Abstract

The Late Jurassic Oberalm beds of the Northern Austrian Alps consist largely of fine-grained limestone with abundant radiolaria and other microplanktonic fossils. Pyrite occurs mainly in microscopic spheroidal aggregates (framboids) within radiolarian tests. These framboids are normally lodged in the lower part of the fossil sphere, suggesting that the pyrite was precipitated within the fossil and sank to the bottom of the sphere during early diagenesis. Subsequent precipitation of mineral matter filled the remaining void space. Thus, the pyrite as seen in thin section provides evidence of the gravitational field at the time of diagenesis and is a potential tool for recognition of tectonic overturning. In penecontemporaneous slump folds the pyrite either formed subsequent to folding, or, more likely, adjusted its position to the gravitational field in the overturned limb. Details of pyrite framboids are illustrated in electron photomicrographs.

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