Abstract

In order to date the beginning and the climax of the Jurassic oolitic-bioclastic sedimentation in the proximal basin facies of the Gran Sasso area, the stratigraphic sections of Pizzo Cefalone and Mount Portella (western part of the Gran Sasso range) have been studied. Ammonite, benthic foraminifer and calcareous nannoplankton biostratigraphy provide evidence that the input of a large amount of oolitic-bioclastic sediments, exported from the Apennine Carbonate Platform, began in the Toarcian and reached the climax from Bathonian to early Kimmeridgian. Analogous events of resedimentation during the Jurassic are documented in the proximal facies of other basins, both in the Southern Apennines (Lagonegro-Molise Basin) and the Southern Alps (Belluno Basin). This implies that shedding from carbonate platforms was very high in response to some event of regional or global relevance, but any attempt of correlations and interpretation need more detailed stratigraphic data. Secondarily, the existence of a calcarenitic cherty formation (Calcari Diasprigni Detritici) in the western part of the Gran Sasso range is shown. This formation, stratigraphically equivalent to the Umbria-Marche Calcari Diasprigni, has been ignored in the existing geological maps although it is well distinct from both the overlying Maiolica and the underlying Calcari Bioclastici Inferiori because of its high content in biogenic silica. The presence of this formation on the western part of the Gran Sasso range means that the Upper Jurassic biosiliceous facies spread from the Alpine Ocean to the slope of the carbonate Apenninic Platform.

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