Pļaviņas Stage

by K. Kajak

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 Pļaviņas Stage and the Pļaviņas Formation have been named after the exposures in the vicinity of the Town of Pļaviņas in Latvia (Liepinš 1951). Currently, these exposures are under the waters of the Pļaviņas reservoir and, therefore, the outcrops near Izborsk (Irboska) have been selected as the neostratotype for the Pļaviņas Stage.

In Estonia and adjacent areas, the Pļaviņas Stage has a thickness of 27–32 m. In the vicinity of Izborsk it is 37 m thick (Fig. 90). The lower boundary of the stage is lithologically clear — the clayey sandy deposits of the Amata Stage are overlain by carbonate rocks of the Pļaviņas Stage. Based on the palaeontological and lithological characteristics, the Pļaviņas Stage has been subdivided into the Snetnaya Gora, Pskov and Chudovo substages.

The Snetnaya Gora Substage has been named after a type section near the Snetnaya Gora Monastery in the vicinity of the Town of Pskov in Russia. In Estonia, the rocks of the substage crop out at the Peetri River upstream of Karisöödi, at the Pärli River near the Saarlase Mill, in the Rõuge Ööbikuorg, in the environs of Loosi and Vastseliina (Fig. 89). According to the borehole data, the thickness of the substage is ranges from 5.5 to 12 m, and increases from west to east (Fig. 90).

The substage is represented by rhythmically alternating yellowish- and greenish-grey micro- to cryptocrystalline argillaceous silty dolomite (MgO 16%, CaO 24%) and dolomitic marl (insoluble residue 30%, MgO 13%, CaO 19%), less frequently by clay. Dolomites and dolomitic marls contain silty interlayers. The complex is micro- and thin-laminated. Imprints of cubical salt crystals are found in dolomite. In northern regions, thin sand interlayers occur. At the Peetri River, the lower part of the section is composed of clay, and the upper part of dolomite.

The fossils, occasionally found in the section, are represented by the brachiopods Camarotoechia aldoga Nal., conchostracans Asmussia vulgaris Lutk. and the fishes Psammosteus meandrinus Ag., Ctenurella pskovensis (Obr.) and Bothriolepis cellulosa Pand., Grossilepis tuberculata (Gross), Moythomasia perforata (Gross).

The Pskov Substage has been named after the type section on the bank of the Velikaya River near Pskov in Russia. The exposures occur in the same area where those of the Snetnaya Gora Substage are situated. According to the borehole data, the Pskov Substage is 7–13 m thick, on the base of the exposures in the vicinity of the Izborsk Castle (Russia) it is about 17 m thick (Fig. 90).

The Pskov Substage is represented by grey, in the lower part by pale purplish limestone. The rate of dolomitization grows to the west. In the Karisöödi area, the lower part of the substage consists of dolomite (MgO 20%, insoluble residue 6%) with 3–10-cm-thick clay interlayers. In the east (Tsiistre, Hino, Vungi), the substage is mainly represented by thin-layered, often cavernous dolomite (MgO 20%), partly silty-argillaceous (insoluble residue 12–20%), in the upper part of the section it is calcareous in places (CaO 30-35%). On the east margin of the Haanja Heights (Tiirhanna, Parmu), the lower beds are represented by dolomite, the upper ones by limestone.

The Pskov Substage is rich in fossils. The brachiopods Anatrypa micans (Buch), Atrypa velikaya Nal., Ladogia meyendorfii (Vern.), Ripidiorhynchus pskovensis (Nal.) dominate. Calcareous algae have also been found.

The Chudovo Substage was differentiated on the basis of the exposures in the vicinity of the Town of Chudovo, Russia. In Estonia, the Quaternary cover is thick and the rocks of the Chudovo Substage are not exposed. The substage crops out near the Pskov - Riga highway. Based on the key fossils Ripidiorhynchus tschudovi (Nal.) and Anatrypa heckeri Nal., the age of the substage has been established in the Izborsk outcrops in Russia. The thickness of the substage in the boreholes reaches 13 m. In places (Laura, Vungi, Parmu), the lower boundary of the substage is marked by a pyritized discontinuity surface.

In the easternmost part of its distribution area (Vungi, Parmu), the Chudovo Substage is represented by micro- and cryptocrystalline limestones (CaO 44–49%, insoluble residue 5–10%). Dolomitization of rocks increases westwards and the substage consists of micro- and fine-crystalline dolomitic limestones (CaO 32–34%, MgO 16–17%, insoluble residue 3–7%) to coarse-crystalline cavernous dolomites (CaO 28–29%, MgO 20%, insoluble residue 3–7%). Dolomitic facies is spread west of Misso.

The Pskov and Chudovo substages are lithologically very similar and sometimes it is expedient to treat them together as the Izborsk Member (Table 10).