|Etymon||Kukruse, est. (E)|
|Age top (Ma)||458.2|
|Age base (Ma)||460.9|
|Age reference||Cooper & Sadler, 2004|
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 Kukruse Stage (Kuckerssche Schicht by Schmidt 1879, 1881) as a stratigraphical unit comprises the commercially exploited oil shale (kukersite) seams (Chapter X) and the richest and most diverse faunal assemblage in the Ordovician of Estonia represented by more than 330 species and subspecies (Rõõmusoks 1970, table 10).
The stratigraphy of the Kukruse Stage has been dealt with in several papers (Rõõmusoks 1957, Männil 1984, Männil & Bauert 1984, 1986, Bauert 1993, Saadre & Suuroja 1993b). The bed-by-bed stratification of the kukersite complex with special sets of indices for the individual kukersite seams form the base for the correlation of the sequences within the kukersite basin (Männil 1984, fig. 2; Bauert & Puura 1990).
The thickness of the Kukruse Stage (Fig. 41) ranges from about 3 m in western to more than 20 m in eastern Estonia (Saadre & Suuroja 1993a). The stage consists of three lithologically distinct formations. The argillaceous bioclastic limestones with intercalations of kukersite (oil shale) and marls of the Viivikonna Formation (Männil & Rõõmusoks 1984) are distributed northeast of the line Osmussaar Island (southwestern Estonia) - Mehikoorma (south coast of Lake Peipsi) (Fig. 41). Based on the frequency of kukersite seams or the content of the kerogenous component, the Viivikonna Formation is subdivided into the Kiviõli, Peetri and Maidla members (Fig. 42). The Kiviõli (lower) and Peetri (upper) members differ from the Maidla (middle) Member by the occurrence of 10—14-cm-thick kukersite seams, while the middle part of the formation consists of kerogenous and variously argillaceous limestones (Männil et al. 1986, Bauert 1993, figs 3, 4). Due to the facies shift of the kukersite beds (Männil et al. 1986), the boundaries of the Viivikonna Formation are diachronous. As a result, the upper part of the Viivikonna Formation (Peetri Member) is missing in northeastern Estonia, but it is exposed in the vicinity of Tallinn (Fig. 42, Nõlvak & Hints 1996) and is well-known by core sections south of the outcrop area (Männil 1984, Männil & Bauert 1984, Männil & Saadre 1987).
Westwards, the Viivikonna Formation grades into the bioclastic limestones of the Pihla Formation with a thickness of about 3 - 6 m (Saadre & Suuroja 1993b) and southwards into the limestones with dark pyritized skeletal detritus and nodular intercalations of argillaceous marls of the Dreimani Formation (Fig. 41, Springis 1974). The thickness of the latter varies from 7 to 14 m and only in southeastern Estonia it is about 20 m, which is nearly the same as in eastern Latvia (Ulst et al. 1982, fig. 47).
For the lower boundary of the Kukruse Stage, Bekker (1923, 1924b) proposed the base of the lowermost commercially important kukersite seam “A” at the base of the Viivikonna Formation. The renovation of faunal association begins with the appearance of new bryozoans in seam “A”. Somewhat higher, in seam “C” several new species, including the brachiopods Bilobia musca (Öpik), Sowerbyella (S.) liliifera (Öpik), Estonomena estonensis (Bekker), and the trilobites Asaphus (Neoasaphus) nieszkowskii Schmidt, Estoniops exilis (Eichwald), Paraceraurus aculeatus (Eichwald) appear (Rõõmusoks 1970, table 9). In western and southern Estonia, the base of the Pihla or Dreimani Formation is used as the lower boundary of the Kukruse Stage. This level is marked by the appearance of indicator ostracodes Baltonotella kuckersiana (Bonnema), Conchoprimitia leperditioides Thorslund, Euprimites locknensis Thorslund and others, several of which are common with the lower part of the Dalby Limestone in Sweden (Männil 1966, Jaanusson 1976). At the same time, several early Viru taxa, such as Chasmops odini odini Eichwald, Sowerbyella (Viruella) uhakuana (Rõõmusoks), Platystrophia biforata (Schlotheim), Dianulites fastigiatus (Eichwald) and others, disappear close to the lower boundary of the Kukruse Stage (Rõõmusoks 1970, p.156, 157). The graptolite Orthograptus uplandicus whose range zone corresponds to the Kukruse Stage (Männil 1984) and the chitinozoa Cyathochitina savalaensis appear roughly on the lower boundary of the Kukruse Stage. In all likelihood, also the boundary between the North Atlantic conodont anserinus and tvaerensis zones (Männil & Bauert 1986) falls into the lower part of the Kukruse Stage.
The diverse assemblage of Kukruse macrofossils is represented first of all by bryozoans (more than 60 species), brachiopods (about 90 species) and trilobites (about 50 species: Rõõmusoks 1970, table 10) which form about two thirds of the species identified. The most abundant and diverse association occurs in the Kiviõli Member in the lower part of the stage. Still some species, such as Hesperorthis inostrantzefi inostrantzefi (Wysogorski), Echinosphaerites aurantium suprum Hecker, are notable due to their mass occurrence in the upper part of the stage (Rõõmusoks 1970, p. 169). The character of the distribution of some brachiopods and trilobites, such as Estlandia marginata magna Öpik, Otarion planifrons (Eichwald), Pharostoma nieszkowskii (Schmidt) and others, shows a facies shift from the lower part of the Kukruse Stage (Kiviõli Member) in northeastern to the upper part (Peetri Member) in northwestern Estonia.
In the core sections, macrofossils are of secondary importance due to their scarcity, especially in western Estonia (Fig. 40). Still, the occurrence of some species should be noticed. In some northernmost core sections, the brachiopod Kullervo panderi (Öpik) marks the lowermost part of the Kukruse Stage (Rõõmusoks 1970). In the outcrops, this species appears presumably in the kukersite seam “G”, which lies 1-4 m above the lower boundary of the stage. In the southern periphery of the Viivikonna Formation and in the Dreimani Formation, Asaphus (Neoasaphus) ludibundus Törnquist and Bilobia musca (Öpik) appear in the Kukruse Stage and in some areas Echinosphaerites becomes frequent.