Bollettino della Società Paleontologica Italiana Modena, Novembre 1999 Late Silurian conodont biostratigraphy in the northern East Baltic Viive VnRA Institute of Geology Tallinn Technical University KEYWORDS- Conodonts, Biostratigraphy, Upper Silurian, East Baltic. ABSTRACT- Conodont biostratigraphy ofthe late Silurian Paadla to Ohesaare stages ofthe northern East Baltic is described based on the conodont fauna collected over a longperiod from the rock samples ofmany outcrops an d about 20 borehole sections. The co nodo n t distribution from shallow shelf (Kolka borehole) through open shelf (Ohesaare and Ventspils boreholes) to deeper p art (Pavilosta borehole) ofthe Palaeobaltic Basi n is discussed A stratigraphically rather sharp fauna! change took piace on the boundary ofthe Paadla and Kuressaare stages. The studied interval includes the snajdri, crispa and remscheidensis zones. The fast zone is subdivided into four subzones: baccara, eosteinhornensis, canadensis and remscheidensis. The eosteinhornensis Subzone marks the beginning ofthe Pridoli. The detona Zone occupies a short interval in the remscheidensis Subzone. RIASSUNTO - [Biostratigrafia a conodonti del Siluriano superiore nella parte settentrionale del Baltico orientale] - La fauna a conodonti estratta da campioni di molti affioramenti e di circa venti perforazioni consente di tracciare un quadro biostratigrafico del Siluriano superiore della parte settentrionale del Baltico orientale. Viene discussa la distribuzione dei conodonti da ambienti costieri (pozzo Kolka), alla parte più profonda (pozzo Pavilosta) del bacino paleobaltico, passando per tutti gli ambienti intermedi (pozzi Ohesaare e Ventspils). Dal punto di vista stratigrafico, una netta variazione delle faune si riscontra al limite tra i piani Paadla e Kuressaare. L'intervallo studiato comprende le biozone a snajdri, crispa e remscheidensis; quest'ultima viene divisa in quattro sottozone: baccara, eosreinhornensis, canadensis e remscheidensis. La sottozona a eosteinhornensis segna la base del Pridoli; la Biozona a detona è limitata a un breve intervallo nella Sottozona a remschiedensis. INTRODUCTION GEOLOGICAL SETTING Late Silurian conodont faunas, as ali Lower The Upper Silurian deposits of the East Baltic Palaeozoic faunas, are abundant in the northern East formed at the period of the generai regression, during Baltic. A brief review of the distribution and zonation the last infìlling stage of the development of the of Ordovician and Silurian conodonts is found in the Paiaeobaltic Basin (Nestor & Einasto, 1997). The monograph Geology and Minerai resources of Estonia infìlling stage began with a brief transgressive event (Raukas & Teedumae, 1997). The late Silurian during the Kuressaare Stage and is represented by conodont fauna has been studied since the 1970s, but different marlstones and nodular argillaceous the results have been partly published in Russian (Vi ira, biomicritic Iimestones. This late Ludlow transgression 1970, 1982, 1983, 1994; Mannik & Viira, 1993). Por reached its maximum at the beginning of the Pridoli this reason the present research was initiated, with the (early Kaugatuma time). The deeper basin where intention of making the most important results of the calcareous-argillaceous muds deposited was very wide cited papers accessible fora wider audience. Recently a and extended from south-western Latvia to north- paper on the latest Silurian (Ohesaare Stage) conodonts western Poland. In the onshore direction (northern and the position of the detorta Zone was submitted for Latvia, Saaremaa Island), it was gradually followed by pubiication (Viira, in press). In the course of preparing open shelf biomicritic marlstones and bioclastic to the latter manuscript ali Upper Silurian conodont crinoidallimestones of the shoai belt. During the late collections, extant and newly obtained were examined. Kaugatuma and Ohesaare times, the generai facies The aim of this paper is to summarize ali conodont pattern remained unchanged but ali facies belts data and to give the survey of whole late Silurian migrated gradually sourh-westwards. Due to unstability conodont biostratigraphy. The present study deals with of the bottom, the intercalation of deposits of the Ludlow Paadla an d Kuressaare stages an d the Pridoli neighbouring facies often took piace. Kaugatuma and Ohesaare stages in the north-western The East Baltic area has some advantages for Upper Latvia and o n Saaremaa Island ofEstonia (Text-fìgs. l, Silurian conodont studies. Shelf deposits have yielded 2). good fauna, including index species, especially the 300 V V/IRA Ventspils) were sampled and prepared for conodonts and vertebrates together with T. Marss. As a result, thousands of specimens in hundreds of samples from numerous outcrops and boreholes were got. Barren samples were very rare- only a few small samples from boreholes. The samples from outcrop sections weighed on average 4-5 kg and those from the boreholes 500- 1300 g. The index of colour alteration is about l. For the present study four borehole sections chosen to demonstrate the late Silurian conodont distribution from shallow shelf (Kolka) through open shelf (Ohesaare, Ventspils) to deeper part (Pavilosta) of the Palaeobaltic Basin (Text-fig. 3). BIOSTRATIGRAPHY Text-fìg. l - Location map and facies belts (I-III) of the early Pridoli (after Bassett et al., 1989). I- inshore lagoon/restricted shelf, II - shoal, III - open shelf. Borehole sections: l) The distriburion and evolurion of the late Silurian Ohesaare, 2) Kolka, 3) Venrspils, 4) Pavilosta. The conodonts in the northern East Baltic are closely dashed line: the boundary of presenr extension of rocks. correlateci with the development and environmental conditions of this pericontinental sea. The diversity of conodont species is highest in the Ludlow Paadla Stage where a number of species of the genera Ozarkodina, Ozarkodina remscheidensis group. Besides, many Oulodus and Ctenognathodus are widely distributed or borehole sequences penetrate through Silurian make rheir first appearance (Mannik & Viira, 1993). sediments. All this allows us to establish vertical ranges This Ludlow radiation phase was in a way caused by and lateral distribution of conodonts from shallow shelf wide-spread different environments of the shallow shelf to deeper basin. In the present paper the facies concept giving rise to some specific near-shore biofacies and by Nestor & Einasto (1977, 1997) is used. They specific shallow-water conodonts, for example distinguished five facies belts in the Palaeobaltic Silurian Ozarkodina roopaensis (Vi ira, 1994). Besides Saaremaa Basin, but here shoal, open shelf and transitional belts Island this species is in the northern East Baltic receive most attention. identified only in the Kolka section (Text-fig. 3). For the Paadla Stage, Ozarkodina confluens is most common. lt appears first, according to the shallowing MATERIAL of the basin, in the middle Wenlock in the onshore part (Saaremaa Island) and gradually later towards the The conodonts discussed herein come from the basin, until in the Pavilosta section i t occurs already in borehole and outcrop samples collected over a long the upper part of the Ludlovian Pagegiai Formation. period. Some of the borehole sections (Pavilosta, Another facies-depending species in the Paadla Stage EXPLANATION OF PLATE l Figs. 1-4 - Ozarkodina excavata (Branson et Mehl). l) Pa elemenr, section Karala, sample l, Cn 1534, x 65; 2) Pa elemenr, section Karala, sample l, Cn 1535, x 95; 3) Pb elemenr, section Karala, sample 2, Cn 1536, x 100; 4) M elemenr, section Karala, sample 2, Cn 1537, x 80. Figs. 5-8 - Oulodus siluricus (Branson et Mehl). 5) Pb elemenr, section Karala, sample 2, Cn 1538, x 80; 6) Pa elemenr, section Karala, sample l, Cn 1539, x 65; 7) M elemenr, section Karala, sample 2, Cn 1540, x 60; 8) Se elemenr, section Karala, sample l , Cn 1541, x 50. Figs. 9-12 - Coryssognathus dubius (Rhodes) 9) Pa elemenr, section Unimae, sample 2-74, Cn 1542, x IlO; IO) M elemenr, section Unimae, sample 2-74, Cn 1543, x 120; Il) Pb elemenr, section Unimae, sample 2-74, Cn 1544, x 120; 12) coniform elemenr, section Unimae, sample 2-74, Cn 1545, x 100. (Specimens shown in latera! view). V. VIIRA, LATE SILURIAN CONODONT BIOSTRATIGRAPHY IN THE NORTHERN EAST BALTIC P!. l 302 VVIIRA basal cavity. The distribution of the two latter species <n REGIONAL SMREMAA NQRTH-WEST CONOOONT WESTLATVIA 15 STAGES NORTH LATVIA LA TV/A ZONES is somewhat different (Text-fig. 3). In the shallow shelf "' area (Kolka, Ohesaare) Ozarkodina crispa occurs in the KAAV/ HB. detarta =::i OHESAARE DHESAARE FH. TAR GALE FM. JURA FM. --- upper part of the Paadla and Torgu formations and is CJ remscheidensis CJ -- replaced by Ozarkodina s. parasnajdri in the uppermost 0:: " canadensis Ludlow Kuressaare Formation. In the Ventspils section Q KAUGATUMA KAUGATUMA FN. MIN/JA FM. MINIJA FM. :'9"' ----- "§"' eosteinhornensis Ozarkodina crispa makes its appearance in the Ventspils E----- Formation and disappears in the lower part of the KURESSAARE KURESSAARE fN. VENTSPILS FM. baccatajsnajdri Minija Formation which is supposed to be ofPridolian parasnaJdri $ PAGEGIAI FM. age. The range of Ozarkodina s. parasnajdri lies within CJ crispa --..J TORGU FM. MITUVA FM. the Ozarkodina crispa range in this section. In the CJ PAAOLA :::J PAADLA FM. ENGURE FM. snajdri snajdri deeper part of the basin (Pavilosta) Ozarkodina crispa --..J OUBHSA FM. alpha morphs are absent and both subspecies of Ozarkodina snajdri are likely to define together