Evolution of Sofia Basin during the Neogene

The sedimentary fill of Sofia Basin represents a large variety of sedimentary rocks grouped into four lithostratigraphic units – variegated terrigenous formation, Gniljane, Novi Iskar and Lozenec Formations (Kamenov, Kojumdgieva, 1983) that represent the distinct stages of the basin evolution - alluvial, marshy-lacustrine and lacustrine.
   The variegated terrigenous formation is presented by irregular alternation of clayshales, siltshales, sands and sandstones, varying in color. Gniljane Formation comprises conglomerates, sands and sandstones, siltstones and silty clayshales with lignite interbeds In the upper part (Balsha Member). Novi Iskar Formation comprises gray to gray-bluish clays, diatomaceous clays, silty clays and siltshales as well as varves, composed of alternation of carbonaceous and clayey siltshales. Lozenec Formation consists of irregular alternation of clays, siltshales, sands, and conglomerates with lignite interbeds (Novi Han Member) in the lowermost part of the formation and with micrite limestones (Bogiovci Member) in the upper part of the formation (Yaneva, 1998). The thickness of Neogene sediments in Sofia basin varies from 10 m in the borders up to 1400m in the central parts.