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Records of the global Valanginian events in the Getic Carbonate Platform (Southern Carpathians, Romania)
Iuliana Lazar, Ioan I. Bucur, Mihaela Gradinaru, Eugen Gradinaru, Emanoil Sasaran, Alexandru Andrasanu

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-05 03:30 PM – 03:50 PM
Last modified: 2018-12-01

Abstract


Carbonate sequences of the Getic Carbonate Platform crop out in large surfaces on the eastern part of the Southern Carpathians (in Dâmbovicioara zone, Braşov-Codlea zone, Bucegi Mountains, Piatra Craiului Mountains).

The aim of the present study is to reveal the effects of the global Valanginian events recorded in different depositional settings of the eastern part of the Getic Carbonate Platform. In the shallowest setting of the platform the late Tithonian - lower Valanginian deposits (almost 400 m thick) are represented by reef limestones that pass gradationally into intraclastic/bioclastic-dominated shoals and peritidal carbonate deposits (Cheile Dâmbovicioarei Formation). The lower part of the succession dominated by bioconstructions contains corals and diverse microencrusters (e.g. Pseudorothpletzella sp. and Crescentiella morronensis). The rudstones and grainstones following the bioconstructions contain foraminiferal assemblage represented by Coscinoconus alpinus and Bramkampella arabica and calcareous algae (Clypeina sulcata) suggesting a late Tithonian age.

Toward the upper part of the Cheile Dâmbovicioarei Formation, two ammonite specimens have been found and identified as Berriasella (Picteticeras) cf. picteti and B. (P.) aff. elmi, and both are indicative for the Boissieri Zone of the upper Berriasian. The microfossil assemblage from the upper part of the Cheile Dâmbovicioarei Formation is represented by abundant foraminifera and rivulariacean-type cyanobacteria. Among foraminifera, Pfenderina neocomiensis, Haplophragmoides joukowskyi, Montsalevia salevensis, Pseudotextulariella courtionensis, Protopeneroplis ultragranulata and Meandrospira favrei are indicative for the Berriasian-Lower Valanginian. Consequently, based on ammonite and microfossil assemblages, at least a late Berriasian-early Valanginian age for the upper part of Cheile Dȃmbovicioarei Formation can be assumed.

During the latest Berriasian–early Valanginian time interval, extensional tectonic events and a coeval sea-level low-stand determined the emergence and possibly subaerial exposure of the structural highs of the Getic Carbonate Platform.

In the shallowest settings of the carbonate platform (Braşov-Codlea and Dȃmbovicioara zones) the corresponding stratigraphic gap is well constrained to the early Valanginian. Considering the existing biostratigraphic data, we assume a gap of almost 1.5 Ma in the Braşov-Codlea zone and one of almost 2.5 Ma in the Dȃmbovicioara zone.

The drowning event following to the emergence of the platform is documented by the intra-Valanginian discontinuity surface associated with an interval of stratigraphic and taphonomic condensation, phosphatisation, glauconitisation and encrustation with iron oxyhydroxides. This drowning event is also associated with glaucony-rich and pyrite-bearing wackestone infilling of the irregular surface of the discontinuity, as well as forming the matrix of in situ fracture breccias and chaotic collapse breccias.

The onset of the drowning event was highly diachronous (based on ammonites species identified on the sediment above the discontinuity surface): in the Braşov-Codlea zone it started in the middle early Valanginian (Neocomiensiformis Zone), in the Dȃmbovicioara zone in the earliest late Valanginian (Verrucosum zone), and in the Bucegi Mountains in the late Valanginian (upper Verrucosum Zone).

The deepest settings of the carbonate platform from Bucegi Mountains reveal the tectonic model of half-graben tilted blocks. The limestones forming the tilted blocks belong to the Lespezi Formation and are interpreted as successions deposited at the base of the slope. The occurrence of the abundant encrusting Crescentiella morronensis in close association with microbe-sponge carbonates with strong biogenic encrustation, Terebella lapilloides and oncoidic carbonates are interpreted as allochthonous carbonate grains exported from the protected areas of deep outer shelf and/or of upper-middle slope settings. The allochthonous shallow-water skeletal debris is represented by Spirillina, Patellina, Lenticulina, Epistomina, Ophtalmidium, Coscinoconus, Neotrocholina, Meandrospira, miliolids; Crescentiella morronensis, quite frequent; Koskinobulina socialis, Radiomura cautica. The autochthonous background pelagic sediment is represented by bioclastic-lithoclastic packstone-wackestone containing abundant calpionellids (Calpionella alpina, Lorenziella hungarica, Remaniella cadischiana, Calpionellopsis simplex, C. oblonga, Tintinnopsella carpathica and T. longa). This assemblage belongs to the Calpionellopsis standard zone. No specimen of Calpionella elliptica or specimens of Calpionellites have been found, so we suppose that the Eliptica subzone, as well as the Calpionellites zone is missing in the analysed material. The existing calpionellid assemblages indicate a late Tithonian–Berriasian age, from the upper part of the standard zone A to the standard zone D.

During the late Berriasian, the slope of the carbonate platform started to be affected by extensional tectonics, generating normal faulting and tilting of blocks, with their foot-wall uplifts forming escarpments and their hanging-wall dip-slopes developing as small ramps. The top of the Lespezi Formation was affected by tectonic fracturing producing in situ breccia and neptunian dykes. After the faulting, the highest-lying parts of the tilted blocks were probably emergent and preferentially karstified. Relative sea-level lowstands (during the latest Berriasian–early Valanginian) and, locally, emersion of the tilted blocks of the carbonate platform were followed in the late Valanginain (starting with late Verrucossum Zone) by a sea-level rise that led to the final drowning of the platform. The general deepening of the platform is documented by the overlying post-drowning pelagic sediments onlapping the discontinuity surface.

The negative – positive carbon isotope excursions recorded in the studied sections are correlated with an increase of the total organic content. These excursions corresponded to the global palaeoenvironmental perturbations of the carbon cycle related to the Valanginian “Weissert” episode, which is reflected both in shallow-water (the Dȃmbovicioara and Codlea zones) as well in deep-water settings (Bucegi Mounatins) of the Getic Carbonate Platform.