66 Age of radiolarian samples
In this section, we present the radiolarian age and results obtained for the Hallstatt Mélange in four localities at Bad Mittendorf area (Table 2). Only the stratigraphically most important species are discussed. For the complete inventory of all samples, see Table 1.
Kumitzberg.—In this area, the massive dark-gray radiolarite beds (sample EW-158) are intercalated by thin layers of cherty shales. The co-occurrence of Unuma latusicostatus (Aita) with Bernoullius cristatus Baumgartner (Table 1) suggests assignment to UAZ 5 (latest Bajocian–early Bathonian) of Baumgartner et al. (1995a). Pantanellium riedeli Pessagno, which first appears in UAZ 7, was also found. This species has a very large variability that overlaps with other Pantanellium species. Its range is thus ignored in the age determination.
Steinwand north.—In this area two sections accurately deter- mine the age of the radiolarite succession. The most complete section, in the northeastern part of the syncline structure (Fig. 4), records continuous radiolarite deposition from Bathonian to the Oxfordian. The lowermost part of the radiolarite succession (near the entrance of the valley, sample BMW-28, Fig. 4) yielded a radiolarian assemblage of latest Bajocian–early Bathonian age (UAZ 5 of Baumgartner et al., 1995a), defined on the co-occurrence of Protunuma ochiensis Matsuoka with Unuma latusicostatus (Aita). The conflicting range of Pantanellium riedeli Pessagno (UAZ 7–12) was ignored for the same reason as in the aforementioned sample EW-158 from Kumitzberg. The youngest part of the succession is preserved in the core
of the syncline (sample BMW-35, Fig. 4) and yielded a radiolarian assemblage of middle–late Oxfordian to late Kimmeridgian–early Tithonian age (UAZs 9–11 of Baumgart- ner et al., 1995a), based on the occurrence of Archaeodictyomi- tra minoensis (Mizutani), Zhamoidellum ovum Dumitrica, and Emiluvia sedecimporata (Rüst). The interval between these two ages was also recognized. UAZ 7 (late Bathonian–early Callovian) or UAZ 8 (middle Callovian–early Oxfordian) was determined in samples BMW-32 and BMW-34. Striatojapono- capsa conexa (Matsuoka) in sample BMW-32 and Preawillir- iedellum robustum (Matsuoka) in sampleBMW-34 suggest that these two samples are not younger than UAZ 7, but Gongylothorax favosus Dumitrica, which first occurs in UAZ 8 is also associated. At the end of the valley is located the second stratigraphic
section outcropping in this area. The radiolarian assemblage in the youngest investigated sample (BMW-33) indicates a Tithonian age. Early–early late Tithonian UAZ 12 is inferred from the co-occurrence of Eucyrtidiellum pyramis (Aita in Aita and Okada) with Protunuma japonicus Matsuoka and Yao.
Area between Krautmoos and Mischenirwiese.—The thick succession of mass-flow deposits in this area is dated by the assemblages studied in the intercalated radiolarite matrix. The radiolarite sample BMW-26 collected below the first debris-flow deposit is dated latest Bajocian–early Bathonian age (UAZ 5
Journal of Paleontology 91(1):25–72
of Baumgartner et al., 1995a). This age is constrained with Semihsuum amabile (Aita), Saitoum trichylum De Wever, and Mizukidella kamoensis (Mizutani and Kido). The same age (UAZ 5) was obtained from a chert clast (sample BMW-13b) in the upper part of the succession. The age-diagnostic species in this clast are Theocapsomella cordis (Kocher), Eucyrtidiellum pustulatum Baumgartner, and Unuma latusicostatus (Aita). The matrix between the chert clasts (sample BMW-13c) yielded a significantly younger radiolarian fauna of middle Callovian– early Oxfordian age (UAZ 8 of Baumgartner et al., 1995a), as indicated by co-occurrence of Spinosicapsa spinosa (Ožvoldová) with Hemicryptocapsa marcucciae (Cortese). As proven by radiolarian dating, the resedimentation of the
Hallstatt Limestone started in the region of Bad Mitterndorf area since the Bathonian and prevailed at least until the Oxfordian. The area of the Hallstatt Limestone mass flows and slide blocks is separated fromthe coeval radiolarite successionwithoutmass-flow deposits (the Steinwand section) by a younger thrust or fault.
Discussion
Steinwand-Mischenirwiese (Fig. 4) mass flows are missing. In the more northern areas of the Northern Calcareous Alps (Fig. 3), radiolarite deposition starts relatively early, indicating an early deepening event due to the tectonic load of the advancing Hallstatt nappes. First imbricates started to form in the Bathonian– Callovian (I and II in Fig. 5), as indicated by age dating of the
matrix radiolarites. In the Oxfordian (III–IIIa in Fig. 5) the mass flows also contain components of the older radiolarite “mélange” basins and the section Mischenirwiese attained a more basinal position, as indicated by the sedimentological features of the radi- olarite succession. In the late Oxfordian the nappe front reached the depositional area of the Mischenirwiese section, as shown in the overlyingmélange (IV in Fig. 5). In the Kimmeridgian–Tithonian, nappe propagation stopped and the nappe stack was sealed by a carbonate platform. In the Kimmeridgian, coarse reefal debris was shed into this deep-water basin (O´Dogherty and Gawlick, 2008) with a general fining-upward trend.Due to uplift and demise of the southern Platform in the late Tithonian (Fig. 5; Gawlick andMis- soni 2011a), only fine-grained siliceous limestones were deposited in the basin, as provenwith radiolarian dating in this study (see the youngest radiolarian sample of the Steinwand locality).
Acknowlegments
This work is a contribution of the research projects P14131- TEC, P15060, and P16812 (Austrian Science Foundation FWF
The time span of deposition of the radiolarite basin, which con- tains the far-traveled Hallstatt Limestone blocks, is determined as Bathonian to Oxfordian. Redeposition started in the ?late Bathonian and ended in the Oxfordian, as proven by the radi- olarite matrix age and the age of the overlying sediments (O’Dogherty and Gawlick, 2008). Radiolarian ages and compo- nent spectrum define this redeposit as part of the Sandlingalm Basin (Sandlingalm Formation: Fig. 2), one of the oldest basins formed in a relative early stage of compression of the Neotethys (see Gawlick et al., 2007a, 2009, 2012; Missoni and Gawlick, 2011b for details). The depositional areas of the radiolarites of the section
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