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In order to check the immobility and mobility of elements during conversion of acidic volcanic glass to bentonites in normal marine environments, we studied the composition of three altered volcanic ash layers from the Palaeozoic of the Baltoscandian Region, correlated through different facies. Regular changes in element concentrations in accordance with loss and gain of material during the transformation of volcanic ash indicate that Al, Nb, Ti, Zr, Sn, Pt, Ta, Hf and Th were generally immobile and can be used for the interpretation of source magma and correlation of ash layers. Cd behaves similarly with immobile elements and this can be explained with preservation only of the immobile portion of Cd that is fixed in phenocrysts. In bentonites in shales during the formation of kaolinite, the data indicate small-scale mobility of Al and Cd. In lime muds where K-feldspar forms from volcanic ash, Ta, Hf and Th reveal some small scale mobility. These slightly mobile elements must be used with caution for interpretation of thin ash layers with thicknesses of

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 13C 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.

Significantly elevated U, Mo, Zn and Pb contents characterize the Early Ordovician black shales in the Sillamäe area, NE Estonia. The presence of silty interlayers with sulphidic mineralization and authigenic carbonates suggests unique physicochemical conditions for metal enrichment in this location. We investigated metallogenesis of these shallow-water black shales based on high-resolution mapping of element distribution in the Sõtke drill core and nearby Päite outcrop using X-ray fluorescence spectroscopy, inductively coupled plasma mass spectrometry and organic elemental analysis, complemented by optical microscopy and scanning electron microscopy of selected samples. Enriched metals in the black shales of the study area show dissimilar distribution trends with sharp vertical concentration gradients. The recorded variance ranges were 88â275 ppm for U, 70â2467 ppm for Mo, 85â1600 ppm for V, 21â17 283 ppm for Zn and 95â26 549 ppm for Pb, while total organic carbon varied from 0.5 to 13 wt%. In most cases, the metals showed no clear covariance with organic matter or other major compounds such as S or P. The development of a sulphate reduction zone near the sedimentâwater interface with a sharp decrease in Eh, production of H2S, elevated alkalinity and pH, and (re)distribution of phosphorus probably controlled the syngenetic capture of Mo and U. Enhanced transfer of fluids and solutes in coarse-grained permeable beds facilitated the accumulation of metals, while the deposit probably acted as a semi-open geochemical system throughout its geological evolution. Local Zn and Pb enrichment developed due to the intrusion of late diagenetic metal-bearing fluids and the entrapment of metals in beds that contained authigenic carbonates.

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.

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 Al2O3enable 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.

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 20 1 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.

This article presents a dataset of geochemical analyses from Silurian altered volcanic ash beds (bentonites) characterising 132 eruptions and preserved in sedimentary sections of the East Baltic area. TiO2, Nb, Zr, Th, Y, Ce, Cr and V ratios with Al2O3 are used for identifying correlations between ash beds from Scandinavian sections. Some correlated ash beds are illustrated by schematic maps of areal distribution indicating volcanic sources from directions of both Iapetus and Rheic palaeo-ocean margins.

This paper reports on the geochemistry of the mid-Silurian Grotlingbo Bentonite, a ca. 0.1–0.4-m-thick and regionally important bentonite bed in Sweden and the East Baltic area. A series of eight samples, spaced by 5 cm, were taken from the Hunninge-1 drillcore in Gotland, Sweden, and were analysed in order to establish the vertical element composition and variation in the bentonite. The results show that the Gotlingbo Bentonite originates from one source magma and from one single eruption. The lowermost 0.1–0.15 m of the bentonite (compacted) was deposited from air-transport fall-out, whereas the upper portions have been reworked in a shallow-marine environment and re-deposited. Vertical differences in Ti and Zr within the Grotlingbo Bentonite mean that regional correlation of the bed may need several samples at each locality to be reliable.

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The formation of hematite and goethite concretions in different sedimentary rocks including sandstones is an important diagenetic process in the geologic history of the Earth. Its interpretation can also contribute to understanding the diagenetic history of Martian iron hydroxide concretions.

The mineral composition of altered volcanic ash beds, bentonites, deposited in marine environments is typically characterized by smectite or illite–smectite type clay minerals. Silurian bentonites from an unmetamorphosed normal marine sedimentary sequence in the eastern Baltic Palaeobasin, however, show a complex and varied mineral composition with illite–smectite, K-feldspar and kaolinite as the dominant authigenic phases. The whole-rock XRD study of 52 bentonite beds from two drill-cores — Ruhnu and Ohesaare, southwestern Estonia, reveals a range of compositions varying between different beds as well as laterally within a single bed. The observed variations are found to reflect the bentonite development and diversification during initial stages of volcanic ash transformation and diagenesis that was probably overprinted by a later K-metasomatic event.

In the Telychian section of Estonia and Latvia K-bentonites from 45 volcanic eruptions were discovered. The thickness of K-bentonite interbeds varies from a few millimetres to 20 cm. The sodium component concentration in sanidine phenocrysts measured by XRD ranges from 20 to 48 mol% and was used for establishing correlations. The Ti, Zr, Nb, Th and Sr concentrations and ratios show temporal trends indicating fractional crystallization in magma chambers. The analysis of biotite phenocrysts revealed magnesium and iron rich biotites in bentonites. Synthesis of these geochemical data enabled a classification of bentonites into seven geochemical types, which probably originate from seven different volcanic sources. Isopach schemes indicate ash transport from the west and north-west directions.

The composition of altered volcanic ash of the Late Ordovician Kinnekulle bed was studied in geological sections of the Baltic Paleobasin. The composition of altered ash varies with paleosea depth from northern Estonia to Lithuania. The ash bed in shallow shelf limestones contains an association of illite-smectite (I-S) and K-feldspar, with the K2O content ranging from 7.5 to 15.3%. The limestone in the transition zone between shallow- and deep-shelf environments contains I-S-dominated ash with K2O content from 6.0 to 7.5%. In the deep-shelf marlstone and shale, the volcanic ash bed consists of I-S and kaolinite with a K2O content ranging from 4.1 to 6.0%. This shows that authigenic silicates from volcanic ash were formed during the early sedimentary-diagenetic processes. The composition of the altered volcanic ash can be used as a paleoenvironmental indicator showing the pH of the seawater or porewater in sediments as well as the sedimentation rate.

The Baltic Ordovician–Silurian sedimentary succession embodies numerous altered volcanic ash beds that are illite–smectite dominated. Only a few beds rich in chlorite–smectite are known from the Upper Ordovician Pirgu Stage of Estonia. Mixed-layer chlorite–smectite occurs commonly in low-grade metamorphic and hydrothermal environments. However, chloritic K-bentonites of the Pirgu Stage have never been buried deeply and lack signs of metamorphic overprinting. In order to understand the origin of chloritization, three distinct beds were sampled in 14 drillcores from Estonia and Latvia and analysed by means of XRD and SEM. The principal authigenic assemblage of the bulk samples consists of chlorite–smectite (corrensite) together with illite–smectite and K-feldspar. The actual mineral composition of K-bentonites though varies from sample to sample. The clay mineral assemblages range from virtually pure chlorite–smectite to illite–smectite dominated assemblages with minor or no chlorite–smectite. The proportion of chlorite–smectite in K-bentonites shows systematic lateral variations: the share of chloritic phases is highest in the shallower-water part of the palaeobasin and decreases towards the deeper part of the basin. Such regular lateral variations suggest a possible link between chloritization and the configuration of ancient palaeobasin. The present study suggests that the chloritization of primary felsic ashes occurred during early diagenesis and that it was caused by an influx of Mg-rich water probably from a marine sabkha-type environment.

The content of the trace elements Ti, Nb, Zr and Th has been analysed in 34 Upper Ordovician bentonites from the Billegrav-2 drill core, Bornholm, Denmark. The section contains two 80–90 cm thick bentonites, which potentially may represent the Kinnekulle Bentonite, as well as several rather thick but composite bentonite layers with thin terrigenous shale interbeds. Comparison of the four immobile trace elements with data from the Kinnekulle Bentonite reported from other locations in Baltoscandia indicate that the 80 cm thick bentonite between 88.30 and 89.10 m in the Billegrav-2 core represents this marker bed. The other thick (90 cm) bentonite in the Billegrav-2 core, exceeding the thickness of the Kinnekulle Bentonite, belongs to the Sinsen or uppermost Grefsen Series bentonites. Bentonites in the Grefsen Series frequently contain much higher concentrations of trace elements than the Kinnekulle Bentonite.

Nine altered volcanic ash samples from the shoal and lagoonal sediments of the Rootsiküla 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 Kuusnõmme 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.

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.

The sand-rich ejecta containing a considerable amount of mineral grains with impact- induced features was studied in five boreholes drilled around the Kardla impact meteorite structure. Quartz grains with planar deformation features and planar fractures, as well as cracked grains showing lowered crystallinity by X-ray diffractometry, were recorded. The orientations of shock-produced deformation features, with the orientation planes {10 13} (dominating), {10 11},{1012}, and {2241}, suggest the peak pressure of 10-25 GPa. The ejecta deposits of the annular zone 2-5 km outside the crater rim yield specific quartz grains surrounded by C-rich amorphous material. The carbon isotope composition of the particles reminds of that of the Early Ordovician Dictyonema shale, the uppermost part of the target, but also coincides with the general cosmic (meteorite) carbon isotope composition. The ejected detrital minerals, mostly quartz, are often coated with transparent or semitransparent yellowish and dark pyrite-bearing apatite, which is probably of post-impact origin, preceding or partly simultaneous with diagenetic carbonate cementation. Unlike carbon coatings on quartz, known from the Lappajarvi crater, the apatite coatings have not been described anywhere in impact craters. Carbon-rich coatings and particles, and apatite coatings have not been previously found in the Estonian bedrock.

Pyroclastic sanidine composition is used for correlation of the Osmundsberg bentonite (O-bentonite) bed from Estonia to Latvia. Chemical changes during conversion of volcanic ash to authigenic silicates in shallow and deep shelf sediments differ, leading to the formation of kaolinite-rich bentonite in deep shelf and feldspar-rich bentonite in shallow shelf environments. The Rumba Formation in Estonia is proved to be of Lower Telychian age. Firmly based correlation between shallow and deep shelf sediments allows tracing of facies variations that occurred during environmental changes in the Late Llandovery times.

Hematite and goethite were determined by X-ray diffractometry using natural goethite and hematite for calibration. The subtraction of diffractograms before and after ignition at 500[ddot]C was applied for goethite identification and quantification, and ZnO was used as internal standard for the matrix correction. Red terrigenous claystones from deep shelf facies revealed co-occurrences of hematite and goethite, on average 1.7% hematite and 0.6% goethite. Grey claystones from the onshore facies of deep shelf contained yellow goethitic layers with the goethite content from 1.6 to 12.5%. Red metabentonites, layers of altered volcanic ash, contained only hematite, on average 2.7%, and did not reveal goethite above the detection limit. The occurrence of red metabentonites points to the early diagenetic origin of hematite in volcanic ash beds as well as in red terrigenous host rock. Several pathways of hematite and goethite formation are considered for different rock types.

The distribution and composition of dolomitized rocks and stoichiometry of dolomite in southern Estonia in the Ordovician, Silurian, and Devonian were studied on the background of the facies, sedimentary cyclicity (nine shallowing-up cycles), and evolution of the palaeobasins. The composition of rocks and lattice parameters of dolomite were investigated using the X-ray diffraction, X-ray fluorescence, titration and gravimetric analyses, and porosity measurements. The formation of dolostones is directly determined by the cyclic evolution of palaeobasins. Dolomitized rocks belong to the shallow-water inner shelf or tidal/lagoonal facies belt of regressive phases of sedimentary cycles. Sediments of the deep shelf/transitional environment and transgressive phases are not dolomitized. The most stoichiometric is secondary replacive dolomite of Silurian and upper Ordovician dolostones, formed during the early diagenesis of normal-marine (saline) shallow-shelf calcitic sediments. The content of insoluble residue does not affect the stoichiometry. The changes in lattice parameters are induced by the Ca/Mg ratio in the dolomite lattice. The dolomite of the dolostones contacting limestone or containing calcite has an expanded lattice. The primary (syngenetic) dolostone of the lagoonal or tidal flat belt has also an expanded lattice. No dolomitizing effect of the waters of the Devonian palaeobasin on the underlying rocks was revealed. The whole data set of the studied dolostones is consistent with the marine water environment in the palaeobasin at the corresponding time and shows no sign of the inflow of external fluids. It suggests that the microbial model of dolomite formation may characterize the Ordovician, Silurian, and Devonian in southern Estonia. The occurrence of dolostones between undolomitized rocks limits the time of dolomitization to the early diagenetic stage.