Porifera-microbialites of the Lower Liassic (Northern Calcareous ...

Geochemical Analyses 95
In contrast the crust from Plattenkogel hill exhibits a small negative Gd-anomaly (0.82) and a
clearly negative Ce-anomaly (0.45), which is also present in the “Frutexites” structures of the
lumachelle layer (data in the supplement to Figure 46) and at a lower amount also in the matrix
of the overlying spiculite (0.53). Furthermore the crust at Plattenkogel hill is characterized by
high concentrations of barium and a Fe/Mn-ratio of about 3.3 (Fig. 47) that is lower than in most
other Liassic crusts (values of Fe/Mn = 8-150, in accordance with Böhm et al. 1999).
Fig. 46. PAAS-normalized (McLennan 1989) rare earth elements in Fe/Mn-crusts and corresponding sediments
from Steinplatte/Plattenkogel (locality S1) and different localities of the Osterhorn block. Measured by liquid icpmass
spectrometry. For all ICP-MS datas see the supplement.
Ferromanganese crusts and nodules are common features of Lower Liassic condensed
sedimentation in the Northern Calcareous Alps (Wendt 1970; Germann 1971; Drittenbass 1979;
Fels 1995). Their genesis has often been discussed and recently been interpreted as mainly
hydrogenic in origin (Böhm et al. 1999). Although most of the ~1-3 cm-thick crusts are
characterized by the dark laminae of ferromanganese material (x-ray diffraction shows that they
mainly consist of the mineral goethite), the main part of them is formed by calcite of intercalated