Journal of Paleontology, 91(1), 2017, p. 25–72 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2016.96
Middle and Late Jurassic radiolarians from the Neotethys suture in the Eastern Alps
Luis O’Dogherty,1 Špela Goričan,2 and Hans-Jürgen Gawlick3
1Departamento de Ciencias de la Tierra, Universidad de Cádiz, CASEM, E-11510 Puerto Real, Spain ⟨
luis.odogherty@uca.es⟩ 2Paleontološki inštitut Ivana Rakovca, ZRC SAZU, Novi trg 2, SI-1000 Ljubljana, Slovenia ⟨
spela@zrc-sazu.si⟩ 3University of Leoben, Department of Applied Geosciences and Geophysics, Petroleum Geology, Peter-Tunner-Strasse 5, 8700 Leoben, Austria ⟨
Hans-Juergen.Gawlick@
unileoben.ac.at⟩
Abstract.—Cherty limestones, marls and radiolarites are widespread in the Hallstatt Mélange of the Northern Calcareous Alps. The mélange was formed during the Neotethyan orogeny in a series of deep-water basins that progressively developed in front of the advancing nappe front. The low thermal overprint of these rocks favors the good preservation of radiolarians (polycystines) that have been used for dating and reconstructing the Jurassic tectonostratigraphy of the area. This paper describes rich Middle–Late Jurassic radiolarian faunas from four localities in the Hallstatt Mélange near Bad Mitterndorf in Austria. Two different successions, both spanning from the Bathonian to the Oxfordian, are dated. In the first succession, the radiolarites are intercalated between or occur as matrix in mass-flow deposits originating from the accretionary wedge. The second succession is nearly 100m thick but is devoid of mass-flow deposits and documents a continuous radiolarite deposition in greater distance from the nappe stack. Both successions are ascribed to the Sandlingalm Basin, which evolved on a relatively distal continental margin during early stages of the orogeny. The highly diverse and well-preserved radiolarian assemblages have been used for a detailed taxonomic study. Two
new families are described: Minocapsidae n. fam. and Xitomitridae n. fam.; six new genera are described Doliocapsa n. gen., Crococapsa n. gen., Parvimitrella n. gen., Xitomitra n. gen., Campanomitra n. gen., and Mizukidella n. gen. In addition, one new replacement name, Takemuraella (pro Triversus), is introduced, the diagnoses of 6 genera are emended, and two new species are described: Hemicryptocapsa nonaginta n. sp. and Mizukidella mokaensis n. sp.
Introduction
This paper describes rich Middle and Late Jurassic radiolarian faunas from four localities in the Hallstatt Mélange near Bad Mitterndorf in Austria. Radiolarian assemblages arewell preserved, suitable for taxonomic studies, and precise biostratigraphy. Middle and Late Jurassic low-latitude radiolarians have
been relatively well studied in terms of species-level systematics and biochronology (Baumgartner et al., 1995a, b); however, the systematics of genera and families has not been sufficiently elaborated yet. O’Dogherty et al. (2009) presented an illustrated catalogue of type species of Jurassic and Cretaceous genera described so far. All genera were revised, and many were considered as invalid (synonyms, homonyms, nomina dubia). Another group of radiolarians that needs taxonomic revision are those Mesozoic species that still bear inappropriate names of Recent genera. The structure of the catalogue, consisting exclusively of illustrations and synonymy, did not allow us to discuss the distinguishing characteristics of the valid genera or to describe new taxa. The primary aim of this paper is to move towards a more natural taxonomy of Jurassic radiolarians. Six new genera are described, a replacement name for a homonym is proposed, the diagnoses of six valid genera are emended, and remarks for several other genera are provided to clarify
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their definition. For all new and revised genera, a list of included species is presented. Two new families are erected and two new species are described. In addition to radiolarian taxonomy, this paper contributes
to the biostratigraphic data of the Hallstatt Mélange in the Northern Calcareous Alps. Extensive radiolarian dating over the last 15 years has provided a wealth of age constraints in deep-water sediments of the Tirolic units and the Hallstatt Mélange (Suzuki and Gawlick, 2003, 2009; Missoni and Gawlick, 2011a, b and references therein). These data had important implications for the reconstruction of the Jurassic tectonostratigraphy and distinguished several trench-like basins that formed progressively during the propagation of thrusting (see Missoni and Gawlick, 2011a, b; Gawlick et al., 2012 for the latest reviews). However, the structure of this area, especially that of the Hallstatt Mélange, is extremely complex and the proposed tectonostratigraphic model can still be refined with additional biostratigraphic data.
Geological overview
The study area is located in the central Northern Calcareous Alps around 100km southeast of Salzburg (Fig. 1.1). The Northern Calcareous Alps represent a far travelled nappe system
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