The Laurentian Ordovician chitinozoan zonation: some modifications and some remaining problems
Aïcha Achab1, Esther Asselin2 and Azzedine Soufiane3
1 Centre géoscientifique de Québec, Institut national de la recherche scientifique, INRS–ETE, 880 chemin Sainte–Foy, CP 7500, Québec, G1V 4C7. Canada. E–mail: email@example.com
2 Centre géoscientifique de Québec, National Resources Canada, Geological Survey of Canada, GSC–Québec, 880 chemin Sainte–Foy, CP 7500, Québec, G1V 4C7. Canada. E–mail: firstname.lastname@example.org
3 Centre géoscientifique de Québec, Institut national de la recherche scientifique, INRS–ETE, 880 chemin Sainte–Foy, CP 7500, Québec, G1V 4C7. Canada. E–mail: email@example.com
Key words: Chitinozoans. Biozonation. Correlation. Ordovician. Laurentia.
Several studies dealing with Laurentian chitinozoans have been undertaken since Achab (1989) proposed an Ordovician chitinozoan zonation for Quebec and western Newfoundland. These studies have tested and validated the proposed biozones, refined the existing correlation schemes and led to the identification of some remaining problems. During this time the stratigraphic scale of the Ordovician System has evolved as the result of improved definitions of its global stages and series boundaries. These developments have resulted in the revision of the left colum of the chitnozoan zonation. This paper will present some of these modifications.
A new upper Tremadocian chitinozoan zone
The Conochitna symmetrica Zone is the oldest Ordovician chitinozoan zone defined by Achab (1980, 1989) in strata of the Levis Formation belonging to the Tetragraptus approximatus graptolite Zone.
In their report to the ICS–Subcommission on Ordovician stratigraphy, Williams et al., (1999) reported a Tremadocian chitinozoan fauna characterized by Lagenochitina maxima from bed 8 of the Ledge section, Cow Head Peninsula, Newfoundland, belonging to the Aorograptus victoriae graptolite Zone. This zone is situated below the first appearance of T. approximatus. In Quebec we have also recognized the occurrence of L. maxima in samples collected at Osborne’s N locality in the Lauzon cemetery at Levis below an assemblage containing C. symmetrica. From this locality, Maletz (1997) collected a fauna with Clonograptus rigidus and Stellatograptus stellatus, which he tentatively correlated to the A. victoriae Zone of Western Newfoundland, followed by another fauna with Araneograptus murrayi and the lowest specimens of T. approximatus. These new data lead us to propose a Lagenochitina maxima Zone (Figure1) for the upper Tremadocian. This is the oldest chitinozoan zone of Laurentia and is succeeded by the C. symmetrica Zone indicative of the base of the "Second Stage".
Darriwilian chitinozoan zones
Four graptolite zones form the Darriwilian stage. In Quebec, the Shumardia Limestone of the Levis Formation belongs to this stage. From the limestone, Maletz (1997) recognized the Undulograptus austrodentatus, U. dentatus and Holmograptus lentus graptolite zones, while Achab (1982) previously described two chitinozoan assemblages. The first chitinozoan assemblage characterized by Lagenochitina sp. is deemed by Maletz to belong to the upper part of the austrodentatus Zone and the second one, typified by Conochitina pirum, but containing also Conochitina poumoti and Fustichitina langei, would correspond to the U. dentatus Zone. No sample were processed from the H. lentus Zone.
In Newfoundland, in samples from the Table Head Group in the West Bay Centre Quarry, Achab (1983) reported a chitinozoan microfauna characterized by C. pirum in association with C. poumoti and F. langei, but containing also such new species as Rhabdochitina turgida and Conochitina subcylindrica. These strata were correlated by Maletz (1997) to the upper part of the H. lentus Zone. In the Cormorant Formation, Neville (1974) and Albany et al., (2001) reported a similar fauna, but with the addition of Cyathochitina jenkinsi and Belonechitina nevillensis.
C. pirum, C. jenkinsi and B. nevillensis (identified as C. robusta) have also been recovered by Achab (1986) in samples from the Port–Daniel–du–Milieu river outcrops, southern Gaspe Peninsula, belonging to the lower part of the Mictaw Group which had been attributed by De Broucker and Riva (1985) to the G. cf. G. teretiusculus graptolite Zone. Here, Belonechitina hirsuta and Cyathochitina campanulaeformis make their first appearance. The upper part of the Mictaw Group cropping out near the bridge of the Petite–Rivière–Port–Daniel river at Clemville has yielded graptolites from the G. cf. G. teretiusculus and Nemagraptus gracilis zones (Bourque et Lachambre,1980). The chitinozoan fauna at this locality is similar to that from the Port–Daniel–du–Milieu river locality, but the absence of C. pirum and the appearance of Lagenochitina sp. A is worth mentioning. The Darriwilian chitinozoan zonation is modified in light of the new paleontological data (Figure 1).
Upper Ordovician chitinozoan zones
Tanuchitina anticostiensis , Hercochitina crickmayi , Belonechitina gamachiana and Spinachitina taugourdeaui are the index–species of the four upper chitinozoan zones of the Ordovician defined by Achab (1989) from the Vaureal and Ellis Bay formations on Anticosti Island. A reappraisal of the chitinozoan distribution on Anticosti has allowed Soufiane and Achab (2000a) to recognize in the uppermost part of the Ellis Bay Formation a younger zone termed the Ancyrochitina ellisbayensis Zone. In terms of graptolite zones, the T. anticostiensis and H. crickmayi zones were encountered in strata of the Vaureal Formation belonging to the C. prominens (= D. ornatus) Zone (Riva 1969), while Melchin (2002) has correlated the S. taugourdeaui and A. ellisbayensis zones to the N. persculptus Zone.
Figure 1. Proposed changes to the Laurentian chitinozoan zonation.
The T. anticostiensis Zone has also been recognized in Nevada and Arctic Canada (Soufiane and Achab 2000). In Nevada, in strata belonging to the D. ornatus Zone, it is succeeded by the Hercochitina normalis and the Belonechitina tenuispinata zones. The short range of H. normalis permits a correlation to the lower part of the H. crickmayi Zone of Anticosti (Soufiane and Achab 2000b). Higher in the succession, comparisons with Anticosti are difficult as none of the chitinozoan index–species of eastern Canada have been hitherto recognized. The microfauna becomes quite distinct with Ordochitina nevadensis occurring in the P. pacificus graptolite Zone and Nevadachitina vininica restricted to N. extraordinarius and N. perscuptus zones.
These new paleontological data have led to some improvements in the chitinozoan zonation. Additional data, however, are needed to further refine some intervals.
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Received: February 15, 2003
Accepted: June 15, 2003