CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Geology

 

Volume 49 No. 3 September 2000

 

Volcanic interbeds in the Silurian of the southwestern part of the East European Platform; 163–176

Tarmo KIIPLI, Petro D. TSEGELNJUK, and Toivo KALLASTE

Abstract. Sixteen samples of altered volcanic ash interbeds from the Silurian of Volynia, Podolia, and Moldova were investigated by X-ray fluorescence and X-ray diffraction analyses. Two associations of main minerals were established: (1) ordered illite and (2) illite-smectite with K-sanidine. The Nb/Y and Zr/TiO₂ ratios indicate rhyodacite-dacite and trachyandesite as the source magmas. The relative similarity of trace element composition in these volcanic beds point to a common source area of Late Silurian eruptions.

Key words: Silurian, volcanism, K-bentonite, metabentonite, Volynia, Podolia, Moldova.

 

Stratotype of the Porkuni Stage with comments on the Röa Member (uppermost Ordovician, Estonia); 177–199

Linda HINTS, Asta ORASPÕLD, and Dimitri KALJO

Abstract. A revised interpretation of the subdivisions within the unit stratotype section of the topmost Ordovician Porkuni Stage is presented together with an improved description of the section. The boundary beds with the underlying Pirgu Stage in the stratotype section are described for the first time. The “Encrinitenlager”, a crinoidal limestone (dolomite) bed, established by Schmidt (1858, Arch. Naturk. Liv-, Ehst- und Kurl., Ser. I) in the base of his “Borkholmsche Schicht” (Porkuni Stage of current use) is considered to be identical with the Röa Member comprising the lower part of the Ärina Formation. The correlation of the Ärina Formation (including the stratotype of the Porkuni Stage) with the Kuldiga and Saldus formations in South Estonia and Latvia, based on the distribution of chitinozoans and carbon isotope data, shows that the Ärina Formation represents only the lower part of the Porkuni Stage, which in full is correlated with the Hirnantian Stage elsewhere.

Key words: uppermost Ordovician (Hirnantian), unit stratotype, Porkuni Stage, Estonia.

 

Stratigraphic correlation of Late Weichselian and Holocene deposits in the Lithuanian coastal region; 200–217

Albertas BITINAS, Aldona DAMUŠYT, Galina HÜTT, Tõnu MARTMA, GiedrRUPLNAIT, MiglSTANIKAIT, Daiva SAITYT, and Rein VAIKMÄE

Abstract. The Lithuanian coastal region serves as a key area for solving different problems of Quaternary geology, particularly those related to the development of the Baltic Sea in Late Weichselian and Holocene times. Within this research, Late Weichselian and Holocene deposits in three localities in the northern part of the Lithuanian coastal region were studied. The set of methods used included palynological and diatom analysis, palaeontological investigation of subfossil mollusc fauna, measurements of the isotopic composition (d¹³C and d¹⁸O) of mollusc shells, radiocarbon dating of molluscs and organic deposits, and optically stimulated luminescence (OSL) dating of sandy deposits. The studies showed that the coastal deposits in the northern part of the Lithuanian coastal region formed during the Baltic Ice Lake, Ancylus Lake, Litorina Sea, and Post-Litorina Sea stages of the Baltic Sea evolution. The OSL technique, which was used for the first time in dating relatively young marine sandy deposits in Lithuania, yielded the results fitting in with palaeobotanical, palaeontological, and radiocarbon data, particularly in the case of deposits younger than 5000 years.

Key words: Baltic Sea, Late Weichselian, Holocene, luminescence, isotopes, radiocarbon, palynology.

 

Diatom stratigraphy and palaeoenvironment of the Yoldia Sea in northern Estonia; 218–243

Atko HEINSALU

Abstract. The palaeoenvironment of the Yoldia Sea is reconstructed on the basis of 20 diatom stratigraphic records from northern Estonia. The development of the Yoldia Sea is divided into three phases. The initial Yoldia Sea phase (c. 10 300–9900 ¹⁴C years BP) is characterized by a freshwater environment. Brackish-water conditions prevailed during a short period (c. 9900–9750 ¹⁴C years BP) along the north coast of Estonia in the middle phase of the Yoldia Sea stage. During the final phase of the Yoldia Sea (c. 9750–9500 ¹⁴C years BP) a freshwater environment is registered by diatom assemblages. Regression of the shoreline, for at least 25 m, occurred around Tallinn during the Yoldia Sea stage. Postisolation coastal lakes are characterized by a shallow eutrophic hard-water environment.

Key words: diatom stratigraphy, chrysophyte cysts, palaeosalinity, shoreline displacement, Yoldia Sea, Baltic Sea, Estonia.