Geology March 2004 Vol 53/1 []

CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of theEstonian Academy of Sciences.

Geology



Volume 53 No. 1March 2004



Surfaceexposure dating using in situ cosmogenic 10Be; 3–12

RolandZech, Uwe Abramowski, Bruno Glaser, Peter W. Kubik,

and Wolfgang Zech

Abstract. Surface exposure dating using in situ cosmogenic 10Bein quartz is an important new tool in Quaternary research. We give anintroduction into the method and describe, how 10Be surface exposureages are measured and calculated at the Institute of Soil Science and SoilGeography at the University of Bayreuth, Germany.

Key words: in situ cosmogenic 10Be, surface exposuredating.

Fractality in geology: apossible use of fractals in the studies of partial melting processes; 13–27

Alvar Soesoo, Jaan Kalda, Paul Bons, KristjanUrtson, and

Volli Kalm

Abstract. The use of nonlinear and chaotic dynamics and fractal approaches inthe studies of the Earth has increased in the last decade. A large number ofgeological objects show a power-law or fractal distribution, which suggeststhat these objects do not require any characteristic length-scale or time intheir definition. Fractals can be used in studies of magma mixing and mingling,mantle convection, lava flows, percolationproperties of veins and ore mineralization, and to separate geochemicalanomalies. It is shown that the width of migmatitic leucosomes in the Estonianbasement rock follows power-lawdistributions and shows fractal properties. Despite the differences insize and number of measured leucosomes and veins, differences in host rocktypes and formation conditions, the studiedleucosome and vein data set shows good power-law distributions with exponents, m, usually between 1.0and 1.9. The spacing of leucosomes and veins in rock section is not a randomfeature, but fractal (fractal dimension D = 0.77–0.79). Usingthe power-law size distribution for the meltbatches, the total volume of the melt phase, as well as the relativecontributions of the largest batch (dominant for m < 1) andof the smallest batches (dominant for m > 1), are estimated. The relationshipbetween the magmatic leucosome width-distribution exponent, and the meltbatch size-distribution is derived.

Key words:fractality, power-law distribution, self-similarity, migmatite, Estonianbasement.

Possibilities ofstratigraphical interpretation of the Dividal Group in the Kilpisjärvi area (Finnish Lapland),based on lithogenetic characteristics; 28–41

Enn Pirrus

Abstract. Lithological characteristics of the sedimentary rocks of theDividal Group in the sub-Caledonian autochthonous complex outcropping in theLapland area (Finland) are analysed. Based onlithogenetic features, the stratigraphic position of this problematicsedimentary body of Vendian–Cambrian age in the composite section of theEast European craton was specified and new data which would help clarify thepalaeogeography of that time in the northwestern rims of the craton wereobtained.

Due to cleavage accompanying theorogeny, the authigenic minerals, such as pyrite, phosphates, glauconite, carbonates, etc., common in otherregions, are not preserved in local rocks. The conditions have also beenunfavourable for preservation of fossils and ichnofossils. At the same time,there is no rhythmical thin layering ofclaystones, characteristic of the Late Vendian in large areas of the EastEuropean craton.

Rare ichnofossils, including afew vertical pyrite-filled burrows of bioglyphs, as well as some indirectlithogenetical features, indicate that the Dividal Group probably correspondsto the Rovno Formation, the oldest stage of the East European Lower Cambrian.This interpretation is confirmed by fossils from the upper part of the samecomplex in the adjoining areas of Norway and Sweden, as well as in the Finnishinland territories, and by the chemical composition of clay rocks.

Key words: palaeogeography, Vendian, Lower Cambrian, Scandinavia, Finland.

Sedimentary cyclicity anddolomitization of the Raikküla Formation in the Nurme drill core (Silurian,Estonia); 42–62

Aada Teedumäe, Heldur Nestor, and ToivoKallaste

Abstract. Medium-rank sedimentary cyclicity ofthe Raikküla Formation (Raikküla Regional Stage, middle Llandovery) is firstdescribed in the Nurme drill core (mid-western Estonia). Four shallowing up submesocycles are distinguished, which group in pairsinto two mesocycles, treated as the Lowerand Upper Raikküla subformations. In the Nurme drill section, totallydolomitized carbonate rocks ofthe Raikküla Formation are sandwiched between the unaltered limestones of theJuuru and Adavere regional stages. It suggests early dolomitization during theRaikküla Age soon after deposition. X-ray diffraction, X-ray fluorescence, andtitration analyses were applied to study the composition of rocks and lattice parametersof dolomite. Geochemistry of rocks is consistent with dolomitization by normalmarine water. No evidence of hypersalinity of the sedimentary environment or inflow of external fluids has beenobserved. The results of d104 measurements showed a trend of growing dolomite stoichiometry towardsthe top of the regressive, shallowing up sub­mesocycles. The mostcompletely ordered dolomite (< 51 mol% CaCO3) occurs inthe intervals of primarily bioclastic, winnowed sediments, which indicates thepromoting role of the agitated-water environmentin the dolomitization process. Extensive pervasive dolomitization associatedwith regressive phases of the evolution of the Baltic Palaeobasin in theSilurian and was related to a zone of shallow normal-saline inner shelf facies,migrating in space in accordance with sea level changes.

Key words: Silurian, dolomitization, sedimentary cyclicity, dolomite,X-ray diffraction, Estonia.

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