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FRESHENED SEAS OR GIANT LAKES: EUSTACY AND HISTORY OF THE PARATETHYS

Gabor Vakarcs and Imre Magyar
MOL Hungarian Oil and Gas Co.

The term “Paratethys” was coined by Laskarev in 1924 to designate the string of epicontinental basins, originally stretching from the Alps to what is now the Aral Lake, that has been separated from the rest of the Tethys by the uplift of the Alpine-Caucasian mountain chain since the Early Oligocene. The Paratethys was defined as a bioprovince; isolation events from the Mediterranean and the Atlantic and Indian Oceans resulted in restricted marine, brackish lacustrine, and freshwater lacustrine environments, and induced the evolution of endemic species and higher taxa among molluscs, ostracods, fish, and other groups of the biota. The border between the Mediterranean and Paratethyan bioprovinces, however, was dynamic; in extreme cases, the Mediterranean completely invaded the Paratethys (as in some periods of the Early Miocene), or Paratethyan waters drained into the dry Mediterranean basin, as it is supposed for some intervals in the Messinian. Thus the history of the Paratethyan basins has been an intricate story of opening and closing connections (towards each other and the Mediterranean), catastrophic floods, and episodes of desiccation.

Endemic radiations and migration of the fauna are now among the best tools in the reconstruction of past paleogeographic changes. Endemism and isolation from the ocean, however, hinder biostratigraphic correlation and application of the standard marine chronostratigraphic system. As a consequence of these difficulties, regional chronostratigraphic units have been implemented in the Paratethyan area; one for the Western/Central Paratethys, which includes the today terrestrial basins of the Paratethys in Central Europe; another for the Balck Sea basin and its surroundings; and a third for the Late Neogene and Quaternary deposits of the Caspian basin.

The first isolation event of the Paratethys, indicated by the appearance and abundance of endemic molluscs, took place in the late Early Oligocene (Rupelian, Kiscellian, Solenovian). The second such Paratethys-wide isolation event happened during the late Early Miocene (Burdigalian, Ottnangian, Kozahurian), and the third in the late Middle Miocene (Serravallian, Sarmatian, Volhynian). Marine connections have never been re-established in the Western/Central Paratethys since this third event. In the Eastern Paratethys, however, further endemic events took place in the latest Miocene (Messinian, Pontian) to Early Pleistocene (Gurian) in the Black Sea basin and in the Late Pliocene (Akchagilian) to the present in the Caspian basin. Apart from these major endemic events, the nature and effectiveness of connections between Paratethyan basins and the Mediterranean are traceable by other aspects of the fossil record, such as appearance of marine microplankton in “closed” basins, impoverishment of the marine fauna along with spread of euhaline forms due to changes in salinity, etc.

What was the major driving force behind the continuously changing paleogeography of the Paratethys? Not denying the enormous role that tectonics played, we argue that the short-term glacio-eustatic sea level changes often overprinted long-term local tectonics. This is quite obvious in a few examples. The most recent significant change in Paratethyan paleogeography, for instance, which took place about 7500 years ago when Mediterranean seawaters broke through the Bosporus and filled up the freshwater lacustrine basin of the modern Black Sea, was clearly the result of glacio-eustatic sea level rise. Another example is the late Middle Miocene Serravallian age, when a gradually falling global sea level made the fully marine environment in the Paratethys change to restricted marine and then to lacustrine, and eventually led to the migration of mammals, including horses (“Hipparion”), from North America to the Old World.

In order to explore more subtle connections between eustacy and environmental changes in the Paratethys, we carried out a sequence stratigraphic study on Oligocene through Middle Miocene strata of the Central Paratethys in NE Hungary. Sixteen sequence boundaries were identified based on the analysis of published geological descriptions of outcrops and study of 3000 km of 2D reflection seismic profiles and 45 hydrocarbon exploration wells. The seven Central Paratethyan regional stages of the covered time period are bounded by unconformities that show a strong correlation with eight sequence boundaries. Because the regional stages have been based on faunal, thus environmental, changes in the Central Paratethys, and their boundaries typically indicate restriction of connections between the Paratethyan (and Mediterranean) seas, the identified sequence boundaries are clearly associated with basinwide lowstands.

The sequences were stratigraphically positioned on the basis of calcareous nannofossil data from 26 wells. Within the limits of the biostratigraphic resolution in this interval, the stratigraphic position of the sequences correlates well with the previous records of global depositional sequences of Haq et al. (1988) and with the oxygen isotope events of Abreu and Haddad (1998). Four of the regional stage boundaries also correlate with global stage boundaries. These results suggest that global eustacy plays a significant role in the formation of depositional sequences even in such tectonically active areas as the Paratethys, and that eustacy was an important factor in controlling the paleogeographic changes, i.e. the opening and closure of connections between various basins, in the Paratethyan-Mediterranean realm.