Plate tectonics and pelagic facies: Late Jurassic to Early Cretaceous deep-sea sediments of the Ybbsitz ophiolite unit (Eastern Alps, Austria)

Abstract

Pelagic sediments of the Ybbsitz Unit are part of an ophiolite sequence within the Penninic Unit in the Eastern Alps. Cherts and calpionellid limestones of Late Jurassic to Early Cretaceous age were deposited on oceanic basement. Sedimentation near an oceanic spreading center, below the calcium compensation depth (CCD), is represented by chert/clay rhythmites. Cherts display hydrothermal Fe and Mn mineralizations that are characterized by low contents of Co, Ni, and Cu as well as by high Fe 2O 3/TiO 2 and MnO/TiO 2 ratios. As the site of deposition migrated away from the spreading center, hydrothermal influence ceased. Due to the Late Jurassic drop of the CCD, calcareous radiolarian ooze was deposited on intrabasinal topographic heighs. Downslope transport of this ooze to sites below the CCD by turbidity currents resulted in intercalations of calcareous and carbonate-free sediments. Apparently the drop of the CCD was greater than the rate of cooling-induced basement subsidence, as calcareous radiolarites overly this sequence. Carbonate contents gradually increase upsection and micritic limestones terminate purely pelagic sedimentation. At the Jurassic/Cretaceous boundary, plate drift moved the site of deposition towards an active continental margin producing a coarsening upward clastic sequence. This sequence starts with turbidite and breccia intercalations in pelagic limestones followed by the deposition of Early Cretaceous flysch. It marks the onset of subduction of the Ybbsitz Unit under the Austroalpine continental margin.