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


mass flows of the Hallstatt Mélange trench-like basins formed earlier from material derived from the outer shelf facing the Neotethys Ocean (Hallstatt Zone, Fig. 1.2–1.5), whereas the Tauglboden Mélange trench-like basin formed later from material derived of the lagoonal part of the Hauptdolomit/ Dachstein carbonate platform (Fig. 1.4, 1.5). However, both basins formed syntectonically and suggest a substantial relief between the basin axis and the source area. A third type of radiolarite basin, the Sillenkopf Basin (Missoni et al., 2001), remained in the southern part of the Northern Calcareous Alps as a starved basin in the Kimmeridgian (Fig. 2). This basin contains the earliest ophiolitic detritus from the accreted and obducted Neotethys Ocean floor (Missoni, 2003). In the Tirolic units of the Northern Calcareous Alps the


establishment of the shallow-water Plassen Carbonate Platform started at the frontal parts of the rising and advancing nappes (Gawlick et al., 2002, 2005). From this position, the progradation of several independent platforms took place towards the adjacent radiolarite basins (Gawlick and Frisch, 2003; Gawlick and Schlagintweit, 2006; Gawlick et al., 2005, 2007a, 2012). This resulted in a complex basin-and-rise topography with different types of sediments in shallow-water and deep-water areas (Gawlick and Schlagintweit, 2006). In the Kimmeridgian a huge carbonate platform was formed in the upper Tirolic unit, whereas in the lower Tirolic unit shallow- water carbonates were restricted to its northern part (Gawlick et al., 2007a). The whole Plassen Carbonate Platform cycle lasted from the Kimmeridgian until the late early Berriasian platform drowning (Gawlick and Schlagintweit, 2006).


Description of the studied localities


All studied localities belong to the Hallstatt Mélange around the village of Obersdorf north of Bad Mitterndorf (Fig. 3). For a more detailed description of the geology of the area, the Late Triassic to Late Jurassic sedimentary succession, and radiolarian and conodont dating, see O’Dogherty and Gawlick (2008).


Kumitzberg.—TheLateTriassicHallstatt Limestoneblock of Mt. Kumitzberg is surrounded by a grassland area without outcropping sedimentary rocks (Fig. 3).Only small pieces of dark- gray to black radiolarites can be foundatthe westernbaseof Mt. Kumitzberg.During the reconstruction of a small bus station in the excavation hole the contact between the Hallstatt Limestone block and the underlying dark-gray to black radiolarite was visible. The contact between the Hallstatt Limestone block and the radiolarite is erosive. This clearly indicates that the massive limestone block cut deep into the radiolarite succession. Therefore the age of the radiolarites is slightly older than the time of its emplacement. The unlaminated and massive dark-gray radiolarite beds are intercalated by thin layers of cherty shales. Only one sample (EW-158) was collected from this locality.


Steinwand north.—A slightly folded, relative thick radiolarite succession is preserved in a valley between the Steinwand and the Mischenirwiese (Fig. 3), on the southeastern slope of Mount Kampl. This succession occurs on top of the Late Triassic (Rhaetian) lagoonal Dachstein Limestone of the Steinwand (Fig. 4). The overlying Early Jurassic interval is covered by


29


Figure 3. Topography and simplified geology of the study area, showing sample locations (after O’Dogherty and Gawlick, 2008). The plus signs indicate positions of the investigated samples below the Kumitzberg, northwest of Krautmoos, and southeast of the Mischenirwiese and north of the Steinwand. Photo below the map shows the study area as viewed from Mount Kampl to the southwest. The hilly area with dense forest and grassland consists of Jurassic cherty sediments with incorporated mass flows and slides of Hallstatt Limestones. The contact between matrix and blocks or complete sections is visible only in areas with steeper slopes or valleys, or anthropogenic excavations.


Quaternary deposits, but in rare cases some relics of the red nodular limestones of the Adnet Formation occur in the grass- land below the dark-gray bedded radiolarite succession. Lower to Middle Jurassic condensed red limestones are well preserved on the southeastern slope of Mount Kampl on top of the Norian/ Rhaetian lagoonal Dachstein Limestone. This series represents the northeastern part of the syncline structure between the Steinwand and Mount Kampl. The lowermost part of the radiolarite succession (sample


BMW-28, Fig. 4) outcrops near the entrance of the valley and yielded the oldest assemblage, whereas the youngest part is preserved in the core of the syncline (sample BMW-35, Fig. 4). The thickness of this black radiolarite succession is nearly 100meters. Intercalated mass-flow deposits aremissing in contrast to equivalent successions to the east. Radiolarian dating proves a continuous radiolarite deposition fromBathonian to theOxfordian. At the end of the valley, near a spring, a small outcrop of gray bioturbated cherty limestones yielded the youngest radiolarians in this area (sample BMW-33). This clearly demonstrates that the radiolarite succession in the valley is separated fromthe area of the Mischenirwiese by a young fault.


Area between Krautmoos and Mischenirwiese.—The area northwest of Krautmoos (Fig. 3) is characterized by a thick succession of mass-flow deposits with intercalated radiolarite matrix. In a few outcrops, below and between the amalgamated


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