Basal erosion and mudclast character and distribution in hybrid beds deposited in a ponded minibasin (Castagnola Fm, NW Italy)

Hybrid beds are commonplace in many deep-water clastic systems. They are characterised by a vertical association of sedimentary facies thought to be deposited under a turbulent regime (e.g. massive/laminated clean sand) and those deposited under a more transitional or cohesive flow regime (e.g. chaotic mudclast-rich and/or muddy sand), within a single event. Multiple models are proposed for their emplacement, focussing on the rheological changes within a flow, as well as their spatial and temporal occurrence. However our understanding of depositional processes, thus character and distribution of hybrid beds, remains limited. The Lower Miocene Castagnola Fm. (>1000m) records the deep-water infill of a small (a few km2 ) ponded piggyback sub-basin in the eastern part of the Tertiary Piedmont Basin (NW Italy). Six detailed sedimentary logs taken at similar distances along a 3.1 km proximal-to-distal transect encompass a low net-to-gross (0.2), 250 m thick interval. Generally, bed types comprise either thin (<1m), highly structured deposits, or thicker (1- 5m) beds. The latter are commonly hybrid bed-like in character with a poorly structured lower division, overlain by a recessive division frequently enriched in mudclasts whose size range is wide (from cm-sized up to 1-2m) and often bimodal. Erosional scouring and 'frozen' entrainment of mudstone substrate at bed bases allows at least some of the larger mudclasts to be directly related to local substrate rip-up. However, the occurrence of smaller mudclasts, with a vertically variable position within the bed, suggests that these were sourced further upstream and travelled greater distances prior to deposition. Such observations are consistent with other systems where hybrid beds contain fartravelled mudclasts (e.g. Braux system, Marnoso Arenacea system, etc.). Characterising the degree of local erosion below hybrid beds and the origin of mudclasts within might help to better understanding the depositional processes and the character and distribution of hybrid beds.