Advances in the Ordovician graptolite biostratigraphy of the St Petersburg area , Russia

The Ordovician shallow-water shelf deposits rich in benthic fauna are exposed along the Russian part of the Baltic Ladoga Klint. Graptolites occur only at some stratigraphic levels, being comparatively numerous in marls and clays and very rare in carbonate layers. Since the 19th century graptolites have been collected from exposures of the Pakerort, Hunneberg, Billingen, and Volkhov regional stages (TremadocianDapingian), while in the UhakuIdavere stratigraphic interval (Darriwilianlower Sandbian) rare graptolites have been found in boreholes. Up to now, only some collections have been systematically studied. On the basis of new graptolite finds, recently the Pterograptus elegans Biozone was established in the upper Aseri Stage and the Nemagraptus gracilis Biozone in the lower Kukruse Stage. Detailed sampling of the Shundorovo Formation (upper Idavere Stage) resulted in the discovery of numerous diplograptids at five stratigraphic levels, in association with various dendroids. These diplograptids were provisionally assigned to ?Archiclimacograptus antiquus lineatus Elles & Wood. A systematic description of the late Darriwilian and early Sandbian diplograptids is under preparation.


INTRODUCTION
The Ordovician shallow-water shelf deposits with an abundant benthic fauna and conodonts are exposed along the Russian part of the Baltic-Ladoga Klint.Graptolites occur at some stratigraphic levels, being occasionally numerous in marls and clays and very rare in carbonate layers.
Graptolites in the Ordovician sections of the St Petersburg area are known since the 19th century.During the 20th century they were collected by M. Yanishevskij, A. Lesnikova, V. Prinada, T. Alikhova, V. Goryanskij, and Yu.Dmitrovskaya among other researchers.The main graptolite sites are located along the Popovka, Lava, Volkhov, Lomashka, Syas´, Nazya, Izhora, and Narva rivers, also near Lake Dudergof.Graptolites are also found in some drill cores near Vaskelovo, Siverskaya, and Chudovo villages and other core sections (Fig. 1).However, old collections have not been systematically studied and graptolite samples not always reliably tied to the sections.Graptolites, collected by different geologists in the 1950s-1970s, were mainly identified by Prof. A. Obut (Obut et al. 1991).For the first time the authors summarized all graptolite finds, known by that time from drill cores and exposures.Although graptolites were not described and some old collections had no precise stratigraphic documentation, some zonal levels were recognized, among them the levels with Rhabdinopora flabelliformis s.l. in the Pakerort Stage, and Pseudophyllograptus densus Törnquist and Ps.angustifolius tenuis Monsen in the upper Billingen Stage.Some rare Late Tremadocian specimens, such as Araneograptus aff.murrayi (Hall), Kiaerograptus sp., Clonograptus aff.tenellus (Linnarsson), also Gymnograptus linnarssoni (Moberg) from the Uhaku Regional Stage, were recorded from drill cores.The presence of the Nemagraptus gracilis and Diplograptus foliaceus biozones was recognized in the upper part of the Ordovician succession (Fig. 2).
At the beginning of the 1990s, biostratigraphic studies of the Ordovician in the St Petersburg area became more intensive and systematic due to close collaboration between the stratigraphers and palaeontologists from the VSEGEI and St Petersburg University.The lower part of the Ordovician succession, spanning the Pakerort, upper Hunneberg-lower Billingen, and Volkhov regional stages, was studied more thoroughly.Numerous graptolites were collected by P. Fedorov, A. Ivantsov, L. Popov, A. Dronov, and T. Koren′ from the well-studied sections at several localities along the Russian part of the Baltic-Ladoga Klint (Putilovo, Babino, Ivangorod, and Kingisepp quarries, on the Popovka and Lava rivers) (Fig. 1).Presently, this stratigraphic interval is covered by comparatively complete graptolite data.Based on the systematic descriptions and biostratigraphic analysis, the Tetragraptus phyllograptoides and Expansograptus hirundo biozones of Baltoscandia were established (Tolmacheva et al. 2001;Koren et al. 2004) (Fig. 2).However, the Middle and Upper Ordovician exposures above the Volkhov Regional Stage have not been sampled for graptolites, although rare specimens are known from drill cores.
In recent years the present authors made an attempt to fill this gap in the graptolite knowledge.Field work at the Ordovician sections of the St Petersburg area was aimed at collecting graptolites from the middle Darriwilian and Sandbian stages, where only sporadic occurrences of rare specimens were known (Männil 1976;Obut et al. 1991;Nõlvak & Goldman 2004).Some new levels with graptolites were found in sections of the Aseri, Uhaku, Kukruse, and Idavere regional stages.

BIOSTRATIGRAPHY Lower Ordovician
The graptolite biostratigraphy of the Lower Ordovician deposits was studied in more detail.The Pakerort stratigraphic interval usually attracts much attention among stratigraphers.Clays of the Tosna and Koporye formations bear numerous specimens of the Rhabdinopora flabelliformis group.Substantial collections of L. Popov and N. Borovko have been studied and described by D. Kaljo and R. Sobolevskaya (Kaljo et al. 1986), who recognized several subbiozones in the Rhabdinopora flabelliformis Biozone: Rh. f. socialis, Rh. f. norvegica, Rh. f. multithecata, and Rh.f. anglica (Fig. 2).
The Tetragraptus phyllograptoides Biozone is the next well-documented level within the Lower Ordovician of the region.This biozone has been recognized in the upper Hunneberg and lower Billingen corresponding to the Floian Stage.It was better studied in the Putilovo quarry and in the well-exposed section along the Lava River (Fig. 1).Glauconitic sands and silty clays of the Lakity Beds of the Leetse Formation (Fm.) contain graptolites, conodonts, brachiopods, and other groups.Dichograptids described by T. Koren′ include Tetragraptus phyllograptoides Strandmark, Didymograptus rigoletto Maletz, Rushton & Lindholm, and Didymograptus aff.protobalticus Monsen (Fig. 2).The association is impoverished, however, each species is represented by numerous specimens.Didymograptids form a dominant part of the assemblage (Tolmacheva et al. 2001).

Middle Ordovician
The Dapingian and the lowermost Darriwilian stages yield the graptolite fauna of the Expansograptus hirundo Biozone at several stratigraphic levels (Fig. 2).The hirundo Biozone was recognized by the presence of the most representative graptolite assemblage in the Babino, Putilovo, and Kingisepp quarries and in sections on the  Lava River and near Ivangorod City (Fig. 1).Numerous graptolites were collected from several layers within the mud-mound clay beds of the Volkhov Fm., in association with the benthic fauna and conodonts.The graptolite diversity is relatively low and the assemblage includes no more then seven species in all localities: Expansograptus hirundo Salter, E. goldschmidti (Monsen), Xiphograptus svalbardensis (Archer & Fortey), Tetragraptus amii Elles & Wood, T. quadribrachiatus (Hall), T. reclinatus Elles & Wood, and Azygograptus volchovensis Koren (Koren et al. 2004).
The Middle Ordovician deposits of the Darriwilian Stage have the poorest graptolite record within the whole Ordovician succession in the St Petersburg area.Old unpublished data give evidence of the occurrence of Undulograptus austrodentatus (Harris & Keble) within the Kunda Regional Stage, but the location of the sample and its exact stratigraphic position are unknown (Fig. 2).The presence of chemically isolated 3D specimens of Gymnograptus linnarssoni (Moberg) has been mentioned by Obut et al. (1991), but they gave no precise information about the location of the drill core (Fig. 2).Nõlvak & Goldman (2004) revised nemagraptid finds in limestones of the East Baltic and documented Nemagraptus subtilis Hadding in the lower part of the Uhaku Stage in the Chudovo-50 drill core (Fig. 2).
Recently new graptolites were collected in the lower Aseri Stage.On the basis of these finds the Pterograptus elegans Biozone was established (Fig. 2).Rare and usually poorly preserved graptolites were found from nodular limestones in the quarry near Vil´povitsy village (Figs 1, 2).However, some good specimens of Haddingograptus oliveri (Boucek) in 3D preservation were found in association with fragmentary rhabdosomes of Pseudamplexograptus sp.indet.and Didymograptus sp.indet.Dendroid graptoloids are much more numerous, but their specimens are too fragmentary, which prevents even genus-level identification (Fig. 3).
A complete and quite well preserved specimen of Hustedograptus teretiusculus (Hisinger) was found in the Veltsy Fm., at the top of the Uhaku Stage in the Alekseevka quarry (Fig. 2).Limestones in the quarry bear an abundant benthic fauna, while possibilities of finding graptolites are extremely low.
Current studies by one of the present authors (A.S.) revealed new graptolite finds from well-documented sections of the lower Kukruse and upper Idavere stages.
The Kukruse deposits are well exposed and thoroughly studied in the central part of the Baltic-Ladoga Klint.Numerous benthic fauna including trilobites, brachiopods, bryozoans, and other groups occurs in kukersites and carbonate deposits.In the Alekseevka quarry and in the quarry near Dyatlitsy Village, Amplexograptus bekkeri (Öpik) was found in the lower part of the Kukruse succession (Figs 1, 2).Graptolites are exceptionally rare, but comparatively well preserved (Fig. 3).This species, typical of the N. gracilis Biozone, is well known in the lower Sandbian of the Baltic (Strachan 1960;Männil 1976).
The complete sections of the Shundorovo Fm. were studied in three locations near Kas′kovo, Paritsy, and Golubovitsy villages (Fig. 1).The most representative one is the Kas′kovo quarry section.Graptolites occur sporadically at some stratigraphic levels, but they are numerous and quite well preserved.The monospecific association is represented by ?Archiclimacograptus antiquus lineatus in association with numerous dendroid fragments.Dictyonema aff.maennili Obut & Rytzk is very abundant at some levels.Fragments of acanthograptids, Desmograptus sp., ?Ptilograptus sp., and some unidentified fragments of Pterobranchia, are also present (Fig. 3).

CONCLUSIONS
On the basis of present data on the Ordovician graptolites of the St Petersburg Region, we can draw the following conclusions: • graptolite associations are impoverished, typical of shallow-water environments, unfavourable for habitation and preservation of graptolites; • eleven graptolite biostratigraphic zonal assemblages have been recognized; they are tied to the conodont zonation in the same sections and can be correlated with those of Baltoscandia and, especially, of Estonia; • presently, almost all regional stages are characterized by more or less detailed or sporadic graptolite record; • the main goal of further studies is sampling in the Lasnamägi Stage sections, where graptolites have not yet been found; deposits of the Kunda, Jõhvi, and Keila stages require additional sampling.

Fig. 1 .
Fig. 1.Map of the St Petersburg District showing localities with the occurrences of graptolites.

Fig. 2 .
Fig. 2. The composite section of the Ordovician in the St Petersburg area, correlation of the graptolite and conodont biozonation for Russia and Estonia, and the overview of the present knowledge on graptolite biostratigraphy (contributions by different authors shown by numbers 1-3).DAPIN., Dapingian; Hunneb., Hunnebergian; Fm., Formation; Lasnam., Lasnamägi.Vertical ruling marks main gaps in the section.