Ohesaare Stage

by H. Nestor

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 Ohesaare strata were originally established by Bekker (1925) as a substage of the Saaremaa Stage and were raised into the stage rank by Luha (1933). Klaamann (1970a) defined the lower boundary of the stage. Aaloe et al. (1976) distinguished the Kaavi Member. The Ohesaare Cliff is the historical stratotype of the stage. Ohesaare-2 drill core at the depth of 4.10 m has been selected as the boundary stratotype of the stage.

The Ohesaare Stage crops out in the southern part of the Sõrve Peninsula and spreads also on Ruhnu Island under the Devonian cover. The only exposures of the stage are the Ohesaare and Loode cliffs (Fig. 73). In Estonia, the upper limit of the stage is erosional and the stage does not reach its full thickness anywhere. The thickest section (33.7+ m) has been recorded in Kaavi-568 boring.

In Estonia, the Ohesaare Stage is represented by the Ohesaare Formation which mostly consists of argillaceous-dolomitic marlstones or calcareous mudstones with thin intercalations of partly to totally dolomitized bio- to lithoclastic limestones. At the base of the stage there is a rather thick (4-5 m) deposit of various thin-bedded bioclastic to micritic limestones with thin intercalations of marlstone. In the upper part of the sequence the argillaceous-dolomitic marlstones are of red colour and contain silt and sand admixture. This part of the sequence is regarded as the Kaavi Member. The lower boundary of the stage coincides with the junction between the marlstones of the Lõo beds and the platy bioclastic limestones in the basal part of the Ohesaare Stage. Above this level there appear some new elements among ostracodes (Juviella piltenensis, Nodibeirichia protuberans), chitinozoans (Urochitina sp. sp., Eisenackitina lagenicula) and vertebrates (Poracanthodes punctatus, Goniporus alatus), etc.

The Ohesaare Stage contains a rather rich shelly fauna and a diverse association of agnathans and fish remains. Most of the palaeontological records come from the Ohesaare locality and characterize the lowermost part of the stage. From the Kaavi Member (K) only vertebrate fossils have been identified up to now. The species characteristic of the whole stage include Eisenackitina lagenicula (Eisenack), Urochitina cf. simplex Eisenack, U. verrucosa Eisenack, Favosites forbesi ohesaarensis Klaamann, F. vectorius Klaamann, Fistulipora tenuilamellata (Bassler), Eridotrypa parvulipora Ulrich et Bassler, Shaleria dzwinogrodensis (Kozlowski), Collarothyris collaris (Rubel), Grammysia obliqua (McCoy), Tentaculites scalaris (Schlotheim), Lonchidium inaequale Eichwald, Calymene conspicua Schmidt, Eophacops serotinus Männil, Juviella piltenensis Gailite, Nodibeirichia protuberans (Boll), Klodenia leptosoma Martinsson, Orcofabella testata (Gailite), Ozarkodina confluens nasutus (Viira), Poracanthodes punctatus Brozen, Tylodus deltoides Rohon, Goniporus alatus (Gross) (K), Nostolepis alta Märss (K).

The presence of the conodont species Ozarkodina remscheidensis remscheidensis allows to correlate the Ohesaare Stage with the upper Pridoli.