The depositional architecture of Mass Transport Deposits from the Ventimiglia Flysch Fm. (Eocene, NW Italy): implications for seafloor reshaping and turbidite deposition

Submarine mass wasting is recognized as one of the main contributors to the reshaping of seafloors. A number of failure and transport mechanisms, including block and debris slides, slump and debris flow or any combination thereof are reported in the literature, reflecting the degree of disaggregation and the rheology of materials involved. Although recent advances in subsurface and seafloor imaging allow a gross characterisation of erosive and depositional features of mass transport deposits (MTDs), their sub-seismic architecture is still relatively understudied, limiting our comprehension of MTDs process sedimentology. This study investigates an extensive 20-80m thick MTD deposited as part of the Upper Eocene Ventimiglia Flysch Fm., the predominantly turbiditic infill of the Ventimiglia sub-basin of the Alpine foreland basin, NW Italy. The turbidites were sourced from the south and were deposited on top of the hemipelagic marlstones of the Middle Eocene Marnes Bleue Fm. (MB) within an N-S trending elongate depocentre. The MTD occurs around half way up the 1 km-thick preserved turbidite stratigraphy and crops out intermittently across a 5 by 15 km area. Field evidence of possible failure head scarps within MB (e.g. scalloped top and sections with reduced thickness), coupled with the paleotopography inferred from regional studies, suggest that the MTD resulted from a collapse of the western basinal slope via a mechanism of submarine slide block detachment. The main features of the MTD include from bottom to top: i) a substrate zone where the underlying turbidites are deformed and locally detached; ii) a basal MTD surface, which locally cuts up to 20m into the substrate; iii) a clast-supported chaotic megabreccia with irregular top, composed of metre to decametre-scale MB blocks with sharp margins and intact internal stratigraphy and rare rafts of turbidite sandstones and iv) a matrix-supported crudely graded monomictic conglomerate of MB, partially filling the top irregularities of the underlying unit and becoming dominant in MTD distal reaches. In addition outsized MB blocks (up to 10s of metres thick, and 100s of metres in length) occur either in the lower unit or at MTD edges as isolated rafts. The lower unit is interpreted as the product of a submarine debris slide involving blocks of semi-lithified MB, whereas the uppermost conglomerate unit is thought to represent a trailing cohesive debris flow which develops by frictional abrasion of MB clasts and can outrun the main MTD deposit. Lastly, isolated rafts of MB are inferred to represent out-runner blocks. Spatial variations in MTD internal character and thickness allow three zones to be distinguished: i) a proximal zone where erosion prevails over deposition; ii) a dominantly depositional medial zone with positive relief and iii) a quickly tapering distal/marginal zone with highly irregular planform. The MTD erosion/deposition budget detailed here adds to understanding where major ponded accommodation space is likely to occur in association with MTDs.