Keila Stage

L. Hints & 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.

In most of northern Estonia, the Keila Stage (Kegelsche Schicht, Schmidt 1881) comprises the argillaceous bioclastic limestones, with intercalations or occasionally thicker (up to 4 m) intervals of relatively pure limestones of the Kahula Formation (Table 7). Only in a restricted area in northwestern Estonia, the upper part of this formation is replaced by the Vasalemma Formation where the greatest thickness of the Keila Stage (more than 30 m) has been recorded (Fig. 46).

Initially, due to the unclear relationship between the fossilifereous argillaceous limestones of the Keila Stage and the Schmidt‘s “Wassalem’sche Schicht”, the term Keila-Vasalemma Stage was introduced by Bekker (1922, see also Öpik 1930b). Later, Jaanusson (1945) and Männil (1958c, 1963b, 1966) subdivided the Keila Stage into several members and defined the lower boundary of the stage on the level of the thickest K-bentonite (bed “d” by Jürgenson 1958a, see also Jaanusson & Martna 1948, Vingisaar 1972). The composition of the Kahula Formation and the distribution of members overlying the boundary K-bentonite is shown in Figure 47.

The lowermost part of the Keila Stage (Kurtna Member) is represented by argillaceous limestones. The Kurtna Member is overlain by relatively pure limestones, in places with argillaceous intercalations of the Pääsküla Member. This unit, although differently understood by stratigraphers (Nõlvak 1996), can be identified in the core sections of northwestern Estonia as a complex of biomicritic limestones, up to ca. 7 m in thickness (Põlma et al. 1988, Fig. 47). It may be replaced by intercalating argillaceous bioclastic and biomicritic limestones with a thickness of up to 20 m (Ainsaar 1991), seemingly corresponding to a longer time interval than the Pääsküla Member in the sense of Põlma et al. (1988).

The younger part of the Keila Stage comprises the Saue and Lehtmetsa members, the fossiliferous argillaceous limestones and detrital marls with thin layers of argillaceous limestones, respectively. Contemporaneously, the formation of carbonate buildups (interpreted as reefs, Raymond 1916, or bioherms, Männil 1960, or mud mounds, Põlma & Hints 1984) has been developed in northwestern Estonia. They belong to the Vasalemma Formation, nearly constituting the upper half of the Keila Stage in the surroundings of Keila - Vasalemma, whereas a distinct eastward shift of the corresponding facies is recorded during late Keila time (Fig. 48). The Vasalemma Formation, in thickness up to 15 m, consists of several principal lithotypes (Männil 1960, Põlma 1967, Hints 1996). The most characteristic type of rock is the bedded grainstone (cystoid limestone), which is intercalated with clayey limestones in the lower part of the formation. Cystoid limestone consists mainly of skeletal sand particles aggregated with pure calcite cement (content of terrigenous material less than 3%). The grainstones contain irregular buildups, measuring up to 10 m vertically and up to 300 m horizontally and consisting of pure limestones with a low content of skeletal sand (less than 10%) and terrigenous material (up to 6%), occasionally with inclusions of fossiliferous marls. The buildups mostly lack the reef-like framework and are considered as carbonate mounds. Still, in some “mounds” the edrioasteroid Cyathocystis rhizophora Schmidt is frequent and may form frame-like structures. The lower and middle parts of the Vasalemma Formation contain a number of species common with the Kahula Formation - Estlandia pyron silicificata Öpik, Clinambon anomalus (Schlotheim), Horderleyella? kegelensis (Alichova), Sowerbyella (S.) cf. forumi Rõõmusoks a.o., which indicate the Keila Age of corresponding rocks.

In southern Estonia, the Keila Stage comprises the upper part of the bioclastic limestones of the Adze Formation and clayey limestones and marls, which contain some species common with the Blidene Formation in Latvia (Ulst et al. 1982). The lowermost part of the Mossen Formation may also be of Keila Age (Table 7, Meidla 1996). Still, the correlation of the Lukštai and Blidene formations with a unit of siltstone and silty limestone identified in southern Estonia (Ainsaar 1995) needs to be adjusted. Due to this uncertainty, the identification of the Keila Stage is complicated in the transition between the distribution areas of the Kahula and Adze formations.

The total thickness of the Kahula Formation may exceed 30 m, and in northwestern Estonia its main part corresponds to the Keila Stage. In general, the thickness of the Keila Stage part of the formation (mostly 10-15 m) decreases in the southeast direction. In the same direction, the formation becomes lithologically more uniform and argillaceous. In southern Estonia, the thickness of the equivalents of the Keila Stage presumably does not exceed 10(?) m.

A rich and diverse fauna of bryozoans, brachiopods, trilobites, echinoderms and other sedentary and vagile groups (see Rõõmusoks 1970) is distributed in the Kahula Formation. In the upper part of the formation, corresponding to the Keila Stage, several macrofossil taxa are common with the Haljala Stage, but a specific component in this particular association comprises last representatives of several brachiopod genera (Clinambon, Cyrtonotella), trilobites (Asaphus (Neoasaphus) nieszkowskii Schmidt and Toxochasmops maximus (Schmidt)), crinoids (Ristanacrinus marinus Öpik and different baltocrinids) or species characteristic of the Keila Stage only Keilamena occidens (Männil), Longvillia asmusi (Verneuil), Horderleyella? kegelensis (Alikhova). The data on macrofauna come mostly from northern Estonia. In southern Estonia, the Keila Stage is characterised by a brachiopod - trilobite association, which comprises several taxa (Skenidioides, “Sampo”, Eoplectodonta), appearing on a higher stratigraphical level in northern Estonia or being related to the Scandinavian faunas.

The Keila Stage presumably corresponds to the uppermost part of the Diplograptus multidens and the lowermost part of the Dicranograptus clingani graptolite zones (Männil 1990). The lower boundary of the stage, the level of the K-bentonite bed “d” corresponds to the lower boundary of the chitinozoa Angochitina multiplex Subzone (Table 7) and is close to the Northern Atlantic conodont superbus Zone (Männik & Viira 1990).