Compared to coeval successions from the Carpathians, the continuous Jurassic-Cretaceous (J/K) pelagic limestone succession of the Brodno section offers the best possibility to document the J/K passage in a wide area. This section comprises a complete calpionellid, and nannofossil stratigraphic record, that supports the older paleomagnetic data. Moreover, the sequence stratigraphy and stable isotope (δ 18 O, δ 13 C) data gave important results, too, enabling comparison with known key sections from the Mediterranean Tethys area.Key words: J/K boundary, Western Carpathians, regional stratotype, stable isotopes, biostratigraphy, microfossils, pelagic carbonates. Geological context of the Brodno section: an overviewThe Brodno section is situated in an abandoned quarry on the eastern side of the narrow straits of the Kysuca River Valley north of the town of Žilina (known as the "Kysuca Gate", Fig. 1). It yields a record of hemipelagic marine sedimentation in a marginal zone (the Pieniny Klippen Belt) of the Outer Western Carpathians. The lithology, fossil record (including ammonites and aptychi) and stratigraphy were studied by Andrusov (1938Andrusov ( , 1950Andrusov ( , 1959, Scheibner (1961, 1962, 1967), Borza (1969, Scheibnerová (1969), andSamuel et al. (1988). A more detailed description of the Upper Jurassic and Lower Cretaceous litho-and biostratigraphy was provided by Michalík et al. (1990), Vašíček et al. (1992). Houša et al. (1996) introduced the magnetostratigraphy of the Jurassic/Cretaceous (J/K) boundary beds correlated with the microbiostratigraphic data.This paper discusses the results of an integrated biostratigraphic study using three microplankton groups (calpionellids, calcareous dinoflagellates and nannofossils), as well as stable isotope data (δ 18 O, δ 13 C) in the Brodno section, which is proposed here as the candidate for a West Carpathian regional J/K boundary stratotype. The distribution of the stratigraphically important planktonic organisms revealed several coeval calpionellid and nannofossil bioevents recorded in the pelagic carbonate sequence of the Jurassic/Cretaceous boundary age. The stable isotope data underline environmental changes during the interval studied.According to the International Commission on Jurassic Stratigraphy, it is necessary to search for complete sections, which can provide continuous records of both sedimentation and biotic events across stage boundaries. Although the Brodno section lacks ammonite record, it is presented here as a potential candidate considering its continuously well exposed and biostratigraphically properly documented succession, at least for the West Carpathian region. Material and methodsThe Jurassic/Cretaceous boundary succession was studied using an integrated sequence-, bio-and isotope stratigraphy approach from the detailed rock section sampled. A quantitative microfacies analysis was carried in thin sections for the sequence stratigraphic pattern of these pelagic limestones (see Reháková 2000a; Michalík 2007). The calpionellids an...
This paper discusses the results of a study of the Le Chouet section, its lithologies, facies, magnetic properties and fossil record (ammonites, calcareous nannofossils, calpionellids and calcareous dinoflagellates). Data obtained have been applied to give a precise biostratigraphy for this carbonate sequence as well as a paleoenvironmental reconstruction. Its relationship to magnetostratigraphy, based on a modern study of a French site, is important. Investigation of the micro-and macrofossils shows that the site comprises a sedimentary sequence in the Microcanthum to Jacobi ammonite Zones, and the Chitinoidella, Crassicollaria and Calpionella Zones. Several calpionellid and nannofossil bioevents have been recorded on the basis of the distribution of stratigraphically important planktonic organisms. The site allows us to calibrate the levels of various biomarkers and biozonal boundaries, and correlate them with the magnetozones M20n, M19r and M19n.
Abstract:The key objective of investigation of hemipelagic sediments from the Gresten Klippenbelt (Blassenstein Formation, Ultrahelvetic paleogeographic realm) was to shed light on environmental changes around the Jurassic-Cretaceous (J/K) boundary on the northern margin of the Penninic Ocean. This boundary is well exposed in a newly discovered site at Nutzhof. Around the critical interval including the boundary, this new outcrop bears a rich microplanktonic assemblage characterized by typical J/K (Tithonian/Berriasian) boundary faunas. The Nutzhof section is located in the Gresten Klippenbelt (Lower Austria) tectonically wedged into the deep-water sediments of the Rhenodanubian Flysch Zone. In Late JurassicEarly Cretaceous time the Penninic Ocean was a side tract of the proto-North Atlantic Oceanic System, intercalated between the European and the Austroalpine plates. Its opening started during the Early Jurassic, induced by sea floor spreading, followed by Jurassic-Early Cretaceous deepening of the depositional area of the Gresten Klippenbelt. These tectonically induced paleogeographic changes are mirrored in the lithology and microfauna that record a deepening of the depositional environment from Tithonian to Berriasian sediments of the Blassenstein Formation at Nutzhof. The main lithological change is observed in the Upper Tithonian Crassicollaria Zone, in Chron M20N, whereas the J/K boundary can be precisely fixed at the Crassicollaria-Calpionella boundary, within Chron M19n.2n. The lithological turnover of the deposition from more siliciclastic pelagic marl-limestone cycles into deep-water pelagic limestones is correlated with the deepening of the southern edge of the European continent at this time. Within the Gresten Klippenbelt Unit, this transition is reflected by the lithostratigraphic boundary between siliciclastic-bearing marl-limestone sedimentation in the uppermost Jurassic and lowermost Cretaceous limestone formation, both within the Blassenstein Formation. The cephalopod fauna (ammonites, belemnites, aptychi) and crinoids from the Blassenstein Formation, correlated with calcareous microfossil and nannofossil data combined with isotope and paleomagnetic data, indicate the Tithonian to middle Berriasian (Hybonoticeras hybonotum Zone up to the Subthurmannia occitanica Zone; M17r-M21r). The succession of the Nutzhof section thus represents deposition of a duration of approximately 7 . The deposition of the limestone, marly limestone and marls in this interval occurred during tectonically unstable conditions reflected by common allodapic material. Along with the integrated biostratigraphic, geochemical and isotopic analysis, the susceptibility and gamma-ray measurements were powerful stratigraphic tools and important for the interpretation of the paleogeographic setting. Two reverse magneto-subzones, Kysuca and Brodno, were detected within magnetozones M20n and M19n, respectively.
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