Magnetochronology of late Eocene turbidites (Ventimiglia Flysch Fm) from the western Alps foreland basin

The late Eocene Ventimiglia Flysch Fm (FYV) of NW Italy is part of the greater Grès d’Annot system, which spans across the French-Italian border and represents the dominantly deep-water sedimentary fill of the foreland basin system of western Alps. Compared to the Grès d’Annot of SE France, the Ventimiglia Flysch Fm. has received relatively little research interest to date, which makes it timely to revaluate its significance, including age and accumulation rates. Previous studies suggest the up to ca. 1000m-thick FYV was deposited in a structurally confined elongate basin and is characterized by an overall sheet-like architecture (Dallagiovanna et al., 2017; Marini et al., 2022). The age of FYV is loosely constrained based on biostratigraphic age of the hemipelagic marlstone below (late Bartonian) and an earliest Oligocene foraminiferal assemblage characterizing the upmost part of the unit. This contribution presents the preliminary results of a magnetochronology investigation that aims at providing constraints for a depositional age model. A total of 170 samples for palaeomagnetic analysis were collected targeting the mudstone component of a ca. 700m-thick composite stratigraphic section, previously logged bed-by-bed for sedimentary facies. Natural remnant magnetisation (NRM) and anisotropy of magnetic susceptibility (AMS) were measured using a cryogenic magnetometer and an AGICO Kappabridge, respectively. Results of NRM analysis indicate that the composite stratigraphic section spans two magnetozones, namely a lower and ca. 500m-thick reverse-polarity zone and an upper normal-polarity zone. Considering the likely age of deposition of FYV from published biostratigraphy, only three reverse polarity chrones (i.e., C15r, C16r and C17r, having durations between 280 and 400 ky) are plausible matches for the zone identified. Although it was not possible to assign conclusively the reverse-polarity zone in the study interval to one of them, this information can be used to constrain the minimum sedimentation rate for FYV, which is 1200 m/Ma, placing it in the upper end of the range of turbidites deposited in large confined basins in active settings. The results of the AMS analysis indicate a disc-shaped fabric subparallel to palaeohorizontal. The direction of K1 varies within the study area and cannot be reconciled with the relatively uniform N-directed palaeoflow, suggesting the observed fabric is tectonic origin.