Nabala 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 Nabala Stage was distinguished by Männil (1958b) as the lower part of the Schmidt’s (1858) “Lyckholm’sche Schicht”. He subdivided the stage into the lower, Paekna and the upper, Saunja substages (see also Öpik & Laasi 1937, Jaanusson 1994). Nowadays they are used as lithostratigraphical units (formations) which represent the Nabala Stage with a total thickness of 10 to 35 m in northern and partly in central Estonia (Fig. 53). The Paekna Formation is up to 16 m thick and comprises predominantly argillaceous bioclastic limestones intercalating with micritic limestones. The thickness of micritic interlayers is usually about 0.1 m, but occasionally it may reach 1-3 m. The up-to-28-m-thick micritic limestones of the Saunja Formation are lithologically uniform and occur all over Estonia. Their thickness decreases towards the south until it is only 0.3 m (Meidla 1996). South of the Muhu - Mustvee line, the lower Paekna Formation is replaced by the Mõntu Formation. This 3–7-m-thick complex consists of argillaceous bioclastic limestones with rare thin (5–30 cm) layers of micritic limestones containing glauconite (Oraspõld 1995).

On the transition from the Rakvere to the Nabala Stage the shelly fauna undergoes notable renovation. Of about 150 species and subspecies occurring in the Nabala Stage, only one third is common with the Rakvere Stage (Männil et al. 1966). Several new species of brachiopods, including Bekkeromena semipartita (Roemer), Ilmarinia sinuata (Pahlen), Laticrura rostrata Hints, Sulevorthis lyckholmiensis (Wysogorski), Pseudolingula quadrata (Eichwald), appear in the Nabala Stage. Some of these species are missing in the Saunja Formation, but appear again in the overlying Vormsi Stage. The micritic limestones of the Saunja Formation contain a notably abundant and diverse fauna of molluscs (about 30 gastropod and more than 10 cephalopod species).

The lower boundary of the stage is exposed only in the Paekna quarry (Nõlvak & Meidla 1990). It is marked by a series of uneven discontinuity surfaces, above which there appears a new association of chitinozoans, including the zonal Armoricochitina reticulifera (Grahn). The latter can be used as the most reliable fossil for the identification of the lower boundary of the Nabala Stage in the core sections in central and southern Estonia.

The composition of ostracodes changes remarkably on the Rakvere - Nabala transition. Several new taxa, including Disulcina perita explicata Sarv, Tetrada neckajae Meidla, Oepikella luminosa Sarv a.o., appear in the lower part of the Nabala Stage, but mainly somewhat higher of the lower boundary of the Paekna Formation (Fig. 50). For this reason the lower boundary of the stage is marked better in the ostracode record by the disappearance of the species Disulcina perita perita (Sarv) and Daleiella admiranda Meidla (a zonal species) in the uppermost part of the Rakvere Stage (Meidla 1996). Brachiopods are found mostly in the lower Paekna Formation and among them new faunal elements Pseudolingula quadrata (Eichwald) and Sulevorthis lyckholmiensis (Wysogorski) appear close to the lower boundary of the Nabala Stage (Fig. 54).

The Nabala Stage corresponds to the middle part of the Pleurogratus linearis graptolite Zone (Table 7) and the upper part of the North Atlantic superbus conodont Zone. In the ostracode record, the summary differences between the Paekna and Saunja formations are not significant, but the very uneven distribution of ostracodes in the Saunja Formation should be mentioned (Meidla 1996).