Stratigraphic characteristics of Quaternary deposits on the left bank of the Sava River near Belgrade

Quaternary deposits on the banks of the Sava River in the Belgrade area have a significant thickness and they are represented by genetically different formations. The data about these sediments were obtained by exploration of two relatively shallow boreholes, RB 47/P-1 and RB 53/P-1, located on the left bank of the Sava River. Two genetic entities are distinguished: the lacustrine-palustrine deposits of Plio–Pleistocene age and alluvial deposits of Pleistocene and Holocene age. Deposits of the Plio–Pleistocene are clearly different, both lithologically and palaeontologically, from the overlying alluvial deposits. Lithologically similar fluvial deposits of the Pleistocene and Holocene age were distinguished according to their palaeontological characteristics, particulary by the presence of bivalve genera Corbicula and Dreissena.

A more complete picture of stratigraphic characteristics of Quaternary sediments was attained using the data obtained from the boreholes RB47/P-1 and RB 53/P-1, which were bored along the left bank of the Sava River in 2016/17. It may be concluded that both the immediate and the broader areas include various genetic types of the continental Quaternary sediments, originating as products of geodynamic processes under specific paleogeographic conditions.
New data on the Quaternary deposits of this area provide a more complete picture on the existing stratigraphic units, their interrelationships and contacts with the underlying sediments, in this case of the Neogene age. The new data set for Quaternary deposits on the Sava banks reported in this paper improves our understanding of the Quaternary stratigraphy and paleogeography in Central and Southeastern Europe.

Stratigraphic characteristics of deposits
According to the data from the borehole cores RB47/P-1 and RB53/P-1, deposits of the Neogene and Quaternary age can be identified in the investigated area (Fig. 2).

Neogene
The Neogene deposits underlie Quaternary sediments in the borehole RB 53/P-1, from the depth of 27.7 m to the bottom of the hole. They are represented by deposits of Late Miocene (Pannonian) age made up of grey marly clay with ostracod microfauna and rare fossil molluscs: Gyraulus praeponticus (GORJA-NOVIĆ-KRAMBERGER), Congeria banatica HÖERNES, Limnocardium sp. etc. At the contact with the Quaternary sediments, these deposits are greyish-yellow and contain lenses of calcium carbonate.

Quaternary
Based on genesis, superposition and age of the Quaternary deposits, and by using the data obtained from the boreholes RB47/P-1 and RB53/P-1, two types of the Quaternary deposits can be identified in the valley of the Sava River in Belgrade: 1. lacustrine-palustrine sediments of Plio-Pleistocene age and 2. alluvial sediments of Pleistocene-Holocene age. range from 23.0 m to 24.5 m, i.e. to the bottom of the borehole, so their exact thickness is not known. According to the available published data, the thickness of these sediments in the area of Ada Ciganlija is approximately 18.0 m, on the left bank of the Sava Ri-ver the thickness decreases, while in the area of Zemun it dramatically increases to more than 100 m (KNEŽEVIĆ et al., 1998;NENADIĆ, 1997NENADIĆ, , 2001NENADIĆ, , 2003NENADIĆ et al., 2009NENADIĆ et al., , 2010NENADIĆ et al., , 2011NENADIĆ et al., , 2015NENADIĆ et al., , 2016NENA-DIĆ & BOGIĆEVIĆ, 2014).  Lithologically, the oldest Quaternary deposits in the Sava River valley are composed of heterogeneous gray-brown, gray, brown-reddish clays (mainly greasy and hard), sandy clays, clayey sands, gravelly clays, etc. Occurrences of ooliths and lenses of manganese and iron oxides are common, sometimes even in greater quantity, when sediments acquire dark browngray colour. In certain places they contain lenses and concretions of calcium carbonate (KNEŽEVIĆ et al., 1998;NENADIĆ, 1997NENADIĆ, , 2003NENADIĆ et al., 2001).
In the investigated boreholes these sediments are represented lithologically by gray-green silts, varicoloured gravelly silts and clays, with manganese and iron oxides and calcium carbonate concretions. They are palaeontologically sterile, i.e. they contain no fossils, except in the area of Zemun (KNEŽEVIĆ et al., 1988;NENADIĆ, 2003;NENADIĆ et al., 2009), where these deposits contain only rare gastropod genera Lymnaea and Planorbis.
It is assumed that these layers were deposited in palustrine-lacustrine environment. They are found only in certain places on the river island Ada Ciganlija and almost everywhere along the left bank of the Sava River, where their thickness significantly increases (KNEŽEVIĆ et al., 1998).
In the Sava River valley in Makiš, on Ada Ciganlija and along the lef bank of the river in New Belgrade, these deposits discordantly overlie the Miocene sediments of the Paratethys (mostly marls of Pannonian age), while in the area of Zemun, they lie over lacustrine deposits of Pliocene age (KNEŽEVIĆ et al., 1998;NENADIĆ, 1997NENADIĆ, , 2001NENADIĆ, , 2003NENADIĆ et al., 2009NENADIĆ et al., , 2010NENADIĆ et al., , 2011NENADIĆ et al., , 2015NENADIĆ et al., , 2016.
The age of these sediments cannot be precisely determined due to the lack of palaeontological evidence. However, according to their lithostratigraphic position and superposition, it is assumed that they were formed in the Late Pliocene and Early Pleistocene.

Alluvial sediments of Pleistocene-Holocene age
Alluvial deposits of Quaternary age (according to data from the investigated boreholes) consist of two cycles of alluvial sedimentation: -older cycle, so-called polycyclic deposits of the Early-Middle Pleistocene age and -younger cycle, i.e. recent alluvial sediments of the Sava River.

Lower-Middle Pleistocene fluvial polycyclic deposits
The most widespread Pleistocene deposits in the riparian area of the Sava River in Belgrade are fluvial polycyclic deposits. In the earlier published papers they were known as the "Makiš beds" or "beds with Corbicula fluminalis" (LASKAREV, 1938;STEVANOVIĆ, 1977).
On the right bank of the Sava River, they usually discordantly overlie Miocene or, to a lesser extent, pre-Neogene sediments, and in some places they are deposited over lacustrine-palustrine layers of Plio-Pleistocene age. On the left bank of the Sava River, they cover thick lacustrine-palustrine Plio-Pleistocene deposits, and near the confluence of the Sava and Danube Rivers they overlie Upper Miocene (Pannonian) marls (KNEŽEVIĆ et al., 1998;NENADIĆ, 2003;NE-NADIĆ et al., 2015).
The Pleistocene deposits make up the most part of the Quaternary deposits in the investigated boreholes, with thicknesses that vary between 10.0 and 15.0 m. Lithologically, they are represented mostly by the brown-yellow and gray sands, gravelly sands and sandy gravels, silty sands, with intercalations of silts and clays.
Cyclic alternation of typical riverbed deposits (gravels, sands) and sediments of flood phase (silts and rarely clays) is observed in these boreholes. A regular multi-phase gradation of grain size has been formed, with coarser material in the lower and finer material in the upper parts.
The percentage of coarser material is larger around the river itself (investigated boreholes), especially on the right riverbank, and it decreases northwards, below the Srem loess Plateau in the direction of Zemun Polje and Batajnica (KNEŽEVIĆ et al., 1988;NENADIĆ, 2003;NENADIĆ et al., 2001NENADIĆ et al., , 2009NENADIĆ et al., , 2015NENADIĆ et al., , 2016. In these deposits in the wider area of southeastern Srem, two cycles of alluvial sedimentation are observed: the older cycle, in which sands and gravels predominate (polycyclic phase) and the younger cycle, made up primarily of silts (fluvial-palustrine deposits). In the Sava River valley in Belgrade, younger layers of this complex have been eroded, and they are discordantly overlain by alluvial deposits of the Sava River (NENA- DIĆ, 2003).
Pleistocene fluvial deposits contain fossil molluscs,  (KNEŽEVIĆ et al., 1998;NENADIĆ, 2003). An unidentifiable vole tooth and bone fragments of large mammals have been found in these deposits, too.
The lower level of these deposits is usually represented by coarser clastites with very rare faunal remains, most commonly the species Viviparus boeckhi and fragments of unionids (GAUDENYI et al., 2013). The biostratigraphical-palaeontological criterion for setting the boundary between the Pleistocene and recent fluvial deposits is the presence of species Corbicula fluminalis (MÜLLER), Viviparus boeckhi (HALA-VATS), and ostracods Candona neglecta SARS, Ilyocypris bradyi SARS and Scottia browniana (JONES).

Alluvial deposits of Holocene age
Recent fluvial deposits represent the youngest stratigraphical unit in the alluvial package in the boreholes RB 47/P-1 and RB 53/P-1. They cut all older Quaternary sediments, and mostly overlie fluvial polycyclic sediments of Pleistocene age. Deposition of recent alluvial sediments precedes the epsiode of erosion of younger levels of the Pleistocene fluvial deposits, so in the investigated boreholes younger horizons of these deposits (known as fluvial-palustrine sediments) are missing, but they can be observed in the areas of Bežanija and Zemun (KNEŽEVIĆ et al., 1998;NENADIĆ, 2003;NENADIĆ et al., 2009).
Recent fluvial deposits consist of riverbed sediments (sandy gravels and sands) and sediments of floodplain (silts and sandy silts). Lithological units of riverbed sediments, sands and gravels with lenses of sandy silts, are alternating vertically. Lithostratigraphical column in both boreholes ends with floodplain sediments, gray-brown sandy silts, silts and gray-brown sandy-silty clays.
Recent alluvial deposits are underlain by anthropogenic sands, whose thickness varies between 1.0 and 5.9 m (according to data from boreholes).

Discussion
In the areas of Ada Ciganlija, New Belgrade, Zemun and Makiš, recent research (KNEŽEVIĆ et al., 1998;NENADIĆ, 2003;NENADIĆ et al., 2009) revealed the presence of geological deposits that overlie the Neogene deposits and lie below Pleistocene fluvial polycyclic sediments. In the earlier literature these beds were referred as the "Makiš beds" or "beds with Corbicula fluminalis" (LASKAREV, 1926(LASKAREV, , 1938STEVA-NOVIĆ, 1977). However, the deposits were not identified as a separate stratigraphic unit of the Quaternary age, but were defined as "weathering crust" of the Neogene (Pannonian and Pontian) deposits of the Paratethys (ŽIVKOVIĆ 1976). Since 1990s, these deposits have been recognized as a separate unit, due to their extreme thickness, which sometimes exceeds 100 m, and their specific lithological-palaeontological features, of transitional, Plio-Pleistocene age (KNEŽEVIĆ et al., 1988).
Thickness of these sediments rapidly increases from the Sava River riparian area to the Srem loess plateau (Bežanijska Kosa and Zemun loess). Extreme thickness of these sediments has been registered in the Bežanija graveyard (near Kvantaš market), where the described deposits are over 100 meters thick and reach to the bottom of the drilled borehole (260 m), so their real thickness in this part of the area has only been assumed (KNEŽEVIĆ et al., 1998;NENADIĆ, 2003). Since they contain no fossils, biostratigraphical methods can not be used for age determination. As they underlie the upper parts of Lower Pleistocene sediments and overlie Pliocene deposits (in the area of Zemun) it has been assumed that they were deposited during Late Pliocene (Romanian)-Early Pleistocene (KNEŽEVIĆ et al., 1998;NENADIĆ, 2003;NENADIĆ et al., 2009).
In the other parts of the Pannonian Basin (Slavonija, Bačka, southern Banat) these deposits correspond to the layers of biostratigraphical zone with Viviparus vukotinovici (youngest horizon of the lacustrine Paludina beds) and "beds with Viviparus boeckhi" (NENADIĆ, 2003).
Fluvial deposits of the Pleistocene age (fluvial polycyclic deposits or "beds with Corbicula fluminalis") have been the subject of investigation related to Hydrogeological survey of Belgrade for a long time now (LASKAREV, 1938;STEVANOVIĆ, 1977;ŽIVKOVIĆ, 1976;RAKIĆ, 1977a;KNEŽEVIĆ et al., 1998;NENADIĆ, 2003). In the Sava River Valley they have been partly eroded and covered by recent alluvial sediments of this river, while in Bežanijska Kosa and Zemun, they are overlain by loess deposits of the Srem loess plateau.
The same deposits were identified in the territory of Romania (ONČESKU, 1960) as lacustrine deposits of Pliocene age, although they obviously contain fluvial fauna. According to other authors (GROMOV et al., 1960;MOSKVITIN, 1970), such layers in eastern England and glaciated areas of ex-USSR are known from the Riss glaciation and Riss/Würm interglaciation (Alpine geochronological scale).
Although the mentioned deposits in Serbia were mainly considered as lacustrine, alternation of riverbed, floodplain and oxbow deposits undoubtedly indicates fluvial character of the sediments, which are characterized also by an enhanced thickness. Only the younger part of these deposits was formed in a regime of relatively calm water (floodplain-palustrine conditions), with very slow water flow and long-term standstill (RAKIĆ, 1977a;RAKIĆ et al., 1998;NENADIĆ, 2003).
In these deposits, in contrast to limnic fauna of the Paludina beds, there are no sculpted unionids, Viviparus and Fagotia, Corbicula and other recent forms.
Earlier authors (e.g. LASKAREV, 1938;STEVANOVIĆ, 1977) used the presence of the fossil assemblage of molluscs Corbicula fluminalis (MÜLLER) and Viviparus diluvianus (KUNTH.), to determine the age as Middle Pleistocene, i.e. Mindel, Mindel/Riss (after the Pleistocene subdivision for the Alps). It has been later established that Viviparus diluvianus corresponds to species Viviparus boeckhi (KROLOPP, 1983), so the revision of age of the Makiš beds, as well as a more detailed regional correlation, became possible.
The guide fossils presently used for the age determination (Early Pleistocene) are Viviparus boeckhi (HALAVATS) and Esperiana esperi (FÉRRUSAC) in the older coarse-clastic beds, and the Pleistocene Corbicula in the younger gravelly sands.
After recent studies and the revision of the guide species, such as Corbicula fluminalis (MÜLLER), Viviparus boeckhi (HALAVATS) and Esperiana esperi (FÉR-RUSAC), the Pleistocene fluvial deposits that contain the mentioned species have been dated as the Early-Middle Pleistocene (NENADIĆ et al., 2009).
The equivalents of fluvial polycyclic deposits in the area of southern Banat (RAKIĆ, 1985) are gravels and sands with occasional occurrences of silts and clays which cover deposits of Pontian age or Paludina beds (lower part of Upper Pliocene). At Belgrade hill chain, to the south of the Danube and Sava Rivers, lateral equivalents of these deposits are represented by poly-genetic terrestrial and palustrine sediments (NENADIĆ, 2003). Fluvial polycyclic sediments could be correlated with the seventh terrace of the Danube River in the Dacian Basin (with a relative height 110-120 m), made of fluvial sediments of Early and Middle Pleistocene (PEH et al., 1998;ENCIU & BALTEANU, 2002).

Conclusions
This paper presents an overview of the Quaternary deposits on the left bank of the Sava River in the Belgrade area (boreholes RB 47/P-1 and RB 53/P-1), in the part of the terrain where the southern edge of the Pannonian Plain meets the Balkan Peninsula. The oldest horizon of the Quaternary deposits, which was only partially bored (borehole RB47/P-1), is represented by the Plio-Pleistocene sediments which have often been erroneously considered to be Neogene-Upper Miocene in age. According to lithological characteristics and very scarce fossil records, it is assumed that these sediments were formed in an environment of unstable shallow lakes and ponds which sporadically dried up, with significant influence of temporary riverine watercourses.
Material that composes these deposits in the investigated area originated in the hilly region of the Belgrade promontory, representing an area of intensive denudation. Most of this material was previously deposited in loosely cemented Early Neogene (Pannonian and Pontian) deposits. It was later redeposited in the areas of present-day New Belgrade, Zemun and Bežanija, where thickness of these deposits may be substantial (over 100 m) (KNEŽEVIĆ et al., 1988;NE-NADIĆ, 2001, 2003NENADIĆ et al., 2009).
Among the Pleistocene deposits in boreholes RB 47/P-1 and RB 53/P-1, the sediments with the most extensive spatial distribution are polycyclic sediments of Early and Middle Pleistocene, as is generally the case along the banks of the Sava River. Their paleontological indicators include greater abundance of species Viviparus boeckhi in the older and Corbicula fluminalis in the younger horizons. In the floodplain sediments (clays, silts) these species were replaced by dominant forms of Planorbis planorbis (morph with denticula) and the ostracod species Scottia browniana (KNEŽEVIĆ et al., 1988;NENADIĆ, 2003;RAKIĆ et al., 2002).
The polycyclic fluvial deposits of Pleistocene age were formed in the ancient valley of the Sava River, under the influence of multiple tectonic movements, as a product of a large lowland river that changed the shape of its bed in cycles, causally connected to tectonic movements and specific paleoclimate conditions. Large amounts of sand and gravel indicate humid climate and intense precipitation, the largest number of present mollusc genera and species are generally associated with beds of fast and moderately fast rivers with clear water, while there is a much smaller number of species characteristic for a less mobile aquatic environment (NENADIĆ, 2003;NENADIĆ et al., 2009).
The results of the core analyses in the 1990s and 2000s suggest that the Pleistocene Corbicula species of the Sava River riparian basin belongs to the temperate stages (equivalent to the interglacials) of late Early and early Middle Pleistocene age (NENADIĆ et al., 2009;NENADIĆ & BOGIĆEVIĆ, 2014;GAUDENYI et al., 2015).
In addition to their stratigraphic importance, these sediments also have a high economic significance due to their connection to aquifers utilized to provide the City of Belgrade with drinking water. Besides, it is worth mentioning that up to recently the fossils of bivalves and gastropods from investigated sediments were predominantly used for biostratigraphical subdivision, neglecting the importance of palaeoecological analysis and its comparison in the abundance and expansion with the recent representatives. This kind of study, as it is pointed by RADULOVIĆ et al., (2016) should be incorporated in future studies as it could shed light on the questions related to anthropogenic factors and their influence to expansion and invasive character of recent representatives of Corbicula fluminalis and Dreissena polymorpha. Additionaly, the importance of palaeoecological data provided from Quaternary sediments has been acknowledged as crucial in terms of global conservation related to species invasion, biodiversity crisis, natural variability, climate changes, etc. (BIRKS, 1996;WILLIS & BIRKS, 2006).
A large part of the alluvial Pleistocene sediments at the banks of the Sava River were eroded after the formation of its present bed and the recent alluvial plain of the Sava River and its tributaries. From the lithological point of view, the Holocene alluvial sediments are similar to the fluvial deposits of Pleistocene age. The palaeontological criterion for their separation includes species Corbicula fluminalis, present only in older fluvial deposits and species Dreissena polymorpha, which is much more common in the formations of Holocene age.
It is supposed that in the geological past, probably in early Holocene, the beds of the Sava River and its tributaries were situated significantly more northward than the present-day riverbed, as evidenced by numerous abandoned and compressed meanders. In addition to the migration from the north, the riverbed of the Sava River was occasionally also situated more to the south, and meandering from the edge of the Makiš area to Bežanija escarpment, it left behind many marshes and oxbow lakes which are presently visible as remnants of its old bed (NENADIĆ, 2002(NENADIĆ, , 2003. Significance of this movement of the riverbed was in redeposition of the previously deposited Pleistocene or even Holocene sediments, as may be determined according to the data from the investigated boreholes. JEVREMOVIĆ, M. & KUZMIĆ, V. 1999