Sem-observation of calcareousmicro- and nannofossils incertae sedis from the Silurian of Gotland, Sweden: Preliminary results
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Incertae sedis
A number of ammonite genera and species previously unknown in the Lower Jurassic beds of the Cana di an Arc tic are described in this report. These are: Psiloceratinae? (possibly Hettangian), Charmasseiceras and Coroniceras (Primarietites) (Bucklandi Zone of the Lower Sinemurian), Gleviceras plauchuti Frebold sp. nov. (Oxynotum Zone of the Upper Sinemurian), Echioceras ak/avikense Frebold sp. nov. and Echioceras arcticm Frebold sp. nov. (Raricoslatum Zone of the Upper Sinemurian), Amalrhem stokesi ( J. Sowe rby) (Margaritalus Zone of the Upper Pliensbachian), Hi/daires sp. indet. (Falcifer Subzone of the Lower Toarcian), and Zugodacrylites cf. braunianus (d'Orbigny) (Braunianus Subzone of the Middle Toarcian). The generic posit ion of some previously described ammonites is revised: the Late Toarcian species Peronoceras spinatum (Frebo ld) and P. po/are (Frebold) were assigned formerly to Catacoeloceras Buckman. The sequence of Lower Jurassic ammonites in the Canadian Arctic Archipelago is now more complete. Moreover, it is virtually identical to that in the Richardson and British Mountains and in northern Alaska. In addition, the principal gaps in the sequences of the three compared areas (Lower Pliensbachian and upper part of Upper Toarcian) are the same. There are also close similarities to the sequence in northeastern Russia.
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Conodont
Radiolaria
Stratotype
Section (typography)
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This paper is a synthesis of calcareous nannofossil biostratigraphy for the Lower and Middle Jurassic of the Mediterranean Province based on several sections from Northern and Central Italy. Nannofossil events were calibrated with ammonite biostratigraphy and, when necessary, ammonite-controlled sections in South East France were incorporated. Data derive from previously published biostratigraphies and unpublished data of the authors.The large data-set allowed estimates of reliability and reproducibility of single events. As a result, in the Hettangian-Bathonian interval we propose 47 main events based on diagenesis-resistant and common taxa, 17 events based on rare but ubiquitous taxa and 12 potential events requiring further investigations due to taxonomic problems and sporadic occurrence. A biostratigraphic scheme, consisting of 11 zones and 15 subzones, is proposed for the Tethyan Lower and Middle Jurassic. The proposed biostratigraphy is compared to recent schemes compiled for Portugal, Morocco, Switzerland and the Boreal Realm. Only 27 events are reproducible in various regions, but diachroneity of most events seems to derive from different ammonite biostratigraphies applied in different areas. A very high stratigraphic resolution is achieved in Italy/France for the Pliensbachian to Lower Bajocian interval. The Sinemurian and Bathonian are characterized by the lowest resolution, and very few sections with ammonite control and/or favourable lithologies are available for improvement of nannofossil biostratigraphy. This study confirms the potential of calcareous nannofossil biostratigraphy for dating Lower and Middle Jurassic successions as well as for intra- and inter-regional correlations.
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This study presents calcareous nannofossils biostratigraphy of the Campanian‒Danian interval at the Gurpi Formation in the southwest of Ilam Province, in the Zagros Basin. The studied sequence of the Gurpi Formation mainly consists of marly shale and limestones. The thickness of Gurpi Formation was measured as approximately 230 m. Forty-five species belonging to twenty-eight genera were recorded in these strata. The biostratigraphy based on calcareous nannofossil assemblages allows the identification of nannofossil standard zones such as CC18b, CC19, CC20, CC21, CC22, CC25b,c and CC26b, that equivalent to UC14cTP - UC20dTP for Cretaceous, and NP2 - NP3 (equivalent with CNP2, CNP3 and lower part of CNP4) for the Paleogene strata in the Gurpi Formation. The studied section spans the early Campanian to early late Campanian with good continuity. A biostratigraphic gap is identified in the late late Campanian to early Maastrichtian (CC23, CC24, and CC25a), across the late Maastrichtian (CC26a) and at the early Danian (NP1).
Paleogene
Sequence (biology)
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The total span of fossil dinoflagellate research in India stretches within the last three decades. Two distinct phases of this study are recognised, the phase of Reconnaissance and Understanding (1961-70) and the phase of Establishment and Refinement (1971-1990). The significance of dinoflagellate cysts in resolving various biostratigraphic problems of Mesozoic and Cenozoic sedimentary sequences is discussed citing specific examples from Indian basins. Recent approaches towards achieving biostratigraphic precision through integration with smaller planktonic and larger benthonic foraminifera, ammonites, and calcareous nannoplankton, are specifically highlighted focusing the potential of dinoflagellate cysts to resolve the time boundaries. A review of the previous literature has revealed that wrong identifications, especially of marker taxa, have led to major errors in age determination of sedimentary sequences. A few cases are exemplified. An appeal has been made to fellow palynologists to be careful in presenting geological and palynological data to maintain the viability and potential of this group of microfossils in biostratigraphy.
Dinoflagellate
Acritarch
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Trilobite
Paleoecology
Ichnofacies
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The scarcity of records of Early Paleocene radiolarians has meant that while radiolarian biostratigraphy is firmly established as an important tool for correlation, there has been a long-standing gap between established zonations for the Cretaceous and from latest Paleocene to Recent. It has also led to considerable speculation over the level of faunal change across the Cretaceous/Tertiary (K/T) boundary. Consequently, the discovery of rich and diverse radiolarian assemblages in well-delineated K/T boundary sections within siliceous limestones of the Amuri Limestone Group in eastern Marlborough, New Zealand, is of great significance for biostratigraphy and K/T boundary research.
This initial report is restricted to introducing a new latest Cretaceous to mid Late Paleocene zonation based on the radiolarian succession at four of these sections and a re-examination of faunas from coeval sediments at DSDP Site 208 (Lord Howe Rise). Three new Paleocene species are described:Amphisphaera aotea, Amphisphaera kina andStichomitra wero. Six new interval zones are defined by the first appearances of the nominate species. In ascending order these are:Lithomelissa? hoplites Foreman (Zone RK9, Cretaceous),Amphisphaera aotea n. sp. (Zone RP1, Paleocene),Amphisphaera kina n. sp. (RP2),Stichomitra granulata Petrushevskaya (RP3),Buryella foremanae Petrushevskaya (RP4) andBuryella tetradica (RP5). Good age control from foraminifera and calcareous nannofossils permits close correlation with established microfossil zonations. Where age control is less reliable, radiolarian events are used to substantially improve correlation between the sections.
No evidence is found for mass extinction of radiolarians at the end of the Cretaceous. However, the K/T boundary does mark a change from nassellarian to spumellarian dominance, due to a sudden influx of actinommids, which effectively reduces the relative abundance of many Cretaceous survivors. An accompanying influx of diatoms in the basal Paleocene of Marlborough, together with evidence for an increase of total radiolarian abundance, suggests siliceous plankton productivity increased across the K/T boundary. Possible causes for this apparently localised phenomenon are briefly discussed.
Radiolaria
Magnetostratigraphy
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