Stratigraphy and geochemical anomalies of the Early Toarcian oxygen-poor interval in the Umbria-Marche Apennines (Italy)

A multidisciplinary approach is used to demonstrate the occurrence of oxygen-poor sediments in the Tenuicostatum Zone (Early Toarcian) within the clayey-marry pelagic sediments of Marne del Monte Serrone Formation (Umbria-Marche basin). This approach involves examination of some geochemical anomalies (trace elements), measurement of Total Organic Carbon and observation of benthic foraminifera and trace fossils. A distinction between ''black shale facies'' (BSF1) and 'black shale-like sediments'' (BSF2) allows better identification of the variable anoxic conditions present within the same basin. A high degree of organic matter preservation and major anoxia extensions occur in the middle-upper part of the Tenuicostatum Zone (Early Toarcian) probably related to maximum flooding - high stand of the global Early Toarcian sea-level rise. Several subenvironments showing pelagic conditions and high primary productivity of planktonic organisms (mainly radiolarians and calcareous nannofossils), have been defined on the basis of organic matter content and positive geochemical anomalies. The diversified physiography of the Umbria-Marche basin, related to synsedimentary tectonic activity and circulation patterns, probably determined different intensities of ventilation conditions on the sea-floor.