Jaani 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 Jaani Stage was defined by Luha (1933) as a marlstone unit corresponding to the lower part of the “Untere Oeselsche Gruppe (Stufe)” by Schmidt (1858, 1892). Kaljo (1962) separated the lower part of the marlstones (now the Velise Formation), corresponding to the uppermost Llandovery, and joined it with the Adavere Stage. V. Nestor (1984) determined the position of the upper boundary in the subsurface area. Aaloe (1960, 1961) subdivided the Jaani Stage into the Mustjala, Ninase and Paramaja members. Later, Aaloe & Kaljo (1962) distinguished the Tõlla Member for the South-Estonian subsurface area.

A historical stratotype of the Jaani Stage is the sea shore with the Paramaja Cliff in the vicinity of the Jaani Church (Resheniya… 1987, Nestor 1993). The Ohesaare drill core at the depth of 345.8 m may be treated as the boundary stratotype of the stage. The Jaani Stage spreads on Saaremaa and Muhu islands and in the southwestern part of mainland Estonia (Pärnumaa and southern Läänemaa). The outcrop runs along the northern coast of Saaremaa and Muhu islands and the southern coast of Matsalu Bay towards the Vändra Borough (Fig. 68). The main localities are the cliffs at Undva, Suuriku, Ninase, Panga, Liiva, Paramaja, Kautliku, Püssina and Uisu, the quarries at Koguva and Anelema (lower part), and the river bank at Jädivere. The thickness of the stage increases westwards and varies from 24.2 m in the Lihula borehole to 70 m in the Kaugatuma borehole.

The stage consists mainly of various marl- and mudstones. Limestones (skeletal wacke-, pack-, grain- and boundstones) are of minor importance and occur only in the upper half of the stage in the northwestern part of Saaremaa Island (Ninase Member). The lower boundary of the stage has been made congruous with the Llandovery/Wenlock boundary (Kaljo 1962), established by the appearance of the graptolite Cyrtograptus murchisoni in the Ohesaare drill core above the depth level 345.8 m and in other sections by chitinozoans of the Margachitina margaritana Zone (Nestor V. 1994). Lithologically, it usually coincides with a certain increase in the carbonate content of rocks.

The Jaani Stage contains rather rich shelly and planktonic faunas with characteristic deeper-water elements (graptolites, chitinozoans, trilobites). The most typical species are as follows (abbreviations: M - Mustjala Member, P - Paramaja Member, N - Ninase Member, T - Tõlla Member, u.pt. - upper part, l.pt. - lower part): Stromatopora impexa Nestor (M, u. pt.), Halysites senior Klaamann (M), Thecia podolica (P), Neocystiphyllum keyserlingi (Dybowski) (P), Leptaena rhomboidalis (Wahlenberg) (M,P), Eocoelia angelini (Lindström) (N), Pseudobollia krekenawaiensis Neckaja (T,P), Craspedobolbina (C.) mucronulata Martinsson (N,P), Beirichia (B.) suurikuensis Sarv (N,P), Calymene orthomarginata Schrank (T,P), Encrinurus punctatus (Wahlenberg) (P), Conochitina cf. mamilla Laufeld (T,N,P), Calpichitina acollaris (Eisenack) (P), Pterospathodus amorphognathoides Walliser (T,M, l.pt.), Kockelella ranuliformis (Walliser) (M,N,P), Cyrtograptus murchisoni Carruthers (T), Monograptus riccartonensis Lapworth (T), M. flexilis Elles (P). The presence of the index species of graptolites proves that the Jaani Stage spans from the C. murchisoni Biozone to the M. flexilis Biozone.

In Estonia, the Jaani Stage is mainly represented by the Jaani Formation. Only in the southernmost sections, the lower part of the stage has been treated as the Tõlla Member of the Riga Formation (Figs. 69, 70).

The Jaani Formation consists of marlstones and, to a lesser extent, of bioclastic and biohermal limestones. The lower part is formed by the Mustjala Member comprising argillaceous marlstones (Figs. 69, 70), which are often dolomitized, particularly in eastern sections. In the middle of the sequence of the Jaani Stage, the carbonate content increases abruptly and, respectively, the upper half of the Jaani Formation is represented by calcareous marlstones or argillaceous limestones of the Paramaja Member in the eastern part of Saaremaa Island, on Muhu Island and in mainland Estonia. In the northwest direction the Paramaja Member is laterally replaced by bioclastic limestones (wackestones to grainstones) of the Ninase Member containing also bioherms. In many sections, a tongue of the Paramaja marlstones overlaps the Ninase limestones.

In Estonia (Tõlla, Ikla, Ruhnu, Ohesaare drill sections) the Riga Formation is represented only by its lower part - the Tõlla Member, which is characterized by graptolite-bearing grey mudstones. Northwards it is replaced by the greenish-grey marlstones of the Mustjala Member and upwards with the marlstones of the Paramaja Member, both belonging to the Jaani Formation (Figs. 69, 70).