Uhaku Stage

L. Hints

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 Uhaku Stage comprises, in the revised and amended form (Jaanusson 1960a, Männil 1966, 1976, 1990), the Caryocystites Zone (Jaansoon-Orviku 1927, = Uhaku Stage by Orviku 1940) and the upper part of the Building Limestone (Väo Formation). These two parts of the Uhaku Stage are considered also as substages (Männil 1976, 1990).

The thickness of the Uhaku Stage varies from 5-10 m in western to about 20-25 m in eastern Estonia (Fig. 39). In northern Estonia, the hard bioclastic limestones of the Väo Formation, forming the lower part of the Uhaku Stage, are of a rather stable thickness (4-5 m). The upper part of the Uhaku Stage, made up of relatively thin-bedded argillaceous limestones of the Kõrgekallas Formation, is subdivided (Table 7) into the Koljala, Pärtlioru and Erra members (Männil & Rõõmusoks 1984). The thickness of the formation decreases from about 18 m in northeastern to 1-2 m in northwestern Estonia (Figs. 39, 40). The lower boundary of the Kõrgekallas Formation and the Koljala Member, formed of argillaceous limestones with marly intercalations, supposedly coincides with the lower boundary of the Conochitina tuberculata Zone (Männil & Bauert 1986, p. 17; Table 7). In the Pärtlioru and Erra members, the argillaceous intercalations in the bioclastic limestones are partly kerogeneous. In the Oil Shale Basin in northeastern Estonia (Puura 1986), thin kukersite beds (up to 2 cm) occur, or they form together with limestones and marls distinct intervals (up to 1.6 m) between relatively pure limestones (Männil & Bauert 1986). These are the oldest kukersite beds in the Middle Ordovician sequence in northern Estonia. In central Estonia, the kukersite beds appear in the Kukruse Stage (cf. Männil 1966, 1986).

In southern Estonia and also in Latvia, the Uhaku Stage is represented mainly by micritic limestones with intercalation of bioclastic limestones and marls of the Taurupe Formation (= Furudal Formation in Männil 1966) with a thickness of 6 - 19 m. Only in a few sequences in western Latvia, the Taurupe Formation is over 20 m thick (Ulst et al. 1982, fig. 46). The limestones of the transitional belt between northern and southern Estonia are characterised by an interfingering pattern which resembles that of northern Öland in Sweden (Männil 1966). Both goethitic and phosphatic ooids occur in some places in the basal part of the Uhaku Stage testifying to continuous shift of the oolitic lithofacies in time (Põlma 1982, fig. 7, Pärnu and Ikla cores, Fig. 40).

In northern Estonia, the lower boundary of the Uhaku Stage falls into the lithologically rather uniform Väo Formation. In practice, a prominent discontinuity surface is used as a boundary marker above which several new taxa appear, some of them widespread and frequent throughout the Baltic Basin. Mass occurrence of the trilobite Xenasaphus d. devexus (Eichwald) is recorded from the Island of Osmussaar in the west as far as Ingria (L. Popov and R. Einasto, pers. comm., Alichova 1960, 1969) in the east. Of the graptolites of the Hustedograptus teretiusculus Zone, Gymnograptus linnarssoni (Moberg) is identified from the Oslo Region up to the Moscow Syneclise (Männil 1976). According to Männil (1986), the lower part of the Uhaku Stage corresponds to the Eoplacognathus robustus and E. lindstroemi subzones of the Pygodus serra Zone (Table 7). The upper part of the stage corresponds to the Pygodus anserinus Zone. The latter zonal species appears close to the base of the Kõrgekallas Formation. Nevertheless, on the basis of the distribution of conodonts, the boundary between the Lasnamägi and Uhaku stages is unclear, at least on the subzones level (Table 7). The chitinozoan Conochitina clavaherculi Subzone comprises the most part of the Väo Formation, including the strata with the first finds of G. linnarssoni (Männil 1986, fig. 2.1.1).

The Uhaku Stage comprises a varied sedentary benthic fauna, particularly articulate brachiopods, bryozoans and cystoids. Since there is no generally acknowledged interpretation of the Uhaku Stage, the lists of fossils presented by different researchers comprise taxa from different stratigraphical intervals (Rõõmusoks 1960, 1970; Männil 1963a, 1966).

Macrofossils are poorly known in the subsurface area of the Uhaku Stage (Fig. 40). The Taurupe Formation, which is distributed in southern Estonia, includes many elements, such as Nileus and Upplandiops (=Estoniops sp. n. in Männil 1966, fig. 12) among trilobites, and both Alwynella? and Christiania among articulated brachiopods, which are widely distributed in the Furudal limestones in Sweden (Jaanusson 1960a, 1963, Jaanusson & Ramsköld 1993).