Sedimentary and tectonic evolution of Paleocene-Eocene chaotic facies from Western Ligurian Flysch (Italy)

Le résumé fourni par la source

Chaotic deposits are common in convergent margins and are crucial for understanding subduction dynamics and basin evolution. This study re-evaluates key chaotic deposits within the Western Ligurian Flysch, focusing on the Colla Domenica Shales and the overlying Leverone Formation along the European margin of the Piemontese–Ligurian Ocean. Traditionally interpreted as Cretaceous–Paleocene trench sediments influenced by mud diapirism, these deposits are reassessed using an integrated multidisciplinary approach combining fieldwork, sedimentological analysis, biostratigraphy, petrography, and geochemistry. Results indicate that chaotic intervals represent Paleocene–Eocene Mass-Transport Deposits deposited within a remnant oceanic basin developed in response to the subduction of oceanic crust. Key features such as disorganized clast distribution, irregular clast shapes, size variability, erosional bases, and matrix-supported fabrics, point to high-energy, gravity-driven processes at the accretionary prism front. Basalt clasts exhibit a Transitional-MORB signature, thus suggesting early oceanic magmatism later incorporated into the prism and remobilized during mass-failure events. Petrographic observations show a stratigraphic upward increase in sandstone immaturity, indicating shorter transport distances as the basin narrowed. Provenance analysis reveals siliciclastic sediments likely sourced from the Sardinia–Corsica segment of the European margin, while ophiolitic clasts suggest contributions from the prism interior. Despite local tectonic overprinting, structural data do not support a tectonic-mélange origin; instead, these deposits represent a polygenetic mélange formed by mass-transport events interlayered with basin-plain turbidites during trench infilling. These findings refine our understanding of chaotic deposits in collisional settings, shedding light on depocenter migration, sediment routing, and prism dynamics during the closure of the Piemontese–Ligurian Ocean.

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