Sedimentation History of Neogene Lacustrine Sediments of Sueoka Bela Stena Based on Geochemical Parameters(Valjevo-Mionica Basin,Serbia)

Sediments of the western part of the Valjevo-Mionica basin(Serbia)were examined both geochemically and mineralogically to explain,on the basis of their sedimentologicai characteristics,the causes of changes in their qualitative and quantitative composition.A total of 62 samples obtained from the drilihole at depths up to 400 m was investigated.Using correlation of the obtained data,six geochemical zones were defined,two of which being specially distinguished by their mineralogical, geochemical and sedimentological characteristics.The first one,upper zone A,consists of banded marlstones interbedded with clay and oil shales and is characterized by presence of analcite and searlesite.These minerals and very high contents of Na_2O indicate sedimentation in alkaline conditions with increased salinity in arid climate.That provided pronounced water stratification,as well as higher bioproductivity in the basin and sedimentary organic matter preservation.Therefore,the zone A sediments are characterized by high organic matter contents of the type which provides good potential for production of liquid hydrocarbons.Another specific zone,zone F,contains sediments with very high MgO,K_2O and Li concentrations.Their geochemical correlation,as well as almost complete absence of illite in this zone,indicates the presence of interstratified clay mineral type illite-saponite (lithium-bearing Mg-smectite).