Origin and evolution of the Paleocene-Eocene chaotic facies from the Western Ligurian Flysch (Ligurian Alps, Italy)

Chaotic deposits represent ubiquitous features of the Alps and, in general, of convergent margin settings; consequently, their accurate interpretation is a prerequisite for reconstructing the subduction dynamics and basin evolution characteristic of the Alps and their analogues.
The study deal with the chaotic deposits of the Cretaceous-Eocene Western Ligurian (Helminthoid) Flysch of the Ligurian Alps. We focused on the Eocene part of the Flysch succession, deposited between the Briançonnais-Prepiemontese margin and the advancing orogenic wedge.  In particular we provide new mapping and stratigraphic analysis of two formations, i.e., the Colla Domenica Shales and the overlying Leverone Formation, which are exposed between the Prepiemontese and Piemontese-Ligure oceanic domains. The Colla Domenica Shales consist of fine-grained sediments embedding polygenic chaotic layers, stratigraphically overlain by the turbiditic deposits of the Leverone Formation. Traditionally interpreted as Cretaceous trench sediments influenced by mud diapirism, these successions are here reassessed through a comprehensive multidisciplinary approach—integrating field mapping, sedimentology, biostratigraphy, petrography, and geochemistry—to better define the tectonostratigraphic history of this Alpine segment.
New data indicate that these chaotic intervals are Paleocene–Eocene Mass Transport Deposits (MTDs), emplaced during the final stages of oceanic closure.
Key diagnostic features, including matrix-supported fabrics, erosional basal contacts, and disordered clast distribution, point toward high-energy gravity-driven processes triggered by slope instability at the front of a prograding accretionary prism. Geochemical signatures from basalt clasts (Transitional-MORB) suggest that products of early oceanic magmatism were first incorporated into the prism and subsequently remobilized during mass-failure events.  All the evidence supports a polygenetic origin of the mélanges, characterized by a main sedimentary nature due to MTDs interlayered with turbiditic deposits, partially overprinted by subsequent tectonic activity. Provenance analyses on siliciclastic sandstones document a source area located within the European margin, likely corresponding to the Corsica block.  An upward increase in sandstone immaturity indicates a progressively shorter transport distance of the sediments which can be explained by a closing basin. Indeed, the last depocenters of the Piedmont-Ligurian basin were characterized by two source areas located at the European margin (siliciclastic turbiditic sediments) on one side and the accretionary prism (ophiolite-bearing MTDs) on the other side. New findings contribute to understanding the origin and significance of chaotic deposits in collisional settings, improving our knowledge of depocenter migration, sediment-routing pathways, and accretionary prism dynamics during the closure of the Piemontese-Ligure Ocean.