The stratigraphic significance of turbidite deposition under partially ponded conditions: bed statistics and onlap geometries, Castagnola mini-basin (Early Miocene, NW Italy)

Confined depocentres (such as salt withdrawal minibasin or structurally confined basins in active settings) represent important depositional sinks for deep-water sediments. Most classic models of turbidite bed statistics and onlap geometries are underpinned by the assumption that as a confined basin fills, the transition from a fully ponded to a non-confined condition is represented by only a relatively minor proportion of the basin fill. Although tectonic deformation can provide a mechanism to increase the proportion of the fill that records partially ponded conditions, i.e., when basin subsidence and sedimentation rates are similar, this condition might not be very common. However, even in a structurally quiescent basin, flows of different volumes will have different abilities to escape the confining topography. Hence, the range of the flow volume distribution will define the minimum duration of the situation when at least some flows are partially ponded. Moreover, aggradation on the basin margins/sills can occur while deposition in the depocentre takes place. This probably most commonly occurs under partially ponded conditions (when the lower concentration and less erosive part of the flows are able to escape the confinement and deposit material on the basin sill). This process will increase the thickness of basin fill deposited under partially ponded conditions. The Castagnola sub-basin (Early Miocene, Tertiary Piedmont Basin, NW Italy) is a small (10-20 km2) minibasin filled by c. 1 km of southerly-sourced siliciclastic turbidites; there is no evidence of significant tectonic deformation during deposition. The northern and southern basin margins are fully and partially preserved, respectively, allowing different onlap geometries (feathered vs abrupt) to be recognised. The succession is tabular at the km scale, with erosion limited to a small number of thicker beds; net-to-gross evolves from 0.2 at the base to 0.7 toward the top. Thick mud caps in the lower section, along with presence of reflected ripples and overall sheetlike architecture of the turbidite system are suggestive of ponded deposition. While the gross evolution of the basin is that of a transition from dominantly ponded to non-ponded conditions, marked by the disappearance of thick mud caps and sandstone beds with reflected facies, a significant thickness of the stratigraphy (c. 400 m) shows less dramatic but still resolvable trends of net to gross increase and mud cap to sandstone bed thickness decrease, interpreted as a signal of long-lasting partially ponded conditions. The high proportion of the stratigraphy laid down under partially ponded conditions has implication for classic fill-and-spill models, turbidite beds pinch-outs and onalp geometries and for techniques used in the subsurface to infer ponding conditions (such as bed thickness statistics). Understanding the factors controlling the degree of ponding, together with any associated diagnostic signatures in the resulting deposits, represents an advance in establishing predictive models for confined basins.