Biostratigraphy of the Konkian ( Middle Miocene of the Eastern Paratethys ) deposits of Southern Ukraine based on foraminifera

The Konkian (Middle Miocene) foraminiferal assemblages and molluscs from five Wells situated in Southern Ukraine were studied in order to correlate the palaeoecology and biostratigraphy of the coeval palaeobasins with different environmental conditions. The article contains comprehensive analysis of controversial issues of the Konkian stratigraphy of the Eastern Paratethys and additional keys for determination of Konkian development phases by foraminifera and molluscs. Five stenohaline normal-marine, two euryhaline and one mixed foraminiferal assemblage were identified in different levels in the investigated wells with the analysis of their accordance to different molluscs assemblages. The study defines an isochronous foraminiferal assemblage for some wells, suggests a palaeoecological and stratigraphic reconstuction of the middle Miocene sediments in the Eastern Black Sea Region and recognizes two models of development of the Konkian foraminiferal assemblages. These models show the differences between environmental conditions in the shallow-water basin of the Eastern Black Sea Region and deeper water basin of the Kerch Peninsula at that time. While the Eastern Black Sea Region was influenced by frequent and abrupt changes in environmental conditions during almost all Konkian time, a relatively deeper basin of the Kerch Peninsula had a successive phase. The late Konkian time created similar depositional conditions for both basins. certain species of mollusсs were traced in sections, then these deposits were defined as the same age and such mollusc associations were identified as “Beds with molluscs” (e.g. ANDRUSOV, 1917; ARKHANGUELSKY, 1930). Several “Beds with molluscs” were determined in the Konkian deposits: Beds with Ve­ nus konkensis in the Northern Black Sea Region of the Southern Ukraine (SOKOLOW, 1899), were named later as the Veselyanka Beds by MERKLIN (1953); the Pholadidae Beds on the Kerch Peninsula of the Southern Ukraine (ANDRUSOV, 1917); the Kartvel Beds in Georgia (DAVITASHVILY, 1930); the Sartagan Beds in Transcaspian (ZHIZHCHENKO, 1937a, b). The Kartvel Beds are characterized (Fig. 1) by Pholadidae (Barnea pseu­ doustjurtensis BOGATCHEV, B. ustjurtensis (EICHWALD), B. kubanica (ZHIZHCHENKO) B. scrinia (BOGATSCHEV) etc.); the Sartagan Beds are defined as deposits with a rich marine stenohaline Konkian mollusc fauna; the Veselyanka Beds are defined as deposits with euryhaline Konkian molluscs (MERKLIN, 1953; VERESHAGIN & MIRONOVA, 1982; MURATOV & NEVESSKAYA, eds., 1986). Later, such Beds with molluscs were found in the Konkian of other areas of the Eastern Paratethys (e.g. VARENTSOV, 1950; MERKLIN, 1953; MURATOV & NEVESSKAYA, eds., 1986). Shallow-water Konkian deposits often comprise Beds with Ervilia trigonula SOKOLOV or Beds with E. trigonula and Barnea pseudoustjurtensis, B. kubanica in the Southern Ukraine (MOLYAVKO, 1960; BARG, 1969). Therefore, they were called the “Ervilia-Pholadidae Beds” and were also considered as Kartvel Beds (BARG, 1969). The age of the Kartvel Beds has been under discussion. These Beds were defined as belonging to both the Karaganian and the Konkian regiostages based on the mollusc species composition (Pholadidae) (ZHIZHCHENKO, 1937a, 1937b; BURIAK, 1965). They were also considered as the final stage of the Karaganian (SUDO, 1961; NEVESSKAYA et al., 2005) or were Article history: Manuscript received January 30, 2018 Revised manuscript accepted September 18, 2018 Available online October 24, 2018


INTRODUCTION
The Konkian is the Middle Miocene stage of the Eastern Paratethys that corresponds to the NN6-NN7 Nannoplankton Zone (NEVESSKAYA et al., 2005), to the Serravallian of the Global Time Scale and to the upper Badenian (Kosovian) of the Central Paratethys (HILGEN et al., 2012).The Karaganian/Konkian boundary is dated at 13.4 Ma, and the Konkian/Sarmatian boundary is dated at 12.65 Ma (PALCU et al., 2017).The Konkian (upper Badenian) time is characterized by a marine transgression which reactivated connections between the Central and Eastern Paratethys.The preceding Karaganian time was characterized by euryhaline conditions which correspond to the Badenian Salinity Crisis in the middle Badenian of Central Paratethys (e.g.MURATOV & NE-VESSKAYA, eds., 1986;PERYT, 2006).After the Konkian (up per Badenian), the Sarmatian basin was characterized by palaeoenvironmen tal changes and the appearance of an endemic fauna in the Eastern and Central Paratethys.(e.g.MURATOV & NEVESSKA-YA, eds., 1986;NEVESSKAYA et al., 2005;HILGEN et al., 2012).
The aim of this study is to understand how the foraminiferal assemblages developed and to show the differences between the shallow-water and deeper water basins in the Konkian.The results are discussed in the context of temporal and spatial changes of foraminiferal species diversity and presented together with the analysis of molluscs as an additional group in the various Konkian facies of the Southern Ukraine.
Thus, today the terms the Kartvel Beds, the Sartagan Beds and the Veselyanka Beds are not clearly defined, as they can refer to stratigraphic horizons or palaeoecological conditions.So, there are problems in the case of their use for the correlation when it is necessary to compare deposits with the same names but they can have different semantic meanings.Therefore, it is possible to agree with the proposal of ILYINA (2000) not to divide the Konkian stage into several substages and not to call these units by their own names such as the Kartvel Beds, the Sartagan Beds and the Veselyanka Beds.However in the case when a researcher decides to divide the Konkian into substages it is proposed to use the terms: lower, middle and upper Konkian substages.If a re-  ZHIZHCHENKO, 1937a, b;MERKLIN, 1953;KRASH ENINNIKOV, 1959;BARG, 1969;IVANOVA, 2012, etc.).
searcher still decides to use individual names for these substages then it is proposed not to call them as Beds, but rather the Kartvelian substage, the Sartaganian substage and the Veselyankian substage (e.g.MAISSURADZE et al., 2014;PALCU et al., 2017).In addition, since the initial criteria for recognition of the Konkian substages were not universal, it is necessary to clarify the definitions for each of these substages by adding criteria to aid more accurate recognition of them in sections.At the same time, it is necessary to take into account the fact that the different facies of the Konkian deposits in the Eastern Paratethys generally contain stable species assemblages of fauna with certain palaeoecological characteristics that are separate biostratigraphic subdivisions "Beds with the fauna" (according to ZHAMOIDA, ed., 1977;TESLENKO, ed., 1997;GOZHYK, ed., 2012).In this case, it has been proposed to not call these subdivisions names such as the Kartvel Beds, the Sartagan Beds, the Veselianka Beds, but to name them after the characteristic fossils (e.g.limestones with Ervilia trigonula, Barnea pseudoustjurtensis) (VERNY-HOROVA, 2014, 2015a,b, 2016).

Criteria for determination of the initial (early) and final (late) development stages of the Konkian basin by molluscs and foraminifera
Since the different Beds with fauna may occur at different stratigraphic levels of the Konkian of the Eastern Paratethys they are not unique for particular development phases of the Konkian basin.Therefore, additional criteria for determination of the development stages of the Konkian basin are needed.Experience in the study of foraminifera and molluscs in the Konkian deposits from different areas of the Southern Ukraine, the Ciscaucasus and the Mangyshlak Peninsula (VERNIGOROVA et al., 2006(VERNIGOROVA et al., , 2009;;VERNIGOROVA, 2008VERNIGOROVA, , 2009VERNIGOROVA, , 2012;;GOLOVINA et al., 2009;BRATISHKO et al., 2015) allows the definition of additional criteria for determination of the initial (early) and final (late) development stages of the Konkian basin (VERNY-HOROVA, 2015a;POPOV et al., 2016).
An initial stage of the Konkian basin development may be recognized by a specific foraminiferal assemblage with a predominance of the genera Cassidulina and Discorbis (Cassidulina bulbiformis, C. bogdanowiczi KONENKOVA, Discorbis kartve licus etc.) (Fig. 3).This regular trend can be observed in some relatively deep Konkian deposits (clays and marls) of the eastern part of the Crimean Peninsula, the Kerch Peninsula and the Ciscaucasus (KRASHENINNIKOV, 1959;BOGDANOWICZ, 1965;VERNYHOROVA, 2015a;POPOV et al., 2016;PALCU et al., 2017).This stage is clearly visible only in the most complete Konkian sections.Otherwise the stage is difficult to identify, because similar foraminiferal assemblages are present in different Konkian stratigraphic horizons, especially in the shallow-water deposits (VERNYHOROVA 2014(VERNYHOROVA , 2015a(VERNYHOROVA , b, 2016)).Criteria for determining of the initial (early) development phase of the Konkian by molluscs are also not yet clearly defined.The early Konkian may contain Beds with Ervilia and Pholadidae.However, as has been indicated above, they are also present in other stratigraphic horizons.

THE KONKIAN LITHOFACIES OF THE SOUTHERN UKRAINE
The Konkian deposits of different lithofacies features are widespread in the Southern Ukraine (Figs. 4,5) More deep-water deposits (36-155 m thickness) are monotonous dark gray clays, partially laminated with rare sandy admixtures that have accumulated in the Kerch Peninsula and in the south-eastern parts of the Crimean Peninsula.They differ from other Konkian deposits of the Southern Ukraine by their litho-logical peculiarities and occurrence of more deep-water species of fauna (e.g.ANDRUSOV, 1917;OSIPOV, 1927;ARHANGUEL-SKY, 1930;ARKHANGUELSKY, ed., 1940;MOLYAVKO, 1960;BARG & STEPANIAK, 2003;VERNYHOROVA, 2014).These Konkian deposits represent a complete stratigraphic sequence and all of them are combined in the Petrovske Formation (VERNIGOROVA et al., 2012;VERNYHOROVA, 2014).

MATERIALS AND METHODS
The Konkian deposits were studied in five wells from several regions of the Southern Ukraine (Figs. 4, 6).There are four wells from the Eastern Black Sea Region (upper part of the Tymoshivka Formation): Well 8z (altitude -69.5 m; thickness of the Konkian deposits -4.2 m; 26 samples selected); Well 9 (altitude -69.7 m; thickness of the Konkian deposits -1.7 m; 24 samples selected); Well 6 (altitude -30.5 m; thickness of the Konkian deposits -5.0 m; 33 samples selected); Well 8m (altitude -86,0 m; thickness of the Konkian deposits -10.0 m; 47 samples selected) and one Well from the Kerch Peninsula (Petrovske Formation): Well 20 (altitude -17.0 m; thickness of the Konkian deposits -51.0 m; 82 samples selected).The stratigraphic sequence of these deposits was determined on the basis of foraminiferal and mollusc assemblages.Mollusc data from Well 8z were defined also by PRISY-AZHNYUK et al., 2007 and were used in this research.During analysis, data on the Konkian foraminiferal and molluscs assemblages and lithological features of the Konkian deposits from other areas of the Southern Ukraine from published literature (e.g.DIDKOVSKIY, 1959;MOLYAVKO, 1960;BARG, 1993;BARG & IVANOVA, 2000) and industrial-geological reports  were also utilised.
The selection and determination of all species of foraminifera was made from 300 g of sediment sample that was previously washed through a 76 μm sieve (e.g.VERNIGOROVA et al., 2006;VERNIGOROVA, 2008).Descriptions and images of foramini- fera from the Neogene deposits of the Eastern Paratethys were used for their species determination (KRASHENINNIKOV, 1959;BOGDANOWICZ, 1952;DJANELIDZE, 1970;BU-GROVA et al., 2005).Molluscs were extracted first, both from the vicinity of the wells and then during sample processing in the laboratory.Traditional conchological methods were used to identify molluscs (ILYINA, 1993;NEVESSKAYA et al., 1993).
The number of tests of each foraminifera species was counted in each sample for each well.Species were considered dominant when their percentage of tests was more than the percentage of tests of other species.The foraminiferal assemblages were compared based on the similarity of foraminifera species composition and the presence of the same dominant species.A definition of normal-marine, euryhaline, and mixed foraminiferal assemblages became possible after assessment of the palaeoecological characteristics of the foraminifera species composition of each assemblage.Comprehensive analysis of the data obtained from the Konkian stratigraphy of the Southern Ukraine is represented here.Descriptions and graphic presentation of qualitative and quantitative analyses of foraminiferal assemblages in the Konkian deposits from each of these wells is described in detail in: VERNI-GOROVA, 2008, 2009, 2012;VERNIGOROVA et al., 2009.Palaeoecological characteristics of foraminifera and molluscs (their classification as stenohaline, normal-marine and euryhaline species) is based on data from: ARKHANGUELSKY, ed., 1940;MERKLIN, 1953;DIDKOVSKIY & KULICHENKO, eds., 1975;MURATOV & NEVESSKAYA, eds., 1986;DIDKO-VSKIY, 1959;KRASHENINNIKOV, 1959;BOGDANOWICZ, 1965;DJANELIDZE, 1970;ILYINA, 2000;MAISSURADZE et al., 2014;ZHGENTI & MAISSURADZE, 2016, etc. Analysis of changes of foraminifera and molluscs species composition gave possibility to understand the peculiarities of the development of different regions of southern Ukraine in Konkian time.Palaeoecological and stratigraphic reconstruction of the middle Miocene deposits of the Eastern Black Sea Region was created using palaeoecological, bio-and lithostratigraphic data that were obtained both from published literature and the author's research (e.g.ARKHANGUELSKY, ed., 1940;DIDKO-VSKIY, 1959;MOLYAVKO, 1960;BARG, 1969BARG, , 2008;;PRISYAZHNYUK et al., 2007;VERNIGOROVA, 2008VERNIGOROVA, , 2009VERNIGOROVA, , 2012;;VERNIGOROVA et al., 2009;VERNYHOROVA, 2014VERNYHOROVA, , 2015bVERNYHOROVA, , 2016)).

Biostratigraphic reconstruction of the
The analysis of foraminiferal assemblages with different species composition and palaeoecological characteristics revealed a common assemblage (NM1) in three Wells (8z; 9; 6) of the Eastern Black Sea Region (Figs.7;8).Marine molluscs, bryozoans and echinoid radiole were also found together with this assemblage.Small distances between these Wells (43 and 17 km accordingly -see Fig. 6) allow consideration of these deposits with identical fauna as an isochronous level.This level was used as a benchmark for stratigraphic correlation, for identification of hiatuses and also for the reconstruction of palaeoenvironmental conditions during Konkian time in the research area.Namely, if we assume that the HM1 assemblage in different Wells was formed   contemporaneously then deposits with this assemblage represent a single biostratigraphic level and a marker for the stratigraphic evaluation of other horizons.
The final phase of development of the Konkian basin was determined in the upper part of Well 8z (the Eastern Black Sea Region) and Well 20 (the Kerch Peninsula) (Figs. 7,8) based on the presence of the special foraminiferal assemblage (EH2) in these deposits.

СONCLUSIONS
The study of spatial and temporal changes of foraminiferal assemblages in the Konkian deposits of Southern Ukraine indicate that the shallow-water basin of the Eastern Black Sea Region and the more deep-water basin of the Kerch Peninsula had different models of development during most of the Konkian period.
Two foraminiferal assemblages with different palaeoecological characteristics in the studied deposits of the Kerch Peninsula indicate two successive development phases of the relatively deep-water Konkian basin of this area.During the first, longer part of the Konkian, relatively stable marine conditions predominated, while the late period of existence of this palaeobasin is characterized by a sharp change to euryhaline conditions.Changes in foraminiferal assemblages in relatively deeper-water Konkian deposits of the Kerch Peninsula and the Ciscaucasus are similar.This indicates that basins with such types of sediments in the Eastern Paratethys had a similar pattern of development during the Konkian.
Many irregular vertical and lateral changes of foraminiferal assemblages with different palaeoecological characteristics and hiatuses observed in the shallow-water Konkian deposits of the Eastern Black Sea Region indicate that the palaeobasin in this area was influenced by frequent and abrupt changes in environmental conditions during most of the Konkian.Palaeoecological and lithological characteristics of the Konkian deposits of the Eastern Black Sea region are similar to those of other shallow-water coeval deposits of the Southern Ukraine, and it can be concluded that the development of the entire Northern Black Sea Region and most of the Crimean Peninsula were similar during this time period.
The late development phase of the Konkian was the same in both shallow-water deposits of the Eastern Black See region and more deep-water deposits of the Kerch Peninsula deposits.This phase was characterized by the presence of similar euryhaline assemblages of Konkian foraminifera (and molluscs) with a small admixture of the Early Sarmatian species.

Figure 8 .
Figure 8. Distribution of foraminiferal and molluscs assemblages in the Konkian deposits of the studied wells.
the rocks is shown at the Fig.8

Figure 9 .
Figure 9. Palaeoecological and stratigraphic reconstruction of the middle Miocene deposits of the Eastern Black Sea Region.
Typical species of molluscs and foraminifera from the Konkian of the Eastern Parethetys (according to The different understanding of stratigraphic value of the Kartvel Beds, Sartagan Beds and Veselyanka Beds in the Southern Ukraine deposits in strati graphic schemes of the Konkian of the Eastern Paratethys.It is depend on environmental conditions in different areas of the Konkian basin especially in shallow-water deposits.Nonion bogdanowiczi VOLOSHI-NOVA).This regular trend can be observed in the Konkian sediments of the Northern Black Sea Region (including the Konkian stratotype), on the Kerch Peninsula, the Ciscaucasus and the Mangyshlak Peninsula (SOKOLOW, 1899; LIVEROVSKAYA, A variety of molluscs and foraminiferal Konkian assemblages that have different paleoecological characteristics.Konkian characteristic species; reduced species composition of assemblages; normal-marine species; predominance of the genera Cassidulina and Discorbis (e.g. C. bulbiformis, C. bogdanowiczi, D. kartvelicus) Figure 3. Characteristics of the Konkian development phases based on foraminifera and molluscs (VERNYHOROVA, 2015a).kobi(BOGDANOWICZ), (VERNYHOROVA, 2015b(VERNYHOROVA, , 2016))ave accumulated in the Northern Black Sea Region and in most parts of the Crimean Peninsula (e.g.MOLY- AVKO, 1960;BARG & STEPANIAK, 2003).Different types of limestones (Hladkivka Formation, Tarkhankut Formation, Mekenziev Strata) prevail in the western part of the Northern Black Sea Region and in the western and southern parts of the Crimean Peninsula; sands and sandstones (Novokakhovka Formation) are more abundant in the south-east of the Northern Black Sea Region and in the northern and central parts of the Crimean Peninsula; laminated green-grey clays with varying sandy admixtures (Tymoshivka Formation) occur in the north-eastern part of the Northern Black Sea Region(VERNYHOROVA, 2015b(VERNYHOROVA, , 2016)).
. Assemblage NM2 is characterized by a small number of specimens of: Quinqueloculina gracilis KARRER, Q. aff.guri

Konkian deposits of the Southern Ukraine based on foraminifera
tritus of mollusc shells (different levels of Wells 8z; 9; 6).All these foraminiferal assemblages as well as mollusc assemblages have different and irregular positions in the Konkian deposits of the various wells.A similar sequence of such assemblages was found in the Konkian sections of shallow-water deposits in other areas of the Northern Black Sea Region (Hladkivka Formation, Novokakhovka Formation) and in the Crimean Peninsula (Mek-enziev Strata, Tarkhankut Formation, Novokakhovka Formation) (e.g.