Geology September 2001 Vol 50/3 []

CONTENTS & ABSTRACTS

InEnglish. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Geology



Volume 50 No. 3September 2001



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

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Ivar Jaek, Galina HÜtt =,Ilmar Rammo, and

ValeriVasilchenko

Abstract. Carbonates of differentorigin, such as Iceland spar, calcites, and mollusc shells, used as electronspin resonance and thermoluminescence palaeodosimeters, were studied in orderto determine their suitability for optically stimulated luminescence dating.The stimulation/excitation spectra of the afterglow of the samples wererecorded in the wavelength range of 250–1100 nm. The results of the studyshow 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 bandsrecorded). The recorded stimulation spectra revealed no spectral bandssensitive to stimulation by ionizing radiation, which would disappear asa 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 luminescentcrystals, including luminescence (palaeo)dosimeters, and the ways of overcomingthe difficulties in optical dating of natural carbonates are discussed.

Key words: carbonates, opticaldating, optically stimulated afterglow.

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

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Heldur NESTOR,Rein EINASTO, Viiu NESTOR, Tiiu MÄRSS,

and Viive VIIRA

Abstract. The lower Wenlock tomiddle Ludlow section of the Riksu drill core from western Saaremaa Island(Estonia), including the stratotype interval of the Riksu Formation (JaagarahuRegional Stage, middle Wenlock) is described. The lower boundary of theformation is redefined in the type section and is placed on the top of theParamaja Member of the Jaani Formation. Three subunits, the Lower, Middle, andUpper Riksu beds, are established. An intricate multistage sedimentarycyclicity 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 RiksuFormation 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 northernEurasia; 174–189

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Tiiu MÄRSS

Abstract. Localities with the actinopterygian Andreolepis in the northern part of Eurasia were revised. Andreolepispetri sp. nov. was established on the basis of distinct morphology andsculpture of the scales, and exoskeletal platelets in the Tabuska Beds, upperLudlow or lower Přidoli of the Ufa River section, the Central Urals. Andreolepis was excluded from the familyLophosteidae 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 thedolomitization of the Mõhküla Beds (Silurian, Estonia); 190–205

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Aada TEEDUMÄE, Toivo KALLASTE, and TarmoKIIPLI

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

X-raydiffraction, X-ray fluorescence, and titration analyses were applied to studythe chemical composition of rocks and differences in the dolomitizingenvironment. The CaCO3/MgCO3 molar ratio and latticeparameters of dolomites, irrespective of the content of insoluble residue orthe texture of rocks, are close to those of ideal dolomites. Compared tolimestones dolomites are depleted in Sr and enriched in Mn. The concentrationsand correlations of these elements vary by lithological varieties along thesection, which supports the suggestion that there was no unique dolomitizingfluid and the dolomitization began soon after the deposition. The geochemistryof dolomites is almost consistent with dolomitization by normal or modified(dilute) sea water. In general, dolomitization correlates with regression. Thearea favourable for dolomitization is related to the inner shelf, migrating inaccordance with the fluctuation of sea level and oscillation of the shoreline.

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