Silurian

by H. Nestor

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 first stratigraphical classification of the Silurian rocks in Estonia was worked out by Schmidt (1858, 1881, 1892). Bekker (1922, 1925) and Luha (1930, 1933, 1946) established the present nomenclature of the Silurian regional chronostratigraphical units - regional stages. Lithostratigraphical divisions have been adequately defined in the monograph “The Silurian of Estonia” (Kaljo 1970c). Further amendments to the stratigraphical nomenclature and correlation with the sequences of the adjacent areas have been published in the unified regional stratigraphical charts of the Baltic Republics (Resheniya… 1978) and of the East-European Platform (Resheniya… 1987). The latest version of the Silurian stratigraphical chart, approved by the Stratigraphical Commission of Estonia, was published by H. Nestor (1995a) and is followed in the present publication (Table 8 ).

The Silurian sequence in Estonia consists of ten regional stages grouped directly into the series of the global chronostratigraphical standard. In most cases the boundaries of the regional stages and series have been considered more or less congruent, based on the graptolite or conodont datings (Kaljo 1962, Viira 1982). An exception is the Wenlock/Ludlow boundary which is only conventionally fitted with the junction of the Rootsiküla and Paadla stages. The lower limit of the Silurian System coincides with the boundary between the Porkuni and Juuru stages. It is proved by the presence of the Hirnantian trilobites and brachiopods in the Kuldiga and Saldus formations of the Porkuni Stage and records of Stricklandia lens prima Williams from the lowermost beds of the Varbola Formation of the Juuru Stage (Kaljo et al. 1988b).

Based on the sharply expressed lateral, facies changes of the Silurian rocks, the Mid-Estonian and South-Estonian confacies belts have been distinguished (Kaljo 1977). The Mid-Estonian Confacies Belt is dominated by various lime- and dolostones, rich in shelly fauna. The belt covers the islands of the West-Estonian Archipelago and the western and central parts of mainland Estonia (Table 8). In the latter area, the Silurian sequence is less complete; its upper part has undergone severe dolomitization. The South-Estonian Confacies Belt consists mostly of marl- and mudstones with a more unilateral deeper-water shelly fauna, graptolites and planktonic microfossils (chitinozoans). Within the confacies belts separate sets of lithostratigraphical units have been established.

Many parts of the Silurian sequence have a clearly expressed cyclical nature, especially in the more shallow-water Mid-Estonian Confacies Belt. In such cases a cyclostratigraphical unit, the so-called beds consisting of alternating types of rocks with a certain trend of succession, has been distinguished and treated as a subdivision of formation. In some cases formations can be subdivided into members.