GEOCHEMISTRY OF MIOCENE LACUSTRINE SEDIMENTS FROM THE SOKOLOV COAL BASIN (CZECH-REPUBLIC)

Sediments of the upper part of the Sokolov Formation of the Sokolov Br own Coal Basin consist mostly of brown lacustrine clays and claystones enriched in organic matter (2-18% TOC). The tower portion of the stra tigraphic profile is formed mostly by kaolinite-illite clays and clays tones, while the montmorillonite content increases in the upper portio n. The change in the lithology of sediments is caused by the gradual e rosion of the weathering crust in the source areas of the elastic mate rial. Inversion of the weathering profile during erosion is manifested geochemically in a decrease in the Al2O3/Na2O ratio and an increase i n the SiO2/Al2O3, Na2O/K2O and K2O/TiO2 ratios from the base to the to p of the studied part of the Sokolov Formation. The amounts of trace e lements (La, Ce, Nh, Zr, Cr, V, Sn) also decrease in the same directio n. On the other hand, the amounts of Rb and Sr increase. The organic m atter of the upper part of the Sokolov Formation consists primarily of Types I and II kerogen. Only near the base of the studied part of the formation, i.e., in the coal stringer, kerogen of Type III prevails. The extremely low degree of thermal maturity permits determination of the source of the organic matter and characterization of its accumulat ion environment. The organic material in the sediments is predominantl y of algae origin. Relatively high amount of pentacyclic triterpanes o f the hopane series indicates either the occurrence of cyanobacteria a s primary producers or bacterial reworking during sedimentation and di agenesis. Organic matter accumulated under the conditions of a fresh-w ater lake or lake with slightly elevated salinity, in a dysoxic aqueou s environment. The alternation of organic carbon-rich layers with laye rs low in organic matter is explained in terms of changes in the degre e of dysoxia of the aqueous environment and a variation in the intensi ty of oxidation and mineralization of the organic material. (C) 1998 E lsevier Science B.V. Ail rights reserved.