Progressive increase in organic-matter burial and preservation from the “Weissert” event to the Faraoni event in Umbria-Marche (central Italy)

Abstract

The Cretaceous experienced several Oceanic Anoxic Events (or OAEs). Anoxia in these events is indicated by deposits of black shales, enriched in organic matter (OM) compared to the layers below and above, strong carbon isotope perturbations, often with a negative excursion at the onset of the OAEs followed by a positive excursion, and concentration of redox-sensitive trace-elements (RSTE) (Baudin & Riquier 2014). Considered to be the earliest Cretaceous OAE (Baudin & Riquier, 2014), the Faraoni level is a short event first defined in the late Hauterivian sections of the Umbria-Marche Apennines (Cecca et al. 1994). It presents black shales enriched in OM with high concentrations of RSTE but lacks an important positive δ13C excursion (Baudin & Riquier, 2014). This event follows the “Weissert” event, a ca. 2.3 million year carbon isotope perturbation event taking place during the late Valanginian-early Hauterivian (Sprovieri et al. 2006). This latter event is not considered to be an OAE, as anoxia indicators such as RSTE high concentrations or OM-rich layers are not observed at least in the western Tethys (Westermann et al. 2010). In order to link those two seemingly opposite events, sections of Late Valanginian to Early Barremian age were studied in the Umbria-Marche Apennines, Italy. Lesser magnitude black shales preceding the Faraoni level were identified. They were correlated in two sections using magnetostratigraphy (Fig. 1). Rock-Eval and palynofacies analyses reveal that they are part of a longer-term trend of increased organic matter preservation and burial. In the black shales this is hinted by a progressive increase of total organic carbon (TOC) content, of the hydrogen index (HI), and by increasingly better preserved amorphous organic matter (AOM) towards the Faraoni level (Fig.1). This increase starts in the upper part of the M5n magnetochron. This is coeval with an increase in mercury concentration interpreted to be due to volcanic activity that was measured among others in the Bosso section (Charbonnier et al., 2018). Palaeoenvironmental differences between the Bosso and Frontone sections are shown by differences in palynomorphs and in organic matter preservation, and by the presence of slumps found in Frontone only