Tilžė Stage

by A. Kleesment & E. Mark-Kurik

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.

Liepinš was the first to acknowledge the Tilžė beds as an independent stratigraphic unit - the Lower Stoniškiai Formation (Liepinš 1955). Paškevičius (1959) determined the present stratigraphic extent of the stage and gave it the present name. Faunistically, it was determined by Karatajute-Talimaa (1962). The stratotype of the Tilžė Stage is in the interval of 1104.5–1212 m of the drill core Stoniškiai-1 in southwestern Lithuania.

The Tilžė Stage is spread in southeastern Estonia and covered with rocks of the Rēzekne Stage. It has been determined only in the Laanemetsa and Värska drill cores, but is assumed to be present also in the Väimela drill core. The sediments are absent in the Mõniste High. The total thickness of the stage varies from 2.1 to 17.7 m (Fig. 74).

The Tilžė Stage lies with a stratigraphical unconformity on the rocks of different stages of Ordovician age (Figs. 75, 76, 77). It has yielded several thelodonts of stratigraphical value: Turinia pagei (Powrie), Turinia sp., Nikolivia gutta Kar.-Tal. and N. elongata Kar.-Tal. The other fossil fishes (psammosteid heterostracans, acanthodians) are identified only on the group level (Sorokin 1981).

In the Baltic region, the Tilžė Stage is represented by the Tilžė Formation which in Estonia is composed of grey and purplish-grey horizontally bedded silt- and sandstones with interlayers of grey clay and yellowish-grey dolomite. Silt- and sandstones are predominantly strongly cemented with dolomitic or gypsum (Värska) matrix. Siltstone is often mottled, conglomeritic sandstone occurs in the basal part.

The rocks of the Tilžė Formation are quartzose or feldspatic arenites with the content of quartz 60–85%. Micaceous arenites (content of micas up to 60%) occur in some places.

The heavy fraction is dominated by the group of transparent allothigenic minerals (50–70% ). Garnet with a considerable supplement of zircon is prevailing. Tourmaline and apatite are also important (Fig. 78).