Oandu 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.

In northern Estonia, the Oandu Stage comprises rocks of two different lithofacies forming the Vasalemma and Hirmuse formations. The Vasalemma Formation, distributed in northwestern Estonia, consists of bedded fine- to coarse-grained bioclastic limestones with irregular bodies of aphanitic massive limestones (carbonate buildups). These rocks were identified first as the Hemicosmites Limestone (Eichwald 1854a) or Wasalemm’sche Schicht (Schmidt 1881). Vasalemma, as the name of a chronostratigraphic unit was applied also to the rocks of another lithofacies – the argillaceous limestones and marls, named Oandu beds by Öpik (1933) and the Hirmuse Formation by Männil and Rõõmusoks (1984), which are exposed on the banks of the Oandu River in northeastern Estonia (Rõõmusoks 1953, Aaloe et al. 1958). Later studies (Männil 1958c, 1960) showed that the lower and middle parts of the Vasalemma Formation are of Keila Age (Fig. 48) and the name Oandu was proposed for chro-nostratigraphic unit of post-Keila Age. The Oandu Age of the uppermost Vasalemma Formation is presumed by the appearance of the corals Lyopora tulaensis (Sokolov), Eofletcheria orvikui Sokolov and the brachiopods Rhynchotrema? parva Oraspõld, Rostricellula nobilis (Oraspõld), Dactylogonia luhai (Sokolskaya) (Männil 1960, Rõõmusoks 1970), or it is supposed by the disappearance of Leiosphaeridia and brachiopods of the Keila Stage (Rummu core, Põlma et al. 1988). In northern Estonia, the Oandu Stage is restricted in thickness (1-4 m, Fig. 49); only in the limits of the Vasalemma Formation it is up to 6 m thick.

In the stratotype area in northeastern Estonia, the lower boundary of the Oandu Stage and the Hirmuse Formation, is known only by the core sections where it is marked by a sharp discontinuity surface with up-to-35-cm-deep pockets, on the upper boundary of the Kahula Formation (= Kahula Group, Männil & Meidla 1994). Below this level, a great number of Middle Ordovician species and even genera common with several older stages, including the brachiopods Cyrtonotella, Estlandia, trilobites Asaphus (Neoasaphus) nieszkowskii Schmidt, Pseudobasilicus, ostracodes Tetrada (Tetrada) harpa (Krause), Polyceratella spinosa Sarv (Fig. 50) disappear. Notable is the disappearance or sharp decrease in the frequency of the acritarch Leiosphaeridia which is abundant in the Keila Stage (Fig. 51). This fossil seemingly can be used for the preliminary establishing of the above-mentioned boundary in core sections, especially when the uppermost part of the Kahula Formation is more argillaceous and possibly belongs to the Lehtmetsa Member of late Keila Age (Fig. 47, Põlma et al. 1988, fig. 32). A new complex of fossils with the ostracodes Bolbina rakverensis (Sarv), Klimphores minimus (Sarv), Disulcina perita perita (Sarv), brachiopods Howellites wesenbergensis (Alichova), Equirostrata wesenbergensis (Teichert) appears near the lower boundary of the Oandu Stage (Fig. 50, see also Põlma et al. 1988). In some cases, these species are found even below the boundary discontinuity surface, seemingly they occur in the deep pockets filled with deposits of Oandu Age. Due to the essential changes in the faunal composition (Männil et al. 1966, Hints et al. 1989), several authors have suggested to use the lower boundary of the Oandu Stage as the regional subseries or series boundary (Jaanusson 1945, Rõõmusoks 1956).

The Hirmuse Formation thins out within a rather short distance in the southern direction, and in many places in central Estonia the Oandu Stage is represented only by the Tõrremägi Member of the Rägavere Formation with a thickness less than one metre (Fig. 49). This area separates the rich and diverse fauna of bryozoans, brachiopods, echinoderms and trilobites of the Hirmuse Formation in northern Estonia (Põlma et al. 1988) from the relatively rich brachiopod and trilobite fauna in the marls and argillaceous limestone in southern Estonia, corresponding presumably to the Lukštai Formation. Beside some species common for northern and southern Estonia (Howellites wesenbergensis Alichova, Rhactorthis kaagverensis Hints a.o.), several brachiopods (Reushella magna Hints, Laticrura sp. Skenidioides sp., Leptellina? sp.) have been identified only in the latter region and are also known in the Lukštai Formation in Lithuania or in the Moldå Limestone in Sweden (Jaanusson 1982, fig. 7).

Identification of the Oandu Stage is most complicated in southeastern Estonia where the black shales and the overlying marls of the Mossen Formation are distributed (Karula core in Fig. 51). The shales, encountered in various sections have been included to the Keila (Meidla 1996), Oandu (Hints 1975) or Rakvere (Männil 1966) Stage. The overlying marls of the Priekule Member are correlated with the uppermost Oandu and/or the Rakvere Stage. In some sections, the marls below the black shale (about 4 m in the Karula core, Fig. 51) comprise brachiopods known in the other sections (Otepää, Laeva) mainly in the beds presumably of Oandu Age. In favour of this age testifies also the disappearance of Leiosphaeridia at a depth of about 3 m below the shales. At the same time, the ostracode record allows to suppose Keila Age of the lowermost part of the Mossen Formation (Meidla 1996). The contradiction in interpreting the age by macrofossils and ostracodes may be caused by insufficient data available, or it may indicate the patchy distribution of deposits in the Keila - Oandu boundary interval in southeastern Estonia.

Still, in most of southern Estonia, the Oandu Stage can be identified most realiably on the basis of ostracodes. The lower boundary of the Oandu Stage is marked by the appearance of Sigmoopsis granulata Sarv, Bolbina rakverensis Sarv, Pelecybolbina illativis Neckaja and Klimphores minimus (Sarv), and a general rapid faunal change which occurred throughout the Estonian part of the palaeobasin (Meidla 1996).