Jaagarahu 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 present unit was established by Luha (1933) provisionally as the Muhu-Kurevere Stage, later as the Jaagarahu Stage (Luha 1946). It corresponds roughly to the upper half of the “Untere Oeselsche Gruppe (Schicht)” by Schmidt (1858, 1881). The subdivision of the stage has been recurrently changed (Bekker 1925, Luha 1930, Aaloe 1970, Aaloe et al. 1958, 1976, etc.). Recently, some additional units, including the Jamaja and Riksu formations, were introduced (Resheniya… 1987, Nestor 1995a) and the Muhu dolomites by Luha (1930) were re-established as a formation (Nestor 1995a). V. Nestor (1984) determined the scope of the stage in subsurface area.

The historical stratotype of the stage is an abandoned quarry at Jaagarahu supplemented with the Jaagarahu drill core in the interval of 0.3 to 21.4 m (Aaloe 1970). The Jaagarahu Stage spreads on Saaremaa and Muhu islands and in the southwestern part of mainland Estonia (Pärnumaa and southern Läänemaa). The outcrop extends as a 10—30-m-wide belt from Vilsandi and Vaika islands through northern Saaremaa and Muhu as far as Eidapere and Tori at Tallinn - Pärnu railway (Fig. 71). The main localities are the quarries at Jaagarahu, Tagavere, Koguva and Anelema, and recent and ancient coastal cliffs at Vilsandi, Abula, Panga (Photo 22), Pulli (Oiu), Üügu, Püssina, Kesselaid, Salevere and Kirbla (Photo 23). The thickness of the stage is variable and increases southwestwards from 32.3 m in the Viki core to 145.0 m in the Ohesaare core.

In the western part of Saaremaa, the Jaagarahu Stage is dominated by comparatively pure limestones, while dolomites are prevailing in the eastern part of Saaremaa, on Muhu Island and in mainland Estonia. Reefs (bioherms and mounds) are widespread in the Jaagarahu Stage, especially in its lower part (Vilsandi beds and Kesselaid Member). In the South-Estonian Confacies Belt, the lower part of the Jaagarahu Stage is represented by the marlstones of the Jamaja Formation, and the upper part by the nodular biomicritic limestones of the Sõrve Formation. Temporal analogues of the latter formation are absent in northern sequences due to the long stratigraphical hiatus (Nestor & Nestor 1991). In the Mid-Estonian Confacies Belt, the lower boundary of the stage has been drawn by an abrupt increase in the carbonate content of the rocks coinciding with the base of the Jaagarahu and Muhu formations. In the more argillaceous sequences of the South-Estonian Confacies Belt and transition area, the lower boundary is determined by the appearance of chitinozoans of the Linochitina cingulata Biozone (Nestor V. 1994) at the base of the Jamaja and Riksu formations.

The Jaagarahu Stage contains a wide spectrum of fossils from lagoon-related eurypterids and thelodonts to deep-water communities of chitinozoans, ostracodes and trilobites. Severe dolomitization has destroyed skeletal remains over a vast area in the eastern part of the stage (e.g. in the Muhu Formation). The most characteristic fossils are as follows (abbreviations: jg - Jaagarahu Formation, jm - Jamaja Formation, srv - Sõrve Formation, rks - Riksu Formation, mh - Muhu Formation, V - Vilsandi beds, M - Maasi beds, u.pt. - upper part): Vikingia tenuis (Nestor) (jgV), Ecclimadictyon astrolaxum Nestor (jgM), Favosites mirandus Sokolov (jgV), Thecia confluens (Eichwald) (jgM), Coenites juniperinus Eichwald (jgV,M), Acervularia ananas (L.) (jgV), Kodonophyllum truncatum (L.) (jgM), Dolerorthis rustica (Sowerby) (jm), Howellella cuneata Rubel (jg,mh,srv), Encrinurus balticus Männil (jm), Warburgella estonica Männil (jgM), Craspedobolbina insulicola Martinsson (jm), Leptobolbina quadricuspidata Martinsson (srv), Conochitina lagena Eisenack (jm, rks), C. pachycephala Eisenack (jm, srv), C. cribrosa Nestor (srv), Ozarkodina sagitta rhenana Walliser (jgV), Kockelella walliseri Helfrich (jm), K. amsdeni Barrick et Klapper (srv), Monograptus flemingii Salter (jm, srv), Gothograptus nassa (Holm) (srv, u.pt.), Logania taiti (Stetson) (srv). Taking into account the few findings of zonal species of graptolites in the Ohesaare core, it seems that the Jaagarahu Stage probably spans from the M. flexilis Biozone (partly) to the Gothograptus nassa Biozone (Table 8). The main, lower part of the Jaagarahu Stage consists of the Jaagarahu, Muhu, Riksu and Jamaja formations laterally replacing one another (Figs. 69, 70). The upper part of the stage is represented by the Sõrve Formation which is distributed only in the South-Estonian Confacies Belt; in the northern sequences a stratigraphical cap corresponds to it.

The Jaagarahu Formation occurs in northwestern Saaremaa and consists of very variable, prevailingly sparitic limestones of shallow-water origin. Coral-stromatoporoid limestones, including reefs or bioherms, and fine-grained skeletal and pelletal grainstones are the most widespread rocks. In some places they are dolomitized. The formation contains some bands of lagoonal argillaceous dolostones, the so-called eurypterus and pattern dolomites which divide the formation into three subunits: the Vilsandi, Maasi and Tagavere beds. The Vilsandi beds comprise an abundance of large bioherms. The Maasi beds contain biomicritic interlayers of deeper-water genesis. The Tagavere beds are capped by the thickest (5-8 m) deposit of lagoonal dolomites treated sometimes as the Selgase Member. The thickness of the Jaagarahu Formation varies from 32 to 46 m (Fig. 71 ).

The Muhu Formation is distributed in northeastern Saaremaa, on Muhu Island and on mainland Estonia north of the Pärnu latitude. It consists mostly of rather monotonous flaggy dolomites containing numerous large massive reef-mounds (Fig. 70, Photo 23) in its lower part (Kesselaid Member). Almost everywhere the thickness (20 to 40 m) of the formation is uncomplete due to the post-Silurian denudation.

The Riksu Formation bounds the Jaagarahu and Muhu formations from the south, and spreads along the southern coasts of Saaremaa, the Tõstamaa Peninsula and around Pärnu Bay. It is mostly represented by nodular biomicritic to micritic limestones containing layers of argillaceous limestones and marlstones causing a cyclical nature of the sequence. In the eastern part of the distribution area, the rocks are usually dolomitized. In places (Pärnu, Kihnu, Seliste), the Riksu Formation is underlain by a tongue of the Jamaja Formation and, in most places, it is overlain by the tongue of the Jaagarahu or Muhu formations. The thickness of the Riksu Formation varies from 34.5 m in the Nässumaa borehole to 50.8 m in the Kaugatuma borehole.

The Jamaja Formation forms the lower part of the Jaagarahu Stage in the South-Estonian Confacies Belt. The formation is represented by different marl- and mudstones. The thickness of the formation reaches 95.2 m in the Ohesaare section.

The Sõrve Formation overlies the Jamaja Formation in the southernmost sections of Estonia (Ohesaare, Ruhnu, Ikla). It is represented by biomicritic to micritic nodular limestones (pack- and wackestones) similar to the Riksu Formation but lying stratigraphically higher in the sequence and corresponding to the hiatus in northern sequences. The thickness of the formation reaches 49.8 m in the Ohesaare boring.

Nimi	Jaagarahu 1 puurauk
Number	1
Tüüp	puurauk
Riik	Eesti
Vald	Kihelkonna
Asutusüksus	Kurevere küla
Koordinaadid	null, null
Strat. algus	Jaagarahu lade
Kirje lisatud	2008-03-12
Kirje muudetud	2018-04-16