Chitinozoan correlation of the Upper Viruan (Middle Ordovician) rocks of Sweden and the East Baltic area

The distribution of eight Upper Viruan chitinozoan species is demonstrated in the following sections (Fig.): (1) Fjäcka section, Dalarna (after Laufeld, 1967), (2) Kandava boring No. 25, Western Latvia, and (3) Värska boring, South-eastern Estonia. Most of the mentioned species seem to represent good index fossils and their vertical ranges may be considered as zones (e. g., zone of Conochitina tigrina, zone of Eremochitina dalbyensis); in other cases distinct repetitive zones here tentatively referred to as zonules, may be useful in correlations (e.g. zonule A of Desmochitina sp. n.). In the Upper Viruan sequence studied five chitinizoan zonation units are distinguished (Fig., I—V), unit I corresponding to the uppermost beds of the Kukruse Stage (СII), II to the type section part of the Idavere Stage (CIII), III and IV to the Jõhvi Stage (DI), and V to the Keila Stage (DII). The upper part of the unit V of the Kandava-25 and Värska core sections, characterized by the presence of Fungochitina fungiformis and Desmochitina sp. n. (zonule B) seems to correspond to the Saue Beds (DIIS) of the North Estonian sequence. According to the correlation of the conodont zonation of the Fjäcka section with the graptolite zones (Bergström, 1971a, b), the upper boundary of the Nemagraptus gracilis zone must be situated within the limits of ithe upper half of the chitinozoan unit I (= Conochitina tigrina zone). This seems tobe in a good accordance with the finds of characteristic graptoloid species in the units I (Nemagraptus sp., Amplexograptus bekkeri, Ortograptus uplandicus), II (Pseudoclimacograptus scharenbergi f. typica) and III—V (Amplexograpius cf. fallax) in the East Baltic sections.