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Magnetostratigraphy of the Jurassic/Cretaceous boundary interval in the Western Tethys and its correlations with other regions: a review

Grabowski J.

Volumina Jurassica

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2011

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Vol. 9, no. 1

105-128

EN

Magnetostratigraphy is an important method in regional and worldwide correlations across the Jurassic/Cretaceous boundary. The M-sequence of magnetic anomalies, embracing this boundary, provides an easily recognizable pattern which might be identified in biostratigraphically calibrated land sections. The polarity chrons between M21r and M16n are well correlated to calpionellid and calcareous nannofossil stratigraphy in the Tethyan Realm. This results in a very high precision of stratigraphic schemes of pelagic carbonates (ammonitico rosso and maiolica limestones), integrating the two groups of fossils with magnetostratigraphy. The main clusters of the reference sections are located in the Southern Alps and Apennines, but the database was recently enriched by sections from the Western Carpathians and Eastern Alps. Quite a few Jurassic/Cretaceous boundary sections with magnetostratigraphy are known in the Iberian Peninsula and south-eastern France but their importance relies on the integration of magnetostratigraphy also with the Tethyan ammonite zonation. Correlation of Boreal and Tethyan regions still remains a major problem. Just two sections with reliable correlation to the global polarity time scale are documented outside Tethys: a shallow marine to non-marine Tithonian–Berriasian–Valanginian sequence in southern England (Portland–Purbeck beds) and the marine clastic Upper Tithonian–Middle Berriasian (= Middle Volgian–lowermost Ryazanian) sequence at Nordvik Peninsula (Siberia). The Volgian/Ryazanian boundary at Nordvik seems to be located in the lower part of magnetochron M18n, while the most commonly accepted definitions of the Tethyan Jurassic/Cretaceous boundary are situated either within magnetochron M19n (A/B calpionellid zonal boundary, Durangites/Jacobi ammonite zonal boundary), or at the boundary of M19n/M18r (Jacobi/Grandis ammonite subzonal boundary).

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Cephalopod fauna and stratigraphy of the Adnet type red deposits of the Krížna unit in the Western Tatra Mountains, Poland

Myczyński R., Jach R.

Annales Societatis Geologorum Poloniae

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2009

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Vol. 79, No 1

27-39

EN

The Lower Jurassic Adnet type red limestones and marlstones (Kliny Limestone Member, Huciska Limestone Formation) of the Krížna unit in the Tatra Mountains comprise cephalopod fauna represented by ammonites, belemnites and rarely by nautiloids. Ammonites belong to the families Phyloceratidae, Lytoceratidae, Hildoceratidae and Dactylioceratidae and indicate Early Toarcian Serpentinum Zone, Middle Toarcian Bifrons Zone (most probably Sublevisoni and Bifrons Subzones) and Late Toarcian Pseudoradiosa Zone. Hence, the age of Adnet type deposits may be estimated as Early Toarcian-Late Toarcian. Relatively moderate diversity of ammonite assemblage is noticed. Ammonites and nautiloids are preserved mainly as internal moulds, only some specimens display preserved calcified shells. Part of this macrofauna has resedimented character. Studied ammonite assemblage is closely related to that of the Mediterranean Province.

3

Bositra limestones - a step towards radiolarites: case study from the Tatra Mountains

Jach R.

Annales Societatis Geologorum Poloniae

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2007

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Vol. 77, No 2

161-170

EN

Bositra limestones of Aalenian–Lower Bathonian age crop out in the Krížna unit in the Western Tatra Mountains (Poland). They are sandwiched between pelagic red limestones and radiolarites and display lateral facies variation. Four facies were distinguished: (i) Bositra packstones/grainstones, (ii) crinoidal packstones/ grainstones, (iii) Bositra-crinoidal packstones, and (iv) Bositra-radiolarian wackestones. The Bositra packstones/ grainstones were laid down in high-energy setting, while Bositra-radiolarian wackestones in calm condition. Crinoidal packstones/grainstones represent density current deposits. Bositra-crinoidal packstones resulted from intense bioturbation and mixing of crinoidal packstones/grainstones with background Bositra-rich deposits. Topographic gradient affected the lateral facies variation. Taphonomic factors strongly controlled by energy of the sedimentary environment, ecological factors which caused domination of Bositra bivalves in benthos assemblage and dissolution eliminating non-calcitic bioclasts could resulted in formation of the Bositra limestones. The eutrophication of water column and remodelling of the Krížna Basin, which finally led to deposition of radiolarites seem to be of considerable importance. Hence, Bositra limestones can be regarded as the record of the interme- diate stage of the basin evolution towards radiolarite formation.

PL

Wapienie bositrowe wczesnego aalenu–wczesnego batonu, które odsłaniają się w jednostce kriżniańskiej w Polskich Tatrach Zachodnich znajdują się w profilach pomiędzy pelagicznymi czerwonymi wapieniami a radiolarytami (Fig. 1, 2; Lefeld et al., 1985; Gradziński et al., 2004). Wapienie te wykazują wyraźne facjalne zróżnicowanie. Wydzielone zostały następujące facje: (i) bositrowe pakstony/greinstony, (ii) krynoidowe pakstony/greinstony, (iii) bositrowo-krynoidowe pakstony i (iv) bositrowo-ra diolariowe wakstony (Fig. 2, 3). Bositrowe pakstony/greinstony powstały w środowisku o stosunkowo wysokiej energii, a bositrowo- radiolariowe wakstony w warunkach niskiej energii. Krynoidowe pakstony/greinstony są interpretowane jako osady prądów gęstościowych, a bositrowo-krynoidowe pakstony jako osady prądów gęstościowych zbioturbowane i zmiksowane z osadami tła depozycyjnego. Zróżnicowana morfologia basenu wpłynęła na oboczną zmienność facjalną omawianych wapieni i przestrzenny rozkład facji (Fig. 4). Powstanie wapieni bositrowych było warunkowane przez czynniki natury tafonomicznej, związane z energią środowiska sedymentacji, natury ekologicznej decydujące o dominacji bositr w zespole bentosu i procesy rozpuszczania eliminujące nie kalcytowe bioklasty. Postępująca eutrofizacja wód (Bartolini & Cecca, 1999; Cobianchi & Picotti, 2001) i przemodelowanie basenu kriżniańskiego związane z procesami ryftingu Zachodniej Tetydy, które ostatecznie doprowadziły do depozycji radiolarytów, wydają się mieć zasadniczy wpływ na powstanie wapieni bositrowych. Tak więc, wapienie bositrowe reprezentują przejściową fację poprzedzającą w czasie powstanie tetydzkich jurajskich radiolarytów.