Rakvere 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 “Wesenbergsche Schicht” by Schmidt (1881) corresponds roughly to the Rakvere Stage in nowadays understanding (Männil 1958b, 1963a, Kõrvel 1962, Põlma et al. 1988). In northern Estonia, the Rakvere Stage forms the lowermost, relatively thick part of Late Viru and Harju pure micritic (aphanitic) limestones which intercalate with more or less argillaceous varieties. The cycles of different lithotypes generally constitute distinct lithostratigraphical units (Põlma 1982, Hints et al. 1989), whereas the clayey parts of the cycles are characterized by the appearance of abundant new taxa .

The Rakvere Stage consists of the Piilse and Tudu members (Kõrvel 1962) which form the main part of the Rägavere Formation. The stage is at its thickest (28 m) in western Estonia (Fig. 52) and it thins notably in the southeastern direction. The lower, Piilse Member with a thickness of up to 27 m (Rõõmusoks 1983) consists of pure, in places dolomitized micritic limestones with a low content of terrigenous material (3 - 9%) and skeletal sand (less than 5%, Kõrvel 1962, Põlma et al. 1988). The member is characterised by a distinct pyritic pattern, following abundant burrows in the former sediments. The upper, Tudu Member is up to 10 m thick and differs from the Piilse Member in the higher content of skeletal sand (commonly 15 %, Põlma et al. 1988), in the occurrence of thin, up to 3 cm thick kukersite layers and rare and weakly developed pyritic patterns.

Southwards the limestones of the Rägavere Formation become more argillaceous and in southern Estonia they are supposedly replaced by the carbonate marls of the Priekule Member in the upper part of the Mossen Formation (Männil & Meidla 1994, Meidla 1996). On the basis of chitinozoan distribution (Nõlvak & Grahn 1993) it is supposed that in some places (Ruhnu and Ohesaare cores) the Rakvere Stage is missing.

The data on the distribution and composition of macrofossils in the Rakvere Stage, particularly in the Tudu Member is scanty due to relatively few outcrops. The Rakvere Stage with the relatively sparse macrofauna of bryozoans, brachiopods and trilobites is characterized by frequent and diverse association of calcareous algae (Cyclocrinites, Rhabdoporella etc., Kõrts et al. 1990). From the Rakvere Stage up to the end of Ordovician, calcareous algae and their fragments dominate in the composition of skeletal particles (Põlma 1972, 1982) where they may account even for 97.6%.

Unlike macrofossils, the ostracode record of the Rakvere Stage is rich, comprising more than 80 species (Meidla 1996). Several distinct associations have been recorded in the lower part of the stage corresponding approximately to the Piilse Member of the Rägavere Formation (Meidla 1996, fig. 47). Valuable is the ostracode record of the upper part of the stage which contains only sparse macrofauna. This interval, nearly equal to the Tudu Member, corresponds to the Daleiella admiranda Subzone (subzone of Daleiella sp. n. in Meidla & Sarv 1990, Table 10), a range zone prominent in the sections of northern and central Estonia. The appearance of several long-ranging taxa, such as Steusloffina cuneata (Steusloff), Medianella blidenensis (Gailite), Pullvillites laevis (Abushik & Sarv ), etc., has been recorded within this interval (Fig. 50).

The lower boundary of the Rakvere Stage is lithologically more or less distinct in northern Estonia where it coincides with the pyritized discontinuity surface on the top of the Tõrremägi Member in the lower part of the Rägavere Formation. The appearance of several new brachiopods, including Microtrypa estonica Rõõmusoks, Platystrophia lutkevichi satura Oraspõld, P. quadriplicata Alichova, Sowerbyella (Sowerbyella) raegaverensis Rõõmusoks, Vellamo wesenbergensis (Pahlen) and trilobites Chasmops wesenbergensis (Schmidt), Encrinuroides seebachi (Schmidt), Pharostoma pediloba (Roemer) and others, above this boundary shows the renovation of faunal associations. However, macrofossils are very scarce, particularly in core sections, and cannot be used for the purposes of detailed biostratigraphy. The same applies to ostracodes, because most of the species characteristic of the Late Ordovician ostracode fauna appear below this boundary, in the Tõrremägi Member of the Oandu Stage which represents a facies similar to the Rakvere Stage. Also the zonal chitinozoa Fungochitina fungiformis, characteristic of the Rakvere and Nabala stages, appears in the Tõrremägi Member. The suggestion to include the Tõrremägi Member to the Rakvere Stage (Meidla 1996) follows partly the earlier wider interpretation of that stage, according to which the Oandu beds by Öpik were included to the Rakvere Stage (Jaanusson 1945, Alichova 1960).

The Rakvere Stage corresponds roughly to the lower part of the graptolite Pleurogratus linearis Zone and the chitinozoan Cyathochitina angusta Subzone of the Fungochitina fungiformis Zone (Nõlvak & Grahn 1993, Table 7).