of the graptolite and chitinozoan scales for the Altai–Salair Ordovician Basin
Nikolay V. Sennikov1 and Olga T. Obut1
1 Institute of Petroleum Geology, Russian Academy of Sciences, Siberian Branch, 630090, Novosibirsk, Ac. Koptyug av., 3, Russia. E–mail: SennikovNV@uiggm.nsc.ru
Key words: Graptolites. Chitinozoans. Altai–Salair Basin. Ordovician. Zonal scale.
The first attempts to generalize the chitinozoan data for purposes of the global startigraphic correlations were made by the international research group for the Ordovician Systems (Paris, 1992; Paris et al., 1999). It showed that chitinozoan biozonation for the particular regions is not studied enough yet, as well as substantiations for alignments of the chitinozoan zones with the standard graptolite and conodont zones of the International Stratigraphic Scale are required. Establishment of the chitionozoan zonation without regard to the graptolite zones, that is independent detailed stratigraphic scale approved for the Ordovician Systems within the International Stratigraphic Scale, seems to be difficult and can lead to many uncertainties. So the authors propose to use synthesis of both graptolite and chitinozoan scales rather than try to define independent chitinozoan one. Synthesis of graptolite and chitinozoan data and correlation allowed to avoid possible misses in the chitinozoan zonation and make right choice of the index–species. Species whose stratigraphic position was verified after a number of publications and also used for definition of biozones in the other regions as well were proposed as indexes. Data on successions of the chitinozoan associations from the following regions with well studied chitinozoans as Baltica, North Gondwana and Laurentia (North America) Ordovician basins were used for comparison (Paris, 1996, Paris et al., 1999; Nolvak and Grahn, 1993; Nolvak, 1999).
Graptolite zonal scale
The graptolite zonation established for the Ordovician of the Altai–Salair basin (Sennikov, 1996) was recently complete by new data. For example, Undulograptus austrodentatus (Harris et Keble) zone marking (GSSP) the basement of the Darriwilian Stage in the International Stratigraphic Scale was defined. In the Altai–Salair Ordovician basin Graptolite Biozones are established on the basis of complex of species and are assigned to the Asseblage Zones biostratigraphic units (Obut and Sennikov, 1986; Sennikov, 1996). The index–species, as well as 7 to 15, rarely 5–7 taxa presence in the composition of the mentioned zones that is also characteristic for all graptolite zones within the Altai–Salair Ordovician basin. Some index–species from the stratigraphically successive Assemblage Zones, for example densus and hirundo, also respectively belong to the Phyllograptus Hall and Expansograptus Boucek et Pribyl generic phylogenetic chains, on the basis of that in turn the Lineage Zones are defined in the Altai–Salair Ordovician basin
Figure 1. Synthesis of the graptolite
and chitinozoan scales for the Altai–Salair Ordovician basin.
No data – small amount of the studied rock materials and poor preservation of the chitinozoan vesicles.
The revised succession of the Assemblage Zones for the Ordovician of the Altai–Salair basin is proposed (see Figure 1). No zones have been established so far for the Lower Tremadocian of this paleobasin. The only graptolite associations found belong to beds with Denrograptus hallianus (Prout), Callograptus staufferi Ruedemann, Archaeodictyota dragunovi Obut et Zaslavskaya. For the Upper Tremadocian osloensis, hyperboreus zone is defined.
Five graptolite zones are recognized within the Arenigian: 1) approximatus, 2) densus (with balticus and densus subzones), 3) angustifolius elongatus, broggeri, 4) gibberulus, 5) hirundo, sparsus, austrodentatus. Three zones are established for the Llanvirnian: 1) kirgisicus, balhaschensis, 2) geminus, jakovlevi, coelatus, 3) teretiusculus.
Four zones are recognized within the Caradocian: 1) gracilis, serratulus, 2) peltifer, antiquus lineatus, 3) wilsoni, 4) quadrimucronatus (with clingani and linearis subzones). The upper part of the linearis subzone, supernus (with supernus and ornatus subzones) and persculptus zones are established for the Ashgillian Stage.
Three chitinozoan biozones and three complexes are proposed (see Figure 1, 2) for the Ordovician of the Altai–Salair basin, mainly on the basis of Altai data. Biozones are regarded as Assemblage Zones. One Assemblage Zones could be subdivided into three subzones. The middle subzone could be regarded as biozone with the index–species. Thus, totally four chitinozoan zonal biostratigraptical units and three complexes are proposed for the Ordovician of the Altai–Salair basin (Decisions..., 1983; Stratigraphy..., 1984; Sennikov et al., 1988).
In Ordovician of the Altai–Salair the most ancient is the chitinozoan association represented by the following species: Desmochitina minor typica Eisenack, D. minor erinacea Eisenack, coincide with graptolite osloensis, hyperboreus zone. No chitinozoans were found yet from the Lower Tremadocian of the Altai–Salair basin.
The succeeded biozone Con. raymondi is proposed for the Lower Arenigian of the Altai–Salair basin. The association includes index–species Conochitina raymondi Achab, Con. ordinata Achab, Con. turgida (Jenkins), Con. infraspinosa Wilson et Dolly, Laufeldochitina stentor (Eisenack). Mentioned chitinozoans were found together with graptolites of the densus subzone, densus zone.
Chitinozoan biozone Con. raymondi was established in North America (Laurentia) (Achab, 1989; Paris et al., 1999), and align to graptolite balticus zone, Lower Arenigian.
The next Con. parvicolla biozone can be recognized for the Upper Arenigian – Lower Llanvirnian of the Gorny Altai. The lower part of the mentioned zone is defined for the Upper Arenigian, hirundo, sparsus, austrodentatus graptolite zone. It is characterized by Conochitina parvicolla Taugourdeau, Con. infraspinosa Wilson et Dolly, Conochitina sp., Desmochitina minor amphorea Eisenack, D. minor typica Eisenack, Desmochitina cf. minor elongata Eisenack, Lagenochitina aff. deunffi Paris, Hercochitina sp., Rhabdochitina sp.
In the middle part of the mentioned above Con. parvicolla biozone coincide with graptolite kirgisicus, balhschensis zone Cyath. calyx subzone is defined. The following chitinozoan association was obtained: Cyathochitina calyx (Eisenack), Cyath. tuloyensis Obut et Zaslavskaya, Conochitina parvicolla Taugourdeau, Con. oelandica Eisenack, Con. simplex Eisenack, Con. bacillum Obut et Zaslavskaya, Desmochitina minor cocca Eisenack, Rhabdochitina regula Obut et Zaslavskaya (Zaslavskaya et al., 1978).
Conochitina parvicolla Taugourdeau was known previously from the Upper Ordovician of Oklahoma, USA (Taugourdeau, 1965). Cyath. calyx biozone was defined for Ordovician of the North Gondwana (Paris, 1996; Paris et al., 1999), for the transitional Arenigian–Llanvirnian beds (upper part of the graptolite hirundo zone and lower part of the graptolite "artus" zone).
The upper part of the Con. parvicolla biozone is characterized by Conochitina parvicolla Taugourdeau and Con. bacillum Obut et Zaslavskaya found together with graptolites of teretiusculus zone.
The succeeding L. dalbyensis, D. lecantiella biozone is recognized for the Caradocian and Ashgillian of the Altai–Salair basin. The following association was obtained: Lagenochitina dalbyensis Laufeld, Desmochitina lecantiella Eisenack, D. minor cocca Eisenack, D. minor erinacea Eisenack, D. sphaerica Eisenack, Desmochitina aff. juglandiformis Laufeld, Conochitina brevis conica Taugourdeau et Jekhowsky, Con. aff. acuminata Eisenack, Cyathochitina sp.
L. dalbyensis biozone was defined in the Ordovician of the North Gondwana and Baltica (Paris, 1996; Paris et al., 1999; Nolvak, 1999), in Caradocian, lower part of the graptolite wilsoni zone (= middle part of multidens zone). Desmochitina lecantiella Eisenack is known from Caradocian of Dalarna, Sweden and from Middle and Upper Ordovician of Estonia (Laufeld, 1967; Eisenack, 1965, 1968).
The next association contained Conochitina micracantha Eisenack, Tanuchitina ontariensis Jansonius, Cyathochitina sp., together with graptolites belong to supernus subzone, supernus zone was found from middle part of Ashgillian strata. Conochitina micracantha Eisenack was obtained from the uppermost Ashgillian of Altai–Salair Ordovician basin and coincide with persculptus graptolite zone (Obut and Zaslavskaya, 1980).
It should be noted that different stratigraphic position was discovered for two chitinozoan species, Laufeldochitina stentor and Lagenochitina deunffi, previously defined as index–species for the Baltica and North Gondwana regions respectively. In the Baltica paleobasin L. stentor biozone was established for Lower Caradocian, graptolite gracilis zone (Nolvak, 1999; Paris et al., 1999). In the Altai–Salair paleobasin species Laufeldochitina stentor (Eisenack) occurred on the lower stratigraphic level. It was found from in Middle Arenigian, graptolite densus subzone, densus zone. Moreover this species was obtained from the Upper Arenigian strata of the central part of Russian Platform (Moscow Syneclise) (Obut, 2002).
L. deunffi biozone was defined in the Lower Caradocian of the North Gondwana, upper part of graptolite gracilis zone (Paris, 1992, 1996; Paris et al., 1999). In the Ordovician of the Altai–Salair basin species Lagenochitina deunffi Paris is found from the upper Arenigian, together with graptolites belong to hirundo, sparsus, austrodentatus zone.
Figure 2. Range scale of the chitinozoan taxa from the Altai–Salair Ordovician basin.
Studies were supported by the RFBR Grant Nº 02–05–64789.
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Received: February 15, 2003
Accepted: June 15, 2003