Lateral facies changes and architecture of hybrid-like beds deposited in a ponded minibasin (Castagnola Fm, NW Italy)

Hybrid-like beds (HEBs) are a class of gravity flow deposit including a basal sandy division followed by a chaotic division made up of both sand and clay in different proportions sometimes capped by an upper structured clean sandstone division. Although their existence has been documented nearly from the onset of the turbidite concept, HEBs have been the object of renewed attention in the last ten years, mainly as they can occur within hydrocarbon reservoirs. A range of generation mechanisms has been suggested, partly reflecting the wide variety of such deposits. Most models focus on the occurrence of rheological changes within the flow, with the chaotic mud-enriched division being deposited under transitional or laminar conditions. HEBs are thought to be more common at fan fringes or in close proximity to confining topography. However their depositional processes remain an object of on-going research, and thus our understanding of their character and distribution continues to evolve. The Lower Miocene Castagnola Turbidite System (800-1000m thick) records the deep-water infill of a small (a few to tens of km2) ponded piggyback sub-basin in the eastern part of the Tertiary Piedmont Basin (NW Italy). Six detailed sedimentary logs taken along a 3.1 km proximal-to-distal transect encompass a low net-to-gross (0.2), 250m thick interval. Generally, bed types comprise either thin (<1m), highly structured deposits, or thicker (1-5m) beds. The latter are commonly hybrid-like, with a poorly structured lower sandy division, overlain by a recessive division frequently enriched in mudclasts and capped by a sandstone that is usually highly structured. Thanks to the tabular nature of the bedding, all of these beds can be confidently correlated along the entire transect. The relative proportion of mudclast-rich intervals is highly variable, changing from over 75% of the bed to less than 25% within some tens of meters. The hybrid character does not appear to vary consistently with distality or proximity to the confining slope but it commonly occurs throughout the basin. This areal pattern challenges current models of hybrid-like bed distribution and has implications for better understanding the character and distribution of hybrid-like beds in similar settings in the subsurface.