Kunda Stage

T. Meidla

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 Kunda Stage (Kunda Formation by Raymond 1916) is represented by oolitic, glauconitic (Lamansky 1905) and sandy limestones corresponding to the emended Vaginatum Limestone by Schmidt (1897). A three-part subdivision of the strata, based on the trilobite zonation, was introduced already by Lamansky (1905) in the Leningrad Region. He also assumed the absence of the lower unit - the Asaphus expansus Zone in northern Estonia which was afterwards confirmed by several authors (Raymond 1916, Orviku 1960a, Männil 1966, a.o.). Orviku (1958b) proposed to name the Lamansky’s subdivisions in ascending order the Hunderum, Valaste and Aluoja substages, and presented a detailed lithostratigraphical description of the corresponding interval in northern Estonia (Orviku 1958b, 1960a, b; Fig. 33).

In northern Estonia, the Kunda Stage comprises the Valaste (corresponding to the Asaphus “raniceps” Zone) and the Aluoja (zones of Megistaspis obtusicauda and Megistaspis gigas) substages (Fig. 32, Table 7). In most of northern Estonia, the lower boundary of the Valaste Substage is drawn at the base of the oolitic limestone of the Sillaoru Formation, or locally within the unit, being marked by a discontinuity surface on the boundary between the Pada and Voka members, and the disappearance of the glauconite grains, characteristic of the underlying strata (Fig. 33).

In northwestern Estonia, the Kunda Stage is represented by the Pakri Formation, eastwards replaced by the upper part of the Sillaoru Formation and the Loobu Formation (Fig. 33). In northeastern Estonia, the Napa Formation forms the topmost part of the Kunda Stage and grades into the Rokiškis Formation in central Estonia. In southern Estonia, the entire Kundan sequence, including the Hunderum Substage, is represented by the Šakyna and Baldone formations (Fig. 36).

The Pakri Formation (Öpik 1927), up to 4.5 m of yellowish-grey sandy limestones and calcareous sandstones, sometimes with conglomerate beds, occurs in northwestern Estonia in the area west of Tallinn. In the westernmost part of its distribution area, the main, lower part of the Pakri Formation consists of up-to-4m-thick nodular kerogenous calcareous sandstone of the Suurupi Member, overlain by the thin (0.5 m) sandy limestone of the Osmussaar Member. In the surroundings of Tallinn, the formation is represented by limestones with quartz and glauconite grains, locally with a basal conglomerate (Kallaste and Jägala members, up to 0.8 m). On the Island of Osmussaar and, to a lesser extent, in the neighbouring mainland areas, a system of sedimentary dikes cuts through the Pakri Formation and the underlying Volkhov-Billingen strata (Puura & Tuuling 1988). The time of the formation of the dikes is dated as middle-late Kunda.

In most of northern Estonia (except the distribution area of the Pakri Formation) and in central Estonia, the basal part of the Kunda Stage consists of the oolitic limestone of the Sillaoru Formation (Resheniya… 1978, Männil & Rõõmusoks 1984). The main, Valaste time part of the formation (Voka Member, up to 0.6 m) consists of clayey limestones with abundant ferriferous ooids, developed around skeletal particles or glauconite grains (Mägi 1984). Among the skeletal particles, fragments of trilobites and ostracodes dominate (50-70%, Mägi 1984). The Voka Member generally serves as a good marker level in the North-Estonian sequence, although in restricted areas of northern and northeastern Estonia it overlies the thin oolitic Pada Member which differs from the main part of the formation by the presence of glauconite grains and has been included in the Volkhov Stage by Orviku (1960a) and subsequent authors. In northern and northeastern Estonia, the Loobu Formation constitutes the main part of the Kunda Stage. Detailed study of the formation (Orviku 1958b, 1960a) has revealed its two-part subdivision; both the lower and the upper parts consist of two laterally equivalent units. In central northern Estonia, east of Tallinn, the formation is represented by clayey limestone of the Nõmmeveski Member (up to 2 m) and glauconitic limestone of the Ubari Member (up to 2 m, Fig. 33). In northeastern Estonia, the lower part of the formation consists of glauconitic limestone of the Utria Member (up to 3 m), overlain by clayey limestones of the Valgejõgi Member (up to 4.7 m, Männil 1987). In the outcrop area, large nautiloids Cyclendoceras vaginatum (Schlotheim), Estonioceras ariense (Schmidt), Para-cyclendoceras cancellatum Eichwald etc., (Rõõmusoks 1960) are characteristic of most of the Loobu Formation. In northeastern Estonia, the rocks have undergone extensive dolomitization resulting in a mottled red colour and cavernous structure. The Loobu Formation reaches its maximum thickness (7 m) in the central part of northern Estonia (Fig. 36), in central Estonia it is less than 3 m thick (Resheniya… 1987). In that area the formation consists of grey, partly clayey glauconitic limestones, overlying the oolitic limestones and marls of the Sillaoru Formation (0.5 m).

The Napa Formation, an oolitic marl and limestone body (up to 4 m), is supposed to replace the upper part of the Loobu Formation in northeastern and central Estonia (Fig. 33).

The relation of the formations and members forming the Kunda Stage in northern Estonia is well demonstrated by Orviku (1960a). The correlation is largely based on the trilobite evidence. Asaphus “raniceps” Dalman has been identified in the Suurupi Member of the Pakri Formation and in the lower part of the Loobu Formation (Nõmmeveski Member). The Osmussaar Member comprises Pseudoasaphus globifrons (Eichwald), which is known from the upper part of the Loobu Formation (Ubari and Valgejõgi members). The Napa Formation is characterized by Megistaspis gigas Angelin (Resheniya… 1978, Mägi 1990). In terms of conodont zonation, the Valaste and Aluoja substages roughly correspond to the Eoplacognathus variabilis Zone (Fig. 32). The shelly fauna is represented by brachiopods, ostracodes, gastropods and cephalopods (Öpik 1927, Sarv 1959, Rubel 1961, Mägi 1990).

In central Estonia, the Napa Formation grades into the Rokiškis Formation (Fig. 36), which is represented by red mottled oolithic limestone (up to 15 m). The fauna of this unit is poorly known in Estonia. Based on Panderodus cf. sulcatus (Fåraeus) and Pinnatulites procera (Kummerow) recorded by Männil (Resheniya… 1987), the Kunda-Aseri age has been suggested. In southern Estonia, the sequence of the Loobu and Rokiškis formations grades into the Šakyna and Baldone formations, represented by grey glauconitic limestone and clayey red limestone, respectively. Palaeontologically, these units are poorly characterized in Estonia and the age relationship to the northern and central Estonian sequences is obscure. The fauna of the Šakyna Formation in Latvia contains trilobites, more rarely brachiopods and graptolites, the Baldone Formation is more fossiliferous (Gailite & Ulst 1975, Ulst et al. 1982). In this area the succession of the above-named formations comprises the entire Kunda Stage and stratigraphically the section of southern Estonia is the completest in this interval.

The thickness of the Kunda Stage demonstrates an obvious decreasing trend towards northwestern Estonia. In most of northern and central Estonia, it does not exceed 10 m, but in southeastern Estonia may locally reach 20 m (Fig. 36).