Andrei V. Dronov1, Tatiana N. Koren2, Tatiana Ju Tolmacheva3, Lars Holmer3 and Tõnu Meidla4

1 Department of Historical Geology, St. Petersburg State University, Universitetskaja emb. 7/9, 199034, St. Petersburg, Russia. E–mail: dronov@GG2686.spb.edu

2 All–Russian Geological Research Institute (VSEGEI), Sredny pr. 74, 199106 St. Petersburg, Russia. E–mail: koren@vsegei.sp.ru

3 Institute of Earth Science, Department of Paleobiology, Uppsala University, Norbyvägen 22, Se– 75236 Uppsala, Sweden. E–mail: tatolm@hotmail.com, lars.holmer@pal.uu.se

4 Institute of Geology, University of Tartu, Vanemuise 46, 51014, Taru, Estonia. E–mail: tmeidla@math.ut.ee.

Key words: Global Stratigraphy. Stages. "Volkhovian". Ordovician. Baltoscandia.

In 1996 the International Subcomission on Ordovician Stratigraphy voted in favor of a subdivision of the Ordovician System into three series and six global stages (Mitchell et al., 1997). Four of the six stages have yet to be formally defined and named, and the Volkhovian (based on a section on the Volkhov River, near St. Petersburg, Russia) is, perhaps, the most appropriate name for the global third stage of the Ordovician System (Webby, 1998). The Volkhovian is one of the best defined and well known of the Ordovician regional stages in Baltoscandia. Deposits of this age are widespread all over the Russian Platform, and they possess very distinctive lithological and palaeontological characteristics, which make them easily recognizable both in outcrops and drill cores. The most biostratigraphically significant groups in the Volkhovian fauna have traditionally been trilobites, brachiopods, conodonts, and graptolites. However, echinoderms, bryozoans, ostracodes, and acritarchs as well as trace fossils are also important components of the fauna.

The Ordovician succession in St. Petersburg Region has had a long history of investigations. The deposits now recognized as the Volkhovian were first established by Schmidt (1858) as the "Chlorite limestone" and after renamed the "Glauconite Limestone" and indexed as the BII Stage (Schmidt, 1881). At the beginning of the 20th century, a very detailed stratigraphy for the Volkhovian deposits of the St. Petersburg region and Estonia was established by Lamansky (1905), who introduced the a, b and g indexes for the existing BII subdivision of Schmidt (1881). The name Volkhov (or Walkhov Formation) was introduced by Raymond in 1916. Later it was converted into the Volkhov Stage by Balashova and Balashov (1959), and a slightly emended and reduced definition was introduced by Männil (1966). Since that time it has been widely used all over the Baltoscandian basin. During the last decade, the classical Volkhovian sections in the St. Petersburg region have been the subject of intensive multidisciplinary study (Dronov et al., 2000, and references therein).

The limestones of the Volkhovian Stage have been extensively quarried for building purposes ever since the foundation of the City of St. Petersburg. The tripartite subdivision of the Volkhovian Stage reflects its traditional subdivision into the "Dikary Limestone" = BIIa, the "Jeltiaky Limestone"=BIIb, and the "Frizy Limestone"=BIIg by the local quarrymen. The base of the Volkhovian Stage coincides with a prominent hardground surface, which can be traced over a distance of more than 1000 km and represents a first order stratigraphic marker (Dronov and Holmer, 1999). However, erosion of the underlying sediments, as well as facies shift and changes in sedimentary environments below and above this surface have not been documented. The "Dikary Limestone" consists of ten distinctive beds traceable over a distance of more than 250 km. These rocks are relatively coarse–grained bioclastic packstones and grainstones with abundant glauconite grains. The "Jeltiaky Limestone" differs from the underlying rocks in having more argillaceous material and numerous clay layers coloured in red and yellow. Glauconite is usually rare or absent. The unit consists of seven beds and bedsets. The "Frizy Limestone" consists of flysch–like intercalations of greenish gray, glauconitic bioclastic limestones and bluish gray clays. Seven beds and bedsets are recognizable in the succession. The Volkhovian/Kundan boundary is represented by erosionally truncated surface (Dronov et al., 2000) (Figure1). Besides these two distinctive hardground surfaces the Volkhovian interval is characterized by some less significant ones that do not reduce the amount of biostratigraphical information available from the sections.

The Volkhovian cool–water bioclastic limestones of St. Petersburg region are interpreted as calcareous tempestites, which were deposited in a storm–dominated, shallow–marine environment close to a ramp depositional system. The Volkhovian is regarded as a transgressive episode in the history of the basin (Männil, 1966) with a complete cycle of relative sea–level changes forming a well developed depositional sequence (Dronov and Holmer, 1999).

The base of the stage is interpreted as a type 2 sequence boundary. The "Dikary Limestone" corresponds with a lowstand (shelf margin) systems tract, while the "Jeltiaky" and "Frizy" seem to represent transgressive and highstand systems tracts, respectively. A more detailed sea–level curve has been reconstructed for the Volkhovian of the region on the basis of lithofacies analysis. Overall, the curve is comparable with that constructed by Nielsen (1995) for southern Scandinavia, except for major differences in the interpretation of water depth in the middle part of Volkhovian.

Figure 1. Stratigraphic column of the "Volkhovian" succession in St. Petersburg region showing the distribution of graptolites and selected conodonts, sea–level curve and sequence stratigraphic framework. Legend: 1) Limestones with iron oolites; 2) Bioclastic wackestones and packstones; 3) Thalassinoides burrowing systems; 4) Marls; 5) Clays; 6) Quartz sandstones; 7) Quartz sandstones with scattered glauconite grains; 8) Black shales; 9) Hardground surface with Trypanites–like borings; 10) Hardground with pencil–like borings; 11) Uneven hardground surface evolved from firmground; 12) "Steklo" hardground surface with Gastrochaenolites oelandicus borings; 13) Planolites and other trace fossils; 14) Bergaueria at the bottom surface of the beds.

Graptolites. Graptolites occur sporadically in clay beds at several stratigraphic levels within the succession. The taxonomical diversity is relatively low and the Volkhovian graptolite associations include only a total of seven recorded species from all localities. Based on the graptolite zonation, the Volkhovian Stage can be correlated with the D. (E.) hirundo Zone of Baltoscandia and with the upper D. simulans to D. (E.) hirundo Zones of the British graptolite succession.

Correlation and recognition of the global stratigraphic boundaries. Two global correlation levels fall within the Volkhovian interval: 1) the Isograptus victoriae lunatus and Tripodus laevis first appearances, which is suggested for the definition of the lower boundary of the Middle Ordovician Series, and 2) the lower boundary of the Darriwilian Stage.

1) According to the graptolite data, the I. v. lunatus lower zonal boundary might fall within the graptolite bearing interval of the lowemost Volkhovian Stage. The conodont based correlation of T. laevis level is complicated by the markedly different fauna in Laurentia and Baltoscandia during this time interval. The conodonts of the Tripodus lineage appear in Baltoscandia in the upper Tremadocian P. proteus Zone and range up to uppermost Billingenian Stage (Tolmacheva, unpublished). Occurrences of Tripodus species in the Volkhovian sediments have not been reported.

2) The base of the austrodentatus Zone, approved as the base of Darriwilian in the Huangnitang section, is broadly coeval with the top of the local P. originalis Zone of Baltoscandia (Mitchell et al., 1997). However, graptolites show much better potential at the global scale than conodonts because this level is marked by the first appearance of diplograptids, which is one of the most important events in their Ordovician history. A few incomplete finds of early diplograptids have been reported from the Tøyen Shale and other units (correlative with the boundary interval of the simon and limbata trilobite zones) from the mid –upper Volkhovian Stage in southern Norway, Scania, and Latvia. However, the first rich fauna of Darriwilian diplograptids appear in the basal beds of the Kundan Stage. Noteworthy is that the first appearance of diplograptids (e.g., Undulograptus sinodentatus) is recorded in the late Yaapenian (Exigraptus clavus Zone) of China, slightly below the entrance of U. austrodentatus fauna, early Darriwilian in age (Chen Xu et al., 1995).

We assume that the name "Volkhovian" is an appropriate one for the third global stage of the Ordovician System mainly because of the following reasons. (1) The Volkhovian Regional Stage is widely used in correlation over Baltoscandia. Currently its usage expands further beyond the Baltic domain. (2) The available biostratigraphic data evidence that the Volkhovian Regional Stage embraces the unnamed third stage of the Ordovician and the lowermost part of the Darriwilian (Figure 1). However, in the case of adoption of the name "Volkhovian" for the third global stage, the further requirement of reclassifying the regional stage of the same name is open for discussion. (3) The Volkhovian deposits in the historical type area are continuously fossiliferous. The co–occurrence of diverse benthic as well as planktic and nektic faunas allows a detailed subdivision and precise regional correlation. Furthermore, the sedimentary environments are favorable for detection of short–term eustatic sea–level fluctuations, which have a high correlation potential. (4) The Volkhovian sections are well exposed along the Baltic–Ladoga Glint in the vicinity of St. Petersburg and are easily accessible for study.

Accepted: June 15, 2003