Early Cretaceous Oceanic Anoxic Events (OAEs) in Peri-Tethyan shallow-water carbonate systems: Evidence from the Latium-Abruzzi Carbonate Platform (Ernici Mts, Central Italy)

While the effects of OAEs are well known for the pelagic successions of the Tethys Ocean, little is known about their impact on the Peri-Tethyan shallow water carbonate systems. Here we present the preliminary results of a study related to the geological mapping of the sheet 390 – Frosinone of the Geological Map of Italy (CARG Project), focussed on the identification and description of the perturbation induced in the Lower Cretaceous shallow water carbonate succession of the Latium-Abruzzi Carbonate Platform by the well-known Early Cretaceous Oceanic Anoxic Events (OAEs).

In the Ernici Mts. (central Apennines, Italy), an Upper Triassic to Upper Cretaceous shallow-water carbonate succession is exposed (Cosentino et al., 2010; Fabbi et al., 2023). This study specifically examines the Lower Cretaceous "calcari ciclotemici a gasteropodi" fm. (CCG - Berriasian p.p. - lower Aptian p.p.), which mainly consists of whitish limestones with intercalations of light grey dolostones. Within this succession, a layer of black dolostone, about ten centimetres thick, has been observed in several outcrops of the dolomitic lithofacies (CCGa) of CCG, at the same stratigraphic position.

Two stratigraphic sections were measured to characterise the microfacies and compositional variations observed between the light-coloured (whitish to light grey) and black layers. SEM images, along with Energy-Dispersive X-ray Spectroscopy (EDS) and Wavelength-Dispersive X-ray Spectroscopy (WDS) analysis indicated the presence of siderite and pyrite aggregates (Meng et al. 2024). These aggregates appear in high concentration starting from the basal part of the blackish dolostone layer and gently decrease towards the upper part of the study interval. TOC and sulphates show similar trends.

Changes in chemical composition between the whitish and blackish dolostones (CCGa) were investigated in situ using the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) facility at Roma Tre University. The results show a significant increase in elemental concentration of P, Fe, Zn, As, Ba, Pb, and U, as well as in the Fe/Al ratio in the blackish dolostones. These elements are generally considered as redox-sensitive proxies associated with anoxic paleoenvironments (Bodin et al., 2007; Craigie, 2018).

Biostratigraphic calibration performed on the collected samples has established a Hauterivian p.p. age for the investigated CCGa levels. A preliminary attempt for U-Pb dating of the CCGa black dolostone was carried out through LA-ICP-MS investigations at Roma Tre and ETH facilities. In the Tera-Wasserburg diagram, the U-Pb measurements on CCGa black dolostone yielded a lower intercept age of 125.7± 1.8 Ma (MSWD=1.6; N=19). These promising results suggest that the changes in the elemental concentration of the redox-sensitive proxies observed in the CCGa black dolostone were induced by the late Hauterivian Faraoni Oceanic Anoxic Event.