Abyssal Cretaceous Varicolored Clays of the Northern Apennines of Italy: mineralogical, geochemical and paleontological insights

Abstract. The deep seafloor is one of Earth’s least observed environments and modern abyssal plains remain difficult to investigate directly. Here we present an integrated mineralogical, geochemical and paleontological study of the Cassio Varicolored Shales (AVV) of the Northern Apennines, interpreted as Cretaceous deep-marine deposits accumulated below the CCD and later incorporated into an accretionary prism. Although the analyzed shales share a broadly clay-rich composition, they record distinct diagenetic pathways controlled by redox conditions, organic matter availability, sediment input and pore-water circulation. Red shales represent an oxidized, organic-poor abyssal clay end-member, while black shales preserve Mn-carbonate trace fossils and tabular crusts formed through early Mn cycling, microbial organic matter degradation and rhodochrosite-rich carbonate precipitation. The three-dimensional preservation of pellet-rich burrows suggests rapid early cementation linked to macrofaunal activity and microbial Mn reduction. Gray-green shale records a carbonate-rich system influenced by sulphate reduction, with early calcite veins preserving pyrite framboids and carbon filaments. Overall, the AVV provides an ancient analogue for the small-scale heterogeneity of modern abyssal plains and shows how benthic activity, microbial metabolism, redox cycling and authigenic mineralization can be preserved in ancient deep-sea sediments.