Lasnamägi 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.

In northern Estonia, the fairly uniform Early Viru sequence of comparatively thick-bedded, hard bioclastic limestones abounding in discontinuity surfaces (Saadre 1992, 1993), was first distinguished as a separate unit - the Building Limestone (Baukalkstein), by Orviku (Jaansoon-Orviku 1927). Subsequently, this unit, determined mainly by the lithological criteria, was termed (Orviku 1940) the Lasnamägi Stage after the sections in the Lasnamägi quarry in the northeastern part of Tallinn. The Lasnamägi Stage is well-exposed also in some other sections, including Suhkrumägi (Photo 20) and Mäekalda (see Einasto et al. 1996, fig. A16) in the vicinity of the type section. In general lines, Orviku’s interpretation of the Lasnamägi Stage kept valid until the 1970s (Jaanusson 1945, Rõõmusoks 1960, 1970, Männil 1963a). In 1966, Männil (1966) stated that the Building Limestone comprises two distinct successive faunal associations of which the upper one with several characteristic trilobites, such as Xenasaphus devexus devexus (Eichwald), Asaphus (Neoasaphus) lepidus Törnquist, and graptolites including Gymnograptus linnarssoni (Moberg), is closely related to the fauna of the overlying argillaceous limestones of the Uhaku Stage. The beds containing the “upper” fauna were included (Männil 1966, 1976, Resheniya… 1978) to the Uhaku Stage, while the term Lasnamägi Stage was restricted to the lower half of the Väo Formation (Photo 21) in recent use (Männil & Rõõmusoks 1984), corresponding roughly to the Kallaste Substage by Rõõmusoks (1970), and to the lower part of the former Building Limestone.

The 4–10-m-thick Väo Formation (Fig. 39) is subdivided into three units; in ascending order these are the Rebala (the relatively argillaceous part, thickness up to 3 m), Pae (dolomites, up to 1.5 m) and Kostivere ( hard limestones, up to 6 m) members. Besides, in the stratotype area where the formation has a detailed bed-by-bed stratification, each layer has a name of its own given by quarry-workers (Mägi 1990, Einasto et al. 1996). Männil (1976) included to the Lasnamägi Stage the Rebala and Pae members and the lower part of the Kostivere Member, up to the discontinuity surface, above which Gymnograptus linnarssoni appears.

In northern Estonia, the lower, Lasnamägi part of the Väo Formation is up to 4.5 m and in the stratotype area at Lasnamägi up to 4 m thick (Männil 1976, Männil & Saadre 1987, Mägi 1990). Still in many sections the exact level of the upper boundary of the Lasnamägi Stage is not established and in Fig. 39 the total thickness of the Väo Formation is given. In southern Estonia, the Lasnamägi Stage is represented by red (lower part) to grey (upper part) bedded, mostly micritic limestones of the Stirna Formation (Ulst & Gailite 1976), equivalent to the Seby and Folkeslunda limestones of Öland Island and mainland Sweden (Männil & Meidla 1994). The Stirna Formation is up to 15 m thick (Fig. 39) which corresponds to the maximum thickness of the formation in Estonia and northwestern Latvia (Ulst et al. 1982, fig. 45). In the transitional area between the Väo and Stirnas formations in central Estonia, the oolitic lithofacies is developed (Põlma 1982, fig. 7).

In northern Estonia, the lower boundary of the Lasnamägi Stage falls into the upper part of the oolitic limestones, predominantly with goethitic ooids, of the Kandle Formation. It is marked by the discontinuity surface above which there appear brachiopods (Equirostra, Noetlingia), trilobites (Illaenus schroeteri (Schlotheim), Illaenus schmidti Nieszkowski, cephalopods (Lituites sp.) and others (Jaanusson 1945, Rõõmusoks 1970). The level of the appearance of phosphatic ooids in the top of oolitic limestones is used as the lower boundary of the Lasnamägi Stage, if the boundary discontinuity surface is absent or the palaeontological data are insufficient.

The lists of fossils published earlier for the Lasnamägi Stage (Rõõmusoks 1970, table 4) can be used with consideration that only the data from the Kallaste Substage by Rõõmusoks (1970) characterize the Lasnamägi Stage in recent meaning. In northern Estonia, the macrofauna is quantitatively dominated by sedentary forms, particularly articulate brachiopods and bryozoans (Jaanusson 1984). Cephalopods occur mostly in lower quantities, except northeastern Estonia where the lowermost beds of the stage abound in orthocones and where lituitids are also fairly common. The same groups of fossils seem to be relatively abundant in core sections as well (Hints & Põlma 1981).

Important information on the range of North Atlantic conodont zones (see Bergström, 1971 for the reference), graptolites and chitinozoans of the Lasnamägi Stage in northern Estonia is provided by Männil (Männil 1976, fig. 2; 1986, fig. 2.1.1). According to him, the stage is comparable to the Eoplacognathus foliaceus Subzone and the main lower part of E. reclinatus Subzone. Although the chitinozoans are not very dignostic for the distinction of the Lasnamägi Stage, the Cyathochitina sebyensis Zone is a good marker for the Aseri-Lasnamägi boundary beds (Table 7).