Der Löß in Bulgarien
Abstract. The loess in Bulgaria takes up an area of 9,800 km2 and is distributed exclusively in the section along the Danube. The loess forms a mantle over the pre-loessian relief surface forms — Pliocene denudation and sedimentation surfaces and Pleistocene river terraces. Loess formations overlie the Cretaceous, Tertiary and old Pleistocene deposits on the plateaux from which they are separated by a red-brown terra rossa-like clay weathering crust. On the terraces the loess overlies alluvial sediments of various ages, greater than its own age. The thickness of the loess formation depends on the distance from the Danube river and the age of the geomorphologic element which it overlies. On the plateaux in immediate proximity to the Danube river this thickness varies within the range of 50 and 60 m, loess walls reaching 102 m. The thickness drops to 25—30 m about 10 km south of the Danube, whereas in the southern peripheral parts it hardly reaches 4—5 m. The lithofacial composition of the loess formation changes with recedance from the Danube, the individual faciei differences being territorially manifested as narrow strips parallel to each other and to the Danube in the following order: loessial sand, sandy loess, typical loess, clayey loess and loessial clay. Loess stratigraphy is based on fossil soils, showing great stability with respect to number, type and territorial distribution. The number of loess horizons depends on the age of the relief element overlain by the loess complex and on the conditions favouring the preservation of the primary accumulation. The loess complex, overlying pre-Quaternary and Lower-Pleistocene forms of the relief, consists of 6 loess horizons, separated by 5 fossil soils; on the terrace of a relative height of 25—35 m (Riss/Würm I) it consists of three loess horizons, separated by two fossil soils and on the terrace of a relative height of 12—15 m (Wurm II) — by one loess horizon. On the basis of the available paleopedologic, paleontologic and pre-historic data and the principle of stratigraphic analogy we assume that the first loess horizon is deposited during Min-del I, the second-during Mindel II, the third-during Riss (I and II), the fourth-during Würm I, the fifth-during Würm II, the sixth-during Würm IIII, the second and third fossil soils being interglacial and the rest-interstadial. The studies conducted so far provide grounds to assume that the loess in Bulgaria is of eolian origin and terrigenous source — the overflooding of the Danube river.