The Upper Miocene of the Rostov Dome ( Eastern Paratethys ) : Implication of the chronostratigraphy and bivalvia-based biostratigraphy

The Rostov Dome is located in the south of the Russian Platform. In the Late Miocene this area was embraced by the Eastern Paratethys. The implications of a recently developed Neogene chronostratigraphy to the studied area are discussed. The Sarmatian regional stage corresponds to the upper part of the Langhian, the entire Serravalian and the lower part of the Tortonian global stages; the Maeotian regional stage corresponds to the upper part of the Tortonian and the lowermost horizons of the Messinian global stages; the Pontian regional stage corresponds to most of the Messinian and the lowermost Zanclean global stages. A first Bivalvia-based biostratigraphic framework is proposed for the territory of the Rostov Dome. Five biozones were established within the Serravalian–Messinian: Tapes vitalianus, Cerastoderma fittoni–Cerastoderma subfittoni, Congeria panticapaea, Congeria amygdaloides navicula and Monodacna pseudocatillus–Prosodacna schirvanica.


Introduction
The study of the Upper Miocene deposits of the Eastern Paratethys began about 150 years ago (ABICH, 1865;ANDRUSOV, 1884;ANDRUSSOW, 1911), but some important questions on their stratigraphy are still unresolved.Correlation between global and regional stages (1) and the development of macrofauna-based biozonation (2) are among them.
The Rostov Dome is a promising area to study the Upper Miocene stratigraphy of the Eastern Paratethys.It is situated in the southern part of the Russian Platform (Fig. 1).The Upper Miocene sedimentary complexes are wide-spread and cover all its territory.The high abundance of fossil bivalves in the Upper Miocene deposits of the Rostov Dome suggests the use of this group for the development of the first regional biostratigraphic framework.To do this, it was first necessary to implement the recently developed chronostratigraphy of the Upper Miocene in order replace the regional scale of the Eastern Paratethys.POGREBNOV et al., 1970).After its uplift in the Cretaceous-Paleogene, the dome has several times been covered by the sea from the south.
In the Neogene, all the south of the European part of Russia was occupied by a large basin.It was a remnant of the previously existing Neotethys, which, after the orogeny in the alpine regions, became divided into 2 parts -the Mediterranean and the so-called Paratethys, consisting of Western (Pannonian), Central and Eastern Basins.The Paratethys originated at the end of the Paleogene, and its isolation strengthened cyclically from the Oligocene until the Pliocene (ILINA et al., 1976;NEVESSKAJA et al., 1984;NOSOVSKIJ, 2001;RÖGL, 1998RÖGL, , 1999;;ULANOVSKAYA, 1998).The territory of the Rostov Dome is located at the northernmost periphery of the Eastern Paratethys (Fig. 1).It was embraced by sea during the maximums of cyclically repeating transgressions, when a relatively large and wide Tanaiss palaeobay originated (RUBAN, 2002a).Various sediments accumulated during these times -clays, silts, sands, marls, limes.But the most typical Upper Miocene deposits are skeletal limestones, consisting completely of shells of bivalves, less gastropods and their remains of different size.
Although the Upper Miocene deposits are well-exposed in outcrops within the studied area, they have been investigated only occasionally during the XX century by BOGATCHOV (see RODZJANKO, 1970) and later by RODZJANKO (1970RODZJANKO ( , 1986)).Also they have been characterized in few monographs, e.g., PAFFENGOLTS (1959), IVANITSKAJA & POGREBNOV (1962), but these descriptions mostly summarized the results of the above mentioned researchers.
Bivalves from the Upper Miocene deposits were studied (RUBAN, 2002b).Identification of the species was made according to general overviews (with taxonomic descriptions and figures) of Neogene bivalves of the Eastern Paratethys presented by ILINA et al. (1976) and NEVESSKAJA (1986).

Implication of the chronostratigraphic scale to the Eastern Paratethys
Normalization of the general stratigraphic framework of the Eastern Paratethys, i.e. to correlate global and regional stages, is an important task, because this will enable correlations of biostratigraphic units, which may be defined in the Upper Miocene of the Rostov Dome, to be made to adjacent and even far-located regions.
When in the XIX century differences between the Mediterranean and the Paratethys were established, the general problem of Neogene strata correlation between  these territories appeared.Differences in the stratigraphy between the Western and the Eastern Paratethys arose.In the 1980s and 90s, the Mediterranean stratigraphic scale of the Neogene coupled with world-wide data underwent revision by the International Comission on Stratigraphy (ICS) in order to develop a globallysignificant chronostratigraphic scale.This procedure is ongoing, and a new precise chronostratigraphic scale is "under construction".When the development of the recent chronostratigraphic scale began, the difficulties in making a correlation between the global and the regional Eastern Paratethys stratigraphies strengthed again.
The "International stratigraphic Guide" (SALVADOR, 1994) proclaims stages as units with a global sense.Thus, it cannot be defined essentially for a particular region, because the geologic time was not different in the palaeospace.Meanwhile, when stratigraphers begin to define further global units, there is often insuficient data to enable the consideration of globally-recognized horizons in the studied interval.In this way, separate standards of stages appear.Each of them is valid for a single region.The stages in such standards are regional stages.
Nowadays, there are at least two intervals for which regional stages are widely used: Cambrian (PALMER, 1998;ZHURAVLEV, 1995) and Carboniferous (MENNING et al., 2000(MENNING et al., , 2001;;WAGNER & WINKLER PRINS, 1983).Discussions about which regional stages are preferable are ongoing.However, every time, when evidence is obtained which enables larger global units to be defined (as in the case of the Upper Miocene), it is not necessary to use regional stages.It is clear that different chronostratigraphies for particular regions should no The Upper Miocene of the Rostov Dome (Eastern Paratethys): Implication of the chronostratigraphy  longer exist, because the geological time was the same at every point on the Earth's surface.Otherwise, chaotic nomenclature, not representing the true geologic history will result.
In the meantime, Russian stratigraphers traditionally continue to use the regional stratigraphic scale for the Neogene deposits, which includes regional stages differing from chronostratigraphic (i.e., global) stages (Fig. 5).A widely accepted version of such a regional scale was proposed by NEVESSKAJA et al. (1984NEVESSKAJA et al. ( , 1986) ) and NEVESSKAJA (1986) .Therefore, there is an urgent need to correlate global and regional stages and to implicate the chronostratigraphy to the Eastern Paratethys in order replace the regional standard and abandon it forever.
A possible way to correlate the Neogene chronostratigraphic and regional stages is to compare the absolute ages of their boundaries.For the Eastern Parathethys these ages were evaluated precisely by TCHUMAKOV et al. (1992), and then discussed several times (TCHUMA-KOV, 2000a, b).For the recently employed chronostratigraphic units, the absolute ages are recommended by the ICS (GRADSTEIN et al., 2004) and some of them have been defined in the Global Stratotype Sections and Points (GSSPs) (CASTRADORI et al., 1998;HILGEN et al., 1998HILGEN et al., , 2000a, b;, b;RIO et al., 1998;VAN COUVERING et al., 2000).For the formal definition of absolute ages of the Messinian, all the Pliocene stages were preferred.ICS recommendations (GRADSTEIN et al., 2004) were used for the Laghian, Serravalian and Tortonian stages.
The results of a correlation by absolute ages (Fig. 5) suggest the Sarmatian regional stage corresponds to the upper part of the Langhian, the entire Serravalian, and the lower part of the Tortonian global stages.The Maeotian regional stage embraces the upper part of the Tortonian and the lowermost horizons of the Messinian global stages.And finally the Pontian regional stage mostly corresponds to the Messinian with only the uppermost part corresponding to the lowermost Zanclean.It is evident, that Miocene/Pliocene boundary, located at the base of the Zanclean in the global scale, has a different position in the Eastern Paratethys, where check meaning the Zanclean is established at the base of the Kimmerian.

Bivalvia-based biostratigraphy of the Rostov Dome
Abundant bivalves remains are the characteristic feature for all the Upper Miocene strata of the Rostov Dome.The analysis of taxa ranges allows the development of the regional biozonation based on this fossil group.Previous studies of the Eastern Paratethys (ILINA et al., 1976;NEVESSKAJA, 1986;NEVESSKAJA et al., 1986) resulted only from malacofaunal support for the regional stages and their substages and from the occasional identification of specific units, called "beds with", which, in fact, are something like acme-zones or assemblage zones.The present study of the Rostov Dome, however, permits the development of a Bivalviabased biostratigraphy.
The definition of the biostratigraphic units (biozones) was made according to the recommendations of the ICS (SALVADOR, 1994).The difference of terms "first occurence level" (FOL) and "last occurence level" (LOL) from "first appearance datum" (FAD) and "last appearance datum" (LAD) is assumed as the one proposed by PAVIA & MARTIRE (1997).Five distinct biozones have been defined in the Upper Miocene strata of the Rostov Dome (Fig. 6).The correlation established between regional and chronostratigraphic stages helped in the asignment of these biozones to global stages.
Tapes vitalianus Interval Zone corresponds to the interval from the pre-Upper Miocene malacofauna assemblage (not represented in the studied sections) to the LOL of Tapes vitalianus ORBIGNY.Further studies are necessary to revise this zone, as its lower boundary is undefined.Age: Langhian-Serravalian; Lower Sarmatian.Reference sections: Morskaja, Merzhanovo.
Congeria panticapaea Interval Zone corresponds to the interval from the FOL of Congeria panticapaea ANDRUSOV to the FOL of C. amygdaloides navicula ANDRUSOV.It is important to note that the LOL of C. panticapaea ANDRUSOV is above the upper boundary of this zone.Age: Tortonian; "beds with C. panticapaea", lower part of the Upper Maeotian.Reference section: Gnilovskaja.
Congeria amygdaloides navicula Total Range Zone corresponds to the interval from the FOL to the LOL of Congeria amygdaloides navicula ANDRUSOV.Age: Tortonian-Lowermost Messinian; "beds with C. amygdaloides navicula", upper part of the Upper Maeotian.Reference sections: Merzhanovo, 1300 km.
Unzoned intervals include hiatuses (at the base of the Donskaja and Aleksndrovskaja Formations) and short intervals where zonality could not be established because of the scarcity of fossils remains (the transition between Taganrogskaja and Rostovskaja Formations, upper part of Rostovskaja Formation, and Janovskaja Formation).
All the above mentioned zones were defined by characteristic taxa bioevents.The last ones are very easy to be determined in the stratigraphic record.All these events seem to be isochronous at least within the area of the Rostov Dome.

Conclusions
The comparison of absolute ages permits a correlation of the global and regional stages for the Eastern Paratethys to be made.
Studies of the Upper Miocene deposits of the Rostov Dome resulted in the definifion of five distinct Bivalvia-based biozones.The implicated chronostratigraphy coupled with the Bivalvia-based biozonation seems to be a real alternative for replacing the previously developed regional stratigraphy, based on the definition of regional stages.
Further research should be aimed at extending the defined biozones to the entire territory embraced by the Eastern Paratethys.
The Upper Miocene of the Rostov Dome (Eastern Paratethys): Implication of the chronostratigraphy

Fig. 1 .
Fig. 1.Geographical (a) and palaeogeographical (b) locations of the studied area (shaded in a and indicated by an arrow in b).Palaeogeographical reconstruction after NEVESSKAJA et al. (1984).

Fig. 2 .
Fig. 2. Location of the studied sections of the Upper Miocene deposits of the Rostov Dome.

Fig. 3 .
Fig. 3. Generalized lithostratigraphy of the Upper Miocene deposits of the Rostov Dome.

Fig. 4 .
Fig. 4. Correlation of the sections of the Upper Miocene deposits of the Rostov Dome.

Fig. 5 .
Fig. 5. Correlation between the chronostratigraphic units of the Upper Miocene-Pliocene and the regional stages of the Eastern Paratethys (see text for sources of the absolute ages).

Fig. 6 .
Fig. 6.Proposed Bivalvia-based biostratigraphy of the Upper Miocene of the Rostov Dome.