CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Geology

Volume 53 No. 1 March 2004

Surface exposure dating using in situ cosmogenic ¹⁰Be; 3–12

Roland Zech, Uwe Abramowski, Bruno Glaser, Peter W. Kubik,

and Wolfgang Zech

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

Key words: in situ cosmogenic ¹⁰Be, surface exposure dating.

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

Alvar Soesoo, Jaan Kalda, Paul Bons, Kristjan Urtson, and

Volli Kalm

Abstract. The use of nonlinear and chaotic dynamics and fractal approaches in the studies of the Earth has increased in the last decade. A large number of geological objects show a power-law or fractal distribution, which suggests that these objects do not require any characteristic length-scale or time in their definition. Fractals can be used in studies of magma mixing and mingling, mantle convection, lava flows, percolation properties of veins and ore mineralization, and to separate geochemical anomalies. It is shown that the width of migmatitic leucosomes in the Estonian basement rock follows power-law distributions and shows fractal properties. Despite the differences in size and number of measured leucosomes and veins, differences in host rock types and formation conditions, the studied leucosome and vein data set shows good power-law distributions with exponents, m, usually between 1.0 and 1.9. The spacing of leucosomes and veins in rock section is not a random feature, but fractal (fractal dimension D = 0.77–0.79). Using the power-law size distribution for the melt batches, the total volume of the melt phase, as well as the relative contributions of the largest batch (dominant for m < 1) and of the smallest batches (dominant for m > 1), are estimated. The relationship between the magmatic leucosome width-distribution exponent, and the melt batch size-distribution is derived.

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

Possibilities of stratigraphical 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 the Dividal Group in the sub-Caledonian autochthonous complex outcropping in the Lapland area (Finland) are analysed. Based on lithogenetic features, the stratigraphic position of this problematic sedimentary body of Vendian–Cambrian age in the composite section of the East European craton was specified and new data which would help clarify the palaeogeography of that time in the northwestern rims of the craton were obtained.

Due to cleavage accompanying the orogeny, the authigenic minerals, such as pyrite, phosphates, glauconite, carbonates, etc., common in other regions, are not preserved in local rocks. The conditions have also been unfavourable for preservation of fossils and ichnofossils. At the same time, there is no rhythmical thin layering of claystones, characteristic of the Late Vendian in large areas of the East European craton.

Rare ichnofossils, including a few vertical pyrite-filled burrows of bioglyphs, as well as some indirect lithogenetical features, indicate that the Dividal Group probably corresponds to the Rovno Formation, the oldest stage of the East European Lower Cambrian. This interpretation is confirmed by fossils from the upper part of the same complex in the adjoining areas of Norway and Sweden, as well as in the Finnish inland territories, and by the chemical composition of clay rocks.

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

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

Aada Teedumäe, Heldur Nestor, and Toivo Kallaste

Abstract. Medium-rank sedimentary cyclicity of the Raikküla Formation (Raikküla Regional Stage, middle Llandovery) is first described in the Nurme drill core (mid-western Estonia). Four shallowing up submesocycles are distinguished, which group in pairs into two mesocycles, treated as the Lower and Upper Raikküla subformations. In the Nurme drill section, totally dolomitized carbonate rocks of the Raikküla Formation are sandwiched between the unaltered limestones of the Juuru and Adavere regional stages. It suggests early dolomitization during the Raikküla Age soon after deposition. X-ray diffraction, X-ray fluorescence, and titration analyses were applied to study the composition of rocks and lattice parameters of dolomite. Geochemistry of rocks is consistent with dolomitization by normal marine water. No evidence of hypersalinity of the sedimentary environment or inflow of external fluids has been observed. The results of d₁₀₄ measurements showed a trend of growing dolomite stoichiometry towards the top of the regressive, shallowing up submesocycles. The most completely ordered dolomite (< 51 mol% CaCO₃) occurs in the intervals of primarily bioclastic, winnowed sediments, which indicates the promoting role of the agitated-water environment in the dolomitization process. Extensive pervasive dolomitization associated with regressive phases of the evolution of the Baltic Palaeobasin in the Silurian 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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