CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Geology

 

Volume 49 No. 4 December 2000

 

Alteration of zircon and garnet due to low-pressure shock at the Kärdla crater, Estonia; 251–266

Mare KONSA, Anne KLEESMENT, and Väino PUURA

Abstract. Immersion analysis with the polarizing microscope and SEM studies revealed fracturing in zircon and garnet grains from impact breccias of the Kärdla crater. The zircon grains showed impact-related features, including widening of pre-existing fractures in the internal zones, some fresh fractures, loss of inclusions, decrease in transparency, increase in turbidity, and the intensely cracked outermost coat in zoned zircons. The garnet grains were intensely fractured only in the uppermost subcrater basement, especially in allochthonous suevitic breccias. Zircon and garnet grains from different parts of the crater can serve as useful indicators of low shock pressure (< 8 GPa).

Key words: impact craters, shock metamorphism, impact breccia, typology, zircon, garnet, Kärdla crater, Estonia.

 

Carbon isotopic composition of Llandovery rocks (East Baltic Silurian) with environmental interpretation; 267–283

Dimitri KALJO and Tõnu MARTMA

Abstract. Carbon isotope data from six borehole sections of Estonia and West Latvia are presented and analysed against different geological and palaeontological phenomena in order to understand possible reasons for shifts in d¹³C values. Four most important carbon isotopic events are established in the Llandovery (from bottom): (1) a negative shift (– 0.5 to – 1.5‰) in the Coronograptus cyphus Zone (in the Pusku Beds), (2) a positive excursion (+ 3.0 to + 4.0‰) in the Demirastrites triangulatus Zone (Ikla Member), (3) a negative excursion (– 1.3 to – 1.4‰) in the Stimulograptus sedgwickii Zone (Rumba Formation), (4) a small positive shift at the beginning of the Velise Formation in the Spirograptus guerichi–Streptograptus crispus Zone. The positive excursions seem to correspond to certain glacial events, negative ones to humid climatic episodes with rising sea level. In the case of the Rumba Formation the influence of carbon with low d¹³C released due to volcanic activity in the late Aeronian is suggested. The background carbon isotope values are relatively stable during the Llandovery, showing some changes in correlation with facies and climate controlled stratigraphical units.

Key words: carbon isotopes, Llandovery, East Baltic, glaciations, facies changes, climates.

 

Fracture systems in Devonian sandstones, South Estonia; 284–293

Anne KLEESMENT and Enn PIRRUS

Abstract. The results of recent field observations of fracturing in Middle Devonian sandstones outcropping in South Estonia are presented. Fractures in the rocks are mainly vertical, planar, usually cross-cutting the entire outcrop within an observable vertical reach of up to 10 m. Fractures in horizontal direction can occur in caves as 5–15 m long linear stretches. In outcrops some fissures can be seen within a horizontal reach of up to 40 m. The frequency of fractures is variable, the predominant distance between them is 10–20 m. A denser fracture net is characteristic of the upper part of the studied section. The orientation of the fractures determined by 235 measurements shows the existence of two diagonal sets, the one in NE–SW direction being more expressive. Orientation of fracturing differs a little in different stratigraphical units. Main directions and features of fracturing agree with widespread planetary fracturing.

Key words: tectonics, fracturing, sandstones, Devonian, Estonia.

 

Early diagenetic chalcopyrite occurrences in Telychian marine red beds of West Estonia and West Latvia; 294–307

Enli KIIPLI, Tarmo KIIPLI, and Toivo KALLASTE

Abstract. In the Telychian (Lower Silurian) marine red beds of the Aizpute core (West Latvia) and several cores of Saaremaa Island (West Estonia) chalcopyrite crystals up to 0.5 mm in size were found in thin red and grey altered volcanic ash layers and in red and yellow terrigenous claystones and marls. The control of sedimentary facies on the distribution of chalcopyrite suggests its formation in early diagenesis. Chalcopyrite finds are more frequent in metabentonites than in terrigenous rocks, presumably due to better Cu-adsorbing properties of the authigenic clay, and amorphous ferric and supposedly once existent but not preserved by the present time manganese oxides. In the West Latvian Aizpute core the findings of chalcopyrite are more numerous and Cu contents are higher than in Saaremaa sections. Correlation of three metabentonite layers in the Aizpute core and the Viki core of Saaremaa hints at earlier onset of red bed facies in the Aizpute area.

Key words: chalcopyrite, marine red bed facies, metabentonite correlation.