In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.



Volume 50 No. 3 September 2001


Search for the dose-sensitive optically stimulated luminescence response in natural carbonates; 139–148

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Ivar Jaek,  Galina HÜtt =, Ilmar Rammo, and

Valeri Vasilchenko

Abstract. Carbonates of different origin, such as Iceland spar, calcites, and mollusc shells, used as electron spin resonance and thermoluminescence palaeodosimeters, were studied in order to determine their suitability for optically stimulated luminescence dating. The stimulation/excitation spectra of the afterglow of the samples were recorded in the wavelength range of 250–1100 nm. The results of the study show that these spectra present either excitation spectra of Mn2+ ion fluorescence (samples of calcites and Iceland spar, red emission recorded) or the excitation spectra of primary phosphorescence (samples of carbonates, including mollusc shells; short-wave emission bands recorded). The recorded stimulation spectra revealed no spectral bands sensitive to stimulation by ionizing radiation, which would disappear as a result of heating and could thus be related to deep traps in carbonates, needed in dating. The causes of this situation which is unusual in luminescent crystals, including luminescence (palaeo)dosimeters, and the ways of overcoming the difficulties in optical dating of natural carbonates are discussed.

Key words: carbonates, optical dating, optically stimulated afterglow.

Description of the type section, cyclicity, and correlation of the Riksu Formation (Wenlock, Estonia); 149–173

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and Viive VIIRA

Abstract. The lower Wenlock to middle Ludlow section of the Riksu drill core from western Saaremaa Island (Estonia), including the stratotype interval of the Riksu Formation (Jaagarahu Regional Stage, middle Wenlock) is described. The lower boundary of the formation is redefined in the type section and is placed on the top of the Paramaja Member of the Jaani Formation. Three subunits, the Lower, Middle, and Upper Riksu beds, are established. An intricate multistage sedimentary cyclicity of the Riksu Formation is described; the meso-, submeso-, mini-, and sub­minicycles are provisionally equated with 400, 100, 41, and 23 Ka Milankovitch cycles, respectively. The Riksu Formation is roughly correlated with its lateral equivalents – the Jaagarahu, Muhu, and Jamaja formations in Estonia, and with the successive upper Högklint, Tofta (Kopparsvik), and Slite beds on Gotland.

Key words: correlation, cyclicity, type section, Estonia, Wenlock, Silurian.

Andreolepis (Actinopterygii) in the Upper Silurian of northern Eurasia; 174–189

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Abstract. Localities with the actinopterygian Andreolepis in the northern part of Eurasia were revised. Andreolepis petri sp. nov. was established on the basis of distinct morphology and sculpture of the scales, and exoskeletal platelets in the Tabuska Beds, upper Ludlow or lower Přidoli of the Ufa River section, the Central Urals. Andreolepis was excluded from the family Lophosteidae Gross, 1969 and placed into a new family Andreolepididae fam. nov.

Key words: Andreolepis, Andreolepididae fam. nov., order incertae sedis, Actinopterygii, Osteichthyes, upper Ludlow–lower Přidoli, Upper Silurian.

Aspects of the dolomitization of the Mõhküla Beds (Silurian, Estonia); 190–205

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Abstract. The dolomites of the Mõhküla Beds represent the uppermost middle Llandovery in the northeastern, marginal part of the Baltic Palaeobasin. Secondary dolomitization has changed the primary composition of rocks, but according to the geological data available they originate from the normal-marine sediments of the shelf facies.

X-ray diffraction, X-ray fluorescence, and titration analyses were applied to study the chemical composition of rocks and differences in the dolomitizing environment. The CaCO3/MgCO3 molar ratio and lattice parameters of dolomites, irrespective of the content of insoluble residue or the texture of rocks, are close to those of ideal dolomites. Compared to limestones dolomites are depleted in Sr and enriched in Mn. The concentrations and correlations of these elements vary by lithological varieties along the section, which supports the suggestion that there was no unique dolomitizing fluid and the dolomitization began soon after the deposition. The geochemistry of dolomites is almost consistent with dolomitization by normal or modified (dilute) sea water. In general, dolomitization correlates with regression. The area favourable for dolomitization is related to the inner shelf, migrating in accordance with the fluctuation of sea level and oscillation of the shoreline.

Key words: Silurian, dolomitization, lithology, Palaeozoic dolomite, X-ray diffraction, Estonia.