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Insights into the palaeoenvironments, structure and stratigraphy of the lower Miocene of the Eastern Carpathians Bend Zone, Romania

Bercea Răzvan-Ionuţ, Bălc Ramona, Tămaș Alexandra, Filipescu Sorin, Mircea Tămaș Dan, Guillong Marcel, Flavius Szekely Flavius, Lukács Réka

Geological Quarterly

2023

Vol. 67, No. 1

art. no. 2

The ages of several Oligocene to Miocene sedimentary formations from the Eastern Carpathians Bend Zone are poorly constrained due to palaeoenvironmental factors, reworking of fossils, structural complexity and limited exposure. To help overcome these issues, this study integrates calcareous nannoplankton and foraminifera biostratigraphy with isotopic age dating (U-Pb) of volcaniclastic zircons, and sedimentological and structural observations/interpretations. Our study was carried out along an ~6-km-long section made from a series of outcrops along the Bizdidel River which exposes several formations such as the Pucioasa, Fusaru, Vinețișu, Starchiojd and Slon. We show that the Fusaru Formation consists of coarse-grained rocks deposited as confined longitudinal channel successions that migrated laterally. It is bounded by the mud-rich Pucioasa and Vinețișu formations which are lateral equivalents of the Fusaru confined channels deposited as levee/overbank units. These genetically related formations appear to reach younger ages – of the lower to middle Burdigalian based on calcareous nannoplankton and foraminifera biostratigraphy – than previously thought (upper Oligocene to lower Burdigalian). The dominant organic-rich mudstones of the Starchiojd Formation represent pelagites/hemipelagites deposited in anoxic conditions. Their middle Burdigalian age is established by a 17.41 ±0.27 Ma zircon U-Pb age of zircons from the Bătrâni Tuff in the Starchiojd Formation. Based on the similar phenocryst content, zircon U-Pb age and zircon trace element composition, the source of the tuff is suggested to be the 17.3 Ma Eger ignimbrite-forming eruption, which has proximal, near-caldera deposits in the Bükkalja Volcanic Field, Hungary. The mud-rich Slon Formation seems to be related to shelf edge/upslope failure that formed cohesive debrite avalanches resulting from foreland propagation of compression. The Slon Formation extends in this area to at least the upper part of the lower Miocene to middle Miocene. These results highlight the need to revise ages of those parts of the sequence which are poorly constrained or different in other parts of the Carpathian Basin. Such revised ages help to better constrain the understanding of the deformation history of the Carpathians.

Detrital zircon geochronology and provenance of the Proterozoic quartz-rich metasediments of the Mazowsze domain: source areas and regional correlation

Krzemiński Leszek, Krzemińska Ewa, Wiszniewska Janina

Biuletyn Państwowego Instytutu Geologicznego

2019

nr 474

59--72

Drilling at Mońki IG 2 and Zabiele IG 1 in the Mazowsze domain has intersected mature quartz-rich metasedimentary rocks belonging to the basement of NE Poland, described so far as a Biebrza complex. The geochemical composition of these rocks is charac¬teristic of a passive margin. The subarkose–quartz arenite underwent low-T metamorphism, but preserved textures typical for the fluvial sediments. The detrital material in range 1.68–2.11 Ga was provided from surrounding late Paleoproterozoic margins of the Fennoscandia and Sarmatia. The maximum depositional age probably did not exceed 1.6 Ga. A previously suggested correlation with Mesoproterozoic molasse-type deposits of the Jotnian formation has not been confirmed. It seems more likely that the sediments formed after Fennoscandia-Sarmatia collision (i.e. termination of Svecofennian orogeny) but before denudation of the Mesoproterozoic Mazury AMCG intrusions.

W otworach wiertniczych Mońki IG 2 i Zabiele IG 1 na obszarze domeny mazowieckiej rozpoznano dojrzałe, bogate w kwarc skały metaosadowe. Należą one do podłoża krystalicznego północno-wschodniej Polski i były opisywane dotychczas jako kompleks biebrzański. Skład geochemiczny tych skał jest charakterystyczny dla krawędzi pasywnej. Skały sklasyfikowane jako arenity kwarcowe i subarkozy uległy metamorfizmowi niskotemperaturowemu, zachowując jednak struktury typowe dla osadów rzecznych. Materiał detrytyczny o wieku 1,68–2,11 mld lat pochodził z erozji paleoproterozoicznych skał na krawędziach Fennoskandii i Sarmacji. Maksymalny wiek depozycji materiału okruchowego prawdopodobnie nie przekraczał 1,6 mld lat. Sugerowana wcześniej korelacja z osadami molasowymi mezoproterozoicznej formacji jotnickiej nie została potwierdzona. Bardziej prawdopodobne wydaje się, że osady powstały po kolizji Fennoskandii z Sarmacją (wygasanie orogenezy swekofeńskiej), a przed denudacją mezoproterozoicznych intruzji AMCG obszaru Mazur.