20 My hiatus in the Jurassic of Umbria-Marche Apennines (Italy): carbonatecrisis due to eutrophication

A hiatus ranging from Early Bajocian to late Early Kimmeridgian is recorded on swells of Umbria-Marche Apennines (Italy). In morphostructural basins t his time-interval coincides with both biosiliceous sedimentation and a drop in the biodiversity. On swells, precursor microfacies of the hiatus, whose beginning coincides with a delta(13)C positive peak, are characterized by high density oligotypical associations alternatively dominated by crinoids, protoglobigerinids and Bositra buchii. The hiatus, the onset of biosiliceo us sedimentation and the fall of biodiversity are manifestations of a carbo nate productivity crisis, induced by highest eutrophication, which affected the Western Tethys during the Middle Jurassic. ((C) 1999 Academie des scie nces / Editions scientifiques et medicales Elsevier SAS.).