Aseri 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 term Aseri Stage was used first by Bekker (1922, 1923) for the Schmidt´s (1897) Upper Oolitic Limestone (Obere Linsenschicht). In nowadays understanding the Aseri Stage bases in a great deal on the studies carried out by Orviku (Jaansoon-Orviku 1927, Orviku 1929, 1930a, 1940), Rõõmusoks (1960, 1970) and Männil (1966).

In northern and central Estonia, the Aseri Stage is 0.1 - 5 m thick (Fig. 37) and consists of bioclastic limestones with unevenly distributed ooids, predominantly brown ferriferous (goethitic) ooids (Orviku 1940, 1960b). In places, the ooids are frequent in the lower and upper parts of the stage, but in the dolomitic limestones of northeastern Estonia they occur only in the upper part (Fig. 38). White phosphatic ooids are distributed mainly in the westernmost sequences. These, early Middle Ordovician oolitic limestones have been treated as the Kandle Formation (sensu stricto; Männil & Rõõmusoks 1984). Afterwards, Männil (1990, Männil & Meidla 1994) proposed the name Aseri Formation for the oolitic limestones of Aseri Age. Here preference is given to the term Kandle Formation, because in many cases the upper boundary of the Aseri Stage is difficult to determine; it may coincide with the upper boundary of the oolitic limestones or fall into the upper part of it.

The Kandle Formation is subdivided into the Malla (Männil & Rõõmusoks 1984) and Ojaküla (Orviku in Aaloe et al. 1958) members. The lower, Malla Member (Asaphus and Echinosphaerites limestones by Jaansoon-Orviku 1927) is lithologically the most variable part of the Kandle Formation and differs from the predominantly oolitic limestones of the Ojaküla Member (Cephalopod Limestone) in the occurrence of glauconite, e.g. in the surroundings of Jägala, or in the absence of ooids in some places or parts of the sequence. The thickness of the Malla Member decreases from 2.5 m in the eastern to 0.30 m in the central part of the klint area. West of Jägala, the Malla Member is missing and the Aseri Stage is represented by the 10–20-cm-thick sandy oolitic (mainly with phosphatic ooids) limestones of the upper, Ojaküla Member.

The Kandle Formation extends to central Estonia (Fig. 38) with the dominantly grey-coloured limestones of northern sections turning southwards brownish-grey or yellowish-grey. In southern Estonia, the stage is represented by the up-to-9m-thick reddish-brown limestones of the Segerstad Formation (Männil 1966, Männil & Meidla 1994). In the transitional area between the Kandle and Segerstad formations, reddish-brown and mottled limestones with occasional goethitic ooids (Männil 1990, Männil & Meidla 1994) have been distinguished. They belong presumably to the upper part of the Rokiškis Formation (Laškov et al. 1984). In practice, identification of the latter unit in sections seems in a great deal subjective, and its distribution area is difficult to determine.

The lower boundary of the Aseri Stage in recent use was defined by Orviku (Jaansoon-Orviku 1927, Orviku 1929). In contrast to Bekker (1922), he excluded from that stage the lowermost part of the oolitic limestones which comprises several early Ordovician (Oelandian) taxa (Ahtiella baltica Öpik, Antigonambonites sp., Megistaspis sp., Rõõmusoks 1970 p. 30). The boundary is marked by essential changes in the faunal composition, especially in trilobites and cephalopods (Rõõmusoks 1970, table 3, see also Jaanusson 1960a, 1963). Notable is the disappearance of the trilobite genus Megistaspis and appearance of Asaphus (Neoasaphus), represented at least by six species in northern Estonia (Rõõmusoks 1970; table 3). Asaphus platyurus, a characteristic species in the Segerstad Limestone in Sweden and Latvia (Jaanusson 1960a, Männil 1963b, 1966) occurs also in southern Estonia (Karula core, Männil 1966, fig. 12). Of new faunal elements, Echinosphaerites as a quite easily notable fossil is also worth of mentioning (Jaansoon-Orviku 1927, p. 15, 16; Orviku 1929, p. 9-11).

The data published on the distribution of ostracodes in the Aseri Stage in Estonia is scanty (Sarv 1959, Männil 1966). The ostracode Pinnatulites procera Zone of the Kunda Stage is replaced by the Piretella tridactyla Zone in the Aseri Stage (Meidla & Sarv 1990). In several core sections (Männil 1966, figs. 12-14), Euprimites effusus Jaanusson appears close to the lower boundary of the Aseri Stage.

The chitinozoans are known only in the grey-coloured rocks of the Kandle Formation which comprises the Cyatochitina regnelly and C. striata zones (Table 7). The boundary between these zones coincides with the boundary between the Malla and Ojaküla members.

The Aseri Stage corresponds to the lower part of the Didymograptus murchisoni graptolite zone and roughly to the Eoplacognathus suecicus conodont zone (Männil 1990, Männik & Viira 1990, Einasto et al. 1996).