The architecture of Mass Transport Deposits from the Ventimiglia Flysch Fm. (Eocene, NW Italy): insights into erosion/deposition budget of submarine mass wasting

During the last few decades submarine mass wasting and associated mass-transport deposits (MTDs) have been well documented via high-resolution imaging of both seafloors and their subsurface, thereby recognizing their role in... [ view full abstract ]

During the last few decades submarine mass wasting and associated mass-transport deposits (MTDs) have been well documented via high-resolution imaging of both seafloors and their subsurface, thereby recognizing their role in sediment transfer to deep water and seafloor reshaping. A number of failure and transport mechanisms are reported in literature, including block and debris slides, slump and debris flow or any combination thereof, reflecting rheology and progress of disaggregation of the materials initially involved in the failure. Despite recent advancement in geophysical imaging techniques, the internal heterogeneity of MTDs is still poorly detailed, limiting our understanding of their process sedimentology.

This study investigates a number of MTDs from the Ventimiglia sub-basin of the Alpine foreland basin, NW Italy. The basin was predominantly filled by turbidites (Ventimiglia Flysch Fm., VF; Upper Eocene) sourced from the south and deposited on top of the Marnes Bleue Fm. marlstones (MB; Middle Eocene) within a N-S elongate depocentre. MTDs of resedimented MB occur at different levels and locations within the basin fill, suggesting they resulted from collapses of basinal slopes via submarine slide block detachment. The best exposed MTD is up to 80 m-thick and spreads over a minimum area of 30 km2. Main constituents include, from bottom to top: i) a deep basal surface cutting into the substrate; ii) a zone where turbidites are deformed and locally detached; iii) a chaotic megabreccia composed of undeformed MB blocks (1-10s metres in size) and rare folded turbidite beds and iv) a crudely graded conglomerate of MB, infilling the top irregularities of the underlying megabreccia and becoming dominant at MTD peripheries. Outsized MB blocks (up to 10s of metres thick, and 100s of metres in length) occur either in the megabreccia or in association with the conglomerate. Proximal to distal and axial to lateral, rapid transitions (hundreds of metres) are observed from a thick megabreccia deeply eroding the substrate, to rafted MB floating in the conglomerate and sitting on deformed turbidites. Typically the MTD top has a rugosity of few to few tens of metres over lengths of hundreds to few thousands of metres, hosting confined accommodation space to post-mass wasting turbidites. The most plausible architectural interpretation is one of a focused mass-flow axis, with an erosional keel flanked by zones of deformed turbidites, which served to focus late-stage debris flow, that eventually overspilled laterally, and may have facilitated long-range transport of megaclasts.