Viiu Nestor

Integrated biostratigraphy of the lower Silurian of the Aizpute-41 core, Latvia

Integrated graptolite, conodont and chitinozoan biostratigraphical data is presented from the Rhuddanian through to lower Sheinwoodian of the Aizpute-41 core, Latvia. Correlation of the biozonation schemes based upon the three groups is achieved from the cyphus through to lowermost riccartonensis graptolite biozones, except for the upper Aeronian and lower Telychian, which lack both chitinozoans and graptolites, and upper lapworthi through to approximately base murchisoni graptolite Biozone, where there is interpreted to be an unconformity. Datum 2 of the Ireviken Event is correlated with a level at the base of or within the murchisoni Biozone. It is possible that the changes in conodont assemblages at Datum 2 on Gotland are the result of an unconformity here. Streptograptus? kaljoi sp. nov., from the lower spiralis graptolite Biozone, is described.

Integrated biostratigraphy of the lower Silurian of the Kolka-54 core, Latvia

Integrated graptolite, conodont and chitinozoan biostratigraphical data are presented from the Llandovery and Wenlock of the Kolka-54 core, Latvia. Correlations between graptolite and chitinozoan biozones are consistent with those published from other East Baltic sections and the Welsh Basin. While most correlations between graptolite and conodont biozones agree with those presented in previous studies, there are important exceptions. Significantly, we report here the discovery of Distomodus staurognathoides Biozone conodonts in the lowest Aeronian Demirastrites triangulatus graptolite Biozone. The base of the D. staurognathoides Biozone was previously considered to lie much higher in the Aeronian. Also it is shown that Walliserodus survived the late Wenlock Mulde Event, during which it was considered previously to have become extinct.

Integrated graptolite and chitinozoan biostratigraphy of the upper Telychian (Llandovery, Silurian) of the Ventspils D-3 core, Latvia

Integrated graptolite and chitinozoan biostratigraphical data are presented from the upper Telychian (Oktavites spiralis and Cyrtograptus lapworthi graptolite biozones) of the Ventspils D-3 core, Latvia. The base of the Angochitina longicollis chitinozoan Biozone is approximately coincident with that of the spiralis graptolite Biozone, as it is elsewhere in the East Baltic, although in Wales it lies within the upper spiralis graptolite Biozone. Conochitina proboscifera appears in the upper spiralis graptolite Biozone in the Ventspils D-3 core, but at lower and higher horizons elsewhere, presumably reflecting its patchy distribution during the lower part of its stratigraphical range. Ramochitina ruhnuensis appears to be a stratigraphically useful, although geographically restricted, species, appearing at a level close to the base of the lapworthi graptolite Biozone. The most remarkable feature of the Ventspils D-3 chitinozoan record is the very early occurrence, in the upper spiralis graptolite Biozone, of two chitinozoan biozonal index species: Margachitina banwyensis and M. margaritana. Previously, these two taxa were considered unequivocal indicators of the uppermost Telychian to Sheinwoodian or Homerian, respectively.

Composition and correlation of volcanic ash beds of Silurian age from the eastern Baltic

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.

Chitinozoan diversity in the East Baltic Silurian: first results of a quantitative stratigraphic approach with CONOP

Chitinozoans are among the most common and stratigraphically useful microfossils in the Silurian. We examine their regional diversity pattern in the Baltoscandian basin using constrained optimisation (CONOP), based on distributional data of 189 species in 15 sections. The CONOP-derived taxonomic richness curve provides close approximation to the standing diversity, complementing previous analyses and revealing finer details. The results show distinct diversity peaks in the Telychian and Homerian with regional maximum of 25–30 species. Major crises occurred in the early Sheinwoodian and late Homerian, coincident with carbon isotope excursions. The Ludfordian pattern remains controversial awaiting extension of the data-set.

Integrated Upper Ordovician–lower Silurian biostratigraphy of the Grötlingbo-1 core section, Sweden

The biostratigraphical data obtained from the Grötlingbo-1 core section (southern Gotland, Sweden) enable recognition of most of the regional stages known from the Sandbian (Upper Ordovician) to lower Sheinwoodian (Wenlock, Silurian) interval in the East Baltic. The strata corresponding to the Upper Ordovician Keila–Nabala stages have markedly reduced thicknesses in the Grötlingbo-1 core section. Gaps of considerable duration were recognised in some parts of the section: lithological evidence suggest that only lower part of the Pirgu Stage (corresponds to all or part of the Jonstorp Formation) is represented in the section; in the Llandovery, strata corresponding to the Aspelundia? fluegeli and Distomodus staurognathoides Conodont Zones (CZs), and to the Pterospathodus eopennatus CZ (equivalent to the Conochitina alargada and Eisenackitina dolioliformisChitinozoan Zones (CtZs)) are missing. Graptolites from the core indicate that the sedimentary record above the Aeronian to middle Telychian gap recommences in the lower part of the Oktavites spiralis Graptolite Zone (GZ). Co-occurrences of conodonts, graptolites and chitinozoans agree with earlier data suggesting correlation between the lower boundaries of the Cyrtograptus lapworthiGZ and Pterospathodus amorphognathoides amorphognathoides CZ, and between the lower and middle parts of the Ireviken Event and the upper Cyrtograptus murchisoni GZ.