Early Jurassic radiolarians from the Weitenau area (Northern Calcareous Alps, Austria) and their implications for palaeogeography

The Lower Jurassic sedimentary sequence deposited on the outer shelf (Hallstat shelf) of
the Neo-Tethys passive margin occurs mainly as blocks in different mélanges, or in rare
cases, far-travelled nappes formed in the frame of the Middle–Late Jurassic mountain building
process related to ophiolite obduction. Datable Lower Jurassic blocks are very rare in
the Western Tethyan Realm, and are studied in detail for their age and sedimentological
characteristics in order to reconstruct the Jurassic distal parts of the Neo-Tethyan passive
continental margin. One of these exotic blocks, belonging to the marly siliceous limestones
of the Dürrnberg Formation, was discovered in the Haselgebirge Mélange of the Weitenau
Area (Northern Calcareous Alps, Austria). The block yielded a well-preserved radiolarian
fauna, which is assigned here to the uppermost Lower Pliensbachian Gigi fustis–Lantus
sixi Radiolarian Zone. The occurrence of this Lower Jurassic block beside other exotic
blocks in the Upper Jurassic Haselgebirge Mélange proves the allochthony of the Haselgebirge
Mélange in the Weitenau area.