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Integrated study of the uppermost Ordovician Porkuni Stage in the Stirnas-18 core, western Latvia, has revealed one of the most complete Hirnantian successions in the eastern Baltic region. The interval is characterized by two shallowing upwards depositional sequences that correspond to the Kuldiga and Saldus formations. The whole-rock carbon stable isotope curve indicates a long rising segment of the Hirnantian carbon isotope excursion, with the highest peak in the upper part of the Kuldiga Formation. The bioclast carbon and oxygen curves fit well with the whole-rock carbon data. Micro- and macrofossil data enabled seven combined associations to be distinguished within the Hirnantian strata. The early Porkuni fauna of the Spinachitina taugourdeaui Biozone, with pre-Hirnantian affinities, is succeeded by an interval with a Hindella–Cliftonia brachiopod association, a specific polychaete fauna, the chitinozoan Conochitina scabra, and the conodont Noixodontus girardeauensis. The middle part of the Kuldiga Formation is characterized by a low-diversity Dalmanella testudinaria brachiopod association, high diversity of scolecodonts, and the occurrence of the chitinozoan Lagenochitina prussica. From the middle part of the Kuldiga Formation the youngest occurrence yet known of the conodont Amorphognathus ordovicicus is reported. Also typical of the Kuldiga Formation is the occurrence of the trilobite Mucronaspis mucronata. The uppermost Hirnantian Saldus Formation contains no shelly fauna, but yields redeposited conodonts and at least partly indigenous chitinozoans and scolecodonts. Palaeontological criteria and stable isotope data enable correlation of the Stirnas section with other Hirnantian successions in the Baltic region and elsewhere.

Partly dissolved prismatic quartz crystals have been found in an Upper Ordovician volcanic ash bed of the Vasagård section on Bornholm, Denmark. Similar crystals occur also in a volcanic ash in the Adze Formation of the Aizpute core, Latvia. Although biostratigraphic data are insufficient for the correlation of these findings, the abundant occurrence of this rare form of quartz at Vasagård indicates that it can be reliably used as a correlation criterion in future studies. Partly dissolved prismatic quartz crystals were most likely formed during hydrothermal processes in the volcanic source area.

Thirty suspected altered volcanic ash (bentonite) samples from the Nässumaa-825 and Orissaare-859 sections were analysed by the X-ray fluorescence method. Twenty of these samples revealed chemical signs of pure volcanogenic material, one was of mixed terrigenous–volcanogenic origin, and nine were classified as terrigenous claystones. Twenty of the bentonites were correlated, with variable confidence, with bentonites from earlier studied sections; one sample represents a formerly unknown eruption. New and earlier published bentonite correlations were used for tracing the diachronous nature of the Rumba–Velise formations boundary and for composing new isopach schemes of six Telychian bentonites.

Abstract: The late Llandovery (Silurian)--early Lochkovian (Devonian) climate of the tropical zone is tracked considering orogenesis, global glaciation events, and cratonic drift. Mineral and chemical compositions of clay fractions of [...]

Sanidine composition and bulk geochemistry of volcanic ash beds from the East Baltic indicate the subalkaline nature of the volcanism near the margins of the Baltica plate during the Silurian. Several bentonites in the Wenlock include a previously unknown sanidine with 48 to 58 mol % of the Na+Ca component. In contrast to the earlier Telychian volcanism, sodium-rich sanidine occurs in ash beds which originate from relatively moderately evolved dacitic magma. The studied material from two drill cores integrated with previous research enables production of a more complete list of 49 volcanic eruption layers for the lower to middle Wenlock in the East Baltic. This updated list of bentonites characterized by their sanidine compositions forms a good basis for future integrated bio- and chemostratigraphic correlations in northern Europe. Keywords: K-bentonites, bentonites, geochemistry, Wenlock, Silurian, volcanism, East Baltic. doi: 10.1017/S0016756810000294

Immobile trace elements in K‐bentonites indicate three volcanic stages during the Sandbian Stage of the Late Ordovician:Early Grefsen is characterised by high‐Ti source magmas alternating with a few high‐Zr layers.Middle Grefsen is characterised by overall low‐Ti source magmas with multiple high‐Zr layers alternating with less frequent low‐Zr layers.Late Grefsen to Early Katian is characterised by a low content of all studied immobile trace elements (Ti, Zr, Nb, and Th). In all K‐bentonites, the Nb/Al2O3 ratio indicates subalkaline volcanism dominated by rhyolites. In the Early Grefsen stage, andesites and dacites occur. The TiO2/Al2O3 versus Zr/Al2O3 discrimination diagram suggests low‐ and high‐temperature magmatism. Low‐temperature magmatism may have occurred during subduction of water‐rich oceanic sediments in the frontal arc of Avalonia and high‐temperature magmatism during the overriding of a mid‐ocean ridge or hot spot. The coeval end of the volcanic record in Avalonia and Baltica may indicate the collision of Avalonia with a hypothetical continental crust within the Iapetus Ocean. [ABSTRACT FROM AUTHOR]

The late Llandovery (Silurian) - early Lochkovian (Devonian) climate of the tropical zone is tracked considering orogenesis, global glaciation events, and cratonic drift. Mineral and chemical compositions of clay fractions of Canadian (the Franklinian Basin) and Estonian (the Baltoscandian Basin) sedimentary rocks from different sides of the emerging Caledonides were studied, using clay as an index of climatic conditions of the provenance area. Illite is the main clay mineral, with addition of orthoclase, muscovite, and chlorite in both regions. Authigenic chert is present in some Canadian sections. The similarity of clay minerals and elemental composition in the two regions suggest common chemical weathering conditions. Nevertheless, the Al/Ti ratio of the <1 μm clay fraction, the proxy for climate's aridity-humidity and weathering intensity, reveals differences. The Franklinian Basin in Laurentia shows a shift of climate from humid to semi-humid in pace with the uplift of the Caledonides. The sub-meridional mountain chain in the east reduced the amount of rainfall and, therefore, caused drier climate than could have been expected from the low latitudinal position of the area. The material from the Baltoscandian Basin reveals semi-arid and arid climate for most of the Silurian. Although the southern part of the Fennoscandian Shield was situated in desert latitudes, the evidence from the provenance of clay suggests that the climate was not fully arid. In Baltoscandia, covariation between low Al/Ti of clay and negative shifts of δ13C of marine carbonates is recorded at some stratigraphical levels, suggesting a link between humid climatic episodes in mainland and waning of glaciers at the South Pole. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Altered volcanic ash interbeds (bentonites) in the upper Katian of Baltoscandia indicate significant volcanic activity in neighbouring tectonically active areas. Katian bentonites in the East Baltic can be reliably correlated using sanidine phenocryst composition. Ratios of immobile trace elements TiO2, Nb, Zr and Th to Al2O3 enable extension of the correlations to Scandinavia, where late diagenetic alterations could have caused recrystallization of sanidine phenocrysts. At least seven volcanic eruptions were recognized in Baltoscandian sections. Several bentonites found in deep-sea sediments are absent in shallow-sea sediments, indicating extensive breaks in sedimentation and erosion during late Katian and Hirnantian times. The areal distribution pattern of Katian bentonites in Baltoscandia indicates a volcanic source from the north or northwest (present-day orientation) from the margins of the Iapetus Palaeo-Ocean. Signatures of ultra-high-pressure metamorphism in the Seve Nappe (Central Sweden) and intrusions in the Helgeland Nappe Complex in Central Norway have been proposed as potential sources of the magmas that generated the volcanic ashes deposited in the East Baltic in Katian times. Geochemical similarities between Baltoscandian and Dob's Linn bentonites from southern Scotland suggest a common volcanic source in Katian times. [ABSTRACT FROM AUTHOR]

Twenty-six samples from two major altered volcanic ash beds, Kinnekulle and BII Bentonite of the Kuressaare core section (K-3), Saaremaa Island, were explored to record the geochemical and mineralogical heterogeneity of beds. Signs of ash transport fractionation, redeposition of volcanic ash and diagenetic redistribution of material are described and interpreted. In authigenic mineralogy of the Kinnekulle Bentonite illite--smectite dominates with addition of K-feldspar at the margins. The BII Bentonite is composed of chlorite--smectite and illite--smectite. The stability of phenocryst compositions, including that of sanidine and biotite, indicates that both bentonites originate from a single eruption. The observed rather stable pyroclastic sanidine compositions in the cross section of bentonites confirm the reliability of sanidine-based fingerprinting of altered volcanic ash beds. Trace element distribution in bentonites and host rocks indicates that Zr, Ga, Rb, Nb, Ti and Th stayed largely immobile during volcanic ash alteration and reflect primary ash composition. However, some redistribution of Nb and Ti as well as Y has probably occurred near the contacts of bentonite with the host rock. More scattered grain size distribution and immobile element patterns of the Kinnekulle Bentonite support the idea that the primary ash bed had a heterogeneous composition and it was one of the biggest bentonites of the Phanerozoic and most likely records an extended volcanic event. Significant geochemical variations, including a high S content, near the upper and lower contacts of the Kinnekulle Bentonite and elevated Ca and P in host rocks of both bentonites suggest that the studied large ash-falls caused notable perturbations in shallow marine and early post-sedimentary environment. [ABSTRACT FROM AUTHOR]

The article deals with grain size distribution of the insoluble residue of Upper Ordovician carbonate rocks of the Baltoscandian Basin in the West Latvian Aizpute-41 core. Great fluctuations in mean grain size in the sections of the Pirgu (uppermost Katian) and Porkuni (Hirnantian) stages are considered as indications of eustatic sea level fluctuations, pointing to the glaciation in the Hirnantian and its prelude in Pirgu time. An alternative cause of grain size fluctuations might be the up-and-down tectonic movements. Nevertheless, the synchronicity of cessation of fluctuations with the end of the Hirnantian glaciation supports the glaciation-related reasons. [ABSTRACT FROM AUTHOR]

Integrated correlation of bentonites (altered volcanic ashes) and graptolite biozonation is presented. Detailed study of two Lithuanian drill core sections extended previous knowledge of the occurrence and composition of bentonites to the south. Identification of graptolite species allowed bentonites to be assigned their proper stratigraphical position. Silurian bentonites in Lithuania are mostly characterized by wide and very wide XRD 201 reflections of the main component of sanidine phenocrysts. Only fourteen of the 69 samples studied contained sanidine with a sharp reflection, which gave the best correlation potential. In the Lithuanian sections one bentonite was found in the Rhuddanian, five bentonites were recognized in the Aeronian, 17 bentonites in the Telychian, 26 in the Sheinwoodian, 10 in the Homerian and six in the Ludlow. All bentonites found in Lithuania are characterized by the main component of sanidine. A large number of Lithuanian bentonites are not known in Latvia and Estonia, indicating that volcanic ashes reached the East Baltic area from two source regions - the Central European and Norwegian Caledonides. [ABSTRACT FROM AUTHOR]

Copyright of Estonian Journal of Earth Sciences is the property of Teaduste Akadeemia Kirjastus and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Estonian Journal of Earth Sciences is the property of Teaduste Akadeemia Kirjastus and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

For the first time data on nitrogen isotopes from the Ordovician- Silurian sedimentary rocks of the Baltic Basin are reported. supplementary samples come from several regions worldwide. The data reveal the existence of different primary bioproductivity pathways in the Ordovician-Silurian. During the formation of black shale surface waters were oxygen-poor and maintained N2-fixing primary production indicated by δ15N -0.3‰ on average. The average δ15N of kukersite oil shale is +7.4‰. The positive δ15N values are in accordance with the formation of kukersite in oxic waters, showing that Gloeocapsomorpha prisca was a nitrate-using not N2-fixing cyanobacteriumlike organism. The black shale samples from the deep shelf suggest that seawater, including the photic zone, often suffered from deficiency of oxygen. [ABSTRACT FROM AUTHOR]

Abstract: In the Fennoscandian Shield the fluctuations in arid–humid climate conditions are inferred from the Al2O3/TiO2 ratio of the clay fraction of Estonian carbonate rocks of the Baltoscandian sedimentary Basin. The Late Ordovician climate of the Fennoscandian Shield was mainly semi-humid with several humid pulses in the Sandbian, mid-Katian and Hirnantian. The post-Hirnantian climate was fluctuating between humid and arid until stable humid conditions were established in the Middle and early Late Llandovery. Starting from the latest Llandovery, the climate turned semi-arid and arid. The Ordovician–Silurian long-term trend toward aridity was caused by the equator-ward drift of the Baltica plate. Shorter-term fluctuations of Al2O3/TiO2 reflect climate changes of the global extent. [Copyright &y& Elsevier]

Study of volcanic ash beds using biostratigraphy, sanidine composition and immobile elements within bentonites has manifested several well-established and some provisional correlations between Gotland and East Baltic sections. Energy dispersive X-ray fluorescence microanalysis of phenocrysts has revealed bentonites containing Mg-rich or Fe-rich biotite. Sanidine phenocrysts contain, in addition to a major Na and K component, often a few per cent of Ca and Ba. On the basis of new correlations the mapping of the distribution areas of bentonites has been extended from the East Baltic to Gotland. The bentonite distribution can be separated into two parts in North Latvia-South Estonia, indicating the existence of shelf sea currents in the Baltic Silurian Basin. [ABSTRACT FROM AUTHOR]

In three drill cores in Latvia and Lithuania a thin (1 cm thick) altered volcanic ash bed with high concentrations of phosphorus (up to 3%), cerium (1%), lanthanum (0.5%) and strontium (2.5%), has been found close to the Aeronian/Telychian boundary (Llandovery, Silurian, ca 438Ma). Small,millimetre-thick lenses within the ash bed contain up to 12%P and Sr, up to 6%Ce and 3%La. These elements occur as a solid solution of goyazite-florencitemineral. Additionally, authigenic K-feldspar and kaolinite occur in this ash bed. The high concentrations of REE elements, strontium and phosphorus suggest a carbonatite source magma, but the strontium isotope ratio contradicts that origin. Such a unique composition in a volcanic ash layer is a good basis for using it as a marker horizon for correlation of sections. [ABSTRACT FROM AUTHOR]

Nine altered volcanic ash samples from the shoal and lagoonal sediments of the Rootsikilla Stage (Wenlock-Ludlow boundary interval, Estonia) were analysed, compared and correlated with five samples of deep sea environments from Latvia. Volcanic ash correlations indicate that the Wenlock-Ludlow boundary correlates with the boundary of the Viita and Kuusnomme beds, i.e., it is significantly lower than proposed earlier. The distribution of chitinozoans supports this new correlation. Geochemical data indicate subalkaline source magma of volcanic ashes with potassium dominating over sodium. [ABSTRACT FROM AUTHOR]

Seventy-seven Telychian bentonite samples from six drill-core sections were correlated on the basis of their sanidine composition. In total, bentonites from 43 volcanic eruptions, of which six are new discoveries, were established in the Telychian of Estonia. Names and identification (ID) codes were assigned to the bentonites. The different distribution patterns of volcanic ash thicknesses indicate different source volcanoes. Lack of several bentonites near the transition between the Rumba and Velise formations and at the Llandovery-Wenlock boundary indicates sedimentary hiatuses in the eastern part of the studied area. [ABSTRACT FROM AUTHOR]

Dolomites of the Upper Subformation of the Raikküla Formation (middle Llandovery), representing the deposits of the northern marginal part of carbonate shelf consisting of cyclically laminated limestone and lagoonal dolomite were studied. The formation of dolomite was investigated using X-ray diffraction, X-ray fluorescence, and titration analyses of the rocks of the same primary origin in nondolomitized sections and those affected by massive pervasive dolomitization. The CaCO[SUB3]/MgCO[SUB3] ratio and lattice parameters of dolomites are in good agreement with the genesis. The primary lagoonal dolomite near the contact with limestone and dolomite in limestone have the most expanded lattice; the primary dolomite near the contact with pervasively dolomitized limestone has the parameters equal to the minimal for primary dolomites, registered in the centre of the layer; the most altered secondary dolomite is close to stoichiometric. The bimodal frequency distribution of the Ca content in dolomites of different genesis reflects the preferred levels of Ca uptake. The highest stoichiometry of the secondary (replacive) dolomite points to the role of recrystallization and crystallization rate. Compared to limestones dolomites are depleted in Sr and enriched in Mn. Major, minor, and trace element concentrations suggest that seawater (or modified seawater) was the dominant dolomitizing fluid. No evidence of hypersalinity or inflow of outside fluids has been observed. The direct relationship of dolomitization to the migrating inner shelf facies and regressive stages of the evolution of the Silurian Baltic Palaeobasin suggests early dolomitization. [ABSTRACT FROM AUTHOR]

Sanidine composition of 130 samples of bentonites from 12 sections of Telychian (some possibly Sheinwoodian) age was analysed by X-ray diffractometry for correlation purposes. Solid solution of magmatic sanidine contained 20-47% NaAlSi[SUB3]O[SUB8] molecules. The sanidine composition is very individual for many beds and can be used successfully for correlations. [ABSTRACT FROM AUTHOR]

The dolomites of the M&oum;kh&u;la Beds represent the uppermost middle Llandovery in the northeastern, marginal part of the Baltic Palaeobasin. Secondary dolomitization has changed the primary composition of rocks, but according to the geological data available they originate from the normal-marine sediments of the shelf facies. X-ray diffraction, X-ray fluorescence, and titration analyses were applied to study the chemical composition of rocks and differences in the dolomitizing environment. The CaCO[sub3]/MgCO[sub3] molar ratio and lattice parameters of dolomites, irrespective of the content of insoluble residue or the texture of rocks, are close to those of ideal dolomites. Compared to limestones dolomites are depleted in Sr and enriched in Mn. The concentrations and correlations of these elements vary by lithological varieties along the section, which supports the suggestion that there was no unique dolomitizing fluid and the dolomitization began soon after the deposition. The geochemistry of dolomites is almost consistent with dolomitization by normal or modified (dilute) sea water. In general, dolomitization correlates with regression. The area favourable for dolomitization is related to the inner shelf, migrating in accordance with the fluctuation of sea level and oscillation of the shoreline. [ABSTRACT FROM AUTHOR]

Concentrations of eight trace elements were determined in 80 samples representing 70 thin, altered volcanic ash beds collected from seven drill cores in Estonia. All of the samples studied come from the Adavere Stage (Telychian), except for one which belongs to the lower Jaani Stage (lowermost Sheinwoodian). The source magma composition of these volcanic ash beds varies from moderately to highly evolved. By combining the results of geochemical correlations with biostratigraphical data, based on the distribution of conodonts, 31 individual volcanic events were identified, which in turn could be grouped into four distinct volcanic episodes. A bimodal distribution of trace elements generated during the third episode of volcanic activity corresponds to a similar bimodality in the composition of a metabentonite suite from Garntangen in Norway. Comparison of these data with those previously published from Norway and Sweden indicates that the majority of volcanic beds from Garntangen can be ascribed to the Pterospathodus amorphognathoides lithuanicus Zone in Estonia. The volcanic beds from the Ireviken section on Gotland can be correlated with the upper part of the studied interval in Estonia and occur in strata corresponding to the uppermost P. amorphognathoides Zonal Group. [ABSTRACT FROM AUTHOR]