Mariusz Paszkowski

Vegetation-controlled modern anastomosing system of the upper Narew River (NE Poland) and its sediments

Abstract The anastomosing system of the upper Narew River consists of a network of interconnected channels. The channels are relatively deep (width/depth ratio 2–10), straight to sinuous, and they lack natural levees. They are characterised by a low water slope and very low stream power. The river is distinctly bedload-dominated and the transport of suspended clastic fines is minimal. Channel deposits consist almost exclusively of medium- to coarse-grained sand. Laterally extensive interchannel areas are flat and covered with peat-forming vegetation. These stable wetlands are flooded for many weeks during high water stages. Except for the channels, the valley fill consists of peat layer reaching 4 m in thickness. The rate of vertical aggradation of the peat deposit is estimated at 1–1.5 mm/year. The radiocarbon dating indicates that the peat layer is predominantly late Holocene in age. The impact of vegetation on the system is overwhelming. Vegetation produces an erosion-resistant peat layer, stabilizes channel banks and slows down the water flow. Vegetation also stimulates aggradation of bedload material on the channel bottom, and contributes to avulsion by blocking the channels. The channel network owes its origin to repeated though infrequent avulsion. Avulsion in the studied system is a small-scale, gradational and slow process. New channels evolve very slowly because of unfavourable hydrologic conditions and the presence of a resistant peat substratum. A new channel eventually intercepts only a part of the flow, while the old channel is still active, though to a limited extent. Although newly formed channels might subsequently be abandoned, long-lasting ones predominate within the system.

THERMAL HISTORY OF LOWER PALEOZOIC ROCKS ON THE PERI-TORNQUIST MARGIN OF THE EAST EUROPEAN CRATON (PODOLIA, UKRAINE) INFERRED FROM COMBINED XRD, K-Ar, AND AFT DATA

The Upper Silurian–Lower Devonian section of the Dniester gorge in Podolia and samples from boreholes located S and N of this area were studied in order to reconstruct the thermal history of Lower Paleozoic sedimentary rocks in the Dniester segment of the Peri-Tornquist margin of the East European Craton which is the most eastern part of a major shale-gas target in Europe. X-ray diffraction data for illite-smectite from shales and carbonates indicate very advanced diagenesis and maximum paleotemperatures of ~200ºC, higher than interpreted from the ‘conodont alteration index’ (CAI) data. Diagenesis of the Devonian section is slightly less advanced than that of the underlying Silurian section, indicating that it is a regional feature and the result of burial. The regional distribution of the diagenetic grade based on illite matches well with the pattern established from the CAI data. K-Ar dating of illite-smectite from Silurian bentonites and shales gave a consistent set of dates ranging from 390 to 312 Ma. To explain such advanced levels of diagenesis and such K-Ar dates, the extension of the Carboniferous foreland basin (which today is only preserved to the NW of L’viv) toward the SE on the craton margin has to be assumed. The diagenetic zonation pattern of the Carboniferous coals supports this hypothesis. The Carboniferous cover may have been either sedimentary or partially tectonic (Variscan intracratonic duplexes) in origin and the thickness, necessary for the observed level of diagenesis, may have been reduced by an elevated heat flow along the major tectonic zone at the edge of the craton (TESZ). The presence of such cover is confirmed by completely reset Cretaceous apatite fission track (AFT) ages of the Silurian bentonites. The AFT dates also imply a Tertiary heating event in the area.The 10 Å clay mineral present in the dolomitic part of the profile (Silurian), both in bentonites and in other rocks, is aluminoceladonite or intermediate between illite and aluminoceladonite, while in the Devonian shale section only illite was documented. Chlorite is also common in the studied rocks and is at least partially authigenic. It is non-expandable in the samples from boreholes, while often expandable to variable extents in the samples from outcrops, which also contain goethite. Such variation in chlorite is attributed to contemporary weathering.

Age determination of monazites using the new experimental chamber of the Cracow proton microprobe

Abstract The Cracow microprobe was applied to determine the age of small detritalmonazite crystals extracted from the uppermost carboniferous sandstone of Upper Silesia Coal Basin (southern Poland). The method applied follows the technique of chemical dating of U–Th minerals: assuming that the monazite crystal is a closed system and all Pb within grain is radiogenic, it is possible to determine the age of a specimen by solving an equilibrium equation which includes as computational factors the elemental contents of U, Th and Pb and decay times of uranium and thorium. Using this approach, thanks to rather fast Pb accumulation in a crystal, monazite older than 100 My may be dated with relative ease. The investigated specimens were clusters of crystals, 30–100 μm in size, polished and mounted in Epoxy resin. Due to different geological history, the age of individual crystals from one sample may differ significantly, therefore the application of a microprobe has been necessary to extract information concerning single monazite grains. Two age groups were identified: small cluster, connected with Cadomian tectonothermal event (579±30 My) and dominating cluster, connected with Variscan tectonothermal event (329±19 My, 353±41 My). After successful preliminary measurements carried out in a provisory experimental chamber, an optimized chamber was constructed and set into operation for regular studies. Together with results of these investigations, the constructional details of the new chamber are presented.