Raikküla 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 Raikküla beds were originally defined (Schmidt 1858) as the “Intermediate zone” (Zwischenzone) between the strata with Pentamerus borealis and P. oblongus. In 1881, Schmidt introduced the geographical name - Raikküllsche Schicht. Kaljo and Vingisaar (1969) presented the currently used subdivision of the stage for southern Estonia. Perens (1992) and H. Nestor (1995a) modernized the classification for the outcrop area. The Mõhküla beds, earlier attributed to the Adavere Stage, were replaced into the Raikküla Stage as they are separated from the rest of the Adavere Stage by a structural disconformity (Nestor 1995a). However, since the stratigraphical level of the Mõhküla beds was changed only recently, it is not yet reflected in the limit between the outcrops of the Raikküla and Adavere stages on the printed geological maps.

The Raikküla-Paka scarp and Raikküla drill core in the interval of 0.5 to 35.0 m have been defined as the composite stratotype of the stage (Nestor 1993). The Raikküla Stage is distributed on the islands of the West-Estonian Archipelago and in the western, central and southern parts of mainland Estonia. The outcrop extends as a latitudinal, eastward widening belt (6 to 45 km) from southern Hiiumaa as far as the southeastern slope of the Pandivere Upland near Palamuse. The main localities are active or abandoned quarries at Pusku, Orgita, Keava, Mündi, Kalana and Rôstla, and ancient coastal scarps (inland cliffs) at Pakamägi and Raikküla-Paka. The thickness of the stage varies from 16.3 m in the Murika borehole to 176.3 m in the Ikla borehole (Fig. 66) and decreases abruptly in the northwest direction due to the end-Raikküla denudation of the upper layers of rocks.

The Raikküla Stage consists of a variety of carbonate rocks. The most characteristic are micritic (micro- and cryptocrystalline) limestones cyclically interbedding with marl- or mudstones in the south and with different bioclastic limestones (wacke-, pack- and grainstones) in the north. In the northernmost sections of central Estonia, the shallowing-up sedimentary cycles may end with argillaceous primary dolomites. In the southernmost sections, the marl- and mudstones contain graptolites on certain levels. In central Estonia, in the Paide - Pärnu belt of faults and eastwards, the Raikküla rocks are strongly dolomitized.

The lower boundary of the stage coincides with the base of a band of marl- or mudstones overlain by thick deposit of monotonous micritic limestones of the Järva-Jaani beds in the north and Slitere Member in the south. In the area of distribution of the most shallow-water sequences of the Hilliste and Raikküla formations the boundary is less definite. Above the boundary, sparse graptolites of the Pristiograptus cyphus Biozone and chitinozoans of the Conochitina electa Biozone (C. electa, C. maennili, etc.) appear.

Fossils are of uneven distribution in the rocks of the Raikküla Stage. The widespread micritic limestones and different dolostones (from pure dolomite to dolomitic marl) contain occasional macrofossils. The most characteristic species are (abbreviations in brackets: rk - Raikküla Formation, nr - Nurmekund Formation, sr - Saarde Formation, u.pt. - upper part, m.pt. - middle part, l.pt. - lower part): Clathrodictyon clivosum Nestor (rk, u. pt.), Parastriatopora celebrata Klaamann (rk, u. pt.), Borealis pumilus (Eichwald) (nr), Borealis borealis osloensis Mjork (nr), Meifodia ovalis Williams (sr), Hermannina hisingeri (Schmidt) (rk, nr), Bythrocyproidea sarvi Neckaja (nr), Icriognathus cornutus Männik (rk, l. pt.), Kockelella manitoulinensis (Pollack, Rexroad et Mehl), (rk, nr), Conochitina electa Nestor (rk, nr, sr, l. pt.), C. iklaensis Nestor (nr, sr), Spinachitina maennili Nestor (sr), Coronograptus cyphus (Lapworth) (sr, l. pt.), C. gregarius (Lapworth) (nr, sr, m. pt.), Demirastrites triangulatus (Harkness) (sr, m. pt.), D. convolutus (Hisinger) (sr, u. pt.). The presence of zonal graptolites shows that the Raikküla Stage spans from the C. cyphus Biozone to the D. convolutus Biozone.

The Raikküla Stage consists of the Raikküla, Nurmekund and Saarde formations, laterally replacing each other from north to south (Figs. 64, 65). The upper part of the Hilliste Formation is of Raikküla Age (Table 8).

The Raikküla Formation is distributed in central and western Estonia, in the Lääne, Rapla and Järva counties. It is represented by two shallowing-up sedimentation cycles star-ting with biomicritic or micritic limestones, succeeded by ske-letal grainstones, pelletal or coral-stromatoporoid limestones, and ending with argillaceous lagoonal dolostones. These cycles are treated as the lower and upper subformations (Nestor 1995a). The thickness of the formation varies from 30 m in the Kiideva borehole to 56 m in the Käru borehole. The upper layers of the formation have undergone considerable denudation and in the westernmost sections of mainland Estonia the upper subformation thins totally out.

The Nurmekund Formation south and east of the Raikküla Formation consists of five sedimentary cycles which begin with a relatively thin layer of marlstone or argillaceous limestone. The main, middle part of the cycle is represented by wavy-bedded micritic limestone, the upper part by bioclastic limestones containing numerous discontinuity surfaces. In central Estonia, the formation is strongly dolomitized, particularly its upper half. The first, third and fifth cycles from below are thicker and more complete, the second and fourth being thinner and less typical. In ascending order, the cycles are termed the Järva-Jaani, Vändra, Jõgeva, Imavere and Mõhküla beds (Table 8). Westwards the upper beds gradually thin out and on Saaremaa Island only the Järva-Jaani and, partly, the Vändra beds are present. The thickness of the formation ranges from 16 m in the Murika borehole to 73+ m in the Võhma borehole.

The Saarde Formation is distributed in southwestern Estonia. It consists of cyclically alternating deposits of horizontally-bedded micritic lime- and marlstones or mudstones and is subdivided into six members. The lowermost, rather thin mudstone member, comprised mostly of argillites, has no name and was earlier included in the Õhne Formation of the Juuru Stage. In ascending order, the Slītere, Kolka, Ikla, Lemme and Staicele members follow. The shaly mudstone interlayers in the Ikla Member abound in graptolites of the Demirastrites triangulatus Zone. In other members graptolites are less frequent. In its full thickness (176.3 m) the Saarde Formation occurs only in the Ikla borehole.