Acta Musei Moraviae

Modal composition of siliciclastic sediments and its impact on their spectral gamma-ray record: application in provenance and stratigraphic study

The spectral gamma-ray (GRS) logs are usually used as indicators of facies trends both in subsurface and outcrop. However, our outcrop and laboratory GRS data from Lower Carboniferous (upper Viséan) siliciclastic sediments of the Moravice Formation (MF) and Hradec-Kyjovice Formation (HKF), the Nízký Jeseník part of the Culm Basin, revealed very weak dependence on facies. Therefore, alternative sources of their GRS variation should be searched for. Sandstones of the MF have only slightly lower radioaktivity than mudstones while at some stratigraphic levels their radioactivity is even higher. No statistical correlation was observed between field and laboratory GRS data but, considering their different methodologic approach and the numerous external factors affecting the field measurement, it is no surprise. However, both field and laboratory GRS failed to show a significant dependence on facies. A possible explanation can be given by the stratigraphic variation in the concentrations of K-, U- and Thbearing minerals. Sandstone modal composition, cathodoluminescence microscopy and microprobe analyses identified potassium feldspars, muscovite (sericite), biotite and albite as the main carriers of K. The main carriers of U and Th include xenotime, monazite, zircon and apatite. Three stratigraphic segments in can be distinguished in the MF and HKF based on the variations in sandstone modal composition, mineral chemistry and the gamma-ray spectra. The lower segment (Bohdanovice and Cvilín Members of the MF) is characterized by increased concentrations of U and Th/U ratios and low kontrast between the radioactivity of sandstones and mudstones. Sandstones of this segment reflect source from predominantly low-grade metamorphic, sedimentary and volcanic rocks. The middle segment (Brumovice and Vikštejn Members, MF) is characterized by relatively high concentrations of U and partly Th and, generally, higher radioactivity of sandstones in comparison with mudstones. Sandstones of middle segment reveal provenance change towards predominantly magmatic sources with ultrapotassic plutonic rocks (svrchní goniatitová zóna Goα a zóna Goβ). In the upper segment the contents of U, Th and K generally decrease and the gamma-ray spectra reveal relatively stronger dependence on facies. The modal composition of sandstones indicates a provenance shift towards high-grade metamorphic rocks in the Uppermost Visean. The GRS and the detrital provenance data sensitively reflect an extremely rapid exhumation of the deeper crustal parts of the Moldanubian nappe pile of the Bohemian Massif.