NORWEGIAN JOURNAL OF GEOLOGY Arenig conodonts, northwestern Russia 161

The Ordovician Billingen/Volkhov boundary inter­ val (Arenig) at lava River, northwestern Russia

Tatiana Tolmacheva & Petr Fedorov

Tolmacheva, T. & Fedorov, P. The Ordovician BillingenNolkhov boundary interval (Arenig) at Lava River, northwestern Russia. Norsk Geologisk Tidsskrift,Vol. 81, pp. 161-168. Trondheim 200 l. ISSN 0029-196X.

A detailed study of the conodont distribution within the condensed carbonate beds around the boundary of Billingen and Volkhov Stages (Lower Ordovician, Arenig) in northwestern Russia confirms the presence of a continuous succession of conodont biownes recognized in contempora­ neous sequences in Scandinavia. The Baltoniodus triangularis and Microzarkodina sp. A zones are reported for the first time from East Baltica, where carbonates of this interval underwent extensive bioturbation and are characterized by a number of closely spaced and distinctive glauconi­ tic hardground surfaces. It has been revealed that the distribution of conodont elements around the hardground surfaces is strongly affected by bioturbation.

Tatiana Tolmacheva, Department of Historical Geology and Palaeontology, In stitute of Earth Sciences, Un iversity of Up psala, Norbyviigen 22, 75236 Upp sala, Sweden; Petr Fedorov, Department of Historical Geology, St. Petersburg State Un iversity, 7/9 Un iversitetskaja emb., 199034, St. Petersburg, Russia.

lntroduction zonation has not been determined across the Bil­ tingen-Volkhov boundary interval in most sections in In Baltica the boundary between the Billingen and Estonia and western Russia. The main object of this Volkhov Stages (lower Arenig) coincides with the base pa per is to give the first detail ed study of the conodont of Megistaspis polyphemus trilobite Biozone ( =Megis­ distribution and lithostratigraphy of this important taspis lata Biozone of Mannil & Meidla 1994). In prac­ interval. tice, however, it is usually located at a distinctive relati­ vely smooth hardground surface with characteristic borings and burrows. In Russia this level has been referred to the «Steklo» (glass) surface since Lamansky (1905), while in Estonia it has been described as the hardground surface with «amphora-like» borings (Orviku 1960). This hardground can be traced over a distance of almost 500 km along the Bal tie-Ladoga Glint from the Syas River in the east to the Pakri Island in the west, and it represents a first order regional lit­ hostratigraphic marker. In some sections in the vicinity of St. Petersburg (e.g., at Izhora and Nazya rivers) a thin layer of quartzose sand further accentuates the «Steklo» surface. Fedorov & Dro nov ( 1998) have recorded a flat pebble limestone conglomerate in one eastern locality, suggesting sub­ marine erosion. However, at most localities in the eas­ tern part of the glint it is accompanied by several clo­ sely spaced hardground surfaces covered by glauconitic veneers and containing numerous vertical borings. It is commonly difficult to determine which of these hard­ Fig. l. Photo of the sampledsection exposedon the left-handside of ground surfaces represents the "real" «Steklo» surface the Lava River canyon, downstream of the Vasilkovo village (locality (Dronov et al. 1996) since the trilobite and conodont 6817ofPopov et al., 1989). 162 T. Tolmacheva, & P. Fedorov NORWEGIAN jOURNAl OF GEOlOGY

Geological setting, localities and methods faces or thin clay beds. Each unit has a name given by local quarrymen, and most of the units can be traced The lithostratigraphy of the Arenig deposits east of St. over a distance of 200 km along the Baltie- Ladoga glint Petersburg was discussed recently by Dronov & Fedo­ in Russia. The lowermost four and a half units approxi­ rov (1995) and Dronov et al. (1996). These authors mately correspond to the Paite Member of the upper­ noted that uppermost Billingen and lowermost Volk­ most Billingen in Estonia, whereas the remaining l O hov deposits in this region are represented by well-bed­ units correlate to the Saka Member of the lower Volk­ ded bioclastic, glauconitic limestone that is informally hov. The distinctive «Steklo» hardground surface is called the "Dikari" (savages) Limestone. According to within the fifth unit, so-called Zelenyi (green) unit. Dronov et al. (1996) this interval can be subdivided Recent sedimentological studies (Dronov 1998) indica­ into 15 lithological units separated by hardground sur- ted that the bioclastic limestones of the "Dikari" beds

Battoscandian stages and conodontzones (/) E (/) (/) u Q) Q) Q) * ·c O> Conodont CD >. Q) zones en en �

30

Battoniodus navis > St o ..r::. .::t:. 25 813 � <( ei. c 1/) ro l1l ·- o O> 20 812 :§ ·- ·- >c .§l1l o Q) Battoniodus 811 N '- 810 'O triangularis g '- <( .!!2> 89 � l1l o c: 15 (l) Zel -5 .Q 88 c: <( l .2 ei. 1/) 87 Æ l1l c .s;; Q) "' 10 O> 1-- Fig. 2. Stratigraphic .� .§ l1l 86 column composed bythe N ,IJ> CD § photographed polished ::::: � § Q) :::s surfacesof the sampled .Q O> � c: successive levels from the 5 85 � c: ·� boundaryinterval, sho- Bg .... .g fJ) wing biostratigraphic 84 -5 subdivision and distribu- .g .Q æ c: tion of selected conodonts. Oepikodus Thedarker grey in the evae �l1l o column emphasizes the 81 Q) cm parts of the succession that is rich in glauconitic grains. NORWEGIANJOURNAL OF GEOLOGY Arenig conodonts, northwestern Russia 163 are mainly proximal tempestites, deposited somewhat below the seasonal storm wave base during unusually strong storms. The tempestites are separated by hard ground surfaces and sometimes covered by thin beds of silty clay, which probably represent the background deposits. The present study is based on a suite of bed-by-bed samples from a succession approximately 33 cm thick through the Billingen-Volkhov boundary interval, which is exposed in a natural outcrop on the left-hand side of the Lava River canyon. (Figs. l, 2, 5). This loca­ lity was mentioned and illustrated for the first time by Raymond (1916), and it represents a continuous expo­ sure from the Upper Cambrian Ladoga Formation to the Middle Ordovician Obukhovo Formation (Kunda Stage). This outcrop has already been discussed in many papers (Magi et al. 1989, Popov et al. 1989, Dro­ nov et al. 1995, and Pushkin & Popov 1999), and fur­ thermore proposed as a potential stratotype section of the Volkhov Formation (Fedorov, 2000). Four large blocks of limestone from the boundary interval were cut into 0.7 cm thick slabs across the bed­ Fig. 3. Block diagram showing structures of bioturbation in the ding surface (Fig. 2). The matrix between hardground uppermost Dillingen - basal Volkhov. «Steklo» surfaceindicated. surfaces was carefully separated from the rock infilling Samples as in Fig. 2. of vertical borings and sub-horizontal systems of bur­ rows. In total, fourteen limestone samples from these slabs were dissolved in buffered l O o/o acetic acid and ples BS, B6). The uppermost bed contains numerous the abundance of conodont elements in each sample trace fossils, which represented by thin (up to five milli­ was counted from the standard unit of residue (Table metres across) sharp-walled, sub-horizontal burrows of 1). One block (Zelenyi unit) was cut into 23 slabs, all Thalassinoides-typethat were excavated in the semi-con­ surfaces of which were used for a three-dimensional solidated sediment. The rare bioclasts in the uppermost reconstruction of the major sedimentological features bed include abundant calcified spicules of hexactinellid including the shape of hardground surfaces and trace sponges. fossils (Fig. 3). Zelenyi unit (lower part).-Yellowish-grey calcareous mudstone 8 cm thick with extensive bioturbation. Bio­ clasts are concentrated in the lowermost part of the unit and mostly represented by fragments of ostracodes, bra­ chiopods and rare calcified spicules of hexactinellid litho- and biostratigraphy of the Billingen­ sponges. This unit is characterized by the branching sub­ Volkhov boundary interval at lava river horizontal systems of Thalassinoide�type, together with large excavations of other types of trace fossils (Fig. 3). The studied samples cover three of the units recognized There are also more obscure traces of bioturbation, indi­ by Dronov at al. (1996) in the «Dikari» Limestone (Fig. cated by slight colour changes. The samples were collec­ 2). The taxonomic composition of conodont assembla­ ted from the matrix (sample B7) as well as from the cavi­ ges from these units is listed in Table l. The following ties, tunnels, and burrows (sample B8), which are filled sequence was recognized from the base upward: with bioclastic wackstone or packstone rich in glauco­ Lower Ordovician (Arenig), Billingen Stage, Lower Dilcari nite. The upper boundary of this bed coincides with a Member smooth hardground surface with a glauconitic veneer, Beloglaz unit.- Only the upper part of this unit was which is identified here as the «Steklo» surface. The sampled. This part consists of one bed of grey to yellow uppermost 3-5 cm below the hardground surface are calcareous mudstone, about 3 cm thick, with rare glau­ stained by a yellow iron oxide. Volkhov Stage, Up per conitic grains (samples Bl, B2) and numerous Trypani­ Dikari Member tes-type borings. The limestone infilling of the borings Zelenyi unit (upper part).- One bed of yellowish was sampled separately (sample B3). This is overlain by a grey, glauconitic packstone with obscure traces of bio­ 2 cm thick bed of grey calcareous packstone, with nume­ turbation is about 3 cm thick (samples B9-Bll). The rous large (up to l.S mm across) glauconitic grains lowermost part of the unit coincides with a poorly deli­ (sample B4) and the uppermost bed of red calcareous neated hardground surface that is only some millimetres mudstone, about 3-4 cm thick, lacking glauconite (sam- above the "Steklo"surface. 164 T. Tolmacheva, & P. Fedorov NORWEGIAN JOURNAL OF GEOLOGY OF GEOLOGY NORWEGIANJOURNAL ' Arenig conodonts, northwestern Russio 165

Staritskii unit.- One bed of greenish grey calcareous comprises up to 50 o/o of the total number of counted packstone, about 10 cm thick (samples B12-Bl4), with elements. Microzarkodina sp. A (20 o/o of the total num­ some poorly pronounced traces of bioturbation marked ber of elements) is also common. Periodon flabellum by a slight enrichment of glauconite. (Lindstrom) is confined only to the uppermost part of The general succession of conodont biozones and the zone. This interval also contains some rare elements stratigraphic ranges of the distinctive species (Tab le l, of Tripodus sp. and «Semiacontiodus» sp.. Microzarkodina Fig. 4) are closely comparable with those recorded from sp. A occurs in the infillings of borings, which penetrate southern Sweden (Bagnoli, Stouge & Tongiorgi 1988), the hard ground surface at the lower boundary of the central Sweden (Lofgren 1993; 1994), and in particular zone (sample B3). It suggests clearly that the underlying with those from northern Oland (Bagnoli & Stouge deposits have been contaminated due to the activity of 1997). The following four conodont assemblages are boring organisms. recognized in the Lava River section: Baltoniodus triangularis Zone.- This conodont Oepikodus evae Zone.- The matrix of the Beloglaz assemblage occurs only within a 2-3 cm narrow band unit (samples Bl-B2) contains a low diversity conodont above the "Steklo"hardground surface in the upper part fauna dominated by Drepanoistodus forceps (Lindstrom). of the Zelenyi unit (Fig. 2, samples B9-B11; Tab le l). It is Other taxa are rare and represented mostly by lang­ characterized by the first appearance of B. triangularis range species, such as Oistodus Ianceolatus Pander, Scolo­ (Lindstrom), which co-occurs with numerous long­ podus rex Lindstrom, Decoriconus peselephantis Lind­ ranging species, like D. forceps, Protopanderdus rectus strom and Drepanodus arcuatus Pander (Fig. 4). This (Lindstrom), S. rex Lindstrom, D. peselephantis and D. interval is referred formally to the Oepikodus evae Zone arcuatus. Elements of D. forceps are the most abundant in although the assemblage recovered from the samples this assemblage as well as in all other samples, however, lacks the index species. The absence of the O. evae (Lind­ the con tent of P. rectus increases in this zone and reaches strom) is also characteristic for the uppermost part of up to 15 o/o of the total number of conodont elements in this zone in Sweden (LOfgren 1993). There is also no the samples. Again, it is evident that the underlying record of Trapezognathus in the studied interval. Thus deposits have been contaminated by elements of B. tri­ the recognition of the Trapezognathus diprion Zone defi­ angularis, since it occurs sporadically in the vertical ned by the uppermost Billingen of northern Oland (Bag­ borings penetrating the "Steklo" hardground surface noli & Stouge 1997) is hardly possible. (sample B8), but is missing from the matrix of the Microzarkodina sp. A Zone.- This biostratigraphical underlying bed. unit was recognized by Bagnoli & Stouge (1997) in the Baltoniodus navis Zone.- The lower boundary of uppermost Billingen of northern O land. In the section at this zone is definedin the section by the first appearance Lava River Microzarkodinasp. A occurs in the uppermost of Baltoniodus navis (Lindstrom). The assemblage of this part of the Beloglaz unit (Fig. 2, samples B4-B6) and in zone is also strongly dominated by D. forceps, which the lower part of the Zelenyi unit between two major comprises up to 70 o/o of the total number of elements in hardground surfaces enclosing the lower part of the lat­ samples B12-Bl4 (Table 1). This zone ranges up to the ter (Fig. 2, sample B7; Table 1). The most abundant top of "Dikari" Beds and it is more than l.S m thick in conodont species in this assemblage is D. forceps, which the Lava River section.

Fig. 4. Conodonts fromthe Lava section. Figuredspecimens are housed in the Central Researeh Geological ExplorationMuseum (prefixed CNIGR Museum), St. Petersburg. A-D: Mierozarkodina sp. A. A. P element, 13060/120, sample B6, x16. B. Se element, 13060/121, sampleB6, x18. C. Sd element, 13060/122, sample B6, x18. D. Sb element, 13060/123, sample B6, x100. E-I: Periodon jlabellum (Lindstrom). E. Sa element, 13060/124, sample Bl, x61. F. Melement, 13060/125, sample Bl, x53. G. P element, 13060/126, sample B7, x64. H. P element, 13060/127, sample B7, x66. I. Sb element, 13060/128, sample B7, x51. ]-N: Tripodus sp. ]. Sb element, 13060/129, sample B5, x43. K. P element, 13060/130, sample B5, x56. L. Se ele­ ment, 13060/131, sample B5, x33. M. M element, 13060/132, sample B5, x34. N. Sa element, 13060/133, sample B5, x42. 0-R: Oistodus lance­ olatus Pander. O. Sa element, 13060/134, sample Bl, x59. P.M element, 13060/135, sample B7, x44. Q. P element, 13060/136, sample Bl, x45. R. Se element, 13060/131, sample Bl, x43. S-U: Protopanderodus reetus (Lindstrom). S. Symmetrieal element, 13060/138, sample B13, x68. T. Asymmetricalelement, 13060/139, sample B13, x44. U. Symmetricalelement, 13060/140, sample B13, x42. V.Drepanodus areuatus Pander, arcuatiform element, 13060/141, sample B13, x55. W: Deeorieonus cf.D. peselephantis (Lindstrom). W. Symmetrical element, 13060/142, sample B5, x92. X: Deeorieonus peselephantis (Lindstrom). X. Asymmetrieal element, 13060/143, sample B6, x94. Y,Z, AC, AF: Baltoniodus navis (Lindstrom). Y. Pb element, 13060/144, sample B13, x16. Z. Pb element, 13060/145, sample B13, x61. AC. Se element, 13060/146, sam­ ple B13, x92. AF.M element, 13060/141, sample B13, x58. AA, AB, AG: Baltoniodus triangularis (Lindstrom). AA. Sb element, 13060/148, sample B13, x100. AB. Sa element, 13060/149, sample B13, x15. AG. M element, 13060/150, sampleB13, x63. AD, AE. Trapezognathus quadrangulum Lindstrom. AD. Pb element, 13060/151, sample B13, x91. AE. M element, 13060/152, sample B13, x86. AH: "Semiaeontiodus" sp. AH. Asymmetrical element, 13060/153, sample Bl, x85. AI. Toxotodus gabriellae Lofgren. AI. 13060/154, sample B8, x85. A]:Scalpellodus sp. A]. 13060/155, sample B13, x93. AK: Seolopodus cf.S. rex. AK. Symmetrieal element, 13060/156, sample Bl, x56.AL: Seolopodus rex Lind­ strom. AL. Asymmetrieal element, 13060/151, sample Bl, x54. AM:Drepanoistodus cf.D. basiovalis (Sergeeva). AM. Oistodiform element, 13060/158, sample B13, x62. AN:Drepanoistodus forceps (Lindstrom). AN. Oistodiform element, 13060/159, sample B13, x66. AO: Drepan­ oistodus eontraetus (Lindstrom).AO. Oistodiformelement, 13060/160, sample Bl, x43. 166 T. Tolmacheva, & P. Fedorov NORWEGIAN JOURNAL OF GEOLOGY

Table l Conodontelement distribution in. saf!lples.A cross (+ fæ marks rare �peci�s, whichare recordedfrom the �tf:mp l:e, but not inclu- ded the counted conodonts. Greytndtcates samples ta n from the mfillmgofburrows, tunnel$and cavzttes. "' a � ::l "3 "' .9 !)() ::l · "' E 1': "' ...V "' "' 1': "' 0.. "' � ::l -� ... ·� � ...V ...... "' "' V ] < "' 1': A ::l ::l "' "' .... o ...... � "' � fr � Q.. a 1 ::l .B V V ::l :-9 "' 'O =." "' 1': "' o< ] !)() ::l "' '!> "' "' "' "' o ::l "' -� "' ::l ::l � 1': "' ... 1': "' ::l ... � V... 0.. "' =5 .... d � d "' "' "' .... .9 :a � ... o o "' d ,.Sl e ::l"' Q.. "' "' "' "' -5 N ... B "' "' ...... <;:: ::l ::l ::l "' i -� ::l ..!!! ::l "' d "' c "' 1': ... .g d "' � 1': "' "' o c "' ·S o o "'o "'::l .9 .9 c "8 "' :::s 8 o = 0.. ::l 0.. "' -� � c d "' � 'g"' � � "' "' ·� o "'o ...... o ... "å 0.. 0.. 0.. 0.. c o 8 o .8 � ... <;l !)() B B V a V · d � � � � -� ....o .e- ;:::: � ft ... lS �V 8 ... V'l... .:; <;l V -5 § � V'l o V'l V'l Cl Cl Cl Cl V) o u 0.. Cl E!::: = � æ � E!::: � 8

"' Bl4 + + + + + + 8 l 2 184 ;; 154 2 2 13 2 � .... . s:·> 10 "' B13 315 l 5 4 6 + + 26 9 + 11 lO 2 3 2 394 ,8:.;::

B9 222 o 16 l + + + 35 7 + l 3 3 288 �l.S

B8 96 + + 2 l 14 l 6 7 + l 3 3 2 136 "' B7 93 8 + + 8 38 l 8 7 4 2 l 26 l 197 � ..§ ...:: B6 48 + 2 + 7 11 3 4 2 4 l 76 158 1:1�e c:: B5 125 5 7 + 8 23 4 lO 8 4 52 2 248 bl).. � � B4 93 2 12 4 6 13 3 24 6 + 37 200 CQ 3 11 3 10 5 l 82 B3 49 "';;

B2 44 + 4 lO l 4 6 l 70 ]�·- ;.. &" o Bl 36 3 l 5 7 2 5 2 61

Discussion result of their negligible thickness, which does not exceed 10 cm in the examined section. This is the major In Sweden, the lower boundary of the Megistaspis difference from the Swedish sections, where the «Micro­ polyphemus Biozone, as defined by Nielsen (1995), zarkodina sp. A» Zone is up to 80 cm thick, and the B. coincides with or is only slightly above the base of the triangularis Zone is almost l m in thickness (Bagnoli & Baltoniodus triangularis Zone (LOfgren 1994). Howe­ Stouge 1997) (Fig. 5). It is possible that both zones are ver, in Estonia and the Russian part of the East Baltic present in many sections west of St. Petersburg and in the stratigraphic interval corresponding to the B. tri­ North Estonia, but remain unrecognized because of angularis Zone has not previously been recognized at insufficient sampling. the base of the Volkhov Stage. Earlier studies indicated In the Lower Allochton platform margin deposits that Baltoniodus navis appears for the first time just within the Norwegian Caledonides, the upper boundary below the distinctive "Steklo" hardground surface of the O. evae Zone has been defined by the first appea­ (Dronov et al. 1995, Fig. 2). This study undoubtedly rance of Microzarkodina flabellum (Lindstrom) while showed that such early appearance of B. navis is con­ neither Microzarkodina sp. A nor B. triangularis Zone is nected with contamination of the underlying beds by observed (Rasmussen 1994). The first appearance of M. active bioturbation. flabellum has been recorded below the first appearance B. triangularis Zone of the Swedish standard as well as of B. navis in the Horns Udde section of the northern «Microzarkodinasp. A» Zone recently definedby Bagnoli Oland. This was used for the recognition of the M. flabel­ & Stouge (1997) in northern Oland is present also in the lum interval Zone between the first appearance of the eastern part of the Baltoscandian Basin. The difficulty of index species and the first appearance of B. navis (Bag­ their recognition in the East Baltic sections is mainly as a noli & Stouge 1997). However, it is impossible to recog- NORWEGIANjOURNAl OF GEOlOGY Arenig conodonts, northwestern Russia 167

nize the proposed interval zone in the Lava section as M. The sediments within the investigated carbonate succes­ flabellumwas not recorded fromthe studied interval and sion in the Arenig of the East Baltic consist mostly of undoubtedly appears somewhere above the first occur­ bioclasts of suspension feeding organisms including bra­ rence of B. navis. A similar pattern of distribution of chiopods, pelmatozoans, bryozoans, and ostracodes, and these two species has been reported for central Sweden this faunal assemblage persisted more or less unchanged (Lofgren 1993; 1995). during the interval under consideration. The boundary In the Russian part of East Baltie «Microzarkodina sp.A» interval is characterized by numerous hardground surfa­ and B. triangularis zones only are preserved within extre­ ces that do not show any traces of major erosion or suba­ mely narrow intervals. The thickness of the remaining rial exposure. The absence of erosion is also confirmed part of Billingen-Volkhov succession is comparable by the rare clay lenses more than l.S m thick that accu­ throughout Baltoscandia. For example, the Oepikodus mulated within the B. triangularis Zone just on top of evae and Baltoniodus navis zones have approximately the the "Steklo" hardground. These clay lenses represent the same thickness in the sections at Lava River and in the basal part of organic mud mounds developed in several Horns Udde section of northern O land. locations along the Russian part of Bal tie-Ladoga Glint

1. Sjurberg section 2. North of Hems Udde section, Dalarna, Sweden Oland, Sweden ( Lofgren 1994) (Bagnoli & Stouge 1997) B. navis Zone Microzarkodina flabellum Zone , \ \ \ \ \ \ \ B. triangularis \ \ Zone \ 4. Lava section, St. Petersburg region \ 3. Maekalda section, \ Estonia (this paper) \ (Einasto et. al 1996) \ \ B. navis Zone B. tri angularis �' B. navis Zone Zone ' ?·=====----- : ��t1a����ris Zone

. - - - - Microzarkodina sp. A - - - - Microzarkodina ,' - - Z e ____gn, __ • sp. A Zone ,' O. evae Zone l 10 cm O. evae Zone l 1 l l l \ l l 10 cm l '\ l l \ l l \ evae l O. \ l Zone \ l \ l \ l \ l \ l \ l \ l \ l \ 1 \ O. evae \ ,' Zone \ l \ l \ l

Fig. 5. Correlation of the upper Billingen and lower Volkhov interval across Baltoscandia with schematic map, showing the location of the illu­ strated sections. 16 8 T. Tolmacheva, & P. Fedorov NORWEGIAN JOURNAL OF GEOLOG Y

(Fedorov & Dronov 1998). The data presented provide Fedorov, P. & Dronov, A. 1998: Ranneordovikskie organogennye post­ direct evidence for a significant period of non-deposi­ roiki severo-zapada Rossii. l. Gekkerovy gorby v "dikariakh" plit­ nogo kar' era Babino (The Early Ordovician organic mudmounds tion in the East Baltic near the boundary of the Billingen of north-western Russia. l. "Hecker"-type mudmounds of the and Volkhov Stages. Babino quarry). Ve stnik Sankt- Peterburgskogo Un iversiteta, ser.7: Unfortunately, now it is impossible to correlate the Geologia, Geographia 3(14), 23-45. conodont zonation with the local trilobite zonation, Lamansky, W. 1905: Die altesten silurishen Schichten Russlands (Etage because the key trilobite taxa have not yet been found 8). Memoires Comite Geologique. No uvelle Serie 20, 1-223. St. Petersburg. fromthis extremely condensed interval. Lofgren, A. 1993: Arenig conodont successions from central Sweden. Geologiska Foreningens i Stockholm Forhandlingar 115(3), 193-207. Lofgren, A. 1994: Arenig (Lower Ordovician) conodonts and biozona­ Acknowledgements. - T. Tolmacheva acknowledges grants (to Lars E. tion in the eastern Siljan district, Central Sweden. Jo urnal of Pa le­ Holmer) from the Swedish Natura! Science Research Council (NFR) ontology68(6), 1350-1368. and the Royal Swedish Academy of Sciences (KVA) that supported the Lofgren, A. 1995: The middle Lanna/Volkhov Stage (middle Arenig) of work in Uppsala. In Russia, T. Tolmacheva's studies were supported by Sweden and its conodont fauna. Geological magazine 132 (6), 693- the Russian Foundation of Fundamental Sciences (grant no. 99-05- 711. 65-290 ). P. Fedorov was supported by the Russian Foundation of Fun­ Magi, S., Viira, V. & A. Helje. 1989: On thecorrelation of the Tremado­ damental Sciences (grant 98-05-65-065) and by a grant (to Lars E. cian and Arenigian boundary beds in the East Baltic. Izvestia Aka­ Holmer) from the Royal Swedish Academy of Sciences (KVA). We are demii Na uk Estonskoy SSR, Geologiya 38(2), 63-67. grateful to Lars E. Holmer (University of Uppsala) and Leonid Popov Mannil, R. & Meidla, T. 1994: The Ordovician System of the East (National Museum and Galleries of Wales, Cardiff) for helpful com­ European Platform (Estonia, Latvia, Lithuania, Byelorussia, parts ments on earlier versions of the manuscript. This paper is a contribu­ of Russia, the Ukraine and Moldova). In: Webby, B.D., Ross, R.J.Jr. tion to the IGCP project no. 410, <