Distribution of molybdenum and uranium in the Tremadoc graptolitic argillite (Dictyonema shale) of North-Western Estonia

The distribution of uranium, molybdenum and calorific vafue in the Dictyonema Shale is presented basing data obtained through the study of about 130 drill cores. The north-western part of Estonia is divided into two geochemical zones. The boundary between these zones is the most distinct near the Paldiski-Rapla line (Fig. 1), which is also rather clear limit for the distribution of graptolites. In the western zone (zone I) sedimentation took place mainly in the stable anoxic conditions, where the toxic sulphide-rich bottom water was covered by a well-defined denitrified water layer serving as an abundant source of food for graptolites. In the younqer, eastern zone (zone II) the bottom layer was hydrodynamically more active. Fine lamination was not formed in that zone, and due to the higher oxygren content in the bottom water the concentrations of uranium, molybdenum, vanadium etc. had decreased in the sediment.