Acta Geologica Polonica, Vol. 49 (1999), No.2, pp. 133-143 406 Lagoonal to normal maril1e Late Silurian Early Devonian ostracode assemblages of the Eurasian Arctic ANNA F. ABUSHIK & IRINA O. EVDOKIMOVA All-Russian Geological Research Institute (VSEGEI), Srednii pro 74, St. Petersburg, 199/06, Russia. E-mail: [email protected]@IB2567.spb.edu ABSTRACT: ABUSHIK, A.F. & EVDOKIMOVA, 1.0. 1999. Lagoonal to normal marine Late Silurian - Early Devonian ostracode assemblages of the Eurasian Arctic. Acta Geologica Polonica, 49 (2), 133-143. Warszawa. In the Eurasian Arctic the ostracode faunas indicate mostly normal marine environments during the Late Silurian. The Lower Devonian deposits are characterized by ostracode assemblages, which indicate the presence of marine outer shelf to lagoonal environments during the Early Devonian. The dynamics of their taxonomic diversity and some examples of the marine and lagoonal assemblages are shown. Correlation of facially mono- and heterogenous deposits is based on ostracode data. Key words: Late Silurian, Early Devonian, Ostracodes. INTRODUCTION The Upper Silurian of almost the entire Eurasian Arctic is represented by various, usually fossiliferous, limestones. These limestones yield abundant ostracode faunas, reported for the first time by the first geological expeditions to Novaya Zemlya, Vaigach and the Timan-Urals in the thir­ ties, headed by LF. PUSTOVALOV, N.A. KULIK and others. The Upper Silurian successions in this area are represented mainly by monotonous facies, cor­ related by means of ostracode associations. The palaeontological description of this fauna began with the publication of GLEBOVSKAYA (1936) and 80° 1·40000000 is continued by one of us (ABusHIK 1962, 1970, 400 800 1200 11M 1980, 1997). By contrast, the Lower Devonian successions of Fig. 1. Geographical sketch-map of the Eurasian Arctic to show the region are represented by facially heterogeneous the location of studied areas strata, with very complicated inter-relationships. I - Spitsbergen; 2 - Novaya Zemlya; 3 - Vaigach Island; The ostracodes, found in most of the facies rep­ 4 - Dolgii Island; 5 - Timan-Pechora region; 6 - Polar and North resented in the Eurasian Arctic, appeared to be Urals (Kozhim and Shchugor rivers); 7 - Severnaya Zemlya 134 ANNA F. ABUSHIK & IRINA O. EVDOKIMOVA very useful both for stratigraphical purposes and lagoonal to normal marine. In lagoons they were for environmental analysis. Unfortunately, Early usually the dominant faunal element, forming local Devonian ostracodes are still inadequately known, mono specific populations. Although less abundant, in spite of a number of papers devoted'to them they were taxonomically more diverse in marine from different parts of the Eurasian Arctic: SOLLE'S shelf environments, where they were characterized (1935) study on leperdicopes from Spitsbergen; by a more uniform distribution. Shallow-water REIN (1936) and ABUSHIK (1961) worked on the shelf successions contain practically every ostra­ same group from Novaya Zemlya and the Polar code group, with leperditicopids, primitiopsi­ Urals respectively; studies of ostracode assem­ copins, beyrichiocopids and kloedenellocopids blages of Novaya Zemlya, Taimyr and Sette­ being especially numerous. The podocopids were Daban by POLENOVA (1974), and from Timan­ less common and, in contrast, dominated the outer Pechora and the Polar Urals by ABUSHIK (1980, shelf (deeper) settings, where they were associated .1 1982). Early Devonian non-Ieperditicopes of with myodocopids. Leperditicopids contributed to Eurasian Russia are currently being studied by L.L. this fauna only in very limited numbers. SCHAMSUTDINOVA and La. EVDOKIMOVA. This paper is based on a study of collections Both Late Silurian and Early Devonian ostra­ from Spitsbergen, Novaya Zemlya, Dolgii and codes were taxonomically diverse and constituted Vaigach islands, the Timan-Pechora region, the an ecologically variable group. Some of them, e.g. western slope of the Urals and Severnaya Zemlya leperditicopids, were euryfacial, being represented (Text-fig. 1). Data from other regions of Eurasia in a wide spectrum of environments ranging from are also included. The taxonomic concepts adopted 90 10080 ....Kl ......... :::::::::: :::' ... ::: ...... ................. .!R1 "..-~~-.... ........... :::::::::::~::::~ ==~~ ~ 70 ... :::::::::::::::;";"':"'>............ ... :::. .... .... ....: ... ... :::::::::::::::~:::.~.. ======~ 60 :::::::::::::::::::::::::Pr:::::::::,A:i:~: .~. :::===:!Be====;::======:= 50 ::::::::::~:::~:.::::=====~~ 4°i •• ~~ 30~ 20i==~ 10 o Timan-Urals region Dolgii Island Novaya Zemlya Sevemaya Zemlya (Gerd'yu Reg.St.) (Zelenets Beds) (Zapadnokhatanzeya (Ust'spokoinaya Fm) Fm) Palaeocopida Leperditicopida [] (except beyrichiacean ~ and hollinacean) eJ Eridocopida Kloedenellocopida Primitiopsicopina ~ (only kloedenellacean) '" D (only primitiopsacean) Platycopida Hollinocopina ~ II (only hollinacean) [0 Metacopida Beyrichicopina ~ (only beyrichiacean) D Podocopida Fig. 2. Relative abundance of the Ludlow ostracode assemblages at order or suborder level LATE SILURIAN - EARLY DEVONIAN OSTRACODE ASSEMBLAGES 135 here follow the Treatise (1961), with the recent Bispinitia aff. pigmaea, Cryptophyllus sp., emendations of ABUSHIK (1990). Leiocyamus aff. paulus, Signetopsis aff. michailensis, S. sp., Platybolbina angustimargina­ ta, Mesomphalus ? sp., Beyrichia aff. kureikiana, LATE SILURIAN Simplicibeyrichia aff. globifera, S. aff. imperson­ aUs, Calcaribeyrichia sp., Gannibeyrichia? sp., Except for those of Spitsbergen, the Late Bingeria aff. indistincta, B. aff. infrequens, B.jlu­ Silurian deposits of the Eurasian Arctic are repre­ ida, B. microrete, Saccarchites tumefactus, sented by marine successions. Eukloedenella kureikiensis, E. clivula, E. poste­ rioalta, Invisibila sp., Cytherellina inornata, C. emaciata, Kuresaaria ? sp. and Pseudorayella Ludlow ellipsoidea, as well as some new forms (see ABUSHIK, in press). The ostracodes from the Ludlow strata of the The Ludlow ostracode assemblages are repre­ Timan-Urals region, Dolgii Island, Vaigach Island sented by endemic and provincial species of the and Novaya Zemlya are invariably represented by widespread genera of leperditicopids (Leperditia, normal marine assemblages (Text-fig. 9). Schrenckia, Kiaeria), primit~opsicopins (Leio­ In the type section of the Gerd'yu Stage cyamus, Signetopsis, Scipionis) and beyrichio­ [Regional Stage in the Timan-Ural region; equiva­ copids (Asperibeyrichia, Beyrichia, Bingeria), as lent to the Ludlow Stage elsewhere] the ostracode well as some cosmopolitan species. However, fol­ assemblage consists of Tollitina minuta, lowing the Late Ludlow regression, which is well Herrmannina hebes, Kiaeria crassa, Signetopsis recorded in the Eurasian Arctic basins (Upper bicardinata, S. eobicardinata, Leiocyamus Gerd'yu, Zelenets, Zapadnokhatanzeya formations; paulus, L. variabilis, L. enucleatus, L. clausus, Severnaya Zemlya - Ust-Spokoinaya Formation), Sulcyamus parvisulcatus, S. grandisulcatus, ostracode faunal endemism increased. The endemic Scipionis ordinarius, S.? praecox, S.? dorsocosta­ leperditicopids (Tollitina, Bispinitia), primitiopsi­ tus, Beyrichia posterior, Simplicibeyrichia parva, copins (Sylciamus) and beyrichiocopids (Dolgitia, S. aff. globifera, Asperibeyrichia simplex, Eokloedenia, and a new genus of the Hexo­ Calcaribeyrichia conjluens, Bingeria punctulata, phthalmoididae, which remains to be described) B. indistincta, B. aff. infrequens, Eokloedenia characterize this part of the succession. subbacata, Eukloedenella grandifabae, E. In general, Ludlow successions are character­ kureikiensis, E. altifabae, E. posterioalta, E. ini­ ized by wide distribution of shallow water shelf qua, Kloedenella mucronata, K. calva, K. poste­ assemblages. They are characterized by great taxo­ rioalveolata, Eoevlanella dizygopleuroides, E. nomic variability (Text-fig. 2) with leperditicopids, costata, Cavelliria dubia, Cytherellina aff. inor­ beyrichiocopids, primitiopsicopins, kloedenel­ nata, Silenis modestus and S.? acutus (see locopids, platycopids, metacopids and podocopids ABUSHIK 1997). represented in almost equal amounts. In places, A similar assemblage was reported from the monotaxonic assemblages consisting either of lep­ Gerd'yu (Ludlow) of the Polar Urals (Kozhim river erditicopids (Herrmannina, Leperditia) or beyri­ section) by TSYGANKO & CHERMNYKH 1983), as chiocopids (Hexophthalmoididae) or primitiopsi­ well as from the equivalent strata of Dolgii Island copins (Leiocyamus) prevailed. Such assemblages (ABUSHIK 1980). reflect the specific character of short-term shallow­ In Novaya Zemlya (see PI. 1) and Vaigach ing, isolation and change of salinity. The monotax­ Island, the assemblages of the Khatanzeya onic association of Herrmannian aff. nana from Formation are characterized by more diverse lep­ the Upper Klenov Formation of Novaya Zemlya is erditicopids: Leperditia quinqueangulata, L. aff. an example of such an impoverished Ludlow lumea, Herrmannina hebes, H. aff. hebes, H. aff. assemblage (PI. 3, Fig. 1). nana, Schrenckia grandis, Sch. nordenskjoeldi, Bispinitia pigmea, Kiaeria crassa, K. kiaeri, K. ele­ gans etc. (ABUSHIK 1970, NEKHOROSHEVA 1981). Pridoli In Severnaya Zemlya, the assemblage of the Ust' -Spokoinaya Formation consists of In the Greben', the Regional Stage of the Herrmannina nana, H. aff. nana, Tollitina minuta, Eurasian Arctic that corresponds to the Pridoli, the 136 ANNA F. ABUSHIK & IRINA O. EVDOKIMOVA ostracode fauna is