Mlcechvostsky slepenec – datovani a geochemie karbonatoveho tmelu slepence

The fact that the basal layer of Middle Pleistocene Vltava (Moldau) River terrace sediments near Mlčechvosty N of Prague, located about 13.0-14.6 m above present-day water level of the river, is cemented by a crystalline calcite into hard conglomerate, has been known since 19th century. Based on their position above the Vltava River, the age of the gravel and sand terrace deposits was traditionally estimated as Riss in the classical glacial scheme. C and O stable isotope compositions of the calcite cement (813C = -6.16 %0; 818O = -7.76 %0 vs. V-PDB) are within the usual range of Quaternary secondary carbonates. The data indicate that the cement was not deposited by evaporation of groundwater. The results of 14C dating show that the age of the calcite cement is behind the limit of this method, i.e. older than ca. 43 ka BP (if initial 50% amount of fossil carbon in the sample is supposed). The separated calcite cement was dated also by the Th/ U method. Even under high care to separate pure calcite cement, the samples showed low 230Th/232Th ratios, indicating thus significant sample contamination by detrital Th. Assuming the sample contamination only by Th and the B0 values (initial 230Th/232Th in the detritus) between 0.7 ± 0.3 and 1.7 ± 0.7, an age between 109±8ka and 100 ± 20 ka BP can be calculated. If these presumptions are invalid, the age of the samples can be in a more wide range. Nevertheless, all data (position of the terrace above the present-day river, the fact of dead-out 14C, and the 230Th/234U dating) indicate that the calcite cement is not Holocene in age, but significantly older.