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Bentonites are considered to be the most suitable materials for the multibarrier system of high-level radioactive waste repositories. Since BCV bentonite has been proved to be an ideal representative of Czech Ca-Mg bentonites in this respect, it has been included in the Czech Radioactive Waste Repository Authority (SÚRAO) buffer and backfill R&D programme. Detailed knowledge of processes in the material induced by thermal loading provides invaluable assistance regarding the evolution of the material under repository conditions. Samples of both original BCV 2017 bentonite and the same material thermally treated at 200 °C were characterised by means of chemical analysis, powder X-ray diffraction, infrared spectroscopy, thermal analysis, cation exchange capacity, specific surface area (BET) measurements, the determination of the swell index, the liquid limit, the swelling pressure and water retention curves. The smectite in BCV 2017 bentonite comprises Ca-Mg montmorillonite with a significant degree of Fe3+ substitution in the octahedral sheet. Two main transformation processes were observed following heating at 200 °C over 27 months, the first of which comprised the dehydration of the montmorillonite and the subsequent reduction of the 001 basal distance from 14.5 Å (the original BCV 2017) to 9.8 Å, thus indicating the absence of water molecules in the interlayer space. The second concerned the dehydration and partial dehydroxylation of goethite. With the exception of the dehydration of the interlayer space, the PXRD and FTIR study revealed the crystallochemical stability of the montmorillonite in BCV 2017 bentonite under the selected experimental conditions. The geotechnical tests indicated no major changes in the mechanical properties of the thermally treated BCV 2017 bentonite, as demonstrated by the similar swelling pressure values. However, the variation in the swell index and the gradual increase in the liquid limit with the wetting time indicated a lower hydration rate. The retention curves consistently showed the lower retention capacity of the thermally treated samples, thus indicating the incomplete re-hydration of the thermally treated BCV 2017 exposed to air humidity and the difference in its behaviour compared to the material exposed to liquid water.

This study examines the early stages of clastic deposition above the main coal seam in the Most Basin (Ohře Rift, NW Czech Republic). Eight drill cores were subjected to chemical analysis by X-ray fluorescence spectroscopy (XRF) and cation exchange capacity determination (CEC) to corroborate the local chemostratigraphic scheme and extend its temporal and spatial ranges. Additionally, four drill cores were subjected to palaeomagnetic polarity analysis for magnetostratigraphic dating, and two cores were subjected to the frequency analysis of variations in elemental compositions to obtain cyclostratigraphic patterns and more detailed insight into depositional dynamics. We demonstrate that a local lake was present in the Bílina area from the end of palaeomagnetic chron C5Er through chron C5En (18.524-18.056 Ma according to the Astronomically Tuned Neogene Time Scale 2012) to the beginning of the C5Dr chrons. This finding extends the previous age models for the Most Formation by more than 0.5 My (18.6 to 17.9 Ma). The chemostratigraphic scheme, which is based on variations in the concentrations of K and the values of CEC, was successfully extended from the basin centre to its north-east periphery, which reflects the basin-wide environmental changes that terminated the peat accumulation (coal formation) in the basin. The basin-wide lacustrine body formed by the coalescence of local lakes and the flooding of the remaining peatlands ca. 17.8 Ma (during C5Dn), which is coeval with the substantial reduction of the Antarctic ice sheet prior to the onset of the Miocene climatic optimum (MCO). The Most Basin deposits are the most detailed (with respect to temporal resolution) continental sedimentary archive of the time period preceding the MCO that have not been described to date. • Key words: cyclostratigraphy, chemostratigraphy, lake sediments, climate forcing, Burdigalian. CR, v.v.i., 250 68 Řež, Czech Republic; grygar@iic.cas.cz, hosek@iic.cas.cz • Karel Mach, North Bohemian Mines, j.s.c., ul. 5. května 2013, 418 01, Bílina, Czech Republic; mach@sdas.cz • Petr Schnabl, Institute of Geology of the CAS, v.v.i., Prague, the Czech Republic; schnabl@gli.cas.cz • Mathieu Martinez, MARUM, Universität Bremen, Leobener Straße 8, D-28359 Bremen, Germany; mmartinez@marum.de • Magdaléna Koubová, Czech Geological Survey, Geologická 6, 152 00 Praha 5, Czech Republic; magdalena.koubova@geologycz Recent studies of sea level fluctuations during the Cretaceous highest sea level (Uličný et al. 2014) have shown that distinguishing between local/autogenic forcings (i.e. tectonic, palaeotopographic) and global/allogenic forcings (i.e. climatic, eustatic) in pre-Pleistocene sedimentary archives is difficult. Researchers mainly perform this intricate task (distinguishing between local and global controls in basin fills) to improve the temporal resolutions of environmental archives; this may be achieved with sediments that were deposited at higher sedimentation rates than in passive continental margins/ocean floors; however, these provid...