Prvková chemostratigrafie křídy v okolí Letovic (česká křídová pánev, Česká republika)Článek v PDF
Element chemostratigraphy of the Upper Cretaceous in the vicinity of Letovice (Bohemian Cretaceous Basin, Czech Republic)
The study provides new data on a major and trace element composition of the Upper Cretaceous Peruc-Korycany and Bílá Hora formations belonging to the Orlice-Žďár sub-basin of the Bohemian Cretaceous Basin, Czech Republic. Samples were taken in regular vertical spacing (20 and 40 cm) from three sections in the surroundings of Letovice (Březinka, Rudka u Kunštátu – Kříb, Vlkov). Lithology of the sections was logged, and six lithofacies (labelled as F1 to F6) were distinguished. The samples were powdered in an agate mill and measured by the handheld X-ray fluorescence spectrometer. Principal component analysis revealed groups of elements with common geochemical behaviour related to biogenic carbonate (Ca, Sr), detrital (Al, Zr, K, Fe, Zn, Pb), authigenic (Fe, Zn, Mn) and mixed detrital-biogenic (Si) mineral phases. A special attention was paid to examining relationship of detrital elements and facies. The uppermost part of the Peruc-Korycany Formation (Korycany Member) has the highest content of Si, K, and Fe. The basal member of the Bílá Hora Formation is characterised by the highest concentrations of Al, Ti, Zr, Zn, Mn, and Pb, and the lower and upper members of the Bílá Hora Formation have the highest content of Ca and Sr. Trends of linear regressions among Al, Zr and Ti are different for each lithostratigraphic member. The Korycany Member (facies F1), basal member of the Bílá Hora Formation (facies F2), and medium to coarse grained sandstone facies F6 of the upper member have high Fe/Al and low Ti/Al and Zr/Al ratios, compared to the marlstone to medium grained sandstone facies F3, F4, and F5 of the lower and upper members of Bílá Hora Formation. These differences are interpreted as reflecting a change of heavy minerals associations probably related to sorting mechanisms rather than to a change in a provenance. Ti is interpreted as the most typicial element proxies of grain size in the Bílá Hora Formation, where Ti (mostly incorporated in silt and fine grained fraction, and eventually in illite) decreases along with coarsening. The Ti curve reflects general coarsening upward trend represented by two cycles in the Bílá Hora Formation. The lower and upper cycles are tentatively correlated with genetic sequences TUR 1 and TUR 2, respectively, related to sea-level fluctuations in the Bohemian Cretaceous Basin.
Josef Riegel, Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 2, Brno 611 37, e-mail: firstname.lastname@example.org
Tomáš Kumpan, Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 2, Brno 611 37, e-mail: email@example.com