Proceedings of the Estonian Academy of Sciences

CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Geology

Volume 52 No. 3 September 2003

Karl Orviku 100; 133–134

Alvar Soesoo, Anto Raukas, and Dimitri Kaljo

The stratotype of the Orasoja Member (Tremadocian, Northeast Estonia): lithology, mineralogy, and biostratigraphy; 135–154

Heljo Heinsalu, Dimitri Kaljo, Tiia Kurvits, and Viive Viira

Abstract. The Orasoja Member constitutes in Northeast Estonia the upper part of the Kallavere Formation and forms some kind of transition into the Türisalu Formation. The member belongs to the Lower Ordovician part (Karepa Substage) of the Pakerort Stage. The Orasoja Member is composed of alternating beds of light quartzose siltstone and black shale. The share of shale is cyclically variable, but mainly siltstone is prevailing. Mineralogically quartz is dominant, K-feldspar (up to 15%) forming a minor component of the rock. Mineral and grain size composition allows us to distinguish two fairly distinct cycles in the Orasoja Member, which are in correlation with changes in conodont associations. Three conodont biozones are distinguished in the section: Cordylodus proavus, C. lindstromi, and C. angulatus. The first appearance of Iapetognathus sp. in the Rannu Member is interpreted here as indicating the beginning of the Ordovician System. Graptolite occurrences give evidence of the presence of the Rhabdinopora flabelliformis socialis and R. f. anglica subzones. Earlier dendroids are missing, but this is mainly attributed to a taphonomical bias.

Key words: conodonts, graptolites, stratigraphy, lithology, mineralogy, Lower Ordovician, Estonia.

Clastic dikes in Middle Devonian sandstones of the Gauja Formation, southeastern Estonia; 155–178

Anne Kleesment, Väino Puura, and Toivo Kallaste

Abstract. Clastic dikes of sedimentary origin are exotic, but still remarkable minor geological structures in the Lower Palaeozoic sequence of northern Baltoscandia. They often carry essential information about the processes and environments that governed during sedimentary breaks. The first up to 10 cm wide clastic dikes in the Devonian sequence of Estonia were found in sandstones of the Gauja Formation in Piusa and Tabina glass sand quarries. Field observations, lithological, mineralogical, and geochemical studies of dikes revealed that these dikes were formed during a short sedimentary break at the end of the Middle Devonian. Eluvial sediments that were partly cemented in tropical arid conditions were the probable source of sandy matrix of dikes and abundant clasts embedded in it. Gravitational failure and temporary water stream processes were the ways of mass transport into the fractures. However, the amount of water was not large enough to fill the fractures and to cause any kind of stratification during the within-fracture sedimentation.

Local uplift of the area and extensional deformation of rocks with the formation of open fractures in Middle Devonian layers suggest a tectonic phase in the vicinity of the regional Liepaja–Riga–Pskov zone of tectonic dislocations. This regional zone was formed due to the Late Caledonian compression and was afterwards recurrently activated. Opening of the fractures was immediately followed by filling. At that moment the levels observable in the present sandstone quarries must have been located more than 20–30 m above the groundwater level. Subsequent subsidence of the area caused preferable vertical circulation of groundwater along the clastic dikes. In comparison with the surrounding rocks goethite-rich fluids have caused stronger cementation of clastic dikes during the post-dike history. Also some other minor diagenetic alternation processes took place in dikes.

Key words: fracturing, clastic dike, sandstones, Devonian, Estonia.

Hydrogeological model of Estonia and its applications; 179–192

Leo Vallner

Abstract. The hydrogeological model constructed using the code Visual MODFLOW covers the whole territory of Estonia, the surrounding coastal sea, Lake Peipsi, and border districts of Russian Federation and Latvia, all together 88 032 km². The 13 model layers include all main aquifers and aquitards from ground surface to as low as the impermeable part of the crystalline basement. Three-dimensional distribution of groundwater heads, flow directions, velocities, and rates as well as transport characteristics can be simulated by the model. Detailed basinwide or local groundwater budgets can be completed.

Key words: hydrogeological modelling, groundwater flow, base flow, net infiltration, predevelopment conditions, Cambrian–Vendian aquifer system, Estonia.

CHRONICLE

Estonian Commission on Stratigraphy reformed; 193

Olle Hints