Geology June 2000 Vol 49/2 []

CONTENTS & ABSTRACTS

In English. Summaries in Estonia

Proceedings of the Estonian Academy of Sciences.

Geology



Volume 49 No. 2 June 2000



Preface; 67–68

Peep MÄNNIK



The Silurian of the Timan–northern Ural region; 69–84

Anna I. ANTOSHKINA

Abstract. During the Silurian, an extensive carbonate platform formed on the northeastern continental shelf of Baltica. The sediments on it are represented by lagoonal, reef, backreef, restricted shelf, and open shelf facies and, as a rule, are highly fossiliferous.

Three events affected strongly the pattern of sedimentation in the region. First, at the end of the Ordovician, glacioeustatic sea level lowering drained the platform and sedimentation ceased on most of it. Rapid glacial melting, accompanied by a sea level rise in the early Silurian (Yarenej time), returned deposition to the platform. Second, in the upper Llandovery (Filipp¢¢el¢ time) for the first time, and then in the Wenlock (Ust¢Durnayu time) and Ludlow (Padimejtyvis and Sizim times), the east-dipping ramp was converted to a rimmed shelf with reefs on its outer margin, and with slumps, debris-flows, and turbidites on the newly formed slope. The third major event in the basin history was a gradual regression which began in the late Silurian and culminated in the Early Devonian. The sea level fluctuations determined in the Silurian of the Timan–northern Ural region well correlate with the global eustatic curve.

Key words: facies, stratigraphy, palaeogeography, Silurian, Timan–northern Ural region, Russia.



Lower Silurian stratigraphy of the Timan–northern Ural region and eustatic fluctuation; 85–103

Sergej V. MELNIKOV and Valentina A. ZHEMCHUGOVA

Abstract. This article is an attempt to correlate the Lower Silurian strata of the Timan–northern Ural region with the general international stratigraphic scale and to describe some global Silurian climatic events recognized in this area. The study of the distribution of Silurian faunas in the Kozhym River section (Subpolar Urals) allowed definition of 25 datums marked by the appearances and/or disappearances of taxa. Facies and sequence-stratigraphical analysis of strata resulted in identification of six sequences. Relative sea level fluctuations were determined. Considering the succession of sea level and faunal changes in early Silurian strata, six shallow shelf episodes were distinguished. On the basis of the assumption that the eustatic changes in sea level and climatic oceanic episodes (Jeppsson
1990. J. Geol. Soc. London, 147, 663–674) are caused by plate tectonics oceanic and shelf episodes were correlated. The correlation shows that the Lower Dzhagal Substage is Rhuddanian and the upper Dzhagal Substage Aeronian in age, the Filipp¢¢el¢ Stage and the lower part of the Sed¢¢el¢ Stage correspond to the Telychian, and the upper part of the Sed¢¢el¢ Stage to the Sheinwoodian.

Key words: biostratigraphy, sequence stratigraphy, eustasy, oceanic episodes and events, Lower Silurian, Timan–northern Ural region, Russia.



The Llandovery–Wenlock boundary in the Russian Arctic; 104–111

Peep MÄNNIK, Anna I. ANTOSHKINA, and Tatyana M. BEZNOSOVA

Abstract. The position of the Llandovery–Wenlock boundary in the Timan–northern Ural region and Severnaya Zemlya has been under discussion for a long time. Datings based on different groups of fauna seem to be inconsistent; conodonts indicate that in both regions this boundary lies considerably higher in the sequence than considered until now. The conodont datings agree well with correlations based on sea level fluctuations.

Key words: stratigraphy, Llandovery–Wenlock boundary, Silurian, Timan–northern Ural region, Severnaya Zemlya, Russia.



Silurian–earliest Devonian ostracode biostratigraphy of the Timan–northern Ural region; 112–125

Anna ABUSHIK

Abstract. The stratigraphical distribution of the Silurian and earliest Devonian ostracodes in sections of the northern Timan, and Chernov and Chernyshev uplifts in the Timan–northern Ural region is presented. The ostracode successions across the Silurian–Devonian boundary are similar in the Timan–northern Ural region and on Novaya Zemlya. The majority of the previously established Homerian–Lochkovian ostracode biozones have now been recognized in these areas. Ostracode biostratigraphy for the middle Llandovery to middle Wenlock time period in northeastern European Russia is discussed.

Key words: ostracodes, biostratigraphy, Silurian, earliest Devonian, Timan–northern Ural region, Russia.



Silurian brachiopods in the Timan–northern Ural region: zonation and palaeoecology; 126–146

Tatyana M. BEZNOSOVA

Abstract. In the Timan–northern Ural region, the Silurian is represented mainly by various limestones, secondary dolostones, and evaporites. The environments in the basin range from shallow-water coastal (northern Timan) and supratidal–subtidal (Pechora Syneclise, western slope of the Urals) to slope and basinal (western slope of the Urals). In the northeastern Timan and Subpolar Urals brachiopods are one of the most abundant benthic faunal groups and have great importance in local stratigraphy. Brachiopod zonation based on the distribution of pentamerids, athyrids, atrypids, spiriferids, and strophomenids includes 10 zones and allows correlation of the Silurian sequence in the Timan–northern Ural region with those in other Arctic regions of Russia, and with the Baltic area. Changes in the composition and distribution of brachiopod associations in the Timan–northern Ural region reflect highly variable ecological conditions in the basin.

Key words: brachiopods, biostratigraphy, Silurian, Timan–northern Ural region, Russia.



Revision of the Silurian brachiopod Pentamerus samojedicus Keyserling, 1846 from the Timan Ridge, Russia; 147–156

Tatyana MODZALEVSKAYA and Madis RUBEL

Abstract. Revision of the pentamerid brachiopod Pentamerus samojedicus Keyserling, 1846 allows it to be assigned to the genus Borealis Boucot, Johnson & Rubel, 1971, and shows its much wider distribution than originally reported, from the Aeronian to Telychian in the Timan Ridge, Severnaya Zemlya, and the Baltic region. The gradational evolution from Borealis borealis to P. oblongus, as supposed and documented by Mųrk using Norwegian material (Palaeontology, 1981, 24, 537–553), has a parallel branch through B. samojedicus, which occupies a transitional position in the Baltic sections.

Key words: brachiopods, pentamerids, Llandovery, Timan Ridge, Baltic.



Instructions to authors; 157–159

Copyright Transfer Agreement; 160