Jurassic radiolarians from cherty limestones below the Hallstatt salt mine (Northern Calcareous Alps, Austria)
Abstract:In the Northern Calcareous Alps, Alpine Haselgebirge salt deposits of Permo-Triassic age are widely distributed and excavated. Their tectonic position is discussed very controversially as in situ or transported. One of the key points to solve the question of the emplacement of the Alpine salt deposits is to date the surrounding/underlying siliceous sedimentary rocks. We examined the radiolarian age of two samples (BNU and NUS) from the ends of boreholes at the Hallstatt salt mine in the Salzkammergut area, Austria. The samples are cherty limestones of the Ruhpolding Radiolarite Group which underlies the Alpine Haselgebirge. The well-preserved late Middle Jurassic radiolarian fauna from these boreholes consists of more than 63 species, including a new subspecies Gongylothorax favosus oviformis. Taking the occurrence of Zhamoidellum ovum, Stichomitra annibill, Gongylothorax favosus and Tricolocapsa fusiformis into consideration, the sample BNU is correlated to the lower horizon of the Protunuma lanosus subzone in the Lower or Middle Callovian of the Zhamoidellum ovum zone. The age determination clearly confirms that the Alpine Haselgebirge of the Hallstatt salt mine lies tectonically on top of radiolarites of the Bajocian to Lower Callovian to Upper Oxfordian Ruhpolding Radiolarite Group. The salt mine is overlain by the shallowing-upward sequence of the Plassen Carbonate Platform, of which sedimentation had started here in (Early) Kimmeridgian time. Therefore, the emplacement of the Alpine Haselgebirge can be estimated as Late Oxfordian to Early Kimmeridgian.
Document Type: Research Article
Publication date: February 1, 2009
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