Middle to Late Jurassic Basin analysis in the NCA

Research project P14131 Middle to Late Jurassic Basin analysis in NCA Hans-Jürgen GAWLICK 26.6.2000 The objective of the proposed project is to determine Middle to Late Jurassic basin and rise formation along a N-S cross-section through the western central part of the Northern Calcareous Alps (Berchtesgaden area) and to use these data to reconstruct the early tectonic history of the western parts of the Tethys domain. The large scale Early and Middle Jurassic evolution in the Northern Calcareous Alps has been interpreted as that of a rifted continental margin by many authors, although the exact timing and the relative proportions of extensional and lateral movements remained controversial. Several hypotheses have been proposed to explain the late Middle to early Late Jurassic tectonic phase, which was associated with the formation of asymmetric basins, large-scale sliding, halokinesis, and basin inversion. To reconstruct the Middle to Late Jurassic basin evolution and their geodynamic causes, the knowledge of age and facies of the sedimentary sequences and the paleogeographic position of the tectonic units (Juvavic units) is of special interest. Therefore, sedimentological, stratigraphic and facies data, together with diagenetic/metamorphic patterns also will be used to reconstruct the time of the polyphase emplacement of the nappes and to subdivide nappe complexes because the evolution of the Hallstatt Zone and its paleogeographic position relative to the main body of the Northern Calcareous Alps represents a key question for the reconstruction of Eastalpine geodynamics in Jurassic times. Therefore, the conclusions drawn from the Salzburg Calcareous Alps appear to be of general validity for a large part of the Northern Calcareous Alps, although detailed investigation is still lacking in most of these basins, especially in the Berchtesgaden area. Also the emplacement of the Juvavic units are discussed very controversial in this area (Oxfordian, Kimmeridgian, Tithonian or late Early Cretaceous). The results of GAWLICK et al. (1999) in the Salzburg area show that an important orogenic event occurred in Late Jurassic time, probably already starting in late Middle Jurassic. The basin and rise evolution in late Middle to early Late Jurassic in the Berchtesgaden area should be correlate with the basin and rise evolution in adjacent areas to the east (Lammer Basin, Trattberg Rise and Tauglboden Basin). Dating of the cherty limestones and radiolarites and investigations of the breccias in these series can solve the problems and allow to reconstruct the early tectonic history of the western parts of the Tethys domain. The western central part of the Northern Calcareous Alps (Berchtesgaden area) is the best area for the planned investigations. In these part of the Northern Calcareous Alps very good preserved radiolarite sequences are located. Also in this part of the Northern Calcareous Alps a good paleogeographic reconstruction of the Upper Triassic carbonate platform from the inner platform (Hauptdolomite and lagoonal limestone - Dachstein limestone) to the reefal area and to the basin carbonates (Hallstatt Zone divided into ZIambachfacies and Hallstatt Salzbergfacies) is established. Therefore, in this part of the Northern Calcareous Alps it is possible to carry out investigations to solve open questions due to the closure of the Tethys Ocean.

The objective of the project (July 2000 to October 2003) was to determine Middle to Late Jurassic basin and rise formation along a N-S cross-section through the western central part of the Northern Calcareous Alps (Berchtesgaden area) and to use these data to reconstruct the early tectonic history of the western parts of the Tethys domain. In this project the sedimentary record was completely investigated to reconstruct the tectonic history of the Berchtesgaden Calcareous Alps. The new geodynamic concept is in controverse to all former interpretations and based on very much stratigraphic and facies data, and basin analysis. The polyphase Late Jurassic radiolaritic basin evolution in the Berchtesgaden Calcareous Alps (Lammer Basin, Tauglboden Basin and Sillenkopf Basin) show a progressively migration form the southern rim to the central part of the Northern Calcareous Alps. These intracontinental deep sea-trenches in front of advancing napps are filled with carbonate-clastic, radiolaritic flysch. Cherty sediments are the matrix of mass-flow deposits and slides. The resedimented material in each basin differs and was mobilized in different source areas. The sedimentation in the Lammer Basin ends in Late Oxfordian due to nappe stacking. From the early Oxfordian onwards thrusting moves to the north. The Tauglboden Basin contains resedimented material from the front of this nappe (Trattberg Rise). Due to further tectonic shortening a new basin generation (Sillenkopf Basin) was formed in Kimmeridgian times in the area of the recent southern rim of the Northern Calcareous Alps. A Late Jurassic shallow water carbonate platform forms a post-tectonic cover of the late Middle to early Late Jurassic basin evolution. Faults, younger as late Late Jurassic or contemporaneous to the miocene lateral tectonic extrusion, destroyed the original basin configuration forming a modern block puzzle (e. g. Berchtesgaden Block) and not a cretaceous nappe stack as former interpreted. So the Berchtesgaden Block is not a Cretaceous (Juvavic) nappe coming from the south. Its a parautochthonous Block as shown by stratigraphic and facies data and was thrust to the north in Oligocene times. By our results we get a better understanding of sedimentation processes and "nappe emplacement" in orogenic belts and how to reconstruct a polyphase tectonic history by the analysis of polymict breccias. The results are important for the whole Eastern Alpine region and the western Tethyan domain and should be the beginning for a lot of detailed reinvestigations of the alpine mountain belt in Europe.