O’Dogherty et al.—Jurassic radiolarians from the Eastern Alps


in the highest structural position of the Eastern Alps (Tollmann, 1985) and belong palaeogeographically to the Austroalpine domain (Fig. 1.1). In Middle–Late Triassic times the Austroalpine domain as


part of the central and southeastern European shelf show a typical carbonate passive continental margin facies distribution (Fig. 1.2–1.3), whereas in Jurassic times the Austroalpine realm was situated between the Penninic Ocean to the northwest and the Neotethys Ocean in the southeast (Fig. 1.4–1.5). Contemporaneous with progressive Jurassic extension and


opening of the Alpine Atlantic Ocean (and the Penninic realm as part of it; see Missoni and Gawlick, 2011a for details) as an eastward continuation of the Central Atlantic Ocean, closure of parts of the Neotethys Ocean with formation of the Neotethys ophiolite imbricates started in the late Early Jurassic and pre- vailed until the early Late Jurassic (Karamata, 2006). Obduction of the Neotethys ophiolite imbricates started in the Middle Jurassic and the Austroalpine and its northern and southern equivalents attained a lower plate position (Frisch and Gawlick, 2003, Gawlick et al., 2008, Schmid et al., 2008). Westward to northwestward propagating Middle to early Late Jurassic compression led to the imbrication of the Austroalpine domain and its equivalents along the Neotethys suture and resulted in the Neotethyan orogeny (Missoni and Gawlick, 2011a). A characteristic feature of this orogenesis is the formation of deep-water radiolaritic basins in front of the westward propagating nappe stack. Sediment supply in these basins derived from the nappe fronts. Gawlick et al. (1999) interpreted this sedimentation pattern


27


radiolarite basins contain the Hallstatt Mélange as an erosional product of the Juvavic nappes, which are mainly eroded today. Subsequently the trench-like basins became overthrusted and incorporated into the accretionary prism (Missoni and Gawlick, 2011b). This Hallstatt Mélange was therefore formed in the late Early to early Late Jurassic interval as a result of successive shortening of the Triassic to Jurassic distal shelf area (Hallstatt Zone).


Jurassic evolution of the southern Northern Calcareous Alps


as a reflection of nappe movements in the Northern Calcareous Alps in the late Middle to early Late Jurassic and related it to the Kimmeric orogeny according to earlier authors (see ‘Jurassic gravitational tectonics’ in Plöchinger, 1974, 1976; Tollmann, 1981, 1985, 1987; Mandl, 1982). This orogenic event (Lein, 1985, 1987a, b) was related to the closure of the western half of the Neotethys Ocean, today named the Neotethyan orogeny (Missoni and Gawlick, 2011a). In the southern and therefore highest nappe group (Tirolic


units and equivalents, Fig. 1.2–1.3) of the Northern Calcareous Alps the remains of this orogenic event are well preserved with a series of deep-water radiolaritic basins which were formed in sequence in front of the advancing nappe front (e.g., Gawlick et al., 1999, Missoni and Gawlick, 2011b). These southernmost


In the early Early Jurassic, sedimentation was generally con- trolled by the topography of the Late Triassic Hauptdolomit/ Dachstein carbonate platform (Böhm, 2003, Gawlick and Frisch, 2003; Figs. 1, 2). On top of the Rhaetian shallow-water carbonates, red condensed limestones of the Adnet Group (?late Hettangian/Sinemurian to Toarcian: Böhm, 1992, 2003) were deposited, mostly separated by a gap of sedimentation (mainly early Hettangian, partly also late Hettangian; Fig. 2). On top of the Rhaetian Kössen Formation (e.g., Eiberg Basin, Restental Basin, Fig. 2) cherty and marly bedded limestones (Kendlbach Formation; Scheibelberg Formation: Böhm, 1992, 2003; Krainer and Mostler, 1997; Ebli, 1997) were deposited, while in marginal areas of the basins crinoidal or sponge-spicule rich limestones of the Enzesfeld Formation were laid down (Böhm, 1992). In the late Pliensbachian to early Toarcian a horst-and-graben morphology developed (Bernoulli and Jenkyns, 1974, Krainer et al., 1994) and triggered breccia formation along submarine slopes and escarpments (Böhm et al., 1995). The Toarcian and most of the Middle Jurassic are characterized by starved sedimentation, ferro-manganese crusts, or a hiatus on the horsts, whereas the grabens were filled with deep-water carbonates and breccias, which latter formed near fault scarps. Neptunian dykes are found on the horsts. In the newly formed basinal areas gray bedded limestones of the younger Allgäu Formation were deposited, while condensed red limestones of the Klaus Formation formed on the top of the topographic highs (Krystyn, 1971, 1972; Fig. 2). This sedimentation pattern changed dramatically in the late


Middle Jurassic (Gawlick and Frisch, 2003). Sedimentation resumedwith the deposition of radiolarian cherts and radiolaria-rich


Figure 1. Tectonic and paleogeographic maps. (1) Tectonic sketch map of the Eastern Alps and study area (after Tollmann, 1987; Frisch and Gawlick, 2003); GPU Graz Palaeozoic Unit; GU Gurktal Unit; GWZ Greywacke Zone; RFZ Rhenodanubian Flysch Zone. Star indicates study area (Fig. 3). (2) Late Triassic paleogeographic position and facies zones of the Austroalpine domain as part of the northwestern Neotethys passive margin; IAZ = Iberia-Adria Zone transform fault, AAT = future Austroalpine-Adria transform fault, TTT = future Tisza-Tatra transform fault, TMT = future Tisza-Moesia transform fault, AA = Austroalpine, BI = Bihor, BR = Briançonnais, BU = Bükk, C = Csovar, Co = Corsica, DI = Dinarides, DO = Dolomites, DR = Drau Range, HA = Hallstatt Zone, JU = Juvavicum, JL = Julian Alps, ME = Meliaticum, MK = Mecsek, MO = Moma unit, MP = Moesian platform, P = Pilis-Buda, R = Rudabanyaicum, SI = Silicicum, SL = Slovenian trough, SM = Serbo-Macedonian unit, TA = Tatricum, TO = Tornaicum, TR = Transdanubian Range, VA = Vascau unit, WC = central West Carpathians (modified after Haas et al., 1995; Gawlick et al., 1999, 2008). (3) Schematic cross section (for position, see line a-b in 2) showing the typical passive continental margin facies distribution across the Austroalpine domain in Late Triassic time (after Gawlick and Frisch, 2003). (4) Palaeogeographic position of the Northern Calcareous Alps as part of the Austroalpine domain in Late Jurassic time (after Frisch, 1979; Gawlick et al., 2008). In this reconstruction the Northern Calcareous Alps are part of the Jurassic Neotethyan Belt (orogen) striking from the Carpathians to the Hellenides. The Neotethys suture is equivalent to the obducted West-Vardar ophiolite complex (e.g., Dinaric Ophiolite Belt) in the sense of Schmid et al. (2008) = far-travelled ophiolite nappes of the western Neotethys Ocean in the sense of Gawlick et al. (2008) (see Robertson, 2012 for discussion). The eastern part of the Neotethys Ocean remained open = Vardar Ocean (Missoni and Gawlick, 2011a). Toarcian to Early Cretaceous Adria-Apulia carbonate platform and equivalents according to Golonka (2002), Vlahović et al. (2005), and Bernoulli and Jenkyns (2009). (5) Schematic cross section reconstructed for Middle to Late Jurassic times showing the passive continental margin of the Lower Austroalpine domain facing the Penninic Ocean to the northwest (e.g., Tollmann, 1985; Faupl and Wagreich, 2000) and the lower plate position and imbrication of the Austroalpine domain in relation to the obducted Neotethys oceanic crust (after Gawlick et al., 2008; compare with Frisch, 1979). Star indicates position of study area (compare Figure 3).


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