Stratigraphy and lithology of the loess deposits in eastern Srem ( Serbia )

Eolian loess sediments in the eastern Srem area, the youngest Pleistocene deposits, occur either as a loess plateau, or as a slope-type of loess, formed on the slopes of the Fru{ka Gora Mountain. Lithological, sedimentological, and mineralogical characteristics of these sediments, which were studied from several common clay deposits and boreholes, demonstrated differences between them and the underlying pre-loess sediments, and supported the stratigraphic division of the Quaternary sediments.


Introduction
Young horizons of the Pleistocene deposits in the eastern Srem area are composed of an eolian loess formation, known as the Srem loess plateau (Fig. 1).These deposits were defined as loess earlier, which was confirmed by later research.Previously, when describing many loess horizons, researches included not only typical loess deposits but also the underlying polygenetic pre-loess sediments (GORJANOVI]- KRAMBERGER, 1921;LASKAREV, 1938; MARKOVI]-MARJANOVI], 1972; RAKI], 1977).Thus, the age of the loess formation appeared to be older than it is in fact.
In this investigation, the loess deposits from the ^ortanovci, Zemun, Ruma and Irig areas were studied.The paleontology and stratigraphy of representative samples was investigated.The mineral composition (bulk and clay fraction samples) was determined by X-ray diffraction (XRD) and optical microscopy (coarse fractions).Grain-size analysis was done using standard sieves; fractions <0.063 mm were separated by the decantation method.All analyses were performed in the laboratories of the Faculty of Mining and Geology, Belgrade.
According to the lithologic and stratigraphic data from numerous boreholes, the thickness of the eolian loess averages about 20 m (Figs. 2, 3).The loess formation in Zemun consists of four loess sequences (I to IV in decending depth), and three paleosol sequences (Fig. 2).
The I and II loess sequences are typical subaeral deposits, formed by the accumulation of wind-transported silt and sand (with a minor clay content) on dry land, and their thickness is usually 10-15 m.They contain only terrestrial mollusks.The III and IV loess sequences were deposited on wet grassland, under variable climatic conditions.These loess sequences contain more clay particles, and a number of Fe-and Mn-oxyhydroxide nodules.Their thickness is usually less than 10 m.These horizons contain both terrestrial and aquatic mollusks.
Quartz is the most abundant mineral in the loess formation at Zemun (DANGI], 1995).Carbonates, feldspars,  mica, clays, rock fragments, and amphibole (in decreasing order of their appearance) are minor.Calcite is the major carbonate mineral, and occurs either as a finely dispersed matrix, or as nodules of different size.The clay minerals are illite and chlorite with occasionally traces of smectite.

^ortanovci area
Several loess samples from the boreholes drilled in the ^ortanovci area have been studied mineralogically and sedimentologically.
The loess formation here is around 25 m thick (Fig. 2), and consists usually of 3-4 loess sequences with the same number of paleosols.Eolian deposits cover the polygenetic deluvial-proluvial sediments of the "Srem series", represented by clayey silt or silty clay.The grain size distributions of the loess and "Srem series" are completely different, especially when comparing the sand and clay fractions (Table 1).
According to our XRD studies (Fig. 4), quartz, feldspar, calcite, dolomite, and mica (in decreasing order of their appearance) are the main minerals of the loess formation.The clay minerals are illite and chlorite, with minor amounts of smectite, which seems to be more abundant in the lower parts of the loess sequences.In the sand fraction, quartz, carbonates, and mica are dominant, with minor amounts of feldspar, sphen, ore minerals, rutile, altered grains, and rock fragments.

Sremski Karlovci -In|ija area
In the area between Sremski Karlovci and In|ija, the loess formation consists of 2-4 loess sequences and the same number of palaeosol horizons.The total thickness varies between 10 and 22 m, depending on the degree of erosion during the Holocene.Loess sequences are sandy silt or silty sand, with abundant carbonate nodules.Palaeosol horizons are made up of dark grey clayey silt, or brown silty clay, with different amounts of organic matter.The fossil remains are typical for the terrestrial forms of cold biotypes, such as: Vallonia costata (MÜLLER), Punctum pygmaeum DRAPARNAUD, Succinea oblonga DRAPARNAUD, Chondrula tridens DRAPARNAUD, Vitrea cristalina (MÜLLER), Arianta arbustorum LINNE, Pupilla sp., and indicate the Late Pleistocene (Riss and Würm).

Ruma area
The loess formation in the area of Ruma consists of three loess sequences and two palaeosol horizons (Figs.2001).The loess sequences are yellow to grey-brown or yellow-brown, usually with small carbonate nodules (up to 2 cm in diameter), while the palaeosols are brown to red clayey silt.The total thickness of the loess formation is up to 20 m, and the whole formation dips towards SSE at an angle of ~10°.
Both the loess sequences and paleosols consist of quartz, mica, calcite, feldspar, with minor amounts of dolomite, and clay minerals (illite and chlorite, with traces of smectite) (Fig. 6).The clay mineralogy of the  pre-loess sediments is quite different: with smectite as the absolutely dominant clay mineral, with traces of illite and chlorite.
Grain size analyses of the loess sequences, palaeosol, and pre-loess sediments (Table 2), suggest a difference in the provenance of the material and the depositional environment.

Irig area
In the Irig area, the loess formation is composed of four loess sequences, and four palaeosols (Figs. 2, 7), which are underlain by brown plastic clay of the "Srem series" (SIMI], 2004).The loess sequences are yellow to yellow brown, occasionally with levels rich in carbonate nodules (usually up to 10 mm in diameter).The carbonate content of the Irig loess formation is 11-18%, on average ~14%.The average content of sand, silt, and clay fraction is 2.90%, 78.99%, and 18.21%, respectively.
The mineral composition is typical for the loess (Fig. 8): quartz is the dominant mineral, accompanied by calcite (as small nodules, shell fragments, and finely dispersed matrix); feldspar, dolomite, and mica occur in minor amounts; illite and chlorite are the dominant clays, smectite occurs in traces.

Conclusion
Loess is one of the major Quaternary formations in the Srem area.It formed as a result of eolian transport and deposition predominantly of silt, either in a wet grassland environment or on dry land.In the Srem, the loess occurs as the large "Srem loess plateau", or as a slope-type, formed along the Fru{ka Gora Mountain, up to 400 m above sea level.Recent studies demonstrated that the loess formation is made up of 2-4 loess sequences, and 1-4 paleosol horizons, with a total thickness of approximately 20 m (less if eroded).The first two horizons are of subaeral and the two lower ones of aquatic origin.This can be inferred from their lithological and palaeontological contents.
The malacofauna from all the studied areas is nearly identical, and cannot be used for more detailed stratigraphic divisions.Comparing these fauna with the occurrences of remnant of mammals in the loess deposits in the Srem and Belgrade, such as Mammuthus primigenius (BLUMENBACH), Rhinnoceras tichorinus PISCH., Equus cabalus LINNE, and Bos primigenius BOJANUS as typical representatives of the Upper Paleolite or Hazar complex (GROMOV et al., 1960), it can be concluded that the loess formation deposited during Riss and Würm.
The mineral composition of the loess formation is very similar in all the studied areas: quartz, carbonates, feldspar, mica, and clay minerals (illite and chlorite).
Smectite usually occurs in traces.The mineralogy of the loess is quite different from that of the underlying sediments, confirming a totally different source of the primary material, as well as of the mode of transport and deposition.On the basis of the collected data, it could be concluded that the underlying units are genetically different from the overlying aeolian package.They were not formed by aeolian activity, as was generally thought, but by fluvial accumulation of material in the floodplains of steppe rivers or by processes of erosion on the slopes of the Fru{ka Gora.

Fig. 3 .
Fig. 3. General section of the loess formation based on the boreholes in the Be`anija-Zemun-Batajnica area.I-IV: loess sequences.
Fig. 5. Typical cross-section of the loess deposit in Ruma.

Table 1 .
Grain size distribution of the loess formation and "Srem series", ^ortanovci area.