Lower and Middle Ordovician conodonts from the subsurface of SE Estonia and adjacent Russia/Alam- ja Kesk-Ordoviitsiumi konodondid Kagu-Eesti puuraukudes.
The first summary on the subsurface geology of SE Estonia, based on data from deep boreholes, was published by Kajak (1962). A general overview of the stratigraphy, lithofacies and palaeogeography of the region was given in the monograph by Mannil (1966). According to the latter and the following publications (Gailite & Ulst 1975; Ulst 1976; Ulst et al. 1982), southeastern Estonia is located in the deeper part of the Baltic Basin. However, the most detailed data are available in unpublished reports of the Geological Survey of Estonia (Vaarsi et al. 1964; Kajak et al. 1975). New supplementary data on the Ordovician geology of southeastern Estonia were lately presented in special publications of the series Estonian Geological Sections (Poldvere 2001, 2005, 2007). Lower-Middle Ordovician conodonts have been identified in the Mehikoorma-421 drill core (Mannik & Viira 2005) and upper Middle Ordovician conodonts in the Valga-10 drill core (Mannik 2001).
In the present paper the Lower and Middle Ordovician conodont biostratigraphy and diversity changes in the deep-water facies are considered on the basis of the drill core material from SE Estonia. Preliminary results were presented at the Seventh Baltic Stratigraphical Conference in Tallinn, Estonia (Viira & Lofgren 2008).
Mannil (1966) and Jaanusson (1976) divided the Ordovician of the Baltoscandian Basin into confacies belts with specific sedimentological and palaeontological features (Fig. 1). The North Estonian confacies is represented by shallow-water carbonate rocks, while the Central Baltoscandian confacies extending to South Estonia and Latvia has more argillaceous sediments of the deeper shelf settings (Fig. 1). In the latter area the boundary of the lower and upper Tremadoc is connected with the development of a maximal submersion of the Jelgava depression, with continuous deepening up to the end of Kunda time (Mannil 1966; Ulst et al. 1982). Redcoloured calcareous sediments accumulated in the depression and on its slopes. The present study area in SE Estonia embraces the northeastern end of the Jelgava depression.
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The sedimentary bedrock strata of Estonia have a gentle southward dip (6-19'), so that the rocks exposed in North Estonia are in South Estonia available for study only in drill core sections. This regularity was disturbed tectonically in southeastern Estonia where the crystalline basement was uplifted in the late Silurian and Early Devonian, and the younger Ordovician and Silurian sedimentary rocks were eroded.
The lithology and fauna of the eastern part of the Central Baltoscandian confacies belt have mainly been studied in Latvia where a system of formations and members has been established (Mannil 1963; Springis 1974; Ulst & Gailite 1976; Ulst et al. 1984). The lithological units first distinguished in Latvia were introduced for southern Estonia by Mannil (1990) and Mannil & Meidla (1994).
The Ordovician sequence in southern Estonia begins with the Zebre Formation which belongs to the Varangu, Hunneberg and Billingen stages (Fig. 2). In SE Estonia the sediments of the Zebre Formation are represented by greenish-grey and reddish-brown glauconitic argillaceous dolomite with a thickness up to 4 m. A series of discontinuity surfaces occurs in the lower and upper parts of the formation. The succeeding Kriukai Formation corresponds to the Volkhov Stage and consists of reddish-brown argillaceous dolomite with dolomitic marlstone interbeds. The thickness of the unit is 9.1-17.6 m, with a maximum in the Dekshino-328 core section. The Sakyna Formation consists mainly of greenish-grey argillaceous dolomite with dolomitic marlstone interbeds, commonly 3-9 m thick. The lower boundary of the formation is marked by discontinuity surfaces in the Petseri-330 and Dekshino-328 core sections. The formation is of early to middle Kunda age. The Baldone Formation is represented by up to 12.6 m thick reddish-brown and greenish-grey argillaceous limestone and dolomite with marlstone interbeds, and is of late Kunda age. Ten discontinuity surfaces are found in the Petseri-330 section in the interval 418.8-423.3 m. The Segerstad Formation corresponds to the Aseri Stage and consists of up to 5.2 m thick reddish-brown argillaceous limestone and dolomite. In the lowermost part of the formation one or two discontinuity surfaces occur in the Dekshino-328 and Hino-452 core sections. The Stirnas Formation consists of thick greenish-grey mottled reddish-brown argillaceous limestone and dolomite with marl interbeds. The unit is about 3 m thick in the Dekshino-328 section and is of Lasnamagi age. The uppermost Middle Ordovician Taurupe Formation of Uhaku age is about 10 m thick in the Dekshino-328 section. It is represented by grey nodular limestone and argillaceous dolomite with grey limestone and marl interbeds.
Conodonts were studied in the Tsiistre-327, Hino-452, Laanemetsa-70, Petseri-330 and Dekshino-328 core sections, situated in the most southeastern corner of Estonia and adjacent Russia, on the northern slope of the Lokno-Moniste uplift (Puura & Vaher 1997) (Fig. 1). The conodonts from these sections were preliminarily identified during the geological mapping in the 1960s1970s. The sampling interval in these cores varied depending on the lithology and necessity of defining the stage boundaries. All together 73 samples were investigated. The samples were small, with a maximum weight of ~300 g. The smallest sample of 30 g came from the Zebre Formation. For this paper some residues were picked additionally, all conodonts were re-examined and identified in the multielement taxonomy. The illustrated specimens of conodonts belong to the collection No. 594, which is housed in the Institute of Geology at Tallinn University of Technology (institutional abbreviation GIT).
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DISTRIBUTION OF CONODONTS
Petseri-330 core section
Twenty-one samples from the Lower and Middle Ordovician part of the section were studied (Fig. 3). The zonal species Paroistodus proteus was identified in three samples, but the complexes of species in these samples were somewhat different. The lowest sample of greenish-grey glauconitic argillaceous dolomite from 444.5 m contains mixed fauna, rare Paltodus deltifer--the index species of the underlying zone, Paroistodus numarcuatus and rare Paroistodus proteus. The sample of reddish-brown dolomite with glauconite grains from 444.25 m yielded numerous P. proteus and Drepanodus arcuatus, and rare specimens of an early form of Acodus deltatus (Fig. 4E, G). The third sample of violet-brown argillaceous dolomite from 444.0 m with large glauconite grains contains a more diverse conodont fauna, besides numerous P. proteus (Fig. 4A-C), D. arcuatus and A. deltatus (Fig. 4F), the first representatives of the genera: Periodon cf. primus (Fig. 4M, N), Prioniodus sp. (Fig. 4O, S), Scandodus furnishi (Fig. 4K) and Drepanoistodus forceps.
The Kriukai Formation in the interval from 442.1 m up to about 428.0 m includes reddish-brown argillaceous limestone in the upper and dolomite in the lower part. Two samples, from 438.0 and 433.9 m, belong to the Paroistodus originalis Zone where rare and fragmentary Baltoniodus navis occur. Almost complete Pa specimens of Lenodus variabilis (Fig. 5B, C) are found in the sample at 424.0 m, from the grey argillaceous dolomite of the Sakyna Formation.
The conodont fauna changes upwards from the level of 420.4 m, in the reddish-brown argillaceous dolomitic limestone of the Baldone Formation. The change is expressed first of all by the appearance of numerous specimens of Baltoniodus medius (Fig. 6A, B), Semiacontiodus cornuformis and Scalpellodus gracilis. The Eoplacognathus pseudoplanus Zone is defined by the index species (Fig. 5D, E, G-I) and the upper subzone by Microzarkodina ozarkodella (Fig. 7B) in the sample from 416.9 m.
The reddish-brown argillaceous limestone of the Segerstad Formation yielded conodont species of the Eoplacognathus suecicus Zone, nominal species (Fig. 5J-M) in the samples from 411.5 m and 408.4 m and Panderodus sulcatus from 413.0 m upwards. The sample from 410.2 m is rich in Baltoniodus specimens, mostly B. prevariabilis, but transitional specimens to B. medius are also found. The complete Pa and Pb elements of Yangtzeplacognathus foliaceus (Fig. 5N, O) define the Y. foliaceus Subzone in the Petseri-330 section.
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Dekshino-328 core section
Conodonts of the Zebre Formation were studied in three samples (Fig. 8). The lowermost sample (433.15 m) from yellow-grey glauconitic dolomite contains representatives of the Paroistodus proteus Zone, together with conodonts from the underlying Paltodus deltifer Zone. The next two higher samples of violet dolomite from 432.6 m and of greenish-grey dolomite from 431.65 m yielded conodonts of the Oepikodus evae Zone. The interval 411.7-429.3 m of reddish-brown argillaceous limestone and dolomite of the Kriukai Formation is very poor in conodonts. The specimen of biostratigraphic importance is the single M element identified as Baltoniodus triangularis (Fig. 6C) in the sample at 427.15 m in the lowermost part of the Kriukai Formation. This species defines the lower boundary of the Dapingian Stage and the base of the Middle Ordovician. Fragmentary specimens of the genus Lenodus appear in the uppermost part of the Kriukai Formation, at 412.5 m, and occur also in the Sakyna Formation (interval ~405.0-411.7 m). Fragmentary specimens of the nominal species of the Eoplacognathus pseudoplanus Zone were identified in the upper part of the Baldone Formation. Eoplacognathus cf. suecicus, Panderodus sulcatus, Costiconus ethingtoni and Complexodus sp. were identified at 394.8 m in the Segerstad Formation. Upwards in the section the subzonal species Baltoplacognathus robustus was found in the Stirnas Formation at 387.5 m (Fig. 9F-1, F-2) and Eoplacognathus lindstroemi in the Taurupe Formation at 379.6 m (Fig. 9G). The Ordovician sediments are covered by Devonian siltstones at 377.5 m (Vaarsi et al. 1964).
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Hino-452 core section
The Lower and Middle Ordovician sequence representing the whole Ordovician in the Hino-452 core is 30.1 m thick and is covered by Devonian sandstone at 489.0 m according to Kajak et al. (1975) (Fig. 10). Seventeen samples were taken, four of which from the interval 514.0-517.7 m were barren of conodonts. The two lowermost samples from the multicoloured (yellow, violet) glauconitic dolomite of the Zebre Formation yielded conodonts of the Paroistodus proteus Zone. The next three higher samples, from glauconitic dolomite, belong to the Oepikodus evae Zone with the index species illustrated in Fig. 11D. The reddish-brown argillaceous dolomite of the Kriukai Formation contains sparse conodonts. The samples from 503.6 m (Sakyna Formation) and 499.5 m (lower Baldone Formation) mark an interval where fragmentary specimens of the genus Lenodus (Fig. 9H, I) are found. A large number of conodonts were obtained from the reddish-brown argillaceous dolomite in the upper part of the Baldone Formation, from 493.7 m. This diverse conodont fauna, including Eoplacognathus pseudoplanus (Fig. 9D), Histiodella kristinae (Fig. 9N, O) and Microzarkodina ozarkodella, indicates the M. ozarkodella Subzone of the E. pseudoplanus Zone (Lofgren 2004). Complexodus sp., identified at 490.1 m, is usually found in northern Estonia in the Eoplacognathus suecicus Zone (Viira et al. 2001). The highest sample (488.9 m) from the Devonian dolomitic siltstone contains two redeposited conodont specimens and two thelodont (?) scales.
Tsiistre-327 core section
A detailed study of the Tsiistre-327 core has been published earlier (Poldvere 2007). According to Poldvere (2007), in this core section the Ordovician occupies only 8.7 m from the 494.7 m level upwards, lying below the Devonian sedimentary rocks starting at 486.0 m with weakly cemented sandstone.
Conodonts were studied in four samples, three from the glauconitic dolomite of the Zebre Formation and one from the uppermost brownish-red argillaceous dolomite of the Kriukai Formation (Fig. 12). The lowermost sample, from 494.65 m, represents the Paroistodus proteus Zone with P. proteus, Paltodus subaequalis (Fig. 4H, L) and Paltodus deltifer (Fig. 4J). In the next sample (494.45 m) the zonal species Oepikodus evae is found. The sample from 494.1 m represents also the O. evae Zone with quite numerous specimens of Stolodus stola, Oistodus lanceolatus and Drepanoistodus forceps. The fauna of the uppermost sample, from 493.5 m, with numerous Oistodus lanceolatus (Fig. 11H-L, P-R) and Drepanoistodus forceps (Fig. 11C) is similar to that in the underlying sample with the exception that it contains Trapezognathus cf. diprion and Microzarkodina russica. The last mentioned taxon is indicative of the uppermost O. evae and the Baltoniodus triangularis zones (Lofgren & Tolmacheva 2008).
Laanemetsa-70 core section
The conodont elements from the eleven samples of the Laanemetsa-70 core are rather poorly preserved, many specimens are corroded and broken (Fig. 13). Five samples from the reddish-brown and grey argillaceous dolomite of the Kriukai and Sakyna formations (374.7, 372.7, 370.7, 368.9 and 368.4 m) yielded the specimens of Lenodus sp. (Fig. 9E). A sample from the lower part of the Baldone Formation (367.0 m) is characterized by the zonal conodont Yangtzeplacognathus crassus (Fig. 9B). The zonal species Eoplacognathus pseudoplanus is found in the sample from 359.0 m (Fig. 9A). The specimens of Eoplacognathus cf. suecicus, Costiconus and Periodon appear in the conodont fauna of the three highest samples of red-coloured dolomite, at 353.6, 351.9 and 350.0 m. This interval may belong to the E. suecicus Zone.
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The conodonts found in the five borehole sections of southeastern Estonia represent 10 successive conodont zones and six subzones, from the Paroistodus proteus Zone to the Eoplacognathus lindstroemi Subzone, occurring in the stratigraphical interval from the Hunneberg to Uhaku stages. Due to small sample sizes and widely spaced samples, the biozone boundaries and full ranges of the zonal species are not always clearly defined here. Two kinds of biostratigraphical zones were distinguished in this study: taxon-range zones in the lower part of the sequence and lineage-zones of the platform conodonts in the remainder part (Murphy & Salvador 1998).
The Paroistodus proteus Zone is found in the Petseri-330, Dekshino-328, Hino-452 and Tsiistre-327 core sections, in the greenish-grey and reddish-brown glauconitic rocks of the Zebre Formation. The studied samples probably represent different parts of the P. proteus Zone. The species Paroistodus proteus, Paltodus deltifer, Paroistodus numarcuatus, Variabiloconus variabilis, Westergaardodina sp. and Cordylodus sp. are found in the lowermost part of the zone in the Petseri330 (at 444.5 m) and Dekshino-328 (at 433.15 m) cores. These are conodonts of the P. proteus Zone and conodonts from the underlying Paltodus deltifer Zone and even from older biostratigraphic units. Such a mixture of conodonts in the lowermost samples of the Zebre Formation is characteristic of SE Estonia and can be explained by erosion and redeposition of sediments. Redeposited elements are generally discoloured and broken. The edges of the elements are frequently worn away or the elements are recrystallized (Lofgren et al. 2005). The conodonts from the two lowermost samples in the Petseri-330 (at 444.5 m) and Dekshino-328 (at 433.15 m) cores may represent the lower Drepanoistodus aff. D. amoenus Subzone of the P. proteus Zone (Lofgren 1994). The samples of this zone in the Petseri-330 (at 444.0 m), Hino-452 (at 518.9 and 518.73 m) and Tsiistre-327 (at 494.65 m) cores contain, besides the zonal species P. proteus (Fig. 4A-C), also Acodus deltatus, Paltodus subaequalis, Prioniodus sp., Drepanoistodus forceps and Drepanodus arcuatus. These conodont species are characteristic of the second, Tripodus Subzone of Lofgren's (1994) subdivision. The upper two subzones of this subdivision are not found in the studied SE Estonia sections. Conodonts of the P. proteus Zone are known in the Zirni Member and of the Paltodus deltifer Zone in the Lutrini and Kumbri members of the Zebre Formation in western Latvia (Ulst et al. 1982). The P. proteus Zone has been established in the lower part of the Leetse Fomation (Hunneberg Stage) in northern Estonia (Viira 1974; Viira et al. 2001, 2006a; Lofgren et al. 2005).
The index species Prioniodus elegans of the succeeding zone is not found in the studied sections, maybe partly because of the presence of a gap in the sequence, partly because of the big interval between samples. The Prioniodus elegans Zone is poorly developed in most of the Swedish sections or occurs as reworked and fragmentary fauna in the base of the O. evae Zone (Lindstrom 1971; Bergstrom 1988). In northern Estonia the P. elegans Zone is known from the Maekalda and Saka sections (Viira et al. 2001, 2006a).
The Oepikodus evae Zone is defined in four core sections, Petseri-330, Dekshino-328, Hino-452 and Tsiistre-327. The specimens of the nominal species (Fig. 11D) are rare and fragmentary and occur only in samples from the lower part of the zone. The conodont fauna is generally diverse, including Oistodus lanceolatus, Scolopodus striatus, Stolodus stola, Paroistodus parallelus, Drepanoistodus forceps and Scandodus furnishi. The range of Protopanderodus rectus (specimens in Figs 14 and 15) begins in this zone. Ulst et al. (1982) have identified an analogous conodont fauna in the Kalvene Member of the Zebre Formation of western Latvia. In Sweden the O. evae Zone is represented in many sections, usually with the different conodont fauna in its lower and upper parts (Lofgren 1993). In northern Estonia the lower part with O. evae is present in the Maekula Member and the upper part without O. evae in the Paite Member of the Billingen Stage (Viira et al. 2001).
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The interval of reddish-brown argillaceous dolostone in the Kriukai Formation is represented by rare samples and a small number of conodonts. Three zonal species are found in this interval: Baltoniodus triangularis (Fig. 6C) at 427.15 m in the Dekshino-328 core, Baltoniodus navis at 422.8 and 418.4 m in the Dekshino-328 core and Paroistodus originalis at 438.0 and 433.9 m in the Petseri-330 core. This interval is remarkable for occurrences of large, in places numerous, specimens of Drepanodus and Protopanderodus. A specific feature of the fauna is also the small number of Baltoniodus and Microzarkodina specimens. According to R. Ulst (Ulst et al. 1982), B. navis, P. originalis and Microzarkodina flabellum are represented in the red marlstones of the Kriukai Formation in western Latvia. The B. navis and P. originalis zones are known in the Volkhov Stage in many outcrop and borehole sections of Estonia.
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The appearance of representatives of the genus Lenodus in the uppermost Kriukai Formation marks the beginning of the platform conodont lineages whose members become zonal species upwards. Lenodus sp. is determined in the Dekshino-328, Hino-452 and Laanemetsa-70 sections in the uppermost Kriukai, Sakyna and lower part of the Baldone formations. These fragmentary specimens may belong either to Lenodus antivariabilis from the upper subzone of the B. norrlandicus Zone or to the nominal species Lenodus variabilis. Complete specimens of L. variabilis are represented in the Petseri330 core (425.2 and 424.0 m, Fig. 5B, C, F). In northern Estonia the Lenodus variabilis Zone has been established in the Kunda Stage, in the Loobu Formation in the Taga-Roostoja (25A) and Kerguta-565 core sections (Viira & Mannik 1999; Viira et al. 2006b).
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The Yangtzeplacognathus crassus Zone is proved by a find of a complete specimen of the index species (Fig. 9B) at 367.0 m in the Laanemetsa-70 core and of the Sa element at 420.4 m in the Petseri-330 core. From the Y. crassus Zone level upwards, the conodont taxa are represented by more numerous specimens, including species of the genus Baltoniodus, which may indicate the shallowing of the basin (Lofgren 2003). In North Estonia Y. crassus has been identified in the Kerguta-565 core (at 187.7 and 187.2 m), in the lower part of the Loobu Formation of the Kunda Stage (Viira et al. 2006b).
The Eoplacognathus pseudoplanus Zone is represented by the nominal species in four core sections: Petseri-330 (depth 416.9 m), Dekshino-328 (401.5, 399.3 and 396.5 m), Hino-452 (493.7 m) and Laanemetsa-70 (359.0 m). The occurrences of Microzarkodina ozarkodella in the same samples in the Petseri-330 and Hino-452 cores determine the upper subzone of the E. pseudoplanus Zone (Lofgren 2004). The occurrence of M. cf. hagetiana, the index species of the lower subzone in the Y. crassus Zone (Petseri-330), is in accordance with the range of this species from the L. variabilis Zone up to the E. pseudoplanus Zone (Lofgren & Tolmacheva 2008). In southeastern Estonia the E. pseudoplanus Zone is found in the upper part of the Baldone Formation. In the Mehikoorma-421 core section E. pseudoplanus is also found in the upper part of the Baldone Formation (Mannik & Viira 2005). In North Estonia this zone is known from the Pakri, Loobu and Napa formations of the Kunda Stage (Viira et al. 2001, 2006b).
The nominal species of the Eoplacognathus suecicus Zone is found in the Petseri-330 core at 411.5 and 408.4 m and in the Dekshino-328 core at 394.8 and 391.0 m, on the level corresponding to the Segerstad Formation. The illustrated specimens of E. suecicus from the Petseri-330 core (sample from 408.4 m; Fig. 5J-M) are morphologically similar to the specimens from the E. suecicus Zone, illustrated by Zhang (1999, fig. 2). Usually Panderodus sulcatus appears in this zone. In North Estonia E. suecicus is found in the Aseri and uppermost Kunda stages in the Maekalda section and in the Aseri Stage in the Taga-Roostoja-25A section (Viira & Mannik 1999; Viira et al. 2001).
The next two zonal species Pygodus serra and Pygodus anserinus were not found but the nominal taxa of the three following subzones are present in the Petseri-330 and Dekshino-328 sections. The Yangtzeplacognathus foliaceus Subzone was identified by the presence of two complete specimens of the nominal species in the Stirnas Formation of the Petseri-330 core at a depth of 407.3 m (Fig. 5N, O). Yangtzeplacognathus foliaceus has been recognized in Estonia in the lower part of the Vao Formation of the Lasnamagi Stage (Viira 1967, 1974; Viira & Mannik 1999; Mannik & Viira 2005). The broken specimen of Baltoplacognathus robustus (Fig. 9F), identified in the sample from 387.5 m in the Dekshino-328 core, defines the subzone of the same name. The Baltoplacognathus robustus is found in the Taurupe Formation of the Ruhnu-500 core, and in the Vao Formation (Lasnamagi Stage) in many core sections of Estonia, such as Ohesaare, Kerguta-565, iamaa, Taga-Roostoja-25A and Kaagvere (Viira 1967; Viira & Mannik 1999; Mannik 2003; Viira et al. 2006b; unpublished material by the author). A complete Pa element of the subzonal species Eoplacognathus lindstroemi (Fig. 9G) was found at 379.6 m in the Dekshino-328 core, in a sample of grey argillaceous dolostone of the Taurupe Formation. This species is also known from the middle part of the Taurupe Formation in the Ruhnu-500 core (Mannik 2003). In northern Estonia E. lindstroemi has been identified from the upper Vao and Korgekallas formations of the Ohesaare, iamaa, Kerguta-565, Taga-Roostoja25A, Mehikoorma-421 and Kaagvere core sections (Viira 1967; Viira & Mannik 1999; Mannik & Viira 2005; Viira et al. 2006b; unpublished material by the author).
REMARKS ON CONODONT FAUNA
The Lower-Middle Ordovician conodont fauna of southeastern Estonia is comparable to the diverse and abundant shallow shelf fauna of northern Estonia and deeper shelf fauna of Sweden. In southern Estonia the red-coloured sediments of the Volkhov Stage were deposited in the deep shelf of the Central Baltoscandian facies belt (Mannil 1966; Ulst et al. 1982). The southeastern Estonian conodonts should be observed in three successive faunas. The conodont assemblage of the first Hunneberg-Billingen fauna is generally similar to those in northern Estonia (Viira et al. 2001, 2006a; Lofgren et al. 2005). The study of conodonts in the SE Estonian sections starts with the Paroistodus proteus Zone. The lowermost samples have mixed fauna with redeposited conodonts from the underlying Paltodus deltifer Zone. The same situation is recognized in the Uuga section, NW Estonia, where conodont elements of these two zones are mixed in the whole range of the P. proteus Zone (Lofgren et al. 2005). The Prioniodus elegans Zone is absent in the studied sections, possibly because the interval with Prioniodus elegans is often very thin. Another possibility is that this zone together with the upper part of the Paroistodus proteus Zone is absent because of a gap. The Prioniodus elegans Zone is poorly developed in most of the Swedish sections, or occurs as reworked and fragmentary fauna at the base of the O. evae Zone (Lindstrom 1971; Bergstrom 1988). In NW Estonia (Uuga, Keila-Joa) the conodonts of the Prioniodus elegans Zone are redeposited, while elsewhere in northern Estonia (Maekalda, Jagala, Varangu, Narva) the thickness of the zone is about 0.3-0.5 m, reaching 1 m only in the Saka section (Viira et al. 2001, 2006a; Lofgren et al. 2005). The Oepikodus evae Zone is commonly present in all previously investigated sections of Billingen age, both in North Estonian outcrop sections, and South and Central Estonian borehole sections, including unpublished core sections (Uuga, Maekalda, Jagala, Saka, Taga-Roostoja-25A, Kerguta-565, Ohesaare, Kaagvere, Karula, Abja). In Sweden this zone is well represented, and usually the upper part of the zone is without the zonal indicator (Lindstrom 1971; Lofgren 1993). The situation is similar in Estonian sections, where the zonal species occurs in the lower and Periodon flabellum in the upper part of the zone (Viira 1974; Viira et al. 2001).
The SE Estonian conodonts of the Volkhov fauna differ largely from the shallow shelf faunas of northern Estonia. The difference in the studied conodont fauna first of all lies in the small number of Baltoniodus specimens, particularly in the Kriukai Formation. Only single specimens determine the B. triangularis and B. navis zones in the Petseri-330 and Dekshino-328 cores. Usually Baltoniodus species are the most abundant in the shallow shelf settings of northern Estonia and also in deeperwater environments of Scandinavia. According to Lofgren (2003), Baltoniodus has comparable abundance maxima in shallow parts of the basin as well as in deeper parts. Lofgren (2003) also noted that some environmental factor other than water depth may influence its distribution. Rasmussen & Stouge (1995) considered the Baltoniodus biofacies to be typical of the shallow and deeper shelf environment. Secondly, Drepanodus and Protopanderodus found in great numbers and specimens in the upper Kriukai, Sagina and lower Baldone formations are frequently very large (Figs 14, 15). Drepanodus arcuatus and Protopanderodus rectus occur in almost all northern Estonian sections but never in such abundance and large sizes. According to Lofgren (2003, 2004), species of Drepanodus and Protopanderodus of the Swedish conodont faunas preferred the areas representing deeper parts of the epicontinental sea. As stated by Rasmussen & Stouge (1995), the Protopanderodus-Periodon biofacies characterizes the slope environment. It should also be noted that the studied sections contain only few specimens of Microzarkodina species, whereas the representatives of this genus are typical of fairly shallowwater settings (Lofgren & Tolmacheva 2008).
Consequently, the conodont associations in the deep shelf settings of SE Estonia have some peculiarities probably caused by multiple factors. The impoverish ment of the conodont fauna in the Volkhov Stage may be closely connected with maximal submersion of the Jelgava depression and currents in the deep shelf (Kiipli et al. 2009). The Jelgava depression in Latvia came into being in Hunneberg-Billingen time and deepening proceeded up to the end of Kunda time (Mannil 1966; Ulst et al. 1982). The occurrences of large specimens of some species (Drepanodus and Protopanderodus) in the lower Darriwilian could be explained by upwelling activity (Kiipli et al. 2010).
Conodonts of the subsequent Darriwilian fauna of SE Estonia became more variable and abundant, and more similar to the usual deep shelf assemblages, for instance in the Mehikoorma-421 and Tartu-453 sections (Poldvere 1998; Mannik & Viira 2005).
Acknowledgements. A. Lofgren (Lund) is greatly acknowledged for critically reading the manuscript and useful suggestions. The author is grateful to T. Tolmacheva (St Petersburg) and an anonymous referee for invaluable comments on the manuscript. The study was supported financially by the Estonian Ministry of Education and Research (project SF0140020s08) and the Estonian Science Foundation (grant ETF7674).
Received 23 April 2010, accepted 15 November 2010
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Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; firstname.lastname@example.org
Fig. 2. Stratigraphical scheme and conodont zonation of the Lower and Middle Ordovician used in Estonia (according to Nolvak et al. 2006, modified). Subzones of the Paroistodus proteus Zone after Lofgren (1994, 2000). Conodont zones and subzones in brackets are not found in SE Estonia. GLOBAL REGIONAL FORMATION CONODONT STAGE STAGE N Estonia SE Estonia ZONE Korgekallas Pygodus serra DARRIWILIAN UHAKU Vao Taurupe LASNA- Stirnas MAGI ASERI Kandle Segerstad Eoplacognathus suecicus Napa Eoplacognathus pseudoplanus KUNDA Loobu Baldone Yangtzepla- cognathus crassus Sillaoru Sakyna Lenodus variabilis (Baltoniodus norrlandicus) DAPITAN VOLKHOV Toila Kriukai Paroistodus originalis Baltoniodus navis Baltoniodus triangularis FLOIAN BILLINGEN Oepikodus evae (Prioniodus elegans) TREMADOCIAN HUNNE- Leetse Zebre Paroistodus BERG proteus VARANGU Varangu (Paltodus deltifer) GLOBAL CONODONT STAGE SUBZONE Eopl. lindstroemi DARRIWILIAN (Yangtzepl. protoramosus) Baltopl. rohustus (Baltopl. reclinatus) Yangtzepl. foliaceus Microzark. ozarkodella (Microzark. hagetiana) (Lenodus antivariabilis) (Trapezogn. quadrangulum) DAPITAN FLOIAN TREMADOCIAN (Oelandodus elongatus -Acodus deltatus) (Paracordylodus gracilis) Tripodus Drepanoistodus aff. D. amoenus
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|Date:||Mar 1, 2011|
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