Natural radioactivity of sedimentary rocks in the northeastern part of the Moravo-Silesian Palaeozoic Basin

Abstract

The paper provides detailed information on the natural radioactivity of sedimentary rocks in the northeastern part of the Moravo-Silesian Palaeozoic Basin in the Bohemian Massif. The studied area is dominated by the Lower Carboniferous flysch sediments belonging to the Horní Benešov, Moravice, and Hradec-Kyjovice formations (from botttom to the top). These three flysch formations consist of siliciclastic sediments, mainly graywackes, siltstones, silty shales, and clay shales, while conglomerates are less abundant. The overlying sediments of the Ostrava Formation (Namurian) occur on the eastern margin of the studied area. The Ostrava Formation has a character of a paralic coal-bearing molasse with alternating beds of sandstone (or arkose), siltstone and claystone. The sediments often have an elevated content of organic matter, concentrating into coal seams. The rocks of the Ponikev Formation (Devonian) occur in a narrow belt in the western part of the studied area. The Ponikev Formation consists mainly of silicites. Potassium, uranium and thorium contents were measured using a laboratory gamma-ray spectrometer in 1612 rock samples from 672 localities on five map sheets (1 : 50 000 scale). The calculated values of mass activity of 226Ra equivalent (am) demonstrate that the natural radioactivity of sedimentary rocks of all three flysch formations is lower or similar to that of the average Earth crust (about 180 Bq . kg–1). The radioactivity of flysch sediments increases with decreasing grain size from psephites (average am is 122 Bq . kg–1) to psammites (144 Bq . kg–1) and to aleurites + pelites (188 Bq . kg–1). Siltstones and claystones with a low organic matter content (226 Bq . kg–1), coaly siltstones and coaly claystones (240 Bq . kg–1), coaly sandstones and arkose sandstones (232 Bq . kg–1), and coal (202 Bq . kg–1) of the Ostrava Formation show slightly higher average am values. The relatively high radioactivity of these sediments is related to the elevated content of uranium (7.9 ppm on average).