Reports

EXECUTIVE SUMMARY 

Phase III - FINAL : 2023

Extended abstract

Sedimentary suites and associated mafic and ultramafic lithologies preserved in accretionary wedges developed along convergent plate margins may offer valuable information about the main characteristics of the crust of ancient oceanic basins consumed by subduction.

The Ceahlău-Severin Suture (CSS) of the Carpathians consists of remnants of a Jurassic (J3)-Lower Cretaceous (K1) sedimentary basin that includes the Azuga, Sinaia, and Black Flysch stratigraphic units in which diverse fragments of mafic and ultramafic rocks are incorporated.

By employing a series of modern field and laboratory techniques (sedimentology, geochronology, geochemistry, and igneous petrology), here we aim to address two important questions regarding the elusive pre-subduction evolution of the Ceahlău-Severin basin:

(1) How much crust/lithosphere extension is required to explain the dominant characteristics of mafic rocks and sediments, and what was the tectonic environment in which the Ceahlău-Severin Ocean formed? 

(2) How did the sedimentary filling process progress across the basin, and what detrital sources contributed to the formation of the J3-K1 deposits?

The Ceahlău-Severin suture extends over 1000 km, from Ukraine to Bulgaria (Fig 1). The Middle Jurassic rifting separated the Dacia mega-unit (the Getic and Bucovinian alpine nappes) from the European realm, forming the Ceahlău-Severin Ocean (CSO). The European realm represents a collage of several tectonic units, including Moesia and the Danubian units (distal Moesia incorporated into the Carpathian orogen in the Late Cretaceous); the East European Craton/Platform and Northern Dobruja. The Danubian units are extended in Serbia (the Western Balkans) and Bulgaria (Struma unit). The compressional tectonics started at the end of the Early Cretaceous led to the closure of the Ceahlău-Severin Ocean and to the modern configuration of the Carpathian Orogen.

Seven areas with CSS deposits have been investigated in Romania, Serbia, and Bulgaria (Fig. 1). Advanced biostratigraphic, structural, sedimentological, geochemical, and geochronological analyses were performed. The resulted database can be consulted at https://osf.io/yjxuw/

The biostratigraphic analyses confirmed the J3 and K1 ages for the Azuga and Sinaia formations (Fig. 2), whereas the structural analysis revealed the intra-formational character of some basaltic flows in the Severin and Maramureș areas. 

The Lower Member of the Sinaia Formation consists of Upper Jurassic shales, marlstones, cherts, and thin sandstones. Its base and partially lateral variation, the Azuga Member, includes distinctive red shales, radiolarites and thin marlstones and sandstones. Collectively, these sediments suggest pelagic suspension settling, carbonate and siliceous productivity alternating with low-density turbidite currents on the basin plain. Basalts and serpentinites are restricted to the Azuga Member, but most of the stratigraphic relationships are obliterated tectonically. Locally in the Southern and Southeastern Carpathians, pillow basalts in conformity with the sedimentary stratification can be interpreted as intraformational submarine lava flows. Likewise, in the northern part of the Eastern Carpathians, basalts from the Black Flysch Unit display intimate stratigraphic relationships with Oxfordian–Kimmeridgian deep-sea limestones. 

Whole-rock REE and Ta/Yb–Th/Yb systematics of 40 samples analyzed in the framework of the project indicate that the basalts are confined to the global MORB (mid ocean ridge basalts) array and reflect various degrees of melting of their mantle source along oceanic ridge segments with different spreading rates (Fig. 3). According to this systematics, normal N-MORB-like basalts from the Southeastern Carpathians formed along a fast-spreading oceanic ridge, whereas enriched E-MORB-like basalts from the Southern Carpathians reflect a slower spreading regime. Oceanic Island Basalts, OIB-like basalts from the northern segment of the Eastern Carpathians, suggest low-degree mantle melting beneath a considerably thicker - possibly continental - lithosphere.

The U-Pb ages of the detrital zircons extracted from 20 J3-K1 sandstone samples collected over ~1000 km along the Carpathians, from Maramureș to Bulgaria (Fig. 4), suggest a massive contribution of the two passive margins of the basin - Dacia and the European edge - in contrast to only a minor cotribution of the distant East-European Craton sources with typical ages > 1Ga. The significant frequency of ~460 Ma zircons indicate the contribution of the Dacia continental margin, whereas the 290-340 Ma and 540-620 Ma intervals are rather typical for the European units, located on the opposite margin of the basin. (Fig. 5). 

The variable rates of extension along the rift system were sufficient to accommodate the abyssal marine deposits—red radiolarite clays of the Azuga Formation and thin chert limestones of the Lower Sinaia Member (J3-K1). 

The Lower Cretaceous (Berriasian - Hauterivian), specific to the middle and upper members of the Sinaia Formation, is characterized by coarsening up sequences, suggesting the interference of prograding turbiditic systems, fed from both edges - Dacia and Europe - of the CS sedimentary basin.

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14 undergraduate theses and master dissertation theses have been financially and logistically supported by the project.

5 ISI papers (one of them Q1) supported financially by this project were published, 2 articles are under review, (one of them Q1). 

The team members made 9 presentations at international conferences. 

Most of the ideas and data resulting from the current project will be incorporated in a synthesis paper to be submitted for publication in the forthcoming months. 

Partial results were presented during a workshop organized in October 2023 at the University of Bucharest. 

In July 2024, an educational field trip, “Finding the Lost Oceans”, was organized in Romania and Serbia on the topic of the project. –The field trip was attended by Romanian and Polish students.  Besides the team members, guides from Poland and Serbia were also invited and new connections aiming future collaborations were established. 

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References:

Balintoni, I., Balica, C., Ducea, M.N. & Stremţan, C. (2011). Peri-Amazonian, Avalonian-Type and Ganderian-Type Terranes in the South Carpathians, Romania: The Danubian Domain Basement. Gondwana Research, 19, 945-957. doi: https://doi.org/10.1016/j.gr.2010.10.002.

Balintoni, I., Balica, C. (2013). Carpathian Peri-Gondwanan Terranes in the East Carpathians (Romania): A Testimony of an Ordovician, North-African Orogeny. Gondwana Research, 23, 1053-1070. doi: https://doi.org/10.1016/j.gr.2012.07.013.

Balintoni, I., C. Balica, M. N. Ducea, and H. P. Hann (2014), Peri-Gondwanan terranes in the Romanian Carpathians: A review of their spatial distribution, origin, provenance and evolution, Geoscience Frontiers, 5, 395–411, doi:10.1016/j.gsf.2013.09.002.

Săndulescu, M., Kräutner, H.G., Balintoni, I., Russo-Săndulescu, D. & Micu, M. (1981a). The Structure of the East Carpathians (Moldavia-Maramures Area). Carp.-Balc. Assoc., XII Congr., Guide to Excursions B1, 92 p.

Săndulescu, M., Ștefănescu, M., Butac, A., Pătruț, I. & Zaharescu, P. (1981b). Genetical and Structural Relations between Flysch and Molasse (the East Carpathians). Carp.-Balc. Assoc., XII Congr., Guide to Excursions A5, 95 p.Schmid, S.M., D. Bernoulli., B. Fügenschuh, N. Georgiev, A. Kounov, L. Mațenco, R Oberhänsli, J. Pleuger, S. Schefer, R. Schuster, B. Tomljenović, K. Ustaszewski, and D.J.J. van Hinsbergen, (2020), Tectonic Units of the Alpine Collision Zone between Eastern Alps and Western Turkey. Gondwana Research, 78, 308-374, doi.org/10.1016/j.gr.2019.07.005.

Van Hinsbergen, D.J.J., Torsvik, T.H., Schmid, S.M., Maţenco, L.C., Maffione, M., Vissers, R.L.M., Gürer, D. & Spakman, W. (2020). Orogenic Architecture of the Mediterranean Region and Kinematic Reconstruction of Its Tectonic Evolution since the Triassic. Gondwana Research, 81, 79-229. doi: https://doi.org/10.1016/j.gr.2019.07.009.

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EXECUTIVE SUMMARY 

Phase II : 2023

Ceahlau-Severin "Ocean" was traditionally defined based on poorly characterized "ophiolites" from a geochemical point of view, currently associated with sedimentary formations of J3-K1 age (Late Jurassic - Early Cretaceous), the Azuga and Sinaia stratigraphic units from de Severin, Ceahlău and the Black Flysch tectonic units. Later, during the Cretaceous main thrusting phases, these units were incorporated into the Carpathian Orogen, forming the Ceahlau-Severin Suture (CSS). Most of the crust was consumed through the Cretaceous-Miocene subduction. Thus, the early evolution of this basin is not fully understood. For this reason, this study intends to characterize geochemically the types of mafic and ultramafic rocks, and further, the original tectonic setting. The stratigraphic relationship between the late Jurassic sediments of the host sedimentary formations and the incorporated mafic bodies is also focused. Another objective is the reconstitution of sedimentary sources and depositional systems until the moment of the mid-Cretaceous initiation of subduction.

The lithostratigraphic framework was refined in this second stage of the project by the constraints of the Berriasian and Valaginian ages in different outcrop locations of the Sinaia Formation, in the Comarnic-Bușteni area. The inter-stratification relationship between the Azuga deposits and the basic lava flows found in the Coșuștea Valley, Severin Nappe and Ceahlău Nappe, Mărului Valley, Azuga - Busteni, implicitly suggests an upper Jurassic age of basalts. The contemporary relationship between the lower Sinaia Member and the Azuga member is still debatable due to the lack of fossiliferous content. Whole-rock REE and Ta/Yb–Th/Yb systematics indicate that the basalts are confined to the global MORB array and reflect various degrees of melting of their mantle source along oceanic ridge segments with different spreading rates. According to this systematics, N-MORB-like basalts from the Southeastern Carpathians formed along a fast-spreading oceanic ridge, whereas E-MORB-like basalts from the Southern Carpathians reflect a slower spreading regime. OIB-like basalts from the northern segment of the Eastern Carpathians, suggest low-degree mantle melting beneath a thicker - possibly continental - lithosphere. The variable rates of expansion along the rift system were sufficient to host abyssal deposits - the red clays with radiolarites of the Azuga Member and the laminated limestones with cherts of the Sinaia Member. The Berriasian-Valanginian interval of the Middle Sinaia Member is dominated by mixed siliciclastic and carbonate turbidite currents of low and high density, in turbidite lobe facies, and the Hauterivian deposits of Upper Sinaia Member even contains marine conglomerate channel fills. The general coarsening-up trend of the whole sequence suggests progradation by increasing the input of clastic sediments. All the seven analyzed J3-K1 detrital zircons (DZ) samples (~ 770 U-Pb ages) argued the massive contribution of the passive margins of the Ceahlău-Severin Basin (CSB) and in contrast to the distal East-European cratonic sources, having typical ages of > 1Ga.The significant frequency of ~ 460 Ma zircons represent an argument for the contribution of the Dacia continental margin. The intervals 290-340 Ma and 540-620 Ma, although typical for the European- derived units located on the opposite passive margin, are still characteristic for the Dacia Block as well. For this reason, the discrimination between the two margins will be done in the 3rd stage of the project by multiplying the isotopic analyzes from the Dacian basement units and the Triassic-Jurassic sedimentary covers. The analysis of chrome-spinels from the J3-K1 detrital sediments also suggests mafic and ultramafic source areas. According to the Mg#/Cr# and Cr#/TiO2 diagrams, the results show a complete match with ophiolitic but also back-arc types of rocks and a partial match with ocean floor peridotites and MORB basalts. Although these data are still inconclusive, the source of chrome-spinels, besides the expansion areas of the CSO, must also be found in the surrounding external areas. All this information will contribute to the refinement of the J3-K1 paleogeographic setting of the CSB.

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EXECUTIVE SUMMARY 

Phase I : 2022

The project aims to evaluate the relationship between the Upper Jurassic-Lower Cretaceous sediments (J3-K1) and associated mafic and ultramafic rocks from the Ceahlau-Severin Suture of the Carpathians and reconstruct the history of the early evolution of the basin. A key factor that needs to be quantified in this respect is the amplitude of oceanic extension that accommodated the J3-K1 deposits. Considering the previously published works and maps of the broader region, 5 study areas were chosen, in the Southern, South-Eastern, and Eastern Carpathians: A. Severin Nappe, Mehedinti area, B. Danubian Nappes, Transalpina-Lotru area; C. Ceahlău Nappe, Azuga- Comarnic area; D. Ceahlău Nappe, Bicaz-Ghimes-Ciuc area; E. Black Flysch and Ceahlau Nappes, Maramures area. During the fieldwork campaigns in these areas, the structural relationships between the mafic bodies and the J3-K1 sediments were observed, detailed lithological logs were produced, and more than 110 samples were collected for petrographic, geochronological, and geochemical analyses. The database can be consulted at https://osf.io/yjxuw/

The only place in which fairly clear depositional relationships between the magmatic rocks and sediments were found is the Maramureș Zone, Black Flysch Nappe, where the lithological succession records basalts capped by J3 limestone. The high-degree deformation of the sedimentary formations limited our sedimentological mapping to relatively short stratigraphic sequences, especially in the well-exposed Azuga-Sinaia-Comarnic area. The micropaleontological determinations supporting the study are in progress and will be reported in the project’s second phase. The detailed sedimentological logs of the Sinaia Formation, Median Member, record siliciclastic and carbonate facies. The Lower Member stands out for the presence of cherts, and the Azuga Member for its red shales, thin sandstones, marls, and radiolarites. These facies, shale and carbonate mudstones, and radiolarites form abyssal basin plain associations, while mostly incomplete Bouma-type turbiditic sequences are specific to deep water lobes and lobe fringe associations.

More than 90 samples from the mafic and ultramafic bodies and from the sedimentary sequences were petrographically analyzed to provide the appropriate petrographic diagnosis and allow sample selection for chemical and isotopic analyses.13 sandstone samples were collected and prepared for geochronological analysis on U-Pb detrital zircons. The bulk of results will be reported in the second phase of the project, but we have already analyzed 2 samples from the Median Member of the Sinaia Formation in the Azuga-Comarnic, and Bicaz-Ciuc sectors. The southern sample (Sinaia), indicates a balanced input both from the inner edge of the basin, the Dacia unit, and the external Danubian unit, while the northernmost sample from the Eastern Carpathians (Bicazu Ardelean), suggests a pronounced internal source area. Petrographic analyses on the mafic and ultramafic samples confirm the presence of basalts and serpentinized peridotites in the Severin Nappe. The poorly metamorphosed detrital and pyroclastic rocks along with intercalations of serpentinized peridotites and gabbroic intrusions with oceanic affinities currently attributed to the uppermost Danubian domain are suspected to belong to the Ceahlău-Severin Suture. Basaltic lavas are also present in the Sinaia and Ciuc areas of the Ceahlău Nappe and the Black Flysch Nappe. The geochemistry of basalt olistoliths embedded in the sediments of the Sinaia and Azuga formations from Ceahlău Nappe suggests the production of oceanic crust in a relatively mature (evolved) ocean, while the suite of basalts from Obârșia Cloșani (Severin Nappe) represents a juvenile oceanic crust (undeveloped). A paper resulting from this study has been published in Basin Research (Roban et al., 2022a) and 2 works have been presented at international meetings (Roban et al., 2022b, c). Additional information about the project can be found at https://sites.google.com/view/devobas