Composition of carbonate rocks of the Idavere, Jõhvi and Keila regional stages (Viruan, Ordovician) in East Estonia

The composition of Estonian Paleozoic carbonate rocks has been studied for long period on the level of major components. During the official geological mapping of the territory of Estonia, taken place from 1960-s to 1980-s, a huge amount of analyses was made for many microelements from hundreds of boring cores. Most of these analyses, mainly semi-quantitative spectral analyses with abbreviated chemical analyses, were made in the laboratory of Geological Survey of Estonia in Tallinn. This material is partly analyzed statistically and published by P. Vingisaar, T. Kiipli, et al. (Vingisaar et al., 1979; 1981; Kiipli et al., 1984). The aim of this study was to follow the distribution of major and minor elements in one sedimentary complex, limited in North Estonia by considerable discontinuity surfaces in the upper and lower boundary. This complex, including the Idavere, Jõhvi and Keila stages, is described as a distinct macrocyclite (Kõrts et al., 1991) and can be possibly subdivided into two mesocyclites. The upper mesocyclite (upper part of the Jõhvi Stage and Keila Stage) differs from lower one by clear microcyclic subdivision consisting up to 25 limestone-argillaceous limestone (or marl) cycles, traced in the whole extent of North-Estonian confacies belt (Ainsaar, 1992; 1993). Carbonate rocks of the Idavere, Jõhvi and Keila stages formed lithostratigraphically Kahula Formation (Resheniya..., 1978), or Kahula Group (Resheniya..., 1987) by previous stratigraphic charte; later this name was used only for rocks of the Jõhvi and Keila stages (Männil, 1990). In this study the name of Kahula Group is used in its initial size to mark the whole Idavere-Keila complex in North Estonia. The complex is characterized by more or less argillaceous, wavy-bedded or semi-nodular limestone (mainly wackestone) with intercalations of mari and several thin K-bentonite layers. The thickness of the Kahula Group is 30-50 m.