Vormsi Stage

L. Hints & T. Meidla

Original text from: Raukas, A., Teedumäe, A. (eds). 1997. Geology and Mineral Resources of Estonia. Estonian Academy Publishers, Tallinn. 436 pp. ISBN 9985-50-185-3. Available online at: sarv.gi.ee/geology.

The Vormsi Stage (Jaanusson 1944b, = middle part of the Lyckholm’sche Schicht, Schmidt 1858) consists of a facies succession of bioclastics limestones (Kõrgessaare Formation, up to 21 m) in northern Estonia, argillaceous limestones with glauconite (Tudulinna Formation, up to 17.1 m) in central Estonia and black shales (Fjäcka Formation, up to 4.5 m) in southern Estonia. The thickness of the stage decreases from 10 - 20 m in northern Estonia to 0.3 m in southern Estonia (Ohesaare core, Fig. 55). In the transitional area between the Kõrgessaare and Tudulinna formations, interfingering of these units can be followed (Oraspõld 1982a, figs. 3, 4).

The association of the diverse shelly fauna of corals, bryozoans, brachiopods, molluscs and trilobites includes some 200 species (Rõõmusoks 1967) in northern Estonia in the Kõrgesssaare Formation. Southwards this fauna is replaced by a specific and less diverse association.

The Tudulinna Formation is characterized by an association of brachiopods, comprising species of the genera Dicoelosia, Christiania, Skenidioides, Leptellina?, and a facies dependent ostracode association prevailed by Uhakiella curta Sidaraviciene, Medianella blidenensis (Gailite) and Rectella nais Neckaja (Meidla 1996). The Fjäcka Formation comprises a brachiopod association typical of shally facies, consisting mainly of inarticulated small brachipods Paterula, Hisingerella a.o., and of a few articulate brachiopods, such as “Sericoidea” and Onniella (Fig. 56). In general, the association is similar to that in the Mossen Formation.

The lower boundary of the Vormsi Stage coincides with a lithologically sharp boundary in most of Estonia. Above that boundary the frequency of macrofossils and ostracodes increases notably. In the Kõrgessaare Formation, several new species appear, including the corals Proheliolites dubius (Schmidt), Kenophyllum siluricum (Dybowski), Streptelasma (Streptelasma) distinctum Wilson, the brachiopods Eoplectodonta schmidti (Lindström), Equirostra gigas Schmidt, Sampo hiiuensis Öpik, Glyptorthis plana Oraspõld, Triplesia insularis (Eichwald), the first Dicoelosia (Wright 1968) and the trilobites Encrinurus moe Männil a.o. Ostracodes are dominated by the species common with the older strata (Fig. 57). The earliest conodonts of the ordovicicus Zone occur also in the Vormsi Stage (Männik1992b).The zonal species Amorphognathus ordovicicus has been recorded from the basal part of the Kõrgessaare Formation and also from the Tudulinna Formation.

In spite of distinct lower and upper boundaries of the Vormsi Stage, the detailed correlation of the formations belonging to this stage is not yet very clear. The distribution of zonal chitinozoans allows to suppose that the oldest part of the Vormsi Stage is missing in central and southern Estonia (Nõlvak & Grahn 1994). In these areas the Vormsi Stage corresponds to the Tanuchitina bergstroemi Zone which forms the upper part of the stage in northern Estonia, overlying the Fungochitina fungiformis Zone (Table 7) of the lower part of the stage in this area.

The distribution of ostracodes (Meidla 1996) does not support the correlation schemes based on chitinozoans.

The topmost part of the Vormsi Stage correponds to the chitinozoa Acanthochitina barbata Subzone (Table 7). The level of the disappearance of the index species marks well the traditional upper boundary of the Vormsi Stage.