Middle/Late Jurassic Radiolarian Stratigraphy in the NCA

The objective of the proposed project (planned for two years) is to clarify detailed stratigraphy with radiolarians of formations consits of cherty sediments in the central Northern Calcareous Alps in order to explane the geologic structure and tectonic development of them. The dating of the cherty sediments (radiolarites, cherty limestones, marls, manganrich sediments) should determine Middle to Late Jurassic basin and rise formation in the central Northern Calcareous Alps. These data results in a completely new interpretation of the basin formation in late Middle to late Late Jurassic times, especially for the tectonic evolution of the Hallstatt Zones (Juvavicum) of the central Northern Calcareous Alps. In the Hallstatt Zones of the central Northern Calcareous Alps the best preserved sections occur, cherty sediments are common and the radiolarians are partly well preserved. By ways of this reconstruction, new results about control processes of sedimentation in connection with tectonic events are expected, which are very important not only for the tectonic development of the Northern Calcareous Alps but also for our general understanding so sedimentation processes in basins of orogenic belts. Austrias Northern Calcareous Alps are one of the best investigated orogenies of the world. By this, it might be possible to get a better understanding of the onset of radiolarian chert in the context of orogenic processes. The project is planned to finish our work of the last ten years. A lot of undated samples are in my collection all over the study area, but no specialist is available to do the stratigraphic and paleontological work.

The objective of the project (Okt. 2001 to Sept. 2003) was to clearify detailed stratigraphy with radiolarians of formations consits of cherty sediments in the central Northern Calcareous Alps (NCA) in order to explane the geologic structure and tectonic development of them. By dating of the cherty sediments (radiolarites, cherty limestones, marls, manganrich sediments) we determine the Middle to Late Jurassic basin and rise formation in the central NCA. For this, we work out a completely new radiolarian stratigraphy for the NCA. The classical concept of the nappe structure of the central NCA, 100 years old, is in contradiction with modern stratigraphic, structural, metamorphic and geochronological data. A great jurassic tectonic event was unknown for long times and a lot of people neglected it until recent times. The most important new results came from our dating of the cherty sediments and breccia analysis. Our new data results in a completely new interpretation of the basin formation in late Middle to late Late Jurassic times, especially for the tectonic evolution of the Hallstatt Zones (former Juvavicum) of the central NCA. In the Hallstatt Zones of the central NCA the best preserved sections of cherty sediments occur, different types of cherty sediments are common and the radiolarians are partly well preserved. By ways of this reconstruction, we get a lot of new results about control processes of sedimentation in connection with tectonic events, especially the formation of accretionary prism. The most important result of the basin reconstruction is a new nappe division for the NCA and, remnants of the Hallstatt nappes are only represented by components of up to kilometer-size in Middle/Upper Jurassic radiolaritic wildflysch sediments ("Hallstatt Mélange"). The destruction of the continental margin started in Middle to Upper Jurassic time and prograded from the oceanic side towards the shelf. We define the term carbonate clastic radiolaritic flysch basins which were typical formed in front of advancing nappes. Only by exact dating of the cherty sediments and breccia analysis it is possible to decide, which basin type was formed: extensional basins or trenches. In the NCA the radiolarite basins formerly were interpreted as extensional basins, because most radiolaritic sediments were formed due to rifting processes. By our results we get a better understanding of sedimentation processes in radiolaritic basins in orogenic belts and the onset of radiolarian chert in the context of orogenic processes. This is important for the whole region of the western Tethyan domain and should be the beginning for a lot of detailed reinvestigations of the alpine mountain belt in eastern and southeastern Europe and Asia.