New biostratigraphic and chemostratigraphic data from the Chicotte Formation ( Llandovery , Anticosti Island , Laurentia ) compared with the Viki core ( Estonia , Baltica )

Due to the lack of biostratigraphically useful graptolites in the crinoidal and reefal Chicotte Formation on Anticosti Island (Québec, Canada), the precise chronostratigraphic position of the formation is not known. New stable carbon isotope and conodont data and a comparison with the detailed conodont distribution and δC development in the Viki core (Estonia) suggest an early Telychian age (Pterospathodus eopennatus to P. a. angulatus zones) for at least the lower half of the formation. This interval is characterized by a small but distinct positive excursion in δC, with peak values of + 3.2 and + 2.8 VPDB in the lowermost 1015 m of the Chicotte Formation on Anticosti and in the Viki core, respectively.


INTRODUCTION
Anticosti Island, more than 200 km in length, is located in the Gulf of St Lawrence in eastern Canada (Fig. 1).The rocks exposed on the island are largely unaffected by tectonic stress and are extremely well preserved (Lespérance 1981a(Lespérance , 1981b;;Petryk 1981; Copper & Long 1998;Chatterton et al. 2008).The succession on Anticosti consists of approximately 1 km thick, undeformed, fossiliferous limestones, shales, and minor siliciclastics, ranging in age from the Late Ordovician (Katian) to early Silurian (Telychian).The sediments accumulated in a slowly subsiding basin in a tropical setting south of the equator, along the NW margin of the Iapetus Ocean (Petryk 1981; Barnes 1988Barnes , 1989;;MacNiocall et al. 1997; Copper & Long 1998;van Staal et al. 1998;Long 2007).The strata strike WNW-ESE (Fig. 1), with an average dip of less than 2° in a SW direction.
The sequence on Anticosti, with its excellent coastalcliff and river-valley outcrops, represents one of the best-preserved exposures of shallow-water carbonates across the Ordovician-Silurian boundary (Petryk 1981;Barnes 1988).Most of the Llandovery strata were formed below the storm wave base, except the Chicotte Formation (Fm.) and other reefal or shallow-water units (Brunton & Copper 1994).The youngest rocks on Anticosti are represented by the 80-90 m thick Chicotte Fm. consisting of crinoidal limestones interpreted as innerramp shoal deposits, with reefs at three levels (Brunton & Copper 1994;Desrochers 2006).Due to the lack of biostratigraphically useful graptolites, the precise chronostratigraphic position of the Chicotte Fm. is still problematic.Whereas in some early papers at least the upper part of the formation was tentatively attributed to the lowermost Wenlock, today the consensus is that the entire formation is Telychian in age (e.g.Zhang & Barnes 2002;Desrochers 2006; Li & Copper 2006; Holland & Copper 2008).
A rough, preliminary set of stable isotope data from Llandovery brachiopods of Anticosti Island has been published by Azmy et al. (1998).Unfortunately, their data do not include precise information from where the samples have been collected, and therefore a reliable location of some samples in the stratigraphic sequence is difficult.In general, the δ 13 C values do not show much variation, scattering around + 1‰.An increase is observed from ca + 0.5‰ in the upper part of the Jupiter Fm. (Ferrum and Pavillon members) to about 1.5‰ in the overlying Chicotte Fm.
The first data on conodonts of Anticosti Island (a brief note) were published in Sweet et al. (1971).In 1975 a programme was initiated by C. R. Barnes to study conodonts from Anticosti, and the exposed strata (including the Chicotte Fm.) were sampled at roughly 2 m intervals along the main sections along the Jupiter and Salmon rivers and a few coastal bluffs (details in Barnes et al. 1981).The sampling of the Chicotte Fm. was limited to a few road-accessible sections in the lower part of the formation ( Uyeno & Barnes 1983).The last authors assigned six samples from the sections at Pointe du Sud-Ouest and Brisants Jumpers, in the lowest 21 m of the Chicotte Fm., to the Icriodella inconstans Assemblage Zone.Additionally, a sample from a drill hole located north of The Jumpers (Uyeno & Barnes 1983, fig. 1: loc. C-89848) was studied.According to Uyeno & Barnes (1983, p. 7), this sample came from a level of about 24 m above the base of the Chicotte Fm. and is the only one yielding Pterospathodus amorphognathoides amorphognathoides Walliser, the nominal taxon of the P. a. amorphognathoides Zone.
In this study we present new conodont and stable carbon isotope data from four localities in the upper part of the Jupiter Fm. and the lower part of the Chicotte Fm.These data are compared with the detailed conodont biostratigraphy and with partly new δ 13 C information from the Viki core in western Saaremaa, Estonia (Fig. 2).

MATERIAL AND METHODS
For the present study samples from four localities on Anticosti Island (Fig. 1) were investigated for stable carbon isotope geochemistry (studied from brachiopod shells) and for conodonts.The GPS positions of the localities as well as the Geological Survey of Canada (GSC) numbers are summarized in Table 1.In total, 19 brachiopod shells and 7 micritic samples from a reef outcrop were measured for stable isotopes.The brachiopod samples were treated according to the procedure described in Samtleben et al. (2001).Additionally, 27 micritic samples from the Viki core, from the interval between 174.7 and 185.6 m, were investigated.A pilot study carried out by ourselves indicated that this interval correlates with the interval sampled on Anticosti Island.In the Viki core, samples V-1 to V-8 come from the upper Rumba Fm. and the others from the lower Velise Fm. (Table 2).
Homogeneous micritic material, drilled from cleaned, cut rock samples with a dentist's drill, was used in isotope studies of rock samples.Carbonate powders were reacted with 100% phosphoric acid (density > 1.9, Wachter & Hayes 1985) at 75 °C using a Kiel III online carbonate preparation line connected to a ThermoFinnigan 252 mass spectrometer.All values are reported in per mil relative to VPDB by assigning a δ 13 C value of + 1.95‰ and a δ 18 O value of -2.20‰ to NBS19.Reproducibility was checked by replicate analysis of laboratory standards and it was better than ± 0.04‰ (1σ).For the chemostratigraphic correlation presented here, only the δ 13 C values were used.The complete data set including δ 18 O values is given in Tables 1 and 2.

Sample number
Level, m

Formation Regional stage
Traditional methods were used for processing the conodont samples.After dissolving a sample in 7-8% acetic acid, the residue was treated with 10% buffered formic acid to get rid of dolomite grains.The buffering of formic acid is essential to avoid damage to conodonts (Jeppsson & Anehus 1995).Final residues were small enough to be picked directly, without additional treatment with heavy liquid.All figured specimens of conodonts are deposited in collection GIT 583 in the Institute of Geology at Tallinn University of Technology, Estonia.

Stable isotopes
The mean δ 13 C values from Anticosti Island show a rapid increase from about 1.0‰ in the uppermost Jupiter Fm. (upper part of the Pavillon Member, locality A1561a, sample No. 416) to 2.8‰ in the lower part of the Chicotte Fm. (localities A1561b, A1562; samples 408-414 and 417; Fig. 3), with peak values of > 3.1‰ (Table 1 sample No. 415) investigated the values decrease again to around 1.4‰.
In the studied interval of the Viki core the rocks consist of argillaceous limestones, marlstones, and claystones.The claystones did not yield enough carbonate for stable isotope analysis.The δ 13 C values increase from ca + 0.1‰ in the lower Rumba Fm. (?uppermost Aeronian) up to values between + 2.0 and + 3.0‰ in the interval spanning the uppermost part of the P. eopennatus ssp.n. 1 Zone to the lower part of the Upper P. eopennatus ssp.n. 2 Subzone (lower Velise Fm.; Fig. 4).Towards the top of the P. eopennatus ssp.n. 2 Zone the values of δ 13 C decrease gradually and remain more or less constant around + 2.0‰ in the lower part of the succeeding P. a. angulatus Zone (Fig. 4).Higher up in the core these values decrease continuously to about 1.3‰ in the P. a. amorphognathoides Zone (Kaljo et al. 2003).This means that a small positive δ 13 C excursion is observed in the P. eopennatus Superzone (lower Telychian) in the Viki core.It is interesting to note that the isotope excursion correlates with an interval where the otherwise greenish-grey rocks are frequently intercalated by red-coloured limestones, marls, and claystones (between 175 m and 183.5 m), whereas lower no such colour has been observed (Fig. 4; Table 2).Few thin red layers are present up to 153.8 m.In the Velise Fm. red strata form a continuous belt parallel to the shelf edge in the outer shelf region (Jeppsson & Männik 1993).(after Männik 2007a) in the Viki core.The position of the gap at the base of the Rumba Formation, which was positioned slightly higher in Kaljo et al. (2003), has been corrected based on new results.

Conodonts
All samples yielded conodonts.The number of specimens per sample was highly variable, due to differences in sample size (from 0.33 to 14.3 kg) and in facies (Fig. 3).In total, 1068 identifiable conodont specimens were found (Figs 3,5).The preservation of conodont specimens is good, with a conodont colour alteration index CAI = 1.In all samples, particularly in 416 and 417, the faunas are dominated by Panderodus.In sample 415 Walliserodus and Pterospathodus are quite common.This sample provided the richest and most variable fauna and allows quite precise dating of the strata (see Discussion).Sample 415 consists of two different lithologies which were processed separately (indicated as 415a and 415b in Fig. 3).

DISCUSSION
Our data show a small but distinct positive stable carbon isotope excursion in the lower Telychian both on Baltica and Laurentia.Stable isotope data from this time slice are rare, and to our knowledge this excursion has so far been reported only by Kaljo & Martma (2000).Conodont data confirm the contemporaneous nature of the isotope excursions from the two areas.The lowermost sample (No. 416, locality A1561a) comes from the top of the Jupiter Fm. (from the Pavillon Member) and is dominated by specimens of Panderodus.
Here also Ozarkodina cf.aldridgei Uyeno & Barnes (Fig. 5A) and Aulacognathus cf.bifurcatus Bischoff (Fig. 5B) occur, which indicate that this sample comes from the O. aldridgei Zone sensu Zhang & Barnes (2002).This interval was previously referred to as the aldridgei fauna of the Distomodus staurognathoides Zone ( Uyeno & Barnes 1983).According to Zhang & Barnes (2002), the O. aldridgei Zone includes also the top of the Jupiter Fm., which, based on the occurrence of Icriodella inconstans here, Uyeno & Barnes (1983) assigned to the I. inconstans Assemblage Zone.Although no Icriodella was found above the upper boundary of the Jupiter Fm., they also correlated the lower Chicotte Fm., the interval below the level of appearance of P. a. amorphognathoides, with this zone.Zhang & Barnes (2002) discarded the I. inconstans Zone in their revised zonation and stated that the O. aldridgei Zone is overlain by the P. celloni Zone.It seems that Zhang & Barnes (2002) correlate the base of the P. celloni Zone on Anticosti tentatively with the boundary between the Jupiter and Chicotte fms.As Pterospathodus had not been found in the lowermost Chicotte Fm., they evidently used the appearance of Astropentagnathus irregularis Mostler to define the base of their P. celloni Zone.This agrees with data from other regions where A. irregularis appears together with P. eopennatus ssp.n. 1 Männik, together with the oldest form in the Telychian Pterospathodus lineage (Männik 1998(Männik , 2007a(Männik , 2008)).Astropentagnathus irregularis is known from the P. eopennatus ssp.n. 1 Zone only (Männik 2007a).Accordingly, it is evident that the lowermost sample from the Chicotte Fm. (sample C-92675 in Uyeno & Barnes 1983) comes from the P. eopennatus ssp.n. 1 Zone.As O. aldridgei is very rare in the eastern Baltic, the strata below the P. eopennatus ssp.n. 1 Zone are assigned to the D. staurognathoides Zone in this region (Männik 2007b; Fig. 3).
The next sample (417; locality A1561b) comes from the lowermost Chicotte Fm.It is the smallest sample in our collection and yielded only 10 conodonts, mainly Panderodus cf.unicostatus (Branson & Mehl) (Fig. 3).This fauna does not allow precise dating of the level.However, considering the high δ 13 C values from brachiopods of the same sample (2.8‰; Fig. 3), it is most probable that this sample comes from the P. eopennatus Superzone (possibly from the lowermost P. eopennatus ssp.n. 2 Zone; Fig. 3).The lowermost P. eopennatus ssp.n. 2 Zone is also indicated by the low number of conodont specimens in this sample: at the lower boundary of the zone the abundance of conodonts drops considerably (e.g.Männik 2005).
Sample 408-414 (locality A1562) is a composite one and includes seven small pieces of rock, all collected from a bryozoan mound but from different levels (distance between the lowermost and the uppermost sample is 5.5 m).Only 11 conodont specimens were found.Three of them definitely belong to P. amorphognathoides, probably to P. a. angulatus (Figs 3; 5J, K).Panderodus langkawiensis Igo & Koike (Fig. 5E) and poorly preserved specimens of Panderodus and Walliserodus are also found.This sample suggests the P. a. angulatus Zone, probably its lower part (see below; Fig. 3).
The highest sample (415; locality A1522) comes from the same conodont zone, about 25 m above the base of the Chicotte Fm.However, due to different lithologies (processed separately, indicated as 415a and 415b) the number of specimens in these subsamples differs considerably (Fig. 3).Subsample 415a consists of micritic reefal limestone, was 14.3 kg in weight, and yielded 367 conodont specimens.Subsample 415b comes from a bed of unsorted, mainly fine-grained bioclastic limestone in which complete brachiopod shells and big fragments of crinoid stems are common.The weight of 415b was 1.3 kg and it yielded 529 identifiable conodont specimens.Occurring in these samples, P. celloni (Walliser) (Figs 3;5C,D,G) indicates that these strata are older than the P. a. amorphognathoides Zone.Ozarkodina polinclinata estonica Männik (Fig. 5F, H, I) refers the samples to the lower and/or middle part of the P. celloni Superzone (to the P. a. angulatus or P. a. lennarti Zone).Although the elements of P. amorphognathoides in 415b are too poorly preserved to be properly identified, they look most similar to P. a. angulatus (Walliser) (Fig. 5L, N).Another indication of the P. a. angulatus Zone is the occurrence of Pelekysgnathus?sp. (Fig. 5M) in this sample.In Estonia Pelekysgnathus?sp.ranges from the upper P. eopennatus ssp.n. 2 Zone to the lower P. a. angulatus Zone (Männik 2007a).Also in the Cellon section Pelekysgnathus?sp.occurs together with P. a. angulatus (in sample 10D in O. H. Walliser's collection; pers.obs.by P. Männik).Other taxa most common in sample 415 include Aspelundia? fluegeli (Walliser) (Fig. 5O-S In summary, the stratigraphic position of at least the lower part of the Chicotte Fm. ranges from the P. eopennatus ssp.n. 1 Zone to the P. a. angulatus Zone.This time interval is characterized by significant changes in depositional environments, which resulted in extinctions in conodont fauna (Valgu Event, Männik 2005, 2007c; Fig. 4).The Valgu Event, as recognized in Estonia, started in the latest P. eopennatus ssp.n. 1 time (temporal disappearance of Astropentagnathus irregularis, = Datum 1 of the event) and ended in the middle of P. eopennatus ssp.n. 2 time (extinction of Aulacognathus kuehni Mostler, = Datum 6).Besides the extinctions of several taxa, changes in the environment also caused considerable decrease in the abundance of conodonts at the early stage of the event.

Fig. 2 .
Fig. 2. The position of the Viki core and distribution of Silurian sediments.R.S., Regional Stage.

Table 1 .
Stable isotope values from Anticosti

Table 2 .
Stable isotope data of the Viki core (' -' indicates samples where carbonate content was too low for isotope analysis) ).In the youngest sample (locality A1522; gen. et sp.indet Ozarkodina ex gr.gulletensis (Aldridge) Distomodus staurognathoides (Walliser) Panderodus sp.n.N Jeppsson Pelekysgnathus?sp. 13 Conodont ranges Fig. 3. Stable carbon isotope data from the uppermost Jupiter Formation and the Chicotte Formation and distribution of conodont taxa (solid line -continuous occurrence of a taxon; dotted line -sporadic occurrence of a taxon; filled box -reliable identification of a taxon; unfilled box -problematic identification of a taxon), zones and superzones in the Baltic region.