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Stratigraphic re-evaluation of the Oligocene-Lower Miocene formations in the Diapiric Fold Zone, Eastern Carpathian, Romania


The thrust and folded belts are prolific areas for the hydrocarbon exploration as well for the development new geological concepts and processes. The relation between the academia and industry in Romania has been of crucial importance in the exploration of Eastern Carpathian. The southernmost part of the Eastern Carpathians is known as the Diapiric Fold Zone, the place where the salt diapirc concept was first developed and the first commercial oil production, in the word, was reported 150 years ago.

The Eastern Carpathian thrust and folded belt, as part of the Carpathian Orogen has a unique double bend geometry and its present day shape is the product of the Alpine deformation. The East Carpathians can been dived into two domains, the internal thick skined involving the stack of basement nappes with crystalline rocks and a Mesozoic sedimentary cover (Median Dacides), and the external one involving a thin-skinned nappe system, i.e., the Ceahlau nappe (Outer Dacides), Convolute Flysch, Audia/Macla, Tarcau and Marginal Folds and Subcarpathia nappes (Moldavides). Thrusting took place from Late Cretaceous in case of the Outer Dacides to Paleogene-Miocene in case of the Moldavides. Miocene thrusting culminated in the Sarmatian (Middle Miocene) when the Subcarpathian nappe was thrust on top of the sedimentary cover of the apparently “undeformed” foreland (European, Scythian and Moesian platforms). With the Sarmatian, deformation by thrusting ceased in the most of the belt, with the exception of the East Carpathian Bend Zone, where deformation continues during Pliocene-Quaternary, culminating with salt diapirism, some of them piercing the entire sedimentary sequence and are now outcropping (Diapirc Folds Zone). These rather continue deformation, with formation and then destruction of foreland and piggy-back basins, has left its mark over the sedimentary architecture which remarks itself by large lateral and across the belt facies change. Adding to that the endemic evolution of the faunas these makes the stratigraphic correlation even more difficult.

For a long period of time discrepancies existed between academia surface and industry subsurface dating of the Oligocene-Miocene sequences. The first one has been rapidly updated to new stratigraphic schemes, benefiting from the easy access by numerous researchers, while the later one has been tied to the official age of the reservoirs and remain basically unchanged. However from time to time the industry must adjust itself to the recent development, on both stratigraphic and tectonic concepts. This is the case of the Easter Carpathian Bend Zone where recently an important effort by a multidisciplinary team has been undertaken for the stratigraphic re-evaluation of Oligocene-Miocene deposits, based on core data. The re-dating has shown that the previous Oligocene deposits are rather spanning across Oligocene-Lower Burdigalian times. This correlation is in agreement with the outcrop based studies showing that the Maikop facies, wildly recognize in the Eastern Parathetys basins, starts in the uppermost Eocene and ends with Lower Miocene times. The revised Lower Burdigalian age for the salt breccia formation (previous interpreted as Aquitanian in age) has major implications in terms of depositional environment and tectonic evolution. Hence recognizing the same restrictive evaporitic environment in both the East and South Carpathian forelands (e.g. Getic Depression) at equivalent stratigraphic levels, one can interpret that both have been shared a similar depositional environmental conditions and even the same basin. The change in depositional environment from deep sea turbiditic facies, in Oligocene- early Lower Miocene, to shallow water evaporitic in late Early Miocene, can be interpreted as an evidence for tectonic activity that has led to the deposition of Salt Breccia Formation in restrictive foreland and/or piggy back basins with important afflux of detritic material from uplifting hanging-wall or fore bulge uplifted areas. The subsequent compressional deformation used the salt formation as decolement level during Miocene-Quaternary times.