Geology September 2003 Vol 52/3 []

CONTENTS & ABSTRACTS

InEnglish. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Geology



Volume 52 No. 3September 2003



Karl Orviku 100; 133–134

Alvar Soesoo, Anto Raukas, and Dimitri Kaljo

The stratotype of theOrasoja Member (Tremadocian, Northeast Estonia): lithology, mineralogy, andbiostratigraphy; 135–154

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

Abstract. TheOrasoja Member constitutes in Northeast Estonia the upper part of the KallavereFormation and forms some kind of transition into the Türisalu Formation. Themember belongs to the Lower Ordovician part (Karepa Substage) of the PakerortStage. The Orasoja Member is composed of alternating beds of light quartzosesiltstone and black shale. The share of shale is cyclically variable, butmainly 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 distinctcycles in the Orasoja Member, which are in correlation with changes in conodontassociations. Three conodont biozones are distinguished in thesection: Cordylodus proavus, C. lindstromi, and C. angulatus. The first appearance of Iapetognathus sp. in the Rannu Member is interpreted here asindicating the beginning of the Ordovician System. Graptolite occurrences giveevidence of the presence of the Rhabdinoporaflabelliformis socialis and R. f. anglica subzones.Earlier dendroids are missing, but this is mainly attributed to a taphonomicalbias.

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

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

Anne Kleesment, Väino Puura, and ToivoKallaste

Abstract. Clastic dikes of sedimentary origin are exotic, butstill remarkable minor geological structures in the Lower Palaeozoic sequence of  northern Baltoscandia. They often carry essential informationabout the processes and environments that governed during sedimentary breaks.The first up to 10 cm wide clastic dikes in the Devonian sequence ofEstonia were found in sandstones of the Gauja Formation in Piusa and Tabinaglass sand quarries. Field observations, lithological, mineralogical, andgeochemical studies of dikes revealed that these dikes were formed during ashort sedimentary break at the end of the Middle Devonian. Eluvial sedimentsthat were partly cemented in tropical arid conditions were the probable sourceof sandy matrix of dikes and abundant clasts embedded in it. Gravitationalfailure and temporary water stream processes were the ways of mass transportinto the fractures. However, the amount of water was not large enough to fillthe fractures and to cause any kind of stratification during thewithin-fracture sedimentation.

Localuplift of the area and extensional deformation of rocks with the formation ofopen fractures in Middle Devonian layers suggest a tectonic phase in thevicinity of the regional Liepaja–Riga–Pskov zone oftectonic dislocations. This regional zone was formed due to the Late Caledoniancompression and was afterwards recurrently activated. Opening of the fractureswas immediately followed by filling. At thatmoment the levels observable in the present sandstone quarries must have beenlocated more than 20–30 m above the groundwater level. Subsequentsubsidence of the area caused preferable vertical circulation of groundwateralong the clastic dikes. In comparison with the surrounding rocks goethite-richfluids have caused stronger cementation of clastic dikes during the post-dikehistory. Also some other minor diagenetic alternation processes took place indikes.

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

Hydrogeological model of Estoniaand its applications; 179–192

Leo Vallner

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

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

CHRONICLE



Estonian Commission onStratigraphy reformed; 193

Olle Hints

Copyright Transfer Agreement; 194