the R.k.–Oz.h.–D.f. to the W.c.–A.p. I community and lithofacies from carbonate to black shale suggests a transgression in the middle atavus Zone. The D. obliquicostatus (D.o.) community first occurs in the upper acinaces Zone, and is presented in two dolostone samples (cm2c-2, cm2c-3). It is totally different from the W.c.–A.p. I community in that a) all

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the ramiform and pectiniform conodonts and P. unicostatus disappeared; b) its diversity is much lower compared to the W.c.–A.p. I community (see Appendix); and c) it nearly contains the nominate species itself (Fig. 8). This profound change in community composition, diversity and abundance could be explained by either deepening or shallowing that greatly changed the temperature, nutrients, and possibly salinity. Most earlier reports of D. obliquicostatus (Branson and Mehl) have commonly been related to a deep-water facies, such as offshore (Aldridge and Jeppsson, 1984; Barrick, 1983, 1997), shelf margin (Aldridge and Mabillard, 1981), slope (Armstrong, 1990), slope/basin (Jowett and Barnes, submitted for publication), oceanic (McCracken, 1991), and the deepest conodont-bearing environment (Aldridge and Jeppsson, 1999), with few cases from the platform (e.g. Zhang and Barnes, 2002a,b). Thus, this community change was most likely caused by a deepening event, and the appearance of D.o. community represents a brief transgression during the late acinaces Zone. Jowett and Barnes (submitted for publication) noted that the Rexroadus/Ozarkodinabearing community disappeared earlier in Cornwallis than in Anticosti, which may be attributed to this (local?) transgressive event in the Rhuddanian. The graptolite diversity increasing from 3 in upper atavus Zone to 6 in lower acinaces Zone, and then to 16 in upper acinaces Zone (Melchin, 1989) (Fig. 11) appears

to circumstantially support conodont community changes in this transgression. However, the dolostone lithofacies hosting the D.o. community does not seem to favour this interpretation. de Freitas et al. (1999) recognized an end-Rhuddanian minor sequence boundary and related it to the end-Rhuddanian transgression. The conodont community changes probably responded earlier to the transgressive event than to the lithofacies change. 5.4. Gradual regression from late Rhuddanian through end-Telychian Overall, the general pattern of conodont community changes from late Rhuddanian through end-Telychian can be summarized as: a) the conodont communities remained relatively stable and community replacement was much less frequent than in most of the Rhuddanian (Fig. 9); and b) both species diversity and species abundance increased through time within this interval. This overall pattern was probably created by the following shallowing-upward pulses. 5.4.1. The A. petila community in the late cyphus and entire curtus zones, late Rhuddanian–early Aeronian The D. obliquicostatus community disappeared by the end of acinaces Zone, mid–late Rhuddanian; the A. petila (A.p.) community first appeared in the middle

Fig. 11. Graptolite species diversity through Cape Phillips Formation, Canadian Arctic Islands (data from Melchin, 1989, Table 1).

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cyphus Zone, late Rhuddanian, and remained until the end of the curtus Zone, early Aeronian. It remains unclear if this community replacement was caused by a shallowing or deepening event, as the dolostone lithofacies and D. obliquicostatus community in the upper acinaces Zone seem to be contradictory, as mentioned above. Both A. petila and D. obliquicostatus are considered as representative of the furthest offshore to slope/basin environment. McCracken (1991) named the Aspelundia–Dapsilodus fauna, and both Armstrong (1990, p. 35) and McCracken (1991, p. 72) noted the bprolific, low diversity conodont faunas containing Aspelundia, Dapsilodus and Decoriconus almost to the exclusion of other generaQ. This is partly supported by the Cornwallis fauna except for Decoriconus. Among the Panderodus-poor communities, the D.o. community lacks specimens of A. petila; the A.p. community only yields very few specimens of D. obliquicostatus in two samples in which the abundance of the two species shows an antipathetic relationship (Fig. 12); among the samples of W.c.–A.p. II community (see discussion below), six samples produced both A. petila and D. obliquicostatus that also exhibit an antipathetic relationship (Fig. 12). Thus, there was probably no Aspelundia–Dapsilodus fauna at Cornwallis. Although both Aspelundia and Dapsilodus favoured a deep-water environment, the change from the D.o. community to the A.p. community must reflect a subtle environmental change. The Dapsilodus commonly occurs in the deepest conodont-bearing environment (Aldridge and Jeppsson, 1999). In most intervals within the distribution of the Aspelundia species through the Cape Phillips Formation, they are grouped together

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with some other species and form a multispecies community with high species diversity and abundance, which may indicate that the Aspelundia species could tolerate a shallower onshore water depth. However, the A.p. community contains very few other species with very low abundance, therefore, during the time interval from the late cyphus Zone, late Rhuddanian to the curtus Zone, early Aeronian the A.p. community probably represents a highest sea level since early Rhuddanian, with the exception of the late acinaces Zone when D.o. community appeared, which is supported by the highest graptolite diversity in the same time interval (Fig. 11) (Melchin, 1989). A major transgressive event by the end of Rhuddanian was recognized by de Freitas et al. (1999), which is not supported by the conodont community. 5.4.2. Shallowing in the early convolutus Zone, middle Aeronian The W.c.–A.p. II community replaced the A.p. community in the early convolutus Zone, middle Aeronian, and remained stable through the end of the guerichi Zone, early Telychian. In comparison with the A.p. community, W. curvatus reappeared in the W.c.–A.p. II community. Although W. curvatus preferred an offshore environment, it did occur in near-shore settings. The co-occurrence of W. curvatus and A. petila indicates that the sea level in the early convolutus Zone, middle Aeronian, underwent a shallowing pulse, accompanied by a sharp decrease in graptolite diversity from 33 species in upper curtus Zone to 18 species in lower convolutus Zone (Melchin, 1989) (Fig. 11). This lowered sea level remained until the end of the guerichi Zone, early Telychian, indicated by the stable W.c.–A.p.

Fig. 12. The relative abundance (percentage of the total fauna) of Aspelundia petila and Dapsilodus obliquicostatus among four Panderodus-poor communities from the Cape Phillips Formation. Sample numbers are shown on the bottom axis; the order of the samples is same as in Fig. 8; black diamonds and solid line, and white diamonds and dashed line represent D. obliquicostatus and A. petila, respectively.

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II community throughout this time interval (Fig. 9), but graptolite diversity seemed not as stable as conodont community development with a low diversity in lower convolutus and lower guerichi zones (18 and 23 species, respectively), but a high diversity in upper convolutus and upper guerichi zones (31 and 35 species, respectively) (Fig. 11). The present interpretation does not support the minor transgression in the mid-convolutus Zone noted by de Freitas et al. (1999). Another minor transgression by the end of sedgwickii Zone recognized by de Freitas et al. (1999) may occur because of the lithofacies change from dolostone to mudstone, but no conodont collection was made from the sedgwickii Zone (Fig. 9). The A.p. community replaced the W.c.–A.p. II community in the turriculatus Zone, lower Telychian, which represents a short-lived sea level increase; however, the graptolite diversity decreased in this interval (Fig. 11). 5.4.3. Further shallowing in the crispus? and griestoniensis zones, middle Telychian A remarkable change among the conodont communities occurred in the upper crispus? Zone. The Panderodus-rich community re-invaded the Cornwallis area with different features from the early Rhuddanian (Fig. 9). The A. fluegeli–A. borenorensis–W. sancticlairi–P. celloni (A.f.–A.b.–W.s.–P.c.) community first occurred in the lower crispus? Zone. It is a completely different community from the A.p. and W.c.–A.p. II communities stratigraphically below in that there is a rapid increase in conodont abundance and diversity and a sudden change in faunal composition (Fig. 8). This has been interpreted as a migration of bEuropeanQ conodonts into the area, reflected by the establishment of more open-marine conditions (Le Fe`vre et al., 1976), and with an original slope or deeper shelf fauna subsequently migrating into shelf environments as the result of a worldwide transgression (Armstrong, 1990). However, the Cornwallis conodont communities support an opposite interpretation to these hypotheses. The A. fluegeli and P. celloni zones were recognized from Cornwallis, which are correlated with the European P. celloni Zone and crispus? and griestoniensis graptolite zones, whereas the P. celloni Zone on Cornwallis was correlated to middle and upper P. celloni Zone in Europe by Jowett and Barnes (submitted for publication). This supports a local regression that created a shallower water environment on the Arctic margin in the early crispus? Zone, and continued to the late griestoniensis Zone, which was only interrupted shortly by the W. sancticlairi (W.s.) community with low di-

versity and low abundance in the middle griestoniensis Zone (Fig. 9). The graptolite diversity within the crispus? and griestoniensis zones remained stable with a range of 17–25 species, which supports a shallower water environment than the time interval of late Rhuddanian–early Telychian (Figs. 9 and 11). The change from the A.p. and W.c.–A.p. II communities to the A.f.– A.b.–W.s.–P.c. community may have involved the evolutionary change in the species of Aspelundia and Walliserodus; however, the appearance of P. celloni (Walliser) on Cornwallis was later than in Europe. 5.4.4. Lowstand continuing in the sakmaricus Zone, late Telychian The A. fluegeli–A. borenorensis–W. sancticlairi–P. pennatus procerus (A.f.–A.b.–W.s.–P.p.p.) community alternated with A.f.–A.b.–W.s.–P.c. community only near the boundary between the griestoniensis and sakmaricus zones, and then the former occurred consistently through to the end of the sakmaricus Zone (approximately equivalent to the P. amorphognathoides Zone of Jowett and Barnes, submitted for publication) (Fig. 9). The A.f.–A.b.–W.s.–P.p.p. community can be differentiated from the A.f.–A.b.–W.s.–P.c. community in two ways: a) the Pterospathodus species—the community includes P. pennatus procerus (Walliser) whereas the A.f.–A.b.–W.s.–P.c. community contains P. cellon; and b) the diversity and abundance—the samples grouped under the former contains more diverse and abundant conodonts than the latter (see Appendix) (Fig. 8). The A.f.–A.b.–W.s.–P.p.p. community has the highest diversity and abundance among all conodont communities recognized in the Cape Phillips Formation (Fig. 8). This may be due to the beginning of the regression that exposed much of the North American shelf by the early amorphognathoides Zone (Jowett and Barnes, submitted for publication). However, graptolite diversity increased in the lower sakmaricus zone with 32 species and decreased in the upper sakmaricus zone with 22 species (Fig. 11). 5.5. Transgression in the early Sheinwoodian, and highstand in the late instrenuus–kolobus Zone, mid-Sheinwoodian Among the sections investigated in this study, no conodonts were found in the insectus and centrifugus zones that lie between the sakmaricus Zone, upper Telychian and the murchisoni Zone, lower Sheinwoodian. However, a notable community change can be observed in the uppermost sakmaricus, murchisoni and

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lower instrenuus–kolobus zones. In the former two, a single species conodont community, the P. pennatus procerus (P.p.p.) community with a lower species abundance (see Appendix), alternated with the A.f.– A.b.–W.s.–P.p.p. and D.o. communities for a short interval (Fig. 9). This may indicate that all three communities were pressured by the unstable sea level. Finally, the D.o community completely replaced the other two communities in the lower instrenuus–kolobus Zone. It appeared briefly once in the late Rhuddanian, but its significance for sea level events remains uncertain. This community disappeared during the latest Rhuddanian, Aeronian and Telychian, and then reappeared in the early Sheinwoodian and persisted for most of the Sheinwoodian. Within the Sheinwoodian, D. obliquicostatus is the principal component of the fauna with a percentage of ~65–100% in most of samples (Fig. 12). This dramatic change was noted as the Telychian–Sheinwoodian (Ireviken) bioevent straddling the Llandovery–Wenlock boundary and affecting the composition of the Wenlock fauna (Aldridge et al., 1993), and interpreted as a result of a transgression at the base of the Wenlock (Barrick, 1997; Jowett and Barnes, submitted for publication). The replacement among the three conodont communities, the A.f.– A.b.–W.s.–P.p.p., P.p.p., and D.o. communities supports the earlier observations, but further indicates that the initiation of the transgression started in the late sakmaricus Zone. The occurrence of the D. obliquicostatus–N. Gen. n. sp. (D.o.–N. Gen. n. sp.) community in the upper instrenuus–kolobus and basal perneri–opimus zones (Fig. 9) is a mid-Sheinwoodian bioevent. This was interpreted as representing a more offshore environment supported by the co-occurrence with abundant and diverse radiolarians (Jowett and Barnes, submitted for publication). The radiolarians reached their highest diversity (9–12 species) during these intervals since the earliest Silurian (MacDonald, 2004). Combining the persistent high percentage of D. obliquicostatus (~50– 90%) and the abundant and diverse radiolarians, this community may represent the highest sea level on the Cornwallis since the beginning of the Silurian. 6. Comparison of Late Ordovician and Early Silurian sea level events between Cornwallis and Anticosti islands Graptolites are rare on Anticosti. Most intervals on both Anticosti and Cornwallis islands do not produce the same conodonts, probably because of the different facies, which makes the comparison of sea level events

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between the two islands, based on either the graptolite or conodont zonation, difficult. A useful comparison can be developed using a relative age. The boundaries of stages within the series of Late Ordovician and Early Silurian on Cornwallis are welldefined based on the graptolites. On Anticosti, the last appearance of conodont Gamachignathus in the lower member 6, Ellis Bay Formation was taken as top Ordovician, and member 1 to lower member 6 represent the Gamachian (McCracken and Barnes, 1981). The btransitional faunaQ with a mixing conodont species typical of the preceding ordovicicus Zone and those generally regarded as Silurian indicators was correlated to the upper persculptus Zone (Melchin et al., 1991), based on which the samples E48–E50 from about 1.5–2 m interval with bioherms at Ellis Bay section represent this btransitional faunaQ. Thus, the last appearance of Gamachignathus is a good indicator for the Ordovician–Silurian boundary on Anticosti. The thin Merrimack Formation was established and assigned to the late Rhuddanian cyphus Zone by Copper and Long (1989) based on brachiopods. Thus, the boundary between Rhuddanian and Aeronian occurs at or near the upper boundary of the Merrimack Formation. The Jupiter Formation was formally subdivided into six members: Goe´land, East Point, Richardson, Cybe`le, Ferrum and Pavillon members, in ascending order (Copper and Long, 1990). The upper half of the Richardson and the Cybe`le members were placed within the Eocoelia hemisphearica hemisphearica brachiopod Zone, and assigned to the C3 division of the Aeronian; the Cybe`le Member was later reassigned to the lower Telychian (Copper and Long, 1998; Jin and Copper, 1999). The latter opinion is followed herein, because of a) the presence of graptolite M. sedgwickii in the Cybe`le member (Copper, 2004, personal communication); b) the range of this uppermost Aeronian zonal species occurs through the M. sedgwickii and Rastrites maximus zones of the upper Llandovery in Britain (Toghill, 1968); and c) M. sedgwickii in the upper Jupiter Formation is best placed in the R. maximus Zone (Zhang and Barnes, 2004a). M. sedgwickii is not present in Cornwallis, but a graptolite-barren interval between convolutus and minor (guerichi) zones was compared with the sedgwickii Zone interval (Melchin, 1989). Thus, the boundary between Aeronian and Telychian is placed at the base of Cybe`le Member, Jupiter Formation, and correlated to the base of the minor (guerichi) Zone on Cornwallis Island. The cartographic scale of the Anticosti Late Ordovician and Early Silurian sea level curve developed by

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Zhang and Barnes (2002b, fig. 11) has been adjusted in Fig. 13 to match that developed herein for Cornwallis Island. Specifically, the scale of Becscie, Merrimack

and Gun River formations is reduced, whereas that of the Jupiter Formation is enlarged from the original figure (Fig. 5) (Zhang and Barnes, 2002b, fig. 11).

Fig. 13. Comparison between the two sea level curves referred based on the analyses of conodont communities from both Cornwallis and Anticosti. The curves on left and right sides represent the latest Ordovician–Early Silurian sea level history in Cornwallis and Anticosti, respectively.

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Within this biostratigraphic and chronostratigraphic framework, the following is a summary of Late Ordovician and Early Silurian sea level events on both Cornwallis and Anticosti islands. 6.1. Sea level events in the latest Ordovician A profound and rapid sea level rise took place during the persculptus Zone, latest Ordovician, which has been noted by recent studies (Brenchley, 2004; Brenchley et al., 2001; Nielsen, 2004) and has been proved by conodont community and lithofacies changes in Cornwallis in this study. However, the conodont community and lithofacies changes in Anticosti do not seem to fit this pattern. The conodont community analysis recognized four different communities near the Ordovician–Silurian boundary. The Phragmodus undatus (Ph.) community (Figs. 6 and 7) was recognized by Zhang and Barnes (2002b, figs. 5, 6) and was interpreted as representing the deepest water in the Late Ordovician in Anticosti. Both the Gamachignathus ensifer–G. hastatus–Panderodus panderi (G.e.–G.h.–P.p.) and G. ensifer (G.e.) communities were also recognized by Zhang and Barnes (2002b, figs. 5, 6). The former was interpreted representing slightly shallower water than the latter, based on the former being more common in member 1, Ellis Bay Formation with more shale. This present study re-examines the distribution of Panderodus through this boundary interval. Among the three communities Ph., G.e.–G.h.–P.p. and G.e., all samples grouped into the former two communities contain abundant Panderodus species, but seven samples out of eight under the G.e. community have no Panderodus species (Fig. 6). If the G.e. community represented deeper water than the G.e.–G.h.–P.p. community and a shallower water environment than the Ph. community, it would suggest that Panderodus species were lacking where water depths were about 20–30 m (Figs. 6 and 7), which was not noticed by Zhang and Barnes (2002b). Logically, the G.e. community probably indicated slightly shallower water than the G.e.–G.h.–P.p. community. Therefore, from uppermost member 3 to lowest member 6, Ellis Bay Formation, the three communities Ph., G.e.–G.h.–P.p. and G.e. probably indicate a gradual change in water depth from deep to shallow. As mentioned above, the samples E48–E50 that were from a 1.5–2 m interval with bioherms at the Ellis Bay section represent the btransitional faunaQ, which can be correlated to the upper persculptus Zone. These samples are the uppermost three taken at

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the Ellis Bay section that are grouped under the G.e. community; although they contain the Silurian conodont O. hassi (Fig. 6), they are still grouped with other samples from the lower level (E43–E47). It is unclear where the lower boundary of the persculptus Zone occurs stratigraphically on Anticosti, but the change from Ordovician fauna to btransitional faunaQ happened during the regression. It has been interpreted that the reef and bioherms represented the lowest sea level in the Late Ordovician on Anticosti (Copper and Long, 1998), which also supports the present data that indicate a major regression in the latest Ordovician on Anticosti. 6.2. Sea level events in the Rhuddanian The conodont community change from the D. fragilis (D.f.) community hosted by the black shale lithofacies in the persculptus Zone to the R. kentuckyensis– O. hassi–D. fragilis (R.k.–Oz.h.–D.f.) community hosted by the more calcareous shale lithofacies in the acuminatus Zone may indicate a regression in the early Rhuddanian on Cornwallis. The conodont community in the lowest Rhuddanian on Anticosti recognized by this study, the Ozarkodina oldhamensis–W. curvatus (Oz.o.–W.c.) community (Fig. 6), is similar to Oz. oldhamensis–R. kentuckyensis (Oz.o.–R.k.) (Fig. 4), both contain abundant Walliserodus species. The conodont community changes from the G. ensifer (G.e.) hosted by the lithofacies with bioherms in the uppermost Ordovician to the Oz. oldhamensis–W. curvatus (Oz.o.–W.c.), or Oz. oldhamensis–R. kentuckyensis (Oz.o.–R.k.) community hosted by the homogenous, laminated mudstone (Sami and Desrochers, 1992) on Anticosti indicates a significant deepening in the early Rhuddanian. In the middle–late Rhuddanian, on the Cornwallis Island, a) the increased sea level caused replacement of the R.k.–Oz.h.–D.f. by the W.c.–A.p. I community in the atavus Zone; b) the sea level event in the acinaces Zone is unclear because of the conflict between lithofacies and the distribution of D.o. communities; and c) to compare the A.p. community in the cyphus Zone with the W.c.–A.p. I community in atavus Zone, the increased sea level forced out all the shallow water conodonts and introduced the A.p. community in the cyphus Zone, late Rhuddanian. However, the Anticosti sea level scenarios are different: a) an initial transgression is recognized only in the early Rhuddanian, which introduced the Oz. oldhamensis–R. kentuckyensis (Oz.o.–R.k.) community; b) offset by a brief regression in the early–middle Rhuddanian resulting in the Oz.o.– R.k. community being replaced by the Oz. strena–

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Rexroadus nathani (Oz.s.–R.n.) community, and then the Oz.s.–R.n. community by the P. unicostatus (P.u.) community; c) sea level increased in the middle Rhuddanian with greater stability through the middle and late Rhuddanian, following the return of the Oz.s.–R.n. community; and d) a highstand appeared in the latest Rhuddanian with shale deposition and a demise of almost all conodonts from the Anticosti Basin except for P. unicostatus (Fig. 5) (Zhang and Barnes, 2002b). In general, a gradual transgression for most of the Rhuddanian and a late Rhuddanian highstand for Cornwallis are not recognized for Anticosti (except for the unclear acinaces Zone); conversely, the brief regression in the early–middle Rhuddanian and the end-Rhuddanian highstand in Anticosti are not evident for Cornwallis (Fig. 13). 6.3. Sea level events in the Aeronian On Cornwallis, the Aeronian sea level was quite stable and only underwent a slight regression in the early convolutus Zone, which resulted in the steady development of the A.p. and W.c.–A.p. II communities, with the latter replacing the former in the lower convolutus Zone. However, on Anticosti, sea levels for most of the Aeronian were moderately high, which maintained a stable environment for the Icriodella deflecta–Oulodus jeannae–Ozarkodina pirata (I.d.– Ou.j.–Oz.p.) community to develop, but increased briefly in the middle and late Aeronian when the Oz. pirata (Oz.p.) and Panderodus recurvatus (P.r.) communities were introduced. Except for the brief latest Aeronian lowstand with the temporary demise of almost all conodonts from Anticosti, sea level exhibited an overall pattern of deepening upward (Figs. 5 and 13) (Zhang and Barnes, 2002b), compared to a shallowing upward in Cornwallis during the Aeronian (Figs. 9 and 13). The conodont and graptolite barren interval, which was correlated to the sedgwickii Zone (Melchin, 1989) on Cornwallis, is composed of dolostone. There is insufficient evidence to compare this interval to the conodont barren interval that comprises shale representing a local clastic influx resulted from the lowstand on Anticosti (Zhang and Barnes, 2002b), because the conodont fauna below and above this barren interval on Cornwallis shows no significant change (Figs. 9 and 13).

On Cornwallis, the guerichi Zone (Figs. 9 and 13) exhibited a relatively low sea level represented by the W.c.–A.p. II community, whereas the turriculatus Zone experienced a relative high sea level represented by the A.p. community. On Anticosti, the sea level underwent frequent fluctuations in the early Telychian: a) the earliest Telychian sea level fluctuations produced repeated replacement of the P. unicostatus (P.u.), Ozarkodina aldridgei (Oz.a.), P. recurvatus (P.r.), and Oz. pirata (Oz.p.) communities; and b) in the late early Telychian, the former two communities prevailed. This pattern of community replacement reflects the frequent sea level fluctuations restricted in the relatively deeper water on the platform during the late early Telychian than during the earliest Telychian (Fig. 5) (Zhang and Barnes, 2002b), therefore, it is probably related to the high sea level in Cornwallis during the turriculatus Zone (Fig. 13). The sudden appearance of the A.f.–A.b.–W.s.–P.c. community in the crispus? Zone and its stable development through the griestoniensis Zone on Cornwallis reflect a sharp sea level drop followed by a stable low sea level. This interval is questionably correlated to a covered interval in the upper Jupiter Formation, middle Telychian, on Anticosti (Fig. 13), interpreted as representing a regression (Zhang and Barnes, 2002b). The A.f.–A.b.–W.s.–P.p.p. and A.f.–A.b.–W.s.–P.c. communities alternated near the boundary between the griestoniensis and sakmaricus zones, reflecting the frequent sea level fluctuations near this boundary on Cornwallis (Fig. 9). On Anticosti, the change to the P.u. community and the other two short-lived communities, the Apsidognathus tuberculatus–P. celloni–P. pennatus procerus–Carniodus carnulus–Ozarkodina polinclinata (A.–P.–P.–C.–O.) and the Icriodella inconstans– Oz. gulletensis–Aulacognathus bullatus (I.–O.–A.) communities, reflected an unstable sea level near the Jupiter–Chicotte boundary (Fig. 5). These two sea level changes are comparable and further supported by the appearance of the P. celloni–P. pennatus procerus fauna near the two boundaries. During most of the sakmaricus Zone, the A.f.–A.b.–W.s.–P.p.p community maintained a stable development on Cornwallis, which represents a lowstand (Figs. 9 and 13). Although no conodont collections were made from the upper Chicotte Formation, the crinoidal grainstone facies probably indicates a shallowing phase on Anticosti (Uyeno and Barnes, 1983).

6.4. Sea level events in the Telychian 7. Discussion The Telychian sea level curves are different from those of the Rhuddanian and Aeronian as the curves from the two islands are now generally comparable.

The concept of beustaticQ changes in sea level defined the vertical displacements of the ocean sur-

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face occurring relatively uniformly throughout the world (Suess, 1885). It was later extended in establishing global eustatic models through seismic stratigraphy (Vail, 1975; Vail et al., 1977) and local/regional variations expressed by sequence stratigraphy (Posamentier et al., 1988; Posamentier and Vail, 1988). This was termed as the global eustatic paradigm by Miall and Miall (2001) who conveyed doubt on the universal validity of the global-eustasy model. A contrasting paradigm, termed the complexity paradigm, was proposed by Miall and Miall (2001, p. 330), which brepresents a body of ideas focusing on the hypothesis that sea-level change is affected by multiple processes operating simultaneously at different rates and over different ranges of time and space, possibly including eustatic sea-level changeQ. We do not oppose global eustatic sea level changes for certain geological periods, but we agree with Miall and Miall (2001) that sea level change is affected by multiple processes. These may include glaciation and deglaciation, tectonic changes (subsidence or uplift, thermal changes), wind action, tidal variations, and extreme events (storm surges or tsunamis). Glaciation and deglaciation as well as tectonics are relatively long-term factors. It is widely accepted that deglaciation in the latest Ordovi-

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cian–Early Silurian involved the melting of the terminal Ordovician continental ice sheet in the southern hemisphere (western Gondwana). The latest Ordovician– Early Silurian was a time when the closure of the Iapetus Ocean was largely completed. The deglaciation, global warming (thermal expansion of the oceans) and the closure of the Iapetus would have played roles in the latest Ordovician–Early Silurian sea level changes (Barnes, 2004). Here, the discussion will focus on the latest Ordovician–Early Silurian deglaciation effects. bGlacio-isostaticQ effects provide strong evidence to oppose the global eustatic model; however, major glaciations in the geological record are relatively rare. Such effects on sea level changes accompanied the last Pleistocene deglaciation phase, with uplift in areas of ice melting and subsidence in a wide peripheral belt (Daly, 1934). Glacio-isostasy and eustasy are two physical processes, which affect the earthTs lithosphere and hydrosphere involving vertical motions that can be monitored specifically by mean sea level today (Fairbridge, 1983). Isostatic responses to glaciation vary across the globe, and the variations differ in five sea level zones resulting from the retreat of Northern Hemisphere ice sheets during the Holocene (Fig. 14) (Clark et al., 1978, fig. 15; Clark and Lingle,

Fig. 14. Sea level zones and typical relative sea level curves deduced for each zone by Clark et al. (1978), resulting from retreat of Northern Hemisphere ice sheets under the assumption that no eustatic change has occurred since 5000 yr BP (adapted from Clark and Lingle, 1979).

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1979, fig. 1). The variations are distinct and were distinguished as different fields (Lambeck, 1993). In the near field sites (areas within the maximum limit of former ice sheets) and the far field sites (areas well away from the influence of the former ice sheets), the relative sea level curve would show an exponential fall in sea level with time following deglaciation and a gradual rise of sea level with global deglaciation, respectively; the intermediate-field sites correspond to the peripheral bulge around former ice margin which tended to subside in late glacial and postglacial times, and the relative sea level continues to rise even when deglaciation has ceased, though at gradually decreasing rates (Lambeck, 1993; Pirazzoli, 1996). For the more complex situation with today’s ice sheets distributed at both south and north polar areas, each ice sheet has a distinct bsea level fingerprintQ (Clark et al., 2002). In general, sea levels’ rise in the opposite hemisphere responds to the melting ice due to the reduction in the gravitational pull of the ice mass. In the Late Ordovician, the only known continental ice sheet was on Gondwana. Based on glacio-isostatic theory, the latest Ordovician–Early Silurian deglaciation would result in sea level rise in much of the northern hemisphere and the sea level fall in much of the southern hemisphere, especially the near field of Gondwana. Fig. 15 shows the distribution of the Late Ordovician tillites on Gondwana, with Anticosti Island closer to Gondwana than Cornwallis Island. Al-

though it is not known which zone of Clark et al. (1978) or which field of Lambeck (1993) both Anticosti and Cornwallis could be related to, the two islands were about 308 apart from each other in paleolatitude (Figs. 1 and 15), and it is likely that the sea levels at these two localities would not have been similarly affected by the latest Ordovician–Early Silurian deglaciation, because of the isostatic adjustments revealed by the studies of Holocene deglaciation as shown in Fig. 14. Furthermore, the latest Ordovician–Early Silurian sea level was probably not only affected by the melting of the Gondwana ice sheet on the Gondwanaland, but also by the three short-lived Early Silurian glacial episodes (early Aeronian, latest Aeronian–early Telychian, and latest Telychian–earliest Wenlock) (Caputo, 1998; Azmy et al., 1998). These would have caused more complex global sea level changes during the Early Silurian than in the Holocene shown in Fig. 14. The latest Ordovician–Early Silurian conodont data and the conodont community analysis from both Cornwallis and Anticosti, representing both Arctic and Appalachian margins of Laurentia, respectively, provide significant evidence against a universal sea level curve in the latest Ordovician–Early Silurian, especially for the latest Ordovician Rhuddanian and Aeronian. This is supported by the glacio-isostatic adjustments documented by studies of Holocene deglaciation and geodynamics.

Fig. 15. Late Ordovician (Ashgillian) reconstruction (~440 Ma), showing the distribution of tillites in Gondwana. Black contours: modern landmass; dark gray areas: ancient landmass; light gray seas: epicontinental seas; black D: position of Anticosti; white D: position of Cornwallis; +: tillites (modified from http://www.scotese.com/mlordcli.htm).

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8. Conclusions

Acknowledgements

1) Overall, the sea level curves inferred from the distribution of the conodont communities from Anticosti and Cornwallis exhibit frequent and less frequent oscillating patterns, respectively. 2) The sea level patterns on both Anticosti and Cornwallis differ in the following ways: ! During the latest Ordovician, the G. ensifer (G.e.) and D. fragilis (D.f.) communities were developed on Anticosti and Cornwallis, respectively. The former was developed in an extremely shallow and aerobic environment during a regressive event, whereas the latter in a deep and anaerobic environment within a transgressive event. ! During the earliest Rhuddanian, similar conodont communities were developed on both Anticosti and Cornwallis, which were introduced by a transgression and regression, respectively. During the early Rhuddanian, they were replaced by a shallower water community on Anticosti and a deeper water community on Cornwallis, which reflect a regression and transgression, respectively. ! A lowstand interval is virtually devoid of conodonts from Anticosti in the late Aeronian, whereas a deep conodont community remained unchanged on Cornwallis from middle Aeronian to early Telychian. 3) The sea level patterns on both Anticosti and Cornwallis are similar in the following ways: ! During the early crispus? Zone sea level dropped sharply and remained at a stable low sea level until the late griestoniensis Zone on Cornwallis. This interval is questionably related to a covered interval in the middle Telychian on Anticosti. ! Sea level underwent frequent oscillations near the boundary between the griestoniensis and sakmaricus zones, and then attained a lowstand though late sakmaricus Zone. 4) In the Sheinwoodian, early Wenlock, sea level reached the highest level during the Early Silurian on Cornwallis; the strata representing this interval do not occur on Anticosti. 5) The sea level history on both the southern and northern margins of Laurentia, reflected by conodont communities from Anticosti and Cornwallis, is contrary to the standard latest Ordovician–Early Silurian sea level curve. This complexity is supported by studies on Quaternary glaciation/deglaciation events that conclude that the melting of an ice sheet will be accompanied by sea level change and regional isostatic effects that do not generate a uniform global eustatic change.

We gratefully acknowledge financial support from a Pan-LITHOPROBE grant and ongoing support from both the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Earth System Evolution Program of the Canadian Institute for the Advanced Research. G.S. Nowlan, A.D. McCracken, S.L. Duffield and S. Gardiner provided earlier field assistance in developing the Anticosti conodont database. M.J. Melchin and P. Noble provided field support and some stratigraphic data for developing the Cornwallis conodont database. M.J. Melchin kindly reviewed the manuscript and made some valuable suggestions. Appendix A. Description of Late Ordovician–Early Silurian conodont communities from Cornwallis Island (see Jowett, 2000 for detailed lithological descriptions of conodont samples) Amorphognathus ordovicicus (A.o.) community Cluster samples: 3 consecutive samples. Defining taxa: Amorphognathus ordovicicus Branson and Mehl. Range: upper fastigatus Zone–pacificus Zone, upper Richmondian. Lithofacies: dolostone dominant with minor calcareous shale; the uppermost sample possibly represents the paleokarst surface. Diversity and abundance: 2–11 species, with b 1–24 specimens/kg of each species. Decoriconus fragilis (D.f.) community Cluster samples: 2 consecutive samples. Defining taxa: Decoriconus fragilis (Branson and Mehl). Range: bpersculptusQ Zone, uppermost Richmondian. Lithofacies: black laminated calcareous mudstone interbedded with black fissile calcareous shales; platy oblate concretions are present. Diversity and abundance: 1 species, with b1 specimen/kg of each species. Rexroadus kentuckyensis–Ozarkodina hassi–Decoriconus fragilis (R.k.–Oz.h.–D.f.) community Cluster samples: 5 inconsecutive samples separated by conodont barren samples. Defining taxa: Rexroadus kentuckyensis (Branson and Branson), Ozarkodina hassi (McCraken and Barnes) and Decoriconus fragilis.

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Range: acuminatus Zone and lower atavus Zone, lower Rhuddanian. Lithofacies: similar to that related to the Decoriconus fragilis community in the bpersculptusQ Zone, but the bblack shale becomes progressively more calcareous and slightly paler upward, to dark grayishbrown, as they pass into the lowest Silurian acuminatus ZoneQ (Melchin et al., 1991, p. 1856). Diversity and abundance: 2–8 species, with b1–24 specimens/kg of each species. Walliserodus curvatus–Aspelundia petila I (W.c.– A.p. I) community Cluster samples: 1 sample. Defining taxa: Walliserodus curvatus (Branson and Branson) and Aspelundia petila (Nicoll and Rexroad). Range: middle atavus Zone, lower Rhuddanian. Lithofacies: The sample was collected at the transition from carbonate to black shale. Diversity and abundance: 9 species, with b1–24 specimens/kg of each species. Dapsilodus obliquicostatus (D.o.) community Cluster samples: 9 inconsecutive samples. Defining taxa: Dapsilodus obliquicostatus Branson and Mehl. Range: upper acinaces Zone, Rhuddanian, Llandovery; instrenuus–kolobus to perneri–opimus zones, Sheinwoodian, Wenlock. Lithofacies: The two samples contained the D.o. community from the upper acinaces Zone are vuggy, nodular, bituminous dolostone; all other samples contained this community from the interval of instrenuus–kolobus to perneri–opimus zones, Sheinwoodian, Wenlock are interbedded shaly to platy, laminated lime mudstone and calcareous shale. Diversity and abundance: 1–5 species, with b1–24 specimens/kg of each species; most trivial species only with b 1 specimen/kg. Aspelundia petila (A.p.) community Cluster samples: 7 inconsecutive samples. Defining taxa: Aspelundia petila. Range: cyphus Zone, upper Rhuddanian to curtus Zone, lower Aeronian; turriculatus Zone, middle Telychian. Lithofacies: the dominant rock type is interbedded shale and lime mudstone. Diversity and abundance: 1–6 species, with b1–24 specimens/kg of each species; except for the

nominate species, all other species only with b1 specimen/kg. Walliserodus curvatus–Aspelundia petila II (W.c.– A.p. II) community Cluster samples: 13 almost consecutive samples separated by only two conodont barren samples. Defining taxa: Walliserodus curvatus and Aspelundia petila. Range: convolutus Zone, middle Aeronian to guerichi Zone, early Telychian. Lithofacies: the dominant lithology is interbedded shale and lime mudstone. Diversity and abundance: most samples yielding 1–4 species, with b 1–9 specimens/kg of each species. Aspelundia fluegeli–A. borenorensis–Walliserodus sancticlairi–Pterospathodus celloni (A.f.–A.b.–W.s.– P.c.) community Cluster samples: 12 inconsecutive samples separated by conodont barren samples and a sample with W.s. community (see below) at four different levels, with the last sample separated by a sample characterized by the A.f.–A.b.–W.s.–P.p.p community (see below). Defining taxa: Aspelundia fluegeli (Walliser), A. cf. A. borenorensis (Bischoff), Walliserodus cf. W. sancticlairi Cooper, Pterospathodus celloni (Walliser). Range: crispus? Zone to griestoniensis Zone, middle Telychian, Llandovery. Lithofacies: the dominant lithology is lime mudstone and calcareous shale. Diversity and abundance: 1–10 species, with b1–24 specimens/kg of each species. Walliserodus sancticlairi (W.s.) community Cluster samples: 2 inconsecutive samples. Defining taxa: Walliserodus cf. W. sancticlairi. Range: middle griestoniensis Zone, middle Telychian, Llandovery; middle perneri–opimus Zone, upper Sheinwoodian, Wenlock. Lithofacies: one sample representing the W.s. community comes from a concretion within an interval of calcareous shale, which becomes cherty towards top in the middle griestoniensis Zone; the other sample from interbedded shaly to platy, laminated lime mudstone and calcareous shale in the middle perneri–opimus Zone. Diversity and abundance: 1 species, with b1–9 specimens/kg of the species.

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Aspelundia fluegeli–A. borenorensis–Walliserodus sancticlairi–Pterospathodus pennatus procerus (A.f.– A.b.–W.s.–P.p.p.) community Cluster samples: 13 inconsecutive samples separated by conodont barren samples at three different levels, the lower two samples are separated by a sample with the A.f.–A.b.–W.s.–P.c. community (see above), and the upper two samples are separated by a sample with the P.p.p. community (see below). Defining taxa: Aspelundia fluegeli, A. cf. A. borenorensis, Walliserodus sancticlairi, Pterospathodus pennatus procerus (Walliser). Range: upper griestoniensis to sakmaricus Zone, late Telychian, Llandovery. Lithofacies: the dominant lithology is lime mudstone and calcareous shale with few bioclastic grainstone and conglomerate layers; most samples are from lime mudstone. Diversity and abundance: 1–17 species, with b 1–99 specimens/kg of each species. Pterospathodus pennatus procerus (P.p.p.) community Cluster samples: 3 inconsecutive samples separated by those containing the A.f.–A.b.–W.s.–P.p.p and D.o. communities. Defining taxa: Pterospathodus pennatus procerus. Range: upper sakmaricus Zone, upper Telychian, Llandovery; lower instrenuus–kolobus Zone, lower Sheinwoodian, Wenlock. Lithofacies: two of the three samples are oily shale/mudstone. Diversity and abundance: 1 species, with b 1–4 specimens/kg of the species. Dapsilodus obliquicostatus–N. Gen. n. sp. (D.o.–N. Gen. n. sp.) community Cluster samples: 4 inconsecutive samples separated by those containing D.o. community. Defining taxa: Dapsilodus obliquicostatus and N. Gen. n. sp. Jowett and Barnes. Range: upper instrenuus–kolobus Zone to lower perneri–opimus Zone, lower Sheinwoodian, Wenlock. Lithofacies: similar to that contains the D.o. community. Diversity and abundance: 2–3 species, with b 1–24 specimens/kg of each species. References Aldridge, R.J., 1976. Comparison of macrofossil communities and conodont distribution in the British Silurian. In: Barnes, C.R.

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