A 95 m thick succession of grey siliceous limestone and marl on Mount Rettenstein in the Northern Calcareous Alps allowed us to study well-preserved and diverse radiolarian assemblages across the Sinemurian–Pliensbachian boundary. The distribution of 31 most characteristic radiolarian species is presented. The radiolarian data are complemented with calcareous nannofossil biostratigraphy, and carbon and strontium isotope analyses of bulk carbonate samples. Ammonites of the Jamesoni Subzone, the topmost subzone of the Jamesoni Zone, have been previously found in the upper part of the section. A negative stable carbon isotope excursion (CIE) occurs in the middle part of the section and is correlated to the supra-regional Sinemurian–Pliensbachian boundary Event. Radiolarian assemblages below the negative CIE belong to the C anutus rockfishensis – Wrangellium thurstonense and Jacus ? sandspitensis radiolarian zones. Katroma clara and several other species of the subsequent Canutus tipperi – Katroma clara Radiolarian Zone first occur at the beginning of the negative CIE. The first occurrence of the nannofossil Similiscutum cruciulus , which defines the base of the NJ4 Calcareous Nannofossil Zone, was recognized near the top of the negative CIE, 10 m above the inferred stage boundary. The strontium isotope ratio is due to diagenetic alteration of bulk micrite systematically shifted to higher values, but clearly decreases across the stage boundary. This decrease is compatible with the trend of the standard 87 Sr/ 86 Sr curve, which was established in skeletal calcite.The radiolarian zones across the Sinemurian–Pliensbachian boundary are for the first time calibrated to chronostratigraphy outside North America and, also for the first time, tied to calcareous nannofossil zones and chemostratigraphy. The results contribute to the stability of Lower Jurassic radiolarian zones and will allow the recognition of the Sinemurian–Pliensbachian boundary in deep-marine successions, in which ammonites are missing.
A 95 m thick succession of grey siliceous limestone and marl on Mount Rettenstein in the Northern Calcareous Alps allowed us to study well-preserved and diverse radiolarian assemblages across the Sinemurian–Pliensbachian boundary. The distribution of 31 most characteristic radiolarian species is presented. Ammonites of the Jamesoni Subzone, the topmost subzone of the Jamesoni Zone, have been previously found in the upper part of the section. The radiolarian data are complemented with calcareous nannofossil biostratigraphy, and carbon and strontium isotope analyses of bulk carbonate samples. A negative stable carbon isotope excursion (CIE) occurs in the middle part of the section and is correlated to the supraregional Sinemurian – Pliensbachian Boundary Event. Radiolarian assemblages below the negative CIE belong to the Canutus rockfishensis – Wrangellium thurstonense and Jacus? sandspitensis radiolarian zones. Katroma clara and several other species belonging to the subsequent Canutus tipperi – Katroma clara Radiolarian Zone first occur at the beginning of the negative CIE. The first occurrence of the nannofossil Similiscutum cruciulus, which defines the base of the NJ4 Calcareous Nannofossil Zone, was recognized near the top of the negative CIE, 10 m above the inferred stage boundary. The strontium isotope ratio is due to diagenetic alteration of bulk micrite systematically shifted to higher values, but clearly decreases across the stage boundary. This decrease is compatible with the trend of the standard 87Sr/86Sr curve established in skeletal calcite. This is the first time radiolarian zones across the Sinemurian–Pliensbachian boundary in Europe can be tied to calcareous nannofossil zones and chemostratigraphy and also calibrated to chronostratigraphy. These results contribute to the stability of Lower Jurassic radiolarian zones and will allow the recognition of the Sinemurian–Pliensbachian boundary in deep-marine successions where ammonites are absent.
One of the best preserved Early Pliensbachian radiolarian assemblages from the Western Tethys is described from the grey marly limestone exposed at Mount Rettenstein in the Northern Calcareous Alps, south of the Dachstein Massif.Fourty-five genera and 71 species are documented and illustrated here.Four species are newly described: Tozerium filzmoosense Cifer sp.nov., Loupanus pliensbachicus Cifer sp.nov., Thurstonia? robusta Cifer sp.nov., and Ares rettensteinensis Cifer sp.nov.Radiolarian age is in accordance with ammonoid data from the overlying red marly limestone, which was assigned to the upper part of the Lower Pliensbachian.The best equivalent for the radiolarian-bearing lithology is the Dürrnberg Formation, characteristic of the open-marine Hallstatt facies zone.Previously published radiolarian data from the Dürrnberg Formation were re-evaluated and the originally proposed age assignments revised.At two localities, the published Hettangian-Sinemurian age was emended to the early Early Pliensbachian that is in accordance with the age of radiolarians from Mount Rettenstein.We compared the studied fauna from Mount Rettenstein also with two other rich radiolarian assemblages, one from another locality in the Dürrnberg Formation and one from the Gümüslü Allochthon in Turkey, which were assigned to the late Early Pliensbachian and are somewhat younger than the assemblages studied herein.
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