Facies analyses, biostratigraphy and radiometric dating of the Lower–Middle Miocene succession near Zaječar (Dacian basin, eastern Serbia)

2 University of Belgrade, Technical Faculty in Bor, Vojske Jugoslavije 12, 19210 Bor, Serbia. E-mail: mbanjesevic@tfbor.bg.ac.rs Abstract. Lower–Middle Miocene sedimentary succession and the conformable/unconformable relationships between the lacustrine-continental systems (i.e. DLS, SLS) and Badenian marine transgression represents one of the intrigue topics. Herein, we studied five exploration boreholes (eastern Serbia) and analyzed the main facies pattern, biostratigraphic characteristics of the Miocene succession, and applied the U-Pb radiometric dating of volcanic tuffs interstratified in the sedimentary series with coal layers (borehole NRKR17002). The obtained concordia age of 16.9 ±0.2 Ma for all the analysed zircon grains without any inherited cores indicate a single magmatic event. We definite the freshwater series originated during Early Miocene Karpatian (= late Burdigalian). Consequently, for the first time, we demonstrated that age of а part of the Serbian Lake System (SLS) is much older than it was previous reported. In addition, sporadic findings of foraminifers, ostracods and molluscs documented the late Badenian marine transgression in eastern Serbia. If accept this fact the flooding occurred later than in the rest of Serbia (˂ 14.5 Ma). However, the lack of quality data and unclear stratigraphic position of some parts of the clastic succession (? Lower–Middle Badenian) makes this claim uncertain.

In this regard, besides the main facies analysis and biostratigraphic division of the Miocene succession, we applied the method of radiometric dating of volcanic tuff interstratified in the sedimentary series, and tried to define more precisely the age of the Miocene freshwater series with coal as well as the timing of Badenian marine transgression in this part of Serbia.

Geological background
The area of eastern Serbia belongs to the Carpatho-Balkanides as a large tectonic unit (e.g. PETKoVIć & aNDjELKoVIć, 1958;gRUBIć & aNToNIjEVIć, 1961/1962DIMITRIjEVIć, 1997;KRäUTNER & KRSTIć, 2003;SCHMID et al. 2008 and references therein). according to the last mentioned authors, they also include parts of the Serbo-Macedonian Massif (DI - MITRIjEVIć, 1997) and to the west, follow the eastern margin of East Vardar ophiolites. Inside this belt, the Timok fault represents one of the main strikeslip structure which displace the Cretaceous nappe unit along the contact between the Dacia Mega Unit and the Moesian Platform (SCHMID et al., 2008). This fault accommodated oligocene to early Miocene dextral strike-slip motion with total movements up to a hundred kilometers (SCHMID et al., 2008). generally, from the end of the early Miocene up to the older part of the Middle Miocene, the Serbian part of Carpatho-Balkanides was subjected to extensional processes (KRSTIć et al., 1988;KRSTIć, 1991;MaRoVIć et al., 1998MaRoVIć et al., , 2002. It was a secondary effect of a much stronger activity established in the area of the Pannonian Basin which led to the formation of numerous basinal structures in these areas (Ma -RoVIć et al., 1998, 2002. Character of the Carpatho-Balkanides orogen was enabled by the general phase of uplifting manifested from the beginning of the Sarmatian to the recent time (MaRoVIć et al., 2002(MaRoVIć et al., , 2007. During the middle Miocene, the Paratethys marine and brackish water was flooded area of the westernmost part of Carpathian foredeep in the territory of Serbia (aNĐELKoVIć & aNĐEL - KoVIć, 1997). It flooded different Mesozoic and Tertiary rocks depending on morphology of paleorelief (gaNIć, 2005;ĐajIć et al., 2018;RUNDIć et al., 2018). generally, different Mezosoic rocks (mostly Upper Cretaceous flysch, clastites, volcanoclastites and carbonates) as well as older Miocene continental-lacustrine sediments represent the basement rock for the mentioned transgressive sediments. These Upper Cretaceous rock formations are known as the carriers of mettalic raw materials (e.g. ĐoRĐEVIć & BaNjEšEVIć, 1997;ĐoRĐEVIć, 2005;aNToNI -jEVIć & MIjaToVIć, 2014) (fig. 1). In that sense, the studied boreholes were a part of exploration drilling which was performed by Tilva Co. during 2017.

Material and methods
This study presents results of detail mapping of five exploration boreholes (RTK-1501, RTK-1502a, RTK-1503, NRKR-17001 and NRKR-17002) located near Zaječar, eastern Serbia ( fig. 1). More than a hundred samples were taken for different analyses. Petrological and chemical analysis were performed in the laboratories of the Pe trology and geochemistry at the faculty of Mining and geology using standard proce du re (grain-size analysis, calcimetry, thin-section, etc.  VeSeLinoVić et al., 1967, modified Zircon grains were separated from the tuff samples no. 93 and 94 (NRKR-17002) using standard mineral separation techniques. The rocks were crushed to coarse sand size with a tungsten carbide jaw crusher. Subsequently, zircons were concentrated by sieving, magnetic separation and heavy liquids. a representative batch of the zircon crystals with grain sizes of 150-300 μm was manually picked and mounted in araldite 2020 resin.
after polishing, cathodoluminescence (CL) images were taken using a CL detector connected to Scanning Electron Microscope at University of Belgrade, faculty of Mining and geology. The CL investigations were carried out using a jEoL jSM-6610LV SEM (with a W-filament as a beam source), coupled with an X-Max EDS. The samples were coated with carbon using a BaLTEC-SCD-005 sputter coating device. The results were recorded under high vacuum conditions with acceleration voltage was 20 kV.
Zircons were dated with the U/Pb method using Laser ablation-Inductively Coupled Plasma-Mass Spectrometry (La-ICP-MS). U, Th and Pb isotopes were measured at the Institute for geosciences, University of Mainz using an agilent 7500ce quadrupole ICP-MS system coupled to an ESINWR193 arf excimer laser system with a 193 nm output wavelength. The laser system is equipped with a two volume sample chamber (10 cm × 10 cm). after pre-ablation, analyses were conducted using a spot size of 30 μm, 20 s warm up time, 30 s dwell time and 20 s washout time. The repetition rate was 10 Hz at pulse energy of 3.5 jcm-2. The instrument was tuned for ma ximum sensitivity at low oxide formation rates of <0.5 %. The dwell times for individual masses are 10 ms for masses 232 and 238, and 30 ms for 202, 204 and 208. Dwell ti mes of 40 and 60 ms were used for masses 206 and 207, respectively. for a first step data reduction, the time-resolved signal was proces sed using the program gLITTER (www.glittergemoc.com, Macquarie University, Sydney, australia). Time-dependent laser and mass spectrometer induced inter-element fractionation (Pb/U), mass fractionation, as well as common lead, were corrected afterwards using an Excel spreadsheet of ComPbcorr (aNDERSEN, 2002). The inter-element fractionation during ablation was corrected linearly. for this purpose, ablation conditions such as spot sizes and ablation times were kept constant during each session. The interference of 204 Hg on 204 Pb was corrected by measuring 202 Hg and calculating 204 Hg using the natural 204 Hg/ 202 Hg ratio of 0.2299. ages, uncertainties and concordia diagrams were produced using Isoplot3 for Excel (LUDWIg, 2003). Concordia ages are plotted with 2 uncertainty ellipses and discordia intercept ages are given at 95% confidence. analyses were calibrated using a gj-1 (gEMoC) zircon (SLaMa et al., 2008). Reproducibility and accuracy were controlled by repeated analyses of Plesovice and 91500 reference zircons, from jaCKSoN et al. WIEDENBECK et al. (1995), respectively, treated as unknown samples; measured values deviate <2% from the published values.

Facies analyses
all of the boreholes drilled the different Mio cene units with total thickness up to 600 m (e.g. RTK-1501). generally, Miocene succession is composed of clastic rocks. They unconformably overlies the various Cretaceous sediments and volcanoclastics. older units include the Valanginian limestone, albian glauconitic sandstone and siltstone, Cenomanian fine-grained clastics and the coarse-grained andesitic epiclastites of the upper Cretaceous, known as Metovnica formation (ĐoRĐEVIć & BaNjE -šEVIć, 1997;aNToNIjEVIć & MIjaToVIć, 2014). In vertical succession, the Miocene sediments were deposited in different environments: non-marine, marine and brackish ones (from the bottom to the top).
The lacustrine succession starts with (a) basal clastites or coarse-grained clastites. Clasts are derived from the Valanginian limestones, re-deposited weathering crust of andesites and andesite epiclastites of Metovnica fm. (U. Cretaceous) . at the same time, a part of that could be clastites from the marginal parts with gravel and sand sequences. The thickness of the basal clastites is from several meters (RTK-1503 and NRKR-17002) up to 35 meters (RTK-1502a, fig. 2). fossils not observed.
b. Colorful fine-grained clastites of muddy plains This lithofacies is made of finest clastics (silty clay and silt) with rapid color shifting in vertical direction (figs. 2, 3). The colour can be marked by tones of red, grey, green, yellow with all the passes. The lithofacies thickness is variable and ranges from 10 to 65 m . It suggests a shallow habitat with occasional subaeric conditions. as effect of this, thin levels of the paleosoil was developed ( fig. 2). Note that whole the Miocene series in the borehole RTK-1503 is located much more upward than in borehole RTK-1502a.
c. Lithofacies of gray fine-grained clastites This lithofacies, from 5 to 40 m thick, represents the gradual transition from the lithofacies of varicolored fine-grained clastites into a marsh facies . Regarding sedimentology, it suggests on the deepening of the lake basin. Regarding petrology the fine-grained clastites in shades of grey, brown, grayish-brown and green color prevail. They consider clayey silt, and silty clays with more or less sandy or calcareous component. If the content of calcite exceeds 35% the sediments transit into marlstones. These sediments contain carbonized floral detritus as well as the debris from fresh-water mollusk molds (e.g. RTK-1502a and RTK-1503 -figs. 2, 3).
Interpretation: Marginal-basinal facies d. Marsh facies Sediments of marsh facies were identified in the northern part of the area of interest. This is, regard-ing petrology, the most complex unit due to rapid vertical and lateral changes of conditions in depositional environment. generally, it consists of clastic sediments, coal, coal-bearing sediments, carbonate rocks and tuffs . The succession discovered by drilling may contain from two to 10 levels of coal that are from 10 cm to 4 m thick such as in borehole NRKR-17002. Lithologically, marsh facies succession include homogeneous marls, a layer of coarse sand that has intraformation fragments of marl in the top (framed by a polygon), gray marls with small deformations, thin layered and laminated marls/marls with freshwater gastropods and coal (figs. 3, 4). generally, marl contains the assemblage of fossil fauna, particularly of gastropods and ostracods (RTK-1503, fig. 3). a lot of thin, transparent ostracode specimens belongs the Candona group. It has been observed and rare large forms reaching over 1.8 mm. They have slightly perforated ornamentation resembling Camptocypria. There are also fragments of slightly accentuated sculpture such as ? Dinarocythere, ilyocypris but the carapace is straighter. Rare, tiny suboval forms similar to Cypria have also been found. fossil remains are found in carbonate rocks, commonly revealing the lamination. a peculiarity of this facies is the presence of a tuff layer which is exposed at the different position within the coal series. The observed thickness ranges of the marsh facies from 25 to 90 meters (NRKR-17002, fig. 4).

e. Marginal-lacustrine facies
Marginal lacustrine (lake) facies is generally shallow-water facies. It was developed along the margins of the basin, and gradually towards the central parts of the lake, crossing the lake facies . It is characterized by deposition of fine grained clastics and marl (e.g. RTK-1503, fig. 3). If it is closer to the alluvial-lake facies, it occasionally contains medium-sized and coarse clastics.
f. alluvial-lacustrine facies alluvial-lacustrine facies developed at place where river enters a lake exhibiting alluvial fans or deltas (oBRaDoVIć & VaSIć, 2007;VaSIć et al., 2018). The most  abundant are coarse-and medium-gra ined clastites. Material was brought by alluvial flows from broader coastal area. Coarse-grained clastites were periodically brought into deeper and farther basin parts by torrential flows (flooding events) giving rise to marginal-lacustrine or basin facies. Marginal-lacustrine facies is generally a shallow-water. It is juxtaposed against basin margins and transits gradually towards the central lake parts into lacustrine facies.
The estimated thickness of alluvial-lacustrine facies is 170-190 m (RTK-1501, fig. 5). Main lithology are gravel, sand, and their transitions. Vertical and lateral distribution is in sequences, which may be of two or three members. 40 sequences from 1-2 m to over 10 m thick were recognized vertically . general characteristic of the all of sequences is the decreasing coarseness upward. Due to erosional contacts between sequences, a part of them may be shortened and lack of upper, sandy sequence part. Signi ficant thickness of given facies and its gravelly pattern suggest on a relatively strong alluvial system that formed a deltaic mo del at river mouths (oBRa DoVIć & VaSIć, 2007;VaSIć et al., 2018). Clastic material was re-deposited into other facies du ring extreme flooding events . That was also the source area for the present marginal-lacustrine environment. In terms of lithology, the marginal-lacustrine facies is generally made by fine-grained clastites with bodies of sand and gravel ( fig. 6).

Marine and marinebrackish facies (Middle Miocene)
This sedimentary package consists of different size clastic roks and covers the previous mentioned lacustrine-continental sediments. Main criteria for the separation was the presence of marine-brackish fauna . Lithologicaly, it can be divided onto three lithofacies: a) the lithofacies association of finegrained clastites with sand and gravels, b) gravel and sand, and c) the upper lithofacies association of fine-grained clastites with sand and gravels (VaSIć et al., 2018).
a. Lower lithofacies association of fine-grained clastites with sand and gravels The measured thickness of the given lithofacies is within range of 60-160 m (e.g. RTK-1501, fig. 7). Regarding sedi mentology is the unit very similar with the succession of marginal-lacustrine facies as the fine-grained clastites are also dominating. The more emphasized frequency and thickness of sand and gravel bodies suggests on the proximity of the main source of clastic material, i.e. to a strong alluvial system. fine-grained clastites were referred silty clays, clayey silts, sandy-clayey silts, clayey-silty sands, clayey-sandy silts. all sediments are generally calcite-poor, with the contents of 10-20% of CaCo 3 . Sands vary in grain size from fine-to coarse-grained and their composition is inherited from gravel fraction. gravels consist of pebbles that have been derived from andesite complex or from Metovnica fm., from older limestones or clastic units .

b. Lithofacies of gravel and sands
This unit is 60-120 m thick (RTK-1501, fig. 7). gravels and sands are the most abundant lithology.  In terms of sedimentology, the unit is organized in clastic sequences whose thickness is up to 12 m ( fig. 8). Sequences are commonly from three members (gravel-sand-fine-grained clastites), less common of two members (gravel-sand). gravelly se diments (lag gravels, gravels, sandy gravels and gravelly sands) occur in the base of sequences. They display the upward transition into sandy sediments (gravelly sands and sands of variable coarseness). The very top of sequences, which is the thinnest, build horizontally or cross laminated fine-grained sands and silts. general characteristic of the all of sequences is the upward decrease in coarseness. Their relations with basal sequences are constantly erosional .
c. Upper lithofacies association of fine-grained clastites with sand and gravels The estimated thickness of this lithofacies is 80-90 m (e.g. RTK-1501, see fig. 7). In this succession is the domination of fine-grained clastites striking, whereas the sands and gravels are subordinated. general characteristic of fine-grained clastites is that they represent two-or three-component systems from sand-silt-clay, silt-clay, clay-silt, and sand-silt. The content of calcite is very low, i.e. less than 10% . fine-grained clastites display textures that were governed by the system energy, which has been low. Consequently, hori zontal and cross lamination developed. fossil remains include tiny debris of mollusks . Carbonized flora is occasionally present. gravels and sands are quite the same as those in the lithofacies described above.
Interpretation: Shallow-water facies with the occasional basinal influence.  cies, three different stratigraphic units could be separated: late Lower Miocene (Karpatian), early Middle Miocene (Badenian) and late Middle Miocene (Sarmatian). The oldest one unit is marked by poor and endemic assemblages of fossil mollusks, ostracods, fish remains and very scarce charophyta gyrogonites within the marsh facies with coal (e.g. borehole NRKR-17002, RTK-1501, RTK-1502a, and RTK-1503). Small and often fragmented planispiral forms such as Gyraulus sp., Planorbarius sp. and Planorbis sp. ( fig. 9) represent typical freshwater fauna. only few findings of Prososthenia sp. and Fossarulus sp. were noted (NRKR-17002). Very often along the thin layers there are the "coquina" with operculums of freshwater gastropods ( fig. 9). Similarly, poorly preserved ostracods such as Candona sp., Fabaeformiscandona sp., ilyocypris sp., ?Dinarocythere sp., Candonopsis sp., ?Cypria sp. were observed in so-called the ostracod marls . Somewhere, very rare and few fragments of charophyta could be seen. Based on these fossils, it is impossible to give a precise stratigraphic designation of the mentioned series. However, the marsh facies contains tuff layers, which are exposed across the whole facies (at the base of coal series, inside it as well as in its covering part, too). Radiometric dating of zircon grains from the vitroclastic tuff was obtained (borehole NRKR-17002). This is the first evidence concerning the age of the coal series (freshwater equivalents of Karpatian, Lower Miocene, 16.9 Ma) in this part of SLS. The total thickness of continental-lacustrine Mio cene reaches from 110 to 260 m RUNDIć et al., 2018).

Zircon U/Pb dating by LA-ICP-MS
The zircons from the volcanic tuff from borehole NRKR-17002 that is positioned on the top of coal series ( fig. 12) were dated. We analysed around 20 zircon grains, each grains are analysed in the core and in the rim ( fig. 13). Results from the zircon U/Pb are presented in Table 1 and corresponding concordia plots and age results are presented in fig. 14. We ob-tained concordia age of 16.9 ± 0.2 Ma for all analysed grains without any inherited cores, meaning that we were able to document a single magmatic event. This age in general corresponds to similar ages obtained the Central Pannonian basin (SEgHEDI et al., 2013, LU -KáCS et al., 2018 when a voluminous magmatism was syn chronous with the development of main sedimentary basins during the main Mio cene-quaternary tectonic events in Carpathian-Pannonian Region. Therefore, we interpret this to represent their crystallization age and therefore as the age of emplacement of this tuff.

Discussion and interpretation
During the Early to Middle Miocene large parts of Central Europe were covered by the Paratethys Sea that had phases of good and partial connectivity to the Mediterranean. Paratethys reduced and expand ed through time by a complex combination of climate variability, sea level change, and geodynamic pro cesses of alpine tectonics (e.g. RögL, 1998;TER BoRgH et al., 2014;SaNT et al., 2017). In the latest Early Miocene, the Central Parate thys mainly occupied the northwestern part of the Pannonian Basin in Slovenia, austria, Slovakia and Hungary. During the early Middle Mio cene this sea extended to the southeastern Pannonian Ba sin in Croatia, Bosnia and Her zegovina and Serbia, to the Transylvanian Basin in Romania, and to the Carpa thian fore deep (e. g. ćoRIć et al., 2009;PEZELj et al., 2013;TER BoRgH et al., 2014;SaNT et al., 2017SaNT et al., , 2018joVaNoVIć et al., 2019joVaNoVIć et al., , 2019aMaNDIC et al., 2019). During this marine water expansion, there were a few corridors between Central and Eastern Paratethys Sea (NEU -BaUER et al., 2015). Similar scenario was observed in eastern Serbia and so-called the Trans-Carpathian seaways across the Djerdap, Tekija and Borska Slatina were recognized (PoPoVIć, 1968;DoLIć, 1977;STEVaNoVIć, 1977). In the same time span a series of
The endemic fauna, together with the lack of magnetostratigraphic and radioisotopic data, led of misunderstanding of the age and evolution of the Serbian basins (SaNT et al., 2018;RUNDIć et al., 2018).
Miocene sediments of eastern Serbia were a subject of interest from long time ago. The first observation on Timok tertiary basin and determination of some mollusks taxa (Venus, Cerithium) comes from ami Boué, the founder of geology of Balkan Peninsula, during his trip in Serbia between 1836-1837 (žIVKoVIć, 1893). a half century later, jovan žujović, the founder of geology of Kingdom of Serbia informed that during the Miocene the Mediterranean Sea flooding the Timok valley in eastern Serbia (žUjoVIć, 1889). His conclusion was based on analysis of five fossiliferous localities (including Zvezdan near Zaječar). Since that time, the Miocene freshwater sediments with coal layers were already known from the Zvezdan coal mine which was opened in 1889. Very soon, RaDoVaNoVIć & PaVLoVIć (1891) and žIVKoVIć (1893) gave very important contributions concerning the Neogene of Timok basin and, especially its geological relationships as well as systematics and taxonomy of Tertiary fossils. The last one author mentioned very interesting geological realtionship between freshwater Tertiary sediments with coal and marine deposits near Zvezdan (žIVKoVIć, 1893). He found a very abundant assemblage of freshwater taxa (Melania serbica, Melanopsis sandbergeri, Prososthenia suessi, etc.). a    (neUBAUeR et al., 2015). note that during the early Miocene there are no data from the Serbian territory except the no.40 -Trijebine (illyrian province). Fig. 16. Palinspastic maps of europe freshwater system during the Middle Miocene (neUBAUeR et al. 2015). note the Balkan lacustrine province existed:  White ellipse marks the studied area of eastern Serbia. lania macedonica, Ancylus serbicus, etc.) from the samples which žujović gave him for identification. Much more later, the great field campaign in frame of the basic geological mapping of eastern Serbia (VESELINoVIć et al., 1967) shows that a close stratigraphic position of these Miocene freshwater series is impossible and there are no good biostratigraphic markers. Similar studies by DoLIć (1977DoLIć ( , 1998 and STEVaNoVIć (1977) confirmed that the mentioned freshwater fossils have the early-middle Miocene age.
However, based on lithostratigraphic correlation, biostratigraphy and radiometric measurements, our study confirms that the freshwater fossiliferous series from the studied boreholes (NRKR-17002, RTK-1501, RTK-1502a, RTK-1503) corresponding to the late Early Miocene equivalents of the Karpatian regional stage (= late Burdigalian). fossil material that was found within the marsh facies with coal layers has been deposited during the lacustrine phase of the late Early Miocene (SLS). The tiny, planispiral forms of Gyraulus-type gastropods, the rare tower-shaped and sculptured forms like Fossarulus and Prososthenia, and the numerous operculums (mostly Bithynia) show no orderliness in terms of orientation. In addition, there are many ostracods, especially many molds of elongated forms that are difficult to determine (most belong to the genus Candona). Their carapaces do not show orderliness i.e. no dominant orientation indicating in situ association. This lake (Lubnica Lake) was an endemic lake settled by various, more or less endemic biota. Sculptured gastropods (Fossarulus, Prososthenia) as well as ornamented ostracods (ilyocypris,?Dinarocythere) indicate shallow-water, but more dynamic water regime. Contrary, tiny, transparent ostracod carapaces (Candona, Cypria, Fabaeformiscandona, etc.) live in a quiet, low turbulence depositional conditions in sheltered or profundal part of lake (SaNT et al., 2018). This environmental depended life pattern is more or less recognized within all freshwater lakes during the Miocene time. for that reason, the clear stratigraphic range of the mentioned lake based on fossil content couldn't be defined. However, for the first time, our radiometric age dating (16.9 Ma) of tuff located above the coal series give the precise stratigraphic position of these sediments. on other words, the famous fresh-water series from the Zvezdan and Lubnica coal mines that underlie the Badenian marine sediments was deposited during the Karpatian (= late Burdigalian). Chronologically, it could be correlated with Pag, Livno and Sinj basins within the Dinaridic Lake System (DLS) which are originated during the Miocene Climatic optimum (DE LEEW et al., 2012). age of the tuff can be correlated with well-known Miocene syn-extensional volcanism across the Pannonian and Dacian basin (CVETKoVIć & PECSKay, 1999;CVETKoVIć et al., 2004;LUKáCS et al., 2018;MaRKoVIć et al., 2018). our results prove that SLS was developed much early than the latest study reported (KRSTIć et al., 2012, SaNT et al., 2018. Based on ostracods and diatoms taxa (ogNjaNoVa-RUMENoVa, 2006;ogNjaNoVa-RUMENoVa & KRSTIć, 2007) as well as some mollusks comparison, KRSTIć et al. (2012) concluded that majority of the freshwater basins in Serbia (SLS) were developed during the early Middle Miocene (including Popovac and Zaječar basins). Similarly, in the palinspastic map of Balkan area during the Early Miocene there are no data about lacustrine basins in Serbia (NEUBaUER et al., 2015). only exception is the Levač basin (central Serbia), which according to the flora remains was developed during the very Early Miocene (KRSTIć et al., 2012). However, recent study from Popovac basin in central Serbia (SaNT et al., 2018) and obtained radiometric age of 14.4 Ma (ar/ar dating of tuff from lacustrine marls) show that all the mentioned basins are significantly different age. all the conventional methods (lithostratigraphy, biostratigraphy, etc.) are not enough for a precise age determination of these lakes. fossil fauna have endemic character and it haven't biostratigraphic potential . It needs more independent age control methods (magnetostratigraphy, radiometric age dating) which can obtain real data concerning timing of SLS (RUNDIć et al., 2013SaNT et al., 2018). otherwise, its relationship towards the younger Badenian marine transgressive sediments is subject of debate (e.g. MaNDIC et al., 2019). Somewhere, this relation can be more or less obvious (e.g. SaNT et al., 2018). Some authors reported that the age of the SLS is older than the marine Middle Badenian but there is concordant relation between them (KRSTIć et al., 2012). Facies analyses, biostratigraphy and radiometric dating of the Lower-Middle Miocene succession near Zaječar (Dacian basin, eastern Serbia) a few of the studied boreholes documented the Badenian marine transgression (e.g. RTK-1501, RTK-1502a). There is good comparison between our results and previous researches (PETRoVIć, 1961(PETRoVIć, , 1969DžoDžo-ToMIć, 1963, 1970VESELINoVIć et al., 1969VESELINoVIć et al., , 1975PoPoVIć, 1968;PoPoVIć & gagIć, 1969;DoLIć, 1977;STEVaNoVIć, 1977;gaNIć, 2005). The transgression occurred accross the Timok bay which was developed along the Timok strike-slip fault reactivated at the beginning of Middle Miocene (KRäUTNER & KRSTIć, 2003;KRSTIć et al., 2012). In eastern Serbia, the area of Zaječar was the southernmost prolongation of the Paratethys Sea (STE -VaNoVIć, 1977;gaNIć, 2005). During the Middle Badenian, marine facies reached to the town of Zaječar (KRSTIć et al., 2012). However, in the Upper Badenian (= Konkian, regional stage in eastern Paratethys), the topmost part of succession have a Sarmatoid character and classified as the "Buglovian layers" (LaSKaREV, 1934;DžoDžo-ToMIć, 1963) or "Veselyankian horizon" (KóKay, 1984, p. 41). Based on these studies, the topmost Badenian and transitional Badenian/Sarmatian sediments have Eastern Parathetian affinity. However, it is not easy to reconstruct marine gateways between Central and Eastern Paratethys (KóKay, 1984;TER BoRgH et al., 2014).. The existence of small oases of Badenian rocks within so-called the Trans-Carpathian corridor (Serbian part of Carpathians) could be a connection route (DoLIć, 1977;STEVaNoVIć, 1977). Contrary, other authors mentioned that the Badenian Sea of so-called the Vienna type, mainly comes through SW Romania to the north-western part of Bulgaria (KRSTIć et al., 2012). Recently, similar paleogeographic reconstructions based on biochronology and ar/ar radiometric age of tuffs from the NW Transylvanian basin and SE Carpathian foredeep indicate that the marine transgression of the Paratethys occurred during Middle Badenian between 14.9-14.4 Ma (SaNT et al., 2019).
our results from a few samples contain microfauna confirm existence of late Badenian to early Sarmatian phases of evolution of the Paratethys Sea in the studied area. Based on foraminifer and ostracod assemblages and scarce molluscan remains found in these samples as well as lithological succession (e.g. RTK-1501, RTK-1502a) there is an in-dication about the late Badenian transgressive event. It could be correlative with global 3 rd order cycle TB 2.5 of sea-level fluctation that is already known form Central Paratethys (ćoRIć et al., 2009;PEZELj et al., 2016). Species such as Ammonia ex gr. beccarri, A. cf .viennensis, Bolivina cf. dilatata, Bulimina elongata, Hanzawaia boueana, Heterolepa dutemplei, Cassidulina laevigata etc. indicate full marine conditions (PoPoVIć, 1968;PoPoVIć & gagIć, 1969;PETRoVIć, 1969PETRoVIć, , 1988. However, large portion of clastites within all the studied boreholes indicate a regressive trend and transition towards the restricted marine and brackish environment (STEVa -NoVIć, 1977;SCHWaRZHaNS et al., 2015;VaSIć et al., 2018;RUNDIć et al., 2018). Indeed, fossil fauna such as Granulolabium bicinctum, Acteocina lajonkaireana, Mohrensternia cf. pseudoangulata, ervilia cf. dissita, irus cf. gregarious, Abra sp., Hydrobia sp., and Cerithium sp. suggest that changes. additionally, small foraminifers such as rare anomalinoides and ammonia with small-size shell as well as scarce ostracod species indicate the shift from the marine to shallow-water brackish environment (LaSKaREV, 1934;DžoDžo-ToMIć,1963, 1970PoPoVIć & gagIć, 1969;gaNIć, 2005). afterwards, we recognized a faunistic sterile package of clastites between the freshwater lacustrine sediments and marine-brackish sediments on the top of some boreholes (RTK-1501, RTK-1502a, and RTK-1503 - fig. 17). It represents the transitional part between these units and it is impossible to give it more precise stratigraphic age. So, it could correspond to time interval from late Karpatian to late Badenian. Based on the mentioned results and spatial and temporal relation between the studied boreholes RTK-1501 and RTK-1502a, the next stratigraphic range within the marine and marine-brackish succession could be determined ( fig. 18).
finally, based on all the collected data from the studied area, we sketched a hypothetical model of development of the lateral alluvial-lacustrine and marginal-lacustrine facies during the early Miocene ( fig. 18). Vertical alteration of the facies in the studied wells and their basic sedimentological and paleontological properties give some elements to the sketch and suggest about the mode of deposition in such a system. The arrangement of individual facies (marginal-lacustrine, lacustrine, prodelta, etc.) and their distribution over the surface indicate to the depositional mode. The thickness of the clastic sediments from the alluvial facies (aL) decreases to basin wards. generally, this model of facies distribution indicates the direction of transport from north to south. Two of transparent arrows mark the direction of the redeposited clastic material to the mar ginal facies.
on the other hand, there is clear evidence that during the middle Miocene marine water of Paratethys flooded this area. It is clearly observed in the boreholes RTK-1501 and RTK-1502a. However, these sporadic evidences of the late Badenian transgressive event (no evidence on older Badenian flooding) and more or less uniform pattern of distribution of the clastic sediments across the basin make too difficult to create another one sketch map of facies distribution during the Badenian and Sarmatian time. So, we think that more detailed analyses of the mentioned core-wells as well as the future nearby boreholes can better support this attempt to reconstruct the direction of the marine Badenian flooding as well as following regressive trend during Sarmatian.

Conclusions
This study is obtained the first exact data regarding the stratigraphic position of freshwater Miocene clastic sediments and pyroclastites in the Zaječar area. The oldest Miocene unit corresponds to the continental-lacustrine series of so-called the Serbian Lake system (SLS) that was originated during the late early Miocene whereas overlying marine and restricted marine sediments belong to the middle Miocene (Badenian and Sarmatian).
Six different major lithofacies within the continental-lacustrine deposition (SLS) and three major   (VASić et al., 2018, modified). marine and restricted marine facies (marine-brackish Paratethys) indicate rapid evolution of the area during the early and middle Miocene.
U-Pb radiometric analyses of zircon grains from the tuff located above the coal series (borehole NRKR-17002) indicates a mean radiometric age of 16.9 Ma. It is the first data that precisely indicate the Lower Miocene age of coal series in eastern Serbia. Previously, it understood as the Lower-Middle Miocene.
analyses of mollusk and foraminifer assemblages from the different clastic sediments which overlay the mentioned freshwater series indicate a marine and marine-brackish development that corresponds to the time interval from late Badenian to early Sarmatian.
Based on data from the studied boreholes, it is impossible to date the timing of marine Badenian transgression in this westernmost part of Dacian basin. Namely, biostratigraphic analyses of the studied material show that the older biostratigraphic zones within Badenian could not be determined. only up per Badenian Bolivina-Bulimina Zone is recognized. No determined stratigraphic range for different clastites which separate the lower, freshwater series of clastites with coal (Lower Miocene) from the upper Badenian and Lower Sarmatian sediments on the top of Miocene succession. They have a potential stratigraphic range from the Karpatian to Upper Badenian.
Comparative lithostratigraphic analysis as well as a geological model of the studied area, point that Badenian flooding sediments or Sarmatian restricted marine clastites cover different older geological units (e.g. Lower Miocene, Lower or Upper Cretaceous, etc.) depending of the morphological characteristics of paleorelief. серије са угљем у Лубници односно Звездану.